Photosynthesis

Question 1

Write out the overall chemical reactions for photosynthesis and respiration.

Answer 1

Photosynthesis- 6CO2 + 6H2O ↔ C6H12O6 + 6O2

Respiration- C6H12O6 + 6O2 → 6CO2 + 6H2O

Question 2

Describe the relationship between photosynthesis and respiration, and use this to explain the importance of photosynthesis for consumers as well as producers.

Answer 2

Photosynthesis produces glucose
Respiration uses the glucose to produce energy.
This energy or the glucose is passed to consumers when they eat producers.

Question 3

Use the concept of “bond energy” to explain why photosynthesis requires energy from the sun and stores energy whereas respiration releases energy that can be used to make ATP.

Answer 3

In respiration the large organic molecules are broken down forming small inorganic molecules. So the total energy required to break all the bonds in a complex organic molecule is less than the total energy released in the formation of all the bonds in the smaller inorganic products. This reaction releases energy.

In photosynthesis the reverse happens so organic molecules are made from small inorganic molecules so energy is required from the sun to build these molecules.

Question 4

Draw, label and annotate a diagram of a chloroplast.

Answer 4

1. Double membrane structure
2. The fluid enclosed is called the stroma
3. Thylakoids- network of membranes, provides large surface area needed for enzymes, proteins and pigments in photosynthesis
4. Granum- several thylakoids stacked together
5. Lamellae- membranes which join the grana
6. Chlorophyll- in the grana
7. Contain DNA and ribosomes

Question 5

Name the two main stages of photosynthesis and state where each occurs in a chloroplast.

Answer 5

1. Light-dependent stage- in the photosystems in the thylakoid membrane
2. Light-independent stage- stroma

Question 6

Define photosynthetic pigment

Answer 6

Pigment molecules absorb specific wavelengths of light and reflect others.
Different pigments absorb and reflect different wavelengths and this is why they have different colours.

Question 7

Define light harvesting system/ antennae complex

Answer 7

A group of protein and chlorophyll molecules found in the thylakoid membranes of the chloroplasts in plant cell.
The role of the system is to absorb or harvest light energy of different wavelengths and transfer this energy quickly and efficiently to the reaction centre.

Question 8

Define reaction centre

Answer 8

Chlorophyll a is located in the reaction centre, which is where the reactions involved in photosynthesis take place.
The light harvesting system and reaction centre are collectively known as a photosytem.

Question 9

Define photosystem

Answer 9

Protein complexes involved in the absorption of light and electron transfers in photosynthesis.

Question 10

Name the photosynthetic pigment in the reaction centre of a photosystem.

Answer 10

Chlorophyll a

Question 11

Name 3 types of photosynthetic pigments found in the antennae complex.

Answer 11

chlorophyll b, xanthophylls and carotenoids.

Question 12

Explain why it is useful for photosynthetic organisms to have many different photosynthetic pigments.

Answer 12

Because different pigments can absorb different wavelengths of light.
This allows them to adjust to different intensities in light- orange leaves in water
In autumn plants stop producing chlorophyll.

Question 13

Label and annotate an absorption spectrum graph to explain what it shows.

Answer 13

1. Wavelength on bottom and absorbance on y-axis
2. Peaks at dark blue and red
3. Troughs at green and light blue.
4. This shows that plants are most suited to absorbing red/ dark blue light and reflect green light
5. look at it in book

Question 14

Describe the purpose of chromatography.

Answer 14

1. The purpose is to show the different photosynthetic pigments present in plant extract, by separating them out.
2. The pigments have different solubilities so move up the paper by the solvent at different rates and distances.
3. Moves up by capillary action.
4. Rf value means retardation factor
5. Rf= Distance moved by pigment/ Distance moved by solvent
6. The ratio will help you identity different pigments.
7. Least soluble will move least difference and so will have smallest rf value

Question 15

Describe a step by step method for conducting thin layer chromatography to separate and identify photosynthetic pigments.

Answer 15

1. draw pencil line on chromatography paper
2. Plant extract is ground with propanone until a smooth dark pulp is achieved.
3. Transfer pigment with a capillary tube to teh strip creating a small spot on the pencil line
4. Put the strip in a chromatography tube with the spot at the bottom- solvent shouldn’t reach the spot.
5. Leave and remove the strip once the solvent is about 1 cm from the top. Mark the solvent line and each pigment line with pencil.
6. Then calculate rf values to identify the pigments.

Question 16

Explain what determines how far a particular molecule travels in chromatography. recap

Answer 16

Depends on the solubility and interactions (hydrogen bonds)

More soluble means it moves up further.

Question 17

Draw a diagram to summarise the light-dependent stage of photosynthesis and state where this occurs.

Answer 17

Non cycling photo-phosphorylation:

1. Light is absorbed by pigments in photosystem 2.
2. The light absorbed excites electrons at the reactions centres of the photosystems.
3. The excited electrons are released from the reaction centre of PS 2 and are passed to an electron transport chain. Transfers to PS 1 (higher wavelength)
4. ATP is produced by the process of chemiosmosis.
5. The electron lost from the reaction centre of PS2 are replaced photolysis
6. Excited electrons are released from the reaction centre of PS1 passed to another electron transport chain, and ATP is again produced by chemiosmosis.
7. These lost electrons are replaced by the electron coming from PS2.
8. The electrons leaving the electron transport chain following PS1 are accepted, along with hydrogen ion, by the coenzyme NADP, forming reduced NADP.

Question 18

Name the two useful products, the waste product, and the requirements, of the light-dependent stage of photosynthesis.

Answer 18

1. Useful- ATP and reduced NADP
2. Waste product- Oxygen
3. Light is needed for energy to excite electrons

Question 19

Define the term phosphorylation

Answer 19

The addition of phosphate group to a molcule

Question 20

Define the term photophosphorylation

Answer 20

The synthesis of ATP from ADP and phosphate that occurs in a plant using light energy absorbed during photosynthesis.

Question 21

Define the term cyclic photophosphorylation

Answer 21

Synthesis of ATP involving only photosystem 1

Question 22

Define the term non-cyclic photophosphorylation

Answer 22

Synthesis of ATP and reduced NADP involving PS1 and PS2

Question 23

Define the term photolysis

Answer 23

Water molecules are split into hydrogen ions, electrons and oxygen molecules using energy from the Sun.
H2O →2H+ + 2e- + 1/2 O2

Question 24

Describe the process of cyclic photophosphorylation

Answer 24

1. The same as non-cyclic
2. But the electrons leave the electron transport chain after PS1 are returned to PS1 instead of being used to form reduced NADP.
3. This means PS1 can still lead to the production of ATP without any electrons being supplied from PS2

Question 25

Describe the process of photolysis and what the products of photolysis are used for.

Answer 25

1. There is an enzyme as part of PS2 which catalyses the breakdown of water.
2. Electrons released replace the electrons lost from the reaction centre of PS2
3. Oxygen gas is released as a by-product
4. The protons are released into lumen of the thylakoids, increasing the proton concentration across the membrane.
5. As they move back through the membrane down a concentration gradient through the ATP synthase to the stroma.
6. This catalyses the reaction to convert ADP and phosphate to ATP at the active site of the enzyme.
7. Once the hydrogen ion has returned to the stroma they combine with the NADP and an electron from PS1 to form reduced NADP.
8. This removed the hydrogen ions from the stroma so it helps to maintain the proton gradient across the thylakoid membranes.

Question 26

Summarise the light-independent stage of photosynthesis and state where this occurs.

Answer 26

1. Calvin cycle takes place in the stroma
2. CO2 diffuses into stroma from the atmosphere through the stomata and spongy mesophyll layer
3. They CO2 combines with the 5-carbon molecules ribulose bisphosphate(RuBP).- carbon fixation as the carbon in the CO2 is fixed.
4. Enzyme ribulose bisphosphate carboxylase RuBisCO catalyses the carbon fixation producing an unstable 6-carbon intermediate.
5. RuBisCO is an inefficient enzyme as it is competitively inhibited by oxygen so a lot is needed to carry out photosynthesis.
6. The unstable 6-carbon compound formed immediately breaks down forming two 3-carbon glycerate 3-phosphate (GP) molecules.
7. Each GP molecule is converted to another 3-carbon molecule, triose phosphate (TP), using a hydrogen atom from reduced NADP and energy from ATP (ATP forms ADP + P). (TP is GP with hydrogens)
8. Triose phosphate is a carbohydrate the majority of it is recycled to regenerate RuBP so that the Calvin Cycle can continue.
9. Triose phosphate is the starting point for many complex biological molecules e.g. carbs, lipids, proteins and nucleic acids.

Question 27

What is required for the light independent cycle to progress

Answer 27

1. CO2- atmosphere
2. ATP- Light dependent stage
3. Reduced NADP- Light dependent stage
4. RuBP- regenerated in the Calvin cycle

Question 28

What are the products of the Calvin Cycle

Answer 28

1. ADP +P
2. RuBP
3. Triose phosphate
4. NADP+

Question 29

What is returned to the light dependent reaction from the Calvin Cycle

Answer 29

1. ADP + P

2. NADP

Question 30

Define RuBisCO and state its long name

Answer 30

Ribulose bisphosphate carboxylase

The key enzyme involved in the first step of carbon fixation in photosynthesis

Question 31

Define carbon fixation

Answer 31

When carbon is incorporated into an organic molecule

Question 32

Describe the roles of ATP and reduced NADP in the Calvin cycle.

Answer 32

ATP- provides energy to form TP from GP and energy to regenerate RuBP
Reduced NADP- provides a hydrogen atom to form TP from GP-(needs to be H atom not H+ as if there is no electron preset a bond will not form).

Question 33

Explain how RuBP is regenerated in the Calvin cycle.

Answer 33

1. For one glucose molecule to be produced 6 CO2 have to enter the Calvin cycle, resulting in 6 turns of the cycle.
2. This produces 12 TP molecules two of which are removed to produce glucose molecule
3. 10 TP molecules are left- they are recycled to regenerate 6 RuBP molecules.
4. This uses ATP for energy (maybe provides phosphate).

Question 34

Label and annotate an action spectrum graph to explain what it shows.

Answer 34

1. Wavelength on bottom
2. Relative effectiveness in photosynthesis
3. Same sort of pattern as relative absorption graph
4. Dip at green wavelength

Question 35

Describe the difference between an absorption spectrum graph and an action spectrum graph.

Answer 35

1. Action spectrum graph- shows how the rate of photosynthesis varies at different wavelengths
2. Absorption spectrum graph- shows how different pigments absorb different wavelengths of light

Question 36

Define the term “limiting factor”.

Answer 36

Factor that limits the rate of a process

Question 37

State 5 factors that affect the rate of photosynthesis.

Answer 37

1. Light intensity
2. Carbon dioxide concentration
3. Temperature
4. Water
5. pH
6. Mineral ions

Question 38

Explain why wavelength of light cannot be a limiting factor for photosynthesis whereas light intensity, temperature and carbon dioxide concentration can be.

Answer 38

Because chloroplasts contain many different photosynthetic pigments e.g chlorophyll a/b, xanthophylls and carotenoids

Question 39

Describe how light intensity affects the rate of photosynthesis

Answer 39

1. Light is needed for an energy source for the light dependent reaction.
2. As light intensity increases, ATP and reduced NADP are produced at a higher rate
3. If there isn’t any- then no ATP or reduced NADP will be made so the Light independent reaction can’t take place.

Question 40

Describe how CO2 concentration affects the rate of photosynthesis

Answer 40

1. Carbon dioxide is needed as a source of carbon, so if all other conditions are met, increasing the carbon dioxide concentration increases the rate of carbon fixation in the Calvin cycle and therefore the rate of TP production.

Question 41

Describe how temperature affects the rate of photosynthesis

Answer 41

1. Affects the rate of enzyme-controlled reactions.
2. As temp increases, the rate of enzyme activity increases until the point at which the proteins denature.
3. An increase in temperature increases the rate of the enzyme controlled reactions in photosynthesis- carbon fixation etc.

Question 42

Explain how graphs of the rate of photosynthesis under different environmental conditions can show what is limiting the rate of photosynthesis under a particular set of conditions.

Answer 42

1. If there are different varying conditions and separate lines on the graph which plateaued out at different points, look at what is the difference between where it has plateaued, and that is the limiting factor for the line which plateaued earlier.

Question 43

Explain how water stress limits the rate of photosynthesis but water availability itself is not considered to be a limiting factor.

Answer 43

1. Although it is required for photosynthesis it is never considered to be limiting because for the water potential to have become low enough to limit the rate of photosynthesis the plant will have already closed its stomata and ceased to photosynthesise.
2. Plants except those with adaptations to drought conditions are unlikely to survive these conditions.

Question 44

Describe how the rate of photosynthesis can be measured.

Answer 44

1. Using a data logger that records data over time using sensors
2. The rate can be estimated by calculating the rate of oxygen produced, carbon dioxide used or increase in dry mass of a plant.

Question 45

Describe a step by step method to investigate how one environmental factor affects the rate of photosynthesis.

Answer 45

1. Choose the factor which can be recorded useing the data loggers.
2. Use sodium hydrogen carbonate as the source of CO2
3. Pond weed should be kept in the dark- so it can finish photosynthesising and use up its reactants,
4. The apparatus should be left to equilibriate for 10 minutes before readings are taken.
5. The oxygen sensor may need to be calibrated using the oxygen concentration of air.
6. The software can be used to take readings at desired intervals for the required time length.

Question 46

Define the term “compensation point” in relation to light intensity and describe how it can be determined for a particular photosynthetic organism.

Answer 46

1. Reducing light intensity will reduce the rate of the light-dependent stage of photosynthesis. This will reduce the quantity of ATP and reduced NADP produced.
2. ATP and reduced NADP are needed to convert GP to TP.
3. The concentration of GP will increase and concentration of TP will decrease.
4. As there will be less TP to regenerate RuBP, the concentration of RuBP will also decrease.
5. The reverse will happen when light intensity is increased.
6. Its a compromise because even if you increase light intensity there won’t be enough of the CO2 so the Calvin cycle will be slow- CO2 concentration would also need to be increased

Question 47

Explain how changing carbon dioxide concentration affects the concentration of RuBP, TP and GP in the Calvin cycle.

Answer 47

1. As CO2 is an essential substrate of the Calvin cycle, low concentrations will lead to reduced concentrations of GP (as there is less CO2 to be fixed) and TP
2. The concentrations of RuBP will increase as it is still being formed from TP but not being used to fix CO2- for a little until TP runs out.

Question 48

Explain how changing temperature affects the concentration of RuBP, TP and GP in the Calvin cycle.

Answer 48

1. All reactions in the Calvin cycle are catalysed by enzymes
2. At lower temperatures enzyme and substrate molecules have less kinetic energy resulting in fewer successful collisions and a reduced rate of reactions.
3. This means there will be lower concentrations of GP, TP and RuBP.
4. The same effect will be seen at high temperatures as enzymes will be denatured.

Question 49

Describe the methods for increasing the rate of photosynthesis in a greenhouse

Answer 49

1. Sunlight- artificial lighting may be used in winter, blinds can be rolled up or down to control the amount of light entering
2. Humidity- if too high it encourages the growth of fungi and other organisms, but plants like humidity- to raise it using a misty spray of water and lowered by opening ventilators or increasing temperature.
3. CO2- extra CO2 can be provided by burning gas (paraffin) or it can be released form cylinders
4. Temperature- can be controlled by opening and closing ventilators and by adjusting the heaters
5. Water in soil- must get exact amount- so plant doesn’t wilt or become waterlogged.