Photosynthesis

Question 1

Structure of chloroplasts.

Answer 1

Flattened organelles with a double membrane.

Thylakoids are stacked up into structures called grana which are linked together by lamellae.

Contain photosynthetic pigments: chlorophyll a, b and carotene. These absorb light energy needed for PTS. Found in thylakoid membranes attached to proteins. Protein and pigment is called a photosystem.

There are 2 photosystems.

Stroma surrounds the thylakoids. It contains enzymes, sugars and organic acids.

Question 2

Light-Dependent Reaction

Answer 2

1. Light energy excites e- in the chlorophyll. E- move to a higher energy level. They are released from the chlorophyll and move down the ETC to PSI.
2. As e- move down the ETC, they are replaced. Light energy splits water into protons, e- and oxygen.
3. Excited e- lose energy as they move down the ETC. Energy is used to transport protons into thylakoid, forming a proton gradient across membrane. Protons move down conc gradient through ATP synthase, combinig ADP with Pi.
4. Light energy absorbed by PSI. Excites e- again to a higher energy level. E- transferred to NADP, along with a proton from the stroma, to form reduced NADP.

Question 3

Where does the Calvin Cycle take place?

Answer 3

Stroma of the chloroplasts.

Question 4

What is the product of the Calvin Cycle?

Answer 4

Triose phosphate.

Question 5

Calvin Cycle

Answer 5

1. Carbon dioxide enters the leaf through the stomata and diffuses into the stroma of the chloroplast. It’s combined with RuBP, this reactin is catalysed with the enzyme rubisco. This gives an unstable 6-carbon compound which breaks down into two molecules of glycerate 3-phosphate (GP).
2. Hydrolysis of ATP provides energy to convert GP into a triose phosphate (TP). This reaction also requires H+ ions which come from reduced NADP, which is recycled to NADP. Some TP is converted into useful organic compounds and some continues in the Calvin cycle to regenerate RuBP.
3. 5 out of every 6 molecules of TP produced in the cycle aren’t used to make hexose sugars, but to regenerate RuBP. Regenerating RuBP uses the rest of ATP produced by the light-dependent reaction.

Question 6

What are triose phosphate (TP) and glycerate 3-phosphate (GP) converted to?

Answer 6

• Carbohydrates - hexose sugars (e.g. glucose) are made by joining 2 triose phosphate molecules together.
• Lipids - made using glycerol, which is synthesised from TP, and fatty acids, which are synthesised from GP.
• Amino acids - some amino acids are made from GP.

Question 7

How many times does the Calvin cycle need to turn to make one hexose sugar?

Answer 7

6 times.

3 turns produces 6 molecules of TP, because 2 molecules of TP are made for every one carbon dioxide molecule used.

5 out of 6 TP molecules are used to regenerate RuBP.

This means that for every 3 turns of the cycle, only 1 TP is produced that’s used to make a hexose sugar.

6 turns of the cycle need 18 ATP and 12 reduced NADP from the light-dependent reaction.