Photosynthesis
Purpose and Reaction
• Photosynthesis is the process in which plants use
sunlight to produce glucose which is food for the
plant. This is converted and stored as starch
(converts solar energy into chemical potential
energy, which is then available to other
organisms).
* Glucose is modified into other substances eg cellulose for the cell walls
Chloroplasts –
Where Photosynthesis Happens
Chloroplasts –
Where Photosynthesis Happens
• The chloroplast is the organelle where photosynthesis occurs. Chloroplasts
have thin membranes / large surface area for absorption of light.
• The organelle is surrounded by a double membrane (membranes are thin
for rapid diffusion, and with a large surface area for absorption of light and
other substances). Inside the inner membrane is a complex mix of enzymes
and water. This is called stroma and is important as the site of the lightindependent reactions. Stroma is a clear fluid, which doesn’t block the light.
• Embedded in the stroma is a complex network of stacked sacs. Each stack is
called a granum and each of the flattened sacs that make up the granum is
called a thylakoid. The large surface area created by this arrangement,
allows the maximum amount of light to be captured, and allows the
enzymes that control the reaction to have the best opportunity to reach the
reactants and maximise the rate of reaction.
Light Dependent vs Light Independent
Parts of a Leaf
Parts of a Leaf
Key: 1) cuticle 2) upper epidermis 3) palisade mesophyll 4) spongy mesophyll 5) lower
epidermis 6) stoma 7) guard cells 8) xylem 9) phloem 10) vascular bundle
Functions of the Parts of a Leaf
•
The epidermis (2 = upper, 5 = lower) is usually transparent (epidermal cells lack chloroplasts) and
coated on the outer side with a waxy cuticle that prevents water loss which would decrease the rate
of photosynthesis
•
Cells containing most chloroplasts are found near the top of the leaf in (palisade) long cylindrical
cells (3), with the chloroplasts close to the walls of the cell. These factors combine and lead to more
light being received where it can be used to maximise the rate of photosynthesis. NOT CLOSER TO
THE SUN! – just more light intensity. The slight separation of the cells provides maximum absorption
of carbon dioxide.
Beneath the palisade layer is the spongy mesophyll (4). The cells of the spongy layer are more
rounded and not so tightly packed. There are large intercellular air spaces. This maximises the rate
of diffusion of the gases during photosynthesis. (CO2 entering the leaf and O2 leaving it.)
Specialised guard cells (7), mainly on the lower leaf epidermis, function to control the movement of
gases and water loss through the stomata (6).
Vacuoles push chloroplasts closer to cell edges (MUST NOT SAY PUSH OUTSIDE THE CELL!). This
reduces diffusion distance of CO2 and O2 and may help prevent shading by other organelles?
The combination of the cell and organelle structure and location collectively helps to keep
photosynthesis at its maximum potential rate.
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Factors Affecting the Rate of
Photosynthesis
Limiting factor: factor that is low in
abundance thus lowering the rate of reaction
Temperature affects the rate of photosynthesis. Because reactions are enzyme
controlled, photosynthesis has an optimum temperature.
ie. Greater rate of photosynthesis at midday than early morning or evening, and little or
no photosynthesis at night; also greater rate in summer compared to winter
Thinking…
Explain the ‘saw tooth’ pattern
Carbon Dioxide
The annual saw-tooth
pattern in the carbon dioxide
graph to the right is
produced by growing
vegetation that consumes
carbon dioxide in the spring
and summer, and releases it
in the fall and winter. The
long-term upward trend
results from carbon dioxide
that is released by the
combustion of fossil fuels.
Limiting Factors: Phytoplankton Blooms
Spectacular evidence of what
happens when a limiting factor is
removed is seen in this image
taken from the ESA Enviro Sat.
So what’s happening? Well, in
marine phytoplankton, iron is
often a limiting micronutrient,
along with other mineral
elements.
These tend to sink to the bottom
of the ocean, but upwelling
currents can push them up to the
near-surface, making them
available for use and resulting in
a population explosion.