GRADE 11 LIFE SCIENCES
SESSION 1: PHOTOSYNTHESIS
Key Concepts
In this session we will focus on summarising what you need to know about:
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process of photosynthesis using words and symbols:
importance of photosynthesis
effects of variable amounts of light, carbon dioxide and temperature on the rate of
photosynthesis
The role of carbon dioxide enrichment, optimum light and optimum temperatures in
greenhouse systems to improve crop
Role of ATP as an important energy carrier in the cell
Photosynthesis – the process
What is photosynthesis?
Process whereby green plants use the radiant energy of the sun and carbon dioxide to manufacture
their own food.
chlorophyll
Carbon dioxide+ Light + Water
Sugar + Oxygen
enzymes
6CO2 + 6H2O → C6H12O6 + 6O2
Photosynthesis takes place mainly in the leaf. These contain chloroplasts, which are tiny objects
containing chlorophyll.
Cross-section through a leaf cell
Photosynthesis occurs mainly in the palisade mesophyll, which contains chloroplasts, which are
organelles containing chlorophyll.
Explain this organelle structurally suited for photosynthesis.
The membrane is permeable to water and carbon dioxide
The stroma contain enzymes for the photosynthesis – dark phase.
The stroma has ribosomes which manufacture enzymes.
The lamella/thylakoids contain chlorophyll to trap radiant energy.
The grana consist of stacks of thylakoids to increase the surface area to absorb sunlight.
Two reactions make up photosynthesis:
1.Light Reaction or Light Dependent Reaction Produces energy from solar power (photons) in the form of ATP and NADPH.
2. Dark Phase/Calvin Cycle or Light Independent Reaction
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Uses energy (ATP and NADPH) from light reaction to make sugar (glucose).
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O2 is released
The Light phase
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Occurs in the Thylakoid membranes
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During the light reaction, there are two possible routes for electron flow:
A.
Cyclic Electron Flow
Here the energy is used to form the energy carrier, ATP
ADP + P
ATP
Noncyclic Electron Flow
Here the energy is used to split water molecules into high energy hydrogen atoms
and oxygen.
The hydrogen atoms combine with coenzymes which take them to into the stroma for
the dark phase.
The oxygen is released into the atmosphere as a by-product.
B.
The Dark Phase
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Occurs in the stroma
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Carbon dioxide combines with the high energy hydrogen atoms in a series of
reactions to form energy-rich carbohydrates like glucose and starch.
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The high energy hydrogen and ATP from the light phase provide the energy
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All reactions are controlled by enzymes.
A Summary of the process:
Question One
1.1
Why is photosynthesis important to organisms?
Maintains the oxygen balance in the atmosphere.
Maintains the carbon dioxide balance in the atmosphere.
Production of food - all of our food comes directly or indirectly from photosynthesis.
1.2
Draw a labelled diagram to illustrate the general structure of the chloroplast.
1.3
A learner performed an investigation in which a green leafy twig from an aquatic plant was placed in a
beaker containing sodium bicarbonate solution. The apparatus was exposed to a light source for a
period of time and the intensity of light from the light source was increased at regular intervals.
Gas bubbles were released. The number of gas bubbles released per minute was observed and
recorded. The results of the investigation are shown in the table below. Study the table and answer the
questions that follow.
(Nated 2008 hgp1)
Light intensity (lux)
10
20
30
40
50
60
70
Number of gas
bubbles released
5
12
18
24
26
30
30
What is the aim of the experiment?
Name the gas that is released in the investigation.
What is the function of the sodium bicarbonate solution in this investigation?
Explain the relationship between the light intensity and the number of gas bubbles released per
minute as shown in the table.
How would you set up a control for this investigation?
Factors limiting photosynthesis
Three factors can limit the speed of photosynthesis - light intensity, carbon dioxide
concentration and temperature. The following graphs illustrate this. Can you use the graphs to
explain how each factor affects the rate of photosynthesis?
Maximising growth
Farmers can use their knowledge of these limiting factors to increase crop growth in
greenhouses.
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They may use artificial light so that photosynthesis can continue beyond daylight
hours, or in a higher-than-normal light intensity they can use shade netting to reduce
light
The use of paraffin lamps inside a greenhouse increases the rate of photosynthesis
because the burning paraffin produces carbon dioxide, and heat too.
Temperature can be increased by using heaters and reduced by using fans and/or
shade nets.
All the above create ideal conditions within the greenhouse and crop yield may improve.