Campbell Ch 7 "Membrane Structure and Function"

Fluid Mosaic Model and Selective Permeability

Cell Membrane is a lipid bilayer embedded with proteins that is constantly moving

Passes Easily: Small nonpolar/hydrophobic molecules (O2, CO2)

Requires Transport Proteins: Polar/hydrophilic molecules, ions, large molecules (H2O, glucose, Na+)

Passive Transport: Diffusion, Osmosis, & Facilitated Diffusion

Passive meaning no ATP required; moves solutes down the concentration/electrochemical gradient (high to low)

Channel Proteins (aquaporins, ion channels) allow solutes to pass freely unless they are selective to open only in certain conditions (gated channel proteins)

Carrier Proteins hold onto solutes in one shape and change shape to transport them across the membrane

Tonicity (Plant vs. Animal Cells)

Hypertonic (High solute outside): Water leaves cell. Animal cell shrivels; Plant cell plasmolyzes

Isotonic (Equal solute): Water leaves and enters the cell at equal rates. Animal cell stay normal (ideal); Plant cell goes flaccid

Hypotonic (Low solute outside): Water enters cell. Animal cell lyses (bursts); Plant cell becomes turgid, meaning pressurized (ideal)

Sodium-Potassium Pump

Three sodium ions have a high affinity with the pump and attach

Signals for ATP to drop a phosphate to change its shape

The sodium ions detach and the pump have a high affinity with 2 potassium ions

The potassium ions signal for the phosphate to detach, reverting the shape of the protein

The potassium ions detach into the cell

This keeps a negative gradient inside the cell (net loss of one positive charge per cycle)

Electrochemical Gradient & Cotransport

Combined chemical (concentration) and electrical (charge) forces acting on an ion.

The proton pump is the main electrogenic pump in plants/fungi/bacteria (H+ pumped out)

Cotransport is when an ion moving down from its gradient (which was moved against it via active transport) powers another solute to move against its gradient

An example is the sucrose hydrogen pump, which uses the energy from an H+ moving back in the cell to cause a sucrose to go in the cell against its gradient.

Bulk Transport (Exocytosis vs Endocytosis)

Exocytosis: usage of vesicles that merge with the cell membrane to expel its contents

Endocytosis: membrane pinches in to create a vesicle with contents from outside the cell

Phagocytosis: A type of endocytosis where the membrane pinches around a large particle such as a cell (taking a big bite)

Pinocytosis: A type of endocytosis where the membrane pinches small vesicles of solute from outside the cell (sipping a drink)

Receptor Mediated Endocytosis: Like pinocytosis but with receptors that specialize in attaching to a specific solute, allowing cells to gather uncommon solutes in bulk (picky)