Photosynthesis

In which cell organelles does photosynthesis occur?

Chloroplasts

Where in the light-dependent stage of photosynthesis take place?

In the thylakoid

Where in the light-independent stage of photosynthesis take place?

In the stroma

Why are there may different photosynthetic pigments?

Each pigment absorbs light efficiently at different wavelengths having multiple pigments allows for more energy from the sun to be captured

Which photosynthetic pigment is found in all photosynthetic plants?

Chlorophyll a.

Which colours of light do the chlorophyll pigments absorb, and which do they reflect?

Chlorophyll a and b absorb red light (wavelength 650-700nm) and blue light (400-450nm)
The pigment reflect green light (500-550nm)

Plants containing carotenoid pigments tend to be which colours?

Red, orange and yellow

The carotenoids absorb blue light (440-450nm) and reflect red (550-700nm)

Explain the difference between an absorption spectrum and an action spectrum

Absorption spectrum measures which wavelengths of light are absorbed by the chlorophyll pigments
Action spectrum measures the rate of photosynthesis occurring at particular wavelengths

Name the technique used to separate chloroplast pigments

Chromatography

How is the Rf value calculated in chromatography?

Rf= Distance travelled by dot/Distance travelled by solvent

Define photoactivation

The process by which two electrons in the chlorophyll molecule become excited due to the absorption of light energy which causes them to leave the molecule

This results in ionisation of the chlorophyll molecule

What happens to the electron that leave the chlorophyll molecule during photoactivation?

The two electrons are taken up by an electron acceptor

Then they move down electron carriers within the thylakoid membrane in the ETC and energy is released as they move

How is the energy from the ETC used in the light dependent stage?

The energy lost from the electrons is used to pump two H+ ions from the stroma into the thylakoid space

This creates a H+ gradient across the thylakoid membrane

How is the H+ gradient used to make ATP?

• The H+ ions in the thylakoid space exit an ATP synthase enzyme in the thylakoid membrane

• The enzyme catalyses the formation of ATP, using the gradient

• When in the stroma, NADP accepts the H+ ions and becomes NADPH

What happens to water during the light-dependent stage of photosynthesis?

Water splits using light energy (photolysis)

2H2O ——> 4H+ + 4e- + O2

State what happens to the products of photolysis of water

• H+ ions are taken up by a hydrogen acceptor, NADP

• Electrons replace those lost from chlorophyll molecules during photoactivation

• O2 is given off as a waste gas

Outline cyclic photophosphorylation

• Excited electron from PS(I) enter the ETC

• Energy from electrons is used to produce ATP. The electrons return to PS(I)

• There is no reduction of NADP and no need for photolysis of water to replace electrons

• An extra ATP is formed for immediate energy

Outline non-cyclic photophosphorylation

• Electrons from PS(II) are excited and enter the ETC and ATP and NADPH are produced via the making of the proton gradients and subsequent movement through the thylakoid membrane

• The electrons then move to PS(I). Photolysis of water is required to replace lost electrons from PS(II)

• ATP and NADPH are produced and used in the light independent stage

What are the two important products of the light-dependent stage that are used in the light-independent stage?

ATP and reduced NADP - they transfer energy from the light independent stage which is used later

Name the three stages of the Calvin cycle in the light-independent stage

1. Fixation of CO2

2. Reduction

3. Regeneration

During fixation, which molecule does CO2 combine with?

Ribulose bisphosphate (RuBP 5C)

Name the enzyme that catalyses the fixation of CO2

Rubisco (in the stroma)

What is the product of the fixation of CO2

2 molecules of glycerate 3-phosphate (GP 3 carbons)

Describe the reduction step of the light-independent stage

• Two molecules of GP are reduced to two molecules of triose phosphate (TP) and ATP and NADPH are required

• ATP is hydrolysed to provide energy for the conversion and NADPH is used to reduce GP

• This results in two ADP molecules and two NADP molecules. NADP returns to the light-dependent stage to be reduced again

What happens during the regeneration step of the light-independent stage?

TP is converted into RuBP, to be used again for fixation of CO2

ATP is required

What other molecules besides RuBP can TP be converted into?

• Sugars (glucose, starch, cellulose)

• Amino acids

• Lipids

Explain the term ‘limiting factor’

A factor that limits the rate of a reaction often because it is in short supply regardless of the levels of other factors

Name the potential limiting factors of photosynthesis

• Light intensity

• CO2 concentration
  Temperature

Explain how light-intensity affects the rate of photosynthesis

• At low light intensity, the rate of the light-dependent stage is low and little ATP and NADPH are produced so the light-independent stage ends up being slow

• As light intensity increases so does the rate. photosynthesis rate and respiration rate are equal, this is the compensation point

• Past this point, light intensity increases photosynthesis until another factor becomes limiting

Explain how changing CO2 concentration affects the rate of photosynthesis

• CO2 is required to combine with RuBP in the light-independent stage

• Increasing CO2 will increasing the rate of photosynthesis until another factor becomes limiting

How can knowledge of limiting factors be used to optimise crop yield in greenhouses?

• Light intensity is increases by installing lamps and using wavelengths of light that chloroplasts can absorb

• The temperature is controlled

• CO2 concentration may be increases

What is DCPIP?

DCPIP is a redox indicator

When it is reduced by the H+ ions from the photolysis of water it changes from blue to colourless

Describe how the structure of chloroplasts are adapted to their function

• The thylakoid membranes are arranged in a stack which provides a large surface area for the absorption of light and photopigments

• The double membrane is permeable to gases and products of photosynthesis

• The thylakoid space accommodated the H+ ions; the thylakoid membrane maintains the gradient

• The enzymes for the light-independent stage are all found in fluid stroma