AQA Biology GCSE B3.1 - first section only

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B3.1 Exchange of Materials
Active Transport
Key Points:
Substances are sometimes absorbed against a concentration gradient by
active transport.
Active transport uses energy from respiration
Cells can absorb ions from very dilute solutions and move molecules
through cell membranes using active transport.
Osmosis = the net movement of water from an area
of high water potential to an area of low water
potential through a partially permeable membrane
down a concentration gradient
Diffusion = the net movement of a substance from an
area of high concentration to an area of low
Active Transport = the movement of cells from an area of low concentration
to an area of high concentration. It requires energy from respiration to take
Osmosis and Diffusion depend on a concentration gradient in the right
direction to work. Substances are moved against
a gradient by active transport which uses energy
produced by mitochondria.
How does active transport work in a cell?

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Active transport allows cells to move substances from an area of low
concentration to an area of high concentration. Cells can absorb ions
from very dilute solutions and it also makes it possible for them to move
substances like sugars and ions from one place to another through the
cell membranes. It happens by taking energy for the active transport
system to carry a molecule across the membrane and then return it to its
original position.…read more

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The job of your breathing system is to move air in and out of your
lunges. It brings in oxygen-rich air and removes air containing waste carbon
The alveoli are tiny air sacs which have a large surface area, which is kept moist.
They also have a rich blood supply which maintains a concentration gradient in
both directions (in and out of your lungs). Due to this, the gas exchange is rapid
and effective because it takes place along the steepest concentration
gradients possible.…read more

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These increase the surface area. Each
individual villus is covered in microvilli which increases the surface area for
diffusion. The small intestine is only 7 metres long but because of the villi, it
creates a surface area of 2000m ²
Exchange of Materials in Other Organisms
Key Points
Whatever the organism, gas and solute exchange depends on a large
surface area, moist surfaces, short diffusion distances and a large
concentration gradient
Wherever gas or solute exchange is important, certain adaptations will
always be seen.…read more

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They have a large surface area and are very effective at gas
Exchange in Plants
Key Points
Plants have stomata which allow them to obtain carbon dioxide from the
Carbon dioxide enters the leaf by diffusion. Leaves have a flat thin shape
and internal air spaces to increase the surface area available for diffusion
Most of the water and mineral ions needed by a plant are absorbed by
the root hair cells which increase the surface area of the roots.…read more

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Key Points
The loss of water vapour from the surface of a plant leaves is known as
Water is lost through the stomata which are opened and closed by guard
cells to let in carbon dioxide for photosynthesis
Water is pulled up through the xylem from the roots to replace the
water lost from the leaves in the transpiration stream
Transpiration is more rapid in hot, dry, windy or light conditions.…read more

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