Topic 5—A: Photosynthesis And Respiration- 2. Photosynthesis And The Light Dependant Reaction

Question 1

Where does photosynthesis take place?

Answer 1

In the chloroplasts of plant cells

Question 2

Chloroplasts

Answer 2

• Small, flattened organelles surrounded by a double membrane
• Thylakoids (fluid-filled sacs) are stacked up in the chloroplast into structures called grana which are linked together by lamellae (bits of thylakoid membrane)
• Chloroplasts contain photosynthetic pigments (chlorophylls)
• These are coloured substances that absorb certain wavelengths of light and reflect others needed for photosynthesis
• Chlorophyll is found in the thylakoid membrane in funnel shaped structures called photosystems which is a type of protein
• Different chlorophyll absorb different frequencies of light
• Contained within the inner membrane of the chloroplast and surrounding the thylakoids is a gel like substance called the stroma which contains enzymes, sugars and organic acids

Question 3

Leaf structure (adaptations) for photosynthesis

Answer 3

• Large surface area (long)
• Large SA:V ratio- allow exchange of gases efficiently

Question 4

Tissues (leaf)

Answer 4

• Spongy layer (large surface area) provided by air spaces
• Mesophyll (lots of chloroplasts)
• Cuticle- reduces water loss
• Upper epidermis- transparent
• Lower epidermis- stomata open which allows gas exchange

Question 5

What are redox reactions?

Answer 5

• Reactions that involve oxidation and reduction
• They occur in photosynthesis and respiration
• If something is reduced it has gained electrons and may have gained hydrogen or lost oxygen
• If something is oxidised it has lost electrons and may have lost hydrogen or gained oxygen
• Oxidation of one molecule always involves reduction of another molecule

Question 6

What is a coenzyme?

Answer 6

• A molecule that aids the function of an enzyme
• They work by transferring a chemical group from one molecule to another

Question 7

Example of a coenzyme used in photosynthesis?

Answer 7

NADP

Question 8

What does NADP do?

Answer 8

• Transfers hydrogen from one molecule to another
• This means it can reduce (give hydrogen to) or oxidise (take hydrogen from) a molecule

Question 9

The light dependant reaction

Answer 9

• Reactions requires light energy
• Occurs in thylakoid membranes and involves chlorophyll
• Products are ATP and reduced NADP (NADPH)
• Water is required
• Oxygen=waste

Question 10

Light dependant reaction (stages)

Answer 10

1. Light strikes chlorophyll, this excites electrons and is called photoionisation
2. The excited electrons pass along an electron transfer chain which involves proton pumps
3. The electrons provide the energy for protons (H+)to be pumped into the thylakoid space from the stroma
4. An electron carrier takes up the excited electrons and they exit the thylakoid into the stroma
5. The significance of the proton pump is that it builds up a high concentration of protons (H+)Inside the thylakoid which are needed later
6. Light strikes H20 (water) in the thylakoid space and splits it (photolysis). Oxygen diffuses out of the thylakoid (waste). The hydrogen splits into electrons and protons (H+)
7. The addition of H+ from photolysis further increases the concentration of H+ in the thylakoid space
8. The H+ diffuse down their concentration gradient through a channel called ATP synthase. This flow of H+ provides the energy needed to bond ADP + Pi to make ATP which is phosphorylation and it’s linked to chemiosmotic theory
   - The H+ now in the stroma and the e- from stage 4 bond with NADP to make reduced NADP (NADPH)

Question 11

What is conserved after the photoionisation stage in the production of ATP and reduced NADP?

Answer 11

Some of the energy from electrons