StemUp: OCR A A level Biology 5.2.1 Photosynthesis

What is photosynthesis? (2)

- The process by which energy from light is used to make glucose from water and carbon dioxide

- The energy is stored in glucose until it is released by respiration

What is respiration? (2)

- Releases energy from glucose

- Producing carbon dioxide and water

How do animals obtain glucose for respiration? (2)

- Animals cannot make their own food

- They obtain glucose by eating plants or other animals

Why is energy important for biological processes? (2)

- Energy is required for processes like photosynthesis, active transport, DNA replication, cell division, and protein synthesis

- Without energy, these processes would stop, and the organism would die

What are some similarities between photosynthesis and respiration? (4)

- Both involve the transfer of energy

- Both use and produce ATP

- Both involve redox reactions using coenzymes and processes controlled by enzymes

How is ATP produced in both photosynthesis and respiration? (2)

- By the flow of electrons down the electron transport chain

- Which creates a proton gradient that drives ATP synthase

Why do plants need to produce ATP through both photosynthesis and respiration? (5)

- Photosynthesis cannot occur in the dark, so no ATP is produced

- Some tissues cannot photosynthesise and do not produce ATP this way

- ATP cannot be moved between cells or stored

- Plants use more ATP than photosynthesis produces

- ATP is needed for active transport and synthesis of molecules

What is a coenzyme, and how does it function in photosynthesis and respiration? (3)

- A coenzyme is a molecule that aids the function of an enzyme by transferring a chemical group

- In photosynthesis, NADP transfers hydrogen between molecules, reducing or oxidising them

- In respiration, NAD, FAD, and coenzyme A transfer hydrogen or acetate between molecules

What are the adaptations of a leaf for photosynthesis? (5)

- Thin structure allows rapid light penetration

- Waxy cuticle prevents water loss

- Upper epidermis is transparent to allow light through

- Chloroplasts in the palisade mesophyll can move to optimise light absorption

- Spongy mesophyll has air spaces to allow rapid gas exchange

What are the two main stages of photosynthesis and where do they take place in the chloroplast? (2)

- Light-dependent reactions = takes place in the thylakoid membranes

- Light-independent reactions (Calvin Cycle) = occur in the stroma

How does the shape of the chloroplast contribute to its function? (2)

- The small, flattened disc shape

- Provides a large surface area for light absorption

What is the role of the chloroplast envelope? (2)

- The chloroplast envelope is a double membrane

- That controls the substances entering and leaving the chloroplast

Why do chloroplasts contain DNA and ribosomes? (2)

- To quickly manufacture proteins involved in the light-dependent reaction

- Such as RuBisCo

What is the function of thylakoids in the chloroplast? (4)

- Thylakoids are fluid-filled sacs stacked to form grana, which are linked by lamellae

- The thylakoid membranes provide a large surface area

- For the attachment of chlorophyll, electron carriers, and enzymes required for the light-dependent reactions

- They are selectively permeable, allowing a concentration gradient to exist

What is the role of the stroma in the chloroplast? (3)

- a fluid-filled matrix containing all the enzymes and organic acids needed for the light-independent reaction

- It also contains enzymes required for starch synthesis and breakdown

- As carbohydrates produced by photosynthesis and not used immediately are stored as starch grains

How does the stroma support the light-dependent reactions? (2)

- Surrounds the grana

- Allowing the products of the light-dependent reactions to diffuse into the stroma easily

Where do the light-dependent reactions of photosynthesis occur? (1)

In the thylakoid membranes of the chloroplast

Where do the light-independent reactions (Calvin Cycle) of photosynthesis occur? (1)

In the stroma of the chloroplasts

Draw and label a diagram of the structure of chloroplasts (7)

What are photosynthetic pigments, and why are they important in photosynthesis? (3)

- Coloured substances that absorb light energy for photosynthesis

- Having more than one pigment allows the plant to absorb a wider range of wavelengths

- E.g., chlorophyll a, chlorophyll b, carotene

Where are photosynthetic pigments found, and how are they organised? (3)

- Found in the thylakoid membranes of the chloroplasts

- Where they are attached to proteins

- Together, the pigment and the protein form a photosystem

What are the two types of pigments in photosystems? (4)

- Primary pigments

- Reaction centres where electrons are excited during the light-dependent reactions

- Accessory pigments

- Make up the light-harvesting system, surrounding reaction centres and transferring light energy to boost electron excitement

What are the two types of photosystems used by plants? (4)

- Photosystem I (PSI):

- Absorbs light best at a wavelength of 700nm

- Photosystem II (PSII):

- Absorbs light best at a wavelength of 680nm

What is thin layer chromatography (TLC) used for in photosynthesis? (1)

Used to separate photosynthetic pigments

What are the mobile phase and stationary phase in TLC? (2)

- Mobile phase = A liquid solvent that moves

- Stationary phase = A solid plate with a thin layer of gel (the chromatography plate) where the pigments cannot move

How are photosynthetic pigments separated using TLC? (2)

- The solvent moves up the TLC plate, carrying the dissolved pigments

- Different pigments travel different distances, separating them on the plate

How is the Rf value calculated in TLC? (1)

Rf value = Distance travelled by the pigment spot ÷ Distance travelled by the solvent

What is the first step in the TLC experiment for separating photosynthetic pigments? (2)

- Crush the leaf (e.g., spinach) with anhydrous sodium sulphate and propanone

- To extract the pigments

How do you prepare the chromatography plate with the pigment? (3)

1. Draw a horizontal pencil line near the bottom of the plate

2. Build up a single concentrated spot of liquid pigment from the plant extract

3. Letting each layer dry between applications

What happens after the pigment is applied to the plate? (2)

1. Stand the plate in a beaker of solvent, making sure the point of origin is just above the solvent level

2. Cover the beaker to prevent evaporation and allow the solvent to develop

How do you finish the TLC experiment? (2)

1. Mark the solvent front with a pencil when the solvent reaches near the top

2. Then calculate the Rf values to identify the pigments by comparing them to a database

What is photophosphorylation? (1)

The addition of phosphate in the presence of light to produce ATP

Describe cyclic photophosphorylation (3)

- Only produces ATP

- Electrons from chlorophyll are passed back to PSI and are recycled

- No reduced NADP or O₂ is produced

Describe non-cyclic photophosphorylation (1)

Produces ATP, reduced NADP, and O₂

What happens during photoionisation of chlorophyll in the light-dependent reaction? (2)

- Light energy absorbed by PSII excites electrons in chlorophyll

- High-energy electrons move along the electron transport chain to PSI

What is photolysis, and what are its products? (2)

- Photolysis is the splitting of water using light energy

- It produces protons (H⁺), electrons, and oxygen (H₂O → 2H⁺ + ½O₂)

How is ATP produced in the light-dependent reaction? (5)

1. Electrons move down the electron transport chain, losing energy at each stage

2. This energy is used to pump protons into the thylakoid space, creating a proton gradient

3. Protons move back into the stroma via ATP synthase

4. Producing ATP from ADP + Pi

5. This process is called chemiosmosis

What happens when light energy is absorbed by PSI? (2)

- Electrons are excited to a higher energy level and are transferred to NADP, along with a proton (H⁺)

- Forming reduced NADP

Where do the products of the light-dependent reaction go? (2)

- ATP and reduced NADP diffuse into the stroma

- Where they are used in the light-independent reaction (Calvin cycle)

What happens during CO₂ fixation in the Calvin cycle? (3)

1. CO₂ combines with ribulose bisphosphate (RuBP) (5C) in the presence of the enzyme RuBisCo

2. This forms an unstable 6C compound

3. Which breaks down into two molecules of glycerate-3-phosphate (GP) (3C)

How is GP converted into triose phosphate (TP)? (2)

1. GP is reduced to TP using reduced NADP from the light-dependent reaction and energy from ATP

2. The resulting NADP is recycled back to the light-dependent reaction

How is ribulose bisphosphate (RuBP) regenerated in the Calvin cycle? (2)

1. 5 out of 6 molecules of TP are used to regenerate RuBP using ATP

2. The remaining 1 out of 6 TP is used to produce useful organic substances like glucose

What substances are used in the light-dependent stage of photosynthesis? (4)

- Water

- Inorganic phosphate

- ADP

- NADP

What substances are used in the light-independent stage of photosynthesis? (4)

- Reduced NADP

- ATP

- Carbon dioxide

- Ribulose bisphosphate

What are the end products of the light-dependent stage of photosynthesis? (3)

- ATP

- Reduced NADP

- Oxygen

What are the end products of the light-independent stage of photosynthesis? (4)

- NADP

- ADP

- Inorganic phosphate

- Carbohydrate

Why is high light intensity important for photosynthesis? (4)

- Light provides the energy for the light-dependent reactions.

- Higher intensity provides more energy for photosynthesis

- Chlorophyll a, chlorophyll b, and carotene absorb red and blue light but reflect green light, which is why plants appear green

- Having multiple photosynthetic pigments allows plants to absorb more light and increase the efficiency of photosynthesis

Why is an ideal temperature of 25°C crucial for photosynthesis? (4)

- Photosynthesis relies on enzymes like RuBisCo and ATP synthase

- Enzymes become inactive below 10°C and denature above 45°C

- High temperatures can cause stomata to close, limiting CO₂ intake, and may damage the thylakoid membranes, chloroplast membranes, or chlorophyll, reducing the rate of photosynthesis

- Greenhouses help maintain the optimum temperature for photosynthesis

What is the ideal CO₂ concentration for photosynthesis? (3)

- The atmospheric CO₂ concentration is normally 0.04%

- Increasing it to 0.4% can increase the rate of photosynthesis, but if it gets too high, stomata may close, limiting CO₂ intake

- Growers can increase CO₂ levels by burning propane

How do high temperatures affect photosynthesis? (3)

- Can cause stomata to close, limiting CO₂ intake

- Thylakoid membranes and chloroplast membranes may be damaged, reducing light-dependent reactions and enzyme activity in the Calvin cycle

- Chlorophyll may be damaged, reducing the plant's ability to absorb light.

How do factors limiting photosynthesis affect plant growth? (3)

- Light intensity, temperature, and CO₂ concentration

- Need to be at the right levels for photosynthesis to occur as quickly as possible

- If any one factor is too low or too high, it will limit the rate of photosynthesis and, consequently, plant growth

How does water stress limit photosynthesis? (2)

- When plants don't have enough water, the stomata close to conserve water

- This reduces the amount of CO₂ entering the leaf for the Calvin cycle, slowing down photosynthesis

How does light intensity affect the levels of GP, TP, and RuBP in photosynthesis? (3)

- Low light intensity reduces the supply of ATP and reduced NADP from the light-dependent stage

- This slows the conversion of GP to TP and RuBP

- As a result, GP levels rise (as it can still be made), but TP and RuBP levels fall

How do temperature and CO₂ concentration affect the levels of GP, TP, and RuBP in the Calvin cycle? (5)

- Low temperatures slow all reactions in the Calvin cycle

- Causing levels of GP, TP, and RuBP to fall

- High temperatures denature enzymes, leading to a similar drop in GP, TP, and RuBP

- Low CO₂ concentrations slow the conversion of RuBP to GP

- Causing RuBP levels to rise and GP and TP levels to fall

How can Canadian pondweed be used to investigate factors affecting the rate of photosynthesis? (3)

1. Used in a test tube with water connected to a capillary tube full of water and a syringe

2. A light source is placed at a specific distance, and the pondweed is left to photosynthesise

3. The oxygen released collects in the capillary tube, and the length of the gas bubble is measured to estimate the amount of O₂ produced

How do you measure the amount of oxygen produced during photosynthesis in the Canadian pondweed experiment? (3)

1. After photosynthesis, the syringe is used to draw the gas bubble in the capillary tube alongside a ruler

2. The length of the gas bubble is measured

3. Which is proportional to the amount of O₂ produced

What variables should be controlled when investigating the effect of light intensity on the rate of photosynthesis? (2)

- Temperature

- The time the pondweed is left to photosynthesise

How can you improve the precision of the experiment on light intensity and photosynthesis? (2)

- Repeat the experiment multiple times and calculate the average length of the gas bubble to make results more precise

- Change the distance of the light source and repeat the experiment to study the effect of light intensity