Download KA 2 Summary Sheet

Biology: Key Area 2
Summary Sheet
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Cell membrane is made up of proteins and phospholipid
molecules.
The cell membrane is selectively permeable.
Small, soluble molecules can easily pass through the membrane.
Large, insoluble molecules must first be broken down.
A concentration gradient can be described as a difference in
concentration of a substance between two solutions or between
two cells or cell/tissue and a solution.
There are two types of passive transport: osmosis and
diffusion.
Passive transport does not require energy and molecules move
down the concentration gradient.
Diffusion is the movement of molecules of a substance from an
area of high concentration to an area of low concentration
across a selectively permeable membrane until concentrations
are equal.
Diffusion is important to all living things. All cells need to take
in food (like glucose and amino acids) and oxygen. Waste
substances like carbon dioxide can be removed by diffusion.
Osmosis is the diffusion of water. Osmosis is the movement of
molecules of water from an area of high concentration to an
area of low concentration across a selectively permeable
membrane until concentrations are equal.
When an animal cell is placed in a solution with a higher water
concentration (HWC) it gains water by osmosis and eventually
bursts. When placed in a solution with a lower water
concentration (LWC) the animal cell loses water by osmosis and
shrinks.
When a plant cell is placed in a solution with a higher water
concentration (HWC) it gains water by osmosis and swells up to
become turgid. Plant cells do not burst as the cell wall prevents
this. When placed in a solution with a lower water concentration
(LWC) the plant cell loses water by osmosis and becomes
plasmolysed.
Active transport is the movement of molecules from a region of
low concentration to a region of high concentration against the
concentration gradient and requires energy.
An example of active transport is the sodium-potassium pump.