Cell Membrane
What do you remember about the cell membrane?
Cell (Plasma) Membrane Separates the internal environment of the cell from the external environment All cells have a cell membrane Selectively permeable: allows some substances to cross, regulates what enters and exits the cell
Selective Permeability Direct consequence of the structure and composition of the cell membrane (fluid mosaic model)
Composition of the Cell Membrane Composition varies among different cell types
Composition of the Cell Membrane Phospholipids Embedded proteins Cholesterol Glycoproteins Glycolipids
Fluid Mosaic Model Phospholipids (rapid) and proteins (slowly) have the ability to shift laterally within the membrane Must be fluid-like to function properly, consistency of olive oil
Phospholipids Form the lipid bilayer (2 layers) Phospholipids have a hydrophobic and hydrophilic regions (amphipathic)
Embedded Proteins Proteins can be hydrophilic, with charged and polar side chains, or hydrophobic, with nonpolar side chains Many different types of proteins Proteins determine the membrane s specific function
Embedded Proteins: Functions Cellular transport (channel and carrier) Enzymatic activity Signal transduction (protein receptors) Cell-cell recognition (glycoproteins) Attachment to the cytoskeleton and extracellular matrix (anchors) Cell-cell attachment (cell junctions)
Cholesterol Stabilizes the cell membrane Maintains fluidity at moderate temperatures Reduces solidification at cool temperatures
Glycoproteins and Glycolipids Function as markers in cell-cell recognition, multicellular development, immune system Located on the outside of the cell membrane Glycoprotein: carbohydrate bonded to a protein Glycolipid: carbohydrate bonded to a phospholipid
Cellular Transport: Regulation of Molecular Traffic
To cross or not to cross, that is the question.. Pass Freely Do Not Pass Freely Small, polar molecules Small, non polar molecules (gases) Large, polar molecules (amino acids, sugars) Macromolecules Ions
Passive Transport Does not require cellular energy (ATP) Substances move from areas of high concentration to areas of lower concentration (with or down the gradient) 1. Simple Diffusion 2. Facilitated Diffusion 3. Osmosis
Simple Versus Facilitated Diffusion
Simple Diffusion Small, nonpolar molecules, such as N 2, O 2 and CO 2 Small, polar molecules
Facilitated Diffusion Large, polar molecules (hydrophilic), such as amino acids and sugars Ions Assisted by transport proteins (channel and carrier)
Transport Proteins Transport proteins allow the passage of substances across the membrane that do not pass freely through the phospholipids Channel proteins (different types) Carrier proteins
Osmosis Diffusion of water across a selectively permeable membrane Uses specialized channel proteins called aquaporins (hydrophilic channel) Water diffuses to balance the solute concentration on both sides of cell membrane
Aquaporin 3 billion water molecules per second
Osmosis External environments can be hypotonic, hypertonic or isotonic to internal environments of cells
Isotonic Concentration of dissolved solutes in the environment is equal to the cell Cell will neither gain nor lose water
Hypotonic Concentration of dissolved solutes in the environment is less than the cell Cells will gain water, swell, and possibly lyse (burst)
Hypertonic Concentration of dissolved solutes in the environment is more than the cell Cell will lose water and shrivel
Plant Cell Osmosis Water moves into the cell creating turgor pressure in hypotonic environments Plasmolysis (Hypertonic) Flaccid (Isotonic)
Osmoregulation Organisms must live in isotonic environments or have adaptations for osmoregulation Osmoregulation: regulation and control of solute concentration and water balance
Osmoregulation in Fish
Osmoregulation in Humans