3. Membrane Transport - mendozascience.com · A. Plasma Membrane - The plasma membrane surrounds the cell and functions as an interface between the living interior ... Transport Across

Transport Across the Cell Membrane 11/5/07

Cell Biology 1 Transport Across the Cell Membrane

"The difference between the internal and external chemical

composition of a cell represents a degree of order, that

can be maintained only by a barrier to free movement

into and out of the cell.

And since cells must also assimilate matter from their

external environment, to grow and reproduce, they must

be able to selectively allow certain molecules and ions

across this barrier, often against concentration gradients,

while restricting and excluding others."

A. Plasma Membrane

- The plasma membrane surrounds the cell and

functions as an interface between the living interior of

the cell and the nonliving exterior.

- All cells have one.

- It regulates the movement of molecules into and out of

the cell.

Membrane Structure

- The membranes of a cell are phospholipid bilayers

that contain numerous proteins embedded within them.

Some of the proteins extend all the way through the

membrane; others do not.

Source: http://en.wikipedia.org/wiki/Cell_membrane

Phospholipids

- Most of the lipids in a membrane are phospholipids.

Notes

Transport Across the Cell Membrane


- Phospholipids contain glycerol, two fatty acids, and a

phosphate group. The phosphate group is polar

(hydrophilic), enabling it to interact with water. The

fatty acid tails are nonpolar (hydrophobic) and do not

interact with water.

Phospholipid Bilayers

- Phospholipids spontaneously form a bilayer in a

watery environment. They arrange themselves so that

the polar heads are oriented toward the water and the

fatty acid tails are oriented toward the inside of the

bilayer (see the diagram below).

- In general, nonpolar molecules do not interact with

polar molecules. This can be seen when oil (nonpolar)

is mixed with water (polar). Polar molecules interact

with other polar molecules and ions. For example table

salt (ionic) dissolves in water (polar).

- The bilayer arrangement shown below enables the

nonpolar fatty acid tails to remain together, avoiding

the water. The polar phosphate groups are oriented

toward the water.

Flexibility

- The fatty acid tails are flexible, causing the lipid

bilayer to be fluid. This makes the cells flexible. At

body temperature, membranes are a liquid similar to

cooking oil in consistency.

Cholesterol



- In animals, cholesterol is a major membrane lipid. It

may be equal in amount to phospholipids.

- It is similar to phospholipids in that it one end is

hydrophilic, the other end is hydrophobic.

- Cholesterol makes the membrane less permeable to

most biological molecules. Source:

http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20100/Bio%20100%20Lectures/Membranes/membrane.htm

Fluid Mosaic Model

- a model of the plasma membrane

- based on the changing location and pattern of

protein molecules in a fluid phospholipids

bilayer

- composed of a mosaic of lipid, protein and

carbohydrate biomolecules.

Parts

Glycolipids (gl)

- are carbohydrate-attached lipids. Their

role is to provide energy and also serve as markers for cellular recognition.

- also to attach cells to form tissues

Glycoproteins

(gp)

- a group of extracellular protein-

carbohydrate compounds, e.g., - mucins (mucous), protective coating

holding in moisture, and preventing proteolysis (digestion by proteases), e.g, in the stomach

- are important for immune cell recognition - antibodies (immunoglobins, not

actually part of the cell membrane) - major histocompatibility complex (or

MHC)



- bind tissues - also include hormones such as,

- Human Chorionic Gonadotropin - Follicle stimulating hormone

- Luteinizing hormone - Thyroid stimulating hormone

Lipoproteins

(lp)

- a combination of fat (cholesterol) and protein that transports lipids, such as cholesterol,in the blood.

- HDL (high density lipoprotein), the good

cholesterols

- LDL (ow density lipoprotein), the bad cholesterols

Carrier Protein

(cp)

- an integral protein that acts as a gateway for water, Na

+ and the like

o e.g., aquaporins, Na+ / K

+

pump, glucose carrier

- Carrier Protein - an integral membrane proteins that bind to a

"substrate" and transport it across the membrane

- aka Carrier Molecule

Channel

Protein (cp)

- an integral membrane protein that acts as “pore”

B. Membrane Transport Processes

1. Diffusion

- the migration of molecules or ions as a result of

their own random movements, from a region of

higher concentration to a region of lower

concentration

2. Osmosis

- is the movement of water through a semi-

permeable membrane (analogy a colander or

sieve)

- movement of water (at constant temp. and

pressure) is from the solution with lower

concentration of solutes to the solution of

higher concentration of solutes (or from the

more pure water to less pure water)

Hi conc. of water —water Low conc. of water

Low conc. of solute —water Hi conc. of solute

99% water

1% solute

90% water

10% solute

The movement of water

follows the concentration

gradient from high

concentration of water to low concentration of water



source: http://wilkes-fs1.wilkes.edu/~terzaghi/BIO-226/lectures/13.html

- the more dissolved particles in solution, the

greater the tendency of water to move into it

and the higher the osmotic pressure

Source: http://www.agen.ufl.edu/~chyn/age2062/lect/lect_06/lect_06.htm

NB. When doing such questions on the Gov’t, check to see

which solutes are actually permeable to the membrane—it

makes a difference

3. Tonicity

- Tonicity refers to the relative concentration of solute

on either side of a membrane

Hypertonic Hypotonic Isotonic



Osmotic Effects

i. Hypertonic Medium

- such a medium is much richer in solutes than the

cell (hypotonic cell)

- as such osmotic flow is outward from the cell

- the cell shrinks as it loses water

- plasmolysis (“fluid breaks”)

e.g., sea water (very salty water)

ii. Hypotonic Medium

- such a medium is poorer in solutes than the cell

(hypertonic cell)

- as such osmotic flow forces water into the cell

- the cell swells as it gains water

- cytolysis (“cell breaks”)

e.g., fresh water

[mnemonic: think hypodermic needle - injects water

into]

iii. Isotonic Medium

- such a medium is in equilibrium with the cell

- neither gaining nor losing water

- the cell is iso-osmotic to the medium

- has the same osmolarity as the cell

- e.g., blood (salty water)

The above diagram shows the movement of water

into or out of the cell.

The example below is a red blood cell (RBC)

Hyperosmotic or Hypertonic

Extracellular Fluid ["hyper" = above, excessive] - has HIGHER osmolarity than

cell; higher osmotic pressure - solute concentration outside

cell > solute conc. inside cell - solution has less H2O & thus

has greater tendency to GAIN it

- cell loses water; shrinks or crenate; water moves into solution

Hypo-osmotic or Hypotonic

Extracellular Fluid ['hypo" = beneath] - has LOWER osmolarity than

cell; lower osmotic pressure - solute concentration outside cell

< solute conc. inside cell

- solution has more water & thus has greater tendency to LOSE it

- cell gains water, may cause lysis; water enters the cell

Osmotic concentration = concentration of all solutes Osmotic pressure = force required to stop osmosis

- Solution with higher osmotic

concentration is hyperosmotic

- Solution with lower osmotic concentration is hypoosmotic

- Solutions with equal osmotic

concentrations are isosmotic

- Cells shrink when hypoosmotic to environment

- Cells swell when hyperosmotic to environment

- Cells without walls want to be

isosmotic with environment



Source: http://www.agen.ufl.edu/~chyn/age2062/lect/lect_06/4_11.GIF

RBC in Isotonic

Solution RBC in

Hypertonic Solution

RBC in Hypotonic

Solution “crenation” “hemolysis”

Some more vocab (just to confuse you some more)

- A Mole of a substance may be defined as the molecular

weight of that substance expressed in grams. o The molecular weight (m wt) of glucose is 180 so

180g of glucose = 1mole. The m wt of NaCl is about 58 so 58g of NaCl = 1mole. If 58g of NaCl are placed into a beaker and water added to a volume of

1l then the result will be a 1 molar solution. Which may be shown as 1mol/l or 1mol l-1 or 1M.

o Moles are used as a mass unit by biologists (and

Chemists) in preference to grams because a mole of any substance contains the same number of particles as a mole of any other substance.

o This greatly simplifies calculations that deal with chemical reactions (including biologically relevant reactions such as buffering) and osmotic effects.

o The number of particles, atoms or molecules in a

mole of any substance is given by Avagadro's number which is about 602204500000000000000000 or 6 x 10

23



- Osmolarity is a measure of the osmotic pressure exerted by a solution across a perfect semi-permeable membrane (one which allows free passage of water and completely prevents movement of solute) compared to pure water.

o Osmolarity is dependent on the number of particles in solution but independent of the nature of the particles.

o For example, 1 mole of glucose dissolved in 1 litre of water has an osmolarity of 1 osmole (osm) /l. If 1 mole of another sugar, such as sucrose were added to the same litre of water, the osmolarity would be 2

osm/l. o It doesn't matter that the solution contains 1 mole of

glucose and 1 mole of sucrose. o If 1 mole of NaCl were dissolved in 1 litre of water it

would produce a 1 mol/l NaCl solution with an

osmolarity 2 osm/l because NaCl dissociates into Na+ and Cl- (two particles) in solution. This is true

of all compounds that dissociate in solution. Na2SO4, which dissociates into Na

+, Na

+ and SO4

2-, to give 3

particles per molecule produces 3 osm/l for every mole dissolved in 1 litre.

o If two solutions contain the same number of particles

they may be said to be iso-osmotic (isosmotic) with

respect to each other. o If one solution has a greater osmolarity than another

solution it is hyperosmotic with respect to the weaker solution. If one solution has a lower osmolarity than another solution then it is hypo-osmotic (hyposmotic) with respect to the stronger solution. Iso, hyper and hypo osmolarity should always be

stated with respect to another solution. For example, a 1 mol/l NaCl solution is hyperosmotic with respect to 1 mol/l glucose solution.

- Because there are more Na+ and Cl-

particles relative to glucose particles

- Tonicity is nearly the same as osmolarity.

o For substance that cannot cross cell membranes, tonicity is practically identical to osmolarity.

o Tonicity is a measure of the osmotic pressure that a substance can exert across a cell membrane, compared to blood plasma. Plasma has an osmolarity of about 0.3 osm/l, therefore a 0.15 mol/l NaCl solution may be said to be isotonic with plasma (Assuming that neither Na+ nor Cl- can cross cell membranes, which is nearly true).

If a substance can cross a plasma membrane, then it cannot exert an osmotic pressure across that membrane. The solute will equilibrate across the membrane instead of forcing water to move. Urea behaves like

this, so a 0.3 mol/l urea solution may be said to be iso-osmotic with plasma but it is not isotonic.

Source : Pete Smith



(http://www.liv.ac.uk/~petesmif/teaching/1bds_mb/notes/common/osmo.htm)

4. Membranes are Differentially Permeable

- The plasma membrane is differentially or

selectively permeable because some particles can

pass through, others cannot.

o It can control the extent to which certain

substances pass through.

- Nonpolar molecules pass through cell membranes

more readily than polar molecules because the

center of the lipid bilayer (the fatty acid tails) is

nonpolar and does not readily interact with polar

molecules.

The following substances can pass through the cell

membrane:

- Nonpolar molecules (example: lipids)

- Small polar molecules such as water

o Via porins, specialized protein channels

Porins are proteins which cross the cellular membrane and

act as a pore through which molecules can diffuse.

- Unlike other membrane transport proteins, porins are

large enough to allow passive diffusion - i.e. they act

as channels.

- Porins are composed of beta sheets (2° proteins)

these form a cylindrical tube, called a beta barrel.

- The amino acid composition of the porin beta

sheets is unique in that polar and nonpolar

residues alternate along them.

- This means the nonpolar residues face outwards so

as to interact with the nonpolar lipid membrane,

whilst the polar residues face inwards into the

centre of the beta barrel to interact with the

aqueous channel.

- And water can pass through

The following substances cannot pass through a cell

membrane without help:

- Ions and charged molecules (example: salts

dissolved in water) – via porins/channels/carrier

The lipid bilayer is permeable to small, uncharged (nonpolar), molecules like oxygen (O2), carbon dioxide (CO2 (these diffuse freely in and out of the cell), and ), other lipids.

Lipid bilayers are not permeable to charged particles and really big particles: - Water (polar) - ions such as

- K+, Na

+, Ca

2+ (called

cations because when subjected to an electric

field they migrate toward the cathode [the negatively-charged electrode])

- Cl-, HCO3- (called anions

because they migrate toward the anode [the positively-charged electrode])

- small hydrophilic molecules

like glucose - macromolecules like proteins

and RNA Note though that the plasma membrane (cf fluid mosaic model) is permeable to water and

other small polar molecules because of porin channels, openings in the phospholipids bilayer.



proteins

- Large polar molecules (example: glucose)

- Macromolecules – via endo/exocytosis Source:

http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20100/Bio%20100%20Lectures/Membranes/membrane.htm

5. Facilitated Transport/Diffusion

- the transport of molecules across a cellular

membrane thru specific protein channels / carrier

molecule (facilitating pathway) from a region of

high conc. to a region of low conc.

- process is driven by conc. differences and does

not require energy

- chief difference from “free” diffusion is that the

membrane is impermeable to the molecule except

for passage thru the carrier channels,

- e.g., the cell membrane, composed of the

phospholipids is impermeable to the passage of

water…

6. “Active Transport” - Against the Gradient

- the other half of Facilitated Transport

- the pumping of molecules or ions across a cellular

membrane thru a carrier protein

- from a region of lower conc. to one of higher

conc.

- therefore against the “current” or concentration

gradient

- such a process requires energy

- = Active transport

source: http://www.accessexcellence.org/AB/GG/

Water and lipids are the two major types of solvent in the body. The lipid cell membrane separates the intracellular fluid from the extracellular fluid. Substances which are water soluble typically do

not cross lipid membranes easily unless specific transport mechanisms are present. It might be expected that water would likewise not cross cell membranes easily. Indeed, in artificial lipid bilayers, water does not cross easily and this

is consistent with our expectation…

but paradoxically, water crosses

nearly all the membranes in the

body with ease!

How? Via aquaporins

These aquaporin proteins form complexes that span the membrane and water moves through these channels passively in response to osmotic gradients. These channel proteins are present in highest concentrations in tissues where

rapid transmembrane water movement is important (e.g., in renal tubules). Source: http://wilkes-fs1.wilkes.edu/~terzaghi/BIO-226/lectures/13.html

Carrier proteins – bind to a substrate to assist it thru the lipid bilayer - facilitated and active

transport Channel proteins – are “pores” through which a particle simply

glides thru - facilitated diffusion



C. Bulk (Vesicular) Transport

- also a form of active (ATP using) transport

1. Endocytosis

- material is engulfed by the plasma membrane and

deposited in the cytoplasm in pockets that are

“pinched off” as vesicles toward the interior of the

cell

- the vesicle is then digested by the cell

- two variations on this theme:

i. Pinocytosis

- Greek “drink cell”s

- cell takes in dissolved material

ii. Phagocytosis

- Greek “eat cell”

- cell takes in solids

- basically large particulate matter is tightly enclosed

by the membrane bound arm of cytoplasm, and

most extracellular fluid is excluded, e.g., amoebae

eating a paramecium

2. Exocytosis

- like endocytosis only the vesicle is not digested,

but transported to and fused with, another part of

the plasma membrane

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3. Membrane Transport - mendozascience.com · A. Plasma Membrane - The plasma membrane surrounds the cell and functions as an interface between the living interior ... Transport Across