Extra questions B8

Question 1

Explain why a leaf kept in the light for 24 hours will turn an iodine solution blue/black, whereas a leaf kept in light for 24 hours and then dark for 24 hours will have no effect on the iodine solution

Answer 1

• Leaf kept in light photosynthesises, making glucose from carbon dioxide and water using energy from light.
• Glucose converted to starch to be stored and used for respiration when leaf is in dark and cannot photosynthesise.
• Iodine solution turns blue‑black in presence of starch, because plant has been photosynthesising for a long time and glucose will have been stored as starch.
• If leaf is then kept in dark for 24 hours it cannot photosynthesise so cells will use stored starch for respiration. Little or no starch left in the leaves so iodine solution unaffected.

Question 2

Describe the path taken by a carbon atom as it moves from being part of the carbon dioxide in the air to being part of a starch molecule in a plant

Answer 2

Carbon atom moves from air into air spaces in leaf, into plant cells, into chloroplasts, joins with water to make glucose, and is converted to starch for storage.

Question 3

Explain in terms of limiting factors why the plants growing in a tropical rainforest are so much bigger than the plants that grow in a UK woodland, and why both are bigger than the plants on the Arctic tundra

Answer 3

• Tropical rainforest: high light intensity, warm temperature, plenty of moisture, carbon dioxide from decaying material
• relatively few limiting factors and rapid growth conditions available all year round, allowing plants and even individual leaves get very large);
• UK woodland: low light intensity, short days in winter, cold temperature
• (most growth takes place in spring and summer with plenty of light and warmth but temperatures still lower temperatures than tropical rain forest, less time for growth so plants smaller);
• Arctic tundra: no light all winter but lots of light in summer, lack of water due to frozen ground
• (low temperatures limiting factor on photosynthesis for most of year, so plants much smaller and slower-
  growing than tropical rainforest or UK woodland)

Question 4

Explain why pitcher plants, sundews and Venus fly traps are often found growing in bogs globally, an environment where not many other plants can survive

Answer 4

• Bogs are wet and peaty and soil contains very few minerals, especially nitrates.
• Plants need nitrates from soil to make amino acids and build them into proteins.
• Many plants cannot grow well in bogs. Carnivorous plants trap insects and digest their bodies, which provide good supply of nitrates and other minerals.
• These plants can grow and thrive in bogs as they do not rely on bog soil for minerals.

Question 5

Why is light needed for photosynthesis?

Answer 5

to transfer energy

Question 6

Plants need energy to make glucose.
How do plants get this energy?

Answer 6

light is trapped/ absorbed by chloroplasts

Question 7

Explain the results when the light intensity was 0 lux.

Answer 7

when there is no light there is no photosynthesis
no oxygen is produced
respiration happens and oxygen is used
overall oxygen production is negative