Membrane Structure and Function: Lipids, Proteins, and Transport in Cells

Membrane Structure and Function

Plasma Membrane: Definition and Role

The plasma membrane (cell membrane) is a vital structure that separates the interior of the cell from its external environment. It acts as a selective barrier, regulating the entry and exit of substances to maintain cellular homeostasis.

• Selective Barrier: Controls which substances can enter or leave the cell.

• Maintains Internal Environment: Keeps the internal conditions of the cell distinct from the external environment.

Lipids: Structure and Function

Definition and Characteristics

Lipids are a diverse group of carbon-containing compounds that are characterized by their insolubility in water due to a high proportion of nonpolar C–C and C–H bonds. Most lipids are hydrophobic.

• Hydrocarbon Chains: Lipids often contain long hydrocarbon chains.

• Hydrophobic Nature: Lipids do not dissolve in water.

How Bond Saturation Affects Hydrocarbon Structure

• Saturated Hydrocarbons: Contain only single bonds between carbon atoms (straight chains). Solid at room temperature.

• Unsaturated Hydrocarbons: Contain one or more double bonds (bent chains). Liquid at room temperature.

Hydrogenation is the process of converting unsaturated lipids to saturated lipids by breaking double bonds and adding hydrogen atoms.

Types of Lipids Found in Cells

• Fats (Triglycerides): Composed of three fatty acids linked to a three-carbon molecule called glycerol.

• Phospholipids: Consist of a glycerol backbone, two fatty acids, and a phosphate group. They are amphipathic, containing both hydrophilic (phosphate head) and hydrophobic (fatty acid tails) regions.

• Steroids: Characterized by a four-ring structure (e.g., cholesterol).

Table: Comparison of Major Lipid Types

Lipid Type	Structure	Function
Fats (Triglycerides)	Glycerol + 3 fatty acids	Energy storage
Phospholipids	Glycerol + 2 fatty acids + phosphate group	Main component of cell membranes
Steroids	Four fused carbon rings	Membrane structure, signaling (e.g., cholesterol, hormones)

Phospholipids and Membrane Structure

Phospholipid Bilayer

Phospholipids spontaneously form bilayers in water due to their amphipathic nature. The hydrophilic heads face outward toward water, while the hydrophobic tails face inward, away from water.

• Foundation of Cellular Membranes: All cellular membranes are primarily composed of phospholipid bilayers.

Artificial Membranes as Experimental Systems

• Liposomes: Artificial membrane-bound vesicles used to study membrane properties.

• Planar Bilayers: Flat, artificial membranes used in experiments.

Selective Permeability of Lipid Bilayers

Cell membranes are selectively permeable, allowing some substances to cross more easily than others.

• Small, nonpolar molecules (e.g., O2, CO2) cross rapidly.

• Small, uncharged polar molecules (e.g., H2O) cross less rapidly.

• Large or charged molecules (e.g., ions, glucose) cross very slowly or not at all without assistance.

Factors Affecting Membrane Permeability

• Saturation of Hydrocarbon Tails: Unsaturated tails (with double bonds) increase membrane fluidity and permeability; saturated tails decrease it.

• Cholesterol Content: Cholesterol decreases membrane permeability by packing between phospholipids.

• Temperature: Higher temperatures increase fluidity and permeability; lower temperatures decrease them.

Transport Across Membranes

Diffusion and Osmosis

• Diffusion: The movement of molecules from an area of higher concentration to an area of lower concentration (down a concentration gradient). It is a passive process and does not require energy.

• Osmosis: The diffusion of water across a selectively permeable membrane.

Types of Solutions and Effects on Cells

Solution Type	Relative Solute Concentration	Effect on Cell
Hypertonic	Higher outside cell	Cell shrinks (water leaves cell)
Hypotonic	Lower outside cell	Cell swells (water enters cell)
Isotonic	Equal inside and outside	No net movement of water; cell maintains shape

Membrane Proteins: Structure and Function

• Integral (Transmembrane) Proteins: Span the membrane and have segments facing both the interior and exterior of the cell.

• Peripheral Proteins: Bind to membrane lipids or integral proteins without passing through the membrane.

Protein Channels and Facilitated Diffusion

• Ion Channels: Specialized transmembrane proteins that allow specific ions to cross membranes.

• Gated Channels: Open or close in response to signals.

• Facilitated Diffusion: Transmembrane proteins assist the passive transport of substances that cannot cross the membrane directly.

Carrier Proteins vs. Channel Proteins

• Channel Proteins: Provide a pore for selective diffusion of molecules.

• Carrier Proteins: Bind to specific molecules and undergo conformational changes to transport them across the membrane.

Pumps and Active Transport

• Active Transport: Moves substances against their concentration gradient using energy (usually ATP).

• Sodium-Potassium Pump (Na+/K+ ATPase): Transports 3 sodium ions out of the cell and 2 potassium ions into the cell per ATP hydrolyzed.

Equation for Sodium-Potassium Pump:

$3\,\text{Na}^+_{\text{in}} + 2\,\text{K}^+_{\text{out}} + \text{ATP} \rightarrow 3\,\text{Na}^+_{\text{out}} + 2\,\text{K}^+_{\text{in}} + \text{ADP} + \text{P}_i$

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose via carrier protein
Osmosis	No	Down water gradient	Water via aquaporins
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump

Additional info:

• Membrane fluidity is essential for proper cell function, affecting processes such as signaling, transport, and cell division.

• Cholesterol acts as a "fluidity buffer," preventing membranes from becoming too rigid or too fluid.

• Defects in membrane proteins can lead to diseases such as cystic fibrosis.