D’YOUVILLE COLLEGE
BIOLOGY 102 - INTRODUCTORY BIOLOGY II
LECTURE 9
PLASMA MEMBRANE TRANSPORT
7. Cell Membrane (Plasma Membrane):
• models of membrane structure: early studies of membranes' chemical content led to the notion of a lipid bilayer composed of amphipathic phosphoglyceride molecules
(fig. 7 – 2 & ppt. 1)
- the protein content was accounted for by suggesting that proteins were arrayed upon the exterior & interior faces of the lipid bilayer (= Davson-Danielli
model) (ppt. 2)
- Davson-Danielli model, although popular for many years failed to account for specific properties of membrane proteins as well as failing to account for membrane permeability
• fluid mosaic model (figs. 7 – 3 & ppt. 3):
- first proposed by Singer & Nicholson, this model has been substantiated by experimental evidence (7 – 4 & ppt. 4)
- phosphoglyceride bilayer: fluid matrix of membrane has variable viscosity due to molecular mobility; cholesterol stabilizes membrane at various temperatures
(figs. 7 – 6, 7 - 8 & ppt. 5)
- proteins (peripheral & integral) are distributed throughout matrix like pieces of a mosaic; proteins can move within the fluid matrix (translational movement) & form
new arrangements & new associations; this may confer specific properties upon membrane functions (fig. 7 – 7 & ppt. 6)
- serve as receptors, channel liners, transport molecules, anchoring molecules, recognition molecules (with carbohydrates), transducers (fig. 7 – 10 & ppt. 7)

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- carbohydrates bind to lipids (glycolipids) or to proteins (glycoproteins); serve as recognition sites on membrane surface
- this model (with modifications) is the currently supported model for membranes (both the external membrane & organelle membranes) (fig. 7 – 5 & ppt. 8)
8. Membrane Permeability and Transport:
• selectively permeable: controls entrance to or exit from cell
• passive transport - diffusion (fig. 7 – 13 & ppt. 9):
- movement from high concentration to low concentration; concentration gradient across cell membrane provides energy, e.g. for nerve impulses
- lipid-soluble materials traverse lipid matrix
- small water-soluble materials (polar &/or ionic) are accommodated by
protein-lined membrane channels (pores) and diffuse in direction of chemical concentration
gradient &/or electrical gradient (electrochemical gradient)
• passive transport - osmosis (fig. 7 – 14 & ppt. 10):
- water diffuses from low solute concentration (high water concentration) to
high solute concentration (low water concentration)
- isotonic = same solute concentration as body fluids
- hypertonic = higher solute concentration than body fluids
- hypotonic = lower solute concentration than body fluids
- water distribution: imbalances across membranes cause potentially damaging
shifts of water (fig. 7 – 15 & ppt. 11)
• passive transport - carrier-mediated transport:
- membrane proteins (carriers) bind substances to facilitate their movement across the plasma membrane; may entail conformation change in protein; some channels may be gated (open in one conformation, closed in another)
- facilitated diffusion (fig. 7 – 17 & ppt. 12): substances “shuttled” across the membrane in accordance with concentration gradient
- saturation kinetics characterizes carrier-mediated transport (ppt. 13)

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• active transport: (“pumps”); require energy and “shuttle” materials against concentration gradient, e.g. sodium-potassium pump & hydrogen ion pump (figs. 7 –
18, 7 – 20 & ppts. 14 & 15)
- cotransport: some substances are transported jointly with others; secondary active transport provides the energy from a concentration gradient to drive a cotransport process (fig. 7 – 21 & ppt. 16)
• summary: most passive transport & all types of active transport is facilitated by membrane proteins (fig. 7 – 19 & ppt. 17)
• bulk transport - endocytosis (fig. 7 – 22 & ppt. 18): includes phagocytosis &
pinocytosis; review lysosomes function in cellular digestion; specific endocytosis involves binding of material to be taken up to specific receptors; invagination of membrane is promoted by proteins such as clathrin (coat protein)
• bulk transport - exocytosis: movement out of the cell facilitated by
endomembrane system (fig. 7 – 12 & ppt. 19)