Cell Physiology

Membrane Transport

• The cell membrane is a differentially permeable barrier

• Substances move through the membrane in two ways:– Passively (without any energy use on the part

of the cell)– Actively (with the cell providing the energy for

the transport)

Passive Processes, 1• Simple diffusion: the movement of molecules

from areas where they are in high concentration to areas where they are in low concentration

• Molecules diffuse down a concentration gradient• A solute is a dissolved particle• Molecules which diffuse through the membrane

are – Oxygen and carbon dioxide– Fat-soluble vitamins– Alcohol

Passive Processes, 2• Osmosis is a special case of diffusion: it is

the diffusion of water through a selectively permeable membrane (water is able to diffuse through the lipid bilayer along its gradient)

Hemodialysis

• Used to treat patients with kidney failure• Process of removing selected substances,

such as urea and various ions, from a patient’s blood by pumping the blood through a membrane

• It is based on the principle of diffusion through a semipermeable membrane

Passive Processes, 3• Facilitated diffusion: passive transport involving

a carrier or a channel– Carriers are transmembrane proteins that are specific

for the molecules they transport. The molecule binds to the carrier causing it to change shape, and then the molecule is transported down its concentration gradient. Molecules that utilize facilitated diffusion include glucose, amino acids and ions

– Channels are tunnels through transmembrane proteins that allow substances to move through, down their concentration gradients. Channels are specific for the substances they transport. They do not change shape, but they do have gates which can open and close to control passage.

Tonicity • The ability of a solution to change the shape of

cells by altering their internal water volume. This is due to the effects of osmosis on cells.

• Isotonic solutions have the same concentration of non-penetrating solutes as that of cells (0.9% saline)

• Hypotonic solutions have lower concentrations of solutes than those of cells, e.g. D5W (= 5% dextrose in water)

• Hypertonic solutions have higher concentrations of solutes than those of cells, e.g. 2% NaCl

Normal red blood cells

Red blood cells placed in a hypertonic solution undergo crenation.

Which are

the

crenated c

ells?

Active Processes, 1

• Active transport: the movement of solutes into and out of cells against their concentration gradients; requires membrane transporters that are proteins which are specific for the transported substances.

• The sodium-potassium pump is an example of active transport—the pump works to maintain potassium concentrations 30-50 times higher inside cells than outside, while moving sodium out of the cell and keeping the concentration of Na low in the cell and high outside the cell.

Na – K Pump

• Pumps 3 Na+ ions out of the cell for every 2 K+ ions that are brought into the cell

• Pump is powered by ATP• Pump is located in the plasma membrane

of all human cells

Active Processes, 2• Muscle cells have calcium pumps• Vesicular transport: the transport of large

particles, macromolecules, and fluids.– exocytosis moves substances outside of the

cell– endocytosis moves substance into the cell– phagocytosis (cell-eating) is a special kind of

endocytosis used by macrophages in engulfing bacteria

Metabolism

• Is the sum total of all the chemical reactions occurring in the cell

• Reactions which break down substances are called catabolic reactions

• Reactions which synthesize substances are called anabolic reactions

• Metabolism = anabolism + catabolism

Catabolism• Cellular respiration is the process in which cells

process glucose, breaking it down into carbon dioxide and water. ATP is formed in the process.

• There are 3 parts: 1) glycolysis – it occurs in the cytoplasm + does not require oxygen. Glucose is broken down into pyruvic acid. 2) the citric acid cycle (also known as the Krebs cycle), 3) the electron transport system. #2 + #3 take place in the mitochondria and require the presence of oxygen (they are aerobic processes).

• In the absence of oxygen (under anaerobic conditions), #2 + #3 can’t occur, and pyruvic acid will be converted to lactic acid

Review• What are the major lipid types found in

plasma membranes?• Cells lose water when they are placed in

___________ solutions.• Name 3 types of RNA. What role does

each type play? • Name the organelle that is the major site

of ATP synthesis.• Name 2 types of organelles that contain

digestive enzymes?

Review, 2

• If a cell loses or ejects its nucleus, what is its fate and why?

• What is the role of the glycoproteins found on the surface of the cell?

• A chemotherapeutic drug, adriamycin, used to treat cancer, binds to DNA and blocks mRNA synthesis. Why is this fatal to cells?