Membranes
Introduction

Topic 1 Cell theory

Topic 2 Prokaryotic cells

Topic 3 Eukaryotic cells

Topic 4 Cell Division

Topic 5 Cell membranes



Cell membranes separate the contents of a cell from their external enviroment, controlling exchange of material such as nutrients and waste products between the two.

Membranes are partially permeable since water, gases, glucose, aminoacids, fatty acids glycerol and ions diffuse through membranes. If only water and gases could cross the cell membrane then it would be called semipermeable.

Membrane are made out of proteins and lipids. This is because only organic compounds can diffuse through it like alchool or chloroforn. This suggests that membranes are non-polar.

Phospholipids:
A phospholipid consist of a polar head and two non-polar hydrocarbon tails from the fatty ads. The phospholipid molecules are unusual because the head is HYDROPHILIC ( water loving ) and the tail are HYDROPHOBIC ( water hating ).
When large amounts of water cover an amount of phospholipids then the hydrocarbon tail is projected inwards adnd the polar lead contacts the water. The structure taht the phospholipids obtain is called BILAYER.
This bilayer structur is one that the plasma membrane has.

This bilayer structure is one that the plasma membrane has.

Features and functions of a membrane:
> 7 nm thick
> The hydrophilic phosphate leads of the phospholipids face outwards into the aqueus enviroment
>The hydrocarbon tail, face inwards and creates a hydrophobic interior
>Phospholipids move by diffusion
>Most protein molecules float about in the phospholipid bilayer forming a fluid mosaic pattern

>Polar molecules and ions cannot go through the membrane
>Membranes have proteins inside them which have a specific shape and a specific function (eg. receptor of hormones and neurotransmiters ).
>Antigens are mage out of glycoproteins ( carbohydrates + lipids ) and they have a specific shape that enables them to be recognized from the immune system.
>Glycolipids act as cell-cell recognistion and are involved in the sticking the correct tissues together.
>Cholesterol, which is found in the membrane, acts as a reducing agent to stop polar molecules going inside the cell.

How does a cell obtain its needs:
>Diffusion
>Osmosis
>Active transport
>Bulk transport