Plasma membrane called selectively permeable membrane because it pass only selected substance in or out through the cell.
What is Plasma Membrane
The plasma membrane is a outermost covering of a cell that is thin, flexible and a living membrane that surrounds the cell and protect the content of the cell from the external environment.
Why is plasma membrane called selectively permeable membrane
The plasma membrane called selectively permeable membrane because the plasma membrane which is outermost covering of the cell that allows to pass selective material inside or outside to the cell. For example: it allow to enter oxygen, glucose , water and allow to exit like carbon dioxide, excess water, toxic substance E.t.c
Everyday Example to Understand Selective Permeability
Imagine a security gate at an airport. This security person is behave like selective permeable membrane that allows only to pass certain type of passengers and prohibit some other person.
- Passengers with valid tickets are allowed to enter to airport.
- Unauthorized persons who have not ticket are stopped.
- Security officers decide who can pass.
For the above example, the plasma membrane acts like a security system that decides which substances can enter or leave the cell. I Hope it will helpful for you and make clear understand about these terms.
Another Example:
Suppose that one day you are alone in your home and all of your family members gone to a picnic. You decided that i lock the main gate of my house and when someone call me then i check it.
Now an unknown two person come and ring the bell you see that person but you don’t know about who is this? can you open the gate, off Couse not, you are not familiar to that person. then you don’t open the gate.
But if your mother or father come then you will open the gate because you know well, That’s similar way a plasma membrane behave like a selectively permeable membrane. It allows only selected substance.
Structure of the Plasma Membrane
you can see clearly in the below image, the plama membrane is made up of these components.
The plasma membrane consists mainly four components
- Phospholipid bilayer
- Proteins
- Cholesterol
- Carbohydrates
These components can work together to regulate the movement of material through the cell membrane.
| Component | Function |
|---|---|
| Phospholipid bilayer | It provide the basic structure of the membrane that helps as a barrier to prevent cell from external environment. |
| Membrane proteins | It Helps to transport substances in or out of the cell. It also provide structural support to the membrane. |
| Cholesterol | It Maintains stability and fluidity of the membrane. It prevent cells to become rigid. |
| Carbohydrates | It Assists in cell recognition and communication to other cells. |
Substances Allowed to Pass Easily
In plasma Membrane the cell allow some substance that are easily pass in to to cell. Below is the list of few.
- Oxygen (O₂)
- Carbon dioxide (CO₂)
- Water molecules
- Small lipid-soluble substances
Substances Restricted from Passing
The substance that cannot be pass in to the cells as they are restricted by plasma membrane. Some of these are;
- Large polar molecules ( sucrose, amino acids )
- Large protein molecules ( Hemoglobin, insulin, antibodies )
- Complex carbohydrates ( Starch, glycogen, cellulose )
- Most ions ( Sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), chloride (Cl⁻) )
- Harmful substances ( Heavy metals (lead, mercury), toxins, many bacteria and viruses )
How Does Selective Permeability Work?
The plasma membrane of our cell passage of the substance through various process such as simple Diffusion, Osmosis, Facilitated Diffusion and Active transport system.
1. Simple Diffusion
The process in which simple molecule like oxygen , carbon dioxide move from an area of higher concentration to lower concentration through the cell membrane.
Examples:
- Oxygen
- Carbon dioxide
2. Osmosis
The water molecule is pass through selectively permeable membrane from a region of higher water concentration to region of lower water concentration then this process is said to be osmosis.
3. Facilitated Diffusion
It is a special type of diffusion where substance can move from higher concentration to lower concentration with the help of special typed os protein. This process doesn’t use their ATP Energy. It is also called as passive transport because need of other protein during Diffusion.
Examples:
- Glucose
- Amino acids
4. Active Transport
A Active transport is the process where substance or material move across the plasma membrane from the higher concentration to lower concentration with the help of ATP Energy. Thus it said to be Active transport.
Examples:
- Sodium ions
- Potassium ions
Examples of Selective Permeability
| Substance | Can Pass Freely? | Method of Transport |
| Oxygen | Yes | Simple diffusion |
| Carbon dioxide | Yes | Simple diffusion |
| Water | Yes (controlled) | Osmosis |
| Glucose | No | Facilitated diffusion |
| Sodium ions | No | Active transport |
| Proteins | No | Cannot pass directly |
Importance of Selective Permeability
Selectively permeable membrane is very important for our cell because it play a vital role of movement of substance.
✔ Maintain the cell environment neutral
The plasma membrane maintains a stable environment of the cell and free from any external changes. I
✔ Obtain Nutrients
The plasma membrane provide essentail nutrients to the cell by allowing entering when it is needed.
✔ Remove Waste Products
It remove the toxic and waste product out of the cell like co2, toxic chemicals.
✔ Protect the Cell
It protect from the harmful substances and toxins.
✔ Regulate Water Balance
The cell maintains the proper water concentration inside the cell that increase or may decrease the concentration of water according to cell need.
✔ Support Cellular Activities
It maintain the proper exchange of substances ensures the cell do proper function.
Difference Between Permeable, Impermeable, and Selectively Permeable Membranes
| Type of Membrane | Description | Example |
| Permeable membrane | It Allows all substances to pass from it. | Filter paper |
| Impermeable membrane | It doesn’t allows the substance to pass from it | Plastic sheet |
| Selectively permeable membrane | It allows only certain substances or material to pass | Plasma membrane |
Key Points to Remember
- The plasma membrane surrounds and provide the structure of the cell.
- It controls the movement of substances like water, carbon dioxide, oxygen, nutrients, e.t.c into and out of the cell.
- It allows only selected materials to cross in or out from the cell.
- This property is known as selective permeability.
- Selective permeability helps to maintain cell stability
- Transport occurs through diffusion, osmosis, facilitated diffusion, and active transport.
Frequently Asked Questions (FAQs)
Q1. Why is the plasma membrane called selectively permeable?
The plasma membrane is called selectively permeable because it allows only certain substances to enter and leave the cell and block the harmful substance or that is not needed.
Q2. What substances can pass through the plasma membrane?
Small molecules such as oxygen, carbon dioxide, and water molecule can pass easily through the plasma membrane. It is also the essential component of our cell.
Q3. Why is selective permeability important?
Selective permeability helps maintain cell stability, provide nutrients, remove waste material, and protect the cell form the external environement.
Q4. Which process involves the movement of water through the plasma membrane?
The movement of water molecule through the selectively plasma membrane by osmosis process.
Q5. Is the plasma membrane completely permeable?
No, plasma membrane is not said to be completely permeable. it is called selectively permeable. As it allows only selected substance.