Biology Notes FA FSc Chapter No 4 Bioenergetics

Q: What do you mean by chemiosmosis?

Answer: Chemiosmosis: "The synthesis of ATP from ADP and inorganic phosphate in the electron transport system through the joint event of chemical and osmotic processes is called chemiosmotic ATP synthesis." Mitochondria are surrounded by a double membrane. The outer membrane is smooth while the inner membrane forms enfolding which are shelf-like projections or protuberances called cristae. The cristae are present in the inner chamber or mitochondrial matrix that is filled with a gel-like substance. The carriers of the electron transport system are present on the cristae. Space is present between the outer and inner membrane called intermembrane space. During the passage of electrons through the electron transport system, certain carriers of the system pump hydrogen ions from the mitochondrial matrix into the intermembrane space. As a result, hydrogen ions accumulate on the outside of the inner membrane in the intermembrane space. space. The difference of hydrogen ion concentration increases across the membrane which develops a gradient of hydrogen ions between the matrix and the intermembrane space i.e. across the inner membrane. Hydrogen ions diffuse down the inner membrane through an electrochemical gradient from the intermembrane space into the matrix. The passage of hydrogen ions through the membrane is coupled to ATP synthesis from ADP and inorganic phosphate through the ATP synthase complex. This process of ATP synthesis is called chemiosmosis because electrochemical and osmotic events are involved.

biology study guide final exam for boards of Khyber Pakhtunkhwa, Fbise, Mardan, Punjab, and Sindh Textbook. Short question, long question, and MCQs.

Bioenergetics Notes for Biology 1st year

Q.2 i) Differentiate between the absorption spectra of chlorophyll “a” and ‘b”.

Answer:

Chlorophyll a	Chlorophyll b
Chlorophyll a is the primary photosynthetic pigment.	Chlorophyll b is the accessory pigment that collects energy and passes it on to chlorophyll a.
Chlorophyll a absorbs energy from wavelengths of blue-violet and orange-red light	Chlorophyll b absorbs energy from wavelengths of green light.
Chlorophyll a absorbs energy at 675nm	Chlorophyll b absorbs energy at 640 nm.
Chlorophyll a is less absorbent	Chlorophyll b is more absorbent
Chlorophyll a has methyl (-CH3) as a functional group	Chlorophyll b has carbonyl (-CHO) as a functional group

Q.2 ii) State the role of carbon dioxide as one of the raw materials of photosynthesis.

Answer:
Role of carbon dioxide as one of the raw materials of photosynthesis:
The carbon of CO₂ is fixed in organic compounds in photosynthesis. Carbon is the most important component of organic compounds. Carbon dioxide is used as one of the raw materials for photosynthesis. In the absence of carbon dioxide, the process of photosynthesis does not occur. Without CO₂ the life of photosynthetic organisms and animals would be impossible, given that CO₂ provides the basis for the synthesis of organic compounds that provide nutrients for plants and animals. Through photosynthesis, organisms with chlorophyll take in atmospheric CO₂ or dissolve CO₂ in water to form more complex molecules, such as carbohydrates, lipids, proteins, and nucleic acids.

Q.2 iii) What is the main difference between cyclic and non cyclic photophosphorylation.

Answer:

Cyclic photophosphorylation	Non-cyclic photophosphorylation
Only the PS I is engaged	Both the PS I and PS II are engaged
ATP molecules are formed	ATP and NADPH2 are formed.
It takes place in stroma lamellae and grana lamellae	It takes place only in grana lamellae
Photolysis of water does not occur so oxygen is not released	Photolysis of water occurs and oxygen is released.
The wavelength of light required is P 700	The wavelength of light required is P 680

Q.2 iv) Define glycolysis.

Answer:
Glycolysis:
“Glycolysis is the breakdown of glucose, a 6-C molecule, in two molecules of pyruvate (3-C molecule) and a net gain of two ATP molecules. It takes place in the cytosol (cytoplasm) and is common in both aerobic and anaerobic respiration. Glycolysis does not need free oxygen.”
“Or”
“Glycolysis is a metabolic process that breaks down carbohydrates and sugars through a series of reactions to either pyruvic acid or lactic acid and releases energy for the body in the form of ATP.”

What do you mean by chemiosmosis?

Answer:
Chemiosmosis:
“The synthesis of ATP from ADP and inorganic phosphate in the electron transport system through the joint event of chemical and osmotic processes is called chemiosmotic ATP synthesis.”

Mitochondria are surrounded by a double membrane. The outer membrane is smooth while the inner membrane forms enfolding which are shelf-like projections or protuberances called cristae. The cristae are present in the inner chamber or mitochondrial matrix that is filled with a gel-like substance. The carriers of the electron transport system are present on the cristae. Space is present between the outer and inner membrane called intermembrane space.

During the passage of electrons through the electron transport system, certain carriers of the system pump hydrogen ions from the mitochondrial matrix into the intermembrane space. As a result, hydrogen ions accumulate on the outside of the inner membrane in the intermembrane space. space.

The difference of hydrogen ion concentration increases across the membrane which develops a gradient of hydrogen ions between the matrix and the intermembrane space i.e. across the inner membrane. Hydrogen ions diffuse down the inner membrane through an electrochemical gradient from the intermembrane space into the matrix. The passage of hydrogen ions through the membrane is coupled to ATP synthesis from ADP and inorganic phosphate through the ATP synthase complex. This process of ATP synthesis is called chemiosmosis because electrochemical and osmotic events are involved.

Q.2 vi) What is photorespiration?

Long Questions

Q.3 i) Describe the role of sunlight in the process of photosynthesis.

Answer:
Role of Sunlight in Photosynthesis:
Sun is the main source of energy for all living organisms. Light is a kind of energy that travels in the form of electromagnetic waves of different wavelengths. It also acts like a beam of particles of different frequencies called photons. There is a wide range of waves for the synthesis of organic food molecules (wavelengths occurring between gamma rays and radio rays).
The energy content of photons is inversely proportional to the wavelengths. Short wavelengths are more energetic i.e. have high energy content than long wavelengths.
Visible spectrum (Light):
A portion of the solar radiation is called the visible spectrum. Human eyes are sensitive to only a small portion of this solar radiation i.e. visible light that ranges from about 390nm to 760 nm in wavelength. Photosynthetic pigments absorb and utilize a portion of the visible spectrum.
Ultraviolet radiations:
Wavelengths shorter than visible light i.e. ultraviolet radiation are more energetic and are dangerous to the cells because they can break organic molecules. Wavelengths longer than visible light i.e. infrared have a low energy content that cannot affect the photosynthetic process.
Wavelengths of the visible spectrum have the right amount of energy absorbed by photosynthetic pigments for photosynthesis.
About forty percent of the total sunlight that enters our atmosphere reaches the earth’s surface. Most of this radiation is within the visible light range. Dangerous higher energy wavelengths are screened out by the ozone layer and upper layers of the atmosphere. Lower energy wavelengths are mostly absorbed or reflected by water vapors and other gases and are scattered in the atmosphere. Of the total sunlight that strikes the green plants only about a fraction is used in photosynthesis. This small portion of sunlight sustains all forms of life on earth.
Types of photosynthetic pigments:
There are two types of photosynthetic pigments i.e. chlorophylls and carotenoids.
Chlorophylls:
Chlorophylls absorb mostly violet-blue wavelengths (390-430 nm) and red wavelengths (670-700nm). Green wavelengths are mostly reflected therefore chlorophyll appears green. The carotenoids which are called accessory pigments absorb light in the visible spectrum ranging between 500nm and 600 nm in wavelengths.

Q.3 ii) Give an account of the events of non-cyclic electron pathway.

Answer:
Non-Cyclic-Electron Transport of Light Reaction:
When sunlight strikes photosystem II (P680) energy is absorbed by the chlorophyll molecules. The activated chlorophyll losses its two electrons and the positively charged chlorophyll molecule are left in the photosystem with a gap of two electrons. Plastoquinone (PQ):
The high-energy electrons instead of falling back into the photosystem are captured by an electron acceptor called Plastoquinone (PQ). The electrons pass along a series of electrons acceptor molecules from one to another in the oxidation-reduction process from plastoquinone.
These electron acceptors include cytochrome ‘be, cytochrome ‘f, and plastocyanin. This transfer of electrons constitutes an electron transport chain. Each molecule in the electron transport chain is alternately reduced when it gains an electron and is oxidized when it loses electrons. When electrons are passed through the electron transport chain, they lose energy. Some of the energy lost by electrons between cytochrome ‘be, and cytochrome ‘f’ is used in making ATP from ADP and inorganic phosphate.
Photophosphorylation:
The process of formation of ATP from ADP and inorganic phosphate using energy from sunlight is called photophosphorylation.
i. Photosystem I (P700):
The electrons from plastocyanin are received by another photosystem called photosystem I (P700). At the same time, light falls on photosystem I and activates its two electrons. Activated electrons are received by Ferredoxin reducing substance (FRS); electron acceptor of PSI.
From FRS electrons are passed to oxidize NADP (Nicotinamide adenine dinucleotide phosphate). The reduced NADP receives hydrogen from water and is converted into NADPH. When photosystem II absorbs light, water molecule splits into OH^– and H⁺. The OH^– ions react to form some water again and release oxygen and electrons. 4H2O → 4H+ + 4(OH) + 2e
4(OH) → 2H2O + O2
Photosystem II:
Electrons from water molecules are accepted by positively charged chlorophyll molecule of Photosystem II, filling the gap produced by the two energized electrons. The electron deficiency of photosystem I have been filled by electrons coming from photosystem II. This transport of electrons is called Non-cyclic electron transport because electrons do not move in a cycle. It involves both the photosystems and follows a zig-zag path.
Z-Scheme (Zigzag Scheme) of light reaction:
In a non-cyclic electron transport system, electrons do not move in a cycle. Both the photosystems are involved and ATP and NADPH are produced which are used in dark reaction. Therefore, it is also called Z-Scheme (Zigzag Scheme) of light reaction. The ATP synthesis during this non-cyclic electron flow is called Non-cyclic Photophosphorylation.

Q.3 iii) Illustrate Calvin cycle.

Answer:
Calvin cycle (Dark reaction) Light-independent reactions:
Light-independent reactions do not require direct energy of sunlight it may occur during day time but are called dark reactions to differentiate them from the light reactions. The sequence of dark reactions in photosynthesis was investigated by Melvin Calvin and his colleagues in 1950. They occur in a series of reactions in the chloroplast’s stroma and take the course of a cycle known as the Calvin-Benson cycle.
Stages of the Calvin cycle:
The Calvin cycle is completed in three stages.
1. Carbon fixation:
The cycle starts when ribulose bisphosphate, a 5-carbon sugar, already present in stroma reacts with CO2 of air to form a 6-Carbon compound. This compound is unstable and soon splits up into two molecules of a 3-carbon compound called Phosphoglycerate (PGA). This process is accelerated by an enzyme known as Rubisco (Ribulose biphosphate carboxylase). This is regarded as the most common protein in nature. The carbon that was part of CO2 molecule is now a part of an organic molecule. This is called carbon fixation. PGA is regarded as the first product of photosynthesis to be identified.
RuBP + CO2 → short-lived 6- Carbon compound
6- C compound → 2PGA
2. Reduction:
PGA formed in the previous step is reduced into phosphoglyceraldehyde (PGAL) in this stage. The products of light reaction i.e. NADPH and ATP are used in the process. Each molecule of phosphoglyceric acid (PGA) receives energy from ATP and hydrogen from NADPH of light reaction, forming phosphoglyceraldehyde (PGAL) and water. ADP and NADP return to the light reaction where ADP is converted into ATP and NADP is reduced into NADPH. In the reduction process, fixed carbon is reduced to a 3-carbon sugar molecule of PGAL.
PGA+ ATP + NADPH → PGAL + ADP + P + NADP + H2O
3. Regeneration of RuBP:
In this stage, RuBP molecules are regenerated to continue the cycle. The PGAL molecules formed in the reduction stage have many alternatives. Out of every six molecules of PGAL formed, only one molecule leaves the cycle to be used by the plant for making glucose and other organic compounds. The other five PGAL molecules are recycled to regenerate 3 molecules of five carbons RuBP using several intermediates including 3-C, 4-C, 6-C, 7-C etc. This process also uses some ATP produced in the light reaction. Ribulose bisphosphate (RuBP) is then available to accept CO₂ and restarts the cycle. With the regeneration of RuBP the Calvin cycle or dark reactions complete.

Q.3 iv) Describe Krebs cycle.

Answer:
Kreb’s Cycle (Tricarboxylic Acid Cycle (TCA)/ Citric acid cycle:
“Acetyl CoA enters a cyclic series of chemical reactions during which the oxidation process is completed. This series of reactions is called the Krebs cycle.”
The acetyl-CoA produced in the linked reaction enters into a cycle called the Krebs cycle. In Krebs cycle, the metabolic pathway takes the course of a cycle where Acetyl CoA (2-C Compound) is completely oxidized into two molecules of CO₂ and hydrogen atoms are removed which reduces NAD and FAD to NADH₂ and FADH₂ respectively.
i- Formation of citrate:
In the first step of the cycle, Acetyl CoA combines with pre-existing oxalo acetic acid (4-C) in the presence of a water molecule to form citric acid (6-C). CoA becomes free and is ready to react with another acetyl group.
Acetyl CoA + oxaloacetate → Citrate + CoA
ii- Conversion of citrate to isocitrate:
Citrate is converted to isocitrate
Citrate → isocitrate
iii- Formation of –ketoglutarate:
Iso-citrate is oxidized to α-ketoglutarate (5-C). In this step, one carbon of isocitrate is oxidized to carbon dioxide and hydrogen is removed which is picked up by NAD reducing into NADH₂.
One carbon of α-Ketoglutarate is oxidized into carbon dioxide. The CO₂ is released from the cycle. This is the second CO₂ molecule produced in the Krebs cycle.
iv- Formation of succinyl CoA:
The two carbons of the acetyl group which was entered into the Krebs cycle are oxidized into two molecules of carbon dioxide. Hydrogen atoms are released which are accepted by oxidized NAD. By accepting hydrogen atoms NAD is reduced to NADH. The α-Ketoglutarate is converted into succinyl group (4-C). CoA reacts with succinyl group forming succinyl CoA.
v- Formation of succinate acid:
In the next step, succinyl CoA is converted to succinate Acid (4-C) and CoA is released. Some of the energy produced in the oxidation is used in the synthesis of ATP. The energy of the substrate used in the generation of ATP is called substrate-level phosphorylation.
vi- Oxidation of succinate to fumarate and malate:
Succinate is oxidized to fumarate (4-C). Coenzyme FAD (Flavin adenine dinucleotide) is reduced in the reaction. Fumarate is converted to malate (4-C).
vii. Oxidation of malate into oxaloacetate:
In the last step, malate oxidized to oxaloacetate. A molecule of NADH is produced during this step. The oxaloacetate is now able to react with another acetyl C0-A and continue the cycle.

Q.3 v) What is photorespiration and what are its disadvantages?

Answer:
Photorespiration:
“Photorespiration is the process in which oxygen combines with ribulose biphosphate (RuBP) in the presence of sunlight and CO₂ is produced. The process is called photorespiration because in the presence of light (photo), oxygen is taken up and CO₂ is evolved (respiration).”
“Or”
“Photorespiration occurs in many higher plants by which they take up oxygen in the light and give out some carbon dioxide, contrary to the general pattern of photosynthesis.”
Photosynthesis needs an optimum concentration of the requirements for normal functioning. If however one of the requirements is present in less concentration than the optimum, photosynthesis is affected and slows down.
In the Dark reaction of photosynthesis, normally CO₂ combines with RuBP (carboxylation) forming PGA molecules. The process occurs in the presence of an enzyme called ribulose biphosphate carboxylase (rubisco). This enzyme can act both as carboxylase and oxygenase. The reaction depends on the concentration of CO₂ and O₂. If the concentration of CO2 is more, then rubisco combines with CO2 and photosynthesis proceeds normally. On the other hand, if the concentration of O2 is more, then rubisco combines with O2 and photorespiration occurs. (Rubisco can act both as carboxylase as well as oxygenase).
Plants have stomata for the exchange of gases. Diffusion of water vapors from leaf to the external environment also occurs through the stomata. In dry and hot weather plants close up stomata to conserve water. In such condition CO₂ cannot enter the leaf and O₂ cannot leave it. Dry and hot conditions are usually accompanied by intense sunlight therefore light reaction occurs with a maximum rate which results in maximum use of CO₂. Since the concentration of CO₂ lowers down in the leaf and photorespiration proceeds.
Steps of photorespiration:
1- Oxygen combines with RuBP (present in the stroma of chloroplast) and a compound called Glycolate is produced.
RuBP + O₂ → Glycolate
2. Glycolate is converted into glycine (simplest amino acid) in the peroxisome.
Glycolate → Glycine
3- Glycine is transported to mitochondria where it is converted into serine and a molecule of CO2 is produced.
Glycine → Serine + CO₂
Disadvantages of Photorespiration (Consequences):

Photorespiration is just the reverse of photosynthesis hampering the fixation of CO, photosynthesis.
The process wastes energy and does nothing to serve the needs of the plant.
Read more: Biology Notes FA FSc Chapter No 3 Enzymes