T5.6 & 5.7 - Photosynthesis & Pigments

Question 1

State what is meant by an absorption spectrum.

Answer 1

A graph that shows which wavelengths of light are absorbed by a photosynthetic pigment

Question 2

Describe the absorption spectrum for chlorophyll a

Answer 2

Chlorophyll a absorbs light most strongly in the blue portion of the visible spectrum, followed by the red portion
Chlorophyll a reflects light most strongly in the green portion of the visible spectrum

Question 3

What pigments absorb light for photosynthesis?

Answer 3

The principle pigment is chlorophyl a

Accsesory pigments e.g. chlorophyll b, carotenoids, xanthophyll,

Question 4

State what is meant by an action spectrum.

Answer 4

A graph showing the rate of photosynthesis against the wavelength of light - shows which wavelengths are actually used in photosynthesis.

Question 5

Compare the absorption and action spectra

Answer 5

There is a strong correlation between the cumulative absorption spectra of all pigments and the action spectrum.
Both display two main peaks – a larger peak at the blue region (~450 nm) and a smaller peak at the red region (~670 nm)
Both display a trough in the green / yellow portion of the visible spectra (~550 nm)

Question 6

Why do many plants have a variety of different photosynthetic pigments?

Answer 6

Accessory pigments absorb light wavelengths from other regions of the spectrum.
Each pigment will absorb different amounts of light at each wavelength and pass the energy onto chlorophyll a.
They broaden the absorption spectrum so that more energy is available for photosynthesis.

Question 7

Why do some tree leaves change colour in Autumn?

Answer 7

Some leaves have accessory pigments which cannot normally be seen when chlorophyll is active. When temperature cools down, chlorophyll breaks down before the accessory pigments, leaving them to show through.

Question 8

Some seaweeds are green, some are brown and some are red.
Explain why these seaweeds have different absorption spectra and action spectra.

Answer 8

Each pigment will absorb different wavelengths.
Each pigment will absorb different amounts of light at each wavelength.
Action spectrum is different because the seaweeds are absorbing different wavelengths of light.
The rate of photosynthesis will therefore be different at each wavelength.

Question 9

Describe the structure of a chloroplast

Answer 9

A chloroplast has a { double envelope/inner and outer membrane}.
It has grana, which are stacks of thylakoid discs, joined by
(intergranal) lamella (thylakoid membrane), within the stroma

Question 10

What structures are found within the stroma?

Answer 10

(70S) ribosomes, (circular) DNA, lipid globules, starch granules, photosynthetic enzymes e.g. RUBISCo

Question 11

Explain how the structure of the chloroplast is adapted to its function

Answer 11

● Inner membrane contains transport proteins which control the transport of molecules between the cell cytoplasm and the stroma
● Many grana (consisting of up to 100 thylakoid discs) create a large surface area for photosynthetic pigments, electron carriers and ATP synthase needed in LDRs.
● Folds in thylakoid membrane allow photosystems and electron carriers to be close together.
● Photosynthetic pigments grouped together in photosystems allows max. absorption of light
● Thylakoid space has low volume, which enables the H+ gradient to be generated rapidly.
● ATP synthases embedded in thylakoid membrane which allow H+ to flow back to the stroma, down the H+ gradient, to produce ATP.
● Fluid-filled stroma contains enzymes needed for LIR
● Grana surrounded by stroma so products made in LDR in grana can pass into stroma to be used in LIR.
● Chloroplast DNA and ribosomes can make some proteins needed for photosynthesis

Question 12

Where is chlorophyll found in a chloroplast?

Answer 12

Thylakoid membrane

Question 13

How are chloroplasts and mitochondria similar?

Answer 13

● Both have double envelopes which compartmentalise them in the cell’s cytoplasm
● Both have membranes - thylakoid in chloroplast and inner mitochondrial membrane - which contain an electron transport chain and ATP synthase to generate ATP via chemiosmosis.
● Both increase surface area of membranes by stacking (grana in chloroplasts) or folding (cristae in mitochondria).
● Both contain low-volume inter-membrane spaces for rapid gerenation of a H+ gradient.
● Both contain a fluid filled medium (stroma for Calvin cycle in chloroplast, and matrix for Kreb’s cycle in mitochondria) for diffusion of molecules and enzymes for cyclic reactions.

Question 14

Describe the differences in structure between chloroplasts and mitochondria

Answer 14

● Chloroplasts are shaped like biconvex lenses, whereas mitochondria are rod shaped.
● Chloroplasts may contain separate stores of starch and lipids but mitochondria do not.
● Chloroplasts contain photosynthetic pigments in photosystems in the thylakoids bur mitochondria do not.

Question 15

Describe the differences in function between chloroplasts and
mitochondria

Answer 15

● Chloroplasts are the site of photosynthesis, whereas mitochondria are the site of aerobic respiration.
● Chloroplasts use the coenzyme NADP to transfer hydrogen, whereas mitochondria use NAD.
● Chloroplasts absorb carbon dioxide, whereas mitochondria release it.
● Chloroplasts release oxygen, whereas mithcondria absorb it.

Question 16

Where do the light-dependent reactions of photosynthesis occur?

Answer 16

Thylakoid membranes

Question 17

Describe non-cyclic photophosphorylation

Answer 17

● Electrons in the chlorophyll a molecules in the RC of PSII are excited by photoactivation
● These electrons are then donated to an electron acceptor, which becomes reduced.
● Photolysis (splitting) of water produces hydrogen ions and hydroxide ions.
● The hydroxide ions react together to form oxygen (which diffuses away) and e-, which replace the ones chlorophyll a has lost:
● The electron acceptor passes the excited electrons along a series of electron carriers (ETC) by a series of redox reactions, which
release the energy required to pump H+ from the stroma into the thylakoid space.
● H+ accumulate, producing a high H+ conc. grad.
● H+ then diffuse back across the thylakoid membrane through ATP synthase, which couples ADP and Pi to produce ATP - chemiosmosis
● Electrons in the chlorophyll a molecules in the RC of PSI are excited by and passed to Ferradoxin
● The electrons are then passed to the electron acceptor NADP along with hydrogen ions from the dissociated water to form NADPH

Question 18

What processes occur in the light-dependent stage?

Answer 18

Photophosphrylation which includes photoactivation, photolysis, and chemiosmosis.

Question 19

State the meaning of the term photophosphorylation

Answer 19

The formation of ATP, by adding Pi to ADP, using light energy

Question 20

State the meaning of the term photoactivation

Answer 20

The excitation of an electron using energy from a photon of light

Question 21

State the meaning of the term photolysis

Answer 21

The splitting of water molecules in LDR

Question 22

State the meaning of the term chemiosmosis

Answer 22

The passive flow of protons down a concentration gradient through ATP synthase, which catalyses the synthesis of ATP

Question 23

Name the enzyme that produces ATP by chemiosmosis

Answer 23

ATP synthase

Question 24

What are photosystems made of?

Answer 24

Many photosynthetic pigment molecules, clustered around two special chlorophyll a molecules in the reaction centre.

Question 25

What is the function of photosystems?

Answer 25

To absorb light to generate excited electrons

Question 26

What is the difference between photosystem I and II?

Answer 26

Photosystem II comes first and the chlorophyll a molecules absorb light with wavelenth 680nm, wheras photosystem I chlorophyll a
molecules absorb light with wavelength 700nm.

Question 27

What are the products of the LDRs?

Answer 27

NADPH, ATP, and oxygen

Question 28

Explain cyclic photophosphorylation

Answer 28

● If NADP runs out the electrons carried by Ferredoxin return back to the ETC instead of being passed to NADP.
● They then flow back along the ETC, resulting in pumping of H+, and production of ATP via chemiosmosis.
● Oxygen and NADPH not are produced.
● Happens when light is NOT limiting - the light-independent reactions are happening more slowly than the light-dependent reactions so the supply of NADP runs out.

Question 29

Compare and contrast cyclic and non-cyclic photophosphorylation

Answer 29

● Both cyclic and non-cyclic involve photoactivation of chlorophyll a in PSII.
● Both result in ATP being produced via chemisosmosis.
● In non-cyclic both PSI and PSII are involved, but in cyclic only PS I is onvolved.
● In non-cyclic electrons from PSII pass to PSI and are accepted by NADP+, whereas in cyclic electrons from PSII travel back to ETC.
● In non-cyclic NADPH is produced, but i ncyclic it is not.
● In non-cyclic photolysis occurs, but in cyclic photolysis does not occur.
● In non-cyclic oxygen is produced as a byproduct, whereas in cyclic it is not.
● Non-cylic is predominant in green platns ,whereas cyclic is predominant in bacteria.

Question 30

Where do the light-independent reactions occur?

Answer 30

Stroma

Question 31

What enzyme found in the stroma is required for the light-independent reactions?

Answer 31

RUBISCO (Ribulose Bisphophate Carboxylase)

Question 32

What name is the LIRs otherwise known as?

Answer 32

Calvin cycle

Question 33

How is carbon fixed in the Calvin Cycle?

Answer 33

RUBISCO catalyses the carboxylation of RuBP (Ribulose bisphosphate) with CO to form 2 GP molecules (glycerate 3-phosphates)

Question 34

Describe the steps in the Calvin Cycle

Answer 34

1) Carbon fixation: RUBISCO catalyses the carboxylation of RuBP (5C) with CO to form 2 x GP (3c) molecules (glycerate 3-phosphates)
2) Reduction: Each GP is reduced by ATP and NADPH to form GALP (3C) (glyceradehyde phosphate)
3) Regeneration: Most GALP is used to regenerate RuBP
4) Synethsis: Some GALP are used to form monosaccharides e.g. glucose, por form amino acids etc.

Question 35

What are ATP and NADPH from th LDRs used for in the LIRs?

Answer 35

Reduction of GP to form GALP

Question 36

Explain the effect of reducing light intensity on the relative concentrations of GP, GALP and RuBP

Answer 36

Less light will result in a lack of NADPH and ATP being produced by the LDRs.
This means GP will not be reduced so it builds up.
Therefore GALP will not be produced so concentration decreases.
and RuBP will no be regenerated so concentration decreases.

Question 37

Name the main abtioic factors that limit the rate of photosynthesis

Answer 37

Low light intensity, extremes of temperature, low CO concentration

Question 38

Explain how does CO concentration limits photosynthesis

Answer 38

If there is no CO available RuBP cannot be converted into GP.
As a result the RuBP starts to build up and no more glucose will be produced.

Question 39

Why is CO usually the limiting factor in the environment?

Answer 39

Atmospheric concentrations vary between 0.03-0.04%, but 0.1% is the optimum concentration.
The rate at which the leaf can be supplied with CO also affects the rate – this depends on the steepness of the diffusion gradient and
the permeability of the leaf.

Question 40

Why does the rate of photosynthesis reach a maximum at high CO concentrations?

Answer 40

RUBISCO becomes saturated, so enzyme concentration becomes limiting factor.

Question 41

Explain which stage of photosynthesis is temperature dependent

Answer 41

The light independent stage is temperature dependent as it is controlled by enzymes e.g. Rubisco, which are affected by temperature

Question 42

Explain how temperature affects the rate of photosynthesis

Answer 42

At high light intensity there is a good supply of ATP and NADPH.
As temperature increases , substrate (RuBP + CO2) and enzyme (RUBISCO) molecules gain more kinetic energy causing substrates to collide with active sites more frequently, this increases the rate of photosynthesis. Temperatures above the optimum denature RUBISCO, so rate will
decrease.
At low light intensity the light dependant stage of photosynthesis is limited - this restricts the supply of ATP and NADPH.
Temperature is unable to speed up the Calvin cycle as more ATP and NADPH are needed.

Question 43

Explain how light intensity affects the rate of photosynthesis

Answer 43

At low light intensities - as light intensity increases, rate increases up to a maximum because more chlorphul molecules get photoactivated, so more ATP and NADPH are made for the LIRs, which produce more glucose.

At high light intensities - increasing the intensity of light has no effect on the rate, because all
available chlorophyll molecules are saturated with light.

In the absence of light - Neither ATP or NADPH will be produced and so the GP (or GALP) cannot be converted into glucose.
This results in the GP building up and the RuBP being used up.

Question 44

State what is meant by the term compensation point

Answer 44

The point at which the rate of respiration equals the rate of photosynthesis and there is no net change in CO2 or O2.