3.5.1 Photosynthesis

Question 1

4 features of oxidation

Answer 1

• loss of electrons
• addition of oxygen
• loss of hydrogen atom
• releases energy

Question 2

4 features of reduction

Answer 2

• gain of electrons
• removal of oxygen
• gain of hydrogen atom
• takes in energy

Question 3

What are coenzymes?

Answer 3

• organic molecules that bind temporarily to the enzyme
• they transfer a chemical group needed for the reaction and ‘help’ the enzyme
• eg NAD, NADP, FAD, co-enzyme A, vitamin c

Question 4

Describe photosystems

Answer 4

• embedded in thylakoid membranes
• contain light absorbing pigments (eg chlorophylls, carotenoids, xynthophylls)
• 2 photosystems (II before I). Both have chlorophyll at their centers

Question 5

Where does the light dependent stage of photosynthesis occur?
Where does the light independent stage occur?

Answer 5

• Thylakoid membranes of chloroplasts
• stroma of chloroplasts

Question 6

Overview of light dependent reaction

Answer 6

• Uses light energy
• to produce ATP, reduced NADP and O2

Question 7

Overview of light independent reaction

Answer 7

• uses ATP and reduced NADP from LDR to produce glucose

Question 8

Full answer: light independent stage of photosynthesis

Answer 8

• light independent reaction/Calvin cycle occurs in the stroma of chloroplasts
• carbon dioxide combines with RuBP (ribulose bisphosphate)
• produces 2 molecules of GP (glycerate 3-phosphate) using Rubisco enzyme
• reduced to triose phosphate
• using reduced NADP
• using energy from ATP
• triose phosphate converted to other organic substance glucose / RuBP
• RuBP reformed using energy and pi from ATP

Question 9

Full answer: light dependent stage of photosynthesis

Answer 9

• occurs in thylakoid membrane of chloroplasts
• chlorophyll absorbs light energy
• excites a pair of electrons from chlorophyll, chlorophyll is photoionsed, chlorophyll is oxidised
• the electrons LOST from chlorophyll move along electron transport chain in a series of redox reactions and energy they release is used to pump protons into thylakoid lumen
• as protons move back into stroma down electrochemical gradient, energy released is used to join ADP and Pi to form ATP by ATP synthase (chemiosmosis)
• photolysis of water produces protons, electrons and oxygen
• NADP reduced by electrons/hydrogen atom

Question 10

Limiting factor graphs: light intensity

Answer 10

• as light intensity increased, rate of photosynthesis increased
• because light is a limiting factor its needed in the light dependent reaction to excite a pair of electrons from chlorophyll
• until … when, as light intensity increases further there is no further increase in rate of photosynthesis- plateau
• because light is no longer limiting- saturation point
• another factor is limiting eg carbon dioxide concentration, temperature

Question 11

Limiting factor graphs: carbon dioxide concentration

Answer 11

• as carbon dioxide concentration increases, rate of photosynthesis increases
• because CO2 is a limiting factor its needed in the light independent reaction to combine with RuBP using Rubisco to eventually produce glucose
• until … when, as CO2 increases further there is no further increase in rate of photosynthesis- plateau
• because CO2 is no longer limiting- saturation point
• another factor is limiting eg temperature, light intensity

Question 12

Limiting factor graphs: temperature

Answer 12

• as light intensity increases, rate of photosynthesis increases
• because light is a limiting factor its needed in more kinetic energy, more collisions, more enzyme substrate-complexes
• until peak at … when, as temp increases further rate of photosynthesis decreases
• because enzymes used in light independent reaction are denatured

Question 13

What is the compensation point?

Answer 13

• rate of photosynthesis = rate of respiration
• no net gas exchange

Question 14

What is the difference between primary and accessory pigments?

Answer 14

• primary pigments act as reaction centres/where electrons are excited
• accessory pigments surround the reaction centre
• transfer energy to primary pigments

Question 15

What is photophosphorylation?

Answer 15

Production of ATP from ADP and Pi

Question 16

Why is high temperature but low light intensity disadvantageous?

Answer 16

• increased temperature increases rate of respiration
• rate of photosynthesis is too low to replace respiratory losses