13.3. Adaptations for photosynthesis

Question 1

Chloroplast Structure

Answer 1

• surrounded by a double membrane (chloroplast envelope)
• membranes in a chloroplast are called lamella
• the membranes contain chlorophyll molecules, arranged into groups called photosystems (PSI and PSII)
• there are enclosed space between pairs of membranes, forming fluid-filled sacs called thylakoids
• Thylakoids are arranged in stacks called grana
• The background material of the chloroplast is called the stroma
• Chloroplasts often contain starch grains and liquid droplets, which are stores of energy-containing substances made in the chloroplast, but are not immediately needed by the cell

Question 2

Where do reactions take place in the chloroplast?

Answer 2

• light dependent reactions occur in the lamella as there are chlorophyll molecules there (with photosystems)
• phosphorylation (formation of ATP using energy from light) occurs in thylakoids
• light independent reactions take place in the stroma

Question 3

C3 Plants

Answer 3

• In the light independent stage of photosynthesis, you may remember that carbon dioxide combines with RuBP to form a six-carbon compound, which immediately splits to form two three-carbon molecules
• Plants that do this are called C3 plants.

Question 4

C4 plants

Answer 4

• However, maize and sorghum plants – and most other tropical grasses – do something different to C3 plants.
• The first compound that is produced in the light independent reaction contains four carbon atoms. They are therefore called C4 plants.

Question 5

What C4 plants do differently to C3 pllants

Answer 5

• they avoid photorespiration
• They keep RuBP and rubisco well away from high oxygen concentrations.
• The cells that contain RuBP and rubisco are arranged around the vascular bundles, and are called bundle sheath cells
• They have no direct contact with the air inside the leaf. - Carbon dioxide is absorbed by another group of cells, the mesophyll cells, which are in contact with air
• The mesophyll cells contain an enzyme called PEPcarboxylase, which catalyses the combination ofcarbon dioxide from the air with a three-carbon substance called PEP.
• The compound formed from this reaction is oxaloacetate
• Still inside the mesophyll cells, the oxaloacetate is converted to malate, and this is passed on to the bundle sheath cells.
• Now the carbon dioxide is removed from the malate molecules and delivered to RuBP by rubisco in the normal way.
• The light independent reaction then proceeds as usual.

Question 6

Enzymes in C4 plants compared to C3 plants

Answer 6

• Enzymes in C4 plants generally have higher optimum temperatures than those in C3 plants.
• This is an adaptation to growing in hot climates.
• For example, in one study it wasfound that in amaranth, which is a C4 plant, the optimum temperature for the activity of PEP carboxylase is around 45 °C. If the temperature drops to 15 °C, the enzyme loses around 70% of its activity.
• By contrast, the same enzyme in peas, which are C3 plants, was found to have an optimum temperature of around 30 °C and could continue to work at much lower temperatures than in amaranth