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CIE Categories Archives: 13 Photosynthesis

Summary of Photosynthesis

4 ATP and reduced NADP are the two main products of the light-dependent reactions of
photosynthesis, and they then pass to the light-independent reactions.
5 In the light-independent reactions, carbon dioxide is trapped by combination with a 5C
compound, RuBP, which acts as an acceptor molecule. This reaction is catalysed by the
enzyme ribulose bisphosphate carboxylase (rubisco), which is the most common
enzyme in the world. The resulting 6C compound splits to give two molecules of a 3C
compound, GP (also known as PGA). GP is reduced to carbohydrate, using ATP and
reduced NADP from the light-dependent reactions. This carbohydrate can be converted
into other carbohydrates, amino acids and lipids or used to regenerate RuBP. This
sequence of light-independent events is called the Calvin cycle.
6 Chloroplasts, palisade mesophyll cells and whole leaves are all adapted for the
efficient absorption of light for the process of photosynthesis.
7 When a process is affected by more than one factor, the rate of the process will be
limited by the factor closest to its lowest value. The rate of photosynthesis is subject to
various such limiting factors, including light intensity and wavelength, carbon dioxide
concentration and temperature.
8 A graph of the particular wavelengths of light that are absorbed by a photosynthetic
pigment is called an absorption spectrum, and a graph of the rate of photosynthesis at
different wavelengths of light is called an action spectrum.
9 The different pigments present in a chloroplast can be separated by paper
chromatography.

 

1. End-of-chapter questions
1 What are the products of the light-dependent reactions of photosynthesis?
A ATp, RuBP and reduced NAD
B ATp, oxygen and reduced NADP
C GP, oxygen and reduced NAD
D GP, reduced NADP and RuBP
2 Where in the chloroplast are the products of photophosphorylation used?
A envelope
B granum
C stroma
D thylakoid
3 In parate
experiments, an actively photosynthesising plant was supplied with one of
two labelled reactants:
• water containing the 18O
• carbon dioxide containing the
17O
In which products of photosynthesis would these isotopes be found?

6 2, 7, 4, 6, 3, 1, 5
All correct = 4, subtract marks for mistakes.
7 a allows chlorophyll and other pigments to be arranged into photosystems;
provides large surface area for pigments;
increases effi ciency of light harvesting; allows electron carriers to be arranged
appropriately; provides structure for proton gradient for chemiosmosis; anchors
ATP synthase;

 

 

 

 

 

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Limiting factors in photosynthesis

Usually, only one of these factors will be the limiting factor in a plant at a certain time.
This is the factor which is the furthest from its optimum level at a particular point in time.
If we change the limiting factor the rate of photosynthesis will change but changes to the
other factors will have no effect on the rate.
If the levels of the limiting factor increase so that this factor is no longer the furthest
from its optimum level, the limiting factor will change to the factor which is at that point in
time, the furthest from its optimum level. For example, at night the limiting factor is likely
to be the light intensity as this will be the furthest from its optimum level. During the day,
the limiting factor is likely to switch to the temperature or the carbon dioxide
concentration as the light intensity increases.

 

In practice, the farmer will need to find the optimum growing conditions for the crop,
given the costs of providing extra lighting, heat and CO2. Paraffin lamps have
traditionally been used in greenhouses. Their use increases the rate of photosynthesis
because as well as the light generated from the lamps, the burning paraffin produces
heat and CO2 too.
Investigating the effect of environmental factors on the rate of photosynthesis
One way to measure the rate of photosynthesis is to measure the rate at which oxygen
is given off by an aquatic plant. There are various ways in which oxygen can be
collected and measured. One method is shown in the diagram below.

 

Alternatively, you can make calcium alginate balls containing green algae and place
them in hydrogencarbonate indicator solution. As the algae photosynthesise, they take
in carbon dioxide which causes the pH around them to increase. The indicator changes
from orange, through red to magenta.
Whichever technique is used, you should change one factor (your independent variable)
while keeping all others constant (the control variables). The dependent variable will be
the rate at which oxygen is given off (measured by the volume of oxygen collected per
minute in the capillary tube) or
the rate at which carbon dioxide is used (measured by the rate of change of colour of

the hydrogencarbonate indicator solution).
The independent variables you could investigate are:
 Light intensity. You can vary this by using a lamp to shine light onto the plant or
algae. The closer the lamp. the higher the light intensity.
 Wavelength of light. You can vary this by placing coloured filters between the
light source and the plant. Each filter will allow only light of certain wavelengths to pass
through.
 CO2 concentration. You can vary this by adcting sodium hydrogencarbonate to
the water around the aquatic plant. This contains hydrogencarbonate Ions, which are
used as a source of carbon dioxide by aquatic plants.
 Temperature. The part of the apparatus containing the plant or algae can be
placed in a water bath at a range of controlled temperatures.
Video: Limiting factors of photosynthesis

 

 

 

 

 

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The light-dependent reactions, Photophosphorilation

Chlorophyll molecules in photosystern I (PSI) and photosystern II (PSII) absorb light
energy. The energy excites electrons, raising their energy level so that they leave the
chlorophyll. The chlorophyll is said to be photo-activated.
PSII contains an enzyme that splits water when activated by light. This reaction is
called photolysis (‘splitting by light’). The water molecules are split into oxygen and
hydrogen atoms. Each hydrogen atom then loses its electron, to become a positively
charged hydrogen ion (proton), H+.
The electrons are picked up by the chlorophyll in PSII, to replace the electrons they lost.
The oxygen atoms join together to form oxygen molecules, which diffuse out of the
chloroplast and into the air around the leaf.

The light- dependent reactions. Credit: Pears education.
The electrons emitted from PSII are picked up by electron carriers in the membranes of
the thylakoids. They are passed along a chain of these carriers, losing energy as they
go. The energy they lose is used to make ADP combine with a phosphate group,
producing ATP. This is called photophosphorylation. At the end of the electron carrier
chain, the electron is picked up by PSI, to replace the electron the chlorophyll in PSI had
lost.
The electrons from PSI are passed along a different chain of carriers to NADP. The
NADP also picks up the hydrogen ions from the split water molecules. The NADP
becomes reduced NADP.
We can show all of this in a diagram called the Z-scheme. The higher up the diagram,
the higher the energy level. If you follow one electron from a water molecule, you can
see how it
• is taken up by PSII
• has its energy raised as the chlorophyll in PSII absorbs light energy
• loses some of this energy as it passes along the electron carrier chain
• is taken up by PSI
• has its energy raised agaln as the chlorophyll in PSI absorbs light energy
• becomes part of a reduced NADP molecule

 

Video: Photosynthesis

 

 

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Photosynthetic Pigments

 

An absorption spectrum is a graph showing the percentage of light absorbed by
pigments, for each wavelength of light.
An example is the absorption spectrum of chlorophyll a and b.
 The best absorption is seen with violet-blue light.
 There is also good absorption with red-orange light.
 Most of the green-yellow light is reflected and therefore not absorbed. This
wavelength of light shows the least absorption.
The action spectrum of photosynthesis is a graph showing the rate of
photosynthesis for each wavelength of light. The rate of photosynthesis will not be the
same for every wavelength of light.
 The rate of photosynthesis is the least with green-yellow light (525 nm-625 nm).
 Red-orange light (625nm-700nm) shows a good rate of photosynthesis.
 The best rate of photosynthesis is seen with violet-blue light (400nm-525nm).
The wavelengths that is does not absorb are reflected from it.
Chlorophyll Is the main pigment contained in chloroplasts. It looks green because it
reflects green light. Other wavelengths (colours) of light are absorbed.
llle diagram shows the wavelengths of light absorbed by the various pigments found
in chloroplasts. These graphs are called absorption spectra.
If we shine light of various wavelengths on chloroplasts containing different pigments,
we can measure the rate at which they give off oxygen. These graphs are called action
spectra.

 

 

 

 

 

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