Cell membranes and Transport

Phospholipids
A substance whose molecules are made up of a glycerol molecule , two fatty acids and a phosphate group; a bilayer of phospholipids forms the basic structure of all cell membranes. Interestingly,when a phospholipid is placed in water, it spontaneously folds upon itself to create a double layer or bilayer.


Here are some examples of Phosholipids in water
a. Spread as a singgle layer of molecules( a monolayer) on the surface of water

b. Forming micelles surrounded by water

c. Forming bilayers

d. Bilayers forming membrane-bound compartments

Structure Of Membrane

Membranes have many different functions with a typical cell, such as keeping unwanted viruses out, buht probably the most valuable is the partitioning of the cell into the functional and segregated compartments.

This is the diagram of the fluid mosaic model of membrane structure.

The Roles of the domponents of cell membranes

1. Phospholipids : This is the basic structure of membrane .They act as barrier to most water-soluble molecules such as sugars,amino acids & proteins cannot leak out of the cell, and unwanted water soluble molecules cannot enter the cell.

2.  Cholesterol : Is a relatively small molecule ,the function is they fit neatly between the phospholipid molecules with their heads at the membrane surface.

3. Glycolipids, glycoproteins and proteins : They form hydrogen bonds with the water molecules and help to stabilise the membrane structure.

Transport across the cell surface membrane 

There are 5 basic mechanisms by which this exchange is achieved : Diffusion, Facilitated diffusion, Osmosis , Active transport and bulk transport.

A. Diffusion : can be defined as the net movement, as a result of  random motion of its molecules or ions, of a substance from a region from its higher concentration to a region of its lower concentration.

A substance diffuses across a membrane depends on a factors , such as :

1.The 'steepness' of the concentration gradient.

2. Temperature

3. The surface area

4. The nature of the molecules or ions 

B. Facilitated Diffusion : It can only cross the membrane with the help of certain protein molecules.Facilitate means to make easy or make possible and this is what the proteins do.

C. Osmosis : is the best regarded as special type of diffusion involving water molecules only.

D. Active transport :as the energy consuming transport of molecules or ions across a membrane against a concentration gradient.

E. Bulk transport :  There are two type of it which is :

1. Endocytosis : engulfing of the material by the cell surface membrane to form a small sac or 'endocytic vacuole' it takes to forms ,they are Phagocytosis (cell eating) and Pinocytosis (cell drinking).

2. Exocytosis : is the process by which materials are removed from cells.

-This is an assignment for my Biology A level-

By : Via noviani