Membrane Permeability and Transport across membranes

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72 Terms

1

How permeable is the plasma membrane?

selectively permeable

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2

what is meant by the plasma membrane being selectively permeable?

- they are permeable to water molecules and some other small molecules, but not to larger molecules

- a barrier to some molecules but lets others across

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3

what affects the rates that substances are transported across a plasma membrane?

- different substances cross different membranes at different rates

- determined by different transport proteins

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4

state substances that can be transported across the membrane

- lipid soluble substances eg vitamin A

- small molecules eg O2 and CO2 dissolve in the phospholipid and diffuse across the membrane

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5

explain why certain substances can move through the cell membrane? (lipid-soluble molecules)

- phospholipid layer is hydrophobic so lipid-soluble molecules move through the cell membrane more easily than water-soluble substances

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6

Explain why water-soluble substances don't move through cell membranes easily?

- polar molecules and ions cannot readily diffuse through the phospholipids and must pass through intrinsic protein molecules, which form water-filled channels across the membrane

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7

state factors affecting membrane permeability

- concentration of organ solvent

- temperature

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8

explain the effect of increasing the concentration of organic solvent on membrane permeability?

- phospholipids dissolve in organic solvents eg ethanol, increasing the concentration increases the permeability of the membrane because more phospholipids will dissolve

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9

Explain the effect of increasing the temperature on the permeability of cell membranes?

- temperature will affect the fluidity of the membrane as it will increase the kinetic energy and so will increase permeability

- temperature will also will also denature the proteins causing an irreversible change to the structure of phospholipids bilayer (at high temps) and proteins will no longer function

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10

what is the differences between passive processes and active processes?

- passive processes require no input of energy by the cell and are due to the random movement of particles

- active processes require input of energy by the cell

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11

give examples of passive processes?

- diffusion

- facilitated diffusion (including co-transport)

- osmosis

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12

define diffusion

- diffusion is the passive net movement of molecules or ions from a region of higher concentration to a region of lower concentration

- movement is down a concentration gradient and molecules are equally distributed

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13

Which molecules are transported across the membrane by simple diffusion - explain

- lipid-soluble, non-polar molecules : membrane is fully permeable to them

- water is a very small polar molecule and is able to diffuse (slowly) across the membrane as small transient gaps open due to the fluidity

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14

how will increasing temperature affect the rate of diffusion of water across the membrane?

- increasing temperature will increase KE increasing the rate of diffusion

- also increases transient gaps so diffusion is more frequent

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15

what are factors that affect the rate of diffusion?

- concentration gradient: the greater the difference in concentration in 2 areas, the more molecules diffuse in a given time

- thickness of the exchange surface or over which diffusion takes place: the thinner the membrane or the shorter the distance, the more molecules diffuse in a given time

- the SA of the membrane: the larger the are, the more molecules have space to diffuse across in a given time

- temperature; increases KE of molecules

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16

what are factors of the diffusing molecules that affect the rate of diffusion?

- size of diffusing molecule: smaller ones dissolve faster

- nature of molecule: lipid-soluble and non-polar molecules diffuse faster than water-soluble and polar ones

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17

state equation for rate of diffusion?

rate of diffusion = (surface area x difference in concentration)/ (length of diffusion pathway)

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18

what is facilitated diffusion

- Passive transfer of molecules or ions down a concentration gradient across a membrane via intrinsic membrane transport proteins

- Provides a way for polar molecules & ions (insoluble in phospholipid bilayer) to cross the membrane

large polar molecules e.g. glucose & amino acids

charged ions e.g. Na+, Cl-

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19

where does facilitated diffusion happen?

- Occurs at specific sites in the membrane where the transport proteins are present

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20

what affects the rate of facilitated diffusion?

- The number & availability of these transport proteins

limit the rate of facilitated diffusion

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21

explain the role of transport proteins?

- Different cells have different transport proteins

- The shape and charge of the transport protein only allow the passage of specific molecules or ions

- Cells can vary the number and type of transport proteins in the membrane at any time so control which substances pass in and out of the cell

- The density of the proteins is the effective surface

area for diffusion

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22

What are channel proteins?

- Form pores lined with polar groups

- Hydrophilic channel allows water-soluble ions through the

membrane

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23

explain how gated channels work?

- Gated channels open & close depending on cell requirements and usually require a signal molecule to bind in order to change shape for the 'gate' to open

- The proteins forming these channels can change conformation (shape) in response to signals

- This allows them to be opened and closed

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24

what are carrier proteins and what is their role?

- Allow diffusion of larger molecules e.g. glucose, amino acids

- The appropriate molecule attaches to the binding site on the carrier protein causing it to change shape and release molecule on the other side of the membrane

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25

Describe and explain the differences in the effect of concentration on the rate of passive and facilitated diffusion

- passive diffusion: steady gradient , proportional

- facilitated diffusion: increases less steeply, then plateaus as concentration gradient is not a limiting factor anymore (number of transport proteins now are)

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26

Why might raising the temperature above a certain temperature e.g. 45oC have greater effect on facilitated diffusion than on simple diffusion?

- high temps will denature transport proteins so rate of facilitated diffusion will decrease

- increases KE so simple diffusion will increase

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27

define active transport

- Requires energy from ATP (made by respiration) to move molecules and ions across the membrane against their concentration gradient

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28

what affects the rate of active transport?

- The rate of active transport will depend on the rate of

respiration

- Factors slowing, stopping or inhibiting respiration will slow or stop active transport

- Low temperature or lack of oxygen e.g. in water-logged soil

- Respiratory inhibitors e.g. cyanide which inhibits aerobic respiration

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29

explain how active transport works, what is required?

- Moves solutes against the concentration gradient

- Requires specific intrinsic carrier proteins

- Molecule/ion binds to specific carrier

- protein on outside of membrane ATP transfers a phosphate group to the carrier protein on the inside of membrane

- The carrier protein changes shape and carries the molecule/ion across the membrane into the cell where it is

released

- The phosphate ion is released from the carrier which returns to its original shape

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30

what affects the rate of active transport?

- The rate of active transport also is limited by the

number and availability of carrier proteins

- E.g. Na+ /K+ pump which is vital in the transmission of

nerve impulse

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31

what are examples of active transport?

- muscle contraction

- nerve impulse transmission

- reabsorption of glucose in the kidney

- mineral uptake into root hair cells

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32

what is co-transport?

- Co-transport is a form of facilitated diffusion bringing

ions and molecules into cells together

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33

explain how glucose is transported into the cell?

- Glucose is transported into the cell against a concentration gradient via a carrier protein with

- Na+ ions that are transported down their concentration gradient

- Glucose & Na+ diffuse through the cell to the opposite membrane

- Glucose passes out of cell by facilitated diffusion and Na+ by active transport

- The Na+ concentration gradient is maintained by Na+/K+

pump

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34

why is the transport of glucose important?

- The sodium-glucose co-transport is important for

the absorption of glucose in the ileum and in the

proximal convoluted tubule of the kidney

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35

Why might this be considered to be active transport of glucose rather than facilitated diffusion?

- no energy used BUT energy needed for Na/K+ which needs to happen for glucose to enter cell

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36

what is osmosis?

Osmosis is the diffusion of water molecules, from a region of higher water potential to a region of lower water potential, through a selectively permeable membrane

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37

define a selectively permeable membrane in terms of osmosis?

Selectively Permeable Membrane: different molecules

cross the membrane at different rates; water is

assumed to cross freely, although in reality the majority

passes through proteins called aquaporins

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38

what is water potential?

- is a measure of the free energy of water molecules and is

the tendency (potential) for water molecules to move

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39

what are the units for water potential?

has the symbol ψ (psi) and is measured in kilopascals (kPa)

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40

what is the water potential in pure water?

- Pure water has a water potential of zero (0 kPa) as there is

no tendency of water to move into pure water

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41

describe and explain what affects water potential of solutions

- The addition of a solute to pure water tends to bring water

molecules in and so lowers the water potential and gives it

a negative value

- In a solution, water molecules are weakly bound to the

solute so fewer are free to move so the system has a lower

potential energy therefore exerting a pulling force on

external water molecules with a higher potential energy =

water potential

- The more concentrated the solution, the greater the pull

and so the greater/more negative water potential

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42

What is solute potential?

- All solutions have a negative water potential which is

called the solute potential (ψs)

- Solute potential measures how easily water molecules move out of a solution

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43

what affects solute potential?

- The higher the solute concentration, the lower the ψs and the greater the tendency to draw in water / the lower the tendency for water to move out

- The effect of adding different solutes is cumulative

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44

define hypotonic?

- a solution with higher water potential (less negative) than the cell will cause the cell to gain water by osmosis

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45

Hypertonic:

a solution with lower water potential

(more negative) than the cell will cause the cell to lose

water by osmosis

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46

Isotonic:

a solution with the same water potential as

the cell therefore there is no net water movement

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47

what is isotonic solution for humans

0.9g NaCl /100 cm3

water and is known as saline

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48

what is (haemo)lysis

The pressure of the cytoplasm on the membrane becomes too

high for the membrane to withstand .... cells swell and burst =

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49

crenated meaning

Water moves out of the cytoplasm .... cells shrink and shrivel

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50

do animal cells have a pressure potential?

Animal cells have no cell wall so pressure potential

does not have to be considered

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51

what is the water potential in animal cells?

The water potential is the same a the solute potential

ψ = ψs

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52

turgid meaning

Cell stiffens but generally retains its shape....

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53

Plasmolysis meaning

Cytoplasm and cell contents shrink and the cell membrane pulls away from the cell wall....

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54

explain the role of osmosis in plant cells

- The presence of a cell wall is an extra factor concerning the movement of water into and out of plant cells

- Water entering by osmosis expands the vacuole and pushes the cytoplasm against the cell wall which can only expand a little so outward pressure increases, resisting the entry of more water, making the cell turgid

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55

what is pressure potential in plant cells?

This pressure is the pressure potential (ψp) and it pushes

outwards so is a positive value

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56

what is water potential determined by in plant cells?

- determined by the sum of two opposing forces:

- Solute potential (ψs) due to the solutes in the cell sap and cytosol pulling water in - which is always negative

- Pressure potential (ψp) of the cell wall pushing against the membrane - which is positive

- The balance of these determines the water potential of the cell and whether water moves in or out

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57

equation for water potential

ψ = ψp + ψs

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58

Incipient plasmolysis:

Cytoplasm is just beginning to pull the membrane away from

the cell wall

Ψp = 0

ψcell = ψs

The bathing solution is isotonic

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59

what is bulk transport

is the movement of materials in bulk across the membrane (not the movement of individual molecules or ions)

includes:

exocytosis which increases the overall area of the cell

membrane

endocytosis which reduces the overall area of the cell

membrane

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60

what does bulk transport need

- relies on the fluidity of the cell membrane which allows

materials to be taken into cells by endocytosis or released

by exocytosis

- is an active process using ATP as the membrane has to

change shape

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61

What is exotyosis?

Exocytosis is the export of macromolecules from the

cell

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62

what is endocytosis + types?

Endocytosis is the import of macromolecules

Phagocytosis

Pinocytosis

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63

what is phagocytosis?

ingestion of solid molecules

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64

Pinocytosis

ingestion of liquids and solutes

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65

what is role of exocytosis ?

A method of secretion (useful substances) or excretion

(unwanted or waste substances) of molecules from cells

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66

explain exocytosis

Vesicle membrane fuses with plasma membrane;

contents of the vesicle are deposited outside the cell

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67

what is exocytosis used for

Exocytosis is used to release:

digestive enzymes from secretory cells

excess water from unicellular aquatic organisms e.g.

Paramecium via contractile vacuole

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68

What are vesicles?

- Small, usually spherical sac of membrane that are normally present in eukaryotic cells

- Dynamic feature in cells - constructed/ moved around/

deconstructed

- Move materials into, around or to the surface of cells:

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69

explain endocytosis?

- A process to engulf material by extensions of the plasma membrane and cytoplasm to surround it

- Forms a vesicle containing material that was outside the cell

- The membrane invaginates (folds in) taking in a small amount of the extracellular fluid and its contents

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70

what is endocytosis used for in body

Endocytosis is used to take in:

antibodies from mothers blood into foetus

food particles e.g. Amoeba & Paramecium

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71

explain phagocytosis?

uptake of solid material that is too large to be taken in by diffusion or osmosis

E.g. when granulocytes engulf bacteria, a lysosome

fuses with the vesicle formed and enzymes digest the

cells

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72

explain pinocytosis

uptake of liquid by the same mechanism, although vesicles are much smaller

E.g.large solutes such as proteins

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