Module 5: Photosynthesis

Question 1

Why is energy important and give examples ?

Answer 1

Living things need energy for biological processes to occur.

Plants: Photosynthesis, active transport. DNA replication and cell division.

Animals: Muscle contraction, Maintenance of body temperature, active transport, DNA replication and cell division.

Microorganisms: DNA replication, cell division, protein synthesis and sometimes movement.

Question 2

What is the equation for Photosynthesis?

Answer 2

6CO2 + 6H2O + ENERGY = C6H12O6 +6O2

Carbon dioxide + water + energy = glucose + oxygen

Question 3

What is the link between photosynthesis and respiration?

Answer 3

Energy is stored in the glucose until the plants release it by respiration. Animals cannot make their own food, so they obtain glucose by eating plants, then respire the glucose to release energy.

Question 4

What are the two types of respiration and state the equation

Answer 4

1) Aerobic respiration.

2) Anaerobic respiration.

C6H12O6 + 6O2 = 6CO2 + 6H2O + ENERGY

Question 5

Describe the synthesis of ATP.

Answer 5

ATP is synthesised from ADP and an 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 the 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.

Question 6

Describe the hydrolysis of ATP.

Answer 6

ATP then diffuses to the part of the cell that needs energy. It is broken down back to ADP and an inorganic phosphate. Chemical energy is released from the phosphate bond and used by the cell. ATPase catalyses this reaction.

Question 7

What are the 5 main properties of ATP?

Answer 7

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

2) It is small and soluble therefore it can be easily transported around the cell.

3) It is easily broken down, so energy can easily be released.

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

5) ATP cant pass out of the cell, so the cell always has an immediate supply of energy.

Question 8

What is the compensation point?

Answer 8

It is the particular level of light intensity at which the rate of photosynthesis exactly matches the rate of respiration.

Question 9

How do you work out the compensation point?

Answer 9

1) Measure the rate at which oxygen is produced and used by a plant at different light intensities.

The rate of CO2 production and use could also be measured - photosynthesis uses CO2 and respiration produces it.

Question 10

What are the 8 main structures of a Chloroplast?

Answer 10

1) Outer/inner membrane of envelope

2) Stroma ( contains enzymes, sugars and organic acids)

3) Granum (thylakoid stack)

4) Starch grain

5) Lamella

6) Thylakoid membrane

7) Thylakoid

8) Circular DNA

Question 11

What are photosynthetic pigments?

Answer 11

These are coloured substances that absorb the light energy needed for photosynthesis.

The pigments are found in the thylakoid membranes- they are attached to proteins.

The protein and the pigment is called a photosystem.

Question 12

What are the 2 types of photosynthetic pigments that a photosystem contains?

Answer 12

Primary pigments: These are reaction centres where electrons are excited during the light-dependent reaction. e.g. Chlorophyll a.

Accessory pigments: These 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 13

What are the 3 types of photosynthetic pigments that chloroplasts contain?

Answer 13

1) Chlorophyll a

2) Chlorophyll b

3) Carotene

Question 14

What are the 2 photosystems used by plants to capture light energy?

Answer 14

1) Photosystem I (PSI) absorbs lights best at a wavelength of 700nm

2) Photosystem II (PSII) absorbs light best at a wavelength of 680 nm.

Question 15

What is a coenzyme?

Answer 15

It is a molecule that aids the function of an enzyme. They work by transferring a chemical group from one molecule to another.

This means that it can reduce or oxidise a molecule.

Question 16

Where in the chloroplast does the light- dependent and the light-independent reaction take place?

Answer 16

Light-dependent reaction: Takes place in the thylakoid membranes.

Light- independent reaction: takes place in the stroma of chloroplasts.

Question 17

Describe the stages of the light- dependent reaction.

Answer 17

1) Light energy from the sun is absorbed by PSII. The light excites electrons within PSII and causes them to move to a higher energy state. The electrons are passed onto a series of electron carriers within the electron transport chain to PSI.

2) The electrons which have been lost from PSII need to be replaced. This happens through the Photolysis of water H2O = 2H +1/2O2(
from roots). The light energy causes a water molecules to split apart and release hydrogen ions + electrons+ oxygen. The electrons from H2O replace lost electrons from PSII.

3) As the electrons move along the electron transport chain, they move from high to low energy (
begin to lose energy). The energy lost by electrons is used to pump hydrogen ions from the stroma into the thylakoids (now has a higher conc. of protons than the stroma) and this generates a proton gradient across the thylakoid membrane.

4) Protons flow down their concentration gradient through ATP synthase into the stroma. The energy from this movement combines ADP + Pi to form ATP (called Chemiosmosis).

5) Light is absorbed by PSI causing another electron to become excited and be passed along the rest of the electron transport chain. Finally, The electrons are transferred to NADP, along with a proton (H+),from the stroma, to form reduced NADP.

6) The ATP + reduced NADP move into the stroma for the next stage of photosynthesis.

Question 18

What is the difference between cyclic photophosphorylation and non-cyclic photophosphorylation?

Answer 18

Non- cyclic photophosphorylation: Produces ATP, NADPH and O2 (all stages are carried out)

Cyclic photophosphorylation: Electrons from chlorophyll molecules are not passed onto NADP, but are passed back to PSI via electron carriers.
Electrons are recycled and repeatedly flow through PSI.
It only produces ATP and only uses PSI

Question 19

Describe the stages of the light-independent reaction (Calvin Cycle)

Answer 19

1) CARBON FIXATION: carbon dioxide enters the leaf through the stomata and diffuses into the stroma. CO2 combines with RIBULOSE BISPHOSPHATE (RuBP) which is a 5 carbon compound. This reaction is catalysed by an enzyme called Rubisco. The 6C molecule is unstable and therefore immediately breaks down to form two 3C compounds called glycerate-3-phosphate (GP)

2) REDUCTION: An isomerisation reaction occurs where GP is reduced to a different 3C compound called triose phosphate (TP). ATP from the light- dependent reaction provides the energy to do this, so it is hydrolysed to ADP + Pi. This reaction also requires H+ ions which come from the reduced NADP (from LDR). Reduced NADP is recycled to NADP to use in LDR again.

REGENERATION: 5/6 molecules of TP will be used to regenerate back into RuBP. 1/6 molecules will be used for an organic compound regenerating RuBP uses the rest of ATP produced by light-dependent reaction.

Question 20

What are the uses of Triose phosphate (TP)?

Answer 20

The Calvin cycle needs to turn 6 times to make one hexose sugar. (2x of TP).

Glucose is made by joining two TP molecules together. The glucose can then be used to build polysaccharides like starch and cellulose.

Amino acids are made from GP.

Glycerol is made from TP and fatty acids are made from GP. Glycerol and fatty acids are joined by ester bonds to form triglycerides-lipids.

Question 21

Describe the practical investigating leaf pigments using chromotography.

Answer 21

Extract pigment from the leaves of a plant – grind up the leaves with anhydrous sodium sulfate then add a few drops of propanone.

Transfer to a test tube and shake with petroleum ether. You should get two separate layers – the top layer contains the pigments. Transfer this top layer into a test tube containing anhydrous sodium sulfate.

Place drops of extract on a pencil line drawn along the bottom of the TLC plate. This is the stationary phase, made of glass containing a thin layer of silica gel.

The TLC plate is placed in a tank containing a solvent (the mobile phase). You can use a mixture of propanone, cyclohexane and petroleum ether as the solvent.

Place a lid on the tank.

As the solvent moves upwards through the gel, the pigments dissolve in the solvent and are carried up with it.

The more soluble the pigment, the further it will travel up the stationary phase. Since different pigments have different solubilities, they separate out. Draw a line at the point on the TLC plate where the solvent has reached – this is the solvent front.

You can identify each pigment by calculating the Rf value and looking it up in a database.

Question 22

What are the optimum condition of light intensity for photosynthesis?

Answer 22

High light intensity means that the light-dependent reaction can work faster. So the more light, the more photosynthesis.

But it needs to be the right wavelength – in the red or blue part of the spectrum (chlorophyll reflects any light in the green part of the visible spectrum).

Gardeners grow plants in transparent greenhouses or polytunnels which let in light. They may also use lamps to provide light at night.

Question 23

What are the optimum condition of temperature for photosynthesis?

Answer 23

Temperatures around 25oC allow photosynthetic enzymes to work quickly. At lower temperatures, enzymes become inactive and at higher temperature they can denature.

At high temperatures, the stomata will also close to conserve water. This stops gas exchange and reduces the rate of photosynthesis.
Greenhouses trap heat energy from sunlight.

Fancier greenhouses will have heaters, cooling systems and air circulation systems to ensure an optimum temperature is maintained year-round.

Question 24

What are the optimum condition of carbon dioxide concentration for photosynthesis?

Answer 24

Atmospheric carbon dioxide concentration is around 0.04%. Concentrations ten times higher (0.4%) can maximise photosynthesis.

Concentrations above 0.4% can cause stomatal closure and a reduction in photosynthesis.

Gardeners can add carbon dioxide to the greenhouse by burning propane.

Question 25

How can light intensity limit the the rate of photosynthesis?

Answer 25

Light intensity – the higher the light intensity, the faster the rate of photosynthesis.

This is because light energy is a required to produce ATP and NADPH in the light-dependent reaction.

Low ATP/NADPH means that there will be less production of GP and TP in the Calvin cycle

Question 26

How can temperature limit the rate of photosynthesis?

Answer 26

Temperature – in general, the higher the temperature the faster the rate of photosynthesis because the enzymes (e.g. Rubisco) and reacting molecules have more kinetic energy so they collide into each other more frequently.

If the temperature becomes too high, the rate of photosynthesis decreases because the enzymes involved in photosynthesis are denatured.

Question 27

How can carbon dioxide concentration limit the rate of reaction?

Answer 27

Carbon dioxide concentration – as the concentration of carbon dioxide increases, the rate of photosynthesis increases because carbon dioxide is a reactant in the first stage of the Calvin cycle (the fixation of carbon dioxide with RuBP.

In the absence of carbon dioxide, RuBP levels increase while TP and GP will fall.

Question 28

How can you calculate the rate of photosynthesis?

Answer 28

We can calculate the rate of photosynthesis by determining the volume of oxygen produced by a plant in a given time. To do this, we read off the y-axis to find the volume of gas produced and divide this by the value on the x-axis, which tells us how long it took for that volume of gas to be formed.

rate = volume of gas/ time