1. Photosynthesis
Is the process whereby light energy from
the sun is transformed into chemical
energy and used to synthesise large
organic molecules from inorganic
substances.
2. 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
Yes No
synthesise
complex organic
molecules from
simple inorganic
ones?
Do they use light
energy?
Yes No
Do they hydrolyse
complex organic
molecules?
Yes Yes
Examples Plants, algae Fungi, animals
4. 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.
5. 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.
6. 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.
7. The Light-Dependant Stage:
Photophosphorylation
This takes place in the thylakoid membranes.
Electron
carrier/accepto
r which contain
iron atoms
8. 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
9. 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.
11. 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.
12. 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.
13. 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
14. 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
15. Carbon Dioxide Concentration
Increasing carbon dioxide will increase the
photosynthesis rate but not indefinitely. At some
point, the rate will plateau.
16. 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.
17. 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.
18. 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.
20. 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)
