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Cells and Their Environment Cell Membrane Phospholipids Lipid Bilayer Fluid Mosaic Integral Proteins Phospholipid Transportation How do substances move into and out of cells? Passive Transport and Active Transport Passive Transport Movement across the cell membrane that does not require energy Some Important Terms Concentration gradient – a difference in the concentration of a substance across a space Equilibrium – concentration of a substance is equal throughout space Diffusion Movement of substance from area of high concentration to low concentration (down its concentration gradient)–resulting in equilibrium Example of Diffusion Small and nonpolar molecules diffusing across the cell membrane down their concentration gradient This is the simplest type of passive transport Osmosis The diffusion of water Movement of a substance down its concentration gradient A simple rule to remember is: Salt is a solute, when it is concentrated inside or outside the cell, it will draw the water in its direction. This is also why you get thirsty after eating something salty. 3 Possibilities for the direction of water movement in a cell 1. Water moves out 2. Water moves in 3. No net water movement The word "HYPER" means more, in this case there are more solute (salt) molecules outside the cell, which causes the water to be sucked in that direction. In plant cells, the central vacuole loses water and the cells shrink, causing wilting. In animal cells, the cells also shrink. In both cases, the cell may die. Water Moves Out – Hypertonic Solution The word "HYPO" means less, in this case there are less solute (salt) molecules outside the cell, since salt sucks, water will move into the cell. The cell will gain water and grow larger. 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. Water Moves In – Hypotonic Solution If the concentration of solute (salt) is equal on both sides, the water will move back in forth but it won't have any result on the overall amount of water on either side. "ISO" means the same Water moves in & out at the same rate! Extra 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. Transport Proteins Provide polar passageways through which ions and polar molecules can move across the cell membrane Each channel is specific for a substance – selectivity – the cell has the Control! How can ions get into a cell? Even though we have added a third party…. This is still a form of passive transport Why? There is no use of energy by the cell required because the ions are moving down their concentration gradient Facilitated Diffusion Passive transport….why? Active Transport How do substances enter a cell when they are working against their concentration gradient? Active transport - Requires the cell to use energy from ATP Carrier Proteins as “Pumps” Why do we like the Na-K Pump? 1. Na ions continuously diffuse into the cell membrane through ion channels (why is this a problem) and the pump prevents these ions from accumulating 2. Maintains the concentration gradient of Na ions and K ions across the cell membrane What about the large molecules (proteins and polysaccharides)? Endocytosis – the movement of a substance into a cell by a vesicle Endocytosis - terms Phagocytosis – cell eating – Membrane sinks in and captures solid particles for transport into the cell – Examples: Solid particles often include: bacteria, cell debris, or food Pinocytosis – cell drinking – Cell brings in a liquid Endocytosis comments Phagocytosis and pinocytosis are not selective – Membrane sinks inward and captures whatever particles/fluid present. – Vesicle forms and merges with the Golgi body… Receptor Mediated Endocytosis Receptor Mediated Endocytosis is a highly specific form of endocytosis. – Receptor proteins on the outside of the cell bind specific substances and bring them into the cell by endocytosis Receptor Mediated Endocytosis 1. Receptor proteins on PM bind specific substances (vitamins, hormones..) 2. Membrane sinks in and forms a pit – Called a coated pit 3. Pit pinches closed to form a vesicle around bound substances Cytoskeleton aids in pulling in the membrane and vesicle formation Example Plasma membrane Coat protein Coated vesicle Coated pit Specific molecule Coated pit Material bound to receptor proteins How to move large substances out of the cell? Exocytosis – the movement of a substance by a vesicle to the outside of a cell (used to export proteins modified by the Golgi) How cells communicate Release signal molecules that carry information to nearby cells and throughout the body Ex. hormones What else is in the cell membrane? Receptor Proteins - proteins in the cell membrane that bind these signal molecules and then “read the message” Marker Proteins – proteins that identify cell type. Cells