Photosynthesis

Question 1

What is the structure of a chloroplast ?

Answer 1

Contains ribosomes
Contains DNA
Contains starch grains
Contains granum, which are stacks of membranes
One individual stack of membrane is a thylakoid
Between grana- intergranel lamallae

Question 2

What is the site of the light dependent and independent stage ?

Answer 2

Light dependent- thylakoid membrane
Light independent- stroma

Question 3

What do thylakoids contain ?

Answer 3

Photosystems I and II- which contain different pigments
Primary pigment- chlorophyll a (at the reaction centre)
Accessory pigment- chlorophyll b, xanthophyll, carotenoids (form the antannae complex)

Question 4

What is photosynthesis and in what type of organisms does it occur?

Answer 4

Autotrophic organisms such as cyanobacteria and plants
Light energy- chemical energy
Inorganic compounds- organic compounds

Question 5

What are the adaptations of chloroplasts to carry out photosynthesis ?

Answer 5

Stroma- contains enzymes that can catalyse the reactions of the light independent stage
Makes the transport of products from the light dependent stage rapid

Grana- large surface area for the presence of many photosystems which allows for the maximum absorption of light
More membrane space for ATP synthase enzymes and electron carrier chains

DNA- contains genes coding for enzymes and proteins used in photosynthesis

Ribosomes (70s)- translation of proteins coded for by DNA

Inner membrane- contains selective transport proteins

Question 6

What is chemiosmosis?

Answer 6

The diffusion of protons from an area of high proton concentration to an area of low proton concentration, through a partially permeable membrane
This releases energy to drive ADP+ Pi= ATP

Question 7

How is a proton gradient established?

Answer 7

Electrons in pigment molecules are excited by absorbing light energy from the sun
They pass into an electron transport chain, from one electron carrier to the next in a series of redox reactions
When passing from electron carriers, energy is released
This is used to pump protons across a membrane by active transport, creating a gradient

Question 8

How do protons move back through the membrane?

Answer 8

Through an enzyme known as ATP synthase (catalyses the formation of ATP by lowering the activation energy)
The flow of protons through this channel provides the energy needed to synthesise ATP
Facilliated diffusion as ATP synthase provides a hydrophillic channel for the protons to move through

Question 9

Describe non- cyclic photophosphorylation

Answer 9

Chlorophyll molecules absorb light energy through photosystems II which excites 2 electrons to a higher energy level
The 2 electrons are accepted by a primary electron acceptor and are passed down the ETC, which generates energy for the production of ATP
Light energy excites 2 electrons from PSI and these are passed through another ETC, producing ATP by chemiosmosis
The 2 electrons that are lost from PSI are replaced by the 2 electrons lost by PSII
The two electrons lost by PSI are accepted, along with 2 hydrogen ions, by NADP
NADP– reduced NADP

Question 10

Describe photolysis

Answer 10

Photolysis occurs to replace the 2 electrons lost by PSII
Splitting up of a water molecule into– 2H+, 2e- and 1/2 O2
Uses energy from the sun
Oxygen gas is released as a by product

Question 11

Describe cyclic photophosphorylation

Answer 11

The electrons leaving PSI can be returned and go back down ETC
No reduced NADP formed
Only PSI involved
Generates less energy
Takes place in the intergranal lamallae

Question 12

What is required for the light independent stage?

Answer 12

The two products of the light dependent stage- reduced NADP and ATP

Question 13

Describe the Calvin cycle

Answer 13

Carbon dioxide enters the leaves by diffusion where it combines with a 5 carbon molecule called RUBP
The carbon dioxide is fixed
This reaction is catalysed by RuBisCO
An unstable 6 carbon intermediate is formed and is immediately broken down in 2 3 carbon GP molecules
GP is reduced into TP (triode phosphate)
This requires a H atom from the reduced NADP and ATP from the light dependent stage of photosynthesis
The majority of TP is recycled back to form RuBP
It can also be used to form carbohydrates, amino acids and lipids

Question 14

What are the benefits of cyclic photophosphorylation

Answer 14

Can occur at lower temperatures as there are no enzymes involved such as NADP reductase so optimal conditions do not have to be maintained
Uses less ATP so more ATP can be used on other processes such as active transport/ cell division and growth
Electrons not needed from PSII
No reduced NADP produced but ATP still produced
Some energy can still be produced when water or light are limiting factors

Question 15

What is the effect of reducing light intensity on the calvin cycle?

Answer 15

Will reduce the rate of the light dependent stage of photosynthesis
Less reduced NADP and ATP produced- which are needed to convert GP to TP
Increase in conc. of GP
Decrease in conc. of TP
Less TP to regenerate RUBP so the conc. of RUBP also decreases

Question 16

What is the effect of reducing carbon dioxide on the calvin cycle?

Answer 16

Increase conc of RUBP as there is less carbon dioxide to be fixed
Reduced conc of GP and TP

Question 17

What is the effect of temperature on the Calvin cycle

Answer 17

Temperature affects the enzyme controlled reactions- eg carbon fixation and RuBisCo
Increase- rate of enzyme activity increases until the point where enzymes denature
Stomata on the leaves close to prevent water loss by transpiration- so Co2 will not be able to enter- reduces rate of Calvin cycle so eventually photosynthesis stops

Lower- enzymes and substrates have less KE, less successful collisions and so reduced ROR, reduced conc of GP and TP