Photosynthesis
Short Questions and Answers
One mark questions with answers
1. Which one of the following is the main product of photorespiration in C3 plants?
(a) Phosphoglycerate
(b) Phosphoglycolate
(c) glycerate
(d) glycocholate
Answer: (b) Phosphoglycolate
2. Which of the following does not evolve oxygen?
(a) photosynthetic bacteria
(b) blue-green algae
(c) green algae
(d) autotrophic plants
Answer: (a) photosynthetic bacteria
3. Compensation point refers to
(a) little photosynthesis
(b) beginning of photosynthesis
(c) the rate of photosynthesis equals the rate of respiration
(d) none of these
Answer: (c) the rate of photosynthesis equals the rate of respiration
4. What is the name of cycle which generates energy and splits water into hydrogen and oxygen?
(a) electron transport chain
(b) glycolysis
(c) glycogenolysis
(d) citric acid cycle
Answer: (a) electron transport chain
5. Which pigment is present in all green plants?
(a) chlorophyll a
(b) chlorophyll b
(c) chlorophyll c
(d) chlorophyll m
Answer: (a) chlorophyll a
6. Carbon dioxide is fixed in
(a) light reaction
(b) dark reaction
(c) aerobic respiration
(d) anaerobic respiration
Answer: (b) dark reaction
7. In photosynthesis oxygen is liberated due to
(a) hydrolysis of carbohydrates
(b) breakdown of proteins
(c) hydrolysis of water
(d) reduction of carbon dioxide
Answer: (c) hydrolysis of water
8. Photosynthetic prokaryote is
(a) cyanobacteria
(b) sulphur bacteria
(c) archaeabacteria
(d) iron bacteria
Answer: (a) cyanobacteria
9. The sunlight controls which reaction of PSII?
(a) photolysis of water
(b) activation of chlorophylls
(c) ATP formation
(d) NADPH2 formation
Answer: (a) photolysis of water
10. Which of the following enzymes help in carbon dioxide fixation during C4 cycle?
(a) RuBP carboxylase
(b) PEP carboxylase
(c) RuBP oxygenase
(d) PGA
Answer: (b) PEP carboxylase
11. Magnesium is an important constituent of which of the following pigments?
(a) phytochrome
(b) chlorophyll
(c) florigen
(d) haemoglobin
Answer: (b) chlorophyll
12. The pigment system which produces sufficient oxidation potential to cause oxidation of water is
(a) pigment system I
(b) pigment system II
(c) carotenoids
(d) phycobilins
Answer: (a) pigment system I
Two marks questions with answers
1. What is carboxylation?
Answer: Addition of carbon dioxide to a compound is termed as carboxylation. In this step of dark reaction, ribulose 1, 5 diphosphate, (also known as ribulose biphosphate), a phosphorylated 5 carbon sugar which acts as carbon dioxide acceptor, combines with carbon dioxide. The enzyme, RUBISCO, acts as a catalyst in this reaction. This results in the formation of 2 molecules of 3-phosphoglyceric acid.
2. How does the anatomy of a typical C4 leaf differs from that of C3 leaf?
Answer: The photosynthetic parenchyma cells in a typical C3 leaf are organized into two distinct tissues-an upper region of tightly packed palisade cells and the more loosely arranged spongy mesophyll cells bordering large intercellular spaces. In C4 leaves there are only one type of mesophyll cells. C4 leaves are generally thinner than C3 leaves. C4 leaves are characterized by the presence of tightly packed, thick-walled bundle sheath cells all around the vascular bundle (Kranz anatomy).
3. What is photorespiration?
Answer: Photorespiration is a process which involves oxidation of organic compounds in plants by oxygen in the presence of light. Like normal respiration, this process also releases carbon from organic compound in the form of carbon dioxide but does not produce ATP. Thus, it seems to be a wasteful process. It occurs in temperate C3 plants such as rice, wheat, barley, bean etc. The process of photorespiration takes place in chloroplast, peroxisome and mitochondria.
4. What is Warburg effect in photosynthesis?
Answer: At a very high oxygen concentration, the rate of photosynthesis begins to decline in all the plants. This phenomenon is called “Warburg effect.” Increase in the concentration of oxygen inhibits the fixation of carbon dioxide by RuBisCO. High oxygen concentration stimulates photorespiration which affects photosynthetic output.
5. Distinguish between photorespiration and respiration.
Answer: Photorespiration occurs inside photosynthetic cells whereas respiration is found in all living cells. Photorespiration takes place only in the presence of light whereas respiration continues both in dark and light. Photorespiration occurs in the chloroplast and may require the help of peroxisomes and mitochondria whereas respiration occurs in cytoplasm and mitochondria.
Three marks questions with answers
1. Explain Emerson effect.
Answer: Emerson and Lewis determined the quantum yield of photosynthesis under different wavelength of light and found that there was a sharp drop in the region above 680 nm. The fall in photo yield beyond red region of spectrum is called “red drop.”
Emerson and his co-worker further extended the previous experiment by supplying additional shorter wavelength of light along with far red light. The monochromatic far red light which was insufficient, when supplied with shorter wavelength of light enhance the photosynthetic yield and recovered the red drop. The enhancement in yield due to combination of shorter wavelength of light with far red light is termed as “Emerson effect.”
2. What is Cyclic photophosphorylation?
Answer: It is a process of synthesis of ATP from ADP in presence of light during cyclic transfer of electron. Absorption of light by pigment system I (PSI) results in the excitation of its reaction centre P700, which emits electron. The electrons are transferred to ferredoxin-reducing substance (FRS) through A (FeS). Reduced FRS transfers its electron to ferredoxin and gets oxidized. If the concentration of NADP+ is low, then FRS instead of transferring its electron to Fd, transfers electron to cytochrome b. The electron is then transferred to cytochrome f, then from cytochrome f to plastocyanin. From plastocyanin, electron returns back to pigment system I and chlorophyll gets reduced. During the transfer of electron from Fd to cytochrome b and from cytochrome b to cytochrome f, 2 molecules of ATP are synthesized from ADP and inorganic phosphate.
3. What do you understand by glycolytic reversal?
Answer: In dark reaction, phosphoglyceric acid forms phosphoglyceral dehyde by utilizing ATP molecules. Phosphoglyceral dehyde converts into hexose sugar. Thus, the entire process is just a reverse of glycolysis in which hexose sugars are first oxidised into phosphoglyceral dehyde and then to carbon dioxide and water, releasing ATP.
The step involves utilization of 2 molecules of ATP for phosphorylation and 2 molecules of NADPH for reduction per carbon dioxide molecule fixed. As glucose is a six carbon compound, thus it requires six turns of Calvin cycle to synthesise its one molecule.
4. What is Kranz anatomy?
Answer: Kranz anatomy occurs in the leaves of C4 plants. In C4 leaves the chloroplasts present in bundle sheath cells are of abnormal type. They are large in size, centripetally arranged and lack well-organized grana. They contain starch grain. The chloroplast of mesophyll cells are normal. Hence, in C4 plants, chloroplasts are dimorphic in nature.
C4 leaves are characterized by the presence of tightly packed, thick-walled bundle sheath cells all around the vascular bundle. Because of the wreath-like configuration of these bundle sheath cells, this arrangement is known as Kranz anatomy .
5. Describe briefly the steps involved in Hatch and Slack pathway/C4 pathway.
Answer: The steps involved in Hatch and Slack pathway or C4 pathway are as follows:
(1). Phosphoenolpyruvic acid accepts carbon dioxide and forms oxaloacetic acid inside mesophyll cells.in the presence of enzyme, phosphoenolpyruvate carboxylase.
(2). Oxaloacetic acid is reduced by NADPH2 to form malic acid in the presence of enzyme, malate dehydrogenase.
(3). Oxaloacetic acid also produces aspartic acid by a transamination reaction in the presence of enzyme, transaminase.
(4). Malic acid is transported to bundle sheath cells, where it is decarboxylated by NADP and specific malic enzyme to produce pyruvic acid and carbon dioxide. Carbondioxide is again fixed inside the bundle sheath cells through calvin cycle.
(5). Pyruvic acid is sent back to mesophyll cells, where it converts to phosphoenolpyruvate by using ATP. This results in the formation of AMP (adenosine monophosphate) instead of ADP (adenosine diphosphate).