1.5.2 Plasma Membrane

Introduction

The plasma membrane is the outer living boundary of the cell. It is also called the cell membrane. It is found in all living prokaryotic and eukaryotic cells. It controls the passage of materials into and out of the cell and regulates the cell's interaction with its environment.

Definition

Plasma Membrane: The plasma membrane is the boundary of the protoplasm and most organelles. It is found in all living prokaryotic and eukaryotic cells. It controls the passage of materials into and out of the cell.

In simple words: The plasma membrane is the living, flexible outer covering of every cell that controls what enters and leaves the cell.

Core Concept

• The plasma membrane is the outer living boundary of the cell.
• It is found in all living prokaryotic and eukaryotic cells.
• It is the boundary of the protoplasm and most organelles.
• It controls the passage of materials into and out of the cell.
• It is about 7 nm thick.
• It is described by the Fluid Mosaic Model.

Detailed Explanation

Composition of Plasma Membrane

Chemically, the plasma membrane consists of:

Structure of Plasma Membrane: Fluid Mosaic Model

The model proposes that:

• The membrane is a phospholipid bilayer in which protein molecules are either partially or wholly embedded.
• The proteins are scattered throughout the membrane in an irregular pattern, just like large icebergs float in the sea.
• The pattern of distribution of proteins can vary from membrane to membrane and also vary on both surfaces of the membrane.
• The membrane is about 7 nm thick.

• The lipid part of the plasma membrane consists of two layers (bilayer) of phospholipids.
• The phospholipids are arranged in such a way that their hydrophobic ends face each other while hydrophilic ends appear on the surface.
• Steroids and cholesterols are wedged into the phospholipid bilayer at some intervals.
• The plasma membrane is asymmetrical — their two surfaces and halves are not identical.

• In general, most membrane proteins drift sideways in the fluid bilayer.
• The proteins within a membrane determine most of the functions of the membrane.

• Carbohydrates are either attached to proteins (glycoproteins) or lipids (glycolipids).
• They are generally on the outer side of the membrane.

• Filaments of the cytoskeleton are also present on the inner surface of the membrane.
• These support the plasma membrane.

• The lipid part of the plasma membrane controls the fluidity of the membrane.
• When the concentration of unsaturated fatty acids in phospholipids becomes greater, the bilayer becomes more fluid, which makes the cell membrane more flexible.
• Cholesterol helps to stabilize the lipid bilayer.
• It also restricts the entry and exit of polar molecules and ions.

Science Tidbits (from textbook): > The fluidity of membrane is dependent on its lipid components, including phospholipids, glycolipids and cholesterol.

Functions of Plasma Membrane Proteins

A great variety of proteins are found in the plasma membrane. They may act as:

• Some proteins have a channel through which a substance simply can move across the membrane.
• Other proteins are carriers that combine with a substance and help it move across the membrane.
• Some proteins in the plasma membrane are receptors that receive signals from other cells. For example, hormones circulate in the blood but bind to specific target cells with specific receptors.
• Some proteins are antigens which enable the cells to recognize other cells.

Roles of Glycolipids and Glycoproteins as Cell Surface Markers

• Mostly glycolipids and glycoproteins act as cell surface markers.
• They are involved in cell-to-cell recognition and sticking the correct cells together in tissues.

Regulation of Cell's Interaction with Its Environment

• The plasma membrane regulates the cell's interaction with its environment by controlling the transport of materials across the cell.
• Transport across the plasma membrane occurs to maintain a suitable pH and ionic concentration within the cell for enzyme activity.

Techniques Used in Studying Cell Membrane

The biophysical imaging techniques used to study biological structures include:

• It gives the morphology and topography data of the prepared membranes.
• It can be used to determine the pore size in the case of a porous membrane.

Intermediate Level

Fluid Mosaic Model in Detail

The Fluid Mosaic Model was proposed by Singer and Nicolson in 1972. It describes the plasma membrane as a fluid structure with a mosaic pattern of proteins.

Key Features:

• Hydrophilic heads face outward (toward water).
• Hydrophobic tails face inward (away from water).
• This arrangement forms a barrier that is selectively permeable.

• Integral proteins – embedded in the bilayer.
• Peripheral proteins – on the surface of the bilayer.
• Proteins can drift sideways (fluid nature).
• Proteins determine most of the functions of the membrane.

• Wedged between phospholipids.
• Stabilizes the membrane.
• Restricts entry and exit of polar molecules and ions.
• Affects membrane fluidity.

• Attached to proteins (glycoproteins) or lipids (glycolipids).
• Generally on the outer side.
• Act as cell surface markers.
• Involved in cell-to-cell recognition.

• Filaments on the inner surface.
• Support the membrane.
• Maintain cell shape.

Why is the membrane called "fluid"? - Because the phospholipids and proteins can move sideways within the bilayer.

Why is it called "mosaic"? - Because the proteins are scattered throughout the membrane in an irregular pattern, like tiles in a mosaic.

Functions of Plasma Membrane Proteins

Regulation of Cell's Interaction with Environment

The plasma membrane regulates the cell's interaction with its environment by:

• Controlling transport of materials across the cell.
• Maintaining a suitable pH within the cell.
• Maintaining ionic concentration for enzyme activity.
• Receiving signals from other cells.
• Recognizing other cells through surface markers.

Techniques Used in Studying Cell Membrane

Important Points

• 1. Plasma membrane is the outer living boundary of the cell.
• 2. It is also called the cell membrane.
• 3. It is found in all living prokaryotic and eukaryotic cells.
• 4. It is the boundary of the protoplasm and most organelles.
• 5. It controls the passage of materials into and out of the cell.
• 6. Composition:
• - Proteins: 60–80%
• - Lipids: 20–40%
• - Carbohydrates: small quantity
• 7. Fluid Mosaic Model:
• - Phospholipid bilayer
• - Proteins embedded (partially or wholly)
• - Proteins scattered irregularly
• - Membrane is about 7 nm thick
• 8. Lipid part:
• - Two layers of phospholipids
• - Hydrophobic ends face each other
• - Hydrophilic ends on the surface
• - Cholesterol wedged into the bilayer
• - Membrane is asymmetrical
• 9. Protein part:
• - Proteins drift sideways in the fluid bilayer
• - Proteins determine most of the functions of the membrane
• 10. Carbohydrate part:
• - Attached to proteins (glycoproteins) or lipids (glycolipids)
• - Generally on the outer side
• 11. Cytoskeleton:
• - Filaments on the inner surface
• - Support the plasma membrane
• 12. Fluidity:
• - Controlled by lipid components
• - Unsaturated fatty acids increase fluidity
• - Cholesterol stabilizes the bilayer and restricts entry/exit of polar molecules and ions
• 13. Functions of plasma membrane proteins:
• - Transport channel
• - Carrier
• - Enzyme
• - Receptor
• - Antigen
• 14. Glycolipids and glycoproteins act as cell surface markers for cell-to-cell recognition.
• 15. Plasma membrane regulates cell's interaction with environment by controlling transport.
• 16. Techniques for studying cell membrane:
• - Atomic Force Microscopy (AFM)
• - Super-Resolution Fluorescent Microscopy
• - Scanning Electron Microscopy (SEM)

Remember

• - Plasma membrane = living boundary of cell
• - Also called = cell membrane
• - Found in = all living prokaryotic and eukaryotic cells
• - Composition: proteins 60–80%, lipids 20–40%, carbohydrates
• - Fluid Mosaic Model: phospholipid bilayer + proteins + cholesterol + carbohydrates + cytoskeleton
• - **Membrane thickness** = 7 nm
• - Lipid part: phospholipid bilayer, hydrophobic tails face each other, hydrophilic heads on surface
• - Cholesterol = stabilizes membrane, restricts entry/exit of polar molecules and ions
• - Unsaturated fatty acids = increase fluidity
• - Membrane proteins: transport, enzyme, receptor, antigen
• - Glycolipids/glycoproteins = cell surface markers
• - Techniques: AFM, Super-resolution, SEM

Common Mistakes

• 1. Thinking the plasma membrane is rigid – The plasma membrane is fluid (Fluid Mosaic Model). Proteins can drift sideways.
• 2. Mixing up the percentages – Proteins are 60–80%; lipids are 20–40%.
• 3. Forgetting that cholesterol stabilizes the membrane – Cholesterol helps stabilize the lipid bilayer and restricts entry/exit of polar molecules and ions.
• 4. Thinking carbohydrates are inside the membrane – Carbohydrates (glycolipids and glycoproteins) are generally on the outer side of the membrane.
• 5. Confusing the functions of different membrane proteins – Channel proteins form pores; carrier proteins bind and change shape; receptors receive signals; antigens enable recognition.
• 6. Forgetting the techniques – AFM, Super-resolution fluorescent microscopy, and SEM are used to study the cell membrane.
• 7. Thinking the membrane is symmetrical – The plasma membrane is asymmetrical — their two surfaces and halves are not identical.
• 8. Confusing fluidity with stability – Fluidity is the ability to move; stability is the ability to maintain structure. Cholesterol affects both.
• 9. Thinking all proteins are embedded – Proteins can be partially or wholly embedded in the membrane.
• 10. Forgetting the cytoskeleton – Filaments of the cytoskeleton are present on the inner surface of the membrane and support it.

Quick Revision

• Plasma membrane is the outer living boundary of the cell.
• It is also called the cell membrane.
• It is found in all living prokaryotic and eukaryotic cells.
• It is the boundary of the protoplasm and most organelles.
• It controls the passage of materials into and out of the cell.
• Composition: Proteins (60–80%), Lipids (20–40%), Carbohydrates (small quantity).
• Fluid Mosaic Model:
• Phospholipid bilayer (hydrophobic tails face each other, hydrophilic heads on surface)
• Proteins embedded (partially or wholly)
• Proteins scattered irregularly
• Membrane is about 7 nm thick
• Cholesterol stabilizes the bilayer
• Carbohydrates (glycolipids/glycoproteins) on outer side
• Cytoskeleton filaments on inner surface
• Functions of plasma membrane proteins: transport channel, carrier, enzyme, receptor, antigen.
• Glycolipids and glycoproteins act as cell surface markers for cell-to-cell recognition.
• Plasma membrane regulates cell's interaction with environment by controlling transport.
• Techniques for studying cell membrane: Atomic Force Microscopy (AFM), Super-Resolution Fluorescent Microscopy, Scanning Electron Microscopy (SEM).

60-Second Revision

• Plasma membrane = living boundary; also called cell membrane
• Found in = all cells (prokaryotic and eukaryotic)
• Composition: proteins 60–80%, lipids 20–40%, carbohydrates
• Fluid Mosaic Model: phospholipid bilayer + proteins + cholesterol + carbohydrates + cytoskeleton; 7 nm thick
• Lipid part: hydrophobic tails face each other, hydrophilic heads on surface
• Cholesterol = stabilizes membrane; Unsaturated fatty acids = increase fluidity
• Membrane proteins: transport, enzyme, receptor, antigen
• Glycolipids/glycoproteins = cell surface markers
• Techniques: AFM, Super-resolution, SEM
• Warning: The plasma membrane is fluid, not rigid. It is asymmetrical. Carbohydrates are on the outer side. Do not confuse the functions of different membrane proteins.