Membrane Transport

Question 1

The body’s internal water is ditributed into two large components

Answer 1

Intracellular Fluid and Extracellular Fluid

Question 2

Majority of cells are in this compartment

Answer 2

Intracellular Fluid

􏰅 Majority is in cells as ICF, the remaining 20% are interstitial fluid
and plasma
􏰅 Two-thirds of total body water

Question 3

Functions of Intracellular Fluid

Answer 3

􏰀 Make, store, and utilize energy
􏰀 Repair itself
􏰀 Replicate
􏰀 Perform cell-specific functions

Question 4

Brings to the cell nutrients (e.g., glucose, fatty acids (FAs), amino
acids (AAs)), oxygen, various ions and trace chemicals, and
hormones
􏰅 Removes CO2, waste products, and toxic or detoxified materials
from immediate cellular environment

Answer 4

Extracellular fluid

Question 5

ECF is charactereized by ___ and ____ content, ____ as major anion

Answer 5

Na+, Ca2+, Cl-

Question 6

Most common extracellular ion

Answer 6

Sodium

Question 7

Most common intracellular ion

Answer 7

Potassium

Question 8

Differences in concentrations forms what

Answer 8

forms gradients that drives pumps to maintain homeostasis between the extracellular and intracellular environments

Question 9

Backbone is glycerol and Amphipathic: nonpolar FA tails, and polar head due to presence of
charged phosphate group

Answer 9

Phospholipids

Question 10

Forms when polar head groups are larger than the hydrophobic tails
􏰁 Relatively small and consequently limited in their potential to form membranes

Answer 10

Micelle

Question 11

Key structure in biological membranes

Answer 11

Bilayer

Polar heads oriented outwards the aqueous environment with the hydrophobic fatty acid chains hidden in between
Impermeable to most ater-soluble molecules

Question 12

What happens when the number of heads is almost equal to tails?

Answer 12

tails associate with other tails and expose polar head groups to aqueous environment and form a sandwich of phospholipid bilayer.

Question 13

The bilayer is driven by this

Answer 13

Hydrophobic Effect

(the tendency of nonpolar molecules
to self-associate in an aqueous environment, while in the process
excluding water))

Question 14

Sometimes the bilayer can engulf the environment forming this with the polar heads oriented inside and non-polar tails still hidden away from water

Answer 14

Liposome

Question 15

Amphiphagic behavior of lipids allow membranes to perform this

Answer 15

Compartmentalization

(Compartmentalization increases the efficiency of many subcellular processes by concentrating the required components to a confined space within the cell.)

Question 16

Molecules that can pass from extracellular to intracellular environment

Answer 16

Small nonpolar hydrophobic & Small uncharged polar molecules

− Small nonpolar hydrophobic (e.g. O2, CO2 and steroids)
􏰅 Can easily permeate through the non-polar layer of phospholipid bilayer, thus can pass easily from the extracellular to intracellular
environment
− Small uncharged polar molecules (e.g. water, glycerol, urea and ethanol)
􏰅 May pass even if they are polar because of their small size

Question 17

Molecules that can not pass from extracellular to intracellular environment

Answer 17

charged ions and large uncharged molecules

− Charged ions
􏰅 Cannot pass through the hydrophobic tails of the bilayer − Large uncharged molecules
􏰅 Cannot pass through bilayer due to size

Question 18

Glycerol-phosphate backbone attached to two FAs via ester linkages and an alcohol

Answer 18

Phosphoglyceride

FA constituents are usually even-numbered carbon molecules (most common: 16 or 18C)
􏰁 Unbranched
􏰁 Can be saturated or unsaturated with one or more double bonds

Question 19

Difference between Sphingomyelin and Phosopglyceride

Answer 19

Phospholipid that contains a sphingosine rather than a glycerol backbone

Question 20

FA of Sphingomyelins are attached by an _____ to the amino group of spingosines, forming ____

Answer 20

amide; ceramide

Question 21

Formed when primary hydroxyl group of sphingosines is esterified to
phosphorylcholine

Answer 21

Sphingomyelin

• prominent in myelin sheaths

Question 22

􏰁Mainly located in the plasma membrane displaying their sugar
components to the cell exterior

Answer 22

Glycospingolipids

Subtype of glycolipids that contain sugar lipids

Include galactosyl- and glucosyl-ceramides (cerebrosides) and the
gangliosides

Question 23

Important in cell membrane fluidity

Answer 23

Cholesterol

Intercalates among the phospholipids of the membrane
− Hydrophilic hydroxyl group at aqueous interface, while remainder stays buried within the lipid bilayer leaflet
􏰁 Not present in plants
􏰁 Can be separated and quantified via column, thin-layer, and gas-liquid
chromatography

Question 24

Majority of cholesterol resides in ____ but small amounts are found in ____

Answer 24

within plasma membrane; mitochondria, golgi complex, nuclear membrane

Question 25

Difference between Glycolipids and Glycosphingolipids

Answer 25

Glycosphingolipid - cermaide backbone; act to modulate membrane-protein function, as well as playing a role in cell-cell communication. Glycolipids - backbone is a sugar; serve a recognition function

Question 26

Major functional molecule of membranes

Answer 26

Proteins Consist of enzymes, pumps and transporters, channels, structural components, antigens and receptors) Interspersed along cell membrane, certain proteins can insinuate themselves − Membrane phospholipids act as a solvent for membrane proteins

Question 27

Type of membrane proteins that interact extensively with phospholipids and span trhough the entire thickness of the bilayer

Answer 27

Integral Membrane Proteins Mostly globular and amphipathic 􏰁 Asymmetrically distributed across the membrane bilayer − Usually they serve as channels or for anchorage of the cell

Question 28

Formation of integtral membrane protein that serves as a receptor for recognition

Answer 28

Alpha helix

Question 29

Formation of integtral membrane protein that sServe as enzymes, transporter, or receptors

Answer 29

Helical bundles

Question 30

Formation of integtral membrane protein that Serve as channels between extracellular and intracellular environment

Answer 30

Beta barrel

Question 31

Specialized areas of the explasmic or outer leafle that is hypothesized to be involved in signal transfuction

Answer 31

Lipid rafts − Enriched in cholesterol, sphingolipids, and certain proteins − Lateral diffusion causes phospholipids (which may carry ligands and receptors) to move to other area 􏰁 Lipid rafts are stabilized through interactions with the actin cytoskeleton

Question 32

May derive from lipid rafts

Answer 32

Caveolae The lipid rafts with ligands can invaginate and form caveolae which pinch and enter cell to where it is needed (usually the

Question 33

− Often located below the apical surfaces of epithelial cells − Prevent the diffusion of macromolecules between cells

Answer 33

Tight junctions − Composed of various proteins, including occludin, various claudins, and junctional adhesion molecules • Other specialized features: desmosomes, adherens junctions, and microvilli

Question 34

Passive Transporrt: Solutes move from ___ concentration to ____ concentration (up/down) electrochemical gradient)

Answer 34

high; low; down

Question 35

Type of passive transport where 􏰅 Solutes move down concentration gradient without the need for receptors or channels

Answer 35

Simple diffusion Limited by three factors: 􏰀 Thermal agitation of the specific molecule 􏰀 Concentration gradient across the membrane 􏰀 Solubility of the solute

Question 36

Type of passive transport where Materials travel down concentration gradient and have to pass a channel of integral proteins

Answer 36

Passive diffusion Ex. Aquaporins forming channels for water passage

Question 37

Type of passive transport that Involves either certain transporters or ion channels

Answer 37

Facilitated Diffusion 􏰅 Ligand must first activate the gated channel before it can move down the concentration gradient

Question 38

Ping Pong Mechanism

Answer 38

􏰀 In the “ping” state, it is exposed to high concentrations of solute, and molecules of the solute bind to specific sites on carrier protein. Binding induces a conformational change that exposes the carrier to a lower concentration of solute (“pong” state) (Harper’s, p. 469)

Question 39

􏰀 Transport against concentration gradient but does not need help past carrier 􏰀 Moves one type of molecule bidirectionally ligand must first activate the gated channel before it can move down the concentration gradient

Answer 39

Uniport Ex. Ex. GLUT transporter in RBC for glucose to enter the RBC

Question 40

Materials are moved against a concentration gradien

Answer 40

active transport

Question 41

Type of active transport that entails pumps and energy from ATP is needed

Answer 41

Primary Ex. Na+-K+-ATPase 􏰀 3 Sodium exits only if 2 ions of potassium are present 􏰀 Restores homeostasis

Question 42

type of active transport that Relies on concentration gradient generated by another system

Answer 42

Secondary (Cotransport)

Question 43

typeof active transport where Both substances must be present for carrier to be activated and transported to the same direction

Answer 43

Symport Ex. SGLT1 in intestines which needs glucose and sodium for carrier to transport both into the cell

Question 44

type of active transport where Both substances have to be present however they will be transported in an opposite manner

Answer 44

Antiport Ex. Sodium Calcium exchanger - during muscle contraction, calcium is released from muscle endoplasmic reticulum. To prevent calcium excess, exchanger brings calcium out of cell when sodium is delivered in

Question 45

Composed of transmamebrane protein subunits, most are highly selective for one ion; allow impermeable ions to cross membranes at rates approaching diffusion limits

Answer 45

Transporters

Question 46

Type of transporter permeases bind solute and undergo conformational changes, transferring solute across the membrane

Answer 46

Carriers 􏰁 Involved in passive and active transport 􏰁 Transport is significantly slower than via ion channels • Examples: Symporters, uniporters, and antiporters

Question 47

Ion channels can be activated

Answer 47

− Voltage-gated: 􏰅 Voltage change in exterior or interior via concentration changes of Na+, K+, Cl- − Ligand-gated: 􏰅 Ligand attached to receptor in or near ion channel can trigger opening − Mechanically-gated: 􏰅 Stress/physical stimuli ex. In the inner ear

Question 48

T/F: Ion channels can require activation (i.e. non-gated)

Answer 48

False: − Some channels are not gated − E.g. Aquaporins, K+ Leak Channels

Question 49

Process by which cells take up large molecules

Answer 49

Endocytosis Provides a mechanism for regulating the content of certain membrane components (e.g. hormones)

Question 50

Type of Endocytosis that ingests large particles such as viruses, bacteria, cells, or cellular debris

Answer 50

Phagocytosis - Plasma membrane engulfs solid particle and brings it to interior of cell as a phagosome − Phagosome contains part of the cell membrane and ingested materials - Occurs only in specialized cells such as macrophages and granulocytes

Question 51

− “Cell drinking” | − Cell membrane Invaginates water and other substances − Transport inside the cell via vesicles

Answer 51

Pinocytosis

Question 52

Uptake of a solute by formation of small vesicles is proportionate to its concentration in the surrounding extracellular fluid

Answer 52

Fluid-phase pinocytosis It’s a nonselective process (Harper’s, p. 1161

Question 53

aka Receptor-mediated endocytosis 􏰀 Primarily responsible for the uptake of specific macromolecules where there are binding sites on the plasma membrane

Answer 53

Absorptive pinocytosis Requires attachment of ligand on cell receptor surface Ligand on receptor moves through lipid rafts to clathrin-coated pit, the cell membrane then invaginates and endocytosis takes place 􏰀 Clathrin will travel where needed and when reached, clathrin is returned to the surface to await new complex 􏰀 Receptors and ligand separate where receptors are reused at the surface and ligands are transported to lysosome or golgi complex

Question 54

Process of releasing macromolecules outside of the cell 􏰁 Also involved in membrane remodeling

Answer 54

Exocytosis

Question 55

3 fates of molecules released by exocytosis

Answer 55

− They remain associated with the cell surface (membrane proteins) − They become part of the extracellular matrix (ex. collagen and glycosaminoglycans) − They can enter the extracellular fluid and signal other cells

Question 56

Structures that permit direct transfer of small molecules between adjacent cell membranes

Answer 56

Gap Junctions

Question 57

Connect cells that are within 2-4mm of each other

Answer 57

Connexins Connexins form connexons (6 connexin molecules make 1 hemiconnexon; 2 hemiconnexons make 1 connexon) 􏰅 One gap junction contains several connexons 􏰅 Open channels enable the diffusion of small solutes thus providing a direct mechanism of cell to cell communication

Question 58

Extracellular vesicles are generated by at least two distinct mechanisms

Answer 58

microvesicles and | exosomes

Question 59

Generated by budding from the plasma membrane of a source cell

Answer 59

Microvesicles

Question 60

Generated from the multivesicular body (MVB) | − Secreted from the source cell upon fusion of the MVB with the plasma membrane

Answer 60

Exosomes

Question 61

Microvesicles and exosomes ultimately fuse to their target cell to deliver a distinct “payload” T/F

Answer 61

True