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Ch. 3 Notes: Membrane Physiology
Cellular Physiology: Membrane Transport
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Membrane Transport – movement of substance into and out of the cell

Transport is by two basic methods
o
Passive transport --no energy is required
o
Active transport -- the cell must provide metabolic energy
Solutions & Transport

Solution – homogeneous mixture of two or more components
o
Solvent – dissolving medium
o
Solutes – components in smaller quantities within a solution

Intracellular fluid – nucleoplasm and cytosol

Interstitial fluid – fluid on the exterior of the cell
Selective Permeability

The plasma membrane allows
some materials to pass while
excluding others

This permeability includes
movement into and out of the cell
Passive Transport Processes

Diffusion -- particles tend to distribute
themselves evenly within a solution
o
Movement is from high
concentration to low concentration, or down a concentration gradient

Types of diffusion:
o
Simple diffusion – unassisted processes

Solutes are lipid-soluble materials or small enough to
pass through membrane pores
o
Osmosis – simple diffusion of water

o
Highly polar water easily crosses the plasma membrane
Facilitated diffusion -- substances require a protein carrier for
passive transport
o
Filtration -- Water and solutes are forced through a membrane
by fluid, or hydrostatic pressure

A pressure gradient must exist

Solute-containing fluid is pushed from a high
pressure area to a lower pressure area
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Active Transport Processes


Transport substances that are unable to pass by diffusion
o
They may be too large
o
They may not be able to dissolve in the fat core of the membrane
o
They may have to move against a concentration gradient
Two common forms of active transport
o
Solute pumping

Amino acids, some sugars and ions are
transported by solute pumps

ATP energizes protein carriers, and in most
cases, moves substances against
concentration gradients
o
Bulk transport


Exocytosis

Moves materials out of the cell

Material is carried in a membranous vesicle

Vesicle migrates to plasma membrane

Vesicle combines with plasma membrane

Material is emptied to the outside
Endocytosis

Extracellular substances are engulfed by being enclosed in a
membranous vescicle

Types of endocytosis
o
Phagocytosis – cell eating
o
Pinocytosis – cell drinking
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