exercise 5- the cell: transport mechanisms and cell permeability

selective, or differential, permeability

valuable cell proteins and other substances are kept within the cell, and excreta or wastes pass to the exterior

passive processes

concentration or pressure differences drive the movement

active processes

the cell provides energy (ATP) to power the transport process

kinetic energy

Molecules possess kinetic energy and are in constant motion.

concentration gradient

is present when molecules are unevenly distributed, resulting in an area of higher concentration and an area of lower concentration.

Diffusion

is the movement of molecules from a region of their higher concentration to a region of their lower concentration.

simple diffusion

The unassisted diffusion of solutes (dissolved substances) through a selectively permeable membrane

facilitated diffusion

The substances move by a passive transport

process

osmosis

The flow of water across a selectively permeable membrane

isotonic solution

Cells retain their normal size and

shape in isotonic solutions (same

solute/water concentration as inside

cells; no net osmosis).

hypertonic

Cells lose water by osmosis and

shrink in a hypertonic solution

(contains a higher concentration

of nonpenetrating solutes than

are present inside the cells).

hypotonic solution

Cells take on water by osmosis

until they become bloated and burst

(lyse) in a hypotonic solution

(contains a lower concentration

of nonpenetrating solutes

than are present in cells).

active transport

requires carrier proteins that combine specifically with the transported substance. Active transport may be primary, driven directly by hydrolysis of ATP, or secondary, driven indirectly by energy stored in ionic gradients.

vesicular transport

fluids containing large particles and macromolecules are transported across cellular membranes inside membranous sacs called vesicles.

endocytosis

vesicular transport moves substances into the cell

exocytosis

vesicular transport moves substances out the cell

three types of endocytosis

phagocytosis, pinocytosis, and receptor-mediated endocytosis

phagocytosis

(“cell eating”), the cell engulfs some relatively large or solid material such as a clump of bacteria, cell debris, or inanimate particles

pinocytosis

(“cell drinking”), also called fluid-phase endocytosis, the cell “gulps” a drop of extracellular fluid containing dissolved molecules

receptor-mediated endocytosis

The main mechanism for specific endocytosis of most macromolecules is receptor-mediated endocytosis.

The receptors for this process are plasma membrane proteins that bind only certain substances.

This exquisitely selective mechanism allows cells to concentrate material that is present only in small amounts in the extracellular fluid.

secretory vesicle

The substance to be removed from the cell is first enclosed in a protein-coated vesicle