5.2.1 Photosynthesis

Question 1

What is the difference between autotrophic and heterotrophic organisms?

Answer 1

• Autotrophic - Produce their own food through photosynthesis, e.g. plants and algae.
• Heterotrophic - Obtain their food by eating other organisms, e.g. animals.

Question 2

🌱What is photosynthesis?

Answer 2

A process where light energy is used to convert carbon dioxide and water into glucose and oxygen.

Question 3

What is cellular respiration?

Answer 3

The breakdown of respiratory substrates, like glucose, to release energy stored temporarily as ATP.

Question 4

What are the three main stages of photosynthesis?

Answer 4

1. Capturing light energy by pigments (chlorophyll)
2. Light-dependent reaction - Non-cylic and Cyclic Photophosphorylation, occurs in thylakoid memb., light energy converted into chemical.
3. Light-independent reaction - Calvin cycle, occurs in stroma, sugars and other organic are produced

Question 5

What is photophosphorylation?

Answer 5

Adding phosphate to a molecule using light

Question 6

What is photolysis?

Answer 6

The splitting of a molecule using light energy.
2H₂O → 4H⁺ + 4e⁻ + O₂

Question 7

What is decarboxylation and dehydrogenation?

Answer 7

Decarboxylation - Removal of carbon dioxide from a molecule
Dehydrogenation - Removal of hydrogen from a molecule.

Question 8

🍃Where are chloroplasts mainly found in plants?

Answer 8

The mesophyll tissue of leaves.

Question 9

Describe the internal of a chloroplast.

Answer 9

• Thylakoids - Flattened sacs contain complexes of pigments like chlorophyll in their membranes to absorb light for the light-dependent reaction.
• Grana - These are stacks of thylakoids.
• Lamellae - These are membranous extensions that connect thylakoids.
• Stroma - This is a fluid surrounding the thylakoids where the light-independent reaction occurs.
• Starch grains - These store sugars.
• Ribosomes - Protein synthesis in the chloroplast.
• Chloroplast DNA - Has genes that code for proteins involved in photosynthesis.

Question 10

What are the main photosynthetic pigments in plants?

Answer 10

Primary pigment = Chlorphyll a
Accessory pigment = Chlorphyll b, Xanthophylls and Carotenoids

Question 11

What is a Primary Pigment?

Answer 11

They are reaction centres, where electrons get excited during light-dependent reaction

Question 12

Why do plants use multiple pigments for photosynthesis?

Answer 12

To broaden the range of wavelengths of light that can be absorbed and used for photosynthesis (exciting electrons).

Question 13

What are the two components of a photosystem?

Answer 13

Light-harvesting complex - Contains accessory pigments
Reaction centres - Contains primary pigment (chlorophyll a).

Question 14

Describe the two photosystems.

Answer 14

• Photosystem I (PSI), absorbs light best at a wavelength of 700nm
• Photosystem II (PSII) absorbs light best at 680 nm

Question 15

What is the role of a photosystem in photosynthesis?

Answer 15

• Absorbs and transfers light energy
• Reaction centre emits high-energy electrons to drive the light-dependent reactions

Question 16

How are photosynthetic pigments in plants seperate and identified?

Answer 16

Thin Layer Chromatography (TLC)

Question 17

How is the Rf value calculated in chromatography?

Answer 17

Rf = Distance moved by pigment / Distance moved by solvent

Question 18

How is the light energy absorbed by photosystems used in the light-dependent reaction?

Answer 18

1. Making ATP from ADP and Pi (Photophosphorylation)
2. Making reduced NADP from NADP
3. Splitting water into protons, electrons and oxygen (Photolysis).

Question 19

What are the two types of photophosphorylation?

Answer 19

• Non-cyclic photophosphorylation – uses PSII and PSI, produces ATP, reduced NADP, and O₂
• Cyclic photophosphorylation – uses PSI only, produces ATP only

Question 20

Outline the process of Non-cyclic photophosphorylation.

Answer 20

1. Light energy is absorbed by PSII (P680), exciting e⁻ in chlorophyll allowing them to move to higher energy level. These high energy e⁻ move along ETC to PSI (P700).
2. As excited e⁻ move along ETC and leave PSII, they must be replaced. Light energy splits H₂O into protons, electrons and oxygen (Photolysis).
   2H₂O → 4H⁺ + 4e⁻ + O₂
3. The excited e⁻ lose energy as they move along ETC. This energy is used to transport H⁺ into thylakoid via proton pumps, conc. of H⁺ in thylakoid is higher than in stroma = proton gradient.
4. Then, protons move down their conc. gradient into the stroma, via ATP Synthase. The energy from this movement combines ADP and Pi to form ATP (Chemiosmosis).
5. Light energy is also absorbed by PSI, which excited the e⁻ to an even higher energy level. Finally, the e⁻ are transferred to NADP, along with a proton from the stroma, to form reduced NADP.

Question 21

Describe the process of Cyclic Photophosphorylation.

Answer 21

1. Uses PSI only
2. Light excites electrons → pass through ETC
3. Energy pumps protons for ATP synthesis
4. Electrons return to PSI (no photolysis or NADP reduction)

Produces: ATP only (no reduced NADP or O₂)
It is called ‘cyclic’ as the e⁻ are not passed onto NADP, but are passed back to PSI via electron carriers.

Question 22

Where in the chloroplast does the Calvin cycle take place?

Answer 22

Stroma

Question 23

What are the main reactants and products of the Calvin cycle?

Answer 23

Reactants: CO₂, ATP, reduced NADP

Products: TP (used to make glucose and other molecules), NADP, ADP + Pi

Question 24

Describe the three main stages of the Calvin cycle.

Answer 24

1. Carbon fixation – CO₂ reacts with 5C compound ribulose biphosphate (RuBP) to form unstable 6C compound, which splits into 2x 3C GP molcules, catalysed by enzyme rubisco.
2. Reduction of GP – GP is reduced into triose phosphate (TP), requires energy so 2ATP → 2ADP + 2Pi and requires protons and electrons from 2 reduced NADP (returns to light-dependent reaction).
3. Regeneration of RuBP – Most TP regenerates RuBP using ATP, the rest is used to make other organic molecules.

For each cycle of the Calvin Cycle, 5 carbons are used to regenerate RuBP (5C), and only one carbon from CO₂ is available to make new organic molecules.
So 6 turns of the cycle make 1 glucose (6C)

Question 25

What can TP be used to make?

Answer 25

* Sugars (e.g. glucose, sucrose, starch, cellulose) * Amino acids * Lipids * Nucleotides

Question 26

How many ATP and reduced NADP would be needed to produce glucose (6C)?

Answer 26

Six turns of the Calvin Cycle (light-independent) 18 ATP and 12 reduced NADP (3 ATP and 2 reduced NADP per cycle)

Question 27

What 5 key requirements for photosynthesis to occur?

Answer 27

* Photosynthetic pigments - Absorb light energy * Carbon dioxide - Source of carbon to make glucose * Water - Provides electrons and source of protons * Light - Provides energy to split the water, produce ATP and to reduce NADP. * Suitable Temperature - Provides kinetic energy for reactions, and optimal conditions for enzyme activity.

Question 28

What does "**limiting factor**" mean in photosynthesis?

Answer 28

The factor in shortest supply that **limits** the rate of photosynthesis. Increasing it will increase the rate, up to a point.

Question 29

What four main factors limit the rate of photosynthests?

Answer 29

Light intensity Carbon dioxide concentration Temperature Water stress (stomatal closure)

Question 30

How does light intensity affect photosynthesis? (draw a graph)

Answer 30

Graph should go up and then plateau * **Low** light intensity - Limits light-dependent reaction, so not much ATP and reduced NADP produced, slowing down light-independent stage, rate is low. * **Intermediate** light intensity - More ATP and NADPH from light-dependent stage→ faster Calvin cycle and faster rate of photosynthesis * **Very high** light intensity - more light than needed, so another factor becomes limiting, rate plateaus..

Question 31

How does **CO₂ concentration** affect photosynthesis?

Answer 31

Low - Limits light-independent stage as less CO₂ is fixed, reducing production of GP and TP, rate is low Intermediate - Allows faster production of GP and TP, increasing rate of photosynthesis Very high - another factor becomes limiting, rate plataeus.

Question 32

How does **temperature** affect rate of photosynthesis?

Answer 32

Low - Provides little kinetic energy, enzymes may be inactive, slowing enzyme-catalysed stages of photosynthesis. Rate of photosynthesis is low Intermediate - Increases kinetic energy supplied, optimal conditions for enzyme activity, rate is high High - Enzymes denature, rate of photosynthesis drops dramatically.

Question 33

How does temperature effect the plant's ability to photosynthesise?

Answer 33

* **Enzymes**: If temp. falls below 10 °C, enzymes become inactive, but if temp. is more that 45 °C they may start to denature. Also at high temperatures: * **Stomata close** to avoid loosing too much water, less CO₂ enters leaf, slowing rate of photosynthesis. * **Thylakoid membranes** may be damaged, could reduce rate of light-dependent stage by reducing number of sites available for electron transfer * **Chloroplast membrane** could also be damaged, enzymes and protons may leave, reducing rate. * **Chlorophyll** could be damaged, reduce amount of pigment that can absorb light energy.

Question 34

💧How does water stress affect the rate of photosynthesis?

Answer 34

1. Water stress causes **stomata to close** to reduce water loss. 2. Closed stomata **limit CO₂** uptake, reducing carbon fixation in the **Calvin cycle**. 3. Less CO₂ → slower production of GP and TP, lowering photosynthesis rate. Prolonged water stress may damage chloroplasts and enzymes, further reducing photosynthesis.

Question 35

🌞What enzymes and coenzymes are involved in each stage of photosynthesis?

Answer 35

Light-dependent reaction (in thylakoid membranes): * ATP synthase * Photolysis enzymes * Reduced NADP Light-independent reaction (Calvin Cycle) (in stroma) * Rubisco * Reduced NADP