Biology - Chapter 11: Photosynthesis

In what organelle does photosynthesis take place?

The chloroplasts

9 adaptations of leaves to maximise the rate of photosynthesis

1. Large surface area - absorb as much sunlight as possible
2. Leaves are arranged in a way that minimises overlapping - avoids shadowing of one leaf by another
3. Thin - most light absorbed on first few um of leaf + short diffusion pathway
4. Transparent cuticle + epidermis - lets light through to photosynthetic mesophyll cells underneath
5. Long, narrow upper mesophyll cells packed with chloroplasts - absorb sunlight
6. Numerous stomata - short diffusion pathway to mesophyll cells
7. Stomata open and close
8. Many air spaces in the lower mesophyll layer - allows rapid diffusion in gas phase of CO2/O2
9. Network of xylem and phloem - brings water to leaf cells and carries sugars away respectively

Photosynthesis equation

6CO2 + 6H2O -> C6H12O6 + 6O2

Name the 2 stages of photosynthesis

The light-dependent reaction and the light-independent reaction

What does the light-dependent reaction produce?

NADPH, ATP and oxygen

Describe the 2 distinct regions in chloroplasts

1. The grana: made up of stacks of thylakoids, which contain chlorophyll. There are inter-granal lamellae - tubular extensions that join up with thylakoids in adjacent grana
2. The stroma: fluid-filled matrix that contains other structures e.g. starch grains

Where does the light-dependent reaction take place?

The thylakoids

Where does the light-independent reaction take place?

The stroma

In the light-dependent reaction, what are the two uses for the light energy absorbed from the sun?

1. To add an inorganic phosphate to ADP, forming ATP
2. Photolysis - to split water into protons, electrons and oxygen

Oxidation

the loss of electrons, loss of hydrogen or gain of oxygen

Reduction

the gain of electrons, gain of hydrogen or loss of oxygen

In oxidation, is energy given out or taken in?

Given out

In reduction, is energy given out or taken in?

Taken in

Describe the process of the light-dependent reaction (7)

1. Photoionisation: chlorophyll absorbs light energy, causing a pair of its electrons to become excited
2. The pair of electrons are taken up by an electron carrier - the electron carrier is reduced, while the chlorophyll is oxidised
3. The electrons pass along a series of electron carriers through a series of oxidation-reduction reactions, causing the electrons to lose energy
4. Photolysis: a molecule of water is split into electrons, protons and oxygen using energy. These electrons are used to replace the electrons in chlorophyll, while the protons remain in the thylakoid space to increase their concentration there
4. This energy is then used by proton pumps in the thylakoid membrane to pump protons (H+ ions) from the stroma into the thylakoid space - this increases the concentration of protons in the thylakoid space
5. This establishes an electrochemical gradient of protons into the stroma
6. This causes the protons to diffuse through ATP synthase channel proteins, causing the structure of ATP synthase to change so it catalyses the formation of ATP from ADP and Pi
7. The protons and electrons (from the ETC) are then taken up by NADP, causing it to form NADPH (reduced NADP)

Give the equation for the photolysis of water

2H2O -> 4H+ + 4e- + O2

What happens to the oxygen after a light-dependent reaction?

It diffuses out of the leaf as a waste product or is used in aerobic respiration

How are the chloroplasts adapted for the light-dependent reaction? (4)

- Thylakoid membranes provide a large surface area for the attachment of chlorophyll, electron carriers and enzymes that carry out the LDR
- Network of proteins in the grana hold the chlorophyll in a precise manner to maximise the absorption of light
- Granal membranes are selectively permeable, so allow the establishment of a proton gradient, and have ATP synthase channels within them which catalyse the production of ATP
- Chloroplasts contain both DNA and ribosomes so they can quickly manufacture the proteins needed for the LDR

What theory is used to describe the precise mechanism behind how ATP is produced in the LDR?

Chemiosmotic theory

NADP

A molecule that carries electrons produced in the LDR

Can the light-independent reaction take place without light?

Yes - but it will rapidly cease as it needs to products of the LDR

Give an alternate name for the LIR

The Calvin cycle

Describe the process of the light-independent reaction (6)

1. CO2 diffuses into leaf via stomata, dissolves in water in mesophyll cells then diffuses across the CSM, cytoplasm and chloroplast membranes into the stroma of the chloroplast
2. CO2 (1C) reacts with ribulose bisphosphate (5C) - RuBP - this reaction is catalysed by rubisco and produces two molecules of glycerate 3-phosphate (3C) - GP
3. NADPH and energy from the hydrolysis of ATP are used to reduce glycerate 3-phosphate to form triose phosphate (3C) - TP
4. This causes the NADPH to be oxidised, forming NADP, which goes back to be used in the LDR
5. 1/6 of the TP are used to form organic molecules (eg. glucose, starch, amino acids, lipis)
6. 5/6 of the TP are used to regenerate RuBP using ATP, which provides phosphate (causing ATP to be hydrolysed)

What proportion of TP is used to make RuBP?

5/6

What proportion of TP is used to make organic molecules?

1/6

Give some examples of the organic molecules TP is used to make

Glucose, starch, cellulose, amino acids, lipids

How are the chloroplasts adapted for the light-independent reaction? (3)

1. The fluid of the stroma contains all the enzymes for the LIR + is membrane-bound so an form an environment with a high concentration of enzymes and substrates can be maintained within it
2. The stroma fluid surrounds the grana so the products of the LDR in the grana can readily diffuse into the stroma
3. It contains DNA and ribosomes so can quickly make the proteins needed for the LIR

Describe the role of ATP in the LIR (2)

1. Provides energy for the reduction of glycerate 3-phosphate to triose phosphate
2. Provides phosphate for the conversion of TP to RuBP

Describe the role of NADPH in the LIR (1)

It provides electrons to reduce glycerate 3-phosphate and form triose-phosphate

Describe the role of chlorophyll in photolysis (4)

1. It absorbs light energy
2. It loses electrons (oxidsed)
3. It accepts electrons from water
4. This causes more water to dissociate

How is light energy converted to chemical energy in the LDR? (3)

1. Electrons are excited
2. They move along the electron carriers in oxidation-reduction reactions
3. This results in the production of ATP

Describe the role of electron transfer chains in the LDR (4)

1. They accept excited electrons from chlorophyll
2. The electrons lose energy along the chain through a series of oxidation-reduction reactions
3. This results in ATP being produced from ADP and Pi
4. It also results in NADPH forming when electrons from the ETC and H+ ions from photolysis combine with NADP

When carrying out chromatography, why should you mark the origin with pencil rather than ink?

The ink and leaf pigments would mix

When carrying out chromatography, where should the level of the solvent be up to?

It should be below the origin

When carrying out chromatography, when should you remove the paper?

Before the solvent reaches the top

Describe the role of light energy in the LDR (3)

1. The light energy is used to excite electrons so they move along the electron transfer chain, releasing energy
2. It is used to join ADP and Pi, forming ATP
3. It is used in the photolysis of water, which produces protons, electrons and oxygen - the electrons and protons are then used to reduce NADP

What would happen if there was a decrease in electron transport (5)? (include impact on CO2 uptake)

- In the light-dependent reaction,
- Less ATP produced as reduced chemiosmotic gradient
- Less NADPH produced
- Less GP converted to TP
- Less TP converted to RuBP, so less CO2 is taken up as it should combine with RuBP

How do the concentrations of CO2 in the air at different heights in a forest above ground in a 24 hour period differ and why? (3)

- High concentration of CO2 at night: no photosynthesis at night as light needed for LDR, plants respire instead, producing CO2
- Lower concentration of CO2 at day: plants use more CO2 than they produce (rate of photosynthesis > rate of respiration)
- CO2 concentration decreases as you go down: at ground level, fewer leaves and less light + there are animals, which respire

How does light intensity affect the rate of photosynthesis and of growth in plants? (5)

- As light intensity increases, ATP and NADPH are produced faster in the LDR
- This causes the LIR to be faster
- So more TP is converted to sugars, which can be used in respiration
- This means there would be more energy for growth
- So organic materials would be synthesised faster

Give 3 features of a calorimeter that ensure that a valid measurement of the heat energy is obtained when measuring biomass

1. Stirrer ensures even distribution of heat energy
2. Air / insulation to prevent heat loss
3. Water has high SHC

When does DCPIP decolourise?

When it is reduced

What colour change occurs when DCPIP is reduced?

It changes colour from colourless to blue

What is the effect of ammonium hydroxide on the rate of decolourisation of DCPIP? (3)

It reduces the rate as the ammonium hydroxide accepts electrons so they don’t pass along to the ETC / DCPIP