11. Photosynthesis

Question 1

What is the equation for photosynthesis?

Answer 1

6CO2+6H2O → C6H12O6 + 6O2

Question 2

Give some leaf adaptations for photosynthesis (7)

Answer 2

• large SA
• minimal overlapping/shadowing
• thin
• transparent cuticle and epidermis
• air spaces in lower mesophyll
• xylem and phloem network
• numerous stomata

Question 3

Where are pigments found?

Answer 3

In clusters called photosystems in the thylakoid membrane

Question 4

What are the 2 main groups of pigments in flowering plants?

Answer 4

Chlorophylls and carotenoids

Question 5

What is an absorption and an action spectrum?

Answer 5

Absorption - a graph showing the absorption of different wavelengths of light by different pigments
Action - a graph showing the effectiveness of different wavelengths of light on the rate of photosynthesis

Question 6

Where does the light-dependent reaction take place?

Answer 6

In the thylakoid membrane of the chloroplasts

Question 7

How are chloroplasts adapted for carrying out the LDR?

Answer 7

• thylakoid membranes provide a large SA for the attachment of chlorophyll, electron carriers and enzymes
• chloroplasts contain both DNA and ribosomes so proteins for the LDR can be quickly and easily produced

Question 8

Summarise the light-dependent reaction

Answer 8

• light strikes a photosystem, embedded in the thylakoid membrane
• energy is absorbed by chlorophyll in the reaction centre
• ‘excites’ two electrons, which are emitted from the chlorophyll molecule in photoionisation
• triggers photolysis - splitting of water into hydrogen, oxygen and electrons (which replace the 2 lost previously)
• high energy electrons pass along the ETC through a series of redox reactions, releasing energy as they pass along
• energy is used to power the electron carriers to double up as proton pumps, so hydrogen ions can diffuse against their conc. gradient into the thylakoid lumen, setting up an electrochemical gradient
• hydrogen ions then have to use ATP synthase to diffuse back down their electrochemical gradient, and the free energy from this is used for the phosphorylation of ADP to form ATP
• the 2 electrons from the ETC is transferred to NADP+ to form reduced NADP
• oxygen is released as a by-product from photolysis and will either be used in respiration or will diffuse out of the leaf as a waste product

Question 9

Where does the light independent reaction take place?

Answer 9

The stroma of the chloroplasts

Question 10

How is the stroma adapted for the LIR?

Answer 10

• contains all enzymes needed
• surrounds grana so products from the LDR can easily diffuse in
• contains DNA and ribosomes so proteins needed can be manufactured easily

Question 11

Summarise the light-independent reaction/calvin cycle

Answer 11

• CO2 diffuses into the leaf/stroma from the atmosphere
• CO2 reacts with ribulose bisphosphate, a reaction catalysed by enzyme rubisco
• this produces 2 molecules of glycerate-3-phosphate (GP)
• reduced NADP from the LDR is used to reduce GP into triose phosphate (TP), using energy supplied by ATP
• NADP is therefore reformed and can go back to the LDR
• some TP molecules are converted into organic molecules that the plant requires, however most TP is used to regenerate RuBP, using ATP from the LDR