Membrane Structure & Function
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The Plasma Membrane
- the fluid mosaic model
- fluid portion is a double layer of phospholipids, called the phospholipid bilayer
Jobs of the Cell Membrane
- Isolate the cytoplasm from the external environment
- Regulate the exchange of substances
- Communicate with other cells
- Phospholipids contain a hydrophilic head and a nonpolar hydrophobic tail.
- Hydrogen bonds form between the phospholipid "heads" and the watery environment inside and outside of the cell.
- Hydrophobic interactions force the "tails" to face inward
- Phospholipids are not bonded to each other, which makes the double layer fluid.
- Cholesterol embedded in the membrane makes it stronger and less fluid.
Proteins Embedded in Membrane Serve Different Functions
- 1. Channel Proteins - form small openings for molecules to difuse through
- 2. Carrier Proteins- binding site on protein surface "grabs" certain molecules and pulls them into the cell
- 3. Receptor Proteins - molecular triggers that set off cell responses (such as release of hormones or opening of channel proteins)
- 4. Cell Recognition Proteins - ID tags, to idenitfy cells to the body's immune system
- 5. Enzymatic Proteins - carry out metabolic reactions
Transport Across Membrane
Simple Diffusion - water, oxygen and other molecules move from areas of high concentration to areas of low concentration, down a concentration gradient.
Facilitation Diffusion - diffusion that is assisted by proteins (channel or carrier proteins)
- diffusion of water. Salt Sucks
- Osmosis affects the turgidity of cells, different solution can affect the cells internal water amountsContractiles Vacuoles are found in freshwater microorganisms - they pump out excess water
- Turgor pressure occurs in plants cells as their central vacuoles fill with water.
- involves moving molecules "uphill" against the concentration gradient, which requires energy
- taking substances into the cell (pinocytosis for water, phagocytosis for solids
- pushing substances out of the cell, such as the removal of waste
- pumps out 3 sodiums for ever 2 potassium's taken in against gradient
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