Section 5: Photosynthesis

Question 1

Where does the Light dependent reaction occur in?

Answer 1

• On the thylakoid membrane in chloroplast

Question 2

Where does the light independent reaction occur in?

Answer 2

• Stroma in the chloroplast

Question 3

What is photoionisation

Answer 3

• Chlorophyll absorbs light energy which excites electrons to a higher energy level, releasing them from chlorophyll

Question 4

How is ATP produced in the light dependent reaction?

Answer 4

• Chlorophyll absorbs light energy which excites electrons to a higher energy level
• Electrons pass down electron transfer chain from photosystem 2 to photosystem 1 via redox reactions, losing energy
• The energy is used to actively transported protons from stroma into the thylakoid
• This creates a proton gradient across thylakoid membrane
• Protons move by facilitated diffusion down the electrochemical gradient into the stroma via ATP synthase
• Energy converts ADP + Pi to form ATP

Question 5

How is reduced NADP formed in the light dependent reaction

Answer 5

• The photosystem 1 electrons are excited and transferred to NADP with a proton from photolysis to reduce NADP

Question 6

What is photolysis?

Answer 6

• Splitting of water using light produces protons, electrons and oxygen

Question 7

What are the products of the light dependent reaction and where do they go?

Answer 7

• ATP - light independent reaction
• Reduced NADP - light independent reaction
• Oxygen - Leaves cell as a by-product or used in respiration

Question 8

What are the steps in the light independent reaction?

Answer 8

• CO2 reacts with RuBP catalysed by enzyme rubisco - 6C
• Produces 2 molecules of glycerate 3 -phosphate (GP) - 3C
• GP reduced to triose phosphate (TP) using energy from 1x ATP and 1x NADPH
• Some TP converted into useful organic substances - 5/6 used to regenerate RuBP uses 1 ATP

Question 9

What is a limiting factor?

Answer 9

• A factor is limiting when it is made a more favourable value, the rate of photosynthesis increases unless it is limited by another factor

Question 10

What are some limiting factors

Answer 10

• Temperature
• Light Intensity
• CO2 concentration

Question 11

What happens when you increase temperature up to optimum - rubisco enzyme

Answer 11

• More kinetic energy
• More enzyme-substrate complexes

Question 12

What happens if you increase temperature above optimum

Answer 12

• H bonds in the tertiary structure break - active site changes shape - enzyme denatured
• Fewer E-S complexes

Question 13

What would happen if light intensity was dramatically reduced?

Answer 13

• Levels of ATP and reduced NADP would fall because
  
  • light dependent reaction limited as less photo ionisation of chlorophyll and less photolysis
• So light dependent reaction would also stop
  
  • GP can not be reduced to TP
  • TP can not regenerate RuBP

Question 14

What would happen if carbon dioxide concentration dramatically decreases?

Answer 14

• Limits light independent reaction
• Less CO2 to combine with RuBP to form GP
• Less GP reduced to TP
• Less TP and GP converted to organic substances e.g hexose and to regenerate RuBP

Question 15

If limiting factors are minimal how will the rate of photosynthesis be increased?

Answer 15

• Faster production of glucose allowing faster respiration
• More ATP to provide energy for growth
• Higher yield so more profit

Question 16

What are some simple ways to increase photosynthesis rate?

Answer 16

• Artificial lighting - to maximise light intensity
• Heating a greenhouse - increase temperature
• Burning fuel such as paraffin burners - to release CO2