Why is a Cell or Plasma Membrane Called Selectively Permeable?

Plasma membrane

All cells have a biological membrane. It separates the protoplasm from the external environment. The plant cells have another outer layer called the cell wall which is not present in animal cells. Both animal and plant cells have a plasma membrane which is required for cellular homeostasis. They may vary in certain characteristics, but both plant and animal cells have a cell wall which is mostly the same in terms of structure and function.

What is a cell or plasma membrane? Where is it found?

The plasma membrane is a biological membrane present in every living cell in order to separate the internal components from the external environment.It protects the cells against the various external substances. It is made up of a phospholipid bilayer, proteins, lipids, carbohydrates,and other components. The different components of the plasma membrane make it a permeable barrier.

It is “semi-permeable” because it controls what goes in and goes out through cellular transport mechanisms. It also regulates the transmission of cellular signals. It lets the red blood cells and white blood cells to mold their shape as they go through narrow capillaries.

Why is it called a plasma membrane?

The cell has a protoplasm which is the semi-fluid living matter. It is placed inside a biological membrane known as the plasma membrane. Because it covers the entire cell, this plasma membrane is known as the cell membrane.

Why is a cell orplasma membrane called selectively permeable?

The cell membrane is made up of a very thin layer of protein and fat. It lets only selective substances to pass through it. That is why it is called a selectively permeable membrane.

It allows only certain molecules to move in and out of the cells. Not all molecules are allowed to pass through. If plasma membrane breaks down, then molecules of some substances will move in and out of the cells.

The plasma membrane serves as a mechanical barrier. Thus, exchange of material from its surroundings through osmosis or diffusion in acell does not take place. As a result, the cell will die due to the disappearance of the protoplasmic material. It lets hydrophobic molecules and small polar molecules pass through the lipid layer, but it does not allow ions and large polar molecules to spread through the membrane.

Proteins in Plasma Membrane

Plasma membrane has two types of protein molecules. They are:

1.Intrinsic proteins- They cover the lipid bilayer.

2.Extrinsic proteins- They are present either on the outer surface or on the inner surface of the lipid layer.

Functions of Plasma Membrane

The plasma membrane acts as a barrier between the inside and the outside environment.

• A physical barrier- The cellular content is physically separated from the external environment by a cell membrane thus protecting all the parts of the cell and allows independent activities to occur within and outside the cell. Same is the case of organelles where their plasma membrane lets only internal activities. Therefore, biological activities take place independently and simultaneously from one another.
• Semi-permeability- Plasma membranes are semi-permeable. It allows only certain molecules to pass through the membrane. The membrane allows free movement of water, oxygen and carbon dioxide. The ions such as potassium and sodium and polar molecules cannot move through the membrane easily. They must move through proper channels or pores in the membrane. In this way, the membrane can control the pace at which the cell can enter or exit such molecules.
• Endocytosis and exocytosis- Endocytosis takes place when a cell consumes greater substances than single ions or molecules that pass- through pores. A cell may ingest many molecules or even entire bacteria from the extracellular fluid by endocytosis. Exocytosis occurs when the substances are released by the cell. A very special role is played by the cell membrane in all these processes. The structure of the membrane differs such that the molecules may enter or leave the cell.
• Cell signaling- Communication and signaling between cells is another important feature of the membrane. This is done by the use of different proteins and carbohydrates in the membrane. Proteins mark the cell so that it can be identified by other cells. The membrane has receptors which helps to do certain tasks as molecules, such as hormones are attached to these receptors.

The function of the cell membrane in an animal cell is to control the movement of what comes in and out of the cell. The transfer of substances through the membrane occurs without the use of cell energy or cause the cell to consume energy while transferring it. Thus, the cell membrane acts as a particular channel which allows only a few substances to enter and leave the cell.

Learn more from the concept ofThe Fundamental Unit of Life from Class 9 Biology