20: Photosynthesis

Question 1

What is the word equation for the overall process of photosynthesis?

Answer 1

carbon dioxide + water – (light energy) (chlorophyll) –> carbohydrates + oxygen

Question 2

How does photosynthesis allow a continuous input of energy to ecosystems?

Answer 2

During photosynthesis, light energy from the sun is captured by plants and algae. It is then converted to chemical energy that is stored in their bodies. The chemical energy is transferred along food chains and is eventually lost as heat to the surroundings.

Question 3

How does photosynthesis maintain the oxygen balance in the atmosphere?

Answer 3

Respiration of organisms consumes oxygen. The oxygen consumed is replaced by the oxygen released from photosynthesis.

Question 4

How does photosynthesis maintain the carbon dioxide balance in the atmosphere?

Answer 4

Respiration of organisms releases carbon dioxide. The carbon dioxide is consumed in photosynthesis, balancing the increase in carbon dioxide concentration in the atmosphere due to respiration.

Question 5

List 3 significances of photosynthesis.

Answer 5

1. It provides the basic food source for most organisms.
2. It maintains energy flow in ecosystems.
3. It maintains the balance of oxygen and carbon dioxide in the atmosphere.

Question 6

List 2 ways to test is photosynthesis has occurred.

Answer 6

1. Detecting the release of oxygen: oxygen is a by-product of photosynthesis and can be tested using a glowing splint. Oxygen is present if the splint relights.
2. Detecting the presence of starch: iodine test can be carried out. Starch is present if the iodine solution turns from brown to blue-black.

Question 7

Why is destarching the plant necessary before an investigation on the requirements of photosynthesis?

Answer 7

If the presence of starch is used as evidence that photosynthesis has occurred, any starch that is present before the investigation in the leaves must be removed. This ensures that the starch detected at the end of the investigation was made during the investigation.

Question 8

How is destarching carried out?

Answer 8

The plant is put in the dark for at least 24 hours. In darkness, the starch in the leaves is broken down to glucose, which is used as an energy source or combined with fructose to form sucrose.

Question 9

What is the main site of photosynthesis in plants?

Answer 9

Leaves

Question 10

List 4 advantages of the adaptations of leaves for photosynthesis.

Answer 10

1. The light absorption is maximised.
2. Gas exchange is facilitated.
3. Transport of materials is facilitated.
4. Water loss is reduced.

Question 11

List and explain 3 ways in which the shape and arrangement of leaves are well adapted for photosynthesis.

Answer 11

1. The leaf blade is often broad and flat to provide a large surface area for absorbing sunlight.
2. The leaf is thin to allow gases and light to reach the photosynthetic cells easily.
3. The leaves are arranged in a way that there is little overlapping among the leaves, allowing each leaf to expose to maximum amount of light.

Question 12

What are the features of the palisade mesophyll in terrestrial dicotyledonous plants?

Answer 12

It consists of cylindrical, tightly packed cells that contain many chloroplasts. It is located on the upper side of the leaf, which is exposed directly to sunlight. These allow effective absorption of sunlight.

Question 13

What are the features of the spongy mesophyll in terrestrial dicotyledonous plants?

Answer 13

It consists of loosely packed cells of irregular shapes with many air spaces. The air space allow gases to diffuse freely inside the leaf. The cells contain chloroplasts for the absorption of sunlight.

Question 14

List 3 differences between the palisade mesophyll and the spongy mesophyll in terrestrial dicotyledonous plants.

Answer 14

1. Arrangement of cells: the palisade mesophyll cells are cylindrical and closely packed, while the spongy mesophyll cells are irregular in shape and loosely packed.
2. Air spaces: The air spaces among palisade mesophyll cells are narrower while there are large air spaces among spongy mesophyll cells.
3. Amount of chloroplast: palisade mesophyll cells contain more chloroplasts and spongy mesophyll cells contain fewer chloroplasts.

Question 15

What are the features of the cuticle in terrestrial dicotyledonous plants?

Answer 15

It is a thin waxy layer covering the upper and lower epidermis. It prevents excessive water loss by evaporation from the leaf. This ensures that water is kept inside for photosynthesis, for keeping the cells turgid and supporting the leaf blade.

Question 16

What are the features of the upper and lower epidermis in terrestrial dicotyledonous plants?

Answer 16

They are covered by a thin layer of easy cuticle. The epidermis is the outermost layer of cells covering the upper and lower surfaces of leaf, which protects the inner layers of cells. In most terrestrial dicotyledonous plants, there are more stomata in the lower epidermis than in the upper epidermis. Epidermal cells have no chloroplasts except the guard cells.

Question 17

What are the features of the stomata in terrestrial dicotyledonous plants?

Answer 17

There are more stomata (stoma) in the lower epidermis than in the upper epidermis. Each stoma is surrounded by a pair of guard cells that contain chloroplasts. Stoma open when the conditions are favourable for photosynthesis, and close when the conditions are less favourable. This regulates the passage of gases and water vapour into and out of the leaf.

Question 18

What are the features of the vascular bundles in terrestrial dicotyledonous plants?

Answer 18

The midrib and veins contain vascular bundles which are made up of xylem and phloem. These allow efficient transport of materials into and away from the leaf. They also support and spread the leaf blade.

Question 19

What are the structural features of chloroplasts?

Answer 19

A chloroplast is bound by a double membrane and is filled with a jelly-like fluid called stroma. In the stroma, there are enzymes that catalyse photosynthetic reactions, starch grains, and other photosynthetic products like lipid droplets. A network of disc-like membranous sacs called thylakoids is suspended in the stroma. They are arranged in stacks called grana (granum). Chlorophyll is located on the thylakoid membranes.

Question 20

List and explain 2 ways in which the features of a chloroplast are well adapted for photosynthesis.

Answer 20

1. A chloroplast has about 50 grana, each consisting of about 50 thylakoids. This provides a large surface area to pack more chlorophyll for absorbing light.
2. The grana are interconnected by extensions from the thylakoids. This allows efficient transport of photosynthetic products within the chloroplast.

Question 21

What are the two main stages of photosynthesis?

Answer 21

Photochemical reactions and carbon fixation (Calvin cycle).

Question 22

What are photochemical reactions?

Answer 22

They are a series of reactions occurring in the thylakoids of chloroplasts, which require light. It involves excitation of electrons in chlorophyll and the splitting of the water molecule, leading to the production of oxygen.

Question 23

What is carbon fixation?

Answer 23

It is a series of reactions occurring in the stroma of chloroplasts, which do not require light. It involves the synthesis of various molecules from carbon dioxide, leading to the production of carbohydrates.

Question 24

Name the 4 main stages of photochemical reactions.

Answer 24

1. Light absorption and electron emission
2. Electron transport
3. Formation of ATP
4. Photolysis of water

Question 25

What happens during light absorption and electron emission in photochemical reactions?

Answer 25

Chlorophyll molecules in the thylakoid membrane capture light energy. Some electrons of the chlorophyll molecule are raised to a higher energy level and are emitted from the chlorophyll molecule.

Question 26

What happens during electron transport in photochemical reactions?

Answer 26

The excited electrons pass through an electron transport chain, which consists of a series of electron carriers of decreasing energy levels in the thylakoid membrane. Energy is released in a step-wise manner.

Question 27

What happens during the formation of ATP in photochemical reactions?

Answer 27

Some energy released by the electrons in the electron transport chain is used to form ATP by combining a phosphate with ADP, which is known as phosphorylation.
Light energy captured by the chlorophyll molecules is converted to chemical energy stored in ATP, which acts as an energy carrier.

Question 28

What happens during the photolysis of water in photochemical reactions?

Answer 28

The light energy captured by chlorophyll molecules is also used to split water molecules to form hydrogen and oxygen. The hydrogen is accepted by NADP which is then reduced to NADPH, while oxygen is released as a by-product of the reaction.

Question 29

List the 3 main stages of carbon fixation.

Answer 29

1. Carbon dioxide fixation and formation of 3-C compound
2. Reduction of 3-C compound and formation of glucose
3. Regeneration of the carbon dioxide acceptor

Question 30

What happens in carbon dioxide fixation of carbon fixation?

Answer 30

Carbon dioxide diffuses from the air spaces in leaves into the photosynthetic cell and then into the stroma of the chloroplast. Under the action of enzymes, one molecule of carbon dioxide combines with one molecule of a 5-C compound to form 2 molecules of a 3-C compound.

Question 31

What happens in the reduction of the 3-C compound in carbon fixation?

Answer 31

Using energy from the breakdown of ATP into ADP and phosphate, and the hydrogen from NADPH, the 3-C compound is reduced to triose phosphate. ADP and NADP are formed and will be reused in photochemical reactions.
Two molecules of triose phosphate combine to form one molecule of glucose.

Question 32

What happens in the regeneration of the carbon dioxide acceptor in carbon fixation?

Answer 32

Most of the triose phosphate formed is used to regenerate the original 5-C carbon dioxide acceptor, so the cycle repeats and forms more glucose. This requires energy from ATP.