Understanding the Cell Membrane and Transport Mechanisms

Fluid Mosaic Model

The fluid mosaic model describes the cell membrane as a flexible, dynamic structure composed of a phospholipid bilayer with embedded proteins, allowing selective transport and communication.

Cholesterol's Effect on Membrane Fluidity

Cholesterol stabilizes membrane fluidity by preventing excessive movement at high temperatures and maintaining flexibility at low temperatures.

Simple Diffusion

Movement of small, nonpolar molecules directly across the membrane without a protein.

Facilitated Diffusion

Movement of larger or polar molecules through a transport protein.

Osmosis

Osmosis is the diffusion of water across a semipermeable membrane from an area of low solute concentration to high solute concentration to balance solute levels.

Red Blood Cell in Hypertonic Solution

The red blood cell shrinks (crenates) because water moves out of the cell due to the higher solute concentration outside.

Facilitated Diffusion Energy Requirement

Facilitated diffusion is a passive process where molecules move down their concentration gradient, so no ATP is needed.

Active Transport

Active transport requires ATP because it moves substances against their concentration gradient (from low to high concentration).

Passive Transport vs Active Transport

Passive transport: No energy required, moves substances down their concentration gradient. Active transport: Requires ATP, moves substances against their concentration gradient.

Receptor-Mediated Endocytosis

Specific molecules bind to receptor proteins before being engulfed by the cell.

Simple Endocytosis

Engulfs extracellular material without receptor specificity.

Role of Channel Proteins

Channel proteins form pores in the membrane that allow specific molecules or ions to pass without using energy.

Electrogenic Pumps

These pumps move ions against their gradient, creating an electrical charge difference across the membrane, essential for nerve signaling and muscle contraction.

Osmoregulation

Cells regulate water balance by controlling solute concentrations using mechanisms like aquaporins, ion pumps, and vesicle transport.

Role of Glycoproteins and Glycolipids

They are involved in cell recognition, signaling, and immune responses by helping cells communicate and identify one another.

Plant Cell in Hypotonic Solution

The plant cell swells but does not burst due to its cell wall, becoming turgid (firm) as water moves in.

Selective Permeability of Plasma Membrane

The phospholipid bilayer and membrane proteins regulate what enters and exits, allowing essential substances in while blocking harmful ones.