Cell Membranes and Membrane Transport: Study Notes for General Biology

The Membrane

Structure and Components of Biological Membranes

Biological membranes are essential structures that separate the interior of cells from their external environment and regulate the movement of substances in and out of the cell. The primary component of these membranes is the phospholipid bilayer, which is interspersed with proteins, cholesterol, and other molecules.

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails; form the basic structure of the membrane.

• Biological Membrane: Composed mainly of phospholipids, with embedded molecules such as proteins and cholesterol.

• Fluid Mosaic Model: Describes the membrane as a dynamic mosaic of phospholipids and proteins that move laterally within the layer.

• Composition: Membranes are comprised of 20-80% proteins by mass.

Example: Membranes are a fluid mosaic of phospholipids, proteins, and cholesterol.

Practice: The fluid mosaic model proposes that membranes consist of a bilayer of phospholipids with embedded proteins and other molecules.

Membrane-Associated Proteins

Proteins associated with membranes play critical roles in cell function and communication. There are two main types:

• Integral Membrane Proteins: Span the entire membrane and are embedded within the lipid bilayer.

• Peripheral Membrane Proteins: Located on the surface (perimeter) of the membrane, not embedded within the bilayer.

Example: Integral proteins cross the membrane, while peripheral proteins are attached to the membrane surface.

Functions of Membrane Proteins

• Recognition: Marks cells for identification.

• Anchorage: Connects cell cytoskeleton to the extracellular matrix (ECM).

• Transduction: Acts as receptors for signal molecules.

• Transport: Facilitates molecular transport across the membrane.

• Linkage: Connects two cells via protein linkage.

• Enzymes: Catalyze various enzymatic processes.

Practice: Not all membrane proteins are involved in transducing signals; some have structural or enzymatic roles.

Concentration Gradients and Diffusion

Concentration Gradient

A concentration gradient refers to the difference in the concentration of a substance between two areas. Molecules tend to move down their concentration gradient, from areas of high concentration to areas of low concentration.

• Definition: The change in concentration of a substance across a space.

• Movement: Molecules move down the gradient (high to low concentration).

• Energy: Movement down a gradient does not require energy; movement against a gradient requires energy.

Example: Diffusion of dye in water demonstrates movement down a concentration gradient.

Diffusion

Diffusion is the passive movement of molecules from an area of higher concentration to an area of lower concentration, driven by the concentration gradient.

• Definition: Movement of substances from high to low concentration.

• Passive Process: Does not require cellular energy (ATP).

Example: Red dye dispersing in water over time until equilibrium is reached.

Practice: Diffusion is not the movement of water across a membrane; that process is called osmosis.

Selective Permeability of Membranes

Permeability and Selectivity

Biological membranes are selectively permeable, meaning they allow certain molecules to pass while restricting others. This property is crucial for maintaining cellular homeostasis.

• Selective Permeability: The ability of the membrane to control what enters and exits the cell.

• Barrier Function: Prevents free diffusion of all molecules.

Example: Only specific molecules can freely cross the membrane without assistance.

Which Molecules Freely Cross Membranes?

• Can Freely Diffuse: Small, uncharged, nonpolar molecules (e.g., O2, CO2).

• Require Facilitation: Large, charged, or polar molecules (e.g., glucose, Na+).

Can Freely Diffuse	Require Facilitation
Small, uncharged, nonpolar	Large, charged, polar/hydrophilic
O2, CO2	Glucose, Na+, proteins

Practice: O2 most easily diffuses across a biological membrane without help from a protein.

Membrane Transport Mechanisms

Types of Membrane Transport

Cells use various mechanisms to transport molecules across membranes, classified as passive or active transport, and bulk transport for large molecules.

Transport Type	Mechanism	Energy Requirement
Passive Transport	Simple diffusion, facilitated diffusion, osmosis	No energy required
Active Transport	Pumps, carrier proteins	Requires ATP
Bulk Transport	Endocytosis, exocytosis, phagocytosis, pinocytosis	Requires energy

Practice: Phagocytosis is a type of bulk transport also called "cell eating."

Equations and Formulas

• Fick's Law of Diffusion:

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

• Where: J = flux, D = diffusion coefficient, dC/dx = concentration gradient

Summary Table: Molecule Permeability

Molecule Type	Permeability	Transport Mechanism
Small, nonpolar (O2, CO2)	High	Simple diffusion
Small, polar (H2O)	Moderate	Facilitated diffusion (aquaporins)
Large, polar/charged (glucose, Na+)	Low	Facilitated diffusion or active transport

Additional info: These notes expand on the original content by providing definitions, examples, and tables for clarity and completeness. All key concepts are explained in the context of General Biology, focusing on cell membranes and transport mechanisms.