Cell Membrane Structure and Transport MD105

Cellular and Molecular Biology MD105

• Dr. C. Michaeloudes teaches at the European University Cyprus, School of Medicine.
• The course covers Cellular & Molecular Biology.

Cell Membrane Structure and Transport

• The cell membrane is a protective layer surrounding all cells.
• It's composed of lipids, proteins, and carbohydrates.
• The membrane is semi-permeable, allowing some molecules to pass through while blocking others.

Lecture Objectives

• Understanding the basic structure and composition of the cell membrane.
• Understanding the structure of the lipid bilayer and factors influencing fluidity.
• Understanding the various mechanisms of molecular transport across the cell membrane.

Cell Membrane Structure and Composition

• Lipids (40%): Phospholipids and cholesterol form a bilayer.
• Proteins (50%): Embedded in the bilayer, performing diverse functions.
• Carbohydrates (10%): Attached to proteins (glycoproteins) and lipids (glycolipids), extending from the membrane surface.

Organelle Membranes

• Intracellular organelles also possess lipid bilayer membranes.
• These membranes have slightly different compositions compared to the cell membrane.
• Nucleus and mitochondria have two lipid bilayers; ER, Golgi, and lysosomes have one.

Lipid Bilayer Structure

• Phospholipids and cholesterol are the major components.
• Phospholipids have a hydrophilic head and hydrophobic tails.
• The hydrophobic tails aggregate away from water, while the hydrophilic heads face water.

Phospholipids and Cholesterol

• Phospholipids are the primary component of cell membranes.
• Cholesterol is embedded within the lipid bilayer.
• Both have hydrophobic tails and hydrophilic heads.

Cell Membrane Lipid Bilayer

• Membrane lipids are exposed to two opposing forces: attraction to water (hydrophilic heads) and repulsion from water (hydrophobic tails).
• The bilayer forms due to these forces.
• Hydrophilic heads face water on both sides of the bilayer; hydrophobic tails reside in the bilayer's interior.

Cell Membrane Fluidity

• Membrane lipids and proteins can move freely within the membrane.
• This creates a fluid mosaic model for the cell membrane.
• Fluidity depends on factors such as temperature, phospholipid composition, and cholesterol levels.

Cell Membrane Phospholipid Movement

• Phospholipids move laterally within a monolayer (lateral diffusion).
• Enzymes catalyze this movement.
• Flip-flop movement (from one monolayer to another) is rare.
• Lipids flex their tails and rotate.
• Fluidity depends on how tightly hydrocarbon tails are packed.

Factors Determining Cell Membrane Fluidity

• Temperature: High temperatures increase fluidity; low temperatures decrease fluidity.
• Phospholipid tail structure: Saturated tails decrease fluidity; unsaturated tails increase fluidity.
• Cholesterol level: Cholesterol acts as a buffer, maintaining fluidity at both high and low temperatures.

Effect of Temperature on Cell Membrane Fluidity

• High temperatures: Phospholipids have more energy, leading to increased movement and spacing between them; increasing fluidity.
• Low temperatures: Phospholipids have less energy, packing more tightly together, decreasing fluidity.

Saturated vs. Unsaturated Fatty Acids

• Saturated fatty acids have only single bonds between carbons in their hydrocarbon chain, resulting in a straight structure and often being solid at room temperature.
• Unsaturated fatty acids have one or more double bonds, creating kinks or bends in their hydrocarbon chains, leading to less interaction between the chains, which result in liquid state at room temperature.
• Cis-unsaturated have hydrogen atoms on the same side of the double bond, whereas trans-unsaturated have hydrogen atoms on opposite sides of the double bond.

Phospholipid Tails

• Phospholipid tails consist of two fatty acids; can be saturated or unsaturated.
• Cis-unsaturated fatty acids are bent, preventing close packing.
• This results in more space between the tails and increased fluidity.

Effect of Phospholipid Structure on Cell Membrane Fluidity

• Length of hydrocarbon chain: Shorter chains reduce interactions, increasing fluidity.
• Double bonds: Double bonds create bends, increasing space between hydrocarbon tails and increasing fluidity.

Effect of Cholesterol on Cell Membrane Fluidity

• Cholesterol fits between phospholipid molecules.
• It regulates membrane fluidity.
• It acts as a buffer to prevent extreme changes in fluidity at low and high temperatures.
• Low temperatures: Increases fluidity, preventing freezing.
• High temperatures: Reduces fluidity, preventing melting.

Importance of Cell Membrane Fluidity

• Cell shape and movement adapt to various conditions.
• Membrane proteins can diffuse and interact—crucial for cell signaling.
• Ensures even distribution between daughter cells during cell division.

Membrane Proteins and Carbohydrates

• Membrane proteins are embedded within the lipid bilayer (integral or transmembrane) or situated on the membrane surface (peripheral).
• Functions: Transporters, receptors, enzymes, and anchors.
• Carbohydrates: Sugars attached to lipids (glycolipids) or proteins (glycoproteins).
• Crucial for cell recognition, damage protection, and lubrication.

Membrane Transport Proteins

• Cells and organelles need to transport hydrophilic molecules.
• Transport proteins—channels and transporters—facilitate this movement.

The Cell Membrane is Semi-permeable

• Hydrophobic interior prevents most hydrophilic molecules from passing through directly.
• The membrane is selective: Small nonpolar molecules and small uncharged polar molecules cross relatively quickly; large polar molecules and charged molecules cross slowly or not at all.

Types of Ion Channels

• Voltage-gated channels: Open in response to changes in membrane potential.
• Ligand-gated channels: Open in response to the binding of a chemical messenger (ligand).
• Mechanically-gated channels: Open in response to a mechanical force.

Resting Membrane Potential

• Differences in ion concentration create a voltage difference across the membrane.
• The inside of the cell is slightly more negative than the outside.
• This stored energy is essential for cell function.

Mechanisms of Membrane Transport

• Concentration gradient: Difference in concentration of a substance across a membrane.
• Diffusion: Driven by a concentration gradient.
• Passive transport: Movement along a concentration gradient, requiring no energy input.

Passive Transport

• Simple diffusion: Small, non-polar molecules directly cross the membrane.
• Facilitated diffusion: Large molecules use transporter proteins across the membrane.

Passive Transport of Charged Molecules

• Two forces driving passive transport of charged molecules: resting membrane potential and concentration gradient.
• The net force is called the electrochemical gradient.

Active Transport

• Movement against the concentration gradient.
• Requires energy input (usually from ATP hydrolysis).
• Facilitated by transporter proteins (pumps).

Summary of Membrane Transport

Osmosis

• Movement of water across a semi-permeable membrane.
• Driven by a concentration gradient of solutes.
• Water moves from low solute concentration to high solute concentration.
• Specialized channels—aquaporins—facilitate water's movement.

This quiz explores the fundamental aspects of cell membrane structure and transport mechanisms. You will learn about the lipid bilayer, protein functions, and the roles of carbohydrates in the cell membrane. Test your understanding of how these components contribute to cellular protection and transport processes.