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Cell Membrane Structure and Function

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eSaral Academic and Editorial Team

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Summary

The cell membrane, also called the plasma membrane, is the thin, selectively permeable boundary that encloses every living cell. Its structure — a phospholipid bilayer embedded with proteins, cholesterol, and carbohydrates — is described by the fluid mosaic model, while its core function is controlling what enters and leaves the cell. The cell membrane is present in the cells of all living organisms, both prokaryotic and eukaryotic.

Cell membrane structure and function diagram showing phospholipid bilayer

eSaral › Class 11 › Biology › Cell Membrane in Cell Biology

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Cell Membrane in Cell Biology: Structure, Functions & Diagram

In cell biology, the cell membrane is the thin, selectively permeable boundary that encloses every living cell, separating its internal contents from the external environment while controlling the movement of substances in and out. Also called the plasma membrane or plasmalemma, it is built primarily of a phospholipid bilayer embedded with proteins, cholesterol, and carbohydrates — a structure described by the fluid mosaic model. Every cell, whether prokaryotic or part of eukaryotic cells, has a cell membrane as one of its two universal features, alongside cytoplasm.

This topic forms the base of the Cell: The Unit of Life chapter for Class 11 Biology, and cell membrane structure is one of the most frequently tested areas in board exams and NEET Biology, contributing multiple questions almost every year. Test how frequently this exact topic appears with NEET Chapterwise PYQ.

What Is the Cell Membrane?

The cell membrane, in cell biology, is defined as the outermost living boundary of the cell that separates the protoplasm from the surrounding environment. It is present in every cell — from the simplest bacterium to the most complex human neuron — and is one of the two basic features shared by both prokaryotic and eukaryotic cells (the other being cytoplasm).

  • The cell membrane is selectively permeable, allowing some substances to pass while blocking others.
  • It is roughly 5–10 nanometres thick and appears trilaminar (three-layered) under an electron microscope.
  • The cell membrane is not a rigid wall — it is fluid, dynamic, and constantly in motion.
  • Unlike the rigid cell wall found in plant cells and bacteria, the cell membrane itself is flexible and present in all cell types.

Cell Membrane – Structure & Functions (Cell Biology)

The structure of the cell membrane in cell biology is best explained by the fluid mosaic model, proposed by S.J. Singer and G.L. Nicolson in 1972. This model describes the membrane as a mosaic of different components — phospholipids, proteins, cholesterol, and carbohydrates — that gives it a fluid, constantly shifting character rather than a fixed, rigid one.

What Are the Components of Cell Membrane Structure?

Component

Role in Cell Membrane Structure

Phospholipid bilayer

Forms the basic two-layered framework, with hydrophilic heads facing outward and hydrophobic tails facing inward

Integral (intrinsic) proteins

Embedded fully within the membrane; often form channels or carriers for transport

Peripheral (extrinsic) proteins

Loosely attached to the inner or outer membrane surface; involved in signalling and support

Cholesterol

Present between phospholipids; regulates membrane fluidity and stability

Carbohydrates (glycoproteins/glycolipids)

Extend from the outer surface; involved in cell recognition and cell-to-cell communication

Why Is the Cell Membrane Called "Fluid Mosaic"?

The term fluid mosaic describes two properties of cell membrane structure at once:

  • Fluid — phospholipids and many proteins can move laterally within the layer, giving the membrane a flexible, liquid-like consistency.
  • Mosaic — proteins are scattered irregularly throughout the phospholipid bilayer, like tiles in a mosaic pattern, rather than being arranged in a fixed order.

What Are the Main Functions of the Cell Membrane?

The functions of the cell membrane in cell biology go far beyond simply enclosing the cell:

  • Selective permeability — regulates which substances enter and exit the cell, maintaining internal stability.
  • Transport of materials — moves ions, nutrients, and waste products across the membrane through passive and active transport.
  • Cell protection — shields the internal organelles and cytoplasm from the external environment.
  • Cell signalling — membrane receptors detect external signals and trigger internal responses.
  • Cell recognition — surface carbohydrates help the immune system distinguish "self" cells from foreign ones.
  • Maintaining cell shape — works with the cytoskeleton to support the overall structure of the cell.

Check the NEET Syllabus to see how Cell Biology's weightage compares to other high-scoring units.

How Does the Cell Membrane Transport Substances Across It?

Movement of substances across the cell membrane happens through two broad mechanisms — passive transport (no energy required) and active transport (requires ATP).

Type of Transport

Example

Energy Required

Diffusion

Movement of oxygen and carbon dioxide across the membrane

No

Osmosis

Movement of water molecules across a selectively permeable membrane

No

Facilitated diffusion

Glucose transport through carrier proteins

No

Active transport

Sodium-potassium (Na⁺/K⁺) pump

Yes (ATP)

Endocytosis

Engulfing large particles or fluid into the cell

Yes (ATP)

Exocytosis

Releasing substances out of the cell

Yes (ATP)

Transport mechanisms — especially active vs passive transport — are a favourite assertion-reason format in NEET Previous Year Question Papers

How Is the Cell Membrane Different in Prokaryotic and Eukaryotic Cells?

The basic fluid mosaic structure of the cell membrane is common to both prokaryotic and eukaryotic cells, but a few differences exist:

Basis

Prokaryotic Cell Membrane

Eukaryotic Cell Membrane

Sterols

Generally absent (except in a few groups)

Cholesterol commonly present

Associated structures

Mesosomes (infoldings) may be present

Connected to an extensive endomembrane system

Cell wall outside membrane

Present in most bacteria (peptidoglycan)

Present only in plant cells (cellulose), absent in animal cells

Complexity of transport proteins

Simpler

More diverse and specialised

Continue Your Cell: The Unit of Life Preparation

Want a structured revision plan for the complete Cell Biology chapter? Explore eSaral's NEET Course, taught by Kota's top Doctor faculty.

Frequently Asked Questions

What is the cell membrane made of?

The cell membrane is made of a phospholipid bilayer embedded with integral and peripheral proteins, cholesterol molecules for stability, and carbohydrate chains attached to proteins or lipids on the outer surface.

What are the main functions of the cell membrane?

The main functions of the cell membrane are selective permeability, transport of materials, protection of the cell's internal contents, cell signaling through receptors, and cell recognition through surface carbohydrates. This is a frequently tested concept in eSaral's NEET and Class 11 Biology study material.

What is the fluid mosaic model of the cell membrane?

The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes the cell membrane as a fluid phospholipid bilayer with proteins scattered throughout it in a mosaic-like pattern, allowing both lipids and many proteins to move laterally.

Why is the cell membrane called selectively permeable?

The cell membrane is called selectively permeable because it allows certain molecules, such as small non-polar substances, to pass through freely while restricting or regulating the passage of larger or charged molecules through specific transport proteins.

How do substances move across the cell membrane?

Substances move across the cell membrane through passive transport methods like diffusion, osmosis, and facilitated diffusion (no energy needed), and active transport methods like the sodium-potassium pump, endocytosis, and exocytosis (which require ATP).

What is the difference between the cell membrane and the cell wall?

The cell membrane is a thin, flexible, selectively permeable layer present in all cells, while the cell wall is a rigid, non-living structure made of cellulose (in plants) or peptidoglycan (in bacteria) found outside the cell membrane in plant cells and most prokaryotes, but absent in animal cells.

Is the cell membrane present in both prokaryotic and eukaryotic cells?

Yes, the cell membrane is present in both prokaryotic and eukaryotic cells biology — it is one of the two universal features of all cells, along with cytoplasm, regardless of whether a nucleus is membrane-bound or not.

Is the cell membrane found in both plant and animal cells?

Yes, the cell membrane is present in both plant and animal cells; in plant cells, it lies just inside the additional rigid cell wall.

What is the definition and function of the cell membrane?

The cell membrane is defined as the selectively permeable boundary enclosing a cell, and its core function is regulating the movement of substances in and out while providing protection, structural support, and a site for cell signaling.

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