Cells Flashcards
plasma membrane
a double layer of phospholipids with proteins and cholesterol that separates the inside of the cell from its outside environment; is present in both prokaryotes & eukaryotes, and it controls what enters and exits the cell. It is flexible (unlike the cell wall which is rigid).
describe the hydrophobic and hydrophilic areas of the plasma membrane
- The plasma membrane is made of an amphiphilic phospholipid molecules; hydrophilic head (charged phosphate groups; has contact w/ the aqueous fluid inside and outside the cell) and hydrophobic tails (non-polar lipid tails)
- The hydrophilic regions of the phospholipids form H bonds w/ water & other polar molecules on both the exterior and the interior of the cell. So the part of the cell that faces the interior and exterior of the cell are hydrophilic, while the middle of the cell membrane is hydrophobic. Therefore, the phospholipids form an excellent lipid bilayer cell membrane.
what do proteins and carbohydrates have to do with the plasma membrane?
Proteins = 2ⁿᵈ major component of plasma membranes
Integral proteins = integrated completely into the membrane structure; have hydrophobic membrane-spanning regions that interact with the hydrophobic region of the phospholipid bilayer.
Some span only part of the membrane (associating with a single layer) while others are exposed on either side as the stretch from one side of the membrane to the other
Proteins are important for transporting larger molecules & charged particles over the membrane layer
Carbohydrates = 3ʳᵈ major component
Always found on the exterior surface of cells & are bound either to proteins (forming glycoproteins) or to lipids (= glycolipids). They form specialized sites on the cell surface, allowing cells to recognize each other
Fluid mosaic model
describes the structure of the plasma membrane as a mosaic of components, giving the plasma membrane a fluid character
◦ The mosaic characteristic of the membrane where the integral proteins & lipids exist in the membrane as separate but loosely-attached molecules; can flow past one another
Lipid rafts
areas of high cholesterol concentration in the membrane where there’s a different composition of the proteins, carbs, and different lipids; micro domains
how does temperature affect membrane fluidity?
High temp = can ↓ membrane fluidity; Low temp = can ↑ membrane fluidity
Fatty acid chains
Fatty acid chains in phospholipids & glycolipids = usually an even # of C atoms (typically 16-20); unsaturated fatty acid chains = nearly always cis orientation
3 main classes of membrane proteins
Transmembrane (integral): includes membrane-spanning proteins w/
‣ 1. A hydrophilic cytosolic domain interacting w/ the interior of the cell ‣ 2. Hydrophobic membrane-spanning domain, anchors it to the cell membrane ‣ 3. Hydrophilic extracellular domain- interacts w/ extracellular environment ‣ Examples = proton pumps, ion channels, G protein-coupled receptors
Peripheral: proteins that are transiently attached to integral membrane proteins or are associated with peripheral regions of the lipid bilayer
‣ Ex- some enzymes & hormones
Lipid-anchored: proteins are covalently bound to lipid molecules, anchoring them w/in the membrane w/o the protein contacting the membrane ‣ Ex- G protein = intracellular membrane-bound structure, helps coordinate the signaling cascade initiated by G protein-coupled receptors
glycosylated proteins
addition of oligosaccharide chains to a peptide chain
O-glycosylation
most common; a glycosidic bond is formed to the oxygen atom in serine & threonine side chains
N-glycosylation
the oligosaccharide binds to a nitrogen on asparagine
Lateral diffusion
when a phospholipid moves side-to-side w/in its layer; very fast
Transbilayer diffusion
(uncatalyzed): when a phospholipid can “flip-flop” to the opposite layer; very slow
Flippase
brings a phospholipid from the outer leaflet to the inner leaflet; requires ATP
Floppase
phospholipid inner leaflet → outer leaflet; requires ATP
Scramblase
brings a phospholipid from the outer leaflet → inner leaflet AND a phospholipid from the inner leaflet → outer leaflet; doesn’t require ATP
Solute
the solid component dissolved in a solvent
Osmosis
water diffusion across a membrane (water movement can ∆ the cell’s volume)
Osmolarity
total [solute] of the solution which gives rise to osmotic pressure
Low osmolarity
H₂O > Solute
Hypotonic solution
{osmolarity} extracellular fluid < fluid inside cell
◦ Solution [solutes] < cell; Water enters the cell (lysis)
Hypertonic solution
{osmolarity} extracellular fluid > fluid inside cell
◦ Water leaves the cell (shrinks)
Facilitated transport
another form of passive transport. A [gradient] exists that would allow materials to diffuse into the cell w/o expending energy. However, when the materials are ions or polar molecules, these are repelled by the hydrophobic parts of the cell membrane. So facilitated transport proteins like channels shield those materials from the repulsion of the membrane, allowing them to diffuse into the cell.
Active transport
requires energy to move substances against a [gradient]; area of [low] → [high]
Sodium-Potassium Pump
- Enzyme oriented towards cell interior; carrier has a high affinity for Na⁺ ions. 3 Na⁺ ions bind to the protein
- The protein carrier hydrolyzes ATP and a low-energy phosphate group attaches to it
- The carrier changes shape as a result and re-orients itself towards the membrane’s exterior. The protein’s affinity for sodium ↓ and the 3 Na⁺ ions leave the carrier
- The shape ∆ ↑ the carrier’s affinity for K⁺ ions and 2 ions attach to the protein. S/p the low-energy phosphate group detaches from the carrier.
- W/ the phosphate group removed and K⁺ ions attached, the carrier protein repositions itself towards the interior of the cell
- The carrier protein (new config) has a ↓ affinity for K⁺, and the 2 ions are released into the cytoplasm. The protein now has a higher affinity for Na⁺ ions; cycle repeats.
ATP-binding cassette (ABC) transporter
uses ATP to transport a variety of substances across cell membranes, usually out of the cell
Both the Na⁺/K⁺ pump & ABC transporters
primary active transport, which uses a chemical energy source (like ATP) to move solutes against their [gradient]
Secondary active transport (aka cotransport)
uses 1 electrochemical gradient to move different molecules against their own [gradient]
Diffusion
a thermodynamically spontaneous process. The movement of the solutes from [high]→ [low] creates a
(-)∆G [aka negative free energy change] so energy will be produced as solutes move. This ↑ system entropy and ↓
free energy. The chemical [gradient] and electrical gradient dictate the direction of diffusion
Facilitated diffusion
simple diffusion; used for molecules that are impermeable to the membrane (large, polar, charged) since the energy barrier is too high for them to cross. This requires integral membrane proteins to serve as transporters (channels) for the substrates.
Active transport
net movement solute against [gradient]; requires energy/ energy is consumed to move a solute against a [gradient] or electrochemical gradient
Carriers
only open to one side of the cell membrane @ any given point
Primary active transport
uses ATP or another energy molecule to directly power molecular transport
Generally involves the use of transmembrane ATPase; maintains membrane potential for neurons
Secondary active transport
(coupled) uses energy for transport but there is no directly coupling to ATP hydrolysis; instead the energy released by a particle going down its electrochemical gradient is harnessed and used to drive a different molecule up its gradient
uses the energy stored in an electrochemical gradient that was created from the primary active transport
Symport
both particles flow in same direction across the membrane
downhill movement for molecules in the same direction and against their [gradient]. Ie = glucose symporter SGLT-1, which co-transporters 1 glucose (or galactose) into the cell w/ the transport of 2 Na⁺ ions
Antiport
particles flow in opposite direction
2 ion species or other solutes, opposite directions. Ie = sodium-calcium exchanger (3 Na⁺ ions ↓ the [gradient] while 1 Ca²⁺ ion out)
Aquaporins
(water channels) = transmembrane proteins selective for water molecules, prevents passage of ions & other solutes; used for movement of large amounts of water into & out of the cell
Gated carrier proteins
binds and facilitates the transport of a single molecule at a time; opens in response to a stimulus
Ligand-gated
responds to the binding of a ligand molecule (ie neurotransmitter); they’re involved in the propagation of signals in response to external stimuli (ie sensory neurons)
Voltage-gated
responds to voltage ∆ across the membrane
Voltage-gated Na⁺ and Ca²⁺
composed of a single polypeptide, 4 homologous domains, each w/ 6 membrane-spanning α-helices w/ 1 acting as the voltage-sensing helix. Has many (+) so it has a high (+) charge outside the cell, repelling the helix, keeping the channel closed. Depolarization inside the cell → helix conformational ∆ → ions flow through the channel
Voltage gated K⁺ channels
composed of 4 separate polypeptide chains, each w/ 1 domain.
transmembrane ATPase enzymes
catalyzes the hydrolysis of ATP, releasing energy. They couple the movement of solutes to ATP hydrolysis; ie Na⁺/K⁺ pump; generally inside cell [K⁺] > outside cell [K⁺] and inside cell [Na⁺] < outside cell [Na⁺]
Can be powered by redox reactions, photon energy, mitochondrial ETC enzymes
Colligative properties
depends only on the concentration of solute particles (amount), not identity of the solute particles
Includes vapor pressure, boiling point, freezing point, melting point, osmotic pressure
Osmotic pressure (∏)
the pressure applied to a pure solvent to prevent osmosis; also used to express the [solution]
◦ ∏=iMRT
‣ M = molarity; R = ideal gas constant (0.08206 L∙atm/mol∙K); T = absolute temp (K); i = van’t Hoff factor (the #
of particles obtained from the molecule when in solution) ‣ Osmotic pressure is ∝ molarity of the solution
tonicity and osmotic pressure
The ↑ the difference in tonicity across the semipermeable membrane = ↑ ∏
capillaries and osmotic pressure
Capillaries: ∏ = blood flow tissues → capillaries; hydrostatic pressure counteracts ∏ and encourages blood flow capillaries → tissue
Channel proteins
spans the membrane, making hydrophilic tunnels across it to allow the target molecules to diffuse through; highly selective; polar & charged compounds are allowed through
Aquaporins
channel proteins for water to cross the membrane quickly
