(notes)cell membrane

8/6/2019 (Notes)Cell Membrane

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Cell Membrane Structure and Function

The Plasma Membrane

--the fluid mosaic model (S.J Singer)-- semi-permeable, differentially permeable (like a fence)

Jobs of the cell membrane

• Isolate the cytoplasm from the external environment

• Regulate the exchange of substances

• Communicate with other cells

• Identification

Phospholipid bilayer

1. The plasma membrane is a phospholipid bilayer with embedded proteins.

2. Phospholipids have both hydrophilic and hydrophobicregions; nonpolar tails (hydrophobic) are

directed inward, polar heads (hydrophilic) are directedoutward to face both extracellular and intracellular fluid.

3. The proteins form a mosaic pattern on the membrane.

4. Cholesterol - stiffens and strengthens the membrane.

5. Glycolipids - protective and assist in various

functions.

6. Glycoproteins have an attached carbohydrate chain

of sugar that projects externally.7. The plasma membrane is asymmetrical; glycolipidsand proteins occur only on outside and cytoskeletalfilaments attach to proteins only on the inside surface.

Proteins embedded in membrane serve different functions

Transport Proteins - regulate movement of substanceChannel Proteins - form small openings for molecules to diffuse throughCarrier Proteins- binding site on protein surface "grabs" certain molecules and pulls them into the cellGated Channels - similar to carrier proteins, not always "open"

Receptor Proteins - molecular triggers that set off cell responses (such as release of hormones or opening of channel protbinding siteRecognition Proteins - ID tags, to identify cells to the body's immune systemEnzymatic Proteins – carry out specific reactions



Membrane Permeability Transport Across Membrane

*Selectively or Differentially permeable – some thing can cross, not others

Passive Transport

Simple Diffusion - water, oxygen and other molecules move from areas of high concentration to areas concentration, down a concentration gradientFacilitation Transport (Diffusion) - diffusion that is assisted by proteins (channel or carrier proteins)

Osmosis - diffusion of water. Salt Sucks

Osmosis affects the turgidity of cells, different solution can affect the cells internal water amounts

Contractile Vacuoles are found in freshwater microorganisms - they pump out excess water

Solution

SoluteSolvent



Turgor pressure occurs in plants cells as their central vacuoles fill with water.

Active Transport - involves moving molecules "uphill" against the concentration gradient, which

requires energy

Endocytosis - taking substances into the cell (pinocytosis for water, phagocytosis for solids)

Receptor-mediated endocytosis, a form of pinocytosis, occurs when specific macromolecules bind tplasma membrane receptors.

Exocytosis - pushing substances out of the cell, such as the removal of wasteSodium-Potassium Pump - pumps out 3 sodiums for ever 2 potassium's taken in against gradient

Demo - Starch in the baggie, iodine in the beaker. What happens and why?

Observation of elodea cells in salt water. What happens and why? Minilab.



Cell Connections and Communication

Tight junctions are composed of protein fibers thatseal adjacent cells to prevent leakage, somethingwhich can be useful in organs such as the bladder andthe lining of the digestive tract. Tight junctions literallyfuse the cells together forming a sheet of cellsrestricting molecules to one side of the sheet or theother. Aka Adhesion junction

Tight junctions can also partition the cells in which theyare found. Certain membrane proteins can be restrictedto one side of the junction, as well, since the tight

junction prevents protein migration within the

membrane.

Desmosomes anchor adjacent cells together bymaking connections that work like staples or rivets thatattach to components of the cytoskeleton. Many epithelialcells must adhere to adjacent membranes to prevent freepassage or free movement, and to not break apart under stress.

*Plants have plasmodesmata - channels between

the cell wall that cytosol can pass through

Gap junctions are protein channels, called

connections, between adjacent cells that permit thetransfer of small molecules, such as nutrient monomers,between the cells. They are common in brain cells,forming the synapse, in many glands, and in cells in theheart muscle that coordinate contraction for heartbeat.Gap junctions can be gated.

(notes)cell membrane

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