Cells

Question 1

plasma membrane

Answer 1

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).

Question 2

describe the hydrophobic and hydrophilic areas of the plasma membrane

Answer 2

• 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.

Question 3

what do proteins and carbohydrates have to do with the plasma membrane?

Answer 3

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

Question 4

Fluid mosaic model

Answer 4

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

Question 5

Lipid rafts

Answer 5

areas of high cholesterol concentration in the membrane where there’s a different composition of the proteins, carbs, and different lipids; micro domains

Question 6

how does temperature affect membrane fluidity?

Answer 6

High temp = can ↓ membrane fluidity; Low temp = can ↑ membrane fluidity

Question 7

Fatty acid chains

Answer 7

Fatty acid chains in phospholipids & glycolipids = usually an even # of C atoms (typically 16-20); unsaturated fatty acid chains = nearly always cis orientation

Question 8

3 main classes of membrane proteins

Answer 8

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

Question 9

glycosylated proteins

Answer 9

addition of oligosaccharide chains to a peptide chain

Question 10

O-glycosylation

Answer 10

most common; a glycosidic bond is formed to the oxygen atom in serine & threonine side chains

Question 11

N-glycosylation

Answer 11

the oligosaccharide binds to a nitrogen on asparagine

Question 12

Lateral diffusion

Answer 12

when a phospholipid moves side-to-side w/in its layer; very fast

Question 13

Transbilayer diffusion

Answer 13

(uncatalyzed): when a phospholipid can “flip-flop” to the opposite layer; very slow

Question 14

Flippase

Answer 14

brings a phospholipid from the outer leaflet to the inner leaflet; requires ATP

Question 15

Floppase

Answer 15

phospholipid inner leaflet → outer leaflet; requires ATP

Question 16

Scramblase

Answer 16

brings a phospholipid from the outer leaflet → inner leaflet AND a phospholipid from the inner leaflet → outer leaflet; doesn’t require ATP

Question 17

Solute

Answer 17

the solid component dissolved in a solvent

Question 18

Osmosis

Answer 18

water diffusion across a membrane (water movement can ∆ the cell’s volume)

Question 19

Osmolarity

Answer 19

total [solute] of the solution which gives rise to osmotic pressure

Question 20

Low osmolarity

Answer 20

H₂O > Solute

Question 21

Hypotonic solution

Answer 21

{osmolarity} extracellular fluid < fluid inside cell

◦ Solution [solutes] < cell; Water enters the cell (lysis)

Question 22

Hypertonic solution

Answer 22

{osmolarity} extracellular fluid > fluid inside cell

◦ Water leaves the cell (shrinks)

Question 23

Facilitated transport

Answer 23

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.

Question 24

Active transport

Answer 24

requires energy to move substances against a [gradient]; area of [low] → [high]

Question 25

Sodium-Potassium Pump

Answer 25

1. Enzyme oriented towards cell interior; carrier has a high affinity for Na⁺ ions. 3 Na⁺ ions bind to the protein
2. The protein carrier hydrolyzes ATP and a low-energy phosphate group attaches to it
3. 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
4. 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.
5. W/ the phosphate group removed and K⁺ ions attached, the carrier protein repositions itself towards the interior of the cell
6. 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.

Question 26

ATP-binding cassette (ABC) transporter

Answer 26

uses ATP to transport a variety of substances across cell membranes, usually out of the cell

Question 27

Both the Na⁺/K⁺ pump & ABC transporters

Answer 27

primary active transport, which uses a chemical energy source (like ATP) to move solutes against their [gradient]

Question 28

Secondary active transport (aka cotransport)

Answer 28

uses 1 electrochemical gradient to move different molecules against their own [gradient]

Question 29

Diffusion

Answer 29

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

Question 30

Facilitated diffusion

Answer 30

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.

Question 31

Active transport

Answer 31

net movement solute against [gradient]; requires energy/ energy is consumed to move a solute against a [gradient] or electrochemical gradient

Question 32

Carriers

Answer 32

only open to one side of the cell membrane @ any given point

Question 33

Primary active transport

Answer 33

uses ATP or another energy molecule to directly power molecular transport
Generally involves the use of transmembrane ATPase; maintains membrane potential for neurons

Question 34

Secondary active transport

Answer 34

(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

Question 35

Symport

Answer 35

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

Question 36

Antiport

Answer 36

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)

Question 37

Aquaporins

Answer 37

(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

Question 38

Gated carrier proteins

Answer 38

binds and facilitates the transport of a single molecule at a time; opens in response to a stimulus

Question 39

Ligand-gated

Answer 39

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)

Question 40

Voltage-gated

Answer 40

responds to voltage ∆ across the membrane

Question 41

Voltage-gated Na⁺ and Ca²⁺

Answer 41

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

Question 42

Voltage gated K⁺ channels

Answer 42

composed of 4 separate polypeptide chains, each w/ 1 domain.

Question 43

transmembrane ATPase enzymes

Answer 43

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

Question 44

Colligative properties

Answer 44

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

Question 45

Osmotic pressure (∏)

Answer 45

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

Question 46

tonicity and osmotic pressure

Answer 46

The ↑ the difference in tonicity across the semipermeable membrane = ↑ ∏

Question 47

capillaries and osmotic pressure

Answer 47

Capillaries: ∏ = blood flow tissues → capillaries; hydrostatic pressure counteracts ∏ and encourages blood flow capillaries → tissue

Question 48

Channel proteins

Answer 48

spans the membrane, making hydrophilic tunnels across it to allow the target molecules to diffuse through; highly selective; polar & charged compounds are allowed through

Question 49

Aquaporins

Answer 49

channel proteins for water to cross the membrane quickly