17.3 - Photosynthesis

Question 1

What is the internal structure of chlorplast?

Answer 1

Network of membranes (with high SA to maximise light absorption). They form flattened sacs called thylakoids.

A stack of thylakoids is called a granum (plural grana), membranous channels called lamellae link grana. Fluid within the chloroplasts in the outside spaces from thylakoids.

THYLAKOIDS – has electron transport chains and ATP synthase (for phosphorylation). Contains chlorophyll. 10-20 thylakoids per grana.

GRANA – flat sacs increase SA:V ratio

STROMA – present outside thylakoid space, contains: Chloroplast DNA; Chloroplast ribosomes; Thylakoid system and proteins

Has enzymes & suitable pH for Calvin Cycle (light dependant reaction)

OUTER MEMBRANE – semi-permeable to small molecules and ions only, not larger proteins.

INNER MEMBRANE – Site of fatty acid, lipid and carotenoid synthesis. Specialised with transport proteins

Question 2

Describe the practical: Investigating Photosynthetic Pigments

Answer 2

• Chromatography is used to separate the pigments in the plant extract
• Mobile Phase: Solution containing pigment mixture
• Stationary Phase: Silica gel, TLC paper
• Pigment solubility affects interactions with stationary phase affecting R_f value

Question 3

What is a photosystem?

Answer 3

Complex proteins involved in the absorption of light and in electron/energy transfer in photosynthesis

Question 4

What are photosynthetic pigments?

Answer 4

• Photosynthetic pigments are organic molecules that absorb light and convert into an energy form used in photosynthesis
• Ranges of photsynthetic pigments absorb different wavelengths of light
• Chlorophyll a is a 1^o pigment, it reflects green so it appears green

Question 5

What is chlorophyll’s role in the reaction centre?

Answer 5

• Chlorophyll b, xanthophylls & carotenoids are embedded in thylakoid membranes. (accessory pigments)
• There is an antennae complex which absorbs and harvests light of different wavelengths to transfer energy to the reaction centre.
• Chlorophyll a is in the reaction centre (it is the site of photosynthesis).
• The light harvesting system and reaction centre is known as a photosystem

Question 6

Describe the process of cyclic phosphorylation.

Answer 6

It is cyclic photophosphorylation because, in PSI, the energy from excited electrons is used to generate ATP, the electron passes back to the same PSI

1. Excited electron (by light) travels down ETC and releases energy
2. Pumps protons across the thylakoid membrane into the thylakoid space
3. Protons diffuse back through ATP synthase, this provides energy to attach ADP to Pi
4. The electron then returns to PSI to replace the lost electron
   
   1. Instead of being used to form NADPH

Question 7

Describe the process of non-cyclic phosphorylation.

Answer 7

• 2 photosystems are involved: Photosystem I (PSI) and Photosystem II (PSII)
  
  • PSI: absorbs higher wavelengths of light (700nm).
  • PSII: absorbs lower wavelengths (680nm)
    
    • Absorbed light excites electrons at the centre of the photosystems
    • The excited electrons are released from the photosystems and passed to electron transfer chains for chemiosmosis.
• Instead of returning to PSI the electron from PSI can combine with a proton (from photolysis) and NADP⁺from the stroma(at the site NADP reductase) to makeNADPH
  
  • ​NB - electron is now lost from PSI

1. An electron is also excited at PSII (P680)
2. This electron travels via an electron carrier to the ETC towards PSI (replacing the lost electron at PSI)
   
   1. ATP is also produced by chemiosmosis
3. PSII is unbalanced as it has lost an electron
   
   1. ​The electron is replaced from photolysis
4. The products of non-cyclic photophosphorylation: ATP and NADPH are used in the Calvin Cycle

Question 8

Describe the process of photolysis.

Answer 8

• Splitting of light
• A water-splitting enzyme is used to split 2 water molecules into 4 protons, 4 electrons and 1 oxygen molecule (a by-product)
• Photolysis occurs in association with PSII

Question 9

Describe the outline of the light independant stage of photosynthesis.

Answer 9

• Takes place in stroma of chlorolpast, uses CO₂, ATP & NADHP (latter 2 are products of light dependent stage)
• Organic molecules are produced, e.g. glucose, in a series of reactions called the Calvin Cycle

Question 10

Describe Stage 1 of the Calvin Cycle

Answer 10

1. CARBOXYLATION: CO₂ enters the stroma of chloroplasts, combines with 5-carbon RuBP (ribulose biphosphate), they will combine eventually forming an organic molecule.
2. (enzyme ribulose biphosphate carboxylase) catalyses the reaction and an unstable 6-carbon intermediate forms.

RuBisCO is an inefficient enzyme, it is competitively inhibited by oxygen so a lot is needed for photosynthesis.

Question 11

Describe Stage 2 of the Calvin Cycle

Answer 11

2. REDUCTION: Unstable 6-carbon intermediate immediately breaks down to form 2, 3-carbon molecules, GP/PGA (glycerate 3-phosphate).

Each GP is converted to another TP (triose phosphate), using the hydrogen atom from NADPH and ATP energy.

TP is recycled to regenerate RuBP.

It is also used in the synthesis of complex biological molecules (carbohydrates, lipids, proteins, nucleic acids)

Question 12

Describe Stage 3 of the Calvin Cycle

Answer 12

3. REGENERATION:

• To produce 1 glucose, 6 carbons are needed, so the cycle occurs 6 times
• This produces 12 TP molecules (as 2 are produced in a single cycle)
  
  • 2 TP molecules (6 carbons) are taken to make 1 glucose leaving 10 TP
• 10 TP molecules have 30 carbons, regenerating 6 RuBP molecules