Cell membrane

Cell membrane

By Assist Prof .Zainab Ali Kadhem for first year medical student

biochemical functions of cell membrane: 

Biochemical functions of cell membrane

The chemical structure of cell membrane

Mainly, membranes formed of lipids , proteins and carbohydrates. These compound are vary in type and percentage , any changes in the percentage or type would be effect cell membrane properties.

Lipid cell membrane

Lipids membrane

Is the membrane rigid or fluid ?

1- Fluidity:the ability of a substance to flow easily.

Significance of fluidity:

As membrane fluidity increase, so its permeability to water and other small hydrophilic molecules increase. The lateral mobility of integral proteins increases as the fluidity of the membrane increase. Many of most basic cellular processes including cell movement, cell recognition, cell division, endocytosis not possible if the membrane rigid not fluid.

2- fluid mosaic model, Mosaic : collection of things stuck to gather 

In 1972, Singer and Nicolson postulated a theory of membrane structure called the fluid mosaic model,
Proteins are interspersed in the lipid bilayer, of the plasma membrane, producing a mosaic effect

5- Selective permeability:

why Ionic and polar substances cannot pass cell membrane freely? . This is due to the hydrophobic nature of the hydrocarbon chain in lipid bilayer. These substances can go out and in of cells by specific membrane proteins.

Membrane of proteins:

Membrane proteins are classified as 1- integral if they are firmly embedded in the bilayer peripheral if they are loosely attached to the outer or inner surface as in the

Membrane carbohydrate 

How substances can pass cell membrane

Types of membrane transport mechanism

Passive Transport or Passive Diffusion 
1- is the process by which molecules move across a membrane without energy (ATP). 2- The direction of passive transport is always from a region of higher concentration to one of lower concentration. 3- There are two types of passive transport as follows: 1. Simple diffusion 2. Facilitated diffusion


Simple Diffusion : Lipid soluble substances , i.e. lipophilic molecules can pass through cell membrane, (e.g. O2, CO2, glycerol) without any interaction with or need carrier proteins in the membrane. Such molecules will pass through membrane along the concentration gradient, i.e. from a region of higher concentration to one of lower concentration and will continue until molecules become equilibrium.

Facilitated Diffusion( or carrier-mediated diffusion) 

• The movement of water soluble molecules and ions. ( e. g glucose and most of the amino acids )They pass through specific carrier proteins (a carrier protein binds to a specific molecule on one side of the membrane and releases it on the other side ). • These diffusion processes are not coupled to the movement of other ions, they are known as uniport transport processes.

Type of transporting across cell membrane

If a molecule moves against a concentration gradient, an external energy source is required.• Substances that are actively transported through cell membranes include, Na+, K+, Ca2+, H+, Cl–, several different sugars and most of the amino acids. Active transport is classified into 2 types according to the source of energy used as follows : i. Primary active transport ii. Secondary active transport. • In both instances, transport depends on the carrier proteins; like facilitated diffusion. However, in active transport, the carrier proteins function differently from the carrier in facilitated diffusion. Carrier protein for active transport is capable of transporting substance against the concentration gradient. Active Transport

Primary Active Transport

The energy is derived directly from hydrolysis of ATP. • Sodium, potassium, calcium, hydrogen and chloride ions are transported by primary active transport. Primary active transport of Na+ and K+ (sodium-potassium pump) • 3 Na+ ions pumps out of the cell and at the same time pumps 2K+ ions from outside to the inside generating an electrochemical gradient. • Carrier protein of Na+-K+ pump has 3 receptor sites for binding Na+ on the inside of the cell and 2 receptor sites for K+ on the outside. The inside portion of this protein has ATPase activity

Primary active transport of Na+ and K+ (sodium-potassium pump ,Na+-K+ ATPase)

Physiological importance of Na+-K+ pump 
The active transport of Na+ and K+ is of great physiological significance. The Na+-K+ gradient created by this pump in the cells 1- controls cell volume. 2 - It carries the active transport of sugars and amino acids.

Transport of Macromolecules Across the Plasma Membrane 

• The process by which cells take up large molecules is called endocytosis and the process by which cells release large molecules from the cells to the outside is called exocytosis . Endocytosis • There are two types of endocytosis: 1. Pinocytosis (cellular drinking) 2. Phagocytosis (cellular eating). Exocytosis• Most of the endocytotic vesicles formed from pinocytosis fuse with lysosomes.

Membrane-related diseases

plasma membrane and other more simple membranes within the cell are damaged in all diseases, chemical or functional changes in these membranes are central to the pathogenesis of the disease: Receptor-related diseases ( e.g. mutation) such as type II familial hypercholesterolaemia, Grave's disease, some lysosomal storage diseases and some forms of diabetes and obesity; Structural instability as manifested by red cell abnormalities and multiple sclerosis; Changes in lipid state as in muscular dystrophy and multiple sclerosis; Altered permeability or transport as in cystic fibrosis, diseases associated with specific transport defects, and the action of many bacterial toxins, and abnormality of the cytoskeleton-membrane interface as in Chediak-Higashi