13. Photosynthesis

Question 1

How is photosynthesis an ‘energy transfer process’?

Answer 1

It converts light energy into chemical potential energy (stored in organic nutrient molecules). This energy is then used for work (eg. respiration).

Question 2

What are photoautotrophs?

Answer 2

Most energy converted to ATP is from light energy used in photosynthesis by photoautotrophs. Examples of these are photosynthetic prokaryotes and protoctists (eg. red, green, brown algae).

Question 3

What are chemoautotrophs?

Answer 3

They use chemical energy sources, eg. nitrifying bacteria (ammonia -> nitrite or nitrite -> nitrate).

Question 4

What is photosynthesis?

Answer 4

The fixation of carbon dioxide and its subsequent reduction to carbohydrate (using hydrogen from the photolysis of water).

Question 5

What is the general equation for photosynthesis?

Answer 5

nCO2 + nH2O -> (CH2O)n + nO2

Arrow = light energy in the presence of chlorophyll.
Hexose sugars and starch are often formed.

Question 6

Into which two stages is photosynthesis split?

Answer 6

Light-dependent and -independent stages. The LDRs occur if suitable pigments are present - these pigments absorb wavelengths of light, the energy of which is used in the photolysis of water and converted into chemical energy (ATP) to be used to reduce carbon dioxide in the LIRs.

Question 7

Where do the LDRs take place?

Answer 7

The thylakoid membranes of the chloroplast.

Question 8

Outline the process of the LDRs.

Answer 8

• Photolysis
• Cyclic photophosphorylation
• Non-cyclic photophosphorylation

Question 9

Describe the process of photolysis.

Answer 9

Light energy is used to split:
H2O -> 2H+ + 2e- + 1/2 O2
It is catalysed by a water-splitting enzyme in PSII.

The protons can be used to reduce NADP -> rNADP, which can be used in the LIRs:
2H+ + 2e- + NADP -> rNADP
The electrons come from PSI.

They can also be used to create an electrochemical gradient, providing electrical potential energy for the synthesis of ATP.

Question 10

Describe the process of cyclic photophosphorylation.

Answer 10

Light is absorbed by PSI and passed to the primary pigment reaction centre. An electron in the chlorophyll molecule is excited to a higher energy level and emitted in a process called photoactivation.
Rather than being reabsorbed into PSI and losing its energy as thermal energy or fluorescence, the electron is captured by an electron acceptor and passed back to chlorophyll by an electron carrier chain.
The energy released by these electrons is used to generate ATP via chemiosmosis.

Question 11

Describe the process of non-cyclic photophosphorylation.

Answer 11

Light is absorbed by PSI and PSII. Excited electrons are emitted from both reaction centre primary pigments. These electrons are passed to electron acceptors and are passed down chains of electron carriers, leaving both photosystems positively charged.
The primary pigment of PSI absorbs the electrons from PSII, and PSII receives replacement electrons from photolysis.
ATP is synthesised as electrons release energy from passing down the electron carrier chains.

Question 12

Describe the Hill reaction.

Answer 12

Isolated chloroplasts have ‘reducing power’, liberating oxygen from water with an oxidising agent present.
It uses a redox agent eg. Fe3+, DCPIP (both blue -> colourless) as an electron acceptor, in the place of NADP.
It is used to test the effect of light intensity/wavelength on the rate of photosynthesis of a chloroplast suspension.

Question 13

What is the Hill reaction using DCPIP?

Answer 13

oxidised DCPIP -> reduced DCPIP

Arrow = H2O -> 1/2 O2 (chloroplasts in light)

Question 14

Where do the LIRs take place?

Answer 14

In the stroma of the chloroplast.

Question 15

Outline the steps of the Calvin Cycle.

Answer 15

• Carbon fixation
• Reduction
• Regeneration

Question 16

Describe the steps of the Calvin Cycle.

Answer 16

CO2 (1C) + RuBP (5C) -> Unstable intermediate (6C)
- catalysed by rubisco
Unstable intermediate -> 2 PGA (3C)

2 PGA -> 2 Triose phosphate

• ATP -> ADP + Pi
• rNADP -> NADP

5/6 Triose phosphate -> RuBP (regeneration)
- ATP -> ADP + Pi
1/6 Triose phosphate -> glucose, lipids, amino acids (via metabolic pathways).

Question 17

What happens to triose phosphates in the Calvin Cycle?

Answer 17

Some condense to hexose phosphates, which are used to make starch, sucrose, cellulose.
Some are converted to glycerol + fatty acids -> lipids, CoA (which can be used in respiration or the production of amino acids).

Question 18

Which factors are required for photosynthesis?

Answer 18

Suitable photosynthetic pigment, supply of CO2, H2O, light energy.

Question 19

Which factors affect the rate of photosynthesis?

Answer 19

Light intensity, light wavelength, temperature, CO2 concentration.

Question 20

What happens to the rate of photosynthesis when light intensity is varied at a constant temperature?

Answer 20

Initially, rate increases as light intensity increases, but the relationship plateaus after a certain point (high light intensity).

Question 21

What happens to the rate of photosynthesis when temperature is varied at constant light intensity?

Answer 21

The rate increases at high light intensities. At low light intensities, temperature has little effect on the rate of photosynthesis.

Question 22

Which two points do these experiments from Blackman demonstrate?

Answer 22

Photochemical reactions are not affected by temperature, yet photosynthesis is affected by temperature.
Therefore, two stages of photosynthesis must occur; a light-dependent stage (photochemical) and a light-independent stage (affected by temperature).
There are also limiting factors which affect the rate of photosynthesis.

Question 23

What is a limiting factor?

Answer 23

The rate of a multi-step reaction is limited by the slowest step. When multiple factors are involved, the factor nearest its lowest value is the limiting factor.

Question 24

What happens to the rate of photosynthesis if CO2 concentration is varied at constant light intensity and temperature?

Answer 24

Increase in CO2 increases the rate, but it plateaus at higher concentrations.

Question 25

Which factor is limiting at low light intensity?

Answer 25

The light intensity itself.

Question 26

Which factor is limiting at high light intensity?

Answer 26

One or more other factors, eg. CO2 concentration, temperature.

Question 27

Which factor is limiting at low CO2 concentration?

Answer 27

The CO2 concentration itself.

Question 28

Which factor is limiting at high CO2 concentration?

Answer 28

One or more other factors.

Question 29

Give an example of plants being grown in protected environments.

Answer 29

Tomato plants in glasshouses. Sensors monitor the light intensity, humidity and CO2 concentration. They are grown hydroponically (roots in a nutrient solution). All these factors are managed by a computer.
These protected environments allow for fungal disease/insect pests to be controlled.
All these factors help to increase the yield of crops.

Question 30

What do C3 plants do?

Answer 30

They use the Calvin Cycle (in the LIS) to fix carbon dioxide from the air.
CO2 + RuBP -> 6C -> 2 3C compounds.

Question 31

What do C4 plants do?

Answer 31

The first compound they produce from CO2 in the LIS is a 4C compound.

Question 32

Where do C4 plants mostly grow?

Answer 32

In areas of high temperature and light intensity - typically low altitude regions in the tropics. Examples of C4 plants include tropical grasses such as maize, sorghum and sugarcane.

Question 33

What reaction can take place under very high temperatures and light intensities?

Answer 33

Photorespiration - rubisco catalyses the combination of RuBP and O2, reducing the rate of photosynthesis as there is less RuBP available to fix carbon dioxide.

Question 34

How are C4 plants adapted to avoid photorespiration?

Answer 34

They have bundle sheath cells containing rubisco and RuBP, which are kept away from the air inside the leaf (high O2 concentration). They do not have direct contact with this air. 
Their enzymes (eg. PEPC) have higher optimum temperatures than in C3 plants (eg. 45°C in amaranth, 30°C in pea plants).
They have specialised mesophyll cells which form a tight ring around the bundle sheath cells, excluding air from them. The cytoplasm of the mesophyll cells fixes CO2. Their chloroplasts capture light and carry out the LDS.

Question 35

Describe how photosynthesis occurs in C4 plants.

Answer 35

The mesophyll cells absorb carbon dioxide and PEP carboxylase catalyses its combination with PEP.
- CO2 + phosphoenolpyruvate -> oxaloacetate

Oxaloacetate is converted to malate which then passes to the bundle sheath cells. Here, carbon dioxide is removed from malate and delivered to RuBP by rubisco.
- Malate -> CO2 + pyruvate

The pyruvate moves back into the mesophyll to regenerate phosphoenolpyruvate, with the aid of the LDRs.

Question 36

Give an outline of the structure of a chloroplast.

Answer 36

• Biconvex disc, 3-10μm diameter.
• Up to 100 in palisade mesophyll cells.
• Double membrane envelope.
• Stroma (ground substance).
• Thylakoids in stacks - grana, joined by lamellae.

Question 37

Describe the membrane system inside a chloroplast.

Answer 37

The membranes of the grana provide a large surface area which contains the pigments, enzymes (eg. ATP synthase) and electron carriers for the LDS.
The grana are made of stacks of flattened, fluid-filled sacs (thylakoids).
The membranes make it possible for many pigments to be arranged in particular light-harvesting clusters for efficient light absorption.

Question 38

Describe the arrangement of pigments in a chloroplast.

Answer 38

Many accessory pigments surround a primary pigment in light-harvesting clusters known as photosystems. The primary pigments act as reaction centres.
The accessory pigments are arranged in funnel-like structures so that energy can be passed down to the chlorophyll a reaction centre.

Question 39

What is contained by the stroma in a chloroplast?

Answer 39

Enzymes for the Calvin Cycle, sugars, organic acids. Starch grains, lipid droplets, 70S ribosomes, circular DNA (codes for some chloroplast proteins, made by the 70S ribosomes).
The stroma bathes the membranes of the grana so it can receive the products of the LDS.

Question 40

What are the two types of pigment, and their subtypes?

Answer 40

Primary pigments (chlorophylls) - chlorophyll a, chlorophyll b.
Accessory pigments (carotenoids) - β-carotene, xanthophyll.

Question 41

What are the colours of the pigments?

Answer 41

Chlorophyll a - yellow-green
Chlorophyll b - blue-green
β-carotene - orange
Xanthophyll - yellow

Question 42

In which regions of the light spectrum does each type of pigment absorb?

Answer 42

Chlorophylls - red and blue-violet regions.

Carotenoids - blue-violet region.

Question 43

What is an absorption spectrum?

Answer 43

A graph of the absorbance of different wavelengths by a particular pigment.

Question 44

What is an action spectrum?

Answer 44

A graph of the rate of photosynthesis at different wavelengths of light (indicates the effectiveness of each wavelength, related to its absorption spectrum and energy content).

Question 45

What is the relationship between wavelength and energy?

Answer 45

Low wavelength = High energy.

Question 46

How is UV light used to identify pigments?

Answer 46

When used on a or b it shows red fluorescence. The UV light is absorbed and electrons are excited to a higher energy level, but they cannot be used (only extracted pigment) so they return to their unexcited state and release their energy as thermal energy or light at a longer wavelength.
This is the energy that drives photosynthesis.

Question 47

How is paper chromatography used to identify pigments?

Answer 47

The solvent travels up the paper, carrying pigments at different speeds. The pigments are separated from one another as they travel different distances.

Question 48

How do you calculate Rf values?

Answer 48

Distance travelled by spot ÷ Distance travelled by solvent.

Question 49

What are the Rf values for each pigment?

Answer 49

Carotenoids - close to 1
Chlorophyll a - much lower
Chlorophyll b - in between.

Question 50

How can an extract of photosynthetic pigment be prepared?

Answer 50

Eg. lettuce or spinach, by liquidising the leaves in ice-cold buffer and then filtering or centrifuging the resulting suspension to remove unwanted debris.

Question 51

Describe an experiment for the Hill reaction.

Answer 51

Using chilled glassware, place tubes of buffered chloroplast suspension with added DCPIP solution in different light intensities/wavelengths.
Assess the blue colour at intervals.

Question 52

How can you tell the rate of photosynthesis by the colour of DCPIP?

Answer 52

The rate of loss of the blue colour can be plotted against time.
To find the colour, you can use a colorimeter or compare the test solutions to known concentrations of DCPIP.

Question 53

How can Elodea be used to measure the rate of photosynthesis?

Answer 53

Altering:

• light intensity – by altering the distance, d, of a small light source from the plants (light intensity is proportional to 1/d^2)
• wavelength of light – by using different colour filters, making sure that they each transmit the same light intensity
• concentration of carbon dioxide – by adding different quantities of sodium hydrogen carbonate (NaHCO3) to the water surrounding the plant
• temperature of the water surrounding the plant – using a large container, such as a beaker, to help maintain the chosen temperatures.

Question 54

How can Elodea be used to measure the rate of photosynthesis? (2)

Answer 54

The elodea must be well illuminated before use and the chosen stem needs to be cut cleanly just before putting it into a test tube. The bubbles given off are mostly oxygen, but contain some nitrogen. To prevent these gases from dissolving in the water, rather than forming bubbles, the water needs to be well aerated (by bubbling air through it) before use.