F214 - Photosynthesis

Question 1

Heterotroph

Definition

Answer 1

Organisms that break down molecules from other organisms to release energy
They rely on other organisms

Question 2

What is light energy used for in photosynthesis?

Answer 2

To produce complex organic molecules

Question 3

Why does respiration in autotrophs and heterotrophs depend on photosynthesis?

Answer 3

Autotrophs release energy through respiration by breaking down complex organic molecules e.g. Glucose
Glucose has to be synthesised from simple molecules and only autotrophs can do this through photosynthesis

Question 4

Where does photosynthesis take place?

Answer 4

Both stages of photosynthesis take place inside chloroplasts

Light Dependent takes place on the thylakoid membranes of chloroplasts in palisade mesophyll cells
Light Independent takes place in the stroma

Question 5

Chloroplast Structure

Envelope

Answer 5

The double membrane which surrounds the chloroplast
There is an inter membrane space about 10-20nm
The outer membrane is permeable to many small ions
The inner membrane is less permeable and has transport proteins imbedded in it

Question 6

Chloroplast Structure

Thylakoids

Answer 6

The inner membrane of the envelope is folded into thin plates called thylakoids

Question 7

Chloroplast Structure

Granum

Answer 7

Thylakoids are stacked like pennies to form grana

Between the grana there are intergranal lamellae

Question 8

Chloroplast Structure

Stroma

Answer 8

A fluid filled matrix containing the enzymes for light independent photosynthesis, starch grains, oil droplets, DNA and ribosomes

Question 9

Adaptations of Chloroplasts

Many Grana

Answer 9

Large surface area of thylakoids membranes for photosynthetic pigments, electron carriers and ATP synthase enzymes

Question 10

Adaptations of Chloroplasts

Photosystems

Answer 10

Photosynthetic pigments are arranged into photo systems to maximise absorption of light energy, they are held in place by proteins embedded in the grana

Question 11

Adaptations of Chloroplasts

Stroma

Answer 11

Contains the enzymes needed to catalyse the light independent reactions
Surrounds the grana so that the products of the light dependent reactions can easily pass into the storms

Question 12

Adaptations of Chloroplasts

DNA

Answer 12

Chloroplasts can make some of the proteins needed for photosynthesis using chloroplast DNA and the chloroplast ribosomes

Question 13

Autotroph

Definition

Answer 13

Organisms that can synthesise complex organic molecules from simple inorganic molecules

Question 14

Photosynthetic Pigment

Definition

Answer 14

Molecules that absorb light energy

Each pigment absorbs a range of wavelengths and has its own distinct peak of absorption

Question 15

How is light harvested in the chloroplast membranes?

Answer 15

Pigments form structures called photo systems
Light it’s the photo system
Photons are absorbed by accessory pigments
This energy excites electrons in the pigment to a higher energy level
When the electrons return to their original state a photon is released
Energy is passed from one pigment to another towards the reaction centre

Question 16

What is chlorophyll?

Answer 16

A mixture of pigments

Consists of a long phytol (hydrocarbon) chain and a porphyrin group

Question 17

What happens when light hits chlorophyll?

Answer 17

A pair of electrons associated with the magnesium in the porphyrin group are excited

Question 18

Chlorophyll in Photosystem I

Answer 18

Chlorophyll A - P700

Peak of absorption = 700nm

Question 19

Chlorophyll in Photosystem II

Answer 19

Chlorophyll A - P680

Peak of absorption = 680nm

Question 20

Accessory Pigments

Answer 20

Cartenoids reflect yellow and orange light and absorb blue light
They do not have a porphyrin group and are not directly involved in the light dependent reaction
They absorb wavelengths that are not well absorbed by chlorophyll a
Carotene (Orange) and xanthophyll (yellow) are the main cartenoid pigments

Question 21

Non Cyclic Phosphorylation

Answer 21

Light energy excites a pair of electrons from photosystem II
They are accepted by an electron acceptor
They are passed along a series of electron carriers taking part in redox reactions ate decreasing energy levels
The energy release is used to bind ADP to a phosphate ion to form ATP
The energy the electrons have left excites a pair of electrons from PSI, they then take their place in Photosystem I
The two new electrons are passed along another redox chain to the enzyme NADP reductase
They are used with the 2H+ from photolysis to reduce NADP

Question 22

Cyclic Phosphorylation

Answer 22

Two electrons are excited from PSI
They go to an electron acceptor
They are passed down a redox chain which releases energy
This energy is used to form ATP from ADP+P
The remaining energy excites a pair of electrons from PSI
The old electrons take the place if these newly excited electrons
The new electrons are passed back to the electron acceptor above PSII and then down the same redox chain again

Question 23

What are the products of the light dependent reaction?

Answer 23

Reduced NADP
ATP
Oxygen (a waste product)

Question 24

Light Independent Photosynthesis

The Calvin Cycle

Answer 24

Carbon fixation - carbon dioxide is combined with ribulose bisphosphate (RuBP) by the enzyme RUBISCO
This briefly forms a six carbon sugar which splits in to two molecules of glycerate phosphate (GP)
Reduced NADP and ATP are used to turn the GP into two triose phosphate molecules (TP)
This also released NADP and ADP+P which can be recycled in the light dependent stage
1/6 of the triose phosphate is used to make glucose
5/6 is used with ATP to regenerate RuBP

Question 25

Photolysis

Answer 25

At Photosystem II some light energy is used to split water molecules H2O -> 1/2O2 + 2e- + 2H+ The oxygen is released as a waste product The hydrogen ions are used by the enzyme NADP reductase to reduce NADP The two electrons replace the electrons that are excited form PSII

Question 26

What is triose phosphate used to make?

Answer 26

1/6 of the TP goes towards synthesising: Carbohydrates Lipids Amino acids The other 5/6 is used to regenerate RuBP

Question 26

What is chemiosmosis?

Answer 26

As electrons move along the electron transport chains, some of the energy released is use by coenzymes to pump protons (H+ ions) into the thylakoid space This creates a proton concentration and electrochemical gradient and a pH gradient all of which causes protons to move back across the membrane through protein channels associated with ATP synthase The flow of hydrogen ions is chemiosmosis and is necessary for the formation of ATP

Question 27

How does carbon dioxide enter the leaf and reach the chloroplasts to take part in the Calvin Cycle?

Answer 27

Carbon dioxide from the air diffuses into the leaf through the open stomata It then diffuses through the air spaces in the spongy mesophyll until it reaches the palisade mesophyll layer It diffuses through the cellulose cell walls, the cell surface membrane, the cytoplasm and the chloroplast envelope into the stroma

Question 28

What are the limiting factors of photosynthesis?

Answer 28

Temperature Light Intensity Carbon Dioxide Concentration Water Availability

Question 29

How does light intensity affect the rate of photosynthesis?

Answer 29

As light intensity increases the rate of photosynthesis increases This is because light causes the stomata to open so that more carbon dioxide can diffuse into the leaf, it excites electrons and splits water molecules

Question 30

How does temperature affect the rate of photosynthesis?

Answer 30

The photochemical reactions of the light dependent stage are not really affected by temperature The enzyme catalysed reactions of the Calvin Cycle are effected Between 0 and 25C photosynthesis rate approximately doubles for each 10C increase Above 25C the rate falls as enzymes work less efficiently and oxygen can more successfully compete with carbon dioxide for the active site of RUBISCO High temperatures also cause more water loss by transpiration causing the stomata to close in a stress response which limits carbon dioxide availability

Question 31

How does carbon dioxide affect the rate of photosynthesis?

Answer 31

Growing forests can absorb carbon dioxide as their rate of photosynthesis is greater than their rate of respiration Mature forests produce CO2 because the net respiration and decomposition is greater than the rate of photosynthesis In greenhouses CO2 levels can be artificially increased by burning oil, enhanced levels of CO2 will increase the rate of photosynthesis providing there are no other limiting factors

Question 32

Limiting Factor | Definition

Answer 32

A factor which prevents a metabolic process from taking l,ace at its maximum rate

Question 33

Low Carbon Dioxide Levels | Affect on Concentrations of GP, TP, RuBP

Answer 33

There would be less CO2 to bind with RuBP so less GP would be formed, all the GP that was available would be turned in to TP All the available TP would be used to make glucose or regenerate RuBP so it's concentration would decrease as there would be no GP to make more from RuBP would build up as there would be little/no CO2 for it to fix, concentration would increase until all of the TP had been used up then it would plateau

Question 34

High Carbon Dioxide Levels | Affect on Concentrations of GP, TP, RuBP

Answer 34

GP concentration would increase, as the number of RuBP molecules combining with CO2 would increase, until all of the RUBISCO enzymes were working at a maximum rate then GP concentration would reach a maximum TP concentration would increase as there would be more GP for it to be made from, it would increase until reduced NADP or ATP concentration became a limiting factor More RuBP would be used to fix CO2 but more RuBP would be regenerated

Question 35

Low Light Intensity | Affect on Concentration of GP, TP, RuBP

Answer 35

Rate of GP production would not be affected as the fixing of carbon by the enzyme RUBISCO is not light dependent TP would decrease as there would be less reduced NADP an ATP to convert the GP into TP GP would build up as a result RuBP would decrease as once the available TP had been used up to regenerate it, the RuBP would continue to combine with CO2 but it wouldn't be replaced

Question 36

High Light Intensity | Affect on Concentration of GP, TP, RuBP

Answer 36

If light intensity was high then the availability of the products of light dependent photosynthesis would not be a problem Providing there were no other limiting factors, photosynthesis would take place at a maximum rate

Question 37

Methods for Measuring for the Rate of Photosynthesis

Answer 37

Volume of oxygen produced per unit time Rate of uptake of carbon dioxide Rate of increase in dry mass of plants

Question 38

Respirometer

Answer 38

- one end of capillary tube stuck into test tube containing plant - plant photosynthesises producing oxygen - oxygen enters the capillary tube - other end of capillary tube can be connected to a syringe, gas pressure pushes syringe and change in volume can be measured - or distance moved by coloured dye in the tube can be measured - volume of gas collected = length of bubble x πr²

Question 39

What are the limitations of using a respirometer to measure the rate of photosynthesis of an aquatic plant?

Answer 39

- some of the oxygen produced in photosynthesis will be used by the plant in respiration - there may be some dissolved nitrogen gas collected - some oxygen may dissolve in the water so it won't be measured