3.5.1 Photosynthesis

Question 1

What adaptations does the leaf have for photosynthesis?

Answer 1

1. large surface area - for maximum sunlight absorption
2. thin - short diffusion distance
3. upper mesophyll is packed with chloroplasts - ensure as much light as possible is collected
4. numerous stomata - means all mesophyll cells are only a short distance away from an opening

Question 2

What adaptations do the chloroplasts have for photosynthesis?

Answer 2

• membranes of grana - create a large surface area to increase the number of light dependent reactions that can occur
• lots of chlorophyll - maximum light absorption
• DNA loop in stroma - codes for chloroplast proteins - produced in 70s ribosomes
• starch grains - store sugars produced

Question 3

Where does the light dependent reaction occur?

Answer 3

in the grana (stacks of thylakoids which contain chlorophyll)

Question 4

Where does the light independent reaction occur?

Answer 4

in the stroma (fluid filled matrix which contains enzymes and sugars) - carbohydrates are also stored as starch grains here

Question 5

Define light dependent reaction

Answer 5

First stage of photosynthesis - light energy and water are used to create ATP and reduced NADP which are needed for the light independent stage

Question 6

What happens during the first stage of the light dependent reaction?

Answer 6

PHOTOIONISATION
1. light energy is absorbed by the chlorophyll -** electrons become excited** and rise up an energy level, leaving the chlorophyll
2. electrons move down the** electron transport chain**

Question 7

What happens during the third stage of the light dependent reaction?

Answer 7

CHEMIOSMOSIS
1. electrons move down the** electron transport chain** (proteins embedded in the thylakoid membrane) - electron carriers are reduced as they gain an electron and oxidised as they lose an electron
2. energy released from electrons is used to pump protons across the thylakoid membranes
3. electrochemical gradient is created - protons pass through** ATP synthase** (facilitated diffusion)- results in the production of ATP

Question 8

What happens during the second stage of the light dependent reaction

Answer 8

PHOTOLYSIS
1. as electrons leave the chlorophyll they must be replaced
2. light energy splits water into protons (picked up by NADP to form reduced NADP and used in LIR), electrons (passed along electron carrier proteins) and oxygen (respiration or diffuses out of the leaf)

Question 9

What happens during the fourth stage of the light dependent reaction?

Answer 9

PRODUCTION OF ATP AND REDUCED NADP
1. protons are taken up by NADP - becomes reduced NADP - used in the light independent reaction

Question 10

How are the chloroplasts adapted for the light dependent reaction?

Answer 10

1. thylakoid membrances provide a large surface area
2. network of proteins hold the chlorophyll for max light absorption
3. granal membranes are selectively permeable - allows proton gradient

Question 11

Define light independent reaction (calvin cycle)

Answer 11

Second stage of photosynthesis - uses carbon dioxide, reduced NADP and ATP to form a hexose sugar

Question 12

What is the role of ATP in the light independent reaction?

Answer 12

hydrolysed to release energy for the reaction

Question 13

What is the role of reduced NADP in the light independent reaction?

Answer 13

donates electrons to reduce the glycerate - 3 - phosphate

Question 14

What is the role of RubisCo in the light independent reaction?

Answer 14

catalyses the fixation of carbon dioxide - which is temperature sensitive as it involves enzymes

Question 15

Describe the main stages of the light independent reaction?

Answer 15

1. carbon dioxide reacts with ribulose biphosphate (RuBP) to form two molecules of glycerate-3-phosphate (GP) - this is catalysed by** Rubisco**
2. GP is reduced to triose phosphate using energy from** ATP and accepting a H from reduced NADP**
3. NADP is reformed and goes back to the LDR to be reduced again
4. one sixth of triose phosphate molecules used to produce useful organic molecules - converted into: hexose phosphates ( produce sucrose, cellulose, starch) converted to glycerol to make lipids, used in amino acid production
5. most triose phosphate is used to regenerate ribulose biphosphate - needs ATP

Question 16

How does light intensity impact rate of photosynthesis?

Answer 16

• wavelength - only certain wavelengths are used in photosynthesis - chlorophyll only absorbs red and blue while green is reflected
• intensity - higher intensity light provides more energy so the light dependent stage is faster - this produces more ATP and reduced NADP for the independent stage - rate of photosynthesis increases until another factor becomes limiting

Question 17

How does temperature impact rate of photosynthesis?

Answer 17

• increased temp increases rate of photosynthesis as it is controlled by enzymes - light independent stage is impacted by temp as Rubisco is needed
  (light dependent reaction just needed sufficient light)
• if temp gets too high enzymes denature and rate of reaction decreases

BUT - increased temp also causes stomata to close to reduce water loss - less CO2 enters - balance needed
- light dependent reaction depends on a proton gradient - high/low temp could impact permeability of membranes

Question 18

How does carbon dioxide concentration impact race of photosynthesis?

Answer 18

• increased concentration increases rate of photosynthesis - as carbon dioxide is needed for the light independent stage of photosynthesis (Carbon dioxide is combined with RuBP) - the faster this rate occurs, the faster the overall rate of photosynthesis

Question 19

H

What can farmers do to overcome the effects of limiting factors?

Answer 19

• artificial lighting to increase light intensity - also green houses
• heaters to increase temperature
• burning fuel such as parafin to release carbon dioxide