5.2- Photosynthesis

Label the structure of a chloroplast

Where does the light dependent stage occur?

Thylakoids in chloroplasts

Where does the light independent stage occur?

Stroma of chloroplasts

How does a plant absorb light?

By chlorophylls (a & b) & carotenoids

Chlorophyll a is a primary photosynthetic pigment while the others are accessory pigments

What is the absorption spectrum?

It shows how much light a particular pigment absorbs at each wavelength

Chlorophyll: Absorbs light in blue-violet & red parts

Carotenoids: Absorbs light from the blue-violet parts

Combining the use of these pigments increases the range over which light is effectively absorbed

What is the action spectrum?

Shows the rate of photosynthesis at different wavelengths

Compare an absorption spectrum with an action spectrum.

The shape of the action spectrum is similar to the absorption spectrum combining photosynthetic pigments

Eg: absorption spectrum shows little green light is absorbed so therefore it’s relatively ineffective in photosynthesis

Outline the main stages of photosynthesis

1. Light harvesting: Photosystems capture light so its energy can raise energy levels of electrons

2. Light dependent reactions: Energised electrons used to produce energy rich compounds light ATP & NADPH

3. Light independent reactions: ATP & NADPH used to reduce CO2 into organic molecules

Explain the process of light harvesting

• Pigments are arranged in photosystems on the thylakoid membranes

• Each photosystem consists of 200/300 accessory pigment molecules surrounding chlorophyll in the reaction centre

• Photosystem absorbs light over a range of wavelengths

• Light energy absorbed by an accessory pigment creates an excitation energy that’s passed along a chain of pigment molecules to the reaction centre- Resonance transfer

What are the two photosystems?

PSII & PSI

PSII is also referred to as P680 bc it has a chlorophyll a molecule in the reaction centre with an absorption peak of 680nm

PSI is also referred to as P&00 bc it has a chlorophyll a molecule in the reaction centre with an absorption peak of 700nm

Describe the light dependent reactions

1. The light energy excites electrons in chlorophyll a raising them to a higher energy level- photoactivation

2. Photoactivation of PSI results in the passage of electrons from PSII to PSI (the Z-scheme) along a series of cytochromes in a sequence of lower energy levels.

3. This electron transfer is coupled with the production of ATP (photophosphorylation) at points where enough energy is released to combine inorganic phosphate with ADP

4. Loss of electrons left chlorophyll (PSII) positively charged so replacement electrons are provided from photolysis

5. O2 is a waste products & the H+ is used to reduce NADP (final acceptor of electrons) forming NADPH

What are the most important consequences of this electron transfer?

• Synthesis of ATP via photophosphorylation & production of NADPH bc both are used in metabolic reactions to synthesise simple carb

• Release of O” as waste & its diffusion out of the chloroplast

Describe the light independent reactions

4 Stages of the Calvin cycle:

1. Fixation of CO2: CO2 diffuses through stroma & combines with ribulose bisphosphate (C5). Catalysed by rubisco. Producing two molecules of glycerate phosphate (C3)

2. Reduction of glycerate phosphate: NADPH reduces the 2 molecules of GP to a triose phosphate & ATP provides the energy

3. Regeneration of ribulose biphosphate: 5/6 of the triose phosphates produced used to regenerate ribulose bisphosphate using the remainder of the ATP from light dependent reactions as a source of phosphate & energy

4. Product synthesis: Remaining 1/6 used of triose phosphate for synthesis of C6 sugars and other compounds

insert picture from guide

How is photosynthesis measured?

As CO2 uptake of O2 production

Define gross & net photosynthesis & the compensation point

Gross photosynthesis: Indication of the carbohydrate made during photosynthesis

Net photosynthesis: carbohydrate gain after respiratory losses are taken into consideration

net photosynthesis + respiration = gross photosynthesis

Compensation point: rate of photosynthesis = rate of respiration

Factors affecting the rate of photosynthesis

Method for DCPIP

1. Put the bowl of a liquidiser in a fridge and, when cold, pour in 200 cm 3 of buffered sucrose

solution. Put a number of centrifuge tubes and a beaker into the fridge to cool.

2. Take fresh spinach, discard the midribs & put the remainder into the

liquidiser.

3. Run liquidiser for 30 seconds. Quickly strain the suspension through 3 layers

of muslin into cold beaker.

4. Quickly stir green suspension in the beaker, pour it into the cold centrifuge tubes.

Spin suspension in a centrifuge at 2500 g for 5 minutes. Remember to balance the

centrifuge.

5. Pour away the supernatant liquid and re-suspend the pellet in a little cold buffered sucrose

solution.

6. Mix equal volumes of chloroplast suspension and DCPIP in 3 boiling tubes. Treat each tube

as follows;

Expose to bright light for 15 minutes

Expose to dim light for 15 minutes

Keep in the dark for 15 minutes

7. For each tube, record a description of the colour at the start and end of the experiment to

allow you to judge how much the DCPIP solution has been decolourised.