Cell Membranes and Transport

• Phospholipid bilayer: hydrophilic heads face out, hydrophobic tails face in
• Fluid: phospholipids move laterally (not fixed in position)
• Mosaic: proteins scattered throughout

Membrane Components

Passive (No ATP)

Simple diffusion: small nonpolar molecules (O₂, CO₂) move down concentration gradient through the bilayer.

Facilitated diffusion: larger or charged molecules move down gradient via channel or carrier proteins.

Osmosis: water moves from high water potential ($\Psi$) to low water potential through a partially permeable membrane. $\Psi = \Psi_s + \Psi_p$

Active (Requires ATP)

Active transport: carrier proteins move molecules against concentration gradient (low → high). ATP changes carrier protein shape.

Co-transport: molecule transported alongside another (e.g. Na⁺-glucose co-transport in gut epithelium).

Endocytosis: cell membrane engulfs large molecules/particles (phagocytosis = solids; pinocytosis = liquids).

Exocytosis: vesicles fuse with membrane to release contents outside cell.

• Temperature: moderate increase → more fluid → more permeable; high temperature → proteins denature → very permeable
• Solvents (e.g. alcohol): dissolve phospholipids → membrane disintegrates

• Glycoproteins and glycolipids on cell surface
• Act as antigens — markers of cell identity
• Immune system recognises self vs non-self antigens
• Important in: organ transplant rejection, immune response, blood transfusions

• Fluid mosaic: phospholipid bilayer with proteins (channel, carrier), cholesterol, glycoproteins
• Passive: diffusion, facilitated diffusion, osmosis (no ATP)
• Active: active transport, co-transport, endo/exocytosis (ATP needed)
• Water potential: $\Psi = \Psi_s + \Psi_p$
• Glycoproteins: cell recognition and immune response