Membranes

Membranes

Learning Objective

  • Understanding the structure and function of biological membranes.

  • Key terminology related to membranes, including definitions and types of transport mechanisms.

Lipid Bilayer

  • Definition: A lipid bilayer is the structural foundation of cell membranes, consisting of two layers of phospholipids arranged back-to-back.

Membrane Composition

  • Major Components:

    • Lipids: Approximately 50% of the membrane composition.

    • Average composition includes:

      • 55% Phospholipids

      • 20% Cholesterol

      • 5% Glycolipids

      • 20% Other lipids

    • Proteins: Also makes up roughly 50% of the membrane composition.

    • Types of Proteins: Include integral and peripheral proteins

Functions of Membranes

  • Structural Boundaries: Define the cells and organelles.

  • Semi-permeable Barriers: Control substance movement into and out of the cell.

  • Enzymatic Activity: Membrane proteins act as enzymes to facilitate reactions.

  • Signal Reception: Receptor proteins bind to specific biologically important substances, triggering biochemical responses.

    • Example: Acetylcholine triggering nerve impulses.

Structure of the Membrane

  • Formed by glycerophospholipid characteristics:

    • Hydrophilic (water-attracting) surfaces face outward.

    • Hydrophobic (water-repelling) tails face inward.

  • Membrane Asymmetry: Lipid distribution is not uniform across the membrane.

Membrane Fluidity

  • Formation: Driven by hydrophobic interactions between lipid molecules.

  • Types of Fatty Acids Affecting Fluidity:

    • Rigid membranes (high saturated fatty acids)

    • Fluid membranes (high unsaturated fatty acids)

  • Cholesterol's Role:

    • Increases membrane rigidity and decreases fluidity due to its rigid ring structure.

Membrane Proteins

  • Types:

    • Peripheral: Attached to the exterior or interior of the membrane.

    • Integral: Embedded in the membrane.

  • Functional Types:

    • Receptor Proteins: Transmit external signals into cells.

    • Transport Proteins: Facilitate the movement of substances across the membrane.

    • Enzymes: Catalyze chemical reactions at the membrane surface.

Fluid Mosaic Model

  • Concept: Describes cell membrane structure as flexible, with proteins and lipids embedded and moving laterally, similar to icebergs in the ocean.

  • Dynamic Structure: Indicates that proteins can move and the membrane is in constant motion.

Transport Across Membranes

Types of Transport Methods

  1. Passive Transport: Movement of molecules down a concentration gradient (high to low), not requiring energy.

    • Examples:

      • Diffusion: Small neutral molecules (O₂, CO₂).

      • Facilitated Diffusion: Uses proteins to help move larger or charged molecules.

      • Protein Channels: Allow ions like Na⁺ to pass.

      • Carrier Proteins: Bind to specific molecules (e.g., GLUT-1 for glucose) and change shape to transport them.

  2. Active Transport: Movement against a concentration gradient (low to high), requiring energy (ATP).

    • Examples:

      • Lactose Permease: Uses the H⁺ gradient to move lactose from low to high concentration.

      • Na⁺/K⁺ ATPase: Actively pumps Na⁺ out and K⁺ into the cell, creating steep gradients.

Transport Classifications

  • Primary Active Transport: Directly consumes ATP.

  • Secondary Active Transport: Uses ion gradients created by primary transport methods to move other substances.

    • Transporter Types:

      • Uniport: Carries one substance (e.g., GLUT-1 for glucose).

      • Symport: Carries two substances in the same direction (e.g., Lactose/H⁺).

      • Antiport: Carries two substances in opposite directions (e.g., Na⁺/K⁺ ATPase).

      • Sodium Glucose Linked Transporter (SGLT): Example of an active symporter using the sodium gradient to transport glucose into cells.

Lipoprotein Transport

Types and Functions of Lipoproteins

  1. Chylomicrons: Transport dietary triglycerides from the intestine to adipose tissue and muscle.

  2. Very Low-Density Lipoproteins (VLDL): Transports endogenous triglycerides and cholesterol from the liver to peripheral tissues.

  3. Low-Density Lipoproteins (LDL): Delivers cholesterol from VLDL to tissues.

  4. High-Density Lipoproteins (HDL): Scavenges excess cholesterol from peripheral tissues back to the liver for disposal.

    • Function: Transfers cholesteryl esters to liver cells for synthesis of steroid hormones and bile acids.

Cholesterol Transport

  • Serum Cholesterol Regulation:

    • High serum cholesterol levels lead to decreased synthesis in the liver and vice versa.

    • Cholesterol is transported in blood encapsulated in lipoproteins due to its hydrophobic nature.

  • Pathway: Cholesterol starts as VLDL from the liver, loses triglycerides in circulation, and then transforms into LDL.

  • Atherosclerosis: Condition characterized by artery narrowing due to cholesterol ester buildup. High LDL and low HDL levels indicate faulty transport and increased risk of atherosclerosis.

Microscopy

  • Scanning Electron Microscopy (SEM): Utilized for displaying particle sizes of different lipoproteins.