1. Big Idea 2B
Growth, reproduction, and dynamic homeostasis require that cells create and maintain internal environments that are different from their external environments
2B1 – Cell membranes are selectively permeable due to their structure. 2B2 – Growth and dynamic homeostasis are maintained by the constant movement of molecules across membranes.

2. Membrane structure, I (Ch7)
Selective permeability – only some substances can cross – separates the internal/external environments
Amphipathic – has both hydrophobic & hydrophilic regions
Singer-Nicolson: fluid mosaic model
Fluid structure w/ various proteins embedded

3. Membrane structure, II Phospholipids – membrane fluidity
Lipids move along membrane - lateral
May move in/out – rare – flip-flop
Unsaturated – kinked tails – increase fluidity
Cholesterol – membrane stabilization
“Mosaic” Structure – collection of different proteins – unique to each cell
Integral proteins – transmembrane proteins – cross entire membrane
Peripheral proteins – surface of membrane
Membrane carbohydrates – cell to cell recognition
Oligosaccharides (cell markers); glycolipids; glycoproteins
Immune system, blood types

4. Membrane structure, III
Membrane protein function:
Transport – channel, shuttle
Enzymatic activity – active site exposed
Signal transduction (receptor) – binds to chem messenger – sends message inside
Intercellular joining – gap/tight junctions
Cell-cell recognition
ECM attachment – maintain cell shape and stability

5. Membrane traffic, I
Passive transport~ diffusion of a substance across a biological membrane – no energy
Diffusion~ tendency of any molecule to spread out into available space
Moves down its concentration gradient
Small, uncharged polar molecules and small nonpolar molecules may pass
Osmosis~ the diffusion of water across a selectively permeable membrane via aquaporins

6. Membrane traffic, II
Facilitated diffusion~ passage of molecules and ions with transport proteins across a membrane down the concentration gradient
Channel proteins: ion channels, gated proteins, carrier proteins
Allows movement of hydrophilic substances – large polar molecules and ions

7. Water Balance, I Osmoregulation – control of water balance
Hypertonic solution – higher concentration of solutes – cell is hypotonic – cell shrinks
Hypotonic solution – lower concentration of solutes – cell is hypertonic – cell swells
Isotonic – equal concentrations of solutes
Cells with Walls:
Turgid (very firm) - hypo
Flaccid (limp) - iso
Plasmolysis – plasma membrane pulls away from cell wall - hyper

8. Water Balance, II
Water potential (Ψ) - movement of water from an area of high concen. to low concen.
Ψ = ΨP + ΨS
Water Potential = Pressure Potential + Solute Potential
At atmospheric pressure - Ψp = 0
Water potential of pure water in an open beaker is zero (ψ = 0)
Water will move from an area of high Ψ to an area of low Ψ
Tonic refers to solute concen., potential refers to water concen.
The Solute Potential of the Solution
Ψs = – iCRT
i = ionization constant (For sucrose this is 1.0 because sucrose does not ionize in water.)
C = molar concentration
R = pressure constant (R = liter bars/mole K)
T = temperature in Kelvin (273 + °C)

9. Active Transport, I
Movement of a substance against its concentration gradient with the help of cellular energy
Sodium-potassium pump – electrochemical gradient – nerve cells
Cotransport – solute pumped out can do work as it diffuses back

10. Active Transport, II
Exocytosis~ secretion of macromolecules by the fusion of transport vesicles with the plasma membrane
Endocytosis~ import of macromolecules by forming new vesicles with the plasma membrane
Phagocytosis – solids
Pinocytosis – liquids
Receptor-mediated
Ligands – bind to receptor