The cell membrane selectively permits movement of substances across it. It is composed of a phospholipid bilayer with embedded proteins. Movement occurs through passive diffusion, osmosis, and facilitated diffusion which move substances down their concentration gradients. Active transport uses energy to move substances against their gradients, such as pumping ions using ATP-powered carrier proteins or endocytosing large molecules. Exocytosis releases secretions and wastes from the cell.

1. Cell Membrane
Movement Across MembranesMovement Across Membranes

2. General characteristics
extremely thin, flexible, and elastic
outpouchings and infoldings for extra
surface area
function -
Selectively permeable -

3. Membrane Structure
composition:
Phospholipid bilayer
hydrophilic
hydrophobic
fluid mosaic model
Cholesterol
Protein
Carbohydrates

4. Functions
physical isolation
Metabolic isolation
Sensitivity
Structural support

5. Permeability
nonpolar moleculesnonpolar molecules
lipidslipids
COCO22 and Oand O22
membrane spanning proteinsmembrane spanning proteins
channelschannels
gatesgates
carrierscarriers

6. Passive Transport
Diffusion - lipid soluble onlyDiffusion - lipid soluble only
greater to lesser - “downhill”greater to lesser - “downhill”
Osmosis - water onlyOsmosis - water only
hypotonichypotonic
hypertonichypertonic
isotonicisotonic
Facilitated Diffusion - ex. glucoseFacilitated Diffusion - ex. glucose
uses carrier proteinsuses carrier proteins

7. Passive cont.
FiltrationFiltration
separate by sizeseparate by size
requires some force but no cellular energyrequires some force but no cellular energy
occurs in capillariesoccurs in capillaries
DialysisDialysis
separate by size and concentrationseparate by size and concentration
done with tubing in hospitals, mimics kidneydone with tubing in hospitals, mimics kidney
functionfunction

8. Active Transport
““Pump” - ionsPump” - ions
carrier proteins change shape with the additioncarrier proteins change shape with the addition
of ATPof ATP
can actively concentrate ions on one side of acan actively concentrate ions on one side of a
membranemembrane
Endocytosis - large moleculesEndocytosis - large molecules
phagocytosisphagocytosis
pinocytosispinocytosis

9. Exocytosis - large moleculesExocytosis - large molecules
wasteswastes
secretionssecretions

10. Exocytosis - large moleculesExocytosis - large molecules
wasteswastes
secretionssecretions