HOMEOSTASIS and the PLASMA MEMBRANE Carroll

Objectives Explain the function of the plasma membrane. Relate the function of the plasma membrane to the fluid mosaic model.

Homeostasis Key – BALANCE! Homeostasis  The equilibrium of an organism’s internal environment that maintains conditions suitable for life.  Example: Human sweating Helps body maintain its proper temperature. Organisms must be able to adjust to changes in their external environment.

Cells and Homeostasis How do cells maintain a balance?  Control what materials enter and leave cell. PLASMA MEMBRANE

Plasma Membrane – Properties The plasma membrane is selectively permeable (semipermeable) barrier around a cell.  Allows some materials to pass.  Water, oxygen, carbon dioxide  Prevents others from passing.  Proteins, carbohydrates Different cells may be permeable to different chemicals or substances.

Plasma Membrane – Functions 1. Support 2. Barrier 3. Protection 4. Transport  Export of wastes and cell products  Import of nutrients Cell Membrane also has methods of Molecular recognition  identify of self vs. nonself

Plasma Membrane – Structure 1.Phospholipid BILAYER (2 layers) 2.Each layer made of lipid molecules 3.Protein molecules embedded in bilayer

Phospholipid Bilayer Lipid Tail  Fats + Oils (fatty acid)  Non-polar (no charges on molecules)  Insoluble in water  Hydrophobic - Not attracted to water Phosphate Head  Phosphate group  Polar (partial charges on molecules)  Water-soluble  Hydro phillic -Attracted to water POLAR HEAD FATTY ACIDS Phosphate Group Glycerol Backbone Water-Soluble Water-Insoluble

QUESTION: Most cells have a watery internal and external environment. Given this information, how might you expect the phospholipids to be arranged? OR OUTSIDE INSIDE OUTSIDE

Arrangement of phospholipid bilayer

Do you “love” or “fear” water? Hydrophilic heads Hydrophobic tails Water Loving Fearing

Phospholipid Bilayer

Fluid Mosaic Model Phospholipid molecules may move sideways through their layer.  “FLUID”

Structure of Membrane Cholesterol – Helps keep fatty acids tails from sticking together Stabilizes membrane

Proteins found in Membrane Most of the functions of the plasma membrane are carried out by proteins. More than 50 found in RBC’s membrane Functions: 1. Determine which particles can pass through membrane (transport proteins) 2. Serve as enzymes (accelerate chemical reactions) 3. Act as markers  identity  Help fight disease

Proteins in Membrane Plasma Membrane Integral Proteins Embedded in membrane Pores to allow nutrients in or out of cell Known as Channel Proteins Peripheral Proteins Not embedded Found on the cytoplasm side of thecell membrane Anchor points for cytoskeleton Glycoprotein Protein with carbohydrate (chain of sugar molecules) Gives cell identity of self or nonself