9th Class Biology Notes Chapter 1 to 5 For Mardan Board
KPK Mardan Board 9th Class Biology Notes Chapter 1 to 5 short questions, and long question chapter wise.
Table of Contents
9th Class Biology Notes Chapter 1 (Introduction to Biology)
Q.1) How the understanding of physics can help the biologist?
Answer: Biology is the study of living organisms and physics is the study of matter and the laws of nature to understand the behavior of matter and the universe. Biophysics is an interdisciplinary study of the understanding of biological phenomena and problems using the principles and techniques of physics. Biophysics is used in physiology, bioenergetics, neurosciences, pharmacology etc. For example:
Physics helps explain how bats use sound waves to navigate in the dark
How wings give insects the ability to move through the air.
Photosynthesis follows the laws of physics during absorption of light.
Q.2) Which career would you like to adopt after studying biology and why?
Answer: After studying biology, I would like to adopt animal science or animal husbandry. It is a branch of biology which deals with the breeding and raising of livestock (domestic animals). Professional courses in this field are taught in many universities. I can also work for veterinary hospitals, the pharmaceutical industry, the livestock department, and the pet food industry or research in livestock and dairy development to fulfil the food requirements.
Q.3) Write the translation of any three verses of the Holy Quran related to the creation of mankind.
Answer: Verses of Holy Quran related to mankind:
Allah is Creator of all things, and He is Guardian over all things.
He created man of clay like the potter’s
Who made all things good which he created, and He began the creation of man from clay
Q.4) Name few Muslim scientists and their contributions in the field of biology and medicine.
Answer: Muslim scientists and their contributions Muslim scientists contributed a lot of knowledge to the science of biology and medicines from the 8th to 15th century. Their views were highly respected. Some of the famous Muslim scientists are: 1- Jabir Bin Hayyan (721 – 815 AD): He was born in Iran and practiced medicine in Iraq. He introduced experimental chemistry and also wrote a number of books on plants and animals. Among these “Al-Nabatat” and “Al-Hayawan” are noteworthy. 2- Abdul Malik Aasmai (740 – 828 AD): He was regarded as a specialist in animal sciences. He wrote many books on animals and plants. One of his books “Al-Kheil” is about horses, another, “Al-lbil” about camels, a third “A-sha” about sheep and fourth “Al-Wahoosh” is about wild animals. In the book “Khalaq-ul-Insan”, he had described different parts and functions of human body. 3- Bu Ali Sina (980 -1037 AD): He was among the greatest Muslims scientists and his most valuable contribution was in the field of medicines. He was an expert in Mathematics, Astronomy, Physics and Palaeontology. He worked on the structure, function and diseases of the eye. He described 130 diseases of the eye in his book. His book “Al-Qanun-fil-Tibb” was translated into many European languages. 4- Abu – Usman Umar Aljahiz: He wrote the famous book “Al- Haywan” which described his own observations on animals, such as the seasonal migration of fish in river Tigris. Similarly, he described the life system of an ant. 5- Al-Farabi: This renowned Hakim and biologist lived in 870 – 950 AD. He wrote two books “Kitab-ul-Nabatat” which is about plants and ” Kitab-ul- Haywanat” which is about animals.
Q.5) What level of organization is represented by volvox?
Answer: Colonial organization: Volvox is a green alga. It is a common example of unicellular colonial organisms. It lives in water. Hundreds of volvox cells make a small volvox colony. Many small colonies make a big colony. Most of the functions are performed by every cell. However, an individual volvox cell cannot live independently as the cells of the colony are dependent on each other. For example: There are cells which are specialized for mobility while other cells are for reproduction. Therefore, the division is above the cellular level and this is a trend towards the more complex division of labour in multi-cellular organisms.
Q.1) How the understanding of biology can be improved through the knowledge of geography, chemistry, and statistic? Give examples.
Answer: Linkage of Biology with other Fields of Study: The functions and metabolic pathways occurring in living organisms follow the principles of other fields of study like physics, chemistry, geography, economics etc. This type of linkage makes inter-disciplinary sciences. Some of the inter-disciplinary sciences are mentioned below: 1- Biogeography: Biogeography is the study of the distribution of organisms in space and time. For example: In biogeography, we study why polar bears are found only in certain parts of the arctic regions or why malaria-causing mosquitoes flourish in damp and warm areas. Scientists also study spread of disease in animals, plants, and humans by finding the source of that disease, to gain knowledge about the preservation of rare species that may have special needs, and to get informed about the changing geography of the world. For solving these problems, a scientist needs to identify the specific climates, foods and geographic features that different organisms acquire. 2- Biochemistry: Biochemistry is concerned with chemical substances and processes that occur in organisms. It involves the structural analysis of the organic compounds that make up cells and of those that play key roles in chemical reactions of life (e.g. photosynthesis and respiration reactions).3-Biostatistics or Biometry: Biostatistics is the application of statistical and mathematical formulas in the study of biology. It involves the design, analysis, and interpretation of research data. Nowadays it is a necessary field because after solving the biological problems results are usually presented statistically. Example: Use of mean, median and mode are simple statistical formulas used in biostatistics.
Q.2) Define biology. How can you describe your own body under different branches of biology?
Answer: Biology: “Biology is the branch of science which deals with the study of life.” The word biology is derived from two Greek words “Bio” means ‘life’ and “Logos” means ‘to study’. In 1736, a Swedish Scientist Carl Linnaeus for the first time used the word biology. Zoology is the study of animals e.g., human beings and all living beings. In zoology, I can study my body morphology, anatomy, histology, physiology, embryology, cell biology, immunology, genetics, biotechnology, and pharmacology. Biology has recently acquired new technological and conceptual tools to investigate, model and understand living organisms at the system level under the field of biotechnology. The bodies of all living things are made of different parts organized in a certain manner that supports life functions. This organization includes: Atoms → molecules → organelles → cells → tissues→ organs→ organ system→organisms
My body is made up of millions of cells. The cell is the structural and functional unit of the human body. When we study cells in a human body it is called cell biology.
If I study, the structure, form, and shape of my body. It is called morphology.
If I study the internal structure of my body like studying my digestive system or nervous system. This field of study relates to the field of anatomy.
When I study the endocrine and nervous systems of my body and how it functions. It belongs to the field of physiology.
If I study the trait of my height or skin colour. This study belongs to the field of genetics.
If I am suffering from a fever and needs to know the reason behind it and how to recover from that disease. This study relates to the field of immunology.
After having a disease when I take medicines to recover from that disease. This belongs to the field of pharmacology.
Q.3) Enlist the various levels of biological organization and explain it with an example.
Answer: Levels of Biological Organization: Millions of organisms are present on the Earth. They are different from each other and range from the simplest organisms i.e. bacteria to the most complex ones i.e. human beings. The bodies of all living things are made of different parts organized in a certain manner that supports life functions. This organization includes: Atoms → molecules → organelles → cells → tissues→ organs→ organ systems →organisms 1- Level 1: Sub-atomic particles and atoms: The living and non-living matter is composed of simple units called atoms. Each atom is further composed of sub-atomic particles (electrons, protons, and neutrons). The bodies of organisms are made of 16 kinds of elements. These elements are called bio-elements. Out of these bio-elements:
Only six (O, C, H, N, Ca and P) makes 99% of the protoplasm
Other 10% includes K, S, Cl, Na, Mg, Fe, Cu, Mn, Zn and I collectively make 1% of the protoplasm.
Level 2: Molecules and compounds: The atoms combine through bonds and form molecules and compounds. Similarly, the atoms of bioelements combine to form biomolecules. Types of biomolecules: There are two types of bio-molecules. i- Micromolecules: The bio-molecules with low molecular weight are called micro-molecules e.g. water, amino acids, glucose etc ii- Macromolecules: The bio-molecules with higher molecular weights are called macro-molecules e.g. starch, protein, lipids etc. Level 3: Organelles: Different bio-molecules combine together in a specific manner and form sub-cellular structures called organelles e.g. mitochondria, chloroplast, ribosome etc. Each organelle performs a particular function. For example, mitochondria provide energy to the cell and ribosomes prepare proteins. Level 4: Cells: The cell is the basic structural and functional unit of an organism. It consists of many organelles such as, Golgi bodies, mitochondria, lysosomes etc. which perform different specific functions inside the cell. In unicellular organisms, a single cell makes the whole body. While in multicellular organisms, there are more levels of biological organization. Level 5: Tissues: In multicellular organisms, the cells which are similar in structure and perform similar functions make groups. Such groups of cells are called tissues. Example: In plants, mesophyll tissue is made of cells which perform photosynthesis. In animals, glandular tissue is made of cells which produce secretions. Level 6: Organs: Different tissues work together to perform a specific activity. Such a group of related tissues is called an organ. Example: The organ stomach is made of two major tissues. Its epithelial (glandular) tissue secretes gastric juice for the digestion of proteins while its muscular tissue contracts and grinds food. Level 7: Organ Systems: The organs, in turn, coordinate and constitute an organ system which performs a particular function e.g. blood circulatory system in animals. The blood circulatory system is composed of organs like the heart, arteries, veins, and capillaries. Level 8: Organism: Different organ systems then collectively form a whole organism. In the case of man, different organ systems such as blood circulatory system, respiratory system, digestive system etc. work in coordination. Similarly, in plants, root and shoot systems constitute plant body.
Q.4) Explain the role of bio-elements for living organisms.
Answer: Bioelements: The living and non-living matter is composed of simple units called atoms. Each atom is further composed of sub-atomic particles (electrons, protons, and neutrons). The bodies of organisms are made of 16 kinds of elements. These elements are called bio-elements. Primary bio-elements: The atoms that make up all living things mostly consist of six different chemical elements: carbon (C), oxygen (O) hydrogen (H), nitrogen (N), phosphorus (P) and sulfur (S). These elements constitute more than 99% of the matter found in living things. They are called primary bio-elements. 1- Carbon (C): Carbon is the main bio-element that constitutes the biomolecules. It has the ability to assemble to form large carbon-carbon chains through single, double or triple bonds, as well as cyclic structures. The carbon atom is probably one of the most important bio-elements since all biomolecules contain carbon. 2- Hydrogen (H): Hydrogenis one of the components of the water molecule, which is essential for life and is part of the carbon skeletons of organic molecules. For example: Fatty acids have more electrons than carbohydrates, so they have the ability to produce more energy when they degrade. 3- Oxygen (O): Oxygen is the other element that makes up the water molecule. It is a very electronegative element that allows greater production of energy through aerobic respiration. 4- Nitrogen (N): Nitrogen is the element that is present in all amino acids. Through nitrogen, amino acids have the ability to form a peptide bond to produce proteins. This bio element is also found in the nitrogenous bases of nucleic acids. It is eliminated by the organism in the form of urea. 5- Sulfur (S): Sulfur is a bio-element that is an essential part of amino acids which form proteins. 2- Secondary Bioelements: Other 10% includes K, S, Cl, Na, Mg, Fe, Cu, Mn, Zn and I collectively make 1% of the protoplasm. These types of elements are also present in all living things but not in the same quantities as the primary elements. They do not form biomolecules but are used in cell concentration gradients, dielectric signaling of neurons and neurotransmitters, stabilize charged biomolecules such as ATP and are part of bone tissue. These bio-elements are calcium (Ca), sodium (Na), potassium (K), magnesium (Mg) and chlorine (Cl). The most abundant are sodium, potassium, magnesium, and calcium. 1- Calcium (Ca): Calcium is essential for living things because plants require calcium to build cell walls. It is an integral part of the human skeleton. Calcium affects muscle neuromuscular excitability (along with K, Na, and Mg ions and participates in muscle contraction.) 2- Magnesium (Mg): Magnesium is a secondary bio-element that is part of biomolecules since it is a cofactor of chlorophyll. It is present in the skeleton (70%) and in the muscles of animals. 3- Sodium (Na): Sodium is an important extracellular cation, participates in the homeostasis of the organism. 4- Potassium (K): Potassium participates in the homeostasis of the organism and in the propagation of the nervous excitation by channels of potassium. 5- Chlorine (Cl): Chlorine is present in the organism of living beings mainly as chloride ion. 6- Trace elements: They are present in some living beings. Many of these trace elements act as cofactors in the enzymes. The trace elements are boron (B), bromine (Br), copper (Cu), fluorine (F), manganese (Mn), silicon (Si), iron (Fe), iodine (i)>
Q.5) Who classify the living organisms into five kingdoms. Explain each kingdom with the living organisms included in it.
Answer: Five Kingdoms system: Robert Whittaker in 1968 classified all living organisms into five kingdoms. This is known as five kingdoms system of classification. These five kingdoms are; 1. Kingdom Prokaryotae 2. Kingdom Protoctista 3. Kingdom Fungi 4. Kingdom Plantae 5. Kingdom Animalia 1. Kingdom Prokaryotae (Monera): This kingdom includes all unicellular and microscopic organisms. These organisms lack a membrane-bounded nucleus and other organelles inside their cells. Their cell wall is made of murein. Examples: Bacteria and Cyanobacteria. 2. Kingdom Protista or Protoctista: This kingdom included eukaryotic organisms which are usually small and having differences with the organisms of other eukaryotic kingdoms. They usually live in aquatic habitats. Example: Algae, slime molds, protozoans (Plasmodium, Amoeba, and Paramecium) are some of the examples of this kingdom. 3. Kingdom Fungi: Fungi include organisms which are all eukaryotic and multicellular. Their cell wall is made of chitin (a polysaccharide). They do not have chloroplast, so they cannot synthesize their own food and are therefore heterotrophic. The food is stored inside the body in the form of glycogen. Example: Some of the examples are Mushrooms, Molds, Puffballs, yeast etc. 4. Kingdom Plantae: This kingdom includes multicellular and eukaryotic organisms which are autotrophic in nature. Their cell wall is made of cellulose and mostly lack centrioles inside their cells. The reserve food in the form of starch. Examples: Brassica, Mulberry, Pinus, Ferns etc. 5. Kingdom Animalia: This kingdom includes multicellular and eukaryotic organisms which are heterotrophic in nature. These organisms lack the cell wall and centrioles are present in the cells. The reserve food is in the form of glycogen. The members of this kingdom can be split into two groups that are vertebrates and invertebrates. Example: Animals include jellyfish, worms, insects, fish, reptiles, birds, mammals etc.
Biology Class 9th Notes Chapter 2 Mardan Board (KPK) (Solving a Biological Problem)
Q.1) What is a science? How does it work?
Answer: Science: The knowledge which is based on observation, experiments and facts is called science. Biology is classified as natural science. Systematic approach to science: Science involves more than just gaining of the knowledge, and thus it is the systematic and organized inquiry into the natural world and its phenomenon. The scientific method is a process for experimentation that is used to explore observations and answer to questions. Scientists use the scientific method to search for the cause, effect, and relationships in nature. In other words, they design an experiment so that changes to one item cause something else to vary in a predictable manner. So, science works on the phenomenon of scientific methods, which helps biologist to focus on science project fair questions, construct a hypothesis, design, execute and evaluate the problem.
Q.2) Control group is important for scientific study, How?
Answer: Control group: “Control group is the standard to which comparisons are made in an experiment.” “Or” “Control group is a group separated from the rest of an experiment where the independent variable being tested cannot influence the results.” It can be positive control groups where the conditions guarantee a positive result and negative control groups where conditions produce a negative outcome. During experiments, a researcher made two arrangements i.e, “experimental group” and “control group”. Both groups are kept under the same condition except for the condition which is being tested. For example: If you want to do an experiment to test the necessity of carbon dioxide for the process of photosynthesis. you will arrange two similar plants and will provide soil, water, and sunlight to both plants. you will not provide carbon dioxide to one plant (experimental group) and provide carbon dioxide to the other plant (control group). The necessity of carbon dioxide will be proved when photosynthesis does not occur in the experimental group but occur in the control group.
Q.3) What deductions were developed during the study of malaria?
Answer: Deductions made during the study of malaria: Biologists further built upon the ancient observations and discovery of Laveran. They developed a hypothesis i.e. “Plasmodium is the cause of malaria.” For testing this hypothesis through experiment, biologists made a deduction; “If Plasmodium is the cause of malaria, then all malarial patients should have Plasmodium in their blood.”
Q.4) How Ronald Ross conducted the experiment to prove that mosquitoes are involved in the spread of malaria?
Answer: Experiment by Ronald Ross: Ronald Ross was a British army physician who worked in India in the 1880s. He performed important experiments to test the deduction that “If mosquitoes transmit Plasmodium, then Plasmodium should be present in mosquitoes.” He also received Nobel Prize for his work on the transmission of Plasmodium. He allowed a female Anopheles mosquito to bite a malarial patient. He killed the mosquito and found Plasmodium multiplying in mosquito’s stomach. In the next experiment, he allows an infected mosquito (having Plasmodium) to bite a healthy person. If the hypothesis was true, the healthy person would have gotten malaria. But he did not use human beings for such a risky experiment. Ross experiment on sparrows: Ross performed his experiment again but used sparrows instead of human beings. He allowed a female Culex mosquito to bite a sparrow suffering from malaria. He killed some of the mosquitoes and studied them. He found that Plasmodium multiplied in the walls of the mosquito’s stomach and then moved into the mosquito’s salivary glands. He allowed infected mosquitoes to bite healthy sparrows. Ross found that these healthy sparrows got malaria. When he examined the blood of these previously healthy sparrows, he found many Plasmodium in it. In this way, biologists had got the solution of the 2nd biological problem (How Plasmodium is transmitted to human beings?). The hypothesis (Mosquitoes transmit Plasmodium.) was proved true and taken as the solution of a biological problem.
Q.5) At what stage of the biological method, hypothesis, is accepted or rejected?
Answer: Experimentation: Biologist performs experiments on his hypothesis and checks the deductions. Through experiments he can find that deductions of some hypothesis have come true while other have not. In this way, the hypothesis are proved as true or false. The false hypothesis are rejected while the true one are accepted. Biologists make new deductions from the accepted hypothesis. Then he perform further experiments and confirms the correctness of hypothesis. There is a possibility that the results of experiments do not support any hypothesis. Under these conditions, new hypothesis are developed and tested.
Q.1) Differentiate between inductive reasoning and deductive reasoning?
Deductive reasoning starts with a general theory, statement, or hypothesis and then works its way down to a conclusion based on evidence.
Inductive reasoning starts with a small observation or question and works its way to formulate a theory by examining the related issues
Deductive reasoning is the observation from general to specific
Inductive reasoning is the observation from specific to general
It moves from broader information to specific information
It moves from specific information to broader information
Moves from theory to observation
Moves from observation to theory
Also called a top-down approach
Also called bottom-up approach
Example: If all organisms have cells and human is also an organism, then we conclude that all humans should have cells
Example: We observe that pigeon has wings and pigeon is a bird, so we conclude that all birds have wings.
Q.2) Explain how biologists use a scientific method to solve the mysteries of addressing the malarial problem?
Answer: Biological Problem: “What is the cause of malaria.?” Step 1: Observations: The people and physicians in ancient times (more than 2000 years ago) had some observations on malaria. For example:
The patients of this disease experienced recurring attacks of chills and fevers.
The disease was more common among people who lived in low marshy areas.
When some volunteers drank the water from marshes, they did not develop malaria. Thus a new observation was made i.e. the disease did not result from drinking the water of marshes.
Then they began thinking that stagnant water of marshes poisoned the air. When people breathed in this bad air, they got malaria. That is why they gave the name malaria to this disease. The Italian words mala means “bad” and aria means “air”. These observations did not help much for solving the problem i.e. “What is the cause of malaria?” Work of Laveran: Two hundred years later, it was proved that many diseases are caused by micro-organisms like bacteria. In 1878, a French Physician Laveran studied the ancient observations and work. He observed the blood of a malarial patient under a microscope. He noticed some microorganisms in the blood. He won the Nobel Prize for his discoveries. Five years after Laveran, the same microorganisms were observed in the blood of malarial patients and this microorganism was given a name of Plasmodium. Step 3: Experiment and Results: In order to test the hypothesis, “Plasmodium is the cause of malaria”, carried out the following experiment.
They examined the blood of 100 malarial patients under a microscope. It was labeled as the experimental group.
They also examined the blood of 100 healthy persons under a microscope. It was labeled as the control group.
Results: It was observed that all the malarial patients had Plasmodium in their blood; whereas the blood of healthy persons was free from Plasmodium. These results supported the hypothesis that Plasmodium is the cause of malaria. So it was taken as the solution to a biological problem.
Q.3) Explain that how mathematics can be used to interpret the data obtained through experimentations.
Answer: Biometry/ biostatistics is the application of statistical and mathematical formulas in the study of biology. so experiments can be interpreted or solved by the application of biometry. Data Organization: The researchers collect and analyze the data (information such as names, dates, numbers, values etc.) at various steps in the scientific method. This data helps them to make a hypothesis from observations and to conclude results from experiments. In order to use data in a scientific method, scientists have to organize and analyze it. Scientists organize the collected data in the form of graphs, tables, flowcharts, maps, and diagrams. Data Analysis: The collected data is analyzed by using statistical methods like ratios and proportions. A ratio is a relationship with respect to relative size between two quantities of the same kind. A pure number to a pure number; an amount of money to an amount of money; a number of people to a number of people. Proportion means to join two equal ratios. a:b=c:d “a” and “d” is called the extremes, “b” and “c” are called the means. The product of the extremes is equal to the product of means. When three values in a proportion are known, the fourth one (a) can be calculated by using this rule. For example: If a biologist wants to know how many sparrows would be infected with malaria if he allows Culex mosquito to bite 50 sparrows. Previously in one of his findings, he already noticed that 6 out of 10 sparrows get malaria if bitten by Culex mosquito. Here he applies proportion rule. a: b= c:d a: 50 = 6:10 a/50 = 6/10 a x 10= 50×6 a= 30 It means that if he allows Culex mosquito to bite 50 sparrows, 30 out of them will get malaria. Proportion method can be used to draw conclusions. As in the above case, we can find the fourth proportion of the data if three proportions are already available.
Biology Class 9th Notes Chapter 3 Mardan Board (KPK) (Biodiversity)
Q.1) How does deforestation lead to desertification?
Answer: Deforestation: “Deforestation is one of the major cause of the loss of biodiversity.” Forests are a vital resource for humans. They are the natural factories for the production of oxygen. Forests provide habitats to wildlife. They are also the source of fuel, timber and also provide recreation to man. Forests control floods, prevent the formation of deserts and keep the air clean. Causes/reasons of deforestation: Some of the major reasons for deforestation are:
Extension of cities requires more land, thus forests are cut to build roads and houses etc.
Ever increasing population need more land for agricultural activities.
Trees are also removed to develop pastures for grazing.
Wood is used both as timber and as fuel wood.
Timber mafia cut the trees for easy cash. All these factors lead to the removal of trees.
Effects of deforestation:
Deforestation leads to soil erosion
Destruction of habitats for wildlife
Extinction of many species
Floods, climatic changes, and desertification (the process of formation of deserts).
Q.2) Why is it important for a biologist to understand biological classification?
Answer: The biologist has identified and described 2 million kinds of organisms (0.5 million types of plants and 1.5 million animals). These are only a small percentage of the total kinds of estimated which actually live on earth. To study a large collection of organisms, biologist classifies them. Classification is a method by which biologists divide organisms into groups and subgroups on the basis of similarities and differences. It is important for a biologist to understand biological classification because this classification allows scientists to identify, group, and properly name organisms according to the rules of taxonomy which is based on similarities found in the organisms DNA/RNA (genetics), their adaptations, and embryonic development. In order to do this, it is very important for scientists to have first agreed upon a standard approach to naming and classifying an organism because without it unknown organisms could be classified differently. However, by only using the modern international taxonomic approach (domain, kingdom, phylum, class, order, family, genus, species) first proposed by Linnaeus, all organisms are named and grouped in the same manner making them more easy to study and keep track of them.
Q.3) What is the status of viruses in classification?
Answer: Classification of a virus: Virus classification is the process of naming viruses and placing them into a taxonomic system. Viruses are mainly classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause. Viruses are unique particles having circular RNA only. They are not considered as living organisms and are not included in the classification system. Living characteristics of viruses: They have some living characteristics. They reproduce in host cells only and contain RNA or DNA, hereditary material which is enclosed in a protein coat. Non-living characteristics of viruses: They also have non-living characteristics. They are not made of cells and do not respire or excrete out the host cells. They live as parasites in all types of organisms and cause diseases in them. They use the metabolism of the cells in which they live. Outside the living host cells, they are inactive. Viruses can be crystallized and this is a property of non-living materials. In crystal form, they can be stored for an indefinite period of time, without any loss of reproductive ability. When the crystallized viruses are placed under favorable conditions, they again start reproducing.
Q.4) How you can differentiate between kingdom Monera and Protista?
Kingdom Monera is unicellular organisms having a prokaryotic cellular structure.
Protistas are also unicellular organisms though possess eukaryotic cellular organization.
These organisms lack membrane-bound organelles
Well defined membrane-bounded cellular organelles are present in organisms of kingdom Protista
Monera are simple in structure, with no complexity
Protista are complex in structure
Monera’s does not possess a true nucleus.
Protists have a well-defined nucleus, bounded with a nuclear membrane
They are heterotrophic but some produce their food by photosynthesis or chemosynthesis
Some protists contain chlorophyll and make their own food while others lack chlorophyll and cannot prepare their own food.
Examples are bacteria and blue green algae (Cyanobacteria)
Examples of protists are Plasmodium, Euglena, Paramecium, Chlymadomonas etc.
Q.5) List down the endangered species of Pakistan and what are the reasons behind their population decrease.
Answer: Endangered species of Pakistan: In Pakistan, the main issues of conservation of biodiversity include deforestation and hunting. We have only 5.2% of forest in our country. Similarly, many species have become extinct or endangered. Causes:
Over-fishing in rivers, lakes, and ponds has resulted in the reduction of many fish species like shermahi, mahasher etc.
Macropolo sheep and Houbara bustard (Markhor) population have been reduced due to overhunting.
Musk deer are killed for their glands used for making perfume.
Asiatic cheetah, Indian horned rhinoceros, swamp deer are near to be extinct in Pakistan.
The most serious threat faced by Indus river dolphin is the release of heavily polluted water into the sea, which is causing the blindness of these dolphins.
Q.1) Differentiate between the two kingdom system and five kingdom system along with their advantages and disadvantages.
Five kingdom system
It was proposed by Carolus Linnaeus in 1751
Five kingdom system was proposed by Robert Whittaker in 1959
Organisms are classified into 2 kingdoms
Organisms are classified into 5 kingdoms
The two-kingdom system is based on nutrition and motility
Five kingdom system is based on the cell structure, complexity of body, mode of nutrition, mode of reproduction and evolution.
Advantages of two-kingdom system:
The two-kingdom system is based on nutrition and motility.
It initiated the systematic classification of living organisms.
Disadvantages of two kingdom system:
This system of classification has not dealt upon the status of viruses.
The placement of organisms like Euglena, bacteria, fungi in Plantae is questionable due to certain reasons
In the two kingdom system, the prokaryotes and eukaryotes are placed together and the difference between them is not clear.
Plants are autotrophs while fungi are heterotrophs but both are placed together in Plantae in two-kingdom system
Advantages of five kingdom system:
Five kingdom system is based on the cell structure, complexity of body, mode of nutrition, mode of reproduction and evolution.
In the five-kingdom system, organisms are better placed on the basis of body, cell structure, and evolution
In this system, prokaryotes are placed in kingdom Monera and eukaryotes are placed separately
Fungi are placed in kingdom Fungi and plants are placed separately in kingdom Plantae.
Disadvantages of five kingdom system:
The main disadvantage of five kingdom system is it does not specify the position of viruses.
The five kingdom does not specify the protists properly because some are heterotrophic and some are autotrophic
Q.2) Take one organism and assign it to different ranks according to the hierarchy of taxonomy.
Answer: Taxonomic Hierarchy of Taxonomy:
Q.3) How human activities affect the biodiversity of the area?
Answer: Impact of human beings on biodiversity: Due to human activities, the biodiversity of many areas and the Earth as a whole is decreasing day by day. Some of these activities are as under 1. Habitat loss and deforestation: All species have specific food and habitat needs. Habitat loss means the destruction of the natural habitat of species. Humans are utilizing land and natural resources and destroying valuable habitats of wildlife. For example: In order to cope with the demands of food, people are clearing forests and developing agricultural lands and residential colonies. In Pakistan, forests cover only 2.5% of the land. The forest area in Pakistan is shrinking very fast. Pakistan has the highest annual deforestation rate in Asia. According to a report of WWF, since 1947 more than 151,500 acres of forest land have been converted to non-forest land. 2. Over-hunting: Over-hunting of animals is also a cause of loss of biodiversity. In Pakistan, various lizards, snakes, crocodiles, and larger mammals are hunted for various purposes. A large number of migratory birds passing through Chitral are hunted and killed during their migration. 3. Introduction or removal of species in a new habitat: When a new species is introduced in an ecosystem, it may prove harmful for the existence of other species living there. For example: Eucalyptus trees were imported to Pakistan from Australia. These trees consume more water and have disturbed the level of underground water. So other smaller plants cannot grow near these trees. The removal of a species from an ecosystem also affects other species. For example: In an ecosystem, starfish eats mussels which are harmful to many other species. If starfish is removed from that ecosystem, the mussels will increase in number and they will be harmful to other species. 4. Rapid industrialization: The chemical pollutants released from industries are harmful to species and ecosystems. Pollution is not recognized as a cause of extinction, but it usually harms species which live in small areas.
Q.4) Explain binomial nomenclature. What are its advantages?
Answer: Binomial Nomenclature: Carolus Linnaeus (1707-1778) introduced the system of binomial nomenclature. Linnaeus listed about 4000 different species of animals and plants and gave a particular description to each of them. Rules of binomial nomenclature:
According to this system, every type of animal and plant should have a particular name consisting of two parts (words).
The first word is the name of the genus while the second word is the name of species. (specific name)
The generic name begins with a capital letter and the specific name begins with a small letter.
When a scientific name is typed, it is Italicized.
When it is handwritten, two parts are separately underlined.
Advantages of binomial nomenclature: Some of the advantages of binomial nomenclature are:
These names are more definite and precise than ordinary names.
In binomial nomenclature, a single scientific name is used in the whole world.
It simplifies the study of species.
The generic name is common to all the species of a particular genus because they all show common features of resemblance with one another, while the specific name is based on certain definite and specific characters, which differentiate the individuals of a particular genus from one another.
Being generally in Latin, they have a universal acceptance by people of all languages.
They indicate the generic and evolutionary relationship of individual animal and plant.
For example: In the animal kingdom, the three carnivorous animals i.e. cat, lion, and tiger are given the same generic name, Felis. They are given different specific names i.e. Felis domestica, Felis leo, and Felis tigris respectively. Canis familiaris is the name for the dog, whereas Canis lupus is the wolf. The first name (Canis) shows that these two species are in the same genus.
Q.5) What are the reasons for the extinction of biodiversity worldwide and what measures are required to conserve the biodiversity of Pakistan?
Answer: Reasons for the extinction of biodiversity worldwide: 1. Habitat loss and deforestation: All species have specific food and habitat needs. Habitat loss means the destruction of the natural habitat of species. Humans are utilizing land and natural resources and destroying valuable habitats of wildlife. 2. Over-hunting: Over-hunting of animals is also a cause of loss of biodiversity. In Pakistan, various lizards, snakes, crocodiles, and larger mammals are hunted for various purposes. A large number of migratory birds passing through Chitral are hunted and killed during their migration. 3. Introduction or removal of species in a new habitat: When a new species is introduced in an ecosystem, it may prove harmful for the existence of other species living there. For example: Eucalyptus trees were imported to Pakistan from Australia. These trees consume more water and have disturbed the level of underground water. So other smaller plants cannot grow near these trees. The removal of a species from an ecosystem also affects other species. 4. Rapid industrialization: The chemical pollutants released from industries are harmful to species and ecosystems. Pollution is not recognized as a cause of extinction, but it usually harms species which live in small areas. Conservation of biodiversity in Pakistan: In Pakistan, the main issues of conservation of biodiversity include deforestation and hunting. We have only 5.2% of forest in our country. Similarly, many species have become extinct or endangered. Precautionary measures to conserve the biodiversity of Pakistan: In Pakistan, national parks are made to conserve the biodiversity of an area. Pakistan has 29 national parks. Hunting, polluting water, cleaning of land for cultivation and destruction of wildlife are banned in conservation parks. However, they are accessible to the public for recreation, education, and research. Following are a few steps taken in Pakistan to conserve biodiversity.
Indus Dolphin Project (IDP) to save Indus Dolphin
Protected Areas Management Project in Machiara in Azad Jammu and Kashmir
Marine Turtle Conservation Project
Ban on the hunting of markhor and urial in Balochistan
Himalayan Jungle Project to protect the biodiversity in the Himalayan region
Conservation of biodiversity of the Suleiman Range, Balochistan
Northern Areas Conservation Project
Conservation of migratory birds in Chitral, Khyber Pakhtunkhwa
Himalayan Wildlife Project to check the hunting of brown bears
Conservation of Chiltan Markhor
Ban on Bear-baiting in Pakistan
Biology Class 9th Notes Chapter 4 (Cells and Tissues)
Q.1) Who proposed the cell theory and what are the main points of the cell theory?
Answer: Cell theory: 1838, a German botanist Mathias Schleiden claimed that all plants were made up of cells. This was supported by a German zoologist, Theodor Schwann, in 1839 who proved that animals were also made of cells. Thus, Schleiden and Schwann proposed that all living organisms are made of cells and presented cell theory. It was the first statement of the cell theory. After the first observations of life under the microscope, it took two centuries of research to formulate the ‘cell theory’. Main points of cell theory: Schwann and Schleiden summarized their observations into three conclusions about cells:
All animals and plants are made up of cells and cell products. Some organisms are unicellular and some are multicellular.
Cells are the structural and functional units of living organisms.
New cells arise from the pre-existing cells by cell division.
Q.2) Differentiate between simple and compound tissues.
Answer: Differences between Simple and Compound tissues:
They are composed of only one type of cells performing a common function
They are composed of more than one kind of cells and type of cells perform a common function
These type of tissues usually cover the surface of external and internal organs
They are found in the vascular bundles
Simple tissues are mainly concerned with growth and variety of functions
Compound tissues mainly involved in the process of conduction in plants
The cells are packed tightly together.
They are scattered throughout the body.
Example: Epidermal (in plants) tissues
Example: Xylem and phloem
Q.3) How cell membrane helps in maintaining equilibrium while exchanging materials with the environment?
Answer: Cell membrane separates the cell content from their external environment. Cell membrane is a selectively permeable membrane which selectively allows the movement of molecules across it. So it controls the movement of materials between cell and environment. It also enables the cell to form separate compartments within the cell in which specialized metabolic processes such as photosynthesis and aerobic respiration can take place. Membranes also act as receptor site for the attachment of hormones, neurotransmitters and other chemicals. Thus, it controls the movement of materials passing through it. It allows only the passage of water and other small molecules such as gases and other small molecules such as glucose, amino acids, fatty acids, glycerol, and ions can diffuse slowly through them. So, maintaining equilibrium by this membrane while exchanging material is actually its innate property.
Q.4) Differentiate between endocytosis and exocytosis.
Endocytosis is a process that brings macromolecules, large particles, and even small cells into the cell.
Exocytosis is the process by which materials packaged in vesicles are excreted from a cell.
In endocytosis, the cell membrane invaginates (folds inward) and takes in the materials from the environment, forming a small vesicle
The initial event in this process is the binding of the vesicle membrane with the cell membrane.
The vesicle separates from the plasma membrane and migrates with its contents to the cell’s interior.
The contents of the vesicle are released into the environment and the vesicle membrane is incorporated into the cell membrane.
Example: Engulfment of bacteria by the white blood cells during defense mechanism
Example: The release of hormones out of the cell
Q.5) How does turgor pressure develop in a plant cell?
Answer: Process of turgor pressure development: When a cell is surrounded by water or hypotonic solution, the water moves into the cell vacuole by osmosis. The vacuole increases in size and pushes the cell contents against the cell wall. This pressure which is exerted by the cytoplasm against the cell wall is known as turgor pressure and the phenomena is called turgidity. In turgid condition, the plant cell does not burst because the cell wall is strong and relatively inelastic.
Q.1) Root hairs are adapted to absorptions and xylem to support. Relate their functions to their structure.
Answer: Adaptation of root hairs to their function: The function of root hairs is to collect water and mineral nutrients present in the soil and take this solution up through the roots to the rest of the plant. To perform this function the root hair cells have specific adaptations.
Root hairs are long and narrow in their shape that increases their surface area to absorb water and minerals efficiently.
The cell sap of root hair has higher salt concentration than surrounding water to absorb water through its movement along the gradient.
Adaptations of xylem tissues to their function: The main purpose of the xylem is to transport water and minerals up the stem, from the roots to the leaves. In order for a xylem to carry out its functions successfully and efficiently, the xylem cells have specific adaptations.
Xylem tissues support the overall structure of the plants because of the presence of two types of cells i.e. vessels and tracheids.
Vessels cells are short, wide and have thick secondary cell walls. These cells are dead and hollow and join together to form long tubes.
Tracheids are long, slender cells with overlapping ends. Water move upwards from tracheids to tracheids.
Xylem tissue is responsible for the transport of water and dissolved substances from roots to upper parts. Due to the presence of lignin in the secondary walls of its cells, xylem tissue also provides support to the plant body.
Q.2) Discuss different types of tissues found in plants. Elaborate your answer with relevant diagrams.
Answer: Plant tissues: “A plant tissue is a cell or a group of cells which are structurally and functionally similar and give rise to a large number of cells.” Types of plant tissue: Plant tissues are mainly classified into two groups. I) Simple tissues II) Compound tissues I) Simple tissues:Simple tissues are composed of only one type of cells. They are further classified into two main types which are: i) Meristematic tissues (Embryonic tissues) ii) Permanent tissues i) Meristematic tissues (Embryonic tissues): Meristematic tissues (Embryonic tissues) are made up of those cells which have the ability to divide and produce new cells at the growing points of a plant. The cells of meristematic tissue have dense cytoplasm, a large central nucleus with small or no vacuoles. Cells are alike, with no intercellular spaces and have thin walls. Types of meristematic tissues: Meristematic tissues are of two types: a) Apical meristem: Apical meristem is present at the apex of root and shoot. Here the cells of meristem divide and re-divide and result in the elongation of stem and root. Such a growth is called primary growth. b) Lateral meristem: Lateral meristem is present on the lateral sides of roots and shoot. Here, the cells divide and result in the increase of the thickness of root and shoot. Such a growth is called secondary growth.
ii) Permanent tissues: The cells of these tissues lack the ability to divide. They originate from the primary meristem. The permanent tissues consist of epidermal, ground, supporting or mechanical tissues. a) Epidermal tissues: These tissues are present as the outermost protective covering of leaf, stem, and roots. Cells of the epidermal tissues are flattened and irregular in shape. They are thickly walled and are closely packed with no intercellular spaces. In the stem, the walls of these cells are covered with a waxy material which prevents loss of water. In leaves, the epidermal tissue has small guarded openings called stomata, for gaseous exchange. b) Ground tissues: This tissue is present in all parts of the plant except the epidermal tissue and vascular tissue. It is composed of thin-walled cells called parenchyma cells. These cells are oval, spherical or polygonal in shape. Their walls are thin and they store food. In leaves, the ground tissue contains chlorophyll and is called mesophyll tissue which prepares food. c) Supporting and mechanical tissues: Plants need support to maintain their shape and remain stable. Supporting tissues provide strength and flexibility to the plant. These are of two types.
Collenchyma tissue is found in young stems where it provides strength to different parts of the plant. The cells of this tissue are living, elongated, and polygonal with tapering ends.
Parenchyma tissues are thick walled dead cells. Walls of these cells are heavily filled with lignin which provides hardness and strength to the cell.
II) Compound tissues: These are the tissues which are composed of more than one type of cells, performing a common function. Xylem and phloem tissue is found in the vascular bundles are examples of compound tissues. i) Xylem tissues ii) Phloem tissues i) Xylem tissues: Xylem tissue consists of two types of cells i.e. vessels and tracheids.
Vessel cells are short and wide and have thick secondary cell walls. These cells are dead and hollow and join together to form long tubes.
Tracheids are long, slender cells with overlapping ends. Water moves upward from tracheid to tracheid.
Xylem tissue is responsible for the transport of water and dissolved substances from roots to the upper parts. Due to the presence of lignin in the secondary walls of its cells, xylem tissue also provides support to the plant body.
b) Phloem tissue: Phloem tissue consists of sieve tube cells and companion cells.
In sieve tube cells, protoplasm has no nucleus and the end walls have small pores called sieve plates. The transport of food occurs through the cytoplasm of the sieve tube cells.
Companion cells accompany the sieve tube cells. They regulate the movement of food through sieve tubes.
Q.3) Describe the nervous, muscular and epithelial tissues.
Answer: 1) Epithelial tissues: The interior of the body is physically separated from the outside world by the skin. The skin is made of epithelial tissue, which is in the form of continuous sheets of cells. Epithelial tissue also lines the gut, lungs and urinary tract. Structurally these cells are flattened and elongated. 2) Muscular tissue: Muscular tissues consist of highly specialized contractile cells or fibers held together by connective tissue. Each muscle fiber is an elongated cell which has the ability to contract and relax. Types of muscular tissues: Skeletal, smooth and cardiac muscles are the examples of these tissues. i) Skeletal muscles: Skeletal muscles are attached to cartilages and bones with the help of connective tissues called tendons. They are composed of striated (striped) cells that are long and cylindrical. The cells of skeletal muscles contain many nuclei. Skeletal muscles are voluntary and produce powerful, rapid contractions. It helps arms and legs to move. ii) Smooth muscles: These are found in the walls of hollow structures such as blood vessels, gut etc. They produce slow contractions. Their contraction moves substances (foodstuff, urine) in the hollow organs. They are composed of spindle-shaped, un-striated muscles and are involuntary in action. iii) Cardiac muscle: Cardiac tissues are found in heart. These are composed of branched fibers and are capable of sustained contraction. Their contraction produces heartbeat that propels blood into vessels. They are also involuntary in action. 3) Nervous tissue: Nervous tissue is composed of nerve cells which are called neurons. Neurons are capable of transmitting nerve impulses to conduct messages in the whole body. Nervous tissues are found in the brain, spinal cord, and nerves.
Q.4) Write a note on the structures of the cell wall, cell membrane, mitochondria, and chloroplast of a plant cell.
Answer: I- Cell wall: Cell wall is present in all bacteria, plants, fungi, and some protists. It is located outside the plasma membrane. It provides shape, strength, protection, and support to the inner living matter (protoplasm) of a cell. The thickness of the cell wall varies from cell to cell. Primary cell wall: The outer layer of the plant cell wall is known as the primary wall. It is made of cellulose. Secondary cell wall: Some plant cells also have secondary walls on the inner side of the primary wall. It contains cellulose, lignin, and other chemicals. An electron microscope reveals that in the primary and secondary cell wall, the cellulose fibers are present in a crisscross manner which provides rigidity. Cell wall structure of different organisms:
The cell wall of Prokaryotes is composed of peptidoglycan (Murein). It is a single large molecule made of amino acids and sugar.
The cell wall of fungi is made of chitin.
The cell walls of protists are made of a variety of chemicals.
II- Cell membrane: Cell membrane or plasma membrane is present in all cells and is the outermost layer of protoplasm. It is the outermost layer of the animal cell. In the cells of bacteria, plants, fungi, and some protists, cell membrane lies beneath the cell wall.
Fluid Mosaic Model of cell membrane: Different models have been presented to understand the structure of cell membrane. Singer and Nicolson proposed “Fluid Mosaic Model” which is the most acceptable model. According to this model, cell membrane consists of a lipid bilayer in which protein molecules float while some stay embedded in the bilayer. Carbohydrate molecules are joined with proteins (in the form of glycoproteins) or with lipids (in the form of glycolipids).
3- Mitochondria: In prokaryotes, mitochondria are absent and it is thought that energy required is produced in the cytoplasm by anaerobic respiration or by mesosomes. Mitochondria in eukaryotic cells: Mitochondria are found in eukaryotic cells. They are bounded by a double membrane. The outer membrane is smooth. The inner membrane is folded inwards, giving rise to extensions called cristae. Cristae increase the surface area on which respiratory process takes place. Function of mitochondria: Mitochondria are the powerhouse of the cell because they carry out respiration in cell and produce energy in the form of ATP.
4- Chloroplast: The most important and abundant plastids are the chloroplasts in plants. These present in green parts of plants, particularly in leaves where photosynthesis takes place. Structure of chloroplast: A chloroplast is bounded by a double membrane. The outer membrane is smooth while the inner membrane gives rise to sacs called thylakoids. The stack of thylakoids is known as granum (plural: grana). The thylakoids contain chlorophyll. The semi-fluid present inside chloroplast is called stroma. Mitochondria and chloroplasts have their own DNA. Mitochondria also have their own ribosomes, which are similar to the ribosomes of bacteria.
Q.1) Define cell cycle and in how many phases it is divided.
Answer: Cell cycle: “Cell cycle is the series of events that take place in a cell leading to its division and duplication of its DNA (DNA replication) to produce two daughter cells.” Phases of the cell cycle: The cell cycle consists of two main phases:
Interphase or resting stage
Division phase (Mitosis or Meiosis)
Q.2) In which type of cells, meiosis takes place and why it is important?
Answer: Occurrence of Meiosis: Meiosis is also called ‘reduction division’, takes place in germ cells (egg and sperm) during sexual reproduction. Importance of Meiosis: 1-Formation of Gametes for sexual reproduction: Meiosis is essential for sexual reproduction and it ensures the correct number of chromosomes in offsprings.
2- Genetic Variations during sexual reproduction: Meiosis helps to create genetic variations among the offspring. During meiosis, the chromosomes of each parent undergo genetic recombination. It produces gametes with different genetic combinations. Gametes fuse to form a zygote with a different genetic combination. Thus meiosis allows a species to bring variations to handle the changes in the environment.
Q.3) How does normal mitosis ensure normal life?
Answer: Normal mitosis ensures normal life. The process of mitosis ensures the equal distribution of genetic material/ information between the newly formed daughter cells, so that life process goes on normally. Equal distribution of the genetic material is necessary for each cell to perform its normal function. Any abnormality in the process of mitosis can be harmful to the cell and eventually life. Example: Unequal distribution of the chromosome can lead to the chromosomal abnormalities which can cause diseases like Down’s syndrome.
Q.4) Give at least four differences between meiosis and mitosis.
In this phase, homologous chromosomes do not form pairs. There is no crossing over.
Homologous chromosomes pair up. Crossing over takes place between these homologous chromosomes.
Single chromosomes align to form metaphase plate.
Homologous pairs align to form metaphase plate.
Chromosomes break and individual chromatids are pulled towards poles.
Individual chromosomes are pulled towards poles.
Daughter nuclei contain a diploid number of chromosomes. Each chromosome has a single chromatid.
Daughter nuclei contain a haploid number of chromosomes. Each chromosome has two chromatids.
Chromosome number remains the same (2n), and yields two diploid cells.
Chromosome number reduced to half (n), and yield four haploid cells.
Occurs in somatic cells.
Occurs in germ cells i.e. egg and sperms.
Q.5) What is chiasma and what is its role in crossing over?
Answer: Chiasma and its role in crossing over: During Prophase-I of meiosis, the two non-sister chromatids of homologous chromosomes become zipped together, forming complexes known as chiasmata (singular chiasma). At chiasma, the non-sister chromatids of homologous chromosomes exchange their parts. This process is known as crossing over. So, chiasma helps in crossing over and crossing over is essential in producing genetic varieties in germ cells (egg and sperm).
Q.1) Describe various phases of interphase of the cell cycle with diagrams.
Answer: Interphase of the cell cycle: “Interphase is the period between the end of one cell division and the start of next division” Events of interphase: Interphase may be divided into three sub-phases. The following events occur during interphase: i. G1-phase ii. S-phase iii. G2-phase i. G1-phase: G1-phase (G stands for gap) is the first and the longest sub-phase of the interphase. In this phase, newly produced cells grow in size and undergo internal chemical changes, which prepare the daughter cells for DNA replication. During G1 phase RNA, ribosomes and several enzymes are synthesized. ii. S-phase (synthesis phase): In this phase, the cell duplicates its chromosomes. For this purpose, the DNA molecule of each chromosome is copied (replication of DNA) and new protein molecules are attached. Each Chromosome consists of two sister chromatids. Cell growth continues throughout S phase. At the same time, synthesis of proteins and enzymes take place which is involved in DNA synthesis. iii. G2-phase: G2-phaseis the period between the completion of DNA synthesis and the beginning of active cell division phase. During G2-phase, cell prepares proteins e.g. microtubule framework (spindle fibers) required for cell division. A final event in interphase occurs with the replication of the centrioles and their movement to either end of the nucleus. At the same time, microtubules appear radiating from the area of centriole at each end.
Q.2) Discuss different events of meiosis-I with the help of diagrams.
Answer: Meiosis-I: Different events of Meiosis-I are: i) Prophase-I ii) Metaphase-I iii) Anaphase-I iv) Telophase-I i) Prophase-I: Prophase-I usually accounts for 90% of the total time spent in meiosis. In this phase, the chromatin material condenses and chromosomes become visible. Tetrad: However, unlike prophase of mitosis, the two homologous chromosomes (chromosome similar in shape and size) pair up or synapse to form tetrads (because a pair of the chromosome has four chromatids). Chiasmata: The two non-sister chromatids of homologous chromosomes become zipped together, forming complexes known as chiasmata (singular chiasma). Crossing over: At chiasma, the non-sister chromatids of homologous chromosomes exchange their parts. This process is known as crossing over. As in the prophase of mitosis, here also the nucleoli disappear and nuclear envelope breaks. Centrioles migrate to opposite poles and make spindle fibers
ii) Metaphase-I: Homologous chromosomes arrange themselves during the metaphase plate. Spindle fibers from one pole attach to one chromosome of each homologous pair while spindle fibers from the opposite pole attach to the other chromosome of the homologous pair. iii) Anaphase-I: During anaphase-I, spindle fibers pull towards the poles and separate the homologous chromosomes. Each member of the pair is pulled toward opposite poles of the cell. During this phase, the sister chromatids remain attached. In this way, one haploid set of chromosomes is formed at each pole. Each chromosome in both haploid sets still contains two sister chromatids. Anaphase-I is a key event in creating new genetic combinations in sexually reproducing organisms. New combinations of genetic material created by any process including crossing-over, is termed as genetic recombination. iv) Telophase-I: After the chromosomes reach their respective poles, the nuclear membrane reappears along with the appearance of nucleoli. Later cytokinesis occurs and two haploid daughter cells are produced.
Q.3) Explain different stage of mitosis with diagrams and at what stage, cytokinesis takes place?
Answer: Mitosis: “Mitosis is a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent cells.” Different stages of mitosis: The process of mitosis can be divided into two main stages: I. Karyokinesis II. Cytokinesis I. Karyokinesis: “The division of nucleus is called karyokinesis.” Stages of karyokinesis: It is further divided into four phases: 1- Prophase 2- Metaphase 3-Anaphase 4- Telophase. 1- Prophase: Chromatin threads condense, coil and shorten to become chromosomes. Each chromosome consists of two sister chromatids attached at the centromere. The chromosomes are now visible under a microscope.
In an animal cell, the two pairs of centrioles reach opposite poles of the cell. They make a network of spindle fibers between the two poles. The complete set of spindle fibers is called the mitotic apparatus.
ln plant cells, there are no centrioles. Their mitotic spindle is formed by the aggregation of spindle fibers present in the cytoplasm.
As a part of the changes of prophase, the nucleolus disappears and the nuclear envelope breaks down. 2- Metaphase: In metaphase, some spindle fibers attach with chromosomes. Two spindle fibers from both sides attach to one chromosome. The chromosomes attached with spindle fibers arrange themselves at the equator of the cell. In this way, a plate is formed called the metaphase plate. 3- Anaphase: The centromere of each chromosome splits. The spindle fibers pull the chromatids apart to opposite poles. Once the chromatids are separated, they are called daughter chromosomes. In early anaphase, the centromere split. Half the chromosomes move to one pole and half to the other pole. In late anaphase, the chromosomes have almost reached to their respective poles. The cell membrane begins to move inward at the center. 4- Telophase: In telophase, spindle fibers break down. A nuclear envelope forms around the chromosomes at each pole of the cell. Thus two daughter nuclei are formed, each with the same number of chromosomes as was present in the parent nucleus.
II. Cytokinesis: “Cytokinesis is the division of the cytoplasm.” Cytokinesis is the last stage of cell division. Nucleolus forms and the chromosomes uncoil to become thin chromatin like network. After the division of the nucleus, the final step is the division of the cytoplasm. The division of cytoplasm is called cytokinesis.
In animal cells, cleavage or furrows appear in the cytoplasm between the two nuclei. The furrows deepen and two identical daughter cells are finally produced.
In plant cells, a cell plate is formed between the two daughter nuclei. It divides the cell into two daughters cells. The cell plate is formed by the fusion of small, fluid-filled vesicles produced by the Golgi apparatus.