Chapter 5 - Membrane Structure & Function

The Plasma Membrane

--the fluid mosaic model (S.J Singer)
-- semi-permeable
--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
  • Identification

Phospholipid bilayer

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

*differentiallty permeable / semipermeable

Passive Transport

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)

Osmosis - diffusion of water. Salt Sucks
Osmosis affects the turgidity of cells, different solution can affect the cells internal water amounts

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

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)
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

 

Demo - Starch in the baggie, iodine in the beaker. What happens and why?
Observation of elodea cells in salt water. What happens and why?

Modification of Cell Sufaces

Tight junctions are composed of protein fibers that seal adjacent cells to prevent leakage, something which can be useful in organs such as the bladder and the lining of the digestive tract. Tight junctions literally fuse the cells together forming a sheet of cells restricting molecules to one side of the sheet or the other.

Tight junctions can also partition the cells in which they are found. Certain membrane proteins can be restricted to one side of the junction, as well, since the tight junction prevents protein migration within the membrane.

Desmosomes anchor adjacent cells together by making connections that work like staples or rivets that attach to components of the cytoskeleton. Many epithelial cells must adhere to adjacent membranes to prevent free passage or free movement, and to not break apart under stress. Desmosome filaments are composed of specialized glycoproteins proteins. Intermediate filaments of keratin in the desmosomes help strengthen the junction. Actin microfilaments can also attach to desmosomes, but have less strength

*Plants have plasmodesmata - channels between the cell wall that cytosol can pass through

Gap junctions are protein channels, called connexons, between adjacent cells that permit the transfer 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 the heart muscle that coordinate contraction for heartbeat. Gap junctions can be gated.