Cell Membrane Structure and Composition

Cell Membrane Structure and Composition

• The cell membrane is a phospholipid bilayer, composed of phospholipid molecules.
• Phospholipids have a hydrophilic head and two hydrophobic tails.
• The hydrophilic heads face the extracellular fluid and cytoplasm, while the hydrophobic tails face each other in the interior of the membrane.
• Cholesterol and glycolipids are also components of the cell membrane.
• Proteins are embedded in the membrane, and are either integral or peripheral.
• Integral proteins span the entire membrane, while peripheral proteins are only on one side of the membrane.
• Carbohydrates are attached to proteins (glycoproteins) and lipids (glycolipids), and face the extracellular fluid.
• This carbohydrate layer is called the glycocalyx.
• The cell membrane's structure as a fluid mosaic model: A flexible sheet composed of lipids and proteins.
• The membrane is composed of a fluid phospholipid bilayer with proteins embedded within it that have various functions.

Hereditary Spherocytosis

• Hereditary spherocytosis is a genetic disorder causing hemolytic anemia.
• It's characterized by red blood cells losing their biconcave shape, becoming spherical, and unable to flow through narrow capillaries efficiently.
• It leads to symptoms like fatigue, dizziness, hair loss, and yellowing of the eyes.
• The disease results from dysfunctional proteins (like spectrin and ankyrin) that attach the cell membrane to its cytoskeleton.

Red Blood Cell Membrane Structure

• Red blood cell membranes are supported by a protein framework called the cell cortex comprised of actin, myosin, and proteins like spectrin and ankyrin.
• The membrane itself is a thin film of fatty lipids and proteins.
• Carbohydrates are attached to the lipids and proteins on the outer surface, forming the glycocalyx.

All Plasma Membranes

• All plasma membranes, surrounding the cell or organelles, consist of lipids and proteins.
• Membrane lipids are arranged in two sheets, called a lipid bilayer.

Plasma Membrane Lipids

• The primary lipids in the plasma membrane are phospholipids, cholesterol, and glycolipids.
• Phospholipids are amphipathic, with a hydrophilic head and hydrophobic tails.
• The arrangement of the hydrophilic heads and hydrophobic tails cause the formation of the bilayer, shielding the hydrophobic tails from the surrounding water.
• Cholesterol and glycolipids further contribute to membrane properties.

Phosphatidylcholine

• Phosphatidylcholine is the predominant phospholipid in biological membranes.
• Its hydrophilic head comprises a choline molecule and a phosphate group.
• The hydrophobic tails are composed of fatty acids.
• These tails can be saturated or unsaturated, affecting overall membrane fluidity.

Phospholipid Bilayer

• The hydrophilic heads of the phospholipids face outward, interacting with water on both sides of the membrane.
• The hydrophobic tails face inward, shielded from water.
• This arrangement forms a stable, sealed compartment.

Membrane Protein Types

• Integral proteins are embedded within the lipid bilayer, often spanning it completely.
• Peripheral proteins are loosely attached to the surface of the membrane.

Transmembrane Proteins

• Transmembrane proteins usually traverse the lipid bilayer as an alpha helix.
• The polypeptide backbone is hydrophilic, allowing it to interact with water.
• Hydrophobic amino acid side chains interact with the lipid bilayer's hydrophobic core.

Membrane Protein Movement

• Membrane proteins can move laterally within the lipid bilayer.
• The movement is restricted by interactions with other cell components.

Carbohydrates on the Cell Membrane

• Carbohydrates are attached to membrane proteins (glycoproteins) and lipids (glycolipids).
• These carbohydrates form the glycocalyx on the outer surface of the cell membrane.
• The glycocalyx plays a role in cell recognition and adhesion.

Cell Adhesion

• The glycocalyx allows for cell-cell adhesion through interactions between carbohydrate regions.

Plasma Membrane Functions

• The cell membrane is involved in cell signaling, transport, cell growth and motility, cell-cell recognition, and intercellular adhesion.
• Membrane proteins have specific roles in these functions.

Membrane Fluidity

• Temperature, lipid composition (including cholesterol), and saturation degree impact membrane fluidity.

Membrane Protein Synthesis and Transport

• Membranes are synthesized in the smooth endoplasmic reticulum.
• Phospholipids are added to the cytosolic side.
• The Golgi apparatus further modifies the membrane and distributes phospholipids asymmetrically.

Membrane Asymmetry

• The distribution of lipids and proteins within the membrane is not even, leading to membrane asymmetry.

Membrane Protein Movement

• Staining membrane proteins allows observation of protein movement.
• Movement may be restricted by factors, such as binding to the cytoskeleton or extracellular matrix.

Learning Outcomes

• Understanding the structure, location, and function of the cell cortex.
• Explaining the formation of the lipid bilayer and its characteristics as fluid and asymmetrical.
• Listing membrane components, explaining their movement, and mechanisms regulating it.
• Visualizing cell membrane structures depicting phospholipids and protein types, and showing membrane asymmetry.
• Predicting the effects of component removal or inhibition.
• Explaining cell fusion experiments and results.
• Classifying transmembrane proteins based on structure.
• Describing how polypeptide chains span the hydrophobic lipid bilayer interior.
• Detailing new membrane synthesis and phospholipid asymmetry.
• Explaining membrane orientation maintenance.
• Defining the glycocalyx, its structure and function in tissue formation.

Keywords for Studying the Lecture

• Amino acids (hydrophobic, hydrophilic, polar, nonpolar)
• Lipids (polar, nonpolar)
• Charges (partial positive, partial negative)
• Bonds (covalent)
• Molecular structure (primary, secondary)