Passive and Active Transport Biology I

Main Idea Cellular transport moves substances within the cell and moves substances into and out of the cell.

Diffusion Particles in gases, liquids, and solids move in random motion (called Brownian motion). This random motion causes diffusion, which is the net movement of particles from an area where there are many particles of the substance to an area where there are fewer particles of the substance.

Diffusion The amount of substance in a particular area is called concentration. Diffusion occurs from areas of high concentrations to areas of low concentration. There must be a concentration gradient, a difference between concentrations across a membrane, for cellular transport to occur. No energy is needed because the particles are already in motion.

Diffusion The particles will continue to move randomly but will not change the concentration difference. At the point where there is continuous movement, but no overall change, dynamic equilibrium is reached. Diffusion at Work

Diffusion All types of diffusion are passive transport mechanisms. Three main factors that affect the rate of diffusion: –Concentration –Temperature –Pressure

Facilitated Diffusion Facilitated diffusion uses transport proteins to move other ions and small molecules across the plasma membrane. Substances move into the cell through a water-filled channel or a carrier protein. Channel proteins open and close, whereas carrier proteins change shape. Requires no extra input of energy

Facilitated Diffusion

Osmosis Special type of diffusion involving water Water can pass freely into and out of the cell. Osmosis is the diffusion of water across a selectively permeable membrane. Very important in maintaining homeostasis!

Osmosis Water molecules will diffuse towards the area of low water concentration. 3 types of solutions possible: –Isotonic –Hypotonic –hypertonic

Osmosis When a cell is in a solution that has the same concentration of water and solutes (ex. Ions, sugars, proteins, etc.) as its cytoplasm, the cell is said to be in an isotonic solution. –Solutes are anything dissolved in water.

Osmosis If a cell is in a solution that has a lower concentration of solute, the cell is in a hypotonic solution. There is more water on the outside of the cell than the inside. Causes swelling as the vacuoles in the cell fill with water

Osmosis When a cell is placed in a hypertonic solution, the concentration of the solute outside the cell is higher than inside. Net movement of water is out of the cell. Vacuoles are emptied of water, causing cells to shrink.

Osmosis

Active Transport Sometimes substances must move from an area of low concentration to high concentration. The movement of substances across the plasma membrane against a concentration gradient is called active transport. Requires energy! Helps maintain homeostasis

Active Transport Occurs with the aid of carrier proteins, called pumps Active Transport Animation Endocytosis is a process by which a cell surrounds and takes in material from its environment. –Does not pass directly through the membrane –Engulfed –Two examples: phagocytosis (eating) and pinocytosis (drinking) Exocytosis is the expulsion or secretion of materials from a cell. –used to expel wastes and secrete hormones

Active Transport