20: Photosynthesis

Question 1

Explain how the distribution of the photosynthetic leaves in these leaves is related to the vertical orientation of the leaves.

Answer 1

• When the leaves are oriented vertically, both sides of the leaves have chance of receiving sunlight
• Therefore, the photosynthetic tissues are evenly distributed on both sides of the leaf
• Such that photosynthesis can be carried out effectively regardless of the change in the orientation of the sun/the direction of illumination from sun during daytime

Question 2

What is the purpose of destarching the plant?

Answer 2

To ensure that any starch detected at the end of the experiment is produced during the experiment

Question 3

Explain how can a plant be destarched.

Answer 3

• Put the plant in the dark for at least 24 hours
• The starch in the leaves will be broken down to
  glucose for respiration or sucrose for transport to other parts of the leaf
• So at the start of the experiment, all starch in the leaves are removed

Question 4

Why should iodine test be carried out on a leaf of the potted plant before the experiment?

Answer 4

To confirm that starch is absent in the leaves at the start of the experiment

Question 5

Describe the procedures of iodine test

Answer 5

1. Boil the leaf in hot water for 1 min
2. Put the leaf in a boiling tube half filled with alcohol, and put the tube into a beaker of hot water to heat up the alcohol.
3. Transfer the leaf to a tube of hot water
4. Put the leaf on a white tile and add iodine solution to the lead

Question 6

What is the purpose of boiling the leaf in hot water for 1 minute?

Answer 6

• Destroys the differentially permeable cell membrane of the leaf cells
• Allows iodine solution to enter the leaf cells

Question 7

Why is it necessary to put the leaf in hot water again?

Answer 7

• alcohol makes the leaf brittle; remove the alcohol + soften the leaf
• leaf must be softened so it can be spread to allow addition of iodine soluiton
• test for the presence of starch on the entire leaf surface

Question 8

What is the purpose of putting the leaf in a boiling tube half filled with alcohol and putting the tube into a beaker of hot water to heat up the alcohol?

Answer 8

• Alcohol dissolves and removes chlorophyll from the leaf - any colour changes of iodine solution can be observed clearly on the decolourized leaf
• Heating: increases rate of diffusion of chlorophyll out of the leaf cells and dissolution of chlorophyll in alcohol.

Question 9

purpose of adding iodine solution to the leaf

Answer 9

• test for the presence of starch

Question 10

fate pf photosynthetic products - glucose

Answer 10

used in respiration to release energy as an energy source
used to make cellulose (main component of cell wall)

Question 11

fate pf photosynthetic products - starch

Answer 11

• synthesised from glucose
• suitable for storage (insoluble → can be stored in leaves without affecting the water potential of the cells)
• can be broken down to form glucose, which may be used as an energy source or to make cellulose (main component of cell walls)
• can be broken down to form sucrose for transport

Question 12

fate pf photosynthetic products - sucrose

Answer 12

• transport to non-photosynthetic tissues through phloem
• suitable for transport (soluble and relatively inactive)

Question 13

fate of photosynthetic products - lipids

Answer 13

metabolic intermediates formed from the breakdown of glucose
- synthesis of fatty acids and glycerol that condense to form lipids
- energy reserves and forming cell membranes

Question 14

fate of photosynthetic products - lipids

Answer 14

metabolic intermediates formed from the breakdown of glucose
- combines with nitrogenous compounds from soil (e.g. NO3-, NH4+) to form amino acids
- amino acids condense to form proteins

Question 15

explanation for negative result in iodine test

Answer 15

• under the absence of ___, photosynthesis cannot occur
• X form glucose
• glucose X accumulate → X form starch
  leaf gives brown colour in iodine test