BIO TOPIC 6

Phospholipid structure

Each phospholipid has a hydrophilic phosphate head and two hydrophobic fatty acid tails

Phospholipid bilayer

Forms when phospholipids arrange with heads facing water and tails facing inward away from water

Bilayer properties

Semi-permeable, fluid, and self-healing due to hydrophobic interactions

Fluid Mosaic Model

Describes the membrane as a flexible bilayer with proteins embedded or attached

Components of membranes

Include phospholipids, proteins, cholesterol, carbohydrates, and glycolipids

Integral membrane proteins

Span the membrane and are involved in transport or signaling

Peripheral membrane proteins

Attach to membrane surfaces and assist in signaling or support

Glycoproteins

Proteins with attached carbohydrates involved in cell recognition

Diffusion

Movement of molecules from high to low concentration until equilibrium is reached

Osmosis

Passive diffusion of water across a semi-permeable membrane

Solute diffusion

Non-polar and small polar molecules diffuse freely through membranes

Water diffusion

Water uses special channels called aquaporins to cross membranes quickly

Membrane transport proteins

Help substances move across membranes if they cannot diffuse directly

Channel proteins

Form pores for specific ions or water to pass through membranes

Carrier proteins

Bind to molecules and change shape to shuttle them across the membrane

Facilitated diffusion

Passive transport using proteins for molecules that can't freely diffuse

Simple diffusion

Passive movement directly through the membrane without proteins

Difference between simple and facilitated diffusion

Simple diffusion occurs directly; facilitated diffusion uses proteins

Active transport

Requires energy to move substances against their concentration gradient

Cotransport

Uses the movement of one molecule down its gradient to drive another molecule against its gradient

Symport

Two molecules move in the same direction across a membrane

Antiport

Two molecules move in opposite directions across a membrane

Ion pumps

Transport ions against their gradient using ATP

Cation-translocating ATPase

A type of ion pump that uses ATP to move positive ions like H⁺, Na⁺, or Ca²⁺

Sodium-potassium pump

Moves 3 Na⁺ out and 2 K⁺ into the cell using ATP

ATP in transport

Used to power active transport and maintain concentration gradients

Exocytosis

Vesicles fuse with the plasma membrane to release contents outside the cell

Endocytosis

Process where cells engulf substances by forming vesicles from the membrane

Phagocytosis

Cell engulfs large particles or cells into a vesicle; "cell eating"

Pinocytosis

Cell engulfs extracellular fluid and dissolved solutes; "cell drinking"

Receptor-mediated endocytosis

Specific molecules bind to receptors before being engulfed into vesicles