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Photosynthesis
and plant nutrition
IGCSE Biology revision notes
Structure of a leaf
Adaptations of a leaf
Feature
Function
Cuticle
Allows light through, prevents loss of water, protection against disease
Upper & lower epidermis
Allow light through (upper), protect leaf from mechanical damage
Palisade cells
Closely packed together, many chloroplasts for efficient light absorption as
they are close to the upper surface
Spongy cells
Large surface area, in contact with air spaces, efficient gas exchange
Air spaces
Allow rapid diffusion of gases
Stomata
Allow carbon dioxide into the air spaces
Guard cells
Open the stomata when conditions are right for photosynthesis (i.e. when
there is enough light) and close them at other times to prevent loss of
water vapour
Shape of leaf: thin and with
a large surface area
Thin: fast diffusion of gases; large surface area means more stomata to
absorb carbon dioxide and more surface to absorb sunlight
Limiting factors: Light, carbon
dioxide
Three factors can limit the speed of photosynthesis: light
intensity, carbon dioxide concentration and temperature.
In the graphs, the limiting factor is the one in short supply:
• when you increase it, the rate of photosynthesis
increases (i.e. between X and Y).
• If you increase it and the rate of photosynthesis does
not change, something else is now the limiting factor.
Y
X
Without enough light, a plant cannot photosynthesise
quickly, even if there is plenty of water and carbon dioxide.
Increasing light intensity boosts the rate of photosynthesis.
Sometimes photosynthesis is limited by the concentration of
carbon dioxide in the air. Even with plenty of light, a plant
cannot photosynthesise if there is too little carbon dioxide.
Y
X
Limiting factors: Temperature
If it gets too cold, the rate of
photosynthesis will decrease,
as the rate of diffusion and of
enzyme activity are affected
by low kinetic energy.
Plants cannot
photosynthesise if it gets too
hot because the enzymes
become denatured.
Greenhouses and photosynthesis
In greenhouses, limiting factors for photosynthesis are easily controlled:
•
•
•
Artificial light allows photosynthesis to continue beyond daylight
hours. Bright lights also provide a higher-than-normal light intensity.
Artificial heating increases the rate of photosynthesis
Additional carbon dioxide released into the atmosphere also allows
photosynthesis to continue at an increased rate.
However, the additional cost of lighting, heat and carbon dioxide has to
be weighed against the increased crop yield.
Paraffin lamps are traditionally used in greenhouses, providing
•
•
•
light
heat
carbon dioxide
Minerals required for plant growth
Mineral
Element
supplied
Why needed
Signs of deficiency
Nitrates
Nitrogen, N
Nitrogen is needed to make
amino acids for proteins
Poor growth, older leaves
turn yellow
Phosphates
Phosphorus, P
Phosphorus is needed to make
DNA and cell membranes
Poor root growth, young
leaves go purple
Potassium
compounds
Potassium, K
Potassium is needed in enzymes
Leaves turn yellow with
involved in respiration and
dead spots
photosynthesis
Magnesium
compounds
Magnesium, Mg
Magnesium is needed to make
chlorophyll
Leaves turn yellow