Cell membrane

The cell membrane is also known as the plasma membrane which surrounds the cell. It is the outermost covering of animal cells while in the plant it is present next to the cell wall.
 
It is thin pliable and elastic. The cell membrane is 7.5 nm to 10 nm thick.

 

membrane

Composition

The plasma membrane is made up of protein and lipids. the composition is
Proteins               55%
phospholipids      25%
Cholesterol           13%
Carbohydrates      3%
Other lipids           4%

Structure

The structure of the cell membrane is that it is lipid bilayer and contains proteins channels. The lipid bilayer is composed of three main types of lipids which are
Phospholipids
sphingolipids
cholesterol

Among the above phospholipids are the most abundant lipids present in the cell membrane.
 
A phospholipid molecule contains two portions which are
Phosphate portion
fatty acids portion

The phosphate portion is soluble in water so it is hydrophilic while the fatty acid portion is soluble in water so it is hydrophobic.
 
Due to the fatty acid portion, the phospholipids molecules are repelled by water but they attracted by each other.
 
The lipid layer is usually impermeable to water-soluble substances such as Glucose etc. While soluble substances such as oxygen and carbon dioxide can penetrate the layer.

Sphingolipids also contain water soluble and fat soluble portions and they are responsible for many functions which are
Protection from the harmful environmental factor
signal transmissions
adhesion site for extracellular proteins.

Cholesterols molecules are also fat-soluble molecules and their main function is to determine the permeability of a lipid bilayer to water-soluble substances. It also controls the fluidity of the membrane.

The functions of the cell membrane

The cell membrane it plasma membrane surrounds the cytoplasm of a cell separating the intracellular component from the extracellular components. It plays a major role in anchoring the cytoskeleton. The plasma membrane gives shape to the cell and holds the cells together to form a tissue.

It is a selectively permeable membrane which means that it regulates what enters and exits the cell.  The plasma membrane facilitates the transport of material which is needed for survival. The movement of molecules across the cell membrane occurs by two mechanisms that are active transport and passive transport.  Active transport is the movement of molecules across the membrane without using energy while active transport uses energy for the movement of molecules across the membrane.

Some substances such as carbon dioxide and oxygen can enter the cell through the process of diffusion. Diffusion is a passive transport process. Diffusion occurs small molecules move freely from high concentration areas to low concentration areas. Diffusion is considered as passive transport because of the movement through the diffusion don not require energy. Molecules movement in diffusion is caused by the concentration gradient.

Transmembrane proteins are the proteins that extend through the lipid bilayer. Molecules such as sugars and amino acids enter the cell passively through the transmembrane proteins.

Molecules also enter the cell membrane by endocytosis. Endocytosis is a process in which the cell absorbs some molecules by engulfing them inward. In the process of endocytosis, the membrane creates a small deformation inward and in the next step, the substances to be transported are captured in this invagination. In the last step, the invagination pinches off from the cell membrane on the inside of the cell creating vesicles. Endocytosis is an active transport because it requires energy.

Prokaryotes and Plasma Membrane

Prokaryotes are divided into two groups that are
1. Archaea
2.  Bacteria

Bacterias are further divided into two groups that are gram-positive and gram-negative bacteria. Gram-negative bacteria have both plasma membrane and outer membrane. Other prokaryotes only have a plasma membrane.

Fluid mosaic model

According to the fluid mosaic model, the membrane is a two-dimensional liquid in which the protein and lipid molecules diffuse more or less easily. The lipid bilayer forms the basis of the membrane. The cell membrane also contains a large number of proteins that form many other structures.

Variations in the cell membrane

The plasma membrane has different compositions in different types of cells present in the body.

Sarcolemma the plasma membrane of myocytes is similar to the plasma membrane of other cells but it has some function that makes it different from others. For example, sarcolemma transmit synaptic signals, and also t tubules are present in it.

Oolemma the membrane of oocytes lacks a bilayer and does not consist of lipids. But protein and channels are present in the oolemma.

Axolemma is the membrane of the axons that contain granular and densely packed lipid bilayer. The axolemma works closely with the cytoskeleton components.

Permeability of cell membrane

Permeability is the rate of passive diffusion through the plasma membrane. Permeability of the plasma membrane depends on
1. electric charge
2.  Polarity of molecules
3.  Molar mass if molecules.

Plasma membrane proteins

The proteins of cell membrane float in the lipid bilayer. There are two types of cell membrane proteins which are

Integral proteins

These proteins protrude all the way through the membrane.

Peripheral proteins

These proteins are attached to the surfaces and do not penetrate all the way.
 
 Integral proteins make specific channels through which water-soluble substances enter the cell. These integral proteins are also responsible for selective diffusion. Some integral protein also acts as carrier proteins that transport water-soluble substances and some time transport substances against their electrochemical gradient.


Some integral proteins act as enzymes.

Peripheral proteins often attached to the integral proteins and act as a controller of the transport of substances.

Membrane carbohydrates

About 3% carbohydrates are present in the plasma membrane but these carbohydrates are present in combination with lipids and proteins making glycolipids and glycoproteins.
These carbohydrates make a loose carbohydrate coat on the outside surface of the membrane which is known is Glycocalyx.

The function of these carbohydrates are
 
They make the outer surface of the cell negatively charged.
Attach cell to another cell sometimes.
receptor substances for binding hormones.

Summary

cell membrane or plasma membrane is the outer layer of a cell in the animals while in the plants it lies next to the cell wall. The structure of the cell membrane is explained by the fluid mosaic model. the plasma membrane contains proteins, lipids, and carbohydrates.