Membrane Structure, Lipids, and Transport in General Biology

Chapter 5: Membrane Transport

Introduction to Biological Membranes

Biological membranes are essential structures that separate the living cell from its surroundings, regulate the movement of substances, and maintain cellular integrity. Membranes are selectively permeable, allowing some substances to cross more easily than others.

• Plasma membrane: Separates the cell from its environment and controls traffic into and out of the cell.

• Selective permeability: Allows some substances to pass while restricting others.

• Fluid mosaic model: Describes the membrane as a mosaic of proteins floating in or on a fluid bilayer of phospholipids.

Concept 5.1: Cellular Membranes and the Fluid Mosaic of Lipids and Proteins

Structure and Function of Lipids in Membranes

Lipids are a major component of membranes, providing the hydrophobic barrier that separates cells from their environment. The most important lipids in membranes are phospholipids, but carbohydrates and proteins also play significant roles.

• Phospholipids: Amphipathic molecules with both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.

• Membrane fluidity: Influenced by temperature and the composition of fatty acids in phospholipids.

• Saturated fatty acids: No carbon-carbon double bonds; pack tightly, making membranes less fluid.

• Unsaturated fatty acids: One or more carbon-carbon double bonds; prevent tight packing, increasing membrane fluidity.

Types of Lipids

• Fats: Large molecules assembled from smaller molecules via dehydration reactions. Fats are energy storage molecules.

• Phospholipids: Major component of cell membranes, consisting of two fatty acids, a phosphate group, and a glycerol backbone.

• Steroids: Lipids with a carbon skeleton consisting of four fused rings (e.g., cholesterol).

Table: Comparison of Fatty Acids

Type	Structure	Effect on Membrane Fluidity
Saturated Fatty Acid	No double bonds; straight chains	Decreases fluidity (packs tightly)
Unsaturated Fatty Acid	One or more double bonds; kinked chains	Increases fluidity (prevents tight packing)

Phospholipid Bilayer

Phospholipids spontaneously arrange themselves into bilayers, with hydrophilic heads facing outward toward the aqueous environment and hydrophobic tails facing inward, away from water.

• Bilayer formation: Driven by the amphipathic nature of phospholipids.

• Fluid mosaic model: Proteins are embedded within or attached to the bilayer, allowing for dynamic movement and function.

Concept 5.2: Membrane Proteins and Their Functions

Types of Membrane Proteins

Proteins are crucial for membrane function, serving as transporters, receptors, enzymes, and structural components.

• Integral proteins: Penetrate the hydrophobic core of the lipid bilayer; often span the membrane (transmembrane proteins).

• Peripheral proteins: Loosely bound to the surface of the membrane; not embedded in the lipid bilayer.

Functions of Membrane Proteins

• Transport: Move substances across the membrane (channels, carriers).

• Enzymatic activity: Catalyze specific reactions at the membrane surface.

• Signal transduction: Relay signals from the external environment to the cell's interior.

• Cell-cell recognition: Allow cells to identify each other.

• Intercellular joining: Connect adjacent cells.

• Attachment to cytoskeleton and extracellular matrix: Maintain cell shape and stabilize membrane structure.

Concept 5.3: Passive Transport Across Membranes

Diffusion and Osmosis

Passive transport is the movement of substances across membranes without energy input from the cell. It relies on concentration gradients and includes diffusion and osmosis.

• Diffusion: Movement of molecules from an area of higher concentration to an area of lower concentration.

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

Equation for diffusion rate:

$J = -D \frac{dC}{dx}$

• Facilitated diffusion: Transport proteins help move substances across the membrane down their concentration gradient.

Table: Types of Passive Transport

Type	Energy Required	Direction	Transport Proteins Involved
Simple Diffusion	No	Down concentration gradient	No
Facilitated Diffusion	No	Down concentration gradient	Yes (channels/carriers)
Osmosis	No	Down water potential gradient	Sometimes (aquaporins)

Concept 5.4: Active Transport and Energy Use

Active Transport Mechanisms

Active transport requires energy (usually from ATP) to move substances against their concentration gradients. This process is essential for maintaining cellular homeostasis.

• Pumps: Transport proteins that use energy to move ions or molecules across membranes (e.g., sodium-potassium pump).

• Electrochemical gradient: Combination of concentration gradient and electrical charge difference across the membrane.

Equation for sodium-potassium pump:

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

Bulk Transport: Endocytosis and Exocytosis

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

• Exocytosis: Cell expels macromolecules by fusing vesicles with the plasma membrane.

Additional info:

• Cholesterol is an important steroid that modulates membrane fluidity in animal cells.

• Plant cells have cell walls that provide additional protection against osmotic stress.

• Transport proteins can be highly specific for the substances they move.