11: Photosynthesis

Question 1

Stages of photosynthesis

Answer 1

• light dependent reaction
• light independent reaction

Question 2

Where does the light dependent reaction occur?

Answer 2

In the thylakoid membranes (granum). These are folded membranes containing photosynthetic proteins and electron carrier proteins are embedded within the membranes

Question 3

Where does the light independent reaction occur?

Answer 3

In the stroma. The fluid centre of the chloroplast which contains enzymes for LIR.

Question 4

The main aim of the LDR

Answer 4

light energy and water is used to create ATP and reduced NADP needed for LIR

Question 5

Four key stages of LDR:

Answer 5

1) Photolysis
2) Photoionisation of chlorophyll
3) Chemiosmosis
4) Production of ATP and reduced NADP

Question 6

Stage 1 of LDR

Answer 6

• Photolysis, light energy is absorbed which splits water into hydrogen (protons), electrons and oxygen.
• the protons are picked up by NADP to produce NADPH.
• the electrons are passed along a chain of electron carrier proteins.
• the oxygen is used for respiration or is diffused out the stomata

Question 7

Stage 2 of LDR

Answer 7

• Photoionisation of chlorophyll.
• light energy is absorbed by the chlorophyll and the energy results in electrons becoming excited and raising up an energy level to leave the chlorophyll.
• the chlorophyll becomes oxidised, the electron carrier has become reduced

Question 8

Stage 3 and 4 of LDR

Answer 8

• Chemiosmosis
• The electrons that gained energy and left chlorophyll move along a series of proteins embedded within the thylakoid membrane
• As they move along they release energy which is used to pump the protons across chloroplast membranes, from the stroma to the thylakoid lumen through the thylakoid membrane.
• An electrochemical gradient is created
• The gradient enables the protons to move by faciliated diffusion, back down the conc grad, back into the stroma
• The protons pass through the enzyme ATP synthase channel proteins (only protein they can attach to), which phosphorylates the ADP into ATP.
• then the protons are taken up by the electron carrier/co-enzyme NADP, to become reduced NADP.

Question 9

How is the chloroplast structured for the LDR

Answer 9

• thylakoid membranes large sa for attachment of chlorophyll, electron carriers and enzymes for ldr
• network of proteins in grana to hold chlorophyll in precise manner for max absorption
• granal membanes selectively permeable
• chloroplasts contain dna and ribosomes to quickly manufacturer proteins for ldr

Question 10

Key role and stages of LIR

Answer 10

• co2, ATP and reduced NADP used to reduce glycerate 3-phosphate. no light required, its a calvin cycle
• it occurs in the stroma, which contains the enzyme RuBisCO which catalyses reaction
• this stage is temperature sensitive (enzymes)

Question 11

Calvin cycle

Answer 11

1) Ribulose biphosphate (5 carbon) and CO2 combines which is catalysed by rubisco
2) this forms 2x 3 carbon compound glycerate 3-phosphate
2) reduced NADP is used to reduce glycerate 3-phosphate to triose phosphate (TP) using energy supplied by ATP and accepting the H from reduced NADP
3) 1 of the 6 carbons is removed to create a hexose sugar (6 carbons needed) so cycle goes 6 times
4) glucose produced can turn into disaccharide (sucrose) or polysaccharide (cellulose/starch). also can be converted to glycerol to combine with fatty acids to make lipids for plants

Question 12

Structure of leaf adaptions for photosynthesis:

Answer 12

• large sa absorb sunlight
• leaves arranged to not overlap
• thin, short diffusion pathway
• transparent cuticle and epidermis to let light through to mesophyll cells
• stomata