Biology 1.3 (Part 1)

The cell membrane

cell membrane

• also known as plasma membrane

• This organelle envelopes around the body of a cell, separating internal and external environments.

• Controls entry and exit of substances across the cell

• Provides structure and cell defence.

Cell membrane structure

• consists of phospholipids (and other lipids) and membrane proteins

• Semi-permeable (solid but can let substances seep in and out

Phospholipid bilayer

2 layers of phospholipids ‘dotted’ with proteins

Fluid mosaic model

describes main characteristics of cell membrane which is:

• phospholipid heads positioned on outer and inner edges of bilayer.

• Cholesterol

• Glycolipids

• membrane proteins

• glycoproteins

Phospholipid components

• bilayer forms when exposed to water

• hydrophobic: when tails face interior membrane (water hating)

• hydrophilic: head groups are in contact with water on surface of membrane (water-loving)

• Low temperatures: packed tightly therefore less permeable

• High temperatures: move apart therefore more fluid and permeable

Cholesterol

• type of lipid found in-between phospholipids

• restricts sideways movement of phospholipids and proteins

• Prevents too solid or too fluid membrane

glycolipids

• carbohydrate + lipid

• projects from exterior surface, allows cells to recognise each other

• Can act as a receptor and aid immune system

glycoproteins

• Proteins that have carbohydrate molecules attached

• They aid recognition (like glycolipids)

membrane proteins

• makes up a large part of the cell membrane

• assists in transport across membrane

• 2 main proteins are integral and peripheral

Integral proteins

penetrate partway or all the way through the membrane

Peripheral proteins

loosely connected to the bilayer or to integral proteins