Transport Across a Cell Membrane

• The makeup of a cell membrane effects its permeability

• Three factors determine whether or not a substance is easily able to cross a cell membrane:

• 1. Polarity

• 2. Charge

• 3. Size

Passive Transport• Passive transport involves the

movement of material across the cell membrane without the input of energy on the part of the cell

• There are three basic forms of passive transport:

• 1. Simple diffusion

• 2. Facilitated diffusion

• Osmosis

Simple Diffusion

• Diffusion is the random movement of particles from areas of high concentration to areas of low concentration

• Simple diffusion of material across a cell membrane occurs only with substances that can move easily across a cell membrane

Facilitated Diffusion

• For molecules that, due to certain characteristics, are not able to simply diffuse across a cell membrane the movement may be aided (facilitated) by carrier proteins that provide an environment acceptable to the movement of these molecule along a concentration gradient

Facilitated Diffusion• Carrier proteins never provide an open

channel across the cell membrane

Passive Transport

Osmosis

• Osmosis is the diffusion of water across a selectively permeable membrane in response to differences in solute concentration between the ECF and the protoplasm

• Water will diffuse from the area of higher water concentration to the area of lower water concentration

Equilibrium

• Once the contraction of water become equal on both sides of the membrane the system has reached equilibrium

• Water molecules will continue to move between the two sides but there will be no net movement of water at equilibrium

Isotonic Solutions

• These are solutions in which the solutes concentration outside the cell is equal to that inside the cell

• The major function of blood is to keep your internal environment in an isotonic balance called homeostasis (the maintenance of a constant environment despite internal and external changes)

Hypotonic Solutions

• A hypotonic solution has a lower solute concentration (higher water concentration) than inside a cell

• This results in a net movement of water into the cell

• If the hypotonic solution is maintained it will result in excess water movement into a cell causing it to explode (hemolysis/turgor)

Hypertonic Solutions

• A hypertonic solution has a higher solute concentration (lower water concentration) and inside a cell

• This results in a net movement of water out of the cell

• If the hypertonic solution is maintained it will result in excess movement of water out of the cell causing it to shrink (crenelation/plasmolysis)

• http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm

Active Transport

• Active transport is the movement of materials across a cell membrane against their concentration gradient with the expenditure of energy in the form of ATP

• There a three main types of active transport:

• 1. Ion pumps• 2. Co-transport• 3. Endocytosis

Ion Pumps• Primary active transport involves using

energy (through ATP hydrolysis) at the membrane protein

• This causes a change in the shape of the protein that results in the transport of the molecule through the protein.

• AN example of this is the Na+-K+ pump. • The Na+-K+ pump is an antiport, it

transports K+ into the cell and Na+ out of the cell at the same time, with the expenditure of ATP

Ion Pump• Ion pump moves sulphur ions across

membrane

Sodium Potassium Pump

Co-Transport• Other transporters use the energy

already stored in the gradient of a directly-pumped ion

• First direct active transport of the ion establishes a concentration gradient.

• Then this concentration gradient uses facilitated diffusion to pass the ion back into the cell

• The ion’ passage through a integral protein pumps some other molecule or ion along with it against its gradient

Co-Transport

• http://www.northland.cc.mn.us/biology/Biology1111/animations/transport1.html

Endocytosis

• Endocytosis is the process by which cells take in very large molecules

• There are two types of endocytosis:

1. Pinocytosis, where cells take up dissolved molecules by engulfing small amounts of the external solution

(Cells of the small intestine take up fat droplets via pinocytosis.)

2. Phagocytosis, the process by which cells engulf solid particles from the external environment.

Exocytosis• Exocytosis is the process by which

large molecules held within the cell are transported to the external environment (waste, proteins and other products of cellular functions)

• Small vesicles break off from the Golgi apparatus and move toward the cell membrane.

• The vesicles fuse with the cell membrane and the material is released into the external environment

Exocytosis