phospholipid membranes

biology ocr

what is the structure of a phospholipid

Made up of a phosphate group, glycerol unit and two fatty acid tails

Formation of a membrane

• Negatively charged (polar) phosphate head is hydrophilic

• The fatty acid tails are non-polar, they are hydrophobic

• If phospholipids are completely surrounded by water they form a phospholipid bilayer

Fluid mosaic model

Fluid: phospholipid bilayer in which individual phospholipids can move so membrane is flexible. proteins can move freely through the bilayer.

Mosaic: Extrinsic and intrinsic proteins are randomly arranged

Movement of fat soluble molecules and polar molecules through bilayer

Fat soluble organic molecules:

• diffuse through the bilayer 

Polar molecules:

• require proteins 

why can’t ions enter the cells

phospholipid bilayer acts as a barrier

Two types of proteins in bilayer

1. intrinsic proteins

2. extrinsic proteins

Intrinsic proteins

• completely span phospholipid bilayer 

• main transport system 

• can form channels, carrier proteins or active pumps

Extrinsic proteins

• on surface of bilayer or partially embedded in it

• provide mechanical support

• act as cell receptors

Types of intrinsic proteins

1. Channel proteins

2. Carrier proteins

Channel proteins

Pores in the membrane that let ions diffuse through 

Carrier proteins

Change shape to let specific molecules through

Glycoproteins

Protein with a carbohydrate molecule attached

• can allows cells to attach to each other (cell adhesion)

• acts as receptors for hormones and neurotransmitters

• cell signalling

• act as antigens

• recognition of cells

Glycolipids

Carbohydrates attached to phospholipid molecules

• act as cell markers or antigens

• cell signalling

• cell recognition

• cell adhesion

Purpose of cholesterol in cell membranes 

• Stabilises the membrane

• regulates its fluidity

• Prevent phospholipids from packaging too closely together and connects them

how do proteins move through lipid bilayer

-proteins move freely but this ease is dependent on the number of phospholipids with unsaturated fatty acids in the bilayer

what is the 3 roles of a membrane in a cell

• partially permeable barriers between the cell and its environment, between organelles and the cytoplasm and within organelles- act as barrier

• compartmentalisation

• sites of cell communication (cell signalling)

• controls what enters and leaves

• creates concentration gradients