Photosynthesis

Question 1

What is photosynthesis?

Answer 1

Conversion of light energy into chemical energy (stored in chemical bonds of ATP and carbohydrates, made from CO2 and H2O)

Question 2

Where does photosynthesis generally occur?

Answer 2

Chloroplast of eukaryotic cells

Question 3

What pigments are involved in photosynthesis?

Answer 3

• chlorophyll a (main)
• chlorophyll b
• carotenoids (such as b-carotene)
• phycobilins
• bacteriochlorophyll (found in some photosynthetic bacteria)

Question 4

Describe the structure of chlorophyll.

Explain how the structures of chlorophyll a and b differ from each other.

Answer 4

• central magnesium atom surrounded by alternating single and double bonds forming a porphyrin ring to which a phytol chain is attached
• chlorophyll a has methyl (CH3) group attached to porphyrin ring
• chlorophyll b has aldehyde (COH) group attached to porphyrin ring

Question 5

How does chlorophyll absorb light energy?

Answer 5

• via process of excitation involving central magnesium atom surrounded by alternating single and double bonds that form a porphyrin ring
• when photons are absorbed, e- in mg atom are excited and this energy is funnelled off through the bonds of the porphyrin ring

Question 6

Describe the function of carotenoids and phycobilins.

Answer 6

Act as accessory pigments and assist in photosynthesis by absorbing wavelengths of light not readily absorbed by chlorophyll.

Question 7

Describe the properties and structure of carotenoids.

Answer 7

Typically yellow/orange in colour

Contains carbon ring and a long hydrocarbon chain in which single and double bonds alternate

Question 8

Describe the function of carotenoids specifically.

Answer 8

• absorb in blue-green spectrum
• protect against photo-oxidation
• extend light-harvesting range
• some herbicides work by blocking carotenoid synthesis

Question 9

Where are phycobilins found?

Answer 9

In red marine algae and some primitive cyanobacteria.

Question 10

What is the light-dependent reaction?

Answer 10

Converts light energy into electrical then chemical energy (in the form of ATP and NADPH) and produces O2 gas as waste product.

Question 11

Where does the light-dependent reaction occur?

Answer 11

Thylakoid membrane of chloroplast

Question 12

What are the two configurations of thylakoid membranes?

Answer 12

• stromal lamellae (individual sacs that traverse the chloroplast and site of PSI)
• granal lamellae (stack of thylakoids and site of PSII)

Question 13

What is the function of the chloroplast envelope?

How does the thylakoid membrane differ?

Answer 13

Chloroplast envelope forms a selective barrier that regulates the transfer of molecules into and out of the chloroplast.
Thylakoid membranes contain light-harvesting complexes which contain pigment molecules, electron transport complexes and ATP-synthesising complexes.
Within these light harvesting complexes are Photosystem I and Photosystem II.

Question 14

What is a photosystem?

Answer 14

A photosystem has light-harvesting pigments that contain proteins, 300-400 chlorophylls, and other pigments.

Question 15

Explain the key differences between photosystem I and II.

Answer 15

Photosystem II:

• contains chlorophyll a special pair called P680
• gets electrons from H2O

Photosystem I:

• contains chlorophyll a special pair called P700
• receives electrons that flow down electron transport chain from PSII

Question 16

Outline the steps involved in the light-dependent reaction (involving the photosystems).

Answer 16

1. In PSII, the P680 special pair absorbs light energy and enters excited (high-energy) state. Then rapidly expels electron. Electron donor then neutralises now positively charged P680 and this donor is then neutralised by removal of electrons from H2O, producing O2 and 4H+ for every 4 electrons lost.
2. These H+ protons accumulate in the thylakoid lumen. The movement of electrons from a higher to a lower energy state also generates energy that is used to pump H+ ions across the thylakoid membrane (from stroma outside into lumen inside), therefore generating H+ concentration gradient (higher concentration inside than outside) that will be used to make ATP.
3. Electrons expelled from PSII are then passed by an electron carrier to another protein complex called cytochrome b/f, which then passes the electrons onto an electron donor molecule in PSI.
4. PSI absorbs a photon, the energy of which allows P700 to enter the excited state and expel an electron to an electron acceptor molecule on the stromal side of the thylakoid membrane.
5. Electrons are removed from PSII and passed onto an acceptor protein called ferredoxin. Ferredoxin then passes electrons onto NADP+ reductase. Two electrons and one H+ proton reduce NADP+ to produce NADPH.
6. Buildup of H+ protons in thylakoid lumen generates electrochemical gradient of protons across the membrane (inside to out i.e lumen to stroma). This provides ATP synthase with the potential energy needed to synthesise ATP from
   ADP+Pi. (Pi is phosphate group) For every 3H+ protons, 1 ATP is produced. This is known as chemiosmosis.

Question 17

What provides the reaction of splitting electrons from H2O with enough energy?

Answer 17

Removal of electron from P680 in PSII. This is facilitated by a cluster of manganese ions bound to reaction centre proteins.

Question 18

What is non-cyclic phosphorylation?

Answer 18

It describes the process of ATP synthesis coupled with the one-way electron traffic from H2O to NADPH (the latter is non-cyclic electron transport).

Question 19

What is cyclic phosphorylation?

Answer 19

Electrons transported back to PSI by ferredoxin and the cytochrome b/f complex. Recycled electrons therefore are not used for NADPH production but still provide energy for ATP synthesis.

Question 20

Why is cyclic phosphorylation important?

Answer 20

It allows plants to increase the ratio of ATP:NADPH in order to meet the varying demands of chloroplast metabolism.

Question 21

Where is ATP synthase located?

What is ATP synthase composed of?

Answer 21

Stromal lamellae.

Proton channel and catalytic site where ATP is synthesised

Question 22

How does the production of NADPH differ in anoxygenic bacteria?

Answer 22

Instead of removing electrons from H2O, H2S is used. Oxygen is therefore not produced by photosynthesis and instead SULFUR is the by-product.

Question 23

Where is chlorophyll located in anoxygenic bacteria?

Answer 23

They lack chloroplasts, therefore chlorophyll is located in plasma membrane.

Question 24

What is the light-independent reaction?

Answer 24

Conversion of CO2 into carbohydrates, with ADP+Pi and NADP+ as byproducts.

Question 25

Where does the light-independent reaction occur?

Answer 25

Stroma of the chloroplast

Question 26

What encodes for the many ribosomes and enzymes needed for photosynthesis (specifically the carbon cycle)?

Answer 26

The stroma of the chloroplast contains multiple copies of the chloroplast genome.

Question 27

Outline the steps of the light-independent reaction.

Answer 27

1. Carbon Fixation:
   Attachment of CO2 to the 5 carbon sugar RUBP. This produces a short-lived 6-carbon intermediate. This reaction is catalysed by the enzyme RUBISCO.
2. This 6-carbon intermediate is rapidly converted into two 3-carbon PGA molecules.
3. The two PGA molecules are first of all phosphorylated using ATP to produce a double phosphorylated compound, leaving behind ADP as a byproduct.
4. PGA is then reduced (gains 2 electrons from NADPH) and dephosphorylated to produce PGAL (also known as G3P), leaving behind NADP+ and Pi as byproducts.
5. PGAL can then follow three pathways:
   - up to 2 out of every 12 PGAL molecules are exported from chloroplast into cytoplasm. Then combined and rearranged into fructose and glucose phosphates. Then condense to form sucrose.
   - PGAL molecules combine to form starch, which is stored in the chloroplast.
   - 10 PGAL molecules used to form six RUBP molecules to complete the cycle. This REGENERATION of RUBP uses six ATP molecules.

Question 28

What is the activity of RUBISCO regulated by?

Answer 28

Regulated by the activating protein RUBISCO activase.

Question 29

What is a key characteristic of RUBISCO?

Answer 29

Constitutes up to 50% of the soluble protein in chloroplasts, and is the most abundant protein on earth.

Question 30

What is photorespiration?

Answer 30

RUBISCO using oxygen as a substrate instead of CO2.

CO2 is produced and ATP is consumed in the process.

Question 31

What makes plants more likely to undergo photorespiration?

Answer 31

High temperatures. Plants will close stomata during hot weather in order to reduce water loss by evaporation, causing O2 from photosynthesis to build up inside the leaf.

Question 32

Is photorespiration a positive or negative thing? Explain why.

Answer 32

Negative. It wastes energy, steals carbon and decreases sugar synthesis.

Plants lose 25% to 50% of the carbon that could be fixed during photosynthesis.

Question 33

Why is it said that most plants undergo C3 photosynthesis?

Answer 33

Because the first product of carbon fixation is a 3C compound.

Question 34

What is C4 photosynthesis? (brief)

Answer 34

First product of carbon fixation is a 4C compound.

Carbon fixation occurs in mesophyll cell, Calvin cycle occurs in bundle sheath cell.

Question 35

Outline the steps of C4 photosynthesis.

Answer 35

1. CO2 is fixed in the mesophyll to form oxaloacetate. This reaction is catalysed by the enzyme PEP carboxylase, which has no tendency to bind to O2.
2. Oxaloacetate is the converted into malate (4C), which is transported using ATP into the bundle sheath cells.
3. Malate is then decarboxylated to form CO2 and pyruvate. Pyruvate is transported back into the mesophyll cells where it is converted back into PEP. Meanwhile, CO2 stays in the bundle sheath where it undergoes the Calvin cycle with Rubisco.

Question 36

What is distinctive about the leaf anatomy of C4 plants?

Answer 36

• vascular bundles are surrounded by a cylinder of bundle sheath cells and outer layer of mesophyll cells
• chloroplasts in bundle sheath and mesophyll are both different in structure and function.

Question 37

What is distinctive about the stomata of C4 plants?

Answer 37

Because they can concentrate CO2 within the plant, their stomatas are not as wide open and less water is consequently lost.

Question 38

How does C4 photosynthesis inhibit photorespiration?

Answer 38

Mechanism for concentrating CO2 within bundle sheath cells, which are relatively impermeable to CO2 and tend to hold it within them.

Question 39

What is CAM photosynthesis?

Answer 39

CAM plants open stomata at night, allowing CO2 to enter. Carbon fixation therefore occurs at night.

Malate is stored in the cytoplasm of the mesophyll cell (no bundle sheath cells involved) until daylight, which is when the Calvin cycle occurs.

Question 40

Why is it beneficial for CAM plants to open their stomata at night only?

Answer 40

At night, humidity is higher and temperature is cooler, therefore preventing water loss from leaves.