Cell Membrane Structure and Transport Mechanisms in Biology

cell membrane or plasma membrane

Surrounds the cell and allows things to travel in and out of the cell. Mostly made of phospholipids, but also contains proteins, carbs, and cholesterol.

phospholipid bilayer

The two-layered structure formed by phospholipids, with hydrophilic heads facing out and hydrophobic tails facing in.

phospholipid Head

The hydrophilic (polar) part of a phospholipid that faces the watery environment inside and outside the cell.

phospholipid tail

The hydrophobic (nonpolar) part of a phospholipid, composed of fatty acids, that faces inward toward the other layer.

Hydrophilic

"Water-loving" or polar; the characteristic of the phospholipid head.

Hydrophobic

"Water-fearing" or nonpolar; the characteristic of the phospholipid tail.

Fluid Mosaic Model

The model describing the cell membrane's fluidity, where phospholipids can move laterally and diverse proteins are embedded like a mosaic.

Cholesterol

A lipid component embedded in the membrane that helps with support structure.

Integral (Intrinsic) Proteins

Proteins that penetrate the hydrophobic core of the lipid bilayer often span the entire membrane.

Peripheral (Extrinsic) Proteins

Proteins located on the inner or outer surface of the membrane, not embedded in the lipid core.

Carbohydrates

Chains attached to the membrane, often functioning in signaling and cell identification (ID).

Selectively Permeable

The property of the cell membrane that allows certain substances to pass through while restricting the movement of others.

Factors Affecting Permeability

The two main factors that determine how easily a substance crosses the membrane: Size and Charge/Polarity.

Easily Permeable Molecules

Molecules that are small and nonpolar (e.g., oxygen, carbon dioxide).

Molecules Requiring Transport

Larger and/or charged molecules (e.g., glucose, ions, polar molecules like urea) that cannot easily pass the hydrophobic core.

Homeostasis

The maintenance of a stable internal environment within the cell, achieved through the regulation of substance movement across the membrane.

Dynamic Equilibrium

The overall goal of the cell membrane; small changes are made to keep things balanced, resulting in no net movement of a substance.

Concentration Gradient

A difference in concentrations of a substance between two regions (e.g., inside vs. outside the cell) that is necessary for diffusion to occur.

Passive Transport

Transport across the cell membrane that requires NO ENERGY (ATP), where substances move from HIGH concentration to LOW concentration (down the concentration gradient).

Diffusion

The general term for the movement of molecules from high concentration to low concentration until equilibrium is reached.

Simple Diffusion

Molecules move directly through the phospholipid bilayer (a semipermeable membrane) without the aid of transport proteins (e.g., oxygen, carbon dioxide).

Facilitated Diffusion

The transport of polar, large molecules, or ions across the membrane that requires transport proteins (like channels).

Osmosis

The specific type of diffusion involving the movement of water across a semipermeable membrane.

Aquaporins

Special transport proteins that facilitate the diffusion of water (osmosis), resulting in a higher rate of water movement.