Is the process whereby light energy from the sun is transformed into chemical energy and used to synthesise large

organic molecules from inorganic substances.

Photosynthesis

Define the terms... Autotrophs: Organisms that use light (photoautotrophs e.g. Plants) or

chemical (chemoautotrophs) energy and inorganic molecules to

synthesise complex-organic molecules.

Heterotroph: Organisms that ingest and digest complec organic

molecules releasing the chemical potential energy stored in them.

Autotroph Heterotroph

Do they respire? Yes Yes

Can they

synthesise

complex organic

molecules from

simple inorganic

ones?

Yes No

Do they use light

energy?

Yes No

Do they hydrolyse

complex organic

molecules?

Yes Yes

Examples Plants, algae Fungi, animals

Chloroplasts

How does the structure of a

chloroplast enable it to perform it’s

functions? The inner membrane contains transport proteins which control the entry

and exit of substances between the cytoplasm and the stroma.

The grana provide surface area for photosynthetic pigments, electron

carriers and ATP synthase. Proteins embedded in the grana hold the

photosystems in place.

The photosynthetic pigments are arranged into photosystems to allow for

maximum absorption of light energy.

The stroma contains enzymes needed to catalyse the reactions in the light-

independent stage.

The stroma surrounds the grana, so the products of the light-dependent

reaction (needed in the light-independent reaction), can readily pass into the

stroma.

They can make some of their needed proteins using genetic information in

the chloroplasts DNA and assemble them in the chloroplast ribosomes.

Define the terms.. Photosynthetic Pigment: Molecules that absorb light energy.

Each pigment absorbs a range of wavelengths and have their own

distinct peak of absorption. Other wavelengths are reflected.

Examples:

chlorophyll a which appears yellow green and absorbs blue light (700nm

- PS1)

chlorophyll b which appears blue-green and absorbs wavelengths of light

of 500nm and 640nm(680nm – PS2).

Accessory Pigment: These aren’t directly involved in the light-

dependant reaction (LDR) as they have no porphyrin group. Instead, they

absorb wavelengths that aren’t absorbed efficiently by chlorophylls and

pass energy they capture to chlorophyll a to use in the LDR. They can be

know as cartenoids, with the two main ones: carotene (orange) and

xanthophyll (yellow) which absorb blue light.

The Light-Dependant Stage: Waters Role

PSII has an enzyme that, when in the presence of light, can split

water into protons and electrons. This process is known as

photolysis: 2H2O 4H+ + 4eˉ +O2

Water is a source of...

Protons: these are used in chemiosmosis to produce ATP. They ar

accepted by NAD so it becomes reduced. Reduced NADP is

used in the light-independent reaction (LIR) to reduce carbon

dioxide and produce organic molecules.

Electrons: to replace those lost by the oxidised chlorophyll

Water keeps cells turgid, so they can function.

The Light-Dependant Stage:

Photophosphorylation This takes place in the thylakoid membranes.

Electron

carrier/accepto

r which contain

iron atoms

Photophosphorylation When light (a photon) hits a chlorophyll molecule, the energy of

the photon is transferred to 2 electrons which become excited.

These electrons will be captured by electron acceptors and

passed along a chain of electron carriers. This generates

energy(as they pass along the chain) and is used to pump

protons across the thylakoid membrane into the thylakoid space

to accumulate. A proton gradient is set up which the protons flow

down through channels associated with ATP synthase enzymes

(chemiosmosis) This produces a force that joins:

ADP + Pi ATP

Cyclic Phosphorylation Non-Cyclic Phosphorylation

•Uses only PS1 (P700)

•The excited electrons pass to an

electron carrier and back to the

chlorophyll a molecule where they

were lost

•No photolysis of water

•No generation of reduced NADP

•Small amounts of ATP formed

(used in the light-independent stage

– or in the guard cells. Guard cells

contain only PS1 to bring potassium

ions in, so water will follow by

osmosis causing the guard cells to

swell and open the stomata)

•Uses PS1 and PS2

•Light strikes PS2 exciting a pair of

electrons that leave the chlorophyll

a molecule from the primary

pigment reaction centre. The

electron pass along a chain of

electron carriers and the energy

released is used to synthesise ATP.

Light has also struck PS1, and a

pair of electrons have also been

lost, but have joined with NADP,

along with protons, to form reduced

NADP. The electrons from PS1

replace those lost at PS2. electrons

from photolysed water replace those

lost by oxidised chlorophyll at PS1.

protons from photolysed water take

part in chemiosmosis to make ATP

and are then captured by NADP in

the stroma. They will be used in the

light-independent stage.

The Light-Independent Stage: Calvin

CycleThe Calvin Cycle takes place in the stroma.

The Calvin Cycle(Carbon Dioxide has diffused in through the stomata into the

stroma). Carbon Dioxide combines with RuBP and this is

catalysed by Rubisco. RuBP becomes carboxylated. The product

of this is 2 GP molecules and CO2 is now fixed (by Rubisco). GP

is reduced and phosphorylated to 2 molecules of TP using ATP

and NADP. 5 out of 6 TP molecules are recycled by

phosphorylation, using ATP, to form RuBP.

What is the fate of the products of

the Calvin Cycle?

Some GP can be made into amino acids and fatty

acids. TP can be converted to glycerol and

combine with the fatty acids to form lipids.

TP pairs can combine to form hexose sugars

such as glucose.

Limiting Factors

The limiting factor is the factor that is present at the

lowest or least favourable value.

The three limiting factors are...

Temperature

Carbon dioxide concentration

Light intensity

Light Intensity At 0 light intensity, there is no photosynthesis.

At low light intensities, as the light intensity increases, so does the rate of

photosynthesis. Therefore, light intensity is the limiting factor.

At high light intensities, the rate plateaus. Another factor (e.g. CO2) must

be limiting.

Overall, as the light intensity increases, so does the rate of photosynthesis.

Light has 3 main effects...

Causes stomata to open, so CO2 can enter

It is trapped by chlorophyll where it excites electrons

It splits water to produce protons

Carbon Dioxide Concentration

Increasing carbon dioxide will increase the

photosynthesis rate but not indefinitely. At some

point, the rate will plateau.

What are the advantages of growing

plants in a greenhouse?

It is easier to control water

supply/humidity/minerals (to prevent wilting)

Easier to control the use of pesticides/pest

control/biological control

The gas/paraffin heaters/burning of fossil fuels

supplies carbon dioxide and heat

The plants won’t be damaged as a result of chill,

wind, frost or hail.

Temperature Increasing the temperature can increase the photosynthesis rate,

but it will reach a plateau. However, at high temperatures,

proteins (such as enzymes in the Calvin Cycle) will denature.

Also, an increase in temperature will lead to more water loss

from the stomata. This leads to a stress response, so the

stomata will close which leads to less carbon dioxide, so less

light-independent reactions take place.

Light Intensity on the Calvin

Cycle

When the light source is switched off, the light reaction stops so no ATP is

produced. GP isn’t converted to TP so it builds up and instead, RuBP is

used to form GP.

A Photosynthometer

What can it measure? How does it

work?This apparatus could measure...

The volume of carbon dioxide produced

The rate of the uptake of carbon dioxide

The rate of increase in dry mass of plants

When being set up, all the joints should be air tight so no air bubbles are

present. The gas given off by the plant (typically Elodea) collects in the

flared end of the capillary tube. The syringe can then be used to move the

air bubble into the part of the capillary tube against the scale. By measuring

the distance moved by the air bubble at each light intensity, the rate can be

worked out (volume/time left)