BIOENERGETICS
TRUE/FALSE
Q.01: Indicate True or False.
(i) Hydroponics are the plants grown in water culture. (FALSE)
CORRECT: Hydroponics is a method of growing plants without soil, using water-based nutrient solutions.
(ii) Calcium is an essential element for chlorophyll formation. (FALSE)
CORRECT: Magnesium is an essential element for chlorophyll formation.
(iii) Chlorosis means yellowing of leaves due to deficiency of certain essential element of plant nutrition. (TRUE)
MULTIPLE CHOICE QUESTIONS
Q.02: Multiple choice questions.
(i) Magnesium is an important nutrient ion in green plants as it is an essential component of:
(a) Cell sap
(b) Protein
(c) Chlorophyll
(d) Glucose
ANSWER: (c) Chlorophyll
EXPLANATION: An atom of magnesium is present in the center of porphyrin ring of chlorophyll molecule. That’s why magnesium deficiency causes yellowing in plants.
(ii) When a green plant performs photosynthesis at its maximum rate:
(a) The rate of water loss is low.
(b) The water content of the plant will be low.
(c) The energy content of the plant will be low.
(d) The energy content will be unaffected.
ANSWER: (b) The water content of the plant will be low.
EXPLANATION: During photosynthesis, plants take in carbon dioxide and release oxygen while converting light energy into chemical energy. Water is a crucial component in this process, and when photosynthesis is occurring at its maximum rate, water is consumed very rapidly. As a result, the water content within the plant is expected to be lower than when the plant is not actively photosynthesizing.
(iii) During the dark reactions of the photosynthesis, the main process which occurs is:
(a) Release of oxygen
(b) Energy absorption by chlorophyll
(c) Adding of hydrogen to carbon dioxide
(d) Formation of ATP
ANSWER: (c) Adding of hydrogen to carbon dioxide
EXPLANATION: During the dark reactions, also known as the Calvin cycle or light-independent reactions, the main process that occurs is the fixation of carbon dioxide into organic molecules. This process involves adding hydrogen atoms (along with electrons) from molecules like NADPH (produced in the light reactions) to carbon dioxide molecules to form carbohydrates like glucose. This step is facilitated by the enzyme Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase).
(iv) Which statement about ATP is not true?
(a) It is used as an energy currency by all cells.
(b) It is formed only under aerobic conditions.
(c) Some ATP is used to drive the synthesis of storage compounds.
(d) It provides the energy for many different biochemical reactions.
ANSWER: (b) It is formed only under aerobic conditions.
EXPLANATION: No, ATP (adenosine triphosphate) can be formed both under aerobic (with oxygen) and anaerobic (without oxygen) conditions. In aerobic conditions, ATP is primarily generated through oxidative phosphorylation in the presence of oxygen during cellular respiration. In anaerobic conditions, ATP can be produced through processes like glycolysis without the need for oxygen. However, the overall efficiency and yield of ATP production are generally higher in aerobic conditions compared to anaerobic conditions.
(v) Glycolysis:
(a) Produces no ATP
(b) Is the same as fermentation.
(c) Takes place in the mitochondrion.
(d) Reduces two molecules of NAD+ for every glucose molecule processed.
ANSWER: (d) Reduces two molecules of NAD+ for every glucose molecule processed.
EXPLANATION: In glycolysis, a process occurring in the cytoplasm of cells, one molecule of glucose is broken down into two molecules of pyruvate. During this process, there is a reduction of two molecules of NAD+ (nicotinamide adenine dinucleotide) to form two molecules of NADH. This reduction reaction occurs when NAD+ accepts electrons and hydrogen ions (H+) from the breakdown of glucose. NADH carries these high-energy electrons to the electron transport chain for further energy production during cellular respiration.
(vi) The citric acid cycle:
(a) Takes place in the mitochondrion.
(b) Is the same as fermentation.
(c) Has no connection with the respiratory chain.
(d) Reduces two molecules of NAD+ for every glucose molecule processed.
ANSWER: (a) Takes place in the mitochondrion.
EXPLANATION: The citric acid cycle, also known as the Krebs cycle, occurs in the mitochondrion. This cycle is a series of chemical reactions that takes place in the mitochondrial matrix, the innermost compartment of the mitochondrion. During the citric acid cycle, acetyl-CoA, derived from the breakdown of carbohydrates, fats, and proteins, enters a series of reactions leading to the production of NADH, FADH2 and ATP.
(vii) Which statement about the chemiosmotic mechanism is not true?
(a) Protons return through the membrane by way of a channel protein.
(b) Protons are pumped across a membrane.
(c) Proton pumping is associated with the respiratory chain.
(d) The membrane in question is the inner mitochondrial membrane.
ANSWER: (c) Proton pumping is associated with the respiratory chain.
EXPLANATION: During chemiosmosis, proton pumping is associated with thylakoid membranes, not with respiratory chain occurring in mitochondria.
(viii) Which statement about oxidative phosphorylation is not true?
(a) Its functions can be served equally well by fermentation.
(b) In eukaryotes, it takes place in mitochondria.
(c) It is brought about by the chemiosmotic mechanism.
(d) It is the formation of ATP during operation of the respiratory chain.
ANSWER: (a) Its functions can be served equally well by fermentation.
EXPLANATION: Oxidative phosphorylation is more efficient in generating ATP than fermentation because it involves an electron transport chain and utilizes oxygen as the final electron acceptor, leading to a higher ATP yield. Fermentation pathways lack these components, resulting in a less efficient ATP production process.
(ix) Before pyruvate enters the citric acid cycle, it is decarboxylated, oxidized and combined with coenzyme A, forming acetyl CoA, carbon dioxide and one molecule of:
(a) NADH
(b) FADH2
(c) ATP
(d) ADP
ANSWER: (a) NADH
EXPLANATION: Before pyruvate enters the citric acid cycle, it undergoes a series of transformations. First, it is decarboxylated, meaning a carbon atom is removed in the form of carbon dioxide. This process is accompanied by oxidation, involving the transfer of electrons to the coenzyme NAD+, producing one molecule of NADH. The remaining two-carbon fragment then combines with coenzyme A to form acetyl CoA. This acetyl CoA is the entry point for the acetyl group into the citric acid cycle, where it participates in further reactions to extract energy and generate more reduced coenzymes.
(x) In the first step of the citric acid cycle, acetyl CoA reacts with oxaloacetate to form:
(a) Pyruvate
(b) Citrate
(c) NADH
(d) ATP
ANSWER: (b) Citrate
EXPLANATION: In the first step of the citric acid cycle, acetyl CoA combines with oxaloacetate, facilitated by the enzyme citrate synthase. This reaction forms citrate, initiating the cycle. The CoA is released, and the two-carbon acetyl group becomes part of the citrate molecule, setting the stage for subsequent reactions in the citric acid cycle.
(xi) When deprived of oxygen, yeast cells obtain energy by fermentation, producing carbon dioxide, ATP and:
(a) Acetyl CoA
(b) Ethyl alcohol
(c) Lactate
(d) Pyruvate
ANSWER: (b) Ethyl alcohol
EXPLANATION: When deprived of oxygen, yeast cells undergo fermentation. In this process, pyruvate, produced during glycolysis, is converted into ethanol (ethyl alcohol) and carbon dioxide. This fermentation pathway regenerates NAD+ to allow glycolysis to continue, producing a small amount of ATP. This enables yeast cells to generate energy in the absence of oxygen, albeit less efficiently than through aerobic respiration.
Q.03: SHORT QUESTIONS
(i) List four features of a leaf which show that it is able to carry out photosynthesis effectively.
ANSWERS:
Some of the characters of leaf which make it an efficient photosynthesizing organ are:
(1) Flatness of surface
(2) Arrangement of conducting vessels
(3) Presence and number of stomata
(4) Arrangement of mesophyll cells
(ii) Summarize the role of water in photosynthesis.
ANSWERS:
Role of Water in Photosynthesis:
Oxygen released during photosynthesis comes from water, and is an important source of atmospheric oxygen used by most organisms. In 1930s, Van Niel hypothesized that plants split water as a source of hydrogen, releasing oxygen as a by-product. This was later confirmed by the scientists during 1940s, when first use of an isotopic tracer (O18) in biological research was made.
Experimental Confirmation:
Water and carbon dioxide containing heavy-oxygen isotope O18 were prepared in the laboratory. Experimental green plants in one group were supplied with H2O containing O18 and with CO2 containing only common oxygen O16. Plants in the second group were supplied with H2O containing common oxygen O16, but with CO2 containing O18. It was found that plants of first group produced O18 but the plants of second group did not. This confirmed that the source of released oxygen during photosynthesis is H2O and not CO2.
Group-1 Plants: CO2 + 2H2O18 ⟶ CH2O + H2O + 18O2
Group-2 Plants: CO218 + 2H2O ⟶ CH2O18 + H2O18 + O2
(iii) What are T.W. Engelmann and Melvin Calvin famous for?
ANSWER:
The first action spectrum was obtained by German biologist, T.W. Engelmann in 1883. He worked on Spirogyra.
The details of the path of carbon in light independent or dark reactions of photosynthesis were discovered by Melvin Calvin and his colleagues at the University of California. Calvin was awarded Nobel Prize in 1961.
(iv) What is the difference between an action spectrum and an absorption spectrum?
ANSWER:
ABSORPTION SPECTRUM:
(1) A graph plotting absorption of light of different wavelengths by a pigment is called ‘absorption spectrum’ of the pigment.
(2) Absorption spectrum of a pigment can be obtained by using spectrophotometer.
(3) Absorption spectrum for chlorophylls indicates that absorption is maximum in blue and red parts of the spectrum, two absorption peaks being at around 430 nm and 670 nm respectively.
ACTION SPECTRUM:
(1) Graph showing relative effectiveness of different wavelengths (colours) of light in driving photosynthesis is called ‘action spectrum’ of photosynthesis.
(2) Action spectrum can be obtained by illuminating plant with light of different wavelengths (or colours) and then estimating relative CO2 consumption or oxygen release during photosynthesis.
(3) Action spectrum shows that photosynthesis is maximum in red and then in blue part of the spectrum.
(v) What is the role of accessory pigments in light absorption?
ANSWER:
ACCESSORY PIGMENTS: “The carotenoids and chlorophyll ‘b’ are called accessory pigments because they absorb light and transfer the energy to chlorophyll a, which then initiates the light reactions.”
Examples: Carotenes (Orange to red), xanthophyll (Yellow), chlorophyll ‘b’ etc.
Functions of Accessory Pigments:
(1) Carotenoids are yellow and red to orange pigments that absorb strongly the blue-violet range, different wave lengths than the chlorophyll absorbs. So, they broaden the spectrum of light that provides energy for photosynthesis.
(2) Some carotenoids protect chlorophyll from intense light by absorbing and dissipating excessive light energy, rather than transferring energy to chlorophyll.
(vi) When and why is there not net exchange of CO2 and O2 between the leaves and the atmosphere?
ANSWER:
At dawn and dusk, when light intensity is low, the rate of photosynthesis and respiration may, for a short time, equal one another. Thus, the oxygen released from photosynthesis is just the amount required for cellular respiration. Also, the carbon dioxide released by respiration just equals the quantity required by photosynthesizing cells. At this moment there is no net gas exchange between the leaves and the atmosphere. This is termed as compensation point.
(vii) What is the net production of ATP during glycolysis?
ANSWER:
During glycolysis, in the course of conversion of one glucose molecules into two pyruvates, two ATP molecules are consumed while four ATP molecules are formed. So, the net production of ATP molecules is four (4).
(viii) What is the main difference between photophosphorylation and oxidative phosphorylation?
ANSWER:
PHOTOPHOSPHORYLATION:
(1) “During light reactions in photosynthesis, when the excited electrons move down the chain of electron acceptors, their energy goes on decreasing and is used by thylakoid membrane to produce ATP. This ATP synthesis is called photophosphorylation because it is driven by light energy.”
(2) Photophosphorylation takes place on thylakoid membrane.
(3) This ATP generated by the light reactions will provide chemical energy for the synthesis of sugar during the Calvin cycle.
OXIDATIVE PHOSPHORYLATION:
(1) “Synthesis of ATP in the presence of oxygen is called oxidative phosphorylation. Normally, oxidative phosphorylation is coupled with the respiratory chain.”
The equation for this process:
NADH + H+ + 3ADP + 3Pi + ½O2 ⟶ NAD+ + H2O + 3ATP
(2) Oxidative phosphorylation takes place along inner mitochondrial membrane.
(3) ATP generated by oxidative phosphorylation is utilized for different cellular activities.
(ix) What is the location of ETC and chemiosmosis in photosynthesis and cellular respiration?
ANSWER:
Electron transport chain and chemiosmosis in photosynthesis take place across thylakoid membranes in chloroplasts. Whereas, respiratory chain in respiration takes place along the inner mitochondrial membrane.
(x) How did the evolution of photosynthesis affect the metabolic pathway?
ANSWER:
The evolution of photosynthesis shifted the metabolic pathway from anaerobic to aerobic. This is because with the emergence of photosynthesis on earth, molecular oxygen began to accumulate slowly in the atmosphere. The presence of free oxygen made possible the evolution of aerobic respiration, which releases greater amount of energy as compared to anaerobic respiration which produces comparatively much lower amount of energy.
(xi) How does absorption spectrum of chlorophyll a differ from that of chlorophyll b?
ANSWER:
ABSORPTION SPECTRUM OF CHLOROPHYLL ‘a’
(1) Absorption spectrum of chlorophyll a has two peaks, showing maximum absorption of light at 430 nm and 662 nm in violet-blue and red regions, respectively.
(2) The absorption peak of chlorophyll ‘a’ is higher in red region than that of chlorophyll ‘b’, so chlorophyll ‘a’ is more effective in red part of the spectrum.
ABSORPTION SPECTRUM OF CHLOROPHYLL ‘b’
(1) Absorption spectrum of chlorophyll ‘b’ has two peaks, showing maximum absorption of light at 453 nm and 642 nm in blue and orange-red regions respectively.
(2) The absorption peak of chlorophyll ‘b’ is higher in blue region than that of chlorophyll ‘a’, so chlorophyll ‘b’ is more effective in blue part of the spectrum.
(xii) Why are the carotenoids usually not obvious in the leaves? They can be seen in the leaves before leaf fall. Why?
ANSWER:
The carotenoids are present in leaves but are not obvious because they are masked by the green colour of chlorophylls. In autumn, the leaves stop food making, the chlorophyll molecules break down, and the carotenoids of yellow to orange-red colour become obvious and visible. So, the leaves appear brown before fall.
(xiii) How is the formation of vitamin A linked with eating of carrot?
ANSWER:
Carrots are rich in beta carotene which is converted into vitamin A by our body on eating.
Q.04: EXTENSIVE QUESTIONS
(i) Explain the roles of the following in aerobic respiration: (a) NAD+ and FAD (b) Oxygen.
ANSWER:
Consult the textbook at page 216 — 217.
(ii) Sum up how much energy (as ATP) is made available to the cell from a single glucose molecule by the operation of glycolysis, the formation of acetyl CoA, the citric acid cycle, and the electron transport chain.
ANSWER:
The total ATP yield from the complete oxidation of one molecule of glucose through cellular respiration is approximately 30-32 ATP molecules. Here’s a brief breakdown:
(1) Glycolysis: Generates 2 ATP molecules (net gain).
(2) Formation of Acetyl CoA: Produces 0 ATP directly.
(3) Citric Acid Cycle: Yields 2 ATP molecules (per glucose molecule; it completes two cycles).
(4) Electron Transport Chain and Oxidative Phosphorylation: Produces approximately 26-28 ATP molecules, depending on the specifics of the cell and conditions.
So, the sum is around 30-32 ATP molecules per glucose molecule through the entire process of cellular respiration.
(iii) Trace the fate of hydrogen atoms removed from glucose during glycolysis when oxygen is present in muscle cells; compare this to the fate of hydrogen atoms removed from glucose when the amount of the available oxygen is insufficient to support aerobic respiration.
ANSWER:
Aerobic Respiration (Presence of Oxygen):
The hydrogen atoms carried by NADH generated in glycolysis are transported to the mitochondria.
In the mitochondria, these hydrogen atoms (electrons) enter the electron transport chain (ETC).
During the ETC, electrons move through a series of protein complexes, and their energy is used to pump protons across the inner mitochondrial membrane.
The final electron acceptor is oxygen, which combines with protons to form water.
The electron transport chain’s activity establishes a proton gradient, and ATP is synthesized through oxidative phosphorylation.
In contrast, when the available oxygen is insufficient to support aerobic respiration (anaerobic conditions), the fate of hydrogen atoms shifts:
Anaerobic Respiration (Insufficient Oxygen):
Since the electron transport chain cannot function effectively without oxygen, an alternative pathway takes place.
In muscle cells, this often leads to lactic acid fermentation.
NADH produced during glycolysis donates its electrons to pyruvate, converting it into lactic acid (lactate).
This regenerates NAD+ so that glycolysis can continue to produce ATP, but without the higher efficiency of oxidative phosphorylation.
In short, under aerobic conditions, hydrogen atoms are efficiently used in the electron transport chain to produce ATP through oxidative phosphorylation. In anaerobic conditions, such as during intense exercise, the cell resorts to fermentation (e.g., lactic acid fermentation), producing ATP without the involvement of the electron transport chain and oxygen. This process is less efficient in terms of ATP yield per glucose molecule.
(iv) Sketch Kreb’s cycle and discuss its energy yielding steps.
ANSWER:
The Krebs Cycle, also known as the citric acid cycle, is a series of chemical reactions that occur in the mitochondria during cellular respiration. It plays a crucial role in extracting energy from molecules derived from carbohydrates, fats, and proteins. Figure
Here’s a summary of the energy-yielding steps of the Krebs Cycle:
(1) Acetyl CoA Formation:
Acetyl CoA, derived from the breakdown of glucose, fatty acids, or amino acids, enters the cycle.
A two-carbon acetyl group combines with a four-carbon oxaloacetate to form citrate.
(2) Decarboxylation and Redox Reactions:
¨ Citrate undergoes a series of decarboxylation and redox reactions.
¨ Carbon dioxide is released, and NAD+ is reduced to NADH in multiple steps.
(3) ATP Synthesis:
GTP (guanosine triphosphate), a molecule similar to ATP, is produced through substrate-level phosphorylation.
GTP later transfers its phosphate group to ADP, forming ATP.
(4) More Redox Reactions:
FAD (flavin adenine dinucleotide) is reduced to FADH2.
More NAD+ is reduced to NADH.
(5) Regeneration of Oxaloacetate:
The remaining carbon compounds go through additional reactions, ultimately regenerating oxaloacetate to keep the cycle going.
The net result for one turn of the Krebs Cycle (which processes two acetyl-CoA molecules) includes the production of 3NADH molecules, 1 FADH2 molecule, 1 GTP (or ATP), and 2 carbon dioxide molecules. Since each glucose molecule results in two acetyl-CoA molecules entering the cycle, these numbers are doubled for the entire glucose molecule. The NADH and FADH2 molecules produced in the Krebs Cycle carry high-energy electrons to the electron transport chain for further ATP production during oxidative phosphorylation.Top of Form
(v) Describe various steps involved in oxidative break down of glucose to pyruvate.
ANSWER:
Consult the textbook at page 224 — 226.
(vi) Sketch respiratory electron transport chain. Discuss the significance of ETC.
ANSWER:
Consult the textbook at page 228 — 229.
(vii) Compare photosynthesis with respiration in plants.
ANSWER:
PHOTOSYNTHESIS
(1) Photosynthesis is the process in which energy-poor inorganic oxidized compounds of carbon (CO2) and hydrogen (from H2O) are reduced to energy-rich carbohydrate (glucose), using light energy, absorbed by chlorophyll and some other photosynthetic pigments.
(2) In photosynthesis, carbon dioxide, water and light are the reactants, while glucose and oxygen are the products.
6CO2+6H2O+Light energy ⟶ C6H12O6 + 6O2
(3) Photosynthesis occurs only during day time.
RESPIRATION
(1) Respiration is the process by which organisms break down complex compounds containing carbon in a way that allows the cells to harvest a maximum of usable energy.
(2) In respiration, glucose and oxygen are the reactants while carbon dioxide, water and energy are the products.
C6H12O6 +6O2 ⟶ 6CO2+6H2O+Energy
(3) Respiration goes on during day and night.
(viii) Explain the difference between the cyclic and non-cyclic photophosphorylation with the help of Z scheme.
ANSWER:
Consult the textbook at page 216 — 218.
(ix) Give an account of light-independent reactions of photosynthesis
ANSWER:
Consult the textbook at page 219 — 221.
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