Module 5 Section 5 - Photosynthesis

Question 1

Why is energy important ?

Answer 1

Plants - photosynthesis, active transport of ions, dna replication and cell division

Animals - muscle contraction, maintenance of body temperature, dna replication and cell division

Microorganisms- DNA replication, cell division, protein synthesis and sometimes motility

Question 2

Overall equation of photosynthesis?

Answer 2

6CO2 + 6H2O + Energy -> C6H12O6 + 6O2

Question 3

Overall equation of aerobic respiration?

Answer 3

C6H12O6 + 6O2 -> 6CO2 + 6H2O + Energy

Question 4

What is the structure of ATP?

Answer 4

ATP is made from the nucleotide base adenine, combined with a ribose sugar and three phosphate groups.

Question 5

How is ATP synthesised?

Answer 5

ATP is synthesised from ADP and inorganic phosphate, using energy. The energy is stored as chemical energy in the phosphate bond. The enzyme ATP synthase catalyses this reaction. This process is known as phosphorylation.

Question 6

How is ATP broken down?

Answer 6

ATP diffuses to the part of the cell that needs energy and is broken down into ADP and inorganic phosphate. Chemical energy is released from the phosphate bond and is used by the cell. The enzyme ATPase catalyses this reaction. This process is hydrolysis as water is required.

Question 7

What are the properties of ATP that make it a good energy source?

Answer 7

1) ATP stores or releases only a small, manageable amount of energy at a time so no energy is wasted

2) it’s a small soluble molecule so is easily transported around the cell.

3) it’s easily broken down, so energy can be easily released.

4) it can transfer energy to another molecule by transferring one of its phosphate groups.

5) ATP can’t pass out the cell, so the cell always has an immediate supply of energy

Question 8

What is the compensation point?

Answer 8

The rate of photosynthesis is equal to the rate of respiration

Question 9

How to work out compensation point?

Answer 9

Measure the rate of oxygen production in a plant at different light intensities. When the net oxygen generation is 0 is the compensation point.

Question 10

What is the structure of chloroplasts?

Answer 10

Chloroplasts are small, flattened organelles in plant cells. They have a double membrane called the chloroplast envelope.
Thylakoids are stacked in the chloroplast into structures called grana.
The grana are linked together by bit of thylakoid membrane called lamellae

Question 11

What are photosynthetic pigments?

Answer 11

1) chloroplasts contain photosynthetic pigments (chlorophyll a, b and carotene). These substances absorb the light energy needed for photosynthesis.

2) the pigments are found in the thylakoid membranes, they’re attached to proteins. The protein and pigment is called a photo system.

Question 12

What are primary pigments?

Answer 12

Primary pigments are reaction centres where electrons are excited during the light-dependant reaction. In most plants this is chlorophyll a.

Question 13

What are accessory pigments?

Answer 13

Accessory pigments make up light-harvesting systems. These surround reaction centres and transfer light energy to them to boost the energy available for electron excitation to take place.

Question 14

What wavelengths do the photo systems absorb?

Answer 14

Photosystem 1 absorbs 700nm
Photosystem 2 absorbs 680nm

Question 15

What is the function of the Stroma?

Answer 15

Stroma is a gel-like substance called the Stroma. It contains enzymes, sugars and organic acids. Chloroplasts have their own DNA. It’s found in the Stroma and is often circular. There can be multiple copies in each chloroplast. Carbohydrates produced by photosynthesis that are not used away are stored as starch grains in the Stroma.

Question 16

What is oxidation and reduction?

Answer 16

Oxidation - lost electrons or gained oxygen
Reduction - gained electrons or lost oxygen

Question 17

What coenzyme is used in photosynthesis?

Answer 17

NADP transfers hydrogen from one molecule to another.

Question 18

How are thylakoid membranes in photo systems linked?

Answer 18

By electron carriers. Electron carriers form an electron transport chain.

Question 19

First stage of non-cyclic photophosphorylation?

Answer 19

1) light energy is absorbed by Photosystem 2. This excites electrons in chlorophyll. The electron moves to a higher energy level and these high energy electrons move along the electron transport chain to Photosystem 1.

2) as the excited electrons from chlorophyll leaves Photosystem 2 to move along the electron transport chain, they must be replaced. Light energy splits water into protons (H+ ions), electrons and oxygen. The reaction is:

H2O -> 2H+ +. 1/2 O2

Question 20

What is the second stage of non-cyclic photophosphorylation?

Answer 20

1) The excited electrons lose energy as they move along the electron transport chain. This energy is used to transport protons (H+) into the thylakoid via membrane proteins called proton pumps, so that the thylakoid has a higher concentration of protons than the Stroma.

2) This forms a proton gradient across the membrane. Protons move down their concentration gradient into the Stroma, via ATP synthase. The energy from this movement combines ADP and inorganic phosphate to form ATP. This is known as chemiosmosis.

Question 21

What is the third stage of non-cyclic photophosphorylation?

Answer 21

Light energy is absorbed by Photosystem 1, which excites the electrons again to an even higher energy level. The electrons are then transferred to NADP, along with a proton from the Stroma, to form reduced NADP.

Question 22

What is cyclic photophosphorylation?

Answer 22

Cyclic photophosphorylation only produces ATP and only uses Photosystem 1. The electrons are not passed to NADP but are passed back to PS1 by electron carriers. This means the electrons are recycled and can repeatedly flow through PS1. This process doesn’t produce any reduced NADP or oxygen and only produces small amounts of ATP.

Question 23

Where does the light independant stage of photosynthesis occur?

Answer 23

In the Stroma

Question 24

What are the three steps of the light independent stage of photosynthesis?

Answer 24

1) formation of glycerate 3-phosphate
2) formation of triode phosphate
3)regeneration of ribulose biphosphate (RuBP)

Question 25

What happens in the first stage of the light independant reaction?

Answer 25

1) carbon dioxide enters the leaf through the stomata and diffuses into the Stroma of the chloroplast.

2) here it combines with RuBP (5C) to give an unstable 6-carbon compound, which quickly breaks down into two molecules of a 3 carbon compound called glycerate 3-phosphate (GP).

3) Ribulose biphosphate carboxylate (RuBisCO) catalyses the reaction between carbon dioxide and RuBP.

Question 26

What happens in the second stage of the light independent reaction?

Answer 26

1) The 3C compound GP is reduced to a different 3C compound called triose phosphate (TP). ATP from light dependant reaction provides energy for this.

2) this reaction requires H+ ions, which come from reduced NADP (from light dependent reaction). Reduced NADP is recycled to NADP (for use in light dependent reaction again).

3) Triose phosphate is then converted into many useful organic compounds.

Question 27

What happens in the third stage of the light independent reaction?

Answer 27

1) five out of six molecules of TP produced in the cycle aren’t used to make useful organic compounds, but to regenerate RuBP.

2) Regenerating RuBP uses the rest of the ATP produced in the light dependant reaction.

Question 28

How are hexose sugars produced from the light independant reaction?

Answer 28

The Calvin cycle needs to turn 6 times to make one hexose sugar. This is because 3 turns produce 6 molecules of TP and 5/6 of these are used to regenerate RuBP. As 2 TP molecules are needed to produce one hexose sugar, the cycle needs to make one hexose sugar. Six turns of the cycle need 18 ATP and 12 reduced NADP from the light dependant reaction.

Question 29

What organic substances are produced from the Calvin cycle?

Answer 29

Carbohydrates - hexose sugars made from 2 TP molecules. Larger carbohydrates are made by joining hexose sugars together in different ways.

Lipids - these are made using glycerol, which is synthesised from TP, and fatty acids which is synthesis from glycerate 3-phosphate.

Amino acids- some are made from glycerate 3-phosphate

Question 30

What are the optimum conditions for photosynthesis?

Answer 30

1) high light intensity of a certain wavelength. Light energy is needed for the light dependant reaction, as the more light there is, the more energy is produced. Only certain wavelengths (red+blue light) are absorbed by chlorophyll a,b and carotene.

2) temperature around 25 degrees. Photosynthesis involves enzymes so if temp falls below 10 degrees, the enzymes become inactive but if they increase above 45 degrees they denature.

3) carbon dioxide at 0.4%. CO2 makes up 0.04% of the gases in the atmosphere and increasing this to 0.4% gives a higher rate of photosynthesis. Any higher and the stomata close.

Question 31

Why is high temperatures bad for the environment?

Answer 31

1) stomata close to avoid losing too much water. This causes the photosynthesis to slow down as less cO2 enters the leaf.

2) the thylakoid membranes may be damaged which could reduce the rate of the light dependant reaction by reducing the number of sites available for electron transfer.

3) the membrane of the chloroplast could be damaged, which could cause enzymes important in the Calvin cycle to be released into the cell which reduces the rate of light independent reaction.

4) chlorophyll could be damaged which would reduce the amount of pigment that could absorb light energy which reduces the rate of light-dependant reactions.

Question 32

How does water stress affect photosynthesis?

Answer 32

When plants don’t have enough water, their stomata close to preserve the little water they have, leading to less CO2 entering the leaf for the Calvin cycle, and slowing photosynthesis down.

Question 33

How does light intensity affect the Calvin cycle?

Answer 33

In low light intensities, the products of the light dependent stage (reduced NADP and ATP) will be in short supply. This means the conversion of GP to TP and RuBP is slow. So GP levels rise and sings being used up as quickly so TP and RuBP will fall as they’re being used to make GP, but aren’t being remade as quickly.

Question 34

How does temperature affect the Calvin cycle?

Answer 34

All reactions in the Calvin cycle have enzymes eg RuBisCO. At low temps all the reactions are slower as the enzymes work more slowly. This means levels of RuBP, GP and TP will fall. GP, TP, and RuBP are all affected in the Same way at high temps as the enzymes begin to denature.

Question 35

How does carbon dioxide concentration affect the Calvin cycle?

Answer 35

At low CO2 concentrations, conversion of RuBP to GP is also slow as there’s less CO2 to combine with RuBP to make GP. The level of RuBP will continue to rise as it’s still being made but isn’t used up. The levels of GP and TP will fall as they are used to make RuBP but aren’t being remade.

Question 36

How does carbon dioxide concentration affect the Calvin cycle?

Answer 36

At low CO2 concentrations, conversion of RuBP to GP is also slow as there’s less CO2 to combine with RuBP to make GP. The level of RuBP will continue to rise as it’s still being made but isn’t used up. The levels of GP and TP will fall as they are used to make RuBP but aren’t being remade.