bio 189 cell_membranes_and_signaling_spr
TRANSCRIPT
Cell Membranes and Signaling
BIOLOGY 189
Biological Membranes• General structure: fluid mosaic model.
• Phospholipids form a bilayer
• Two regions:
Hydrophilic regions—“heads” that associate with water molecules
Hydrophobic regions—nonpolar fatty acid “tails” that do not dissolve in water
Membrane Molecular Structure
Biological Membranes
•May differ in lipid composition as there are many types of phospholipids.
•Phospholipids may differ in:
Fatty acid chain length
Degree of saturation
Kinds of polar groups present
Biological Membranes
•Two important factors in membrane fluidity:
Lipid composition—types of fatty acids can increase or decrease fluidity
Temperature—membrane fluidity decreases in colder conditions
Biological Membranes
•Membranes contain proteins
Peripheral membrane proteins lack hydrophobic groups and are not embedded in the bilayer.
Integral membrane proteins are partly embedded in the bilayer.
Biological Membranes
•Anchored membrane proteins have lipid components that anchor them in the bilayer.
•Transmembrane proteins extend through the bilayer on both sides
Biological Membranes
•Plasma membrane carbohydrates located on the outer membrane serve as recognition sites.
Glycolipid—a carbohydrate bonded to a ________
Glycoprotein—a carbohydrate bonded to a ______
Selective Permeability
•Biological membranes allow some substances to pass or not.
Passive transport does not require energy.
Active transport requires energy.
Diffusion
Passive transport
•Two types of diffusion:
Simple diffusion
Facilitated diffusion through channel proteins or aided by carrier proteins
Diffusion
• Speed of diffusion depends on
Diameter of the molecules—smaller molecules diffuse faster
Temperature of the solution—higher temperatures lead to faster diffusion
The concentration gradient the greater the concentration, the faster a substance will diffuse
Diffusion
Higher concentration inside the cell causes the solute to diffuse out, and a higher concentration outside causes the solute to diffuse in, for many molecules.
Osmosis
•Diffusion of water across membranes.
•Depends on the concentration of solute molecules on either side of the membrane.
•Water passes through special membrane channels.
Hypertonic, Isotonic, Hypotonic
When comparing two solutions separated by a membrane:
• A hypertonic solution has a higher solute concentration
• Isotonic solutions have equal solute concentrations
• A hypotonic solution has a lower solute concentration
Hypotonic
Isotonic
Hypertonic
Hypotonic solution
Animal cell
Plant cell
H2O
LysedH2O
Turgid (normal)
H2O
H2O
H2O
H2O
Normal
Isotonic solution
Flaccid
H2O
H2O
Shriveled
Plasmolyzed
Hypertonic solution
Funny animation
Facilitated Diffusion
Diffusion aided by
•Channel proteins. Integral membrane proteins that form channels across the membrane.
•Substances can also bind to carrier proteins to speed up diffusion.
A Ligand-Gated Channel Protein Opens in Response to a Stimulus
Diffusion
• Water crosses membranes with the help of Aquaporins (specific channels)
• They allow large amounts of water to move against its concentration gradient
• I t may “hitchhike” with ions such as Na+
as they pass through channels.
Carrier Proteins Facilitate Diffusion
Active transport
• requires energy to move substances against their concentration gradients.
• energy source: ATP.
• A substance moves in the direction of the cell’s needs, usually by means of a specific carrier protein.
Active transport
Two types
•Primary active transport involves hydrolysis of ATP for energy.
•Secondary active transport uses the energy from an ion concentration gradient, or an electrical gradient.
Active Transport
•The sodium–potassium (Na+–K+) pump is an integral membrane protein that pumps Na+ out of a cell and K+ in.
•One molecule of ATP moves two K+ and three Na+ ions.
2
EXTRACELLULAR
FLUID [Na+] high [K+] low
[Na+] low
[K+] high
Na+
Na+
Na+
Na+
Na+
Na+
CYTOPLASM ATP
ADP P
Na+ Na+
Na+
P 3
K+
K+ 6
K+
K+
5 4
K+
K+
P P
1
Endocytosis and Exocytosis
•Macromolecules are too large or too charged to pass through biological membranes and instead pass through vesicles.
•Cells use endocytosis or exocytosis
Endocytosis
Three types:
•Phagocytosis, (“cellular eating”)
•Pinocytosis, (“cellular drinking”)
•receptor mediated endocytosis
Endocytosis
Receptor-Mediated Endocytosis
Exocytosis Exocytosis moves materials out of the cell in vesicles.
Summary of cell membrane transport