Cell Membranes
Cell membranes are mosaic in structure and function. The major constituent of membranes are phospholipids. The hydrophilic heads sandwich the hydrophobic tails in a lipid bilayer. Hydrophobic interactions keep the membrane together. The membrane is built inside the ER. The "inside" becomes the "outside".
The membrane is as fluid as salad oil. Lateral movement of phospholipids is quick and frequent. Membrane proteins may drift as well if not anchored by the cytoskeleton.
Fluidity is influenced by the composition of fatty acids, the presence of cholesterol (in animal cells), and the presence of peripheral and integral proteins.
Different proteins appear on the outer and inner face. Their functions are varied and vital to the life of the cell. They include: TRANSPORT, ENZYME ACTIVITY, SIGNAL TRANSDUCTION, INTERCELLULAR INTERACTIONS, CELL-CELL RECOGNITION, and BRIDGE BETWEEN EXTRACELLULAR MATRIX and CYTOSKELETON.
Transport in and out of cell
Living membranes are selectively permeable. The passage of particles through the membrane depends on several factors including the solubility characteristics of the membrane and the presence of specific transport proteins.
What can cross?
a) non-polar molecules pass through the hydrophobic interior easily (eg. oxygen and hydrocarbons). If two molecules are equally soluble, the smaller of the two will cross more quickly.
b)small, polar uncharged molecules like water and carbon dioxide can pass through
c)large polar molecules like glucose require a channel to enter cells
d) charged molecules (all ions), even as small as H+, cannot penetrate the hydrophobic core
Passive transport
-based on random molecular movement. Requires no cellular input of energy.
Simple diffusion: molecules move down their concentration gradient across the bilayer (movement is from an area of high to low concentration) Rates of diffusion are affected by temperature, concentration gradient, and size of molecules.
Facilitated diffusion: channel proteins allow passage through the hydrophobic core of bilayer. Cannot alter direction of movement but can increase the rate
Osmosis: the diffusion of water across a selectively permeable membrane.
vocabulary
solvent dissolves solute
When two solutions have equal concentrations of solute, they are isotonic.
When the concentrations differ, the solution with the higher concentration (A) is hypertonic to the solution with the lower concentration (B). Solution B is described as being hypotonic to A.
Animal and plant cells react somewhat differently to hypotonic and hypertonic environments. In isotonic environments, both types of cells will experience dynamic equilibrium.
In a hypotonic environment, more water enters than leaves the cell. An animal cell may swell and burst. But a plant cell fares much better in this environment. Excess water is stored in the central vacuole and the cell wall can exert back pressure that prevents even more water from entering. At this point, the plant is turgid.
In a hypertonic environment, more water leaves than enters. An animal cell will crenate and die. A plant cell will plasmolyze (its cell membrane shrinks away from the cell wall) and die.