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Plasma Membrane Function Maintains balance by controlling what enters and exits the cell What characteristic of life is this? HOMEOSTASIS Membrane is selectively permeable only allows certain molecules through Plasma Membrane Structure Phospholipid Bilayer (2 layers of lipids) Polar heads face out because they are hydrophilic “water loving” Nonpolar tails face in because they are hydrophobic Fluid Mosaic Model Plasma Membrane Structure Fluid Properties: Unsaturated fatty acid tails Cholesterol in animal membranes Plasma Membrane Structure Mosaic Properties Proteins in the lipid bilayer Integral proteins are inside the membrane Peripheral proteins are only found on one side of the membrane Integral Protein Examples Channel – allow materials to pass through (tunnel) Carrier – transport materials across Receptor – transmit information Marker – identify the cell to other cells Diffusion of Liquids 9 Diffusion Solute particles move from HIGH concentration to LOW concentration This always occurs when there is a concentration gradient It will continue until dynamic equilibrium is reached Diffusion Demonstration 1. What solution is being put into the bag? What color is it? starch & water; cloudy white 2. What solution is in the beaker? What color is it? iodine & water; amber 3. Predict what will happen to the bag and/or beaker color(s). Diffusion Demonstration Results 1. What color is the bag? 2. What color is the beaker? 3. Explain what happened. Iodine molecules were small enough to fit across the membrane. Starch molecules were too large. The solution turned blue when starch & iodine mixed. Diffusion review Movement of particles from HIGH concentration to LOW concentration This is caused by a concentration gradient It will stop when dynamic equilibrium is reached Osmosis Diffusion of water molecules across a selectively permeable membrane Occurs when the solute particles (dots) cannot pass through the membrane Osmotic Solutions Hypotonic = lower concentration (less dots) Hypertonic = higher concentration (more dots) What controls osmosis? Unequal distribution of particles, called a concentration gradient, is one factor that controls osmosis. Before Osmosis Selectively permeable membrane After Osmosis Water molecule Sugar molecule Cells in an isotonic solution water molecules move H2O H2O Water Molecule Dissolved Molecule into and out of the cell at the same rate, and cells retain their normal shape. Cells in an isotonic solution water molecules move H2O H2O Water Molecule Dissolved Molecule into and out of the cell at the same rate, and cells retain their normal shape. Cells in an isotonic solution A plant cell has its normal shape and pressure in an isotonic solution. Cells in a hypotonic solution hypotonic solution: dilute solution thus low solute concentration In a hypotonic solution, water enters a cell by osmosis, causing the cell to swell. H2O H2O Water Molecule Dissolved Molecule Cells in a hypotonic solution Plant cells swell beyond their normal size as pressure increases. (plants prefer this –it makes the leaves firm) Turgor Pressure – water pushing the plasma membrane against plant cell wall Cells in a hypertonic solution hypertonic solution: concentrated solution, thus a high solute concentration In a hypertonic solution, water leaves a cell by osmosis, causing the cell to shrink H2O H2O Water Molecule Dissolved Molecule Cells in a hypertonic solution Plant cells lose pressure as the plasma membrane shrinks away from the cell wall. PLASMOLYSIS – shrinking of cytoplasm away from cell wall due to outward water flow Osmotic Solutions Isotonic = equal concentration Direction of Water Hypotonic solutions = water moves INTO the cell Hypertonic solutions = water moves OUT of the cell Isotonic solutions = NO net movement of water Effects on different cells Animal Cell Solution Plant Cell burst Hypotonic Turgor pressure shrivel Hypertonic plasmolysis normal Isotonic limp Passive Transport Movement of particles from HIGH concentration to LOW concentration Requires NO energy 3 types: 1. Diffusion 2. Osmosis 3. Facilitated diffusion Facilitated Diffusion Diffusion of particles across a membrane using transport proteins Uses both channel and carrier proteins Active Transport Movement of particles from LOW concentration to HIGH concentration Requires cell energy 3 types: 1. Pumps 2. Endocytosis 3. Exocytosis Active Transport Pumps -use carrier proteins to push/pull molecules across the membrane Active Transport Endocytosis - Transport of very large molecules INTO the cell - Forms a vacuole Exocytosis - Release of very large molecules (from a vacuole or vesicle) OUT of the cell