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The Plasma Membrane
fluid mosaic model, semi-permeable (selectively permeable), double layer of
phospholipids with embedded proteins
Jobs of the cell membrane
Isolate the cytoplasm from the external environment
Regulate the exchange of substances
Communicate with other cells
Phospholipids (fats) contain a hydrophilic head and a nonpolar hydrophobic
tail, which creates a barrier.
Cholesterol - stiffens the membrane by connecting phospholipids
Glycolipids - signal molecules
Glycoproteins - have an attached chain of sugar (antibodies)
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, (gated channels)
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
5. Enzymatic Proteins - carry out metabolic reactions
Transport Across Membrane
-The membrane is selectively permeable (also called semipermeable )
- Small particles, or particles with no charge can pass through the bilayer
(carbon dioxide and oxygen)
- Water has a charge, does not easily cross the membrane - a channel
protein, Aquaporin helps water across
Passive Transport
Simple Diffusion - water, oxygen and other molecules move from areas of
high concentration to areas of low concentration, down a concentration
Facilitation Diffusion - diffusion that is assisted by proteins (channel or carrier
Osmosis - diffusion of water
Solutions: Hypertonic | Isotonic | Hypotonic
Contractile vacuoles can help pump out excess water in freshwater organisms
The central vacuole of plants can store excess water, creating a turgor
pressure; plants are less likely to burst due to cell walls
Active Transport
- involves moving molecules "uphill" against the concentration gradient, which
requires energy (ATP)
Endocytosis - taking substances into the cell (pinocytosis for water,
phagocytosis for solids)
Exocytosis - pushing substances out of the cell, such as the removal of waste
Sodium-Potassium Pump - pumps out 3 sodiums for ever 2 potassium's taken
in against gradient