Homeostasis and Cell Transport Chapter 5

HSCE: B2.5h Explain the role of cell membrane as a highly selective barrier (diffusion, osmosis, and active transport)

Passive transport systems Diffusion-Movement of molecules from an area of higher concentration to an area of lower concentration facilitated Osmosis-Movement of WATER molecules across a cell membrane from an area of higher concentration to an area of low concentration. Concentration Gradient-the difference in the concentration of molecules across a distance. Substances naturally travel down their concentration gradient (high to low concentration).

Diffusion In the absence of other influences, diffusion will eventually cause the molecules to be in EQUILIBRIUM Cell membranes allow certain molecules to pass through, but not others. Depends on the size and type of molecule. SIMPLE DIFFUSION- diffusion across a membrane

Facilitated Diffusion Some molecules are too large or are not soluble in lipids (can’t pass the lipid bilayer). They need the help of CARRIER PROTEINS. Does not require any extra energy to transport the molecules because they are still traveling from higher to lower concentration (passive transport). molecule Cell membrane (lipid bilayer) carrier protein

Facilitated Transport Another type of passive transport carries ions across the membrane though proteins called ION CHANNELS. Ions: sodium (Na+), potassium (K+), calcium (Ca2+), chlorine (Cl-) Each type of ion channel is usually specific to a certain ion. Some channels are always open, others have “gates”. IONS Ion channel

Osmosis Passive transport In a solution, a solute (the substance being dissolved) is dissolved in a solvent (substance doing the dissolving). Example: SUGAR (solute) dissolves in WATER (solvent) In cells the solutes are organic molecules and the solvent is water. OSMOSIS: the process by which water molecules diffuse across a cell membrane from an area of higher to lower concentration.

Direction of Water movement across the membrane OSMOSIS Volume increases Volume decreases Direction of Water movement across the membrane

Hyper-, hypo-, & Isotonic The pressure exerted by water inside the cell wall is called TURGOR PRESSURE When turgor pressure is gained and the cell bursts: CYTOLYSIS When turgor pressure is lost the cell shrinks away from the cell wall and the plant wilts: PLASMOLYSIS Plasmolysis External solution is hypotonic to cytosol: water moves in…cell can burst External solution is isotonic to cytosol: No net movement External solution is hypertonic to cytosol: water moves out…cell shrinks

Active Transport systems Active transport moves substances against the concentration gradient (from lower concentration to higher). This requires energy most often supplied by ATP Pumps Sodium-potassium pump Movement in vesicles Move macromolecules and nutrients that are too large to pass the cell membrane or “packs” of small molecules at the same time. Two types Endocytosis Exocytosis

Sodium-Potassium Pump Step 1-3: 3 Na+ bind to protein from inside the cell, a phosphate is removed from ATP binding to the carrier protein and turning ATP into ADP. The protein changes shape releasing the 3 Na+ to the outside. Steps 4-6: The protein then picks up 2 K+ from the inside, the phosphate is removed, changing the shape so that it releases the 2 K+ into the cell.

Sodium-potassium pump Pumps 3 Sodium ions out of the cell Pumps 2 Potassium ions into the cell This difference in charge is important for the conduction of electrical impulses along nerve cells.

Endocytosis Cells ingest (take in) external fluid, macromolecules, and large particles. External materials are enclosed in a “pouch” that pinches off from the cell and becomes a membrane-bound vesicle. Two main types Pinocytosis- transports fluids or solutes Phagocytosis- transports large particles or whole cells.

Phagocytes Many unicellular organisms feed by the process of phagocytosis. Certain cells (Phagocytes) in animals use phagocytosis to ingest bacteria and viruses that invade the body.

Exocytosis Process by which a substrate is released from the cell through a vesicle.

Homeostasis Cell transport Carrier Protein Is aided by which includes Such as Osmosis Diffusion Facilitated diffusion Is aided by Cell transport which includes Passive transport Of substance down their Active transport Of substances against their Endocytosis Exocytosis Sodium-potassium pump Concentration gradient Which use a Carrier Protein