Transport and Cell Membranes

Membranes are very clever barriers around cells. They are a filtering system, allowing us get the right stuff coming in and out of our cells.

Cell membranes are selectively permeable, meaning that they are very picky with what they let in and out of the cells. Permeable means “allowing liquids or gases to pass through.”

Membrane Structure
Cell membranes have two layers of little structures called phospholipids.
These phospholipids are fat molecules. They have a head made out of glycerol, and tails made out of fatty acids. This phospholipid membrane structure is very flexible.
Membranes also have proteins that act as receptors and transporters. Receptors are proteins that respond to chemical messengers outside the cell. Transporter proteins like pumps and channel-forming proteins in the cell membrane allow the transport of molecules in and out of the cell.
Small molecules such as CO2 and oxygen are small and can pass through the phospholipid bilayer of the membrane. Other larger molecules, such as glucose, need to travel through transporter membrane proteins.

Diffusion in Cells
Diffusion is when molecules move from an area of higher concentration to an area of lower concentration. When molecules move from a high concentration to a low concentration, they are said to be moving down the **concentration gradient**.
This is very important for the survival of all life. For example, Oxygen diffuses from an area of high concentration in the air into an area of lower concentration in our blood within our lungs. This allows oxygen to enter our blood and travel around our body.

Osmosis
Osmosis is the travelling of water from an area of high concentration to an area of low concentration across a **selectively permeable membrane**.
In animal cells, you can see a clear effect of osmosis on cells.
If animal cells are placed in water of a high concentration, water enters it and they will burst.
If animal cells are placed in water of low concentration, water leaves the cell and they will shrink.
Osmosis also affects plant cells
If plant cells are placed in water of high concentration, water enters and they will become turgid
If plant cells are placed in water of a low concentration, water will leave the cell and they will become plasmolysed.

Passive and Active Transport
Passive transport is when molecules naturally travel down a concentration gradient, without using energy. This occurs through specific transport proteins that allow passive transport. Diffusion and osmosis are examples of passive transport.
Active transport requires energy to transport molecules across a membrane. This occurs when molecules need to be moved from an area of low concentration to an area of high concentration. As this is movement against the concentration gradient, energy is required. This energy is in the form of a molecule called ATP (adenosine triphosphate).

Key Points!

Cell membranes

Cell membranes are barriers that allow transport of molecules in and out of the cell.
They are selectively permeable
Structure of Membranes

They contain a phospholipid bilayer that is flexible.
The membrane also contains membrane proteins, used as receptors and transporters.
Diffusion

Diffusion is the movement of molecules down the concentration gradient from a higher to lower concentration.
Osmosis

Osmosis is the movement of water from an area of high water concentration to low water concentration through a selectively permeable membrane.
When animal cells are put in high water concentration, they burst.
When animal cells are put in low water concentration, they shrink.
When plant cells are put in high water concentration, they become turgid.
When plant cells are put in low water concentration, they become plasmolysed.
Passive and Active Transport

Passive transport is the movement of molecules across a membrane with no energy being required.
Active transport is the transport of molecules across a membrane with the help of energy in the form of ATP.