5 Energy transfer: 11 Photosynthesis

Question 1

What is the process of the light-dependent reaction?

(The chemiosmotic theory).

Answer 1

1. Chlorophyll absorbs light energy so electrons get excited and leave (photoionisation). (Chlorophyll gets oxidised).
2. The electrons are taken up by an electron carrier (which is therefore reduced).
3. A transfer chain is formed as the electrons are passed along electron carriers in a series of oxidation-reduction reactions.
4. Electrons lose energy at each stage because each new carrier is at a slightly lower energy level than the previous one.
5. Some of the lost energy is used to make ATP by combining inorganic phosphate and ADP and to reduce NADP.
6. Proton pumps in the thylakoid membrane actively transports H+ ions into the thylakoid from the stroma, using energy from the photolysis of water.
7. In the thylakoid, photolysis of water produces more H+ ions (and electrons to replace electrons lost from chlorophyll).
8. Therefore, there’s a high concentration of H+ ions in the thylakoid and a low concentration in the stroma, creating a concentration gradient.
9. H+ ions move through ATP synthase channel proteins in the thylakoid membrane by facilitated diffusion down the concentration gradient from the thylakoid into the stroma.
10. The H+ ions change the structure of these ATP synthase channel proteins, causing them to catalyse ADP + Pi -> ATP.
11. Electrons from the photoionisation of chlorophyll reduce NADP.

Question 2

What does the photolysis of water produce?

Write the equation as well.

Answer 2

Protons, electrons, and oxygen.
2H20 -> 4H+ + 4e- + O2

Question 3

What does the light-independent reaction use reduced NADP and ATP for?

Answer 3

To form simple sugars.
The hydrolysis of ATP provides the additional energy for this reaction.

Question 4

What is the process of the light-independent reaction?

The Calvin Cycle.

Answer 4

1. Atmospheric CO2 diffuses into the leaf through stomata and dissolves in water around the mesophyll cells. Then it diffuses through the cell-surface membrane, cytoplasm, and chloroplast membranes into the stroma.
2. In the stroma, the CO2 reacts with ribulose biphosphate (RuBP), catalysed by ribulose biphosphate carboxylase (rubisco).
3. This reaction produces two molecules of glycerate 3-phosphate (GP).
4. Reduced NADP from the light-dependent reaction is used to reduce GP to triose phosphate, using energy from ATP from the light-dependent reaction.
5. The NADP is re-formed and goes back to the light-dependent reaction to be reduced again.
6. Some triose phosphate molecules are converted to organic substances like glucose.
7. Most triose phosphate molecules are used to regenerate RuBP, using ATP from the light-dependent reaction.

Question 5

What are the environmental factors that limit photosynthesis?

Answer 5

Light intensity, temperature, CO2 concentration.

Question 6

How does light intensity affect photosynthesis?

Answer 6

Increasing light intensity increases the rate of photosynthesis until another factor becomes limiting.

Question 7

How does temperature affect photosynthesis?

Answer 7

As temperature increases, the rate of photosynthesis increases until another factor becomes limiting.

Question 8

How does CO2 concentration affect photosynthesis?

Answer 8

Increasing CO2 concentration increases the rate of photosynthesis until another factor becomes limiting.

Question 9

What are the agricultural practices used to overcome the effect of limiting factors on photosynthesis?

In a glasshouse.

Answer 9

CO2 concentration - CO2 is added to the air by e.g. burning propane in a CO2 generator.
Light intensity - transparent glass allow light through and lamps provide light at night-time.
Temperature - glasshouses trap heat energy from the sun, which warms the air. Heaters and cooling systems can be used to maintain a constant optimum temperature, and air circulation systems make sure the temperature is even throughout.