Unit 2 – The cell membrane

Biology

Plasma Membrane

• It protects and supports the cell and also controls everything that enters and leaves the cell.

• Selective permeability (semipermeability) – ability to allow only certain molecules in or out of the cell.

• *Remember: both plant and animal cells have a plasma membrane. The plant cell has a cell wall then the plasma membrane.

Phospholipid Bilayer

• Consists of fatty acids and glycerol.• Each phospholipid molecule has a head and

two tails.• The head “loves” water (hydrophilic) and the

tails “hate” water (hydrophobic).

What enters the cell?

• Molecules that are hydrophobic can easily pass through the plasma membrane, if they are small enough, because they are water-hating like the interior of the membrane.

• Hydrophilic molecules cannot pass through the plasma membrane. –they need help to pass through.

Other molecules in the plasma membrane

• Plasma membranes also contain other molecules:– Cholesterol (lipid) –

helps the plasma membrane keep its shape.

– Proteins – assist other substances in crossing the membrane.

Carbohydrates

• Carbohydrates (sugars) found attached to proteins or lipids on the outside of a cell membrane.

• Provides cushioning and protection for the plasma membrane.

• Also important in cell recognition – your body can recognize cells and determine if they should be there or not.

• Also acts as a glue to attach cells together.

Proteins

• Peripheral proteins – are located outside of the lipid bilayer. – act as enzymes, receptors for specific molecules, or transport materials.

• Integral proteins – embedded in lipid bilayer.

Transport

• Passive transport – does not require ATP, with concentration gradient – molecules move from high concentration to low concentration

• Active transport – requires ATP, against concentration gradient – molecules move from low concentration to high concentration.

Passive transport

1. Simple diffusion – movement of a substance across a membrane.

2. Osmosis – movement of water across membrane.3. Facilitated diffusion – uses transport proteins to

moves substances across the membrane.a. Channel proteins – tiny holes, allows water molecules

and small ions to pass through.b. Carrier proteins – bind with specific ions or

molecules, changes shape allowing them to carry ions or molecules across the membrane.

Active Transport

• Energy is needed because the substance is moving from an area of lower concentration to an area of higher concentration.

1. Sodium-potassium pump – sodium ions are pumped out of the cell, and potassium ions are pumped into the cell. -- both ions move from areas of lower to higher concentration.

Vesicle Transport

• Big molecules can be moved out or in cell.

1. Endocytosis – vesicle transport that moves a substance into the cell.

a. Phagocytosis – taking in a solid.

b. Pinocytosis – taking in a liquid.

2. Exocytosis – vesicle transport that moves a substance out of the cell.