TRANSPORT ACROSS
CELL MEMBRANE
DR OMIRE-OLUEDO OKEY
OUTLINE
INTRODUCTION
STRUCTURE OF A CELL MEMBRANE
FUNCTIONS OF A CELL MEMBRANE
DIFFUSION
OSMOSIS
ACTIVE TRANSPORT
FACILITATED DIFFUSION
EXOCYTOSIS
ENDOCYTOSIS
BULK FLOW/SOLVENT DRAG
INTRODUCTION
CELL MEMBRANE
All cells is encircled by a cell membrane
Most cells contain an extensive intracellular
membrane system
Membranes fence off the cell interior from its
surroundings
Functions to control what enters and exits the cell
(selectively permeable) to maintain the internal balance
called HOMEOSTASIS.
Cell membranes has pores/holes in it
Also provides protection and support for the cell because
without a membrane the cell content would diffuse into the
surroundings, informations containing molecules would be
lost and many metabolic pathways would cease to work.
THE CELL WOULD DIE.
It is a lipid trilayer
A hydrophilic (polar, water-loving) head
A hydrophobic (non-polar, water-fearing)tail
It is elastic (7.5-10nm)
Contains protein called membrane proteins
Functions of cell membrane
Protection
Selective permeability
Absorption
Exchange of gases
Excretory functions
Maintenance of shape and size of cell
MEMBRANE PROTEINS
INTEGRAL PROTEIN
Part and parcel of membrane structure
Consists of 70% of cell membrane protein
PUMPS
Transfer particles against concentration/electrical
gradients
CHANEL PROTEIN
Opened and closed gated
CARRIER PROTEINS
Transport of substances by binding with them
ENZYME PROTEINS
Takes place in membrane reaction
RECEPTOR PROTEIN
Bear appropriate site for recognition of specific ligands
CARRIER TYPES
Uniport carriers:
Transports only one substance from one side of the
membrane to the other
Symport carriers:
Transports two or more substances from one side of the
membrane to the other, transport of Na+ and glucose
from intestinal lumen to cells
Also called co-transport
Antiport carriers:
Transports substances in opposite direction, one substance
from inside the cell another from inside to outside. Eg Na+k+ pump
TYPES OF TRANSPORT
ACTIVE TRANSPORT
PASSIVE TRANSPORT
Characteristics of passive Transport
Downhill transport
No metabolic energy (no ATP)
SIMPLE DIFFUSION
Oxygen, CO2, urea, ammonia, ions are all examples of substances
that pass through cell membranes by diffusion.
Factors determining rate of diffusion across cell membrane
1) Properties of Substance (concentration and permeability)
Concentration-chemical concentrations or electrical potential
difference
Permeability- lipid solubility(O2,CO2,alcohol has high lipid
solubility), molecular size, presence of charge on molecule
2) Properties of the Membrane
3) Distance- the shorter the distance the more quickly
gradients are eliminated
4) Temperature-
temperature leads to motion of particles
5) membrane surface area- larger the area, faster the
diffusion
OSMOSIS
OSMOTIC PRESSURE
This is the exact amount of energy required to stop osmosis
MOLE:
Molecular weight (MW)of a substance in gram (gram molecular
weight)
Its an SI unit
E.g MW of glucose=180gm, NaCL= 58.5gm
OSMOLE/MILLIMOLE
A unit describing the concentration of osmotically
active particles.
Mole of substance/no of freely moving particles
which each molecule liberates in solution
Physiologic solutions are expressed in milliosmo per
litre (mosm/L)
1 molecule of glucose =1 osmolar active particles=1 osmole
1 molecule of NaCl = 2 osmolar active particles = 2 osmoles
1 molecule of CaCl2 = 3 osmolar active particles = 3 osmoles
1millimole = 1000 osmole
Osmolality : No of osmole dissolved in 1kg of water
Total sum of all particles in plasma determines its
osmolality
>90% of plasma osmolality is due to NaCl
Osmolarity : No of osmole in 1litre of plasma
-Value is affected by volume of
other solute in the solution
NB-
Most important determinant of osmotic pressure
of a solution is the conc of the particles released in
solution(i.e the osmole)- size, shape and charge of the
particles is not important
Osmotic pressure is measured by:
Freezing point depression
Van’t Hoff equation
Measuring the equivalent hydrostatic pressure
Filtration and bulk flow
TONICITY
This is the ability of a solution to affect fliud volume and
pressure with cell
Depends on concentration and permeability of solute
Hypotonic
Low concentration of non-permeable solute than that of
cytosol.(high water concentration)
Cell swell (lysis)
Isotonic
Solution with same solute concentration as that of cytosol. e.g.
Normal saline
Cell neither swell nor shrinks
E.g 5% glucose solution or 0.9%NaCl solution
Hypertonic
High concentration of non permeable solute than that of cytosol.
(low water concentration)
Cell shrinks (crenation)
E.g Saline solution,or 0.45% sodium chloride is hypertonic.
extremely common
TONICITY
ACTIVE TRANSPORT
Active transport requires energy. It is most commonly
accomplished by having a transport protein which changes
shape when it binds with the cell’s “fuel,” a molecule called
ATP.
One type of active transport channel will bind to something it
is supposed to transport – for example, a sodium ion – and
hold onto it until a molecule of ATP comes along and binds to
the protein.
The energy stored in ATP then allows the channel to change
shape, spitting the sodium ion out on the opposite site of the
cell membrane.
CHARACTERISTICS OF ACTIVE TRANSPORT
Uphill transport
Metabolic energy required
Exhibit saturation kinetics
Types of Active transport
Carrier mediated transport – primary
- secondary
vesicular transport – endocytosis - Pinocytosis
-phagocytosis
- exocytosis
Primary active transport
Eg - sodium-potassium pump,
- calcium channel pump,
- hydrogen pump
Secondary active transport
the protein pump does not use ATP itself, but the cell must
expend ATP in order to keep it functioning
Exocytosis:
A cell moves particles outside of itself in large quantities by
wrapping it in a membrane called a vesicle and “spitting out”
the vesicle.
Endocytosis
In endocytosis, a cell “eats” large no of particles by wrapping
and re-forming its membrane around the substance or item,
transporting it into the cell
If material is liquid the process is Pinocytosis
If material is solid, its phagocytosis
Pinocytosis
“Cell drinking” of substances in solution in ECF
More rapid in cell like macrophages
Macro molecules attach to receptors on membrane
surface >>invagination of entire pit >>vesicles detaches
from cell membrane >> fusion of pinocytic vesicle with
one or more lysosomes >> undigested materials are
removed via exocytosis
BULK FLOW/ SOLVENT DRAG
The movement of ions across cell membranes by bulk transport fol
lowing themovement of water rather than being facilitated by ion
channels or cellular pumps.
It generally occurs in the paracellular, rather than transcellular,
pathway between the tubular cells
e.g., NaCl transport in the jejunum via the paracellular route
Solvent drag is responsible for the reabsorption of approximately
60% of previously filtered potassium in the proximal tubule.
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