Cell Membrane Flashcards

Cell Membrane

Learning Objectives

• Define the cell membrane.
• Outline the structure, composition, and function of the cell membrane.
• State the different models proposed for the plasma membrane.

What is the Cell Membrane?

• The cell membrane (also known as the plasma membrane) is a biological membrane that separates the interior of cells from the outside environment, known as the extracellular space.
• It protects the cell from its environment.
• It is present in both prokaryotic and eukaryotic cells.

Cell Membrane Composition

• The cell membrane consists of a lipid bilayer, including cholesterol, which sits between phospholipids to maintain fluidity at various temperatures.
• The membrane also contains membrane proteins:
  • Integral proteins: Span across the membrane and serve as membrane transporters.
  • Peripheral proteins: Loosely attach to the outer side of the lipid bilayer, acting as enzymes and shaping the cell.

Cell Membrane Function: Cell Transport

• The cell membrane controls the movement of substances in and out of cells.
  • It is selectively permeable.
  • Oxygen and carbon dioxide can easily enter and exit through the membrane.
  • Water can also freely cross the membrane, although at a slower rate.
  • Highly charged molecules (ions) and large macromolecules (carbohydrates or amino acids) cannot directly pass through.

Functions of Cell Membrane

• Protection
• Cell adhesion
• Ion conductivity
• Cell signaling
• Serves as the attachment surface for several extracellular structures (e.g., the cell wall, the carbohydrate layer called the glycocalyx, and the intracellular network of protein fibers of the cytoskeleton).

Structure of the Cell Membrane

• Phospholipids are the main component of the cell membrane.
  • These are lipid molecules made up of a phosphate group head and two fatty acid tails.
  • In water or aqueous solutions (like inside the human body), the hydrophilic heads of phospholipids orient themselves to face outwards, while the hydrophobic tails are on the inside of the bilayer.

Chemical Composition of a Cell Membrane

• The cell membrane is composed mainly of:
  • Proteins: 50-55%
  • Lipids: 40-45%
  • A small percentage of oligosaccharides (1-5%) that may be attached to lipids (glycolipids) or proteins (glycoproteins).
• There is a wide variation in the lipid-protein ratio between different cell membranes.

Lipid Fraction of Cell Membranes

• The cell membrane contains a large proportion of lipids.
• The main lipid components are phospholipids (>55%), cholesterol (40-45%), and glycolipids (2%).
  • Cholesterol is embedded in the bilayer:
    • Contributes to the fluidity of the membrane.
    • Decreases the permeability of the bilayer to small water-soluble molecules.
  • Glycolipids protrude outwards:
    • Function in cell-recognition processes.

Carbohydrate Fraction of Cell Membrane

• Functions of carbohydrates in the membrane:
  1. Protection for the cell
  2. Cell recognition

Protein Fraction of Cell Membranes

• Proteins represent one of the most significant fractions of cell membranes.
• They play an important role in the structure of the membrane.
• Also play a role as carriers or channels (cell transport).
• Besides structural proteins, there are also enzymatic proteins, antigens, and various kinds of receptor molecules in the plasma membrane.
• Membrane proteins are amphiphilic, having both hydrophobic and hydrophilic regions.
  • Hydrophobic regions pass through the membrane and interact with the hydrophobic tails of the lipid molecules in the interior of the bilayer.
  • Hydrophilic regions are exposed to water on either side of the membrane.

Types of Proteins in Plasma Membrane

1. Integral proteins:
   • Some integral membrane proteins are responsible for cell adhesion (attaching a cell to another cell or surface).
   • Permanent part of the membrane.
   • Channeling or transporting molecules across the membrane.
   • Cell receptors.
   • Example: transmembrane proteins (extend through the lipid bilayer).
2. Peripheral proteins:
   • Temporarily associated with the membrane.
   • They can be easily removed, which allows them to be involved in cell signaling.
   • Most peripheral membrane proteins are hydrophilic.

Models of the Cell Membrane

Sandwich Model: Danielli and Davson Model (1935)

• Proposed that the plasma membrane consists of two layers of lipid (phospholipid) molecules.
• They arranged themselves with the polar heads facing outwards.
• This model was later rejected because it didn't account for the fluid nature of the membrane and the variety of protein functions.

Unit Membrane Model: Robertson’s Model (1965)

• Robertson noted the structure of membranes as seen in electron micrographs.
• He saw no spaces for pores in the electron micrographs.
• Suggested a more organized structure, with the lipid bilayer sandwiched by proteins.
• It also included the idea of the membrane being more symmetrical.

Fluid Mosaic Model: S.J. Singer and G.L. Nicolson (1972)

• A two-dimensional liquid where all lipid and protein molecules diffuse more or less freely.
  • These proteins could cover the entire membrane or interact with one of the two lipid layers.
  • Some proteins could even be attached to the membrane only through a short lipid chain.
• Singer studied phospholipid bilayers and found that they can form a flattened surface on water, with no requirement for a protein coat.
• Fluid: Individual phospholipids and some proteins can move sideways in the layer, creating a fluid structure.
• Mosaic: A range of different proteins resting on the surface or through the phospholipid layer, therefore creating a mosaic appearance.

Membrane Asymmetry

• Cell membranes are made up of two lipid monolayers.
• Lipids, carbohydrates, and peripheral proteins are present in different types and proportions when both monolayers are compared.
• This unequal distribution of molecules between both monolayers is referred to as membrane asymmetry.
• Carbohydrates are mostly present in the outer monolayer of the plasma membrane, forming the so-called glycocalyx (contributing to the asymmetry).
• The enzymatic activity is different on the two sides.

Membrane Fluidity

• There are multiple factors that lead to membrane fluidity.
• The mosaic characteristic of the membrane helps the plasma membrane to remain fluid.
• The integral proteins and lipids exist in the membrane as separate but loosely attached molecules.
• The membrane is not like a balloon that can expand and contract; rather, it is fairly rigid and can burst if penetrated or if a cell takes in too much water.
• However, because of its mosaic nature, a very fine needle can easily penetrate a plasma membrane without causing it to burst (the membrane will flow and self-seal when the needle is extracted).
• Cell membranes are fluid, meaning they are not fixed in position and can adopt various shapes.
• Membrane fluidity is greater at higher temperatures and is also affected by the composition of the bilayer.
• Phospholipids may vary in the length and relative saturation of the fatty acid tails.
• Shorter fatty acid tails will increase fluidity as they are less viscous and more susceptible to changes in kinetic energy.
• Lipid chains with double bonds (unsaturated fatty acids) have kinked hydrocarbon tails that are harder to pack together.

Cholesterol

• Cholesterol acts as a bi-directional regulator of membrane fluidity.
• At high temperatures, it stabilizes the membrane and raises the melting point.
• At low temperatures, it inserts between the phospholipids and prevents clustering.
• Therefore, cholesterol decreases fluidity at high temperatures and increases fluidity at low temperatures.