Cell 1 & 2

Assoc Prof Rosnah Ismail

Department of Physiology

Faculty of Medicine

22 & 27 July 2010

Introduction

Lecture Objectives

1. describe basic cell structure

2. name cellular organelles and their functions

3. describe the structure of cell membrane

4. explain various transport processes across cell

membranePassive, active, special

Osmosis; osmolarity and calculation for it

5. describe the distribution of ions between

extracellular and intracellular fluids (Na+ & K+)

6. define Resting Membrane Potential and explain how

it is generated

Cell Structure

Cells differ from one type to another

epithelial, muscle, nerve etc

Epithelial cell Bone cellsMuscle cells Nerve cell

BUT all cells have one

BASIC structure:

Membrane; cell membrane,

membrane of organelles

Cytoplasm; mainly water,

electrolytes and proteins

Organelles; nucleus,

mitochondria, Golgi apparatus

etc

Size of cells: 10-20m

Cell Membrane

Functions:

1. Regulates movement of substances in and out of cells

2. Senses neurotransmitter/hormone at the surface of cells

(receptor)

3. Connects adjacent cells

4. Binds various proteins involved in generation of force

and transfer of force

5. Separates the extracellular fluid from intracellular fluid

Semi permeable/ selective permeability

compare permeability towards polar molecule to

that of non-polar molecules??

small molecules compared to big ones??

Characteristics of cell membrane

Causes differences between ICF and ECF

Intracellular Fluid (ICF) versus Extracellular Fluid (ECF)

ECF

ICF

Cell Membrane

Structure: (fluid-mosaic model)

consists of: 1. phospholipid bilayer

2. protein

3.cholesterol

Phospholipid:

Amphiphatic molecule

Phosphate:

hydrophilic

Lipid:

hydrophobic

Membrane thickness: 6-10nm

No chemical bonds between phospholipid molecules

• very fluid

• cholesterol stabilises the membrane

ECF

ICF

Cell Membrane Proteins

Mostly glycoproteins

• integral (intrinsic) proteins

protrude through membrane

channels, carrier, membrane enzyme,

receptors

• peripheral (extrinsic) protein

attached to one surface of membrane, onto integral

protein, does not penetrate membrane

Integral protein

Integral protein

peripheral

protein

Junctions between cells

Transport across Membrane

1. Passive

Simple Diffusion

Facilitated Diffusion

Osmosis

2. Active : Primary, Secondary

3. Special processes:

Filtration

Endocytosis

Exocytosis

Passive Transport

No energy requirement

• Movement of solutes down gradient;

concentration, electrical

due to random motion (Brownian movement)

• Movement of solutes through

lipid bilayer

channels

carrier (facilitated diffusion)

Passive Transport

Q = D(C1-C2)

Factors affecting diffusion

1. Concentration gradient

2. Electrical gradient

3. Size of molecules

4. Lipid solubility

5. Surface area

6. Thickness of membrane

7. Temperature

Q = rate for diffusion

D = diffusion coefficient (depends on

properties of the molecule, membrane

and temperature)

C1-C2 = concentration gradient

Movement of solutes is from

HIGH conc LOW conc

Simple Diffusion

Solute binds to specific carrier on membrane

· carrier undergoes conformational change

· solute transported to opposite surface

· solute unbound from carrier

· carrier back to original conformation

Facilitated Diffusion

Example: transport of glucose into red blood cell

Factors affecting rate of

facilitated diffusion

• Same factors as for simple diffusion

• 2 additional factors

Saturation

competitive inhibition

Shows Saturation

- number of carrier proteins available

- the rate at which binding/unbinding

occurs

Shows Competitive/ non-

competitive inhibition

- presence of two or more types of

molecule transported by same

carrier protein

- they compete/ bind to same carrier

Characteristics of carrier-mediated transport:

Normal

Ra

te o

f tr

an

sp

ort

Concentration of transported molecule

Osmosis

Definition: movement of solvent (water) down its

concentration gradient across a membrane selectively

permeable to it

passive process

• water is a polar molecule

• size 0.3nm

• crosses lipid bilayer of membrane or

through water channels (aquaporin)

Osmosis

Visualise 2 solutions with different concentrations

separated by a membrane permeable

i. to both water and solutes

ii. to water only

solution with low solute conc, has high water conc

solution with high solute conc, has low water conc

What will happen?

Concept of Osmotic Pressure

Osmotic pressure: the pressure required to stop

movement of water (osmosis) is the osmotic pressure

of the solution

Unit = mmHg

solution with high solute concentration

high osmotic pressure

solution with low solute concentration

low osmotic pressure

OSMOSISOsmotic Pressure

Osmotic Pressure

Osmotic pressure depends on:

1. Number of non-penetrating solutes

per unit volume of fluid

DOES NOT depend on size/mass of solutes

the higher the number of non-penetrating solutes in a solution, the higher the osmotic pressure

Number of solutes in a solution depends on:

i. Concentration of the molecule in a solution

(Molarity)

ii. Whether the molecule dissociates in solution

2. Temperature

van’t Hoff Law: P= nRT

V

P= osmotic pressure

R= gas constant

T= absolute temp ( K)

n= no solutes

V= volume of solution(true only for dilute solution)

Osmolality: Concentration of solution in terms of

number of solutes per kg water.

Unit: Osmole/kg

Osmolarity: Concentration of solution in terms of

number of solutes per liter water.

Unit: Osmole/L

Terminologies

1 Osmole = 1000 mOsmole

(Osm) (mOsm)

Osmolarity of body fluid: 285-300 mOsm/l

ECF & ICF

285-300 mOsm/L (0.285 – 0.3 Osm/L)

equivalent to osmotic pressure of

7.3 atmospheric pressure

(5500 mmHg)

Osmolarity of a Solution

Number of molecules in a solution depends on the

molarity of the solution

No of Osmole = Molarity of substance X no. of freely moving

solutes that each molecule liberates in soln

For non-dissociable molecule:

e.g. soln of 0.3 M; will have 0.3 Osm since;

no. of Osm = 0.3 X 1 = 0.3 Osm

For dissociable molecule:

e.g. substance A dissociates into 3 solutes in solution

0.3 M of substance A will have 0.9 Osm since;

no. of Osm = 0.3 X 3 = 0.9 Osm

Examples:

2. NaCl (dissociates into 2 ions: Na+ and Cl-)

What is the osmolarity of a 1 M NaCl solution?

Osm = 1 X 2 = 2 Osm/ L

(In this solution there are: 1 Osm/L Na+ + 1 Osm/L Cl- ions)

1. Glucose does not dissociate in solution.

1 M glucose solution = 1 Osm/L

What is the osmolarity of 0.3 M/L glucose solution?

Osm = 0.3 X 1 = 0.3 Osm/L (300 mOsm/L)

Some Exercises to do

Calculate the osmolarity of the following solutions:

1. 145 mM NaCl

2. 0.15 M CaCl2

3. 0.15 M Na2SO4

4. 0.90% NaCl (MW NaCl= 58.5)

Isosmotic, Hyposmotic & Hyperosmotic

A CB

300

mOsm/L

300

mOsm/L

200

mOsm/L

compare A to B

Compare A to C

Compare C to A or B

isosmotic

hyperosmotic

hyposmotic

Concept of Tonicity

Comparison between osmolarity of a solution to that of

plasma

Body fluid Osmolarity about 300 mOsm/l

Osmolarity ICF = Osmolarity ECF

ICF and ECF in equilibrium

looks at the non-penetrating solutes

considers whether there is a change in cell size

when cells are put into the solution

Isotonic, Hypotonic, Hypertonic

Isotonic solution:concentration of non-penetrating solutes is same as that of plasma

(body fluid), 300mOsm/L

water influx = water efflux (no net movement of water)

no change in cell size

Hypotonic solution:concentration of non-penetrating solutes is less than that of plasma;

< 300mOsm/L

water influx > water efflux (net movement of water into cells)

cells swell

Hypertonic solution:

concentration of non-penetrating solutes is more than that of plasma,

>300 mOsm/l

water influx < water efflux (net movement of water out of cells)

cells shrink (crenation)

300 mOsm/L

NaCl

300

mOsm/L

No change in cell size

NaCl solution with solute concentration of 300 mOsm/L

is isosmotic as well as isotonic compared to cell

NaCl does not penetrate cell membrane

300

mOsm/L

300 mOsm/L

NaCl

300 mOsm/L

urea

Urea penetrates cell membrane easily

Cells swell

This solution of urea with solute concentration of 300

mOsm/L is isosmotic but is NOT isotonic compared to cell

300

mOsm/L

urea

water

300 mOsm/L

urea

300

mOsm/L

+ 300 mOsm/L

urea

What will happen to red blood cells if they are placed in:

1. 150 mOsm/L NaCl solution?

2. A solution containing mixture of NaCl 300 mOsm/L

and urea 150 mOsm/L?

3. 300 mOsm/L glucose solution?

Some More Exercises to do

Measurement of Osmolarity of a Solution

• Using the principle of freezing point depression

• Pure water freezes at 0°C

• Addition of solutes decreases the freezing point of water, < 0°C

• KNOWN:

1 mole of an ideal solution depresses the freezing point of water by 1.86C

• Can use this fact to measure the osmolarity of a solution

e.g if the freezing point of a solution decreases by 0.56C

osmolarity of the solution is 0.56/1.86

about 0.3 Osm/L

Active Transport

Requires energy (ATP)

Uses carrier

Carrier has ATPase enzyme

(to hydrolyse ATP, releases energy)

Substance transported against concentration gradient

Can occur in living cells only

2 major types:

• Primary

• Secondary

Primary Active Transport

Examples:

Na+,K+- ATPase (in all cells)

Ca2+-ATPase (e.g. in sarcoplasmic reticulum of muscles)

H+-K+ ATPase (cells in stomach, kidneys)

H+- ATPase (lysosome)

Uses energy (ATP) directly to transport substances

against electrochemical gradient across cell membrane

Transports one type of solute: uniport

Transports 2 types of solutes in same direction: symport/co-transport

Transports 2 types of solute in opposite directions : antiport/counter-transport

Types of Transporters

Na,K- ATPase

Secondary Active Transport

Indirect use of Energy (ATP)

Carrier has 2 binding sites for two different solutes

one for ion (Na+) and one for another solute

E.g: glucose, galactose, amino acid

in kidneys and small intestine

1. conc gradient for Na+ provides

the energy (High Na+ conc

outside, low inside due to action

of Na+-K+-ATPase pump)

2. Na+ binds to carrier because of

presence of gradient

3. Glucose binds to carrier

4. Carrier undergoes

conformational change

5. Na+ released into cytoplasm,

glucose released from binding

site into cytoplasm

6. Carrier goes back to original

conformation

7. Na+ pumped out (by Na+-K+

ATPase pump; utilising ATP) and

Na+ gradient maintained

Special Transport Process

1. Filtration

Transport by bulk flow across a

membrane due to difference in

pressure

Both solutes and solvent cross membrane

Dependent on Effective Filtration Pressure

e.g Filtration at the renal glomerulus

Filtration of substances across capillary

endothelium into the interstitial space

Endocytosis: Process whereby a substance

is taken up by cell

Phagocytosis

(cell eating):

solid particulate

(dead cell, bacteria)

engulfed by cell

Pinocytosis

(cell drinking) :

liquid substance

engulfed by cell

Exocytosis: secretion of substances from cell out to

the ECF

e.g. secretion of neurotransmitter from vesicle

Selective permeability of Cell Membrane leads to different

ionic distribution between ICF and ECF:

it is relatively more permeable towards K+

(conc K+ ICF >>> K+ ECF; 4.5 mM in ECF compared to 150 mM in ICF)

it has very low permeability towards Na+

(conc Na+ ICF<<< Na+ ECF; 145 mM in ECF compared to 15 mM in ICF)

It is not permeable to proteins

(protein ICF > protein ECF)

Introduction to Resting Membrane Potential

K+

Na+ Na+

Na+

Na+

K+

K+

K+

K+

K+

K+

K+

K+

K+

K+

K+

K+

K+

Na+

K+ Na+

K+

Na+

Na+

prot-

prot-prot-

Efflux K+

Prot cannot pass through membrane

Influx Na+

prot-

Na+

Na+/K+ ATPase

pump

3Na+

2K+

Na+

Na+

Na+ Na+

Na+

Na+

Na+

Na+

Na+

Resting Membrane Potential (Em)

Em log [Ko+]

[Ki+]

(Number of molecules involved in generation of Em is very small)

2. Negatively charged proteins cannot penetrate

membrane

1. Membrane permeability to K+ is relatively high

compared to Na+;

3. Na+/K+ ATPase pump is electrogenic, pumping more

+ve charges out than in (3 Na+ out and 2 K+ ions in),

contributing to the negative RMP

Resting membrane potential is potential difference across

the cell membrane at rest: inside all cells is negative

At rest, inside all cells is negative

Resting Membrane Potential (Em): - 10 to – 100 mV

Em of Excitable cells: - 60 to -100 mV

Em of other cells: - 10 to – 30 mV