Cell Membrane Structure & Permeability

Biology: IB Diploma Higher Level, topic 6.1.4

Structure and Function of Cell Membranes

The fluid mosaic model describes the structure of the plasma membrane as a mosaic of phospholipids, cholesterol, proteins, and carbohydrates. This gives the membrane a fluid character.

Function of the plasma membrane

The plasma membrane defines the borders of cells and most organelles. This creates compartments within cells.
The plasma membrane is partially permeable. This means that the membrane allows some materials to freely enter or leave the cell/organelle, while other materials cannot move freely.
The plasma membrane is made of a phospholipid bilayer, which is 5 - 10 nm thick.

Structure of phospholipids

A phospholipid is a molecule consisting of glycerol, two fatty acids, and a phosphate-linked head group.
The hydrophilic phospholipid head faces outwards towards the aqueous external or internal environments.
The hydrophilic regions of the phospholipids tend to form hydrogen bonds with water and other polar molecules on both the exterior and interior of the cell.
The hydrophobic fatty acids face each other within the inside of the membrance.

Structure of cholesterol

Cholesterol is a lipid that sits with phospholipids in the core of the membrane.
Cholesterol is not found in bacterial cell membranes.
Cholesterol molecules make the membrane more rigid.
This explains why cholesterol helps to maintain the shape of animal cells.

The Fluid Mosaic Model

The phospholipid bilayer forms the ‘fluid’ part of the ‘fluid mosaic’ model.
The ‘mosaic’ part is made up of the various proteins, carbohydrate and lipid molecules that punctuate membranes.

Structure and Function of Cell Membranes - Proteins

Membrane proteins are part of the "mosaic" in the fluid mosaic model. Membrane proteins have a variety of functions.

Function of membrane bound proteins

Membrane-bound proteins are large molecules embedded in the bilayer, they can form open pores that allow the diffusion of large molecules across the bilayer or they can be transport proteins that bind to specific molecules and carry them across the membrane.
ATP-synthase is an example of a membrane-bound protein and catalyzes the production of ATP during oxidative phosphorylation on the inner mitochondrial membrane.

Integral vs peripheral proteins

Integral proteins are integrated completely into the membrane structure, and their hydrophobic membrane-spanning regions interact with the hydrophobic region of the phospholipid bilayer.
Peripheral proteins are found on the exterior and interior surfaces of membranes, attached either to integral proteins or to phospholipids.
Peripheral proteins & integral proteins may serve as enzymes, as structural attachments for the fibers of the cytoskeleton, or as part of the cell’s recognition sites.

Membrane proteins

Integral membranes proteins may have one or more alpha-helices that span the membrane (1 & 2), or they may have beta-sheets that span the membrane (3).

Function of glycoproteins and glycolipids

Some proteins and lipids in cell membranes have carbohydrate chains attached to them.
These chains are vital in interacting with the cell’s immediate environment.
Some glycolipids and glycoproteins are able to form hydrogen bonds with water molecules surrounding the cell, helping to stabilize the membrane.

Cell receptors

Other glycolipids and glycoproteins act as cell signaling receptors and have active binding sites for communication molecules such as hormones and drugs.
When these receptors bind a target molecule, the glycomolecule undergoes a conformational change and initiates a chain of reactions known as a cascade that can lead to a cell-level response to the binding of certain signaling molecules.

Test yourself on Cell Membrane Structure & Permeability

Which of the following statements about cell membranes are true?
What is the normal thickness of the phospholipid bilayer?
What type of protein secondary structure element is represented by 3?

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