Chapter 5 A Closer Look At Plasma Membranes Honors Biology

HOMEOSTASIS……….. It Isn’t Easy Being Single? (why not) Concentrations of ions & other substances outside the cell may rapidly become too high or low. A mechanism is needed to selectively permit substances to enter or leave the cell. What is homeostasis? Homeostasis is the name given to the dynamic processes that enable optimum conditions to be maintained for constituent cells, in spite of continual changes taking place both internally and externally. What mechanism is needed to selectively permit substances to enter or leave the cell? Phospholipid bi-layer with embedded proteins. How does a cell maintain homeostasis? By controlling what enters and leaves the cell. What are 2 examples of homeostasis with the human body? It Isn’t Easy Being Single: (Concentrations on ions and other substances outside the cell may rapidly become too high or low; a mechanism is needed to selectively permit substances to enter or leave the cell. Cell involved in constant chemical activity. Take in: oxygen and nutrients to maintain metabolism (aerobic respiration) Release: Waste take could to toxic to the cell. (carbon dioxide, sodium, enzymes, end products of digestion)

Membrane Structure & Function Phospholipid molecules form a bilayer. Phosphate/glycerol head is polar & hydrophilic, two fatty acid tails are nonpolar & hydrophobic. phospholipid molecule Note: Do hydrophilic substances have an easier time passing thru the membrane? NO lipid bilayer water Cholesterol (right) is another lipid molecule that’s commonly found in the plasma membrane. Question: What is the arrangement of the phospholipids? H – T - T - H

What is the ‘Fluid Mosaic Model’ of the plasma membrane structure? The lipid molecules are free to move and protein molecules float independently in the lipid bi-layer. open channel proteins transport protein gated channel proteins lipid bilayer

oligosaccharide groups A wide variety of protein molecules float around in the lipid bi-layer and perform most of the functions of the membrane. oligosaccharide groups cholesterol phospholipid EXTRACELLULAR ENVIRONMENT cytoskeletal proteins (beneath the plasma membrane) RECEPTOR PROTEIN RECOGNITION PROTEIN ADHESION PROTEIN open channel protein gated channel protein (open) gated channel protein (closed) active transport protein LIPID BILAYER area of enlargement TRANSPORT PROTEINS

7 Types of Membrane Proteins (Note: Most of the plasma membrane functions are carried out by these proteins) 1. Cytoskeletal Proteins - composed of microtubules. Needed for internal support and attachment. 2. Adhesion Proteins - composed of glycoproteins attached to oligosaccharides. Serve as the “glue” to hold cells together. 3. Open-Channel Proteins- needed for passive transport and facilitated diffusion. 4. Gated Channel Proteins - help control the directional flow or transport of ions across the membrane.

7 types of membrane proteins..cont. 5. Carrier Proteins - carry on active transport requiring energy (ATP) to “actively pump” their cargo across the membrane. 6. Receptor proteins - these proteins have binding sites for hormones and enzymes, which allow them to do their work . 7.Recognition Proteins -these proteins are the “fingerprints” of the cells. These self-recognizing proteins identify their own body cells or recognize invaders.

What will happen in the example above? What causes the reaction? What energy creates the reaction below? When will the reaction stop?

Concentration Gradients Molecules are in constant motion, due to their own kinetic energy. If there are more molecules of a substance in one area than another, a concentration gradient exists. Random collisions of molecules cause them to move from an area where they’re highly concentrated to an area of low concentration. Once the molecules have arranged themselves equally throughout two adjoining regions, it’s called “dynamic equilibrium.” At this point, a concentration gradient no longer exists.

Molecular size – smaller molecules move faster than larger ones. Diffusion The net movement of like molecules down their concentration gradient is known as diffusion. The rate at which diffusion occurs can be affected by the following factors: Molecular size – smaller molecules move faster than larger ones. Temperature – heat energy causes molecules to move more rapidly & to collide more frequently. Electrical gradients – the negative side of a membrane will attract positive ions & repel negative ions. Pressure gradients – applying pressure can speed up the rate at which molecules move.

selectively permeable membrane between two compartments Osmosis selectively permeable membrane between two compartments The movement of water across a selectively permeable membrane in response to concentration gradients, fluid pressure, or both is known as osmosis. water molecules protein molecules

Osmosis in a Plant Cell

Tonicity The relative concentrations of solutes in two fluids is known as tonicity. Water tends to move to areas where solute concentrations are greater. When solute concentrations in two fluids are equal, we say the two fluids are isotonic. In an isotonic solution, there is no net movement of water in either direction.

Water moves from a hypotonic solution to a hypertonic solution. compartment 1 compartment 2 HYPOTONIC SOLUTION HYPERTONIC SOLUTION membrane permeable to water but not to solutes fluid volume increases In compartment 2

Tonicity 2M sucrose solution A hypotonic solution has fewer solutes dissolved in it than an adjoining solution does. A hypertonic solution has more solutes dissolved in it than an adjoining solution does. As a result, water molecules will move from a hypotonic solution to a hypertonic solution. HYPOTONIC CONDITIONS HYPERTONIC CONDITIONS ISOTONIC CONDITIONS 1 liter of distilled water 10M sucrose solution 2M sucrose solution

Plasma membranes are selectively permeable. (This means that some substances are able to enter the cell and others are not.) O2, CO2, other small nonpolar molecules, as well as H2O C6H12O6, other large, polar water-soluble molecules, ions (such as H+, Na+, K+, Ca++, CI-) along with H2O X

Passive Transport vs. Active Transport concentration gradient Passive Transport vs. Active Transport high low DIFFUSION ACROSS LIPID BILAYERS lipid-soluble substances as well as water diffuse across PASSIVE TRANSPORT Water-soluble substances, and water, diffuse through interior of transport proteins. No energy boost required. Also called facilitated diffusion ACTIVE TRANSPORT Specific solutes are pumped through interior of transport proteins. Requires energy boost

SPECIALIZED TYPE OF VACUOLE: CONTRACTILE VACUOLES: PROTISTS HAVE THESE ORGANELLES - THEY COLLECT EXCESS WATER AND EXCRETE IT SO THE CELL DOESN’T BURST. (cytolysize) contractile vacuole (emptied) contractile vacuole (filled)

PLASMOLYSIS: LOSS OF WATER FROM A CELL RESULTING IN A DROP IN TURGOR PRESSURE. Turgor pressure: Water pressure inside the plant cell. (turgidity) PASSIVE TRANSPORT: REQUIRES NO ENERGY TO MOVE SUBSTANCES ACROSS A MEMBRANE = DIFFUSION , EXAMPLE: WATER, NON-POLAR MOLECULES, AND LIPID-SOLUBLE SUBSTANCES. LIPID BILAYER KEEPS MOST SUBSTANCES OUT BECAUSE THEY’RE REPELLED OR TOO LARGE.

FACILITATED DIFFUSION: THIS SPEEDS UP THE DIFFUSION RATE! PROTEINS PROVIDE CONVENIENT OPENINGS FOR PARTICLES TO PASS THROUGH - SUGARS AND AMINO ACIDS ARE MOVED THIS WAY... THIS SPEEDS UP THE DIFFUSION RATE!

Facilitated Diffusion

ACTIVE TRANSPORT (Requires energy) FROM LOW CONC. TO HIGHER CONC. In active transport, SOLUTES MOVE AGAINST THE CONCENTRATION GRADIENT! Ex- Sodium-Potassium Pump-helps nerve cells transfer electrical impulses by maintaining an electrical gradient on the plasma membrane. Ex- Calcium Pump - helps keep the calcium concentration high in cells.

ENDOCYTOSIS: a form of active transport in which the cell uses energy to bring materials into the cell by engulfing. A. PHAGOCYTOSIS: (“CELL EATING”) Cell takes in large molecules by pinching in the plasma membrane. B. PINOCYTOSIS: (“CELL DRINKING”) Cell takes in fluid by pinching in the plasma membrane.

Exocytosis

Active Transport EXOCYTOSIS: a form of active transport in which the cell excretes wastes or secretes substances needed elsewhere in the organism. These substances do not move through the plasma membrane itself. Slide 19 EXOCYTOSIS Vesicle in cytoplasm moves to plasma membrane, fuses with it; contents released to the outside