Photosynthesis
STEP Grade 9
Simple Diagram of a Leaf
Cross Section of a Leaf
The Chloroplast
The Chloroplast
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Stroma: The matrix surrounding the grana in
the inner membrane of chloroplasts. The area
between membranes (thylakoids, grana)
inside the chloroplast.
Grana: A series of stacked thylakoid disks
containing chlorophyll; found in the inner
membrane of chloroplasts.
Chlorophyll
Reactions of Photosynthesis
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6CO2 + 6H2O
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The process is a lot more complex
Actually takes place in two separate stages:
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C6H6O6 + 6O2
The Light reactions – occurs in grana
The Dark reactions – occurs in stroma
The Light Reactions

The purpose of the light reactions is to change light
energy into chemical energy in the form of 2
chemicals ATP and NADPH.
 Requires light
 Light energy used to make ATP.
 The light splits water into hydrogen ions (H+) and
oxygen atoms.
 The oxygen is released to the air.
 The H+ ions are used to make NADPH
 ATP carries energy to the dark reactions.
 NADPH carries energy and H+ to the dark
reactions.
The Dark Reactions

The purpose of the dark reactions is to
change CO2 into sugar by adding energy &
the H+ ions from ATP & NADPH.


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Can occur in light or dark.
6 CO2 & lots of ATP & NADPH are needed to
make one sugar molecule.
The sugar is then used for plant growth or stored
for use in fruits and leaves of plants as food for
animals
Light & Dark Reactions
Limiting Factors

Factors affecting the rate of photosynthesis.

As the rate of photosynthesis increases so will the plants
growth rate.

Three factors limit photosynthesis from going any faster:
Light level, carbon dioxide level, and temperature.

Without enough light a plant cannot photosynthesize
very fast, even if there is plenty of water and carbon
dioxide. Increasing the light intensity will make
photosynthesis faster.
Limiting Factors

Sometimes photosynthesis is limited by the
level of carbon dioxide. Even if there is
plenty of light a plant cannot photosynthesize
if it has run out of carbon dioxide.

Temperature can be a limiting factor too. If it
gets too cold the rate of photosynthesis will
slow right down; equally, plants cease to be
able to photosynthesize if it gets
Limiting Factors
Questions
1.
Which graph best shows the effect of increasing
light intensity on the rate of photosynthesis?
2.
Which graph best shows the effect of increasing
carbon dioxide concentration on the rate of
photosynthesis? A
3.
Which graph best shows the effect of increasing
temperature if light and carbon dioxide are not
limiting (i.e. the levels of light and carbon dioxide
are high). A