Section 5 - Photosynthesis

Question 1

Why is energy important?

Answer 1

Living things need energy for biological processes to occur. Without energy, these biological processes would stop and the plant, animal or microorganism would die.

Question 2

Give examples as to why energy is important.

Answer 2

• plants need energy for things like photosynthesis, active transport (e.g. to take in minerals via their roots), DNA replication and cell division.
• animals need energy for things like muscle contraction, maintenance of body temperature, active transport,DNA replication and cell division.
• microorganisms need energy for things like DNA replication, cell division, protein synthesis and sometimes motility (movement).

Question 3

Describe photosynthesis and give its equation.

Answer 3

Plants can make their own food (glucose). They do this using photosynthesis. Photosynthesis is the process where energy from light is used to make glucose from water and carbon dioxide. The light energy is converted to chemical energy in the form of glucose. The overall equation is:
6CO2 + 6H2O + energy ——————-> C6H12O6 + 6O2
Energy is stored in the glucose until the plants release it by respiration. Animals can’t make their won food. So they obtain glucose by eating plants (or other animals), then respire the glucose to release energy.

Question 4

Describe respiration and energy.

What are the two types of respiration?

Answer 4

Living cells release energy from glucose - this process is called respiration. This energy is used power all the biological processes in a cell. There are two types of respiration:
- aerobic respiration - respiration using oxygen.
- anaerobic respiration - respiration without oxygen.
Aerobic respiration produces carbon dioxide and water, and releases energy. The overall equation is:
C6H12O6 + 6O2 ———-> 6CO2 + 6H2O + energy

Question 5

What is ATP and how does it relate to respiration?

Answer 5

As you learned in module 2, ATP (adenosine triphosphate) is the immediate source of energy in a cell.
A cell can’t get its energy directly from glucose. So, in respiration, the energy released from glucose is used to make ATP. ATP is made from the nucleotide base adenine, combined with a ribose sugar and three phosphate groups. It carries energy around the cell to where it’s needed.

Question 6

Describe how ATP is synthesised.

What is this process called and what does it do?

Answer 6

ATP is synthesised from ADP (adenosine diphosphate) and inorganic phosphate using energy from an energy-releasing reaction, e.g. the breakdown of glucose in respiration. The energy is stored as chemical energy in he phosphate bond. The enzyme ATP synthase catalyses this reaction.
This process is known as phosphorylation - adding phosphate to a molecule. ADP is phosphorylated to ATP.
ATP then diffuses to the part of the cell that need energy. Here, it’s broken down back into ADP and inorganic phosphate. Chemical energy is released from the phosphate bond and used by the cell. ATPase catalyses this reaction.
This process is known as hydrolysis. It’s the splitting (lysis) of a molecule using water (hydro).

Question 7

What are ATP’s specific properties?

Answer 7

ATP has specific properties that make it a good energy source.

• ATP stores or releases only a small, manageable amount of energy at a time, so no energy is wasted.
• It’s a small, soluble molecule so it can be easily transported around the cell.
• It’s easily broken down, so energy can be easily released.
• It can transfer energy to another molecule by transferring one of its phosphate groups.
• ATP can’t pass out of the cell, so the cell always has an immediate supply of energy.

Question 8

What is the compensation point?

Answer 8

Plants carry out both photosynthesis and respiration. Both processes can occur at the same time and at different rates. The rate at which photosynthesis takes place is partly dependent on the light intensity of the environment that the plant is in.
There’s a particular level of light intensity at which the rate of photosynthesis exactly matches the rate of respiration. This is called the compensation point for light intensity.

Question 9

How do you work out the compensation point?

Answer 9

One way to work out the compensation point for a plant is to measure the rate at which oxygen is produced and used by a plant at different light intensities. Because photosynthesis produces oxygen and respiration uses it, in this case, the compensation point is the light intensity at which oxygen is being used as quickly as it is produced. The rate of carbon dioxide production and use could also be measured - photosynthesis uses carbon dioxide and respiration produces it.

Question 10

Describe chloroplasts.

Answer 10

Photosynthesis takes place in the chloroplasts of plant cells. Chloroplasts are small, flattened organelles found in plant cells. They have a double membrane called the chloroplast envelope. Thylakoids (fluid-filled sacs) are stacked up in the chloroplast into structures called grana (singular=granum). The grana are linked together by bits of thylakoid membrane called lamellae (singular-lamella).

Question 11

What are photosynthetic pigments and where are they found?

What is the name given to both the protein and the pigment?

Answer 11

Chloroplasts contain photosynthetic pigments (e.g. chlorophyll a, chlorophyll b and carotene). These are coloured substances that absorb the light energy needed for photosynthesis. The pigments are found in the thylakoid membranes - they’re attached to proteins.
The protein and pigment is called a photosystem.

Question 12

What are the two types of photosynthetic pigment contains within a photosystem?

Answer 12

A photosystem contains two types of photosynthetic pigments - primary pigments and accessory pigments.

Question 13

What are primary pigments?

Answer 13

Primary pigments are reaction centres where electron are excited during the light-dependent reaction - in most chloroplasts the primary pigment is chlorophyll a.

Question 14

What are accessory pigments?

Answer 14

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

Question 15

What are the two photosystem used by plants to capture light energy?

Answer 15

There are two photosystems used by plants to capture light energy. Photosystem l (or PSI) absorbs light best at a wavelength of 700nm and photosystem ll (PSII) absorbs light best at 680nm.

Question 16

Describe the stroma.
How is the DNA stored in the chloroplast?
How are carbohydrates stored?

Answer 16

Contained within the inner membrane of the chloroplast and surrounding the thylakoids 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 and not used straight away are stored as grains in the stroma.

Question 17

What are redox reactions and when do they occur?

Answer 17

Redox reactions are reactions that involve oxidation and reduction.
They occur in photosynthesis (and in respiration), so it is important to be your head around them:
- If something is reduced, it has gained electrons (e-), and may have gained hydrogen or lost oxygen.
- If something is oxidised it has lost electrons, and may have lost hydrogen or gained oxygen.
- Oxidation of one molecule always involves reduction of another molecule.

Question 18

What is a coenzyme and how do they work?

Name a coenzyme used in photosynthesis and its function.

Answer 18

A coenzyme is a molecule that aids the function of an enzyme. They usually work by transferring a chemical group from one molecule to another.
A coenzyme used in photosynthesis is NADP.
NADP transfers hydrogen from one molecule to another - this means it can reduce (give hydrogen to) or oxidise (take hydrogen from) a molecule.

Question 19

What are the stages of photosynthesis?

Answer 19

There are actually two stages that make up photosynthesis - the light-dependent reaction and the light-independent reaction.

Question 20

Describe the light-dependent reaction stage.

Answer 20

As the name suggests, this reaction needs light energy. It takes place in the thylakoid membranes of the chloroplasts. Here, light energy is absorbed by photosynthetic pigments in the photosystem and converted to chemical energy. The light energy is used to add a phosphate group to ADP to form ATP, and to reduce NADP to form reduced NADP. (Reduced NADP is an energy-rich molecule because it can transfer hydrogen, and so electrons, to other molecules.) ATP transfers energy and reduced NADP transfers hydrogen to the light-independent reaction. During the process water is oxidised to oxygen.

Question 21

Describe the light-independent reaction (the Calvin cycle).

Answer 21

As the name suggests, this reaction doesn’t use light energy directly. (But it does rely on the products of the light-dependent reaction.) It takes place in the stroma of the chloroplast. Here, the ATP and reduced NADP from the light-dependent reaction supply the energy and hydrogen to make glucose from carbon dioxide.

Question 22

What is the light energy absorbed by the photosystems used for?

Answer 22

In the light-dependent reaction, the light energy absorbed by the photosystems is used for three things:

1. Making ATP from ADP and inorganic phosphate. This is called photophosphorylation - it’s the process of adding phosphate to a molecule using light.
2. Making reduced NADP from NADP
3. Splitting water into protons (H+ ions), electrons and oxygen. This is called photolysis - it’s the splitting (lysis) of a molecule using light (photo) energy.

Question 23

What are the two types of photophosphorylation involved in light-dependent reactions.

Answer 23

The light-dependent reaction actually includes two types of photophosphorylation - non-cyclic and cyclic. Each of these processes has different products.

Question 24

What is non-cyclic photophosphorylation

Answer 24

Non-cyclic photophosphorylation produces ATP, reduced NADP and oxygen. To understand the process, you need to know that the photosystems in the thylakoid membranes are linked by electron carriers. Electron carriers are proteins that transfer electrons. The photosystems and electron carriers form an electron transport chain - a chain of proteins through which excited electrons flow. There are several processes going on all at once in non-cyclic photophosphorylation.

Question 25

Describe the steps of non-cyclic photophosphorylation.

Answer 25

1. Light energy excites electrons in chlorophyll. Light energy is absorbed by PSII. The light energy excites electrons in chlorophyll. The electrons move to a higher energy level (i.e. they have more energy). These high-energy electrons move along the electron transport chain to PSI.
2. Photolysis of water produces protons, electrons and oxygen. As the excited electrons from chlorophyll leave PSII to move along the electron transport chain, they must be replaced. Light energy splits water into protons (H+ ions), electrons and oxygen. (So the oxygen in photosynthesis comes from water.) The reaction is: H2O ——> 2H+ + 1/2O2.
3. Energy from the excited electrons makes ATP. The excited electrons lose energy as they move along the electron transport chain. This energy is used to transport protons (H+ ions) into the thylakoid, via membrane proteins called proton pumps, so that the thylakoid has a higher concentration of protons than the stroma. This forms a proton gradient across the membrane. Protons move down their concentration gradient, into the stroma, via an enzyme called ATP synthase. The energy from this movement combines ADP and inorganic phosphate (Pi) to form ATP.
4. Energy from the excited electrons generates reduced NADP. Light energy is absorbed by PSI, which excites the electrons again to an even higher energy level. Finally, the electrons are transferred to NADP, along with a proton from the stroma, to form reduced NADP.

Question 26

What happens during cyclic photophosphorylation?

Answer 26

Cyclic photophosphorylation only produces ATP and only uses PSI. It’s called ‘cyclic’ because the electrons from the chlorophyll molecule aren’t passed onto NADP, but are passed back to PSI via electron carriers. This means the electrons are recycled and can repeatedly flow through PSI. This process doesn’t produce any reduced NADP or oxygen - it only produces small amounts of ATP.

Question 27

What is the Calvin cycle?

Answer 27

The light-independent reaction is also called the Calvin cycle. It takes place in the stroma of the chloroplasts. It makes a molecule called triose phosphate from carbon dioxide (CO2) and ribulose bisphosphate (a 5-carbon compound). Triose phosphate can be used to make glucose and other useful organic substances. There are a few step in the cycle, and it needs ATP and H+ ions to keep it going. The reactions are linked in a cycle, which means the starting compound, ribulose bisphosphate, is regenerated.

Question 28

Describe the three stages of the Calvin cycle.

Answer 28

1. Formation of glycerate 3-phosphate. Carbon dioxide enters the leaf through the stomata and diffuses into the stroma of the chloroplast. Here, it’s combined with ribulose bisphosphate (RuBP), a 5-carbon compound. This gives an unstable 6-carbon compound which quickly breaks down into two molecules of a 3-carbon compound called glycerate 3-phosphate (GP). Ribulose bisphosphate carboxylase (RuBisCO) catalyses the reaction between carbon dioxide and RuBP.
2. Formation of triose phosphate. The 3-carbon compound GP is reduced to a different 3-carbon compound called triose phosphate (TP). ATP (from the light-dependent reaction) provides the energy to do this. This reaction also requires H+ ions, which come from reduced NADP (also from the light-dependent reaction). Reduced NADP is recycled to NADP (for use in the light-dependent reaction again). Triose phosphate is then converted into many useful organic compounds - e.g. glucose.
3. Regeneration of ribulose bisphosphate. Five out of every six molecules of TP produced in the cycle aren’t used to make useful organic compounds, but to regenerate RuBP. Regenerating RuBP uses the rest of the ATP produced by the light-dependent reaction.

Question 29

What is a heroes sugar?

Answer 29

A heroes sugar is a monosaccharide that has six carbon atoms, e.g. glucose.

Question 30

How is a hexose sugar made?

What can hexose sugars be used to make?

Answer 30

One hexose sugar is made by joining two molecules of triose phosphate (TP) together. Hexose sugars can be used to make larger carbohydrates. The Calvin cycle needs to turn six times to make one hexose sugar. The reason for this is that three turns of the cycle produces six molecules of triose phosphate (because two molecules of TP are made for every one carbon dioxide molecule used). Five out of six of these TP molecules are used to regenerate ribulose bisphosphate (RuBP). This means that for three turns of the cycle only one TP is produced that’s used to make a hexose sugar.
As a hexose sugar has six carbons, two TP molecules are needed to form one hexose. This means the cycle must turn six times to produce two molecules of TP that can be used to make one hexose sugar. Six turns of the cycle need 18 ATP and 12 reduced NADP from the light-dependent reaction.
This might seem a bit efficient, but it keeps the cycle going and makes sure there’s always enough RuBP ready to combine with carbon dioxide taken in from the atmosphere.

Question 31

How does the Calvin cycle make up carbohydrates, lipids and amino acids.

Answer 31

The Calvin cycle is the starting point for making all the organic substances a plant needs. Triose phosphate (TP) and glycerate 3-phosphate (GP) molecules are used to make carbohydrates, lipids and animo acids:
Carbohydrates - hexose sugars are made from two triose phosphate molecules and larger carbohydrates (e.g. sucrose, starch, cellulose) are made by joining hexose sugars together in different ways.
Lipids - these are made using glycerol, which is synthesised from triose phosphate, and fatty acids which are synthesised from glycerate 3-phosphate.
Amino acids - some amino acids are made from glycerate 3-phosphate.

Question 32

What are the optimum condition for photosynthesis?

Answer 32

The ideal conditions for photosynthesis vary from one plant species to another. Most plants in temperature climates, like in the UK, would be happy with the following conditions:
High light Insteon situ of a certain wavelength
Temperature around 25 degreesC
Carbon dioxide at 0.4%

Question 33

Describe how high light intensity of a certain wavelength is an optimum condition for photosynthesis.

Answer 33

Light is needed to provide the energy for the light-dependent reaction - the higher the intensity of the light, the more energy it provides. Only certain wavelengths of light are used for photosynthesis. The photosynthetic pigments chlorophyll a, chlorophyll b and carotene only absorb the red and blue light in sunlight.

Question 34

Describe how the temperature being around 25 degrees C is an optimum condition for photosynthesis.

Answer 34

Photosynthesis involves enzymes (e.g. ATP synthase, RuBisCO). If the temperature falls below 10 degrees C the enzymes become inactive, but if the temperature is more than 45 degrees C they may start to denature (lose structure and function). Also, at high temperatures:

• Stomata close to avoid losing too much water. This causes photosynthesis to slow down because less carbon dioxide enters the leaf when the stomata are closed.
• the thylakoid membranes may be damaged. This could reduce the rate of the light-dependent stage reactions by reducing the number of sites available for electron transfer.
• The membrane around the chloroplast could be damaged, which could cause enzymes important in the Calvin cycle to be released into the cell. This would reduce the rate of the light-independent stage reactions.
• Chlorophyll could be damaged. This would reduce the amount of pigment that can absorb light energy, which would reduce rate of the light-dependent stage reactions.

Question 35

Describe how carbon dioxide at 0.4% is an optimum condition for photosynthesis.

Answer 35

Carbon dioxide makes up 0.04% of the gases in the atmosphere. Increasing this to 0.4% gives a higher rate of photosynthesis, but any higher and the stomata start to close.

Question 36

What are the limiting factors of photosynthesis?

Give examples.

Answer 36

Light, temperature and carbon dioxide can all limit photosynthesis. All three of these things need to be at the right level to allow a plant to photosynthesise as quickly as possible. If any one of these factors is too low or too high, it will limit photosynthesis (slow it down). Even if the other two factors are at the perfect level, it won’t make any difference to the speed of photosynthesis as long as that factor is at the wrong level.
For example, on a warm, sunny, windless day, it’s usually carbon dioxide that’s the limiting factor. Another example is at night, it’s usually the light intensity that’s the limiting factor.
However, any of these factors cold become the limiting factors, depending on the environmental conditions.