chapter 5 - photosynthesis · photosynthesis. plants can carry out photosynthesis because their...

Chapter 5 – Photosynthesis

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Chapter 5 Photosynthesis A leaf is an organ in plants that carries out photosynthesis because it contains the largest amount of chloroplasts. The following diagram shows the cross‐sectional view of a typical leaf structure.

Figure 5.1 Leaf Leaf Structures and its Functions Palisade mesophyll layer: cells are regular, an elongated shape, and contain a lot of chloroplasts for photosynthesis. Spongy mesophyll layer: cells are an irregular shape to provide a lot of air space for gas exchange. Waxy cuticle: a waterproof layer on the upper epidermis, sometimes on the lower epidermis as well, to prevent water loss from the plant. Upper and lower epidermis: a one‐cell thick layer to protect the internal leaf structure Stomata: pores on the lower epidermis that allow substances to move in and out of the leaf. Guard cells: cells located next to the stomata that control the opening or closing of the stomata. Vascular bundle (vein): contains xylem and phloem tissues for water and sugar transport respectively.


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Plants are autotrophic organisms that can produce their own food by converting light energy to chemical energy. The chemical energy is stored in sugar; for example glucose. This process is called photosynthesis. Plants can carry out photosynthesis because their chloroplasts contain chlorophyll, the green pigment that absorbs sunlight.

Figure 5.2 Structure of chloroplast In chloroplasts, chlorophyll is located on the thylakoid membrane. There are different types of chlorophyll, for example chlorophyll a, chlorophyll b and carotenoid. Chlorophyll a is the major photosynthetic pigment. Its color is green because it absorbs all colors (mainly red and blue) except green. Chlorophyll b and carotenoids are regarded as accessory pigments, i.e., they help chlorophyll a to absorb more light energy for photosynthesis. The wavelength that they absorb is slightly different from the chlorophyll a. Therefore, this can broaden the light spectrum that can be used to produce sugar. Photosynthetic pigments are clustered together to form complexes on the thylakoid membrane. The complexes of pigments are called photosystems (PS). There are two different types of photosystems: photosystem I and II. In each photosystem, chlorophyll a is located at the center and is known as the reaction center. Other accessory pigments surround the reaction center to assist light absorption. Photosystem I and II absorb slightly different wavelengths, 700 and 680 nm respectively.


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Figure 5.3 Absorption spectra for different chlorophyll types Overview of Photosynthesis

6CO2 + 6H2OLight

ChlorophyllC6H12O6 + 6O2

Photosynthesis is illustrated by the above equation. The oxygen produced comes from water. This is because water is broken down by sunlight, which is absorbed by chlorophyll. This reaction is called photolysis, which releases oxygen gas. Besides oxygen, photolysis also releases hydrogen from water. This hydrogen is then fixed into carbon dioxide to form sugars. Therefore, photosynthesis is divided into two processes: a light‐dependent reaction, which involves the breaking down of water, and a light‐independent reaction, which involves the fixation of hydrogen into carbon dioxide. They occur at the thylakoids and stroma respectively. As mentioned above, water is broken down, and hydrogen is released in light‐dependent reactions. This hydrogen is carried by NADP+ in a form of NADPH. ATP is also formed in light‐dependent reactions because energy is required to fix hydrogen into CO2 to form sugar in light‐independent reactions.


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Light‐dependent Reaction When light hits photosystem II (PSII) and photosystem I (PSI), electrons from the reaction centers are activated. The excited electrons from PSII release energy when they pass through an electron transport chain (ETC). The energy is for pumping the hydrogen ions (i.e. protons) into the thylakoid space by active transport. This establishes a high concentration of hydrogen ions in the thylakoid. Therefore hydrogen ions diffuse across the ATP synthase, which are also located on a thylakoid membrane, for ATP formation. Since the energy comes from light, the formation of ATP is called photo‐phosphorylation. The excited electrons from PSI can either combine with NADP+ and hydrogen ions to form NADPH, or pass through the ETC to produce more ATP by photo‐phosphorylation. The electrons in PSI either come from PSII or self‐replenish by PSI itself after passing through the ETC. The electrons in PSII, however, come from water, when water is broken down in photolysis. In photolysis, water is broken down into oxygen, hydrogen ions and electrons. The electrons fill up the space in PSII, while hydrogen ions are pumped into thylakoid space for the formation of ATP and NADPH, and oxygen is released for respiration.

Figure 5.4 The energy change in light‐dependent reaction


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Figure 5.5 The light‐dependent reaction in thylakoid Light‐independent Reaction Light‐independent reactions, also called Calvin cycles, occur at the stroma. In this process, CO2, which comes from the atmosphere, is accepted by an intermediate called RuBP in the Calvin cycle. RuBP is a 5‐C molecule containing 2 phosphate groups. An enzyme called rubisco (RuBP carboxylase) is required for this reaction, called carbon fixation. In this carbon fixation, a 6‐C molecule is formed, and this molecule is very unstable. It breaks down immediately into two 3‐C molecules called PGA. PGA is then reduced by NADPH, with the energy from ATP, to form PGAL, which is also called G3P. G3P is regarded as the first product in the Calvin cycle, or photosynthesis. Some G3P is further modified to become other sugars, like glucose; the remaining G3P is regenerated to RuBP, with the energy from ATP.


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Figure 5.6 Calvin cycle Other types of Photosynthesis The plants that use CO2 directly from the air to produce sugars are called C3 plants. This is because the first organic compound produced is a 3‐C compound (G3P). One of the problems in growing C3 plants is that dry weather can reduce photosynthesis, and in turn decrease the crop productivity. This is because, on a hot and dry day, leaves close their stomata, and less CO2 is available for photosynthesis. If CO2 content is very low, and O2 content from light‐dependent reaction builds up, rubisco will combine RuBP with O2 and produce CO2 and H2O. This process which wastes a large amount of energy is called photorespiration. Plants that live in dry environments use alternative processes to prevent photorespiration from occurring. C4 and CAM plants are examples of plants that live in dry environments, and they use different ways to produce sugars.


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C4 plants In C4 plants, two different cells are involved in photosynthesis. They are mesophyll cells and bundle sheath cells. Mesophyll cells carry out normal light‐dependent reactions to produce ATP and NADPH. In addition, CO2 is modified to a 4‐C molecule to act as a shuttle of CO2. This 4‐C molecule, together with ATP and NADPH, move into a bundle sheath cell. The 4‐C molecule is then converted back to CO2 in the bundle sheath for the Calvin cycle to produce sugar. This process can prevent O2 from the light reaction that occurs in the mesophyll cell from reacting with rubisco and RuBP in the bundle sheath. Rubisco then can combine RuBP and CO2 to form sugar.

Figure 5.7 C4 photosynthesis CAM plants CAM, which stands for crassulacean acid metabolism (CAM), is another process that occurs in plants living in hot and dry environments such as cacti. The cactus usually opens its stomata at night and closes them in the daytime because the temperature is relatively low at night. So during the time when the stomata are opened, the plants obtain CO2 from the surroundings. The CO2 is converted to a 4‐C molecule, which acts as a CO2 store. The 4‐C molecules release CO2 when light is available in the daytime. So in the daytime when light is available, ATP and NADPH, which are formed in the light‐dependent reactions, can work with the CO2 in the Calvin cycle.


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Figure 5.8 CAM photosynthesis Practice Questions Questions 1‐3 refer to the following molecules. A. NADH B. Sugars C. CO2 D. O2 E. NADPH 1. It is the product of a light‐dependent reaction that is used in the Calvin cycle. 2. It reacts with RuBP in the Calvin cycle. 3. It reduces PGA into G3P in the Calvin cycle. 4. All of the following are correct about a light‐dependent reaction EXCEPT:

A. It takes place at the stoma. B. It produces ATP and NADPH. C. It releases oxygen in photolysis. D. It includes photo‐phosphorylation. E. It uses light as the ultimate source of energy.

5. Which of the following molecules is/are product(s) of light‐dependent reactions?

A. NADH B. ATP C. O2 D. A and B E. B and C


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6. Green light is the least effective for photosynthesis because

A. carotenoid is green. B. chlorophyll can only reflect red and green. C. green light is used for the light‐dependent reaction only. D. green light is reflected by the chlorophyll. E. green light is absorbed by the mitochondria for respiration.

7. Which of the following molecules is the first product of the Calvin cycle?

A. CO2 B. RuBP C. PGA D. G3P E. NADPH

8. Which of the following is correct about a C4 plant?

A. It uses mitochondria to produce NADPH for the Calvin cycle. B. It uses photorespiration to produce sugars. C. It uses a 4‐C molecule as the source of CO2 for the Calvin cycle. D. It uses a 4‐C molecule as the source of O2 for photorespiration. E. It uses the upper epidermis to carry out the Calvin cycle.

9. Which of the following is the major difference between PSI and PSII?

A. PSI has more pigments than PSII. B. PSI is the first reaction site for light‐dependent reactions, while PSII is the second. C. PSI is more reactive by absorbing 700nm, whereas PSII is more reactive by absorbing 680nm. D. PSII is the reaction center for photolysis, whereas PSI is the reaction center for the Calvin cycle. E. PSII is located on the thylakoid membrane, while PSI is at the stroma.

10. The electron transport chains for light‐dependent reactions

A. are located on the outer membrane of chloroplasts. B. produce O2. C. transport ATP to the stroma for the Calvin cycle. D. transport hydrogen ions into the thylakoid. E. transport O2 to the stroma for forming sugars.

11. The electrons released from the reaction centers of PSII are replaced by the electrons from the

A. PSI. B. other pigments in PSII. C. NADPH. D. ATP. E. H2O.


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Questions 12‐15 refer to the following terms: A. Upper epidermis B. Lower epidermis C. Guard cells D. Vascular bundles E. Palisade mesophyll layer 12. Contains stomata for gases exchange. 13. Controls the opening of the stomata. 14. Contains tissues to transport water and nutrients. 15. Contains cells that have a lot of chloroplasts for photosynthesis. Answers 1. E 2. C 3. E 4. A 5. E 6. D 7. D 8. C

9. C 10. D 11. E 12. B 13. C 14. D 15. E


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chapter 5 - photosynthesis · photosynthesis. plants can carry out photosynthesis because their...

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