biol110 lec 11/12 (not on rec quiz but exam2 material)

types of membrane transport

• passive transport (diffusion)

  • movement of molecules across membrane from high → low conc.

  • doesn’t need energy input from cell

• active transport (anti-diffusion)

  • movement of molecules across membrane from low → high conc.

  • does require energy input from cell

types of passive transport

• simple diffusion

  • if PM is permeable to particular molecules AND there is difference in conc. of that molecule across the membrane

  • then the molecule will simple diffuse across the membrane

    • no energy input nor transport proteins needed

facilitated diffusion (FD)

• molecules that can’t cross membrane on own need this

• requires transport (transmembrane) proteins: channel & carrier proteins

  • selective; only transport certain molecules

• still driven by diffusion; doesn’t need energy input from cell

cells can regulate FD by…

1. regulating conc. of particular transport proteins in membrane

2. regulating activity of particular transport proteins

channel proteins in FD

• like tunnels (don’t touch/bind to molecules they transport)

  • channels can be 1. always open or 2. gated (opened or closed)

• direction of mvmt depends on conc.

  • mvmt doesn’t need energy input from cell

• selectiveness regulated by shape/design of protein

carrier proteins in FD

• must touch/bind to molecules they transport

• direction of mvmt depends on conc.

  • mvmt doesn’t need energy input from cell

• selectiveness regulated by binding sites of protein

3 basic types of carrier proteins (CP)

1. uniporters: CPs that only transport one type of molecule

2. symporters: CPs that transport two types of molecules, same direction at same time

3. antiporters: CPs that transport two types of molecules, opposite direction at same time

channel and carrier protein kinetics (rate of transport)

• channel proteins: linear kinetics

  • as solute conc. inc, rate constantly inc

• carrier proteins: saturation kinetics

  • slow inc, then rapidly inc, then levels off (S-shape)

active transport (anti-diffusion)

• used by cells to build up conc. gradient of a molecule across a plasma membrane

  • bc conc. gradients are critical for some biological processes

• moving molecules against conc. gradient needs 1. carrier proteins and 2. energy input from cell

membrane transport summary

transport across plasma membranes

• passive transport

  • simple diffusion (no transport proteins or energy input)

  • facilitated diffusion (no energy input)

    • channels

    • carriers

• active transport (needs energy input)

  • carriers

photo-bleaching experiments

unlabeled cell

→ membrane proteins labeled w flourescent dye

→ laser beam bleaches an area of cell surface

importance of maintaining fluidity level

• membrane is too fluid: will not serve as a barrier; will fill apart

• too solid: transmembrane proteins can’t flex (change shape) and carry out functions

  • ex. transmembrane transport and signaling proteins

how can cells regulate membrane fluidity, and what are the two ways for this

changing fatty acid chains of phospholipids

1. cell can generate phospholipids w/ more/fewer unsaturations in FA chains

2. cell can generate phospholipids w/ longer/shorter FA chains

FA saturation and membrane fluidity: MORE UNSATURATED FA

• pack less tightly

• if membrane is too solid, cell will inc. conc. of phospholipids w/ more unsaturated FA

FA saturation and membrane fluidity: MORE SATURATED FA

• pack more tightly

• if membrane is too fluid, cell will inc. conc. of phospholipids w/ more saturated FA

cells can _ the percentage of phospholipids that have more _ or more _ FA chains

change; saturated; unsaturated

FA length and membrane fluidity: shorter FAs

• pack less tightly (less SA/less interaction)

• if membrane is too solid, cells will inc. conc. of phospholipids w/ short FA chains

FA length and membrane fluidity: longer FAs

• pack more tightly (more SA/more interaction)

• if membrane is too fluid, cells will inc. conc. of phospholipids w/ long FA chains

cells can _ the percentage of _ with _ or _ FA chains, and when do these modifications happen

• change; phospholipids; long; short

• both at same time

if cells are exposed to lower temps, what happens to PM and how can the cell compensate?

• less fluid/ too soft

• make phospholipids w/ FA chains that are 1. shorter or 2. more unsaturated

if cells are exposed to higher temps, what happens to PM and how can the cell compensate?

• more fluid/ not soft enough

• make phospholipids w/ FA chains that are 1. longer or 2. more saturated

cholesterol and membrane fluidity

• cholesterol: ampipathic lipid w/ hydrophilic head group and hydrophobic ring structure and HC tail

  • present in plasma membranes of eukaryotic cells

  • helps regulate fluidity of plasma membranes

how can cholesterol help maintain solidity

steroid ring structure of cholesterol interacts with / partially immobilizes phospholipid FA chains

how can cholesterol help maintain fluidity

prevents phospholipid FA chains from packing together too tightly

hydrophobic interior

major barrier for molecules crossing a PM

• PM is permeable to nonpolar molecules

• PM is less permeable to small polar molecules

• PM is not permeable to large polar molecules or ions