As you study for your A levels, you'll frequently encounter membrane-bound organelles. These organelles, found in both plant and animal cells, include the nucleus, Golgi body, endoplasmic reticulum, mitochondria, lysosomes, and chloroplasts (which are only found in plants). Understanding the structure of the cell membrane is key to understanding the function of these organelles. So, make sure to pay attention in class and review your notes on cell membrane structure. It will make a big difference in your understanding of biology.

What is the purpose of cell membranes?

Cell membranes serve three main purposes:

Cell communication

Compartmentalisation

Regulation of what enters and exits the cell

What is the cell membrane structure?

Let's take a closer look at the different components.

Phospholipids

Phospholipids are a type of molecule that have both hydrophilic and hydrophobic regions. This means that part of the molecule is attracted to water, while another part repels it. This property makes phospholipids important building blocks of cell membranes.

The structure of a phospholipid consists of a hydrophilic head and a hydrophobic tail. The head is made up of a phosphate group and a glycerol molecule, while the tail is composed of two fatty acid chains. The phosphate group and glycerol are polar, which means they are attracted to water and make up the hydrophilic head of the molecule. The fatty acid chains, on the other hand, are nonpolar and repel water. These chains form the hydrophobic tail of the molecule.

When phospholipids are placed in water, they spontaneously arrange themselves into a bilayer, with the hydrophilic heads facing outwards and the hydrophobic tails facing inwards. This bilayer forms the basic structure of cell membranes, and the amphipathic nature of phospholipids is essential for maintaining the integrity and selective permeability of the membrane.

Membrane proteins

The phospholipid bilayer of a cell membrane contains two types of proteins: integral proteins (also known as transmembrane proteins) and peripheral proteins.

Integral proteins span the entire length of the bilayer and play a crucial role in transporting molecules across the membrane. There are two types of integral proteins: channel proteins, which form channels or pores for small molecules to pass through, and carrier proteins, which bind to larger molecules and transport them across the membrane by undergoing a conformational change.

Peripheral proteins, on the other hand, are found only on one side of the bilayer, either on the extracellular or intracellular side. These proteins can function as enzymes, receptors, or help maintain the shape of the cell.

Both integral and peripheral proteins are essential for the proper functioning of the cell membrane, and their specific arrangement and distribution play a key role in the selective permeability of the membrane.

Glycoproteins

Glycolipids

Antigens also make up the different blood types. This means whether you are type A, B, AB or O, is determined by the type of glycolipid found on the surface of your red blood cells; this is also cell recognition.

A glycolipid positioned in a cell membrane.

Cholesterol

The structure of the cell membrane is vital for its proper functioning, and several factors can affect its structure. Some of the main factors that can affect the structure of the cell membrane include:

1. Temperature: Changes in temperature can affect the fluidity of the membrane. At low temperatures, the membrane becomes more rigid, while at high temperatures, it becomes more fluid. This can affect the ability of the membrane to function properly, as it may become too stiff or too loose.
2. Lipid composition: The composition of the lipids in the membrane can also affect its structure. Different types of lipids have different properties, and the composition of the lipids in the membrane can affect its fluidity and stability.
3. Presence of cholesterol: As discussed earlier, cholesterol plays a crucial role in regulating the fluidity of the membrane. The presence of cholesterol in the membrane can affect its structure by making it more stable and less fluid.
4. pH: Changes in the pH of the environment surrounding the cell can affect the structure of the membrane. This is because changes in pH can affect the charges on the membrane proteins and lipids, which can in turn affect their ability to interact with each other.
5. Pressure: Changes in pressure can also affect the structure of the membrane. This is particularly relevant in organisms living in high-pressure environments, such as deep-sea organisms. The structure of their membranes is adapted to withstand high pressures and maintain their integrity.

Overall, the structure of the cell membrane is crucial for its proper functioning, and any changes to its structure can affect its ability to maintain homeostasis and perform its vital functions.

Solvents

The phospholipid bilayer is arranged with the hydrophilic heads facing the aqueous environment and the hydrophobic tails forming a core away from the aqueous environment. This configuration is only possible with water as the main solvent. Water is a polar solvent and if cells are placed in less polar solvents, the cell membrane can be disrupted. For example, ethanol is a nonpolar solvent that can dissolve cell membranes and therefore destroy cells. This is because the cell membrane becomes highly permeable and the structure breaks down, enabling the cell contents to leak out.

Temperature

What's more, the membrane proteins involved in transport can also become denatured if the temperature is high enough. This also contributes to the breakdown of the cell membrane structure. At lower temperatures, the cell membrane becomes stiffer as the phospholipids have less kinetic energy. As a result, cell membrane fluidity decreases and the transport of substances is hindered.

Investigating cell membrane permeability

In summary, the cell membrane structure is essential for its proper functioning, and disruptions to this structure can affect its permeability. The cell membrane is made up of phospholipids, membrane proteins, glycolipids, glycoproteins, and cholesterol, which form a fluid mosaic model. Solvents and temperature can affect the structure and permeability of the cell membrane.

To investigate how temperature affects cell membrane permeability, beetroot cells can be used. The pigment responsible for the red color of beetroot, betalain, leaks out of disrupted cells, indicating increased permeability. By placing beetroot cells in distilled water at different temperatures and analyzing the water samples with a colorimeter, researchers can determine the effect of temperature on the permeability of the cell membrane. A higher absorbance reading indicates more pigment is present in the solution and the cell membrane is more permeable.

Cell Membrane Structure

What are the major components of the cell membrane?

The major components of the cell membrane are phospholipids, membrane proteins (channel proteins and carrier proteins), glycolipids, glycoproteins and cholesterol. 

What is the structure of a cell membrane and what are its functions?

The cell membrane is a phospholipid bilayer. The hydrophobic heads of the phospholipids face the aqueous environments while the hydrophobic tails form a core away from the aqueous environments. Membrane proteins, glycolipids, glycoproteins and cholesterol are distributed throughout the cell membrane. The cell membrane has three important functions: cell communication, compartmentalisation and regulation of what enters and exits the cell. 

What structures allow small particles to cross cell membranes?

Membrane proteins allow the passage of small particles across the cell membranes. There are two main types: channel proteins and carrier proteins. Channel proteins provide a hydrophilic channel for the passage of charged and polar particles, like ions and water molecules. Carrier proteins change their shape to allow particles to cross the cell membrane, such as glucose.

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