• Leaves enable photosynthesis to occur. Photosynthesis
is the process by which leaves absorb light and carbon
dioxide to produce carbohydrate (food) for plants to
grow. Leaves are adapted to perform their
function, e.g. they have a large surface area to absorb
• Plants have two different types of 'transport'
tissue, xylem and phloem, that move substances in and
around the plant. When water evaporates from the
leaves, resulting in more water being drawn up from
the roots, it is called transpiration.
Structure of a leaf
• The function of a leaf is photosynthesis – to absorb
light and carbon dioxide to produce carbohydrates. The
equation for photosynthesis is:
• Carbon dioxide and water → glucose and oxygen
• Did you know:
• Leaves are the source of all of food on the planet
• Leaves recycle all of the world's carbon dioxide in the
• Leaves contain the world's most abundant enzyme
Large surface area To absorb more light
Thin Short distance for carbon dioxide
to diffuse into leaf cells
Chlorophyll Absorbs sunlight to transfer
energy into chemicals
Network of veins To support the leaf and transport
water and carbohydrates
Stomata Allow carbon dioxide to diffuse into
Leaves are adapted in several ways to help them
perform their function.
Features of leaves
• The internal structure of the leaf is
also adapted to promote efficient
Epidermis is thin and transparent To allow more light to reach the palisade
Thin cuticle made of wax To protect the leaf without blocking out
Palisade cell layer at top of leaf To absorb more light
Spongy layer Air spaces allow carbon dioxide to
diffuse through the leaf, and increase
the surface area
Palisade cells contain many chloroplasts To absorb all the available light
• Three factors can limit the speed of
photosynthesis: light intensity, carbon
dioxide concentration and temperature.
• Without enough light, a plant
cannot photosynthesise very
quickly, even if there is plenty of
water and carbon dioxide.
Increasing the light intensity will
boost the speed of photosynthesis.
• Sometimes photosynthesis is
limited by the concentration of
carbon dioxide in the air. Even if
there is plenty of light, a plant
cannot photosynthesise if there is
insufficient carbon dioxide.
• If it gets too cold, the rate of photosynthesis will
decrease. Plants cannot photosynthesise if it
gets too hot.
• If you plot the rate of photosynthesis against the
levels of these three limiting factors, you get
graphs like the ones above.
• In practice, any one of these factors could limit
the rate of photosynthesis.
• Farmers can use their knowledge of these
limiting factors to increase crop growth in
greenhouses. They may use artificial light so
that photosynthesis can continue beyond
daylight hours, or in a higher-than-normal light
intensity. The use of paraffin lamps inside a
greenhouse increases the rate of
photosynthesis because the burning paraffin
produces carbon dioxide, and heat too.
• No heart, no blood and no circulation, but
plants do need a transport system to move
food, water and minerals around. They use
two different systems – xylem moves water
and solutes from the roots to the leaves –
phloem moves food substances from leaves
to the rest of the plant. Both of these systems
are rows of cells that make continuous
tubes running the full length of the plant.
• Xylem cells have extra reinforcement in
their cell walls, and this helps to support
the weight of the plant. For this reason, the
transport systems are arranged differently
in root and stem – in the root it has to resist
forces that could pull the plant out of the
ground. In the stem it has to resist
compression and bending forces caused by the
weight of the plant and the wind.
• Stem – the xylem and phloem are
arranged in bundles near the edge of
the stem to resist compression and
• Root - xylem and phloem in the centre
of the root to withstand stretching
Comparison of xylem and phloem
Tissue Process What is moved Structure
Xylem Transpiration Moves water and
roots to leaves
Phloem Translocation Moves food
leaves to rest of
Columns of living
Root hair cells
• Plants absorb water from the soil by osmosis. Root hair
cells are adapted for this by having a large surface area
to speed up osmosis.
• The absorbed water is transported through the roots to
the rest of the plant where it is used for different
• It is a reactant used in photosynthesis
• It supports leaves and shoots by keeping the cells rigid
• It cools the leaves by evaporation
• It transports dissolved minerals around the plant
• Leaves are adapted for photosynthesis by having a
large surface area, and contain openings,
called stomata to allow carbon dioxide into the leaf.
Although these design features are good for
photosynthesis, they can result in the leaf losing a lot of
water. The cells inside the leaf have water on their
surface. Some of this water evaporates, and the water
vapour can then escape from inside the leaf
• To reduce loss the leaf is coated in a wax cuticle to
stop the water vapour escaping through the epidermis.
Leaves usually have fewer stomata on their top
surface to reduce this water loss.
• Plants growing in drier conditions tend to have
small numbers of tiny stomata and only on
their lower leaf surface, to save water loss.
Most plants regulate the size of stomata
with guard cells. Each stoma is surrounded
by a pair of sausage-shaped guard cells. In
low light the guard cells lose water and
become flaccid, causing the stomata to close.
They would normally only close in the dark
when no carbon dioxide is needed for
• Most plant cells are turgid at all times. This
supports the weight of the plant, which is
especially important where there is no woody
tissue, such as leaves, shoot and root tip. If the
plant loses water faster than it can be absorbed
the cells lose turgor pressure and become
flaccid. This causes the plant to wilt.
• You should be able to explain why most plants
will wilt if they get flooded by sea water. (Hint:
sea water contains many chemicals in
solution, such as salt. Osmosis will move water
across the plant cell membrane, from the weaker
to the stronger solution.)
• Osmosis is the movement of water molecules
from an area of high concentration of water to
an area of lower concentration of water through
a partially permeable membrane. This can be the
cell membrane. An example is the flooding of
plants by sea water. Sea water contains many
chemicals in solution, such as salt. Osmosis will
move water across the plant cell
membrane, from the weaker to the stronger