Membrane Structure and Function

Overview

The plasma membrane is a fundamental component of all cells, responsible for regulating the movement of substances into and out of the cell. Its structure and function are essential for maintaining cellular homeostasis and enabling communication with the environment.

Key Concepts and Vocabulary

• Amphipathic: Molecules that have both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions. Phospholipids are amphipathic, with hydrophilic heads and hydrophobic tails.

• Fluid Mosaic Model: Describes the structure of the plasma membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

• Selective Permeability: The property of the membrane that allows some substances to cross more easily than others.

Ways the Plasma Membrane Regulates Inbound and Outbound Traffic

Types of Transport

• Passive Transport: Movement of small molecules across the membrane without energy input. This can occur via diffusion or with the help of transport proteins.

• Active Transport: Movement of small molecules against their concentration gradient, requiring both energy (usually from ATP) and a transport protein.

• Bulk Transport: Movement of large molecules (such as proteins and polysaccharides) via vesicles, including exocytosis (out of the cell) and endocytosis (into the cell).

Structure of Cellular Membranes

Components

• Phospholipids: Form a bilayer with hydrophobic tails facing inward and hydrophilic heads facing outward, creating a semi-permeable barrier.

• Proteins: Embedded within or attached to the membrane, responsible for various functions such as transport, signaling, and cell recognition.

• Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids), important for cell recognition and signaling.

Fluid Mosaic Model

• The membrane is dynamic, with lipids and proteins able to move laterally within the layer.

• Proteins are not randomly distributed; they often form functional groups.

• Membranes are held together mainly by weak hydrophobic interactions.

Membrane Fluidity

• Unsaturated fatty acids: Increase membrane fluidity due to kinks in their tails, preventing tight packing.

• Saturated fatty acids: Decrease fluidity by allowing tighter packing of phospholipids.

• Cholesterol: Acts as a fluidity buffer in animal cells, restraining movement at high temperatures and preventing solidification at low temperatures.

Types of Membrane Proteins

• Peripheral Proteins: Bound to the surface of the membrane.

• Integral Proteins: Penetrate the hydrophobic core; those that span the membrane are called transmembrane proteins.

Membrane proteins are involved in transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix.

Role of Membrane Carbohydrates

• Function as markers for cell identification.

• Glycolipids and glycoproteins are involved in cell-cell recognition.

Selective Permeability of the Lipid Bilayer

• Hydrophobic (nonpolar) molecules: Such as hydrocarbons, CO2, and O2, can dissolve in the lipid bilayer and pass through rapidly.

• Hydrophilic (polar) molecules: Such as sugars, water, and ions, pass through slowly or require transport proteins.

Transport Proteins

• Channel Proteins: Provide hydrophilic channels for specific molecules or ions (e.g., aquaporins for water).

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

Transport proteins are specific for the substances they move, contributing to selective permeability.

Passive Transport

Diffusion

• Movement of particles from an area of higher concentration to an area of lower concentration (down their concentration gradient).

• Does not require energy input.

• At dynamic equilibrium, molecules continue to move but there is no net change in concentration.

Osmosis

• Diffusion of water across a selectively permeable membrane.

• Water moves toward the area of higher solute concentration until equilibrium is reached.

Tonicity and Water Balance

• Isotonic: Solute concentration is equal inside and outside the cell; no net water movement.

• Hypertonic: Higher solute concentration outside the cell; cell loses water.

• Hypotonic: Lower solute concentration outside the cell; cell gains water.

Osmoregulation

• Control of solute concentrations and water balance is essential for cells in environments that are not isotonic.

• Example: Paramecium uses a contractile vacuole to expel excess water in a hypotonic environment.

Facilitated Diffusion

• Transport proteins (channels and carriers) speed up passive movement of molecules across the membrane.

• Channel proteins can be gated, opening or closing in response to stimuli.

• Carrier proteins undergo shape changes to move substances down their concentration gradients.

Active Transport

• Requires energy (usually from ATP) to move substances against their concentration gradients.

• Performed by specific carrier proteins.

Sodium-Potassium Pump (Na+/K+ Pump)

• Maintains high K+ and low Na+ concentrations inside animal cells.

• ATP transfers a phosphate group to the pump, changing its shape and driving ion transport.

Equation:

Membrane Potential

• Voltage across a membrane due to differences in ion distribution.

• Inside of the cell is typically negative relative to the outside.

• Electrochemical gradient combines the chemical and electrical forces driving ion movement.

Electrogenic Pumps

• Transport proteins that generate voltage across a membrane.

• Animals: Sodium-potassium pump; Plants, fungi, bacteria: Proton pump (H+).

Cotransport

• Active transport of one solute indirectly drives transport of another.

• Example: Proton gradient created by a pump is used to drive the uptake of sucrose in plants.

Bulk Transport

• Large molecules cross the membrane in bulk via vesicles.

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell (e.g., secretion of insulin).

• Endocytosis: Cell takes in macromolecules by forming vesicles from the plasma membrane.

Types of Endocytosis

• Example: Uptake of cholesterol via LDLs (low-density lipoproteins) by receptor-mediated endocytosis.

Summary Table: Types of Membrane Transport

Practice Questions

• If a cell is hypertonic to the solution, water will move into the cell, potentially causing it to swell or burst (lyse in animal cells, become turgid in plant cells).

• If a solution is hypotonic to the cell, water will move out of the cell, causing it to shrink (crenate in animal cells, plasmolyze in plant cells).