Cell Membrane

Cell membrane

Primary structure is a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

Selectively permeable

The membrane allows some substances to pass while blocking others.

Cholesterol in the membrane

Stabilizes membrane fluidity and prevents fatty acid chains from packing too tightly.

Integral proteins

Proteins embedded within the membrane that often span the entire bilayer.

Peripheral proteins

Proteins attached to the membrane surface, not embedded within the bilayer.

Fluid mosaic model

Describes the membrane as fluid with proteins moving within a flexible lipid bilayer.

Passive transport

Movement of substances across the membrane without energy input.

Diffusion

Movement of molecules from high to low concentration without energy.

Facilitated diffusion

Passive movement of molecules through transport proteins.

Osmosis

Diffusion of water across a selectively permeable membrane.

Active transport

Movement of substances against their concentration gradient using ATP.

Sodium-potassium pump

An active transport protein that exchanges 3 Na⁺ out of and 2 K⁺ into the cell using ATP.

Endocytosis

The process of engulfing material into the cell via vesicles.

Exocytosis

The release of substances from the cell via vesicles fusing with the membrane.

Glycoproteins and glycolipids

Carbohydrate-attached proteins and lipids used for cell recognition.

Protein channels

Allow specific molecules to pass through the membrane.

Molecules passing easily through the membrane

Small, nonpolar molecules like O₂ and CO₂.

Molecules requiring transport proteins

Large, polar, or charged molecules like glucose and ions.

Membrane composition and fluidity

More unsaturated fats and cholesterol increase fluidity.

Importance of membrane fluidity

Allows for membrane flexibility, self-repair, and proper function of proteins.