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Flashcards in Chapter 4: Movement of molecules Deck (52):
1

Purposes of membranes

- membranes serve as boundaries between the cell interior and extracellular fluid (plasma membrane) and between organelle interiors and the cytosol
--> regulate what substances enter and leave cells/organelles, how much and how fast

2

Passive mechanisms of membrane transport

- don't require energy
- diffusion, osmosis, facilitated diffusion

3

active mechanisms of membrane transport

- require energy
- active transport, exocytosis, endocytosis
- exo and endocytosis are both bulk transport

4

Active transport and facilitated diffusion

- both require carrier molecules and are considered transport mechanisms

5

Diffusion

- movement of molecules from one location to another as a result of random thermal motion; movement of molecules from higher to lower concentration (decrease the concentration gradient)
- used to move substances in and out of capillaries and blood and cells
- most common process most likely
- may or may not occur across a semi-permeable membrane
- uniform concentration is eventually reached

6

Flux

- amount of material crossing a surface in a unit of time
- magnitude determined by concentration gradient

7

net flux

- the difference between two one-way fluxes
- the net amount of material transferred from one location to another
- net flux always occurs from regions of higher concentration to regions of lower concentration
- concentration difference determines the magnitude of net flux
- at a given concentration gradient, also influenced by temperature, mass, surface area, and medium

8

diffusion rate

affected by distance

9

diffusion time

square of the distance over which molecules diffuse

10

diffusion through lipid bilayer

- plasma membrane is semipermeable/differentially permeable
- some, not all, molecules can diffuse across the plasma membrane
- small, uncharged, lipid-soluble molecules diffuse easily across lipid bilayer (nonpolar)
- charged (polar) molecules have more difficulty diffusing across lipid bilayer...slowly or not at all
- macromolecules can't freely cross membrane

11

Polar molecules and membrane

- lower permeability and harder to cross membrane

12

Protein Channels

- used by ions because harder for them to pass because charge
- made of integral membrane proteins, either a single one or usually a protein aggregate
- small diameters
- ion selectivity

13

Electrochemical gradient

- intracellular fluid is usually negatively charged
- extracellular fluid usually positively charged
- influences ion movement
- direction and magnitude of ion fluxes across membranes depend on both the concentration difference and membrane potential (electrical difference)

14

membrane potential

- separation of electrical charge across the plasma membrane

15

regulation of diffusion through ion channels

channel gating
- ligand-gated channels
- voltage-gated channels
- mechanically-gated channels

16

ligand-gated channels

- open and close in response to a certain binding chemical ligate to protein of channel

17

voltage-gated channels

- open and close in response to membrane potential

18

mechanically-gated channels

- open and close due to the stretching of the membrane

19

Mediated transport mechanisms

- responsible for the transport of some ions, and for polar molecules such as amino acids and glucose that are too large to move through channels
- use transporters/carrier molecules (integral mem proteins, undergo conformational change, can move molecules in either direction across mem, chemical specificity)

20

how mediated transport mechanisms work

- the solute binds to a specific site on the transporter protein
- the protein changes shape, and the solute is released on the other side of the membrane

21

magnitude of solute flux determined by

- extent of saturation of transporter binding sites (solute concentration and affinity of transporters for the solute)
- number of transporters in membrane
- rate at which conformational change in the transport protein occurs
- finite number of tranporters in any mem for a given solute --> maximal flux (saturation)

22

mediated transport vs simple diffusion

- in mediated transport, the # of available transporters places an upper limit on the flux magnitude
- in simple diffusion, flux magnitude is limited only by the concentration gradient

23

2 types of mediated transport

- facilitated diffusion
- active transport (primary and secondary)

24

Facilitated diffusion

- uses carrier molecules/transporters to transport substances from areas of higher to lower concentration
- doesn't require ATP

25

active transport

- uses carrier molecules to transport substances from areas of low to high concentration (against gradient!)
- i.e. sodium/potassium pump
- primary active transport requires ATP
- secondary active transport uses an electrochemical gradient across a membrane to drive the process

26

NA/K pump

- maintain proper concentrations
- usually high sodium out and potassium in
- concentration moves sodium in and potassium out but need to work against gradient
- uses ATP

27

cotransport

- form of secondary active transport
- sodium and second solute cross membrane in same direction (symport)

28

countertransport

- form of secondary active transport
- sodium and second solute move in opposite directions across membrane (antiport)

29

Osmosis

- the diffusion of water through a semipermeable membrane (movement of water from high to low concentration)
- responsible for much of the movement of fluids
- as solute increases, the water decreases. Water concentration determines the direction of osmosis

30

aqua porins

- membrane proteins that create channels for water

31

osmolarity

- the total solute concentration of a solution
- 1 osmo = 1 mol of solute particles
- 1 M solution NaCl contains 2 osmol of solute per liter; osmolarity=2OSM

32

Osmotic pressure

- the pressure that must be applied to the solution to prevent the net flow of water into it
- attractive force to water
- the higher osmolarity, the higher the osmotic pressure, the more water will move into solution via osmosis
- a tendency of a solution to attract water to move into it via osmosis

33

extracellular osmolarity and cell volume

- non-penetrating solutes can't cross plasma membrane (Na+, Cl-, K+ behave as nonpenetrating solutes
- penetrating solutes cross plasma membrane
- at equilibrium, the intracellular and extracellular fluids have an osmolarity of ~300 mOsm

34

tonicity

- the concentration of nonpenetrating solutes in a solution relative to the concentration of those solutes within the cell

35

isotonic solution

- has same nonpenetrating solute concentration as the cell
- perfect concentration, stays the same

36

hypotonic solution

- has a lower nonpenetrating solute concentration than the cell
- cell swells and bursts

37

hypertonic solution

- has a higher nonpenetrating solute concentration than the cell
- cell gets skinny and shriveled and shrinks

38

isoosmotic, hypoosmotic, hyperosmotic

- osmolarity of a solution relative to that of the cell in regard to all solutes

39

Exocytosis

- moves larger molecules or groups of molecules OUT of the cell (bulk transport)
- uses vesicles which fuse with plasma mem
- requires energy ATP
- secretions ~ cells release subs that they generate into extracellular fluid

40

endocytosis

- moves larger molecules, whole cells, or groups of molecules INTO the cell
- requires ATP

41

3 mechanisms endocytosis may occus

- pinocytosis
- receptor-mediated
- phagocytosis

42

pinocytosis

- cell drinking
- invagination of the plasma mem forms a vesicle that encases a portion of the extracellular fluid and substances suspended in it
- nonselective

43

receptor-mediated endocytosis

- requires specific receptors
- specific and selective

44

phagocytosis

- cell eating
- cells engulf whole cells, pathogens, parts of cells, or large organic molecules

45

transcytosis

- endocytosis and exocytosis move a molecule across a cell
- capillary exchange of fatty acids, albumin, and hormones such as insulin

46

luminal/apical/mucosal membrane

lines the hollow organs

47

basolateral/serosal membranes

opposite the luminal mem, usually adjacent to blood

48

lateral membrances

face adjacent epithelial cells

49

epithelial transport pathways

- paracellular and transcellular

50

paracellular pathway

- diffusion between adjacent epithelial cells

51

transcellular pathway

- movement through an epithelial cell (involves movement of the substance across the luminal and basolateral mem)

52

epithelial cells

- form lining of hollow organs and tubes and therefore epithelial cells regulate absorption/secretion across those surfaces