Transport Across Cell Membrane Flashcards
4 modes of passive transport ?
a). Simple diffusion
b). Facilitated diffusion
c). Osmosis
d). Filtration
2 types of active transport
a). The “pumps”
b). Endocytosis/ Exocytosis
What is simple diffusion
“ Movement of solute from an area of high concentration to an area of low concentration in solution i.e. down a concentration gradient”
6 things that affect rate of diffusion
-Magnitude of the concentration gradient
-Viscosity of the fluid
-Temperature - increased temperature increased rate
of diffusion
-Solubility of solvent
-Gases diffuse rapidly; liquids more slowly
-Molecular size
e.g Glycerol diffuses faster than larger fatty acids
2 places where diffusion occur and example
- Lipid bilayer
- Membrane channels
( Integral channel proteins )
+example = Some nutrients enter the cell / some waste products leave the cell by diffusion.
E.g. - Oxygen from blood stream diffuse into the body cells - Carbon dioxide diffuses out of the cell into the blood stream
What is facilitated diffusion
-Carrier mediated process
-Moves substances into or out of cells from a
high to a low concentration ( i.e. Down a
concentration gradient )
-It DOES NOT require Energy
2 things affecting rate of facilitated diffusion with examples
- cocentration gradient
-Saturation of the carrier molecule
E.g. Glucose carrier of RBC
Glucose carrier of skeletal muscle &
adipose tissue( Insulin dependent)
What is osmosis ?
-Diffusion of water through a semipermeable membrane.
-Water can move freely across cell membrane of
cells, either through protein channels or by
slipping between the lipid tails of membrane.
-However, it is the concentration of solutes within the water that determine whether or not water will be moving into the cell, out of the cell, or both
- Osmosis occurs when there is an imbalance of solutes outside of a cell versus
inside the cell.
-More solute a solution contains, the greater the osmotic pressure that solution will have
What is hypotonicity
-A solution with a lower concentration of solutes than another solution is hypotonic
- Cells in a hypotonic solution will take on too
much water and swell, with the risk of eventually bursting.
What is hypertonicity
Ø A solution with a higher concentration of solutes than another solution is said to be hypertonic.
Ø Water molecules tend to diffuse into a hypertonic solution because the higher osmotic pressure pulls water in.
What is isotonicity
A solution with same concentration of solute as other solution
What is filtration and example
Filtration through a porous membrane refers to bulk flow of a solvent along with
substances dissolved in it ( except for molecules that are larger than the pores)
Ex. Glomerular membrane of Kidney
What is active transport
Carrier mediated process that moves substances across cell membranes against a concentration gradient at the expense of Energy ( ATP)
What affects rate of active transport
Ø Number of carrier molecules
Ø Availability of adequate energy i.e. ATP
3 examples of active transport
- Na+/K+ ATPase
- Calcium pump
- Proton pump (ATP dependent H pump)
Function of Na+ / K+ transporting ATPase system
Most mammalian cells maintain a higher concentration of K+ inside the cell while the Na+ concentration is higher outside the cell
Na+. K+. Cl- Plasma 142. 4. 103 Intra cellular fluid 10. 152
If no mechanism is there to maintain this difference, Na+ & Cl- will enter the cell down its conc. gradient
Creation of an osmotic gradient
Water will move into the cell
Cells will swell while pressure inside them balanced the influx
But this does not occur. Because Na+/K+ pump maintains the
normal solute levels of these two ions
What can inhibit Na+/K+ pump
ØCardiac glycosides Digitalis/Oubain inhibit this pump by acting on the extra cellular surface of the membrane
What does endocytosis and exocytosis do
Transport macromolecules thru membrane
What is endocytosis
small region of plasma membrane folds inwards / invaginates until it has formed a new intracellular vesicle with the macromolecule inside.
This is drawn into cytoplasm
3 types of endocytosis ?
a. Pinocytosis
b. Receptor Mediated Endocytosis
c. Phagocytosis
What is pinocytosis
Non specific uptake of small droplets of extracellular fluid
( Along with any material dissolved in it e.g. amino acids, Glc, Vitamins, low molecular weight nutrients )
e.g. Kidney cells
2 types of pinocytosis ?
Absorptive pinocytosis / receptor mediated endocytosis
Fluid phase pinocytosis
What is Receptor Mediated Endocytosis
Involves a specific receptor on plasma membrane that recognizes the extracellular macromolecule and binds it. Substance bound to receptor ( Referred to as ligand )
Region of plasma memebrane that contains the receptor-ligand complex undergoes endocytosis
4 examples of receptor mediated endocytosis
Hepatitis virus ( affecting liver cells )
Poliomyelitis virus (Affecting motor neurons )
AIDS virus ( Affecting T-Cells )
LDL( ligand ) and its receptor
What is phagocytosis
Active movement that allows large molecules and other cells to pass through the plasma membrane into the cytoplasm
Phagocytosis ( Cell eating Gr.)
When large molecules make contact with membrane of a phagocyte
The membrane begins to form a pocket around the particle by sending out PSEUDOPODIA (False feet )
Pocket closes to form a Phagosome Phagosome with particle enter the cytoplasm.
Example of phagosome
WBC engulfing a harmful bacteria
What is exocytosis ?
In some cells secretions accumulate within vesicles.
The secretary vesicle then moves to the plasma membrane
The contents of the vesicle are expelled from the cell.
3 examples of exocytosis
Secretion of digestive enzymes
Secretion of mucous by salivary glands
Secretion of milk by mammary glands
What are ionophores
Ionophores are chemicals that facilitate the transport of monovalent and divalent cations across membranes.
2 examples of ionophores
Antibiotics -Valinomycin
Thermogenin
What are 2 types of ionophores
- Ion carriers
- Ion channels ( depending on their mechanism of action )
Describe structure of ion carrier
cyclic structure or has the ability to form a cyclic structure.
-Inside of the ring contains several oxygens that can chelate a cation.
-Outside of the ring is hydrophobic
Function of ion carrier
complex made up of a cation and the ionophore is capable of diffusing across the membrane.
Example of ion carrier
Ex. Valinomycin
(carries K+ across membranes.)
Function of ion channel
Ion channel ionophores insert themselves into
the membrane in such a way as to create a pore or channel through which ions can flow
Example of ion channel
Ex. 1. Antibiotic Gramicidin A
Describe structure of ion channel
forms a helical structure in the membrane
ØHas a central hydrophilic pore large enough
to accommodate ions.
ØThe ions diffuse through the membrane via
the hydrophilic pore.
4 types of transport systems based on functional sense according to no. Molecules moved and direction of movement . With example
- Uniport - Moves one type of molecule bi- directionally
- Cotransport - Here the transfer of one solute depends on the simultaneous / sequential transfer of another solute
3.Symport - The two solutes moves in the same direction
e.g.- Na+ - amino acid transport
- Na+ - sugar transporters
( Glc, Mannose, Galactose etc.)
4.Antiport - Moves the two molecules in opposite directions
e.g. Na+ / K+ ATPase etc.
What is jeevanee ?
Oral rehydration solution
Used to rehydrate persons who are dehydrated.
Composition of jeevanee . With total osmolarity
Composition:
grams/ litre
Sodium chloride
2.6
Glucose anhydrous
13.5
Potassium chloride
1.5
Trisodium citrate, dihydrate
2.9
mmol/litre
Sodium
75
Chloride
65
Glucose anhydrous
75
Potassium
20
Citrate, dihydrate
10
Total osmolarity
245
3 things required for endocytosis
- Energy usually from hydrolysis of ATP
- Ca2+
- Contractile elements in cell
How does jeevanee work
In dehydration giving a saline solution (water plus Na+) by mouth has no beneficial effect (as normal mechanism by which Na+ is absorbed by the healthy intestinal wall is impaired in the diarrhoea).
Ø Thus as Na+ is not absorbed water is not absorbed as well.
Ø Furthermore, excess Na+ in the lumen of the intestine causes increased secretion of water and diarrhoea may even worsen.
Ø If glucose is given along with Na+ and water sodium glucose co-transporter comes into play.
Ø Glucose is absorbed through the intestinal wall and in conjunction Na+ is carried through by the co-transport coupling mechanism (in a glucose : Na+ ratio of 1:1)
Glucose does not co-transport water. It is the now increased relative concentration of Na+ across the intestinal wall which pulls water inside.
What is fluid phase pinocytosis
Non selective process
Uptake of solute by formation of small vesicle is simply proprotionate to conc. in surrounding extracellular fluid. Formation of vesicles is an extremely active process.
process occurs more rapidly than membranes are made. The surface area and volume of a cell do not change much, so membranes must be replaced by exocytosis or by being recycled as fast as they are removed by endocytosis.
Example of fluid phase pinocytosis
Fibroblasts, for example, internalize their plasma membrane at about one-third the rate of macrophages
