1.5 Cell Physiology

What determines the mode of transport?

size

polarity

concentration on either side of membrane

Which molecules can pass directly across phospholipids unaided?

Fat soluble molecules (hormones, glycerol)

Very small molecules (water, oxygen)

Diffusion

Net movement of substance from a higher concentration to a lower concentration

Passive

Simple Diffusion

Possible for very small molecules

Facilitated Diffusion

Requires transmembrane proteins - either channel proteins or carrier proteins

Down concentration gradient

Passive

Channel Proteins

Ions are charged and can’t pass between the phospholipid molecules

Channel proteins have a central pore which enables ions to pass through

Some are permanently open - non-selective

Some are gated - selective

Carrier Proteins

**Large Polar molecules** depend on protein carriers

They take in the diffusing molecule, **change shape,** and release the molecule on the other side of the membrane

Protein Carriers have specific receptor sites which are **complementary** to the molecule being carried - **selective**

Active Transport

Transports substances against the concentration gradient

Requires ATP

involves the use of Specific Protein carriers - confers selectively

Carriers change of shape to move the substance across the membrane

Cytosis

Allows molecules to enter or leave the cell without having to pass through the membrane itself

Important in molecules too big for carriers, bulk transport eg water

Endocytosis

Cell surface membrane invaginates around substances entering the cell

Forms vesicle (membrane bound sac)

Vesicle pinches off on the inside of the membrane

Phagocytosis

transport of solid material into cell

Pinocytosis

transporting fluid into cell

Exocytosis

Secretory vesicles move to cell surface membrane and fuse with it

Contents of vesicle are released outside the cell

Secretion of proteins