Membrane Structure and Function

Overview

This section covers the structure and function of biological membranes, focusing on the plasma membrane. It explains the fluid mosaic model, membrane permeability, and the mechanisms by which substances are transported across membranes.

Membrane Models and Components

• Fluid Mosaic Model: The plasma membrane is described as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids. This model emphasizes the dynamic and heterogeneous nature of the membrane.

• Phospholipids: The main component of membranes, phospholipids are amphipathic, meaning they have both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions.

• Membrane Fluidity: Membrane fluidity is maintained by weak hydrophobic interactions among lipids and proteins. Lateral movement of phospholipids is rapid, while flip-flopping across the bilayer is rare.

Factors Affecting Membrane Fluidity

• Temperature: Membranes solidify at lower temperatures. The temperature at which this occurs depends on the types of lipids present.

• Unsaturated Hydrocarbon Chains: Increase fluidity because kinks prevent tight packing.

• Saturated Hydrocarbon Chains: Decrease fluidity due to tight packing.

• Cholesterol: Acts as a "fluidity buffer," resisting changes in membrane fluidity caused by temperature shifts.

Membrane Proteins

• Integral Proteins: Penetrate the hydrophobic core of the lipid bilayer; often involved in transport and signal transduction.

• Peripheral Proteins: Loosely bound to the membrane surface; often involved in cell recognition and signaling.

Functions of Membrane Proteins

Membrane Carbohydrates

• Glycolipids: Carbohydrates covalently bonded to lipids.

• Glycoproteins: Carbohydrates covalently bonded to proteins.

• Important for cell-cell recognition, such as sorting cells into tissues and organs in embryos and immune responses.

Labeling the Animal Cell Membrane

• Glycolipid: Cell recognition

• Glycoprotein: Cell recognition

• Integral Protein: Transport, signal transduction, cell recognition

• Peripheral Protein: Cell recognition, signal transduction

• Cholesterol: Fluidity buffer

• Phospholipid: Forms bilayers, boundaries for cells/organelles

• ECM Fibers: Support, interaction around animal cells

• Cytoskeleton Microfilaments: Mechanical support, transport, signaling

• Integrins: Connect cytoskeleton to extracellular matrix

Membrane Permeability and Transport

• Selective Permeability: The membrane allows some substances to cross more easily than others due to its structure.

• Channel Proteins: Provide hydrophilic channels for specific molecules or ions.

• Carrier Proteins: Bind to molecules and change shape to shuttle them across the membrane.

• Transport Protein Specificity: Each transport protein is specific for the substance it moves (e.g., aquaporins for water).

Types of Membrane Transport

• Passive Transport: Movement of small molecules without energy input (e.g., diffusion, osmosis).

• Active Transport: Movement of molecules against their concentration gradient, requiring energy (ATP).

• Bulk Transport: Movement of large molecules via vesicles (endocytosis, exocytosis).

Diffusion and Osmosis

• Diffusion: Movement of molecules from high to low concentration.

• Concentration Gradient: Difference in concentration across a space.

• Passive Transport: Diffusion across a membrane without energy input.

• Osmosis: Diffusion of water across a selectively permeable membrane.

Osmotic Solutions

Facilitated Diffusion

• Transport proteins help move substances across the membrane without energy input.

• Includes channel proteins (e.g., aquaporins) and carrier proteins.

Plant vs. Animal Cells in Osmotic Solutions

• Animal cells burst in hypotonic solutions due to lack of a cell wall.

• Plant cells become turgid but do not burst because the cell wall provides structural support.

Key Equations

• Osmosis:

Where is the flux, is the permeability coefficient, and is the concentration difference across the membrane.

• Diffusion Rate (Fick's Law):

Where is the rate of diffusion, is the diffusion coefficient, and is the concentration gradient.

Summary Table: Types of Membrane Transport

Additional info: These notes expand on the provided outline by including definitions, examples, and key equations relevant to General Biology students.