Cell Membrane Structure

&

Transport across Cell Membrane

Lecture 2- foundation

Prof. Hisham Al-Matubsi

Outline

Plasma Membrane

Extracellular Environment

Movement Across Plasma Membrane

Osmosis

Membrane Transport Systems

Is basic unit of structure & function in body Is highly organized molecular factory Has 3 main components: plasma membrane,

cytoplasm & organelles

Surrounds & gives cell form, is selectively permeable

It is thin, can be seen by electron but not light microscope.

Separates cell from extracellular fluid.

Exchange of material takes place across plasma membrane.

Maintains difference in ion concentration between interior & exterior of cell.

These ionic difference (ions have charge) are important for electrical activity of cell.

Cells have membrane potential (M.P. i.e difference in

changes between in & out of the cell)

Cells have negative charge inside.

Nerve & Muscle cell have ability to change their M.P. upon stimulation.

Plasma membrane has mostly lipids & protein plus small amount of -CHO.

Formed by a double layer of phospholipids Which restricts passage of polar compounds

Lipid layer (mostly phospholipids & little cholesterol) is not rigid but flexible & responsible for fluid nature of cell membrane.

Phospholipids forms basic structure of membrane, it has hydrophilic head (polar = water liking) outside & hydrophobic tail (non-polar =water fearing) inside.

Proteins customize membrane Provide structural support Different protein in plasma membrane have

different specialized functions. Serve as transporters (ion channel; e.g. Na+, K+,

Ca++., carrier), enzymes, receptors, cell adhesion molecules (CAMs), & with carbohydrate are identity markers.

Carbohydrates in form of glycoproteins & glycolipids are part of outer surface ◦ Impart negative charge to surface

Includes all constituents of body outside cells

67% of total body H2O is inside cells (=intracellular compartment); 33% is outside cells (=extracellular compartment-ECF) ◦ 20% of ECF is blood plasma

◦ 80% of ECF is interstitial fluid contained in gel-like matrix

Non polar(lipid soluble): can easily pass through membrane e.g. steroid hormones

Small molecules that have polar covalent bond but

uncharged (e.g., H2O , urea, CO2 ) are able to penetrate phospholipid bi-layer, diffusion occur when gradient concentration exist

Large polar molecules (glucose) can not pass, require

specific carrier protein Inorganic charged molecules (Na+, K+) can not pass

through; require ion channel to permit passage of these ions

Large molecules (proteins, nucleotides) can not pass

through; require specific process of endocytosis and exocytosis (bulk transport)

Plasma membrane is selectively permeable-allows only certain kinds of molecules to pass

Many important molecules have transporters & channels

Do molecules required to pass need a carrier

◦ Carrier-mediated transport involves specific protein

transporters such as facilitated diffusion & active transport

◦ Non-carrier mediated transport occurs by diffusion, osmosis

Passive transport moves compounds down concentration gradient; requires no energy (i.e. cellular energy)

Active transport moves compounds up a concentration gradient; requires energy & transporters

Is random motion of molecules

If two solutions separated by permeable membrane, movement will take place from high concentration to low concentration until equilibrium. i.e Net movement is from region of high to low

concentration (down its concentration gradient)

Passive transport; No energy is required

Example: O2 & CO2 transferred across lung by diffusion.

Rate of diffusion (Fick’s law) depends on: ◦ Magnitude of its concentration gradient

◦ Lipid solubility

◦ Surface area of membrane

◦ Thickness of membrane

◦ Molecular weight of substance

Factor Effect on rate of net diffusion ↑Conc. Gradient of substance ↑

↑ Surface area of membrane ↑

↑ Lipid solubility ↑

↑ M.Wt. of substance ↓

↑ Distance ↓

Diffusion

Slide number: 1

Solute

Solvent

Diffusion Slide number: 2

Solute

Solvent

Diffusion Slide number: 3

Diffusion Slide number: 4

Is net diffusion of H2O across a selectively permeable membrane ◦ H2O diffuses down its

concentration gradient ◦ H2O is less concentrated

where there are more solutes

Solutes have to be osmotically active (induces osmosis of water across a membrane)

i.e. membrane is impermeable to substance

Fig 6.5

H2O diffuses down its concentration gradient until its concentration is equal on both sides of membrane

Some cells have water channels (aquaporins) to facilitate osmosis

Fig 6.6

6-14

Is force that would have to be exerted to stop osmosis (i.e stop volume change)

◦ Indicates how strongly H2O wants to diffuse

Is proportional to solute concentration

Water tend to move by osmosis into glucose solution, thus creating hydrostatic

pressure that will push the membrane to the left and expand the volume

of glucose solution.

The amount of pressure that must applied to just counteract this change

is equal to the osmotic pressure of glucose solution

1 molar solution (1.0 M) = 1mole of solute dissolved in 1L of solution (i.e. up to1L) ◦ Doesn't specify exact amount of H2O ◦ Glucose (C6H12O6) Mwt= 180; and Saline NaCl Mwt=

58.5 (Mwt= sum of atomic weight). i.e NaCl need more water to make 1L compare to glucose (need less water)

Since the exact ratio of solute to water is critical in osmosis we use molality

1 molal solution (1.0 m) = 1 mole of solute dissolved in 1 kg H2O ◦ e.g 180 g of glucose + 180 g of fructose dissolved in

Kg of water Osmotic pressure=360 g/l

◦ 1m of glucose + 1m of fructose = 2 Osmol/L= 2 Osm

Osmolality (Osm) is total molality of a solution ◦ E.g. 1.0m of NaCl yields a 2 Osm solution

Because NaCl dissociates into Na+ + Cl-

Osmolality (Osm) is total molality of a solution ◦ E.g. 1.0m of NaCl

yields a 2 Osm solution

◦ Because NaCl dissociates into Na+ & Cl-

Measurement of Osmolality

1 mole of solute/L depress freezing point of water by –1.86 oC

1m of glucose freeze at temp. of –1.86 oC

1m of NaCl freeze at temp. of –1.86 x2 = - 3.72 oC

Plasma freeze at –0.5 oC Osm= -0.5/-

1.86 = 0.3 Osm

This is equivalent to 0.3 m of glucose (5%) and

to 0.15 m of NaCl (0.9%= normal saline)

0.3 m glucose which is = 0.3 Osm & 0.15 m saline which is = 0.3 Osm, both have same osmolality & osmotic pressure as plasma

Isosmotic solutions have same osmolality as plasma Tonocity means concentration of extracellular fluid. i.e if isosomatic glucose is separated from plasma

by a membrane permeable to water, but not for glucose, osmosis will not occur → the solution is isotonic to plasma

Isotonic slns have same osmotic pressure;

extracellular fluid (ECF) is isotonic to intracellular fluid (ICF).

Isotonic slns have same osmotic pressure ◦ A solution may be isosomatic but not isotonic as

0.3 m urea, RBC membrane is permeable to urea, urea diffuses into the cells until its conc. on both sides becomes equal → the solutes within the cells (which is osmotically active) → causes osmosis of water into the cells RBCs will thus eventually burst.

Hyper-osmotic solutions have higher osmotic

pressure (i.e have a higher total conc. of solutes) than plasma

Hypertonic slns have higher osmotic pressure & solutes are osmotically active

Hypo-osmotic slns have lower osmotic pressure (i.e have a lower total conc. of solutes) than plasma

Hypotonics have lower osmotic pressure & solutes are osmotically active

Effects of

tonicity on

RBCs

crenated

Blood osmolality maintained in narrow range around 300m Osm

If dehydrated, osmoreceptors in hypothalamus stimulate:

◦ ADH (antidiuretic hormone) release

Which causes kidney to conserve H2O

◦ & thirst

6-20

Molecules too large & polar to diffuse are transported across membrane by protein carriers

Protein carriers exhibit: ◦ Specificity for single molecule e.g glucose carrier interact

with glucose and not with any other monosaccharides

◦ Competition among substrates for transport

So rate of transport for each is lower when they present together than it would be if each were present alone.

◦ Saturation when all carriers are occupied

This is called Tm (transport maximum).

As conc. of transported molecules increase, rate of transport will increased up to max. Beyond Tm any further increase in conc. of transported molecules DO NOT increase transport rate

Is passive transport down concentration gradient by carrier proteins

No energy is required.

E.g glucose transport in cell