Chapter 3
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The plasma membrane
o Selectively permeable
o Regulates exchange of materials in and out of cell
o Internal membranes- organelles
Fluid Mosiac Model
o 1972 singer- Nicolson
 The structure of the membrane is dynamic
o Made up of phospholipid bilayer
 Hydrophyilic head- “h2o loving”
 Hydrophobic tail- “h2o hating”
 Results in an amphilic molecule
Membrane Lipids:
o Phospholipids:
 The two long fatty acid chains are hydrophobic non-polar tails
 Made up of C and H and they have no charge- which means they
don’t like h2o
 The plor head is made up of a phosphate group and some O2
bonds
 Because of the charge on this end- the molecules don’t share
electrons
o Glycolipids: have a sugar monomers attached at the head
 Usually found on outer surface
o Cholesterol: found in animal membranes, it helps to “pack” phospholipids
giving more rigidly to the membranes
Membrane Proteins
o Make up half the membrane by weight
o Integral vs. peripheral
 Transportation and support
o Helps maintain fluid oligosacchalides (many sugar monomers
[carbohydrates])
o Attached the extra cellular side of the bilayer
Protein function:
o Transport:
 Channel protein- opened or gated, serves as a pore where ions,
h2o, and soluble substances can move
 Carrier proteins- bind specific substances to change shape to get
materials across
 Some passive, some “pump” (require energy activation)
Recognition: identifies the cell as a certain type.
Receptor has binding sites for hormones that can trigger changes in the cell
action and growth
At Work:
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o Uniport- one molecule is transported
o Co- transported- two molecules transported
 Symport: 2 molecules transported in same directions
 Antiport: 2 molecules transported in different directions
 Solution: homognous mixture of 2 or more substances
 Solute: substance that is dissolved in a solution (usually solid)
 Solvent: dissolving agent, usually a liquid (h2o- universal solvent)
Diffusion:
o Occurs high to low concentration
o Gradients in pressure temp and electron charge can also affect movement
Simple Diffusion:
o Diffusion of like molecules or ions down concentration gradient
 Substances diffuse and independent of each other (dye in water)
 Dynamic equilibrium: no gradients- no net movement
 Rate dependent on concentration temp (higher= faster) and
molecules size
 Smaller=faster
Bulk flow:
o Tendency of different substances in fluid to move together in the same
direction due to a pressure gradient (as seen in animal circulatory system)
o Shrinks the distance that molecules travel; enhancing diffusion rates
Osmosis:
o The passive transport of water across a differentially permeable
membrane
o Response to concentration or pressure gradient
Tonicity- ton of cell
o Is the ability of a solution to change the shape or “tone” of cells by
alternating their internal water volume
3 conditions:
o Isotonic: no net movement of water relative solute concentrations are
equal
o Hypotonic: cells swell, solute concentration of fluid lower than that of cell
o Hypertonic: cell shrivel (crenate) solute concentration fo fluid higher than
that of cells
Movement across membrane:
o Simple diffusion: small molecules, O2, CO2, and “h2o” can cross lipid
bilayer unaided
o Facilitated diffusion: move larger molecules, like glucose, h2o (with
aquaporis) and charged ions, like Na+, Ca+, and h2o with aid of a
transport protein (passive, no need for ATP, need carrier)
Active Transport:
o Similar to facilitated, but not limited by concentration gradients
o Carrier proteins work as “rumps” to work against gradient
o ATP required
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o Is independent on the presence and activities of membrane proteins
o Shows specify (only a certain molecule or group of molecules moved
across membranes by a given active transport)
o Na-K ATPase pump: (sodium- potassium pump)
 Uses the energy from the hydrolysis of ATP
 Actively transport 3 Na+ ions out the cell
 For each 2K+ ions pumped into the cell
 Helps with the resting potential and osmosis balance
Vesicular Transport:
o Small sacs made of membranes can transport and store substances within
cytoplasm
Exocytosis: move substances “out of cell” (require energy) cytoplasm and into
the extracellular space
Endocytosis: “into the cell”
o Infolded into plasma membranes
o Form vesicle and move to cytoplasm part then digest
Phagocytosis: “cell eating”
o Cytoplamsic extensions envelope large particles
o This phagosome fuses with lysosome for digestion
o Examples:
 Macrophages
 White blood cells
 “eat” intrudes such as bacteria
Pinocytosis: “cell drinking” taking in liquids
o Liquid first becomes bound or absorbed on the cell membrane
o It invaginates (form a pocket) and pinches off