Cell Membrane – Definition, Function and Structure

Cell Membrane was discovered by Swiss botanists Carl Naegeli and C. Crammer in 1855.

Introduction:

The cell membrane is also called the plasma membrane, cytoplasmic or protoplasmic membrane. Cell The membrane is the second layer in plant cells below the cell wall, while it is the first in animal cells. Cell Membrane surrounds the cytoplasm and other organelles in it.

Structure:

In 1972 two scientists, S.J Singer and CL Nicolson proposed a fluid mosaic model explaining the structure of the cell membrane.

Fluid Mosaic Model

According to this model, “Cell membrane comprises phospholipid by layer and proteins.
Some Cholesterol and Carbohydrates are also present in the cell membrane. The phospholipid
bilayer forms a Fluid Sea in which proteins are floating.

Phospholipids:

Phospholipids have two Ends.

1) Head:

Polar, spherical heads are located over the cell surface. They have a phosphate group. They
are called hydrophilic (Water Loving)

2) Tail:

They are also called Non-polar ends. They face each other in the middle of the bilayer. Tails of both layers attract, reach other and repel water. So they are Hydrophobic. (Water Hating).

Cholesterol:

Cholesterol is also present in the cell membrane. They make the membrane less permeable
for water-soluble substances. It makes the membrane a rigid structure.

Carbohydrates:

Carbohydrates are present in two forms, i.e., Glycolipids and Glycoproteins.

Proteins:

The cell membrane contains two types of protein.

1) Peripheral Proteins:

These Proteins are also called Extrinsic Proteins. These are attached to the inner-outer membrane
surface.

2) Integral Proteins:

These Proteins are also called Intrinsic Proteins. These are embedded in the lipid bilayer. These
Proteins perform the following functions.

• Some link to sugar-Protein markers on the cell surface.
• Some move ions or molecules across the membrane.
• Some attach the membrane to the cell’s inner Cytoskeleton.

Functions of Cell Membrane

1. It gives shape and protection to cells.
2. Transport material into and out of the cell.
3. Act as a receptor site and recognize chemicals, hormones and neurotransmitters, and help in signaling.
4. The boundary separates the part of the cell from the outer environment.
5. It helps in exocytosis and endocytosis.
6. Regulates material moving into and out of the cell and from one part to another.

The fluidity of the Cell Membrane

Cell membranes are fluid, meaning they are not fixed in position and can adopt amorphous
shapes. Membrane fluidity is enhanced at higher temperatures, i.e., an increase in temperature
decreases the Fluidity of the Cell membrane, and it is also affected by the composition of the
bilayer.

Cholesterol:

As the amount of cholesterol increases, the fluidity of the cell membrane decreases.

Fatty acids:

Saturated fatty acids are straight, while unsaturated fatty acids bend down. Saturated fatty acids decrease the fluidity, while unsaturated fatty acids increase the membrane’s fluidity.

Polar and Nonpolar Substances:

Polar substances increase the fluidity of the membrane, while Nonpolar Substances
decreases the fluidity.

Asymmetrical Nature of Cell Membrane

Asymmetrical means two sides of the membrane are not the same.

The cell membrane tends to have different compositions on one side of the membrane than on the other. The differences can be caused by the different ratios or types of amphipathic lipid-based molecules, the different positioning of the proteins (facing in or facing out), or the fixed orientations of proteins spanning the membrane.

Additionally, there are different enzymatic activities in the outer and inner membrane surfaces.

Upper phospholipids are phosphatidylcholine and sphingomyelin.

Lower Phospholipids are phosphatidyl serine and phosphatidyl ethanolamine.

Proteins are also different on both sides.

Transport Across Plasma Membrane

1. Active transport is the Movement of molecules or ions against a concentration gradient, i.e., from an area of lower concentration to an area of higher concentration with energy expenditure. In contrast, Passive transport is the movement of molecules or ions toward the concentration gradient, i.e., from an area of higher concentration to an area of lower concentration without energy expenditure.
2. Active transport is also called the uphill movement of molecules, while Passive transport
   also called the downhill movement of molecules.
3. Equilibrium maintenance is not necessary for active transport, while Equilibrium
   maintenance is required in passive transport.
4. Active transport is faster, while passive transport is slower.
5. Active transport is affected by O2 and cynoid concentration, while passive transport
   is unaffected.
6. Active transport is a unidirectional process, while passive transport is a bidirectional process.
7. Macromolecules like proteins, carbohydrates (sugars), lipids, and large cells are a few of the materials which are transported by active transport,
8. While Oxygen, monosaccharides, water, carbon dioxide, and lipids are the few soluble materials transported through passive transport.
9. Examples of Active transport are Endocytosis, Exocytosis, Proton pumps and Sodium potassium pumps, while examples of passive transport are osmosis, diffusion and facilitated diffusion.

Diffusion

Diffusion is the movement of molecules from the more concentrated solution to the less concentrated solution through the permeable membrane. The cell membrane does not
spend energy when molecules through it.

Facilitated Diffusion

The movement of molecules which involves the proteins as their helpers is called facilitated diffusion. Many molecules cannot diffuse through the cell membrane due to their size or charge; such molecules are taken into or out of the cell with the help of transport protein in the cell membrane.

The process is facilitated diffusion When transport proteins help move molecules from high to low concentration without energy expenditure.

Osmosis

It is the Diffusion of solvent (water) through a semi-permeable membrane. It is controlled by the relative concentration of solutes in the water on both sides of the membrane. Water always moves from a hypotonic solution (with a lower concentration of solutes) to a hypertonic solution (with a higher concentration of solutes).

A few examples of Osmosis are the following:

1. When a cell is placed in a hypotonic solution (which has a lower solute concentration than the cell), the water movement rate inside the cell is more. In such conditions, animal cells swell and may rupture due to the absence of a cell wall, while the plant cells become turgid (due to their hard cell wall).
2. When a cell is placed in an isotonic solution (a solution in which the concentration of solutes is equal to that of the cell), the rate of osmosis outward is equal to the rate of osmosis inward. In such a condition, animal cells retain their volume constant while plant cells become flaccid (loose), because the net uptake of water is not enough.
3. Water moves out when a cell is placed in a hypertonic solution (with a higher salt concentration than the cell). In such conditions, animal cells shrink in size. In plant cells, the cytoplasm shrinks within the cell wall.

Endocytosis and Exocytosis

Endocytosis is a general term for bringing macromolecules, large particles and even small cells into the cell.

In endocytosis, the cell membrane invaginates (folds inward) and takes in the materials from the environment, forming a small pocket. The pocket deepens, forming a vesicle. This vesicle separates from the plasma membrane and migrates with its contents to the cell’s interior.

The initial event in this process is the binding of the vesicle membrane with the cell membrane. Then, the vesicle’s contents are released into the environment, and the vesicle membrane is incorporated into the cell membrane.

Turgor

When a plant cell is surrounded by water or a hypotonic solution, the water moves into the cell vacuole by osmosis. The vacuole increases in size and pushes the cell contents against the cell wall.

This pressure exerted by the cytoplasm against the cell wall is known as turgor pressure, and the phenomenon is called turgor. The plant cell does not burst in turgid conditions because the cell wall is strong and relatively inelastic.

The importance of turgor in plants is as follows:

• It plays an important role in maintaining the plant’s shape.
• It provides support to plants, especially in young tissues.
• It also helps in the closing and opening of the stomata.
• Some flowers open during the daytime and close at night. This is also due to changes in turgor in the cells of the sepals of flowers.

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Read Also:

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• Mitochondria – Definition, Function & Structure
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