Membrane Structure and Function

Key Concepts

The plasma membrane is a fundamental structure in all living cells, responsible for maintaining the internal environment and mediating interactions with the external environment. This section covers the composition, properties, and functions of biological membranes.

• Cellular membranes are fluid mosaics of lipids and proteins.

• Membrane structure results in selective permeability, allowing some substances to cross more easily than others.

• Passive transport is the diffusion of a substance across a membrane without energy investment.

• Active transport uses energy to move solutes against their concentration gradients.

• Bulk transport across the plasma membrane occurs by exocytosis and endocytosis.

Prefixes and Suffixes in Membrane Biology

Understanding common prefixes and suffixes helps in interpreting biological terms:

Membrane Structure

The Fluid Mosaic Model

The fluid mosaic model describes the structure of cell membranes as a mosaic of various proteins floating in or on the fluid lipid bilayer. This model explains both the flexibility and the selective permeability of membranes.

• Phospholipids form a bilayer with hydrophilic heads facing outward and hydrophobic tails facing inward.

• Proteins are embedded within or attached to the lipid bilayer, serving various functions such as transport, signaling, and structural support.

• Carbohydrates are attached to proteins (glycoproteins) or lipids (glycolipids) on the extracellular surface, playing roles in cell recognition.

Synthesis and Sidedness of Membranes

Biological membranes have distinct inside (cytoplasmic) and outside (extracellular) faces, which is crucial for their function.

• Lipid bilayers may differ in composition between the two leaflets.

• Proteins have a specific orientation, with certain domains always facing the cytoplasm or the extracellular space.

• Glycoproteins and glycolipids are found on the extracellular side, originating from the inside face of the endoplasmic reticulum (ER) and Golgi apparatus during synthesis.

Example: The synthesis of membrane components involves the ER and Golgi, which determine the final orientation of proteins and lipids in the plasma membrane.

Membrane Permeability and Transport

Selective Permeability

Cell membranes are selectively permeable, allowing some molecules to cross more easily than others. This property is essential for maintaining homeostasis.

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

• Hydrophilic (polar) molecules and ions (e.g., sugars, water, Na+, K+) cannot easily cross the hydrophobic core and require transport proteins.

Transport Proteins

Transport proteins facilitate the movement of hydrophilic substances across the membrane.

• 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. They are specific to the substance they transport (e.g., glucose transporter).

Example: The glucose transporter only transports glucose, not fructose.

Passive Transport

Diffusion

Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration, down their concentration gradient. This process does not require energy.

• Each substance diffuses down its own concentration gradient.

• Examples: O2 and CO2 diffusion across cell membranes.

Equation:

Where is the flux, is the diffusion coefficient, and is the concentration gradient.

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane. Water moves from areas of higher free water concentration (lower solute concentration) to areas of lower free water concentration (higher solute concentration).

• Water balance is crucial for cell survival.

• Osmosis can cause cells to swell or shrink depending on the surrounding solution.

Tonicity and Water Balance

Tonicity describes the ability of a surrounding solution to cause a cell to gain or lose water.

• Osmoregulation is the control of solute concentrations and water balance (e.g., contractile vacuole in Paramecium).

Facilitated Diffusion

Facilitated diffusion is passive transport aided by proteins. Channel and carrier proteins help hydrophilic molecules and ions cross the membrane down their concentration gradient.

• Does not require energy.

• Examples: Aquaporins (water channels), glucose transporters.

Active Transport

Mechanism and Importance

Active transport moves solutes against their concentration gradients, requiring energy (usually from ATP). This process is essential for maintaining concentration differences across membranes.

• Transport proteins involved are always carrier proteins.

• Allows cells to maintain internal concentrations of small molecules that differ from concentrations in their environment.

Sodium-Potassium Pump

The sodium-potassium pump is a classic example of active transport in animal cells. It exchanges three Na+ ions out of the cell for two K+ ions into the cell, using energy from ATP hydrolysis.

• Maintains high K+ and low Na+ inside the cell.

• ATP hydrolysis provides the energy for the pump to change shape and transport ions.

Equation:

Membrane Potential and Electrogenic Pumps

Membrane potential is the voltage difference across a membrane, resulting from the unequal distribution of ions. Electrogenic pumps generate this voltage.

• Sodium-potassium pump is the main electrogenic pump in animal cells.

• Proton pump is the main electrogenic pump in plants, fungi, and bacteria, transporting H+ out of the cell.

• The electrochemical gradient combines the concentration gradient and membrane potential, influencing ion movement.

Cotransport

Cotransport is a type of active transport where the diffusion of one solute down its concentration gradient drives the transport of another solute against its gradient, using a cotransporter protein.

• ATP indirectly provides the energy by establishing the gradient.

• Example: H+/sucrose cotransport in plant cells loads sucrose into phloem cells.

Summary Table: Types of Membrane Transport

Key Terms

• Phospholipid bilayer: Double layer of phospholipids forming the core of all cell membranes.

• Integral protein: Protein embedded within the membrane.

• Peripheral protein: Protein attached to the membrane surface.

• Glycoprotein: Protein with carbohydrate chains attached.

• Glycolipid: Lipid with carbohydrate chains attached.

• Osmoregulation: Regulation of water and solute balance.

• Electrochemical gradient: Combined effect of concentration gradient and electrical potential.