* Photosynthesis (Ch 20)

Question 1

Leaf blade

Answer 1

• Broad and flat to provide large surface area to maximize absorption of sunlight
• Thin, shortens diffusion distance between the leaf cells and the atmosphere so that carbon dioxide and oxygen can diffuse rapidly in and out of the leaf; maximizes the penetration of light into mesophyll cells

Question 2

Petiole

Answer 2

• Holds the leaf blade in the position that can maximize the exposure to sunlight
• Carries vascular bundles

Question 3

Upper epidermis

Answer 3

• Covered by a layer of waxy cuticle which reduces water loss from the leaf surface by evaporation
• Cells are transparent to allow light to penetrate to the mesophyll cells

Question 4

Palisade mesophyll

Answer 4

• Consists of cylindrical cells which are closely packed to maximize light absorption for photosynthesis
• Located at the upper position of the leaf that allows it to maximize light absorption for photosynthesis
• Contain a high density of chloroplasts that allows them to receive more sunlight for photosynthesis

Question 5

Spongy mesophyll

Answer 5

• Consists of irregular cells which are loosely packed with many large air spaces between them, facilitates diffusion of carbon dioxide, oxygen and the rate of gas exchange
• Contains less chloroplasts than palisade mesophyll cells

Question 6

Guard cells

Answer 6

• The cell wall of the inner side of the guard cell is thicker
• Outer side of the cell wall is thinner
• When the cells become turgid, outer side of the cell wall stretches more
• Guard cells are curved outwards and the stoma opens

Question 7

When a leaf is placed in hot water

Answer 7

• Bubble are found to evolve from the surface of the leaf: air in the air space expands on heating -> escapes from the leaf
• More bubble are found on the lower surface: more stomata are present in the lower epidermis of the leaf

Question 8

Double membrane of chloroplast

Answer 8

• Outer membrane: regulate the passage of substances into and out of the organelle, providing an isolated environment, energy transformation process will not be interfered by other cellular processes
• Inner membrane: increase substrate concentration by entrapping the necessary raw materials inside the organelles

Question 9

Thylakoid

Answer 9

• Thylakoids are stacked one over the others, forming grana. This provides large surface area ti pack more chlorophyll molecules for trapping light and carriers of electron transport and enzymes for ATP synthesis within limited amount of space

Question 10

Photochemical reactions

Answer 10

• In thylakoid membrane

Photophosphorylation:
- When the chlorophyll in the thylakoids absorb light, it emits high energy electrons
- These electrons pass down an electron transport chain. Energy is released in a stepwise manner to produce ATP by adding a phosphate group to ADP

Photolysis of water
- The light energy absorbed by the chlorophyll splits water into hydrogen and oxygen
- The hydrogen is accepted by hydrogen carrier NADP to form NADPH
- Oxygen is released to the atmosphere as a by-product

Question 11

Calvin cycle

Answer 11

• In stroma
• Carbon dioxide combines with a 5-C compound which acts as a carbon dioxide acceptor to fix the carbon dioxide. This forms a very unstable 6-C compound which splits into 2 molecules of 3-C compound quickly
• The 3-C compound is reduced by NADPH to triose phosphate. The energy needed is provided by ATP produced in photochemical reactions. Both ADP and NADP are regenerated and reused in photochemical reactions
• Two molecules of triose phosphate combine to form glucose and other biomolecules needed by the plant

Question 12

Changes in 3-C and 5-C compounds when the light is switched off

Answer 12

• When the light is switched off, level of 3-C compound rises rapidly and level of 5-C compounds falls rapidly, since the regeneration of acceptor 5-C compound from triose phosphate in the dark because regeneration requires ATP and NADPH generated in photochemical reaction.
• 3-C compound synthesis continues for a short time in the dark, resulting in an accumulation of 3-C compound until the pool of 5-C compound generated in the light is used up

Question 13

Abundant carbon dioxide

Answer 13

• When carbon dioxide is abundant, the steady level to 3-C compound is twice that 5-C compound. One molecule of 5-C compound combines with one molecule of carbon dioxide to produce two molecules of 3-C compounds

Question 14

Insufficient carbon dioxide

Answer 14

• When there is insufficient carbon dioxide, rate of 5-C compound formation is much greater than the rate of 3-C compound formation, the amount of 5-C compound accumulates as little of it can be used to fix carbon dioxide, with a corresponding decline is 3-C compound