Cell Membrane Flashcards

Cell Membrane

Introduction

• Cell Membrane (Plasma Membrane): A biological membrane that separates the interior of cells from the outside environment (extracellular space).

• Function: Protects the cell from its environment.

• Presence: Found in both prokaryotic and eukaryotic cells.

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.

Structure and Composition

• Main components:

  • Lipids (40-45%)

  • Proteins (50-55%)

  • Carbohydrates (1-5%)

• Lipid Bilayer:

  • Phospholipids: Main component; lipid molecules with a phosphate group head (hydrophilic) and two fatty acid tails (hydrophobic).

    • In aqueous solutions, hydrophilic heads face outwards, and hydrophobic tails face inwards.

  • Cholesterol: Sits between phospholipids to maintain fluidity at various temperatures.

    • Contributes to membrane fluidity.

    • Decreases the permeability of the bilayer to small water-soluble molecules.

  • Glycolipids: Protrude outwards and function in cell-recognition processes.

• Membrane Proteins:

  • Integral Proteins: Go across the membrane, serving as membrane transporters (e.g., transmembrane proteins).

    • Some are responsible for cell adhesion.

    • Can act as channels or transport molecules.

    • Can act as cell receptors.

  • Peripheral Proteins: Loosely attach to the outer side of the lipid bilayer, acting as enzymes shaping the cell. Temporarily associated with the membrane.

    • Involved in cell signaling.

    • Easily removed.

    • Mostly hydrophilic.

• Oligosaccharides: Small percentage, attached to lipids (glycolipids) or proteins (glycoproteins).

Chemical Composition

• Cell membrane mostly contains:

  • Proteins

  • Lipids

  • Small percentage of oligosaccharides (glycolipids and glycoproteins)

• There is a wide variation in the lipid-protein ratio between different cell membranes.

Lipid Fraction

• Phospholipids (>55%)

• Glycolipids (2%)

• Cholesterol (40-45%)

Carbohydrate Fraction

• Functions:

  1. Protection for the cell.

  2. Cell recognition.

• Mostly present in the outer monolayer of the plasma membrane, forming the glycocalyx.

Protein Fraction

• Significant fraction of the membrane.

• Important role in the structure of the membrane.

• Role as carriers or channels for cell transport.

• Also contains enzymatic proteins, antigens, and receptor molecules.

• Amphiphilic: Having both hydrophobic and hydrophilic regions.

  • Hydrophobic regions interact with the hydrophobic tails of the lipid molecules.

  • Hydrophilic regions are exposed to water on either side of the membrane.

Functions of the Cell Membrane

• Protection.

• Cell adhesion.

• Ion conductivity.

• Cell signaling.

• Attachment surface for extracellular structures (e.g., cell wall, glycocalyx) and intracellular network of protein fibers (cytoskeleton).

• Controls the movement of substances in and out of cells (selectively permeable).

  • Oxygen and carbon dioxide can easily enter and exit.

  • Water can also freely cross, but at a slower rate.

  • Highly charged molecules (ions) and large macromolecules (carbohydrates, amino acids) cannot directly pass through.

Membrane Models

1. Sandwich Model (Danielli and Davson, 1935)

• Proposed two layers of lipid (phospholipid) molecules with polar heads facing outwards.

• Rejected because it didn't account for the fluid nature of the membrane and the variety of protein functions.

2. Unit Membrane Model (Robertson, 1965)

• Organized structure with a lipid bilayer sandwiched by proteins.

• Suggested a more symmetrical structure.

• Observed from electron micrographs with no spaces for pores.

3. Fluid Mosaic Model (Singer and Nicolson, 1972)

• Two-dimensional liquid where lipid and protein molecules diffuse more or less freely.

• Proteins can cover the entire membrane or interact with one of the two lipid layers.

• Some proteins can be attached through a short lipid chain.

• Fluid: Phospholipids and some proteins can move sideways, creating a fluid structure.

• Mosaic: Range of different proteins resting on the surface or through the phospholipid layer, 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.

• Unequal distribution of molecules between monolayers is referred to as membrane asymmetry.

• Carbohydrates are mostly present in the outer monolayer forming the glycocalyx.

• Enzymatic activity differs on the two sides.

Membrane Fluidity

• Multiple factors lead to membrane fluidity.

• Mosaic characteristic helps the membrane remain fluid.

• The integral proteins and lipids exist as separate but loosely attached molecules.

• The membrane is fairly rigid and can burst if penetrated or if a cell takes in too much water.

• A fine needle can easily penetrate without causing it to burst, as the membrane will flow and self-seal.

• Fluidity is greater at higher temperatures and is affected by the composition of the bilayer.

• Shorter fatty acid tails increase fluidity.

• Lipid chains with double bonds (unsaturated fatty acids) have kinked hydrocarbon tails that are harder to pack together.

Cholesterol as a Regulator

• 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.