Photosynthesis

Question 1

What is the site of photosynthesis?

Answer 1

The leaf is the main photosynthetic structure in eukaroytic plants.
Chloroplasts are the cellular organelles within the leaf where photosynthesis takes place.
Light is absorbed by chlorophyll and then transferred into the chemical energy of the molecules formed during photosynthesis.

Question 2

What are the adaptations of a leaf - light?

Answer 2

A large surface area that absorbs as much sunlight as possible.
An arrangement of leaves on the plant that minimises overlapping and so avoids the shadowing of one leaf by another.
A transparent cuticle and epidermis that let light through to the photosynthetic mesophyll cells beneath.
Long, narrow upper mesophyll cells packed with chloroplasts that collect sunlight.
Stomata that open and close in response to changes in light intensity.

Question 3

What are the adaptations of the leaf - carbon dioxide and oxygen?

Answer 3

Thin, as most light is absorbed in the first few micrometres of the leaf and a short diffusion distance for gases.
Numerous stomata for gaseouse exchange so that all mesophyll cells are only a short diffusion pathway from one.
Many air spaces in the lower mesophyll layer to allow rapid diffusion of carbon dioxide and oxygen.
A network of xylem that brings water to the leaf cells, and phloem that carries away the sugars produced during photosynthesis.

Question 4

What is the overall equation for photosynthesis?

Answer 4

6CO2 + 6H2O —light—-> C6H12O6 + 6O2

Question 5

What are the three main stages of photosynthesis?

Answer 5

Capturing of light energy by chloroplast pigments such as chlorophyll.
The light dependent reaction, where some of the light energy absorbed is conserved in chemical bonds. An electron flow is created by the effect of light on chlorophyll, causing water to split (photolysis) into protons, electrons and oxygen. The products are reduced NADP, ATP and oxygen.
The light independent reaction, the protons (H ions) are used to produce sugars and other organic molecules.

Question 6

What are the grana?

Answer 6

The grana are stacks of up to 100 disc-like structures called thylakoids where the light dependent stage of photosynthesis takes place.
Within the thylakoids is the photosynthetic pigment called chlorophyll.
Some thylakoids have tubular extensions that join up with thylakoids in adjacent grana, inter-granal lamellae.

Question 7

What is the stroma?

Answer 7

A fluid filled matrix where the light-independent stage of photosynthesis takes place.
There are other structures such as starch grains.

Question 8

What is the energy from the light-dependent reaction used for?

Answer 8

To add an inorganic phosphate molecule to ADP, making ATP.
To split water into H^+ ions (protons) and OH^- ions. Photolysis.

Question 9

What is oxidation?

Answer 9

When a substance gains oxygen or loses hydrogen.
The substance to which oxygen is added has been oxidised, and loses electrons.
Energy is given out.

Question 10

What is reduction?

Answer 10

When a substance loses oxygen, or gains hydrogen.
The substance gains electrons.
Energy is taken in.

Question 11

What is photoionisation?

Answer 11

When a chlorophyll molecule absorbs light energy, it boosts the energy of a pair of electrons in the chlorophyll, and raises them to a higher energy level.
The electrons are in such an excited state, that they leave the chlorophyll molecule.
The chlorophyll becomes ionised.

Question 12

What is an electron carrier?

Answer 12

The electrons that leave the chlorophyll are taken up by an electron carrier.
Having lost a pair of electrons, the chlorophyll has been oxidised.
The electron carrier gains electrons, and is so reduced.

Question 13

What do the electron carriers do with the electrons?

Answer 13

The electron carriers pass the electrons along in a series of oxidation-reduction reactions.
These electron carriers form a transport chain located in the thylakoid membranes.
Each new carrier is at a lower energy level, so the electrons lose energy at each stage.
Some of this energy is used to combine an inorganic phosphate with ADP to make ATP.

Question 14

What is the chemiosmotic theory?

Answer 14

Each thylakoid is in an enclosed chamber into which protons are pumped from the stroma using protein carriers in the thylakoid membrane, called proton pumps.
The energy driving this comes from electrons released in photolysis of water, which also produces protons, further increasing their concentration in the thylakoid.

Question 15

What is the chemiosmotic theory - concentration gradient?

Answer 15

This creates and maintains a concentration gradient of protons across the membrane with a high concentration inside and a low concentration in the stroma.
The protons can only cross the thylakoid membrane through ATP synthase channel proteins.
These channels form small stalked granules on the membrane surface.

Question 16

What is the chemiosmotic theory - ATP formation?

Answer 16

As the protons pass through these ATP synthase channels they cause changes to the structure of the enzyme which then catalyse the combination of ADP with Pi to form ATP.

Question 17

Why must photolysis of water occur?

Answer 17

The loss of electrons when light strikes chlorophyll leaves it short of electrons, so these must be replaced.
These are provided from water molecules that are split using light energy.
It also yields protons.

Question 18

What is the basic equation for photolysis of water?

Answer 18

2H2O —> 4H+ + 4e- + O2
water —> protons + electrons + oxygen.

Question 19

What do the protons do in photolysis of water?

Answer 19

These protons pass out of the thylakoid space through the ATP synthase channels and are taken up by electron carrier NADP, which becomes reduced.
Reduced NADP enters the light-independent reaction with the electrons from the chlorophyll molecule.

Question 20

What happens to oxygen in photolysis of water?

Answer 20

The oxygen by-product is either used up in respiration or diffuses out of the leaf as a waste product.

Question 21

Why is reduced NADP important?

Answer 21

It is the main product of the light dependent stage and it is a further potential source of chemical energy to the plant.

Question 22

How are chloroplast structurally adapted for catching light and the light dependent reaction?

Answer 22

The thylakoid membranes provide a large surface area for the attachment of chlorophyll, electron carriers and enzymes.
Chloroplasts contain both DNA and ribosomes so they can quickly and easily manufacture some of the proteins involved in the light dependent reaction.

Question 23

How are chloroplast structurally adapted for catching light and the light dependent reaction - grana?

Answer 23

A network of proteins in the grana hold the chlorophyll in a very precise manner to allow for maximum light absorption.
The granal membranes have ATP synthase channels within them, which catalyse ATP production.
They are also selectively permeable which allows establishment of a proton gradient.

Question 24

What is the light independent reaction?

Answer 24

ATP and reduced NADP from stage 1 are used to reduce glycerate 3-phosphate in the second stage.
It does not require light directly but it does require the products of stage 1 so rapidly ceases when light is absent.
It takes place in the stroma and is referred to as the Calvin Cycle.

Question 25

What is the first part of the Calvin Cycle?

Answer 25

Carbon dioxide diffuses into the leaf through stomata and dissolves in water around the walls through the mesophyll cells. It then diffuses through the cell-surface membrane, cytoplasm and chloroplast membranes into the stroma.
Here, CO2 reacts with the 5 carbon compound ribulose bisphosphate (RuBP) catalysed by rubisco.
This produces 2 molecules of 3 carbon glycerate 3-phosphate (GP).

Question 26

How is NADP used in the Calvin Cycle?

Answer 26

Reduced NADP from stage 1 is used to reduce GP to triose phosphate (TP) using energy supplied by ATP.
The NADP is re-formed and goes back to the light-dependent reaction to be reduced again by accepting more protons.

Question 27

How are the products of the Calvin Cycle used?

Answer 27

Some TP molecules are converted to organic substances required by the plant such as starch, cellulose, lipids, glucose, amino acids and nucleotides.
Most TP molecules are used to regenerate ribulose biphosphate using ATP from the light dependent reaction.

Question 28

How is the chloroplast adapted for the light independent reaction?

Answer 28

The fluid of the stroma contains all the enzymes needed to carry out the light independent reaction.
Stromal fluid is membrane-bound in the chloroplast which means a chemical environment which has a high concentration of enzymes and substrates can be maintained within it - as distinct from the environment of the cytoplasm.

Question 29

How is the chloroplast adapted for the light independent reaction - speed?

Answer 29

The stroma fluid surrounds the grana and so the products of the light dependent reaction in the grana can readily diffuse into the stroma.
It contains both DNA and ribosomes so it can quickly and easily manufacture some of the proteins involved in the light independent reaction.