5.1 - Photosynthesis

Question 1

Where do the light-dependent & light independent reactions occur in plants?

Answer 1

• Light-dependent: in the thylakoids of chloroplasts
• Light-independent: stroma of chloroplasts

Question 2

Explain the role of light in photoionisation

Answer 2

Chlorophyll molecules absorb energy from photons of light. This ‘excites’ 2 electrons (raises them to a higher energy level), causing them to be released from the chrolophyll.

Question 3

Name the 2 main stages involved in ATP production in the light-dependent reaction

Answer 3

1. Electron transfer chain
2. Chemiosmosis

Question 4

What happens in the electron transfer chain (ETC)?

Answer 4

Electrons released from chlorophyll move down a series of carrier proteins embedded in the thylakoid membrane & undergo a series of redox reactions which releases energy.

Question 5

How is a proton concentration gradient established during chemiosmosis?

Answer 5

Some energy that is released from the ETC is coupled to the active transport of H+ ions (protons) from the stroma into the thylakoid space.

Question 6

How does chemiosmosis produce ATP in the light-dependent stage?

Answer 6

H+ ions (protons) move down their concentration gradient from the thylakoid space into the stroma via the channel protein ATP synthase.
ATP synthase catalyses ADP + Pi –> ATP

Question 7

Explain the role of light in photolysis

Answer 7

Light energy splits molecules of water 2H2O –> 4H+ + 4e- + O2

Question 8

What happens to the products of the photolysis of water?

Answer 8

• H+ ions: move out of thylakoid space via ATP synthase & are used to reduce the coenzyme NADP
• e-: replace electrons lost from chlorophyll.
• O2: used for respiration or diffuses out of leaf as waste gas.

Question 9

How and where is reduced NADP produced in the light-dependent reaction?

Answer 9

• NADP + 2H+ + 2e- —-> reduced NADP
• Catalysed by dehydrogenase enzymes.
• Stroma of chloroplasts

Question 10

Where do the H+ ions and electrons used to reduce NADP come from?

Answer 10

• H+ ions: photolysis of water
• Electrons: NADP acts as the final electron acceptor of the electron transfer chain

Question 11

Name 3 main stages in the Calvin cycle

Answer 11

1. Carbon fixation
2. Rduction
3. Regeneration

Question 12

What happens during carbon fixation?

Answer 12

• Reaction between CO2 & ribulose biphosphate (RuBP) catalysed by rubisco.
• Forms unstable 6C intermediate that breaks down into 2x glycerate 3-phosphate (GP).

Question 13

What happens during reduction (in the Calvin cycle)?

Answer 13

• 2 x GP are reduced to 2 x phosphate (TP)
• Requires 2 x reduced NADP & 2x ATP
• Forms 2 x NADP & 2 x ADP

Question 14

What happens during regeneration (in the calvin cycle)?

Answer 14

• After 1C leaves the cycle, the 5C coumpound RuP forms
• RuBP is regenerated from RuP using 1 x ATP
• Forms 1 x ADP

Question 15

How does the light-independent reaction result in the production of useful organic substances?

Answer 15

1C leaves the cycle (i.e. some of the TP is converted into useful organic molecules).

Question 16

State the roles of ATP & (reduced) NADP in the light-independent reaction.

Answer 16

• ATP: redction of GP to TP & provides phosphate group to convert RuP into RuBP.
• (reduced) NADP: coenzyme transports electrons needed for reduction of GP to TP.

Question 17

State the number of carbon atoms in RuBP, GP & TP.

Answer 17

RuBP: 5
GP: 3
TP: 3

Question 18

Describe the structure of a chloroplast.

Answer 18

• Usually disc-shaped
• Double membrane
• Thylakoids: flattened discs stack to form grana
• Intergranal lamellae: tubular extensions attach thylakoids in adjacent grana
• Stroma: fluid-filled matrix

Question 19

How does the structure of the chloroplast maximise the rate of the light-dependent reaction?

Answer 19

• ATP synthase channels within granal membrane.
• Large surface area of thylakoid membrane for ETC.
• Photosystems position chlorophyll to enable maximum absorbtion of light.

Question 20

How does the structure of chloroplast maximise the rate of the light-independent reaction?

Answer 20

• Own DNA & ribosomes for synthesis of enzymes e.g. rubisco.
• Concentration of enzymes & substrates in stroma is high.

Question 21

Define ‘limiting factor’.

Answer 21

Factor that determines maximum rate of a reaction, even if other factors change to become more favourable.

Question 22

Name 4 environmental factors that can limit the rate of photosynthesis.

Answer 22

• Light intensity (light-dependent stage)
• CO2 levels (light-independent stage)
• Temperature (enzyme controlled steps)
• Mineral/ magnesium levels (maintain normal functioning of chlorophyll)

Question 23

Outline some common agricultural practices used to overcome the effect of limiting factor in photosynthesis.

Answer 23

• Artificial light, especially at night.
• Artificial heating
• Addition of CO2 to greenhouse atmosphere

Question 24

Why do farmers try to overcome the effect of limiting factors?

Answer 24

• To increase yield
• Additional cost must be balanced with yield to ensure maximun profit

Question 25

Suggest how a student could investigate the effect of a named variable on the rate of photosynthesis.

Answer 25

Dependent variable: rate of O2 production/ CO2 consumption 1. Use a potometer 2. Place balls of calcium alginate containing green algae in hydrogencarbonate indicator (colour change orange --> magenta as CO2 is consumed & pH increases)

Question 26

State the purpose and principle of paper chromatography

Answer 26

Molecules in a mixture are seperated based on their relative attraction to the mobile phase (running solvent) vs the stationary phase (chromatography paper).

Question 27

Outline a method for extractiong photosynthetic pigments

Answer 27

Use a pestle and mortar to grind a leaf with an extraction solvent e.g. propanone

Question 28

What are Rf values? How can they be calculated?

Answer 28

- Ratios that allow comparison of how far molecules have moved in chromatograms - Rf value = distance between origin and centre of pigment spot/ distance between origin and solvent front.

Question 28

Outline a how paper chromatography can be used to seperate photosynthetic pigments.

Answer 28

1. Use a capillary tube to spot pigment extract onto pencil 'start time' (origin) 1cm above bottom of paper. 2. Place chromatography paper in solvent (origin should be above solvent level). 3. Allow solvent to run until it almost touched the other end of the paper. Pigments move different distances.

Question 29

Can you explain how the plant leaf is adapted to carry out photosynthesis?

Answer 29

- Large SA: absorb as much sunlight as possible - Arrangement of leaves: minimises overlapping and so avoids shadowing of on leaf over another - Thin: most light is absorbed in 1st few micrometres of lead + diffusion pathway distance for gases is kept short - Transparent cuticle and epidermis: let light through to p.s mesophyll cells beneath - Long, narrow upper mesophyll cells packed w/ chloroplasts: collect sunlight - Numerous stomata for gaseous exchange: all mesophyll cells are only a short diffusion pathway from one - Stomata that open and close: responds to changes in light intensity - Many air spaces: in lower mesophyll layer to allow rapid diffusion in gas phase of CO2 and O2 - Network of xylem: brings water to leaf cells + phloem: carries away sugars produce during p.s

Question 30

Can you describe the main stages of photosynthesis?

Answer 30

1) Capturing of light energy: by chloroplast pigments such as chlorophyll 2) Light-dependent reaction: light energy absorbed is converted in chemical bonds. During process electron flow created by effect of light on chlorophyll, causing water to split (photolysis) into protons, electrons and oxygen. Products are reduced NADP, ATP and oxygen 3) Light-independent reaction: protons (H+) used to produce sugars and other organic molecules

Question 31

Can you explain the processes of oxidation and reduction?

Answer 31

OILRIG: electrons Oxidation: loss of hydrogen or gain of oxygen Reduction: gain H or lose O2

Question 32

Can you explain how ATP is made during the light-dependent reaction?

Answer 32

1. PSII Photoionisation: Light excites electrons in PSII, which move down the electron transport chain to PSI. 2. Photolysis: Light splits water into H⁺, electrons (replacing those lost in PSII), and oxygen. 3. H⁺ Transport: Electron energy is used to move H⁺ across the membrane, forming a proton gradient. 4. ATP Synthesis: H⁺ flows through ATP synthase, generating ATP from ADP and Pi. 5. PSI Photoionisation: Electrons from PSI reduce NADP to NADPH for the next reaction stage. 6. Chemiosmosis: Electron movement creates a proton gradient that drives ATP production.

Question 33

Can you describe the role of photolysis in the light-dependent reaction?

Answer 33

- Source of Electrons: Provides replacement electrons to Photosystem II. - Oxygen Production: Releases O₂ as a by-product. - Proton Gradient: Increases H⁺ concentration in the thylakoid lumen, essential for ATP synthesis. Photolysis, therefore, is vital because it enables ATP production, provides electrons for NADPH formation, and releases oxygen into the atmosphere.

Question 34

Can you explain how chloroplasts are adapted to carry out the light-dependent reaction?

Answer 34

- The thylakoid membrane provides a large SA for chlorophyll, electron carriers, and enzymes for the light-dependent reaction. - Protein networks in grana optimize chlorophyll placement for maximum light absorption. - ATP synthase channels in the membrane produce ATP and establish a proton gradient. - Thylakoids contain DNA and ribosomes to rapidly produce proteins needed for the reaction.

Question 35

Can you explain how carbon dioxide absorbed by plants is incorporated into organic molecules?

Answer 35

- Carbon dioxide diffuses through stomata, dissolves in water around mesophyll cells, and enters the chloroplast stroma. - In the stroma, CO₂ reacts with RuBP, catalyzed by rubisco, to form GP. GP is then reduced to TP using ATP and reduced NADP from the light-dependent reaction. - NADP is recycled back to the light-dependent reaction, and most TP regenerates RuBP using ATP, while some TP forms organic substances like glucose.

Question 36

Can you describe the roles of ATP and reduced NADP in the light-independent reaction?

Answer 36

-The reduced NADP provides the reducing power (hydrogen) and is converted back to NADP which is then reduced again in the light-dependent reactions. -ATP is also used to provide energy for the conversion. It is converted into ADP + Pi, which are reconverted into ATP in the light-dependent reactions.

Question 37

Can you describe the events of the Calvin cycle?

Answer 37

1) CO2 from atmo. Diffuses into leaf through stomata + dissolves in H2O around walls of meso. Cells. Then diffuses through cell-surface membrane, cytoplasm and chloroplast membranes into stroma of chloro. 2) In the stroma: CO2 reacts w/ 5C RuBP, the reaction catalysed by rubisco 3) This provides 2 molecules of 3C GP 4) Reduced NADP (NADPH) - from light dep- used to reduce GP to TP using energy supplied by ATP 5) NADP is re-formed and goes back light-dependent reaction to be reduced again by accepting more protons 6) Some TP molecules converted to organic substances that plant requires (starch, cellulose, lipids, glucose, amino acids, nucleotides) 7) Most TP used to regen RuBP using ATP from light-depen reaction