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Cell Membrane • The composition of nearly all cell membranes is a double-layered sheet called a lipid bilayer. The phospholipid bilayer gives cell membranes a flexible structure that forms a strong barrier between the cell and its surroundings. The cell membrane regulates what enters and leaves the cell and also provides the cell with protection and support.
Structure of Cell Membrane • Has a phospholipid bilayer (2 layers of fat).
The Cell Membrane • Maintains homeostasis for the cell by allowing waste and other products to leave the cell. • Selectively Permeable – allows only some materials in and out of the cell. • Water and oxygen move freely across the cell's membrane.
The phospholipids have a: • hydrophilic head (water loving) • two hydrophobic tails (water fearing)
Cell Membrane • The Cell Membrane is also called “The Fluid Mosaic Model” • Because the membrane is flexible and the lipids move within the membrane. (like water molecules move in the currents of a lake). • Also because proteins create a “mosaic” or pattern on the surface of the membrane.
Another type of lipid in the cell membrane is CHOLESTEROL that makes the membrane more fluid. Embedded in the phospholipid bilayer are PROTEINS that also aid in diffusion and in cell recognition. Proteins called INTEGRAL proteins go all the way through the bilayer, while PERIPHERAL proteins are only on one side. Large molecules like PROTEINS or carbohydrates use proteins to help move across cell membranes. Some of the membrane proteins have carbohydrate chains attached to help cells in recognize each other and certain molecules
Passive Transport- moving molecules across cell membrane • Does NOT require energy. • Three Types: • Diffusion • Osmosis • Facilitated Diffusion
Diffusion • As a result, the particles tend to move from an area of greater concentration to an area of lower concentration. • EQUILIBRIUM- when the molecules are even throughout a space. • Diffusion depends upon random particle movements, across membranes without requiring extra energy
http://www.youtube.com/watch?v=E7kor5nHtZQ&feature=endscreen&NR=1http://www.youtube.com/watch?v=E7kor5nHtZQ&feature=endscreen&NR=1
Facilitated Diffusion • Movement of large molecules across the cell membrane using PROTEINS!! • Cell membranes have transport proteins that act as carriers, making it easy for certain molecules to cross. • Each channel is specific for 1 type of molecule • facilitating, or helping, the diffusion of molecules across the membrane, and this is called facilitated diffusion.
Osmosis • The movement of WATER across the cell membrane from high concentration to low concentration.
Huddle Up • What part of the cell membrane is hydrophilic? What part is hydrophobic? • What is diffusion? • What is osmosis?
Three Types of Solutions • Isotonic • Hypotonic • Hypertonic • Recall a Solution = Solute + Solvent • Solute – substance being dissolved • -Solvent – substance that does the dissolving • http://www.youtube.com/watch?v=nd2jtwviyC8&feature=related
Isotonic Solution- “ISO” means same • If the concentration of solute (salt) is equal on both sides, the water will move back and forth equally.
Hypotonic Solution- “HYPO” means less • There are less solute (salt) molecules in the solution so water will move into the cell. • Cell gets larger and could burst and die. In plant cells, the central vacuoles will fill and the plant becomes stiff and rigid, the cell wall keeps the plant from bursting. In animal cells, the cell may be in danger of bursting, organelles called CONTRACTILE VACUOLES will pump water out of the cell to prevent this.
Hypertonic Solution- “Hyper” means more • There are more solute (salt) molecules in the solution, so the water is sucked out of the cell. • Cell shrinks, causes cell to die.
Osmotic Pressure • For organisms to survive, they must have a way to balance the intake and loss of water.
Can Drinking Salt Water Hurt You? • Yes! This is why it is dangerous to drink sea water. • Its a myth that drinking sea water will cause you to go insane, but people marooned at sea will speed up dehydration (and death) by drinking sea water. • This is also why "salting fields" was a common tactic during war. It would kill the crops in the field, thus causing food shortages.
Active Transport • REQUIRES ENERGY b/c materials are moving against the concentration gradient. • Molecules from an area of LOW concentration to HIGH concentration.
Active Transport • The active transport of small molecules or ions across a cell membrane is generally carried out by transport proteins or “pumps”that are found in the membrane itself. • Larger molecules and clumps of material can also be actively transported across the cell membrane by processes known as endocytosis and exocytosis.
Types of Active Transport • Endocytosis- moving large particles INTO the cell by the cell membrane engulfing them. • Phagocytosis: engulf large FOOD particles • Pinocytosis: engulf large LIQUID particles • Exocytosis – moving large particles OUT of the cell.
Endocytosis(en-doh-sy-TOH-sis) is the process of taking material into the cell by means of infoldings, or pockets, of the cell membrane.
Phagocytosis means “cell eating.” In phagocytosis, extensions of cytoplasm surround a particle and package it within a food vacuole. The cell then engulfs and digests it.
Exocytosis(ek-soh-sy-TOH-sis) is the process by which cells release large amounts of material from the cell.
