Membrane Permeability and Transport across membranes

How permeable is the plasma membrane?

selectively permeable

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

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

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

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

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

state factors affecting membrane permeability

- concentration of organ solvent

- temperature

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

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

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

give examples of passive processes?

- diffusion

- facilitated diffusion (including co-transport)

- osmosis

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

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

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

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

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

state equation for rate of diffusion?

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

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-

where does facilitated diffusion happen?

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

what affects the rate of facilitated diffusion?

- The number & availability of these transport proteins

limit the rate of facilitated diffusion

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

What are channel proteins?

- Form pores lined with polar groups

- Hydrophilic channel allows water-soluble ions through the

membrane

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

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

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)

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

define active transport

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

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

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

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

what are examples of active transport?

- muscle contraction

- nerve impulse transmission

- reabsorption of glucose in the kidney

- mineral uptake into root hair cells

what is co-transport?

- Co-transport is a form of facilitated diffusion bringing

ions and molecules into cells together

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

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

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

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

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

what is water potential?

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

the tendency (potential) for water molecules to move

what are the units for water potential?

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

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

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

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

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

define hypotonic?

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

Hypertonic:

a solution with lower water potential

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

water by osmosis

Isotonic:

a solution with the same water potential as

the cell therefore there is no net water movement

what is isotonic solution for humans

0.9g NaCl /100 cm3

water and is known as saline

what is (haemo)lysis

The pressure of the cytoplasm on the membrane becomes too

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

crenated meaning

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

do animal cells have a pressure potential?

Animal cells have no cell wall so pressure potential

does not have to be considered

what is the water potential in animal cells?

The water potential is the same a the solute potential

ψ = ψs

turgid meaning

Cell stiffens but generally retains its shape....

Plasmolysis meaning

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

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

what is pressure potential in plant cells?

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

outwards so is a positive value

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

equation for water potential

ψ = ψp + ψs

Incipient plasmolysis:

Cytoplasm is just beginning to pull the membrane away from

the cell wall

Ψp = 0

ψcell = ψs

The bathing solution is isotonic

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

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

What is exotyosis?

Exocytosis is the export of macromolecules from the

cell

what is endocytosis + types?

Endocytosis is the import of macromolecules

Phagocytosis

Pinocytosis

what is phagocytosis?

ingestion of solid molecules

Pinocytosis

ingestion of liquids and solutes

what is role of exocytosis ?

A method of secretion (useful substances) or excretion

(unwanted or waste substances) of molecules from cells

explain exocytosis

Vesicle membrane fuses with plasma membrane;

contents of the vesicle are deposited outside the cell

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

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:

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

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

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

explain pinocytosis

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

E.g.large solutes such as proteins