MEMBRANE DYNAMICS and Cell Signaling
Cell Membrane
phospholipid bilayer
cholesterol
proteins: integral/transmembrane and peripheral
glycolipids
glycoproteins
Passive Transport (doesn't require energy)
simple diffusion across a permeable membrane
allows for movement of lipophilic substances
requires conc. gradient (substances move ⬆ to ⬇ conc.)
Factors that influence diffusion rate
general properties: uses kinetic energy of molecular movement and uses conc. gradient
MEMBRANE properties
SOLUTE PROPERTIES
S.A.
Thickness
M.W. (size)
Lipid Solubility
Lipid Conc.
Uses Membrane Transport Proteins
cylinder shaped (move water & ions) 💧
gated (open or closed)
channel proteins (generally do not change shape)
chemically gated 👩🏾🔬
voltage gated ⚡
carrier proteins (bind substances & carry them across membrane; undergo conformational change) 🛄
Types
Uniport: transports 1 substance
Symport: 2+ substances in same direction
Antiport: 2+ substances in opposite directions
Primary Active Transport (Uses ATP directly 💥)
1) Molecule (or ion) binds to transport protein
2) ATP is hydrolyzed to ADP and P gets released
3) Phosphate binds to transport protein. Phosphorylation causes protein to change shape, substrates are on other side of membrane
4) Transport protein releases phosphate & returns to reg. shape
Secondary Active Transport (uses potential energy from conc. gradient as energy source)
needed ATP to set up conc. gradient 💥
glucose is transported against its conc. gradient when Na+ moves down its conc. gradient (SYMPORT)
sodium-glucose transporter [Na+ gradient created by pump]
PROPERTIES OF CARRIER MEDIATED TRANSPORT
Binding Affinity: depends on formation of a set of weak, non-covalent bonds
Competition: Diff substances may compete for binding sites on transporters
Specificity: protein can only bind to 1 or 2 other molecules (Transport proteins only bind to specific molecules)
Terms
Saturation: rate of transport
Antagonist: binds to receptor but doesn't activate it, blocks agonist activity
Agonist/ Ligand: substance fully activates receptor that it binds to
VESICULAR TRANSPORT (transports molecules too lrg. for protein channels or carriers)
endocytosis- transports materials into cells via membrane bound vesicles
exocytosis - releases lrg. molecules out of cell
RESTING MEMBRANE POTENTIAL
due mostly to potassium (K+) ions conc. gradient created by Na+/K+ pump
Cell membrane separates electrical charges in the body
an uneven distribution of ions (electrical charge) between the ICF and ECF
CREATION OF A MEMBRANE (ex. with an artificial cell)
✅cell is initially electronically neutral (NO MEM POTENTIAL) (+)ions= (-) ions
✅Insert K+ leak channel into the cell...K+ leaks outside the cell down its conc. gradient resulting in a net negative charge inside the cell and a net positive charge outside the cell. The cell now has a membrane potential diff. The resulting (-) membrane potential attracts K+ back into the cell, however K+ wants to move out due to conc. gradient. K+ ions eventually stop moving which results in an equivalent potential of (Ek= -90mV)
✅Insert a Na+ leak channel into the cell. Cl- contributes to a (-) charge outside of the cell. Na+ moves back out of the cell attracted to a (-) charge. (Ena= +60mV)
NOTE: Cells are permeable to more than 1 ion. Most cells are more permeable to K+. Human body cells are 40x more permeable to K+ so resting membrane potential is -70 mV