Cell Transport All cells must maintain HOMEOSTASIS balance

Cell Transport All cells must maintain HOMEOSTASIS (balance). What types of substances must be balanced in a cell?

First Step: Cell Membrane • Cell membrane is semi- or selectively permeable – not all molecules can pass through.

Concentration gradient • The difference in concentration of a given molecule between two points is called the concentration gradient. • The larger the gradient, the greater the net movement of the molecules. • Molecules continue to move until evenly distributed (gradient = 0).

Diffusion • Diffusion occurs whenever molecules move from areas of high concentration to low concentration. • The movement is caused by the random kinetic energy of the molecules, so does not require any energy input.

Passive Transport • Does not require energy. • Molecules move from high concentration to low concentration. • Three major types: – Simple Diffusion – Osmosis – Facilitated Diffusion

Simple Diffusion across a Membrane • Molecules pass directly through lipid bilayer. • Molecules generally small with little or no electrical charge (O 2, CO 2, H 2 O, etc. ).

Facilitated Diffusion • Molecules move through special proteins. • Move from high concentration to low. • Does not require energy.

Osmosis • Special type of diffusion. • Involves water moving across a selectively permeable membrane. • Very important in living organisms.

Tonocity • During osmosis, water moves from high concentration to low concentration. • Water is called a solvent, molecules dissolved in water are called solutes. • There are three terms to describe the direction in which water molecules will move during osmosis: – Hypotonic – Isotonic – Hypertonic

Hyper- and Hypotonic • Hypo = less solute • Hyper = more solute • These are relative terms like “greater than” or “less than” Water moleclue Solute molecule

Effects of Tonicity on Cells

Active Transport • Moves molecules against their gradient (from low to high concentration). • This requires energy. • Two major types: – Carrier Proteins – Endocytosis

Carrier Proteins • Special membrane proteins that use energy to move molecules across the membrane.

Endocytosis • Process by which large molecules or large volumes of liquid are taken into cells. • Endo- means inside, cytosis refers to cell • Phagocytosis = “cell eating” • Pinocytosis = “cell drinking” • Requires energy.

Phagocytosis • Cell membrane folds around and brings particle into cell. • The membrane around the particle pinches off and forms a vesicle. • The vesicle will eventually fuse with a lysosome for digestion.

Pinocytosis • Cell membrane folds around and brings liquid into cell. • The membrane around the liquid pinches off and forms a vesicle.

Exocytosis • The opposite of endocytosis is called exocytosis (exo = outside). • This is what happens to the packages produced by the Golgi bodies that leave the cell. Cell Membrane