Transport Across Cell Membrane

Question 1

4 modes of passive transport ?

Answer 1

a). Simple diffusion
b). Facilitated diffusion
c). Osmosis
d). Filtration

Question 2

2 types of active transport

Answer 2

a). The “pumps”
b). Endocytosis/ Exocytosis

Question 3

What is simple diffusion

Answer 3

“ Movement of solute from an area of high concentration to an area of low concentration in solution i.e. down a concentration gradient”

Question 4

6 things that affect rate of diffusion

Answer 4

-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

Question 5

2 places where diffusion occur and example

Answer 5

• 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

Question 6

What is facilitated diffusion

Answer 6

-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

Question 7

2 things affecting rate of facilitated diffusion with examples

Answer 7

• cocentration gradient
  -Saturation of the carrier molecule
  E.g. Glucose carrier of RBC
  Glucose carrier of skeletal muscle &
  adipose tissue( Insulin dependent)

Question 8

What is osmosis ?

Answer 8

-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

Question 9

What is hypotonicity

Answer 9

-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.

Question 10

What is hypertonicity

Answer 10

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.

Question 11

What is isotonicity

Answer 11

A solution with same concentration of solute as other solution

Question 12

What is filtration and example

Answer 12

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

Question 13

What is active transport

Answer 13

Carrier mediated process that moves substances across cell membranes against a concentration gradient at the expense of Energy ( ATP)

Question 14

What affects rate of active transport

Answer 14

Ø Number of carrier molecules
Ø Availability of adequate energy i.e. ATP

Question 15

3 examples of active transport

Answer 15

• Na+/K+ ATPase
• Calcium pump
• Proton pump (ATP dependent H pump)

Question 16

Function of Na+ / K+ transporting ATPase system

Answer 16

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

Question 17

What can inhibit Na+/K+ pump

Answer 17

ØCardiac glycosides Digitalis/Oubain inhibit this pump by acting on the extra cellular surface of the membrane

Question 18

What does endocytosis and exocytosis do

Answer 18

Transport macromolecules thru membrane

Question 19

What is endocytosis

Answer 19

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

Question 20

3 types of endocytosis ?

Answer 20

a. Pinocytosis
b. Receptor Mediated Endocytosis
c. Phagocytosis

Question 21

What is pinocytosis

Answer 21

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

Question 22

2 types of pinocytosis ?

Answer 22

Absorptive pinocytosis / receptor mediated endocytosis
Fluid phase pinocytosis

Question 23

What is Receptor Mediated Endocytosis

Answer 23

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

Question 24

4 examples of receptor mediated endocytosis

Answer 24

Hepatitis virus ( affecting liver cells )
Poliomyelitis virus (Affecting motor neurons )
AIDS virus ( Affecting T-Cells )
LDL( ligand ) and its receptor

Question 25

What is phagocytosis

Answer 25

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.

Question 26

Example of phagosome

Answer 26

WBC engulfing a harmful bacteria

Question 27

What is exocytosis ?

Answer 27

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.

Question 28

3 examples of exocytosis

Answer 28

Secretion of digestive enzymes
Secretion of mucous by salivary glands
Secretion of milk by mammary glands

Question 29

What are ionophores

Answer 29

Ionophores are chemicals that facilitate the transport of monovalent and divalent cations across membranes.

Question 30

2 examples of ionophores

Answer 30

Antibiotics -Valinomycin
Thermogenin

Question 31

What are 2 types of ionophores

Answer 31

1. Ion carriers
2. Ion channels ( depending on their mechanism of action )

Question 32

Describe structure of ion carrier

Answer 32

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

Question 33

Function of ion carrier

Answer 33

complex made up of a cation and the ionophore is capable of diffusing across the membrane.

Question 34

Example of ion carrier

Answer 34

Ex. Valinomycin
(carries K+ across membranes.)

Question 35

Function of ion channel

Answer 35

Ion channel ionophores insert themselves into
the membrane in such a way as to create a pore or channel through which ions can flow

Question 36

Example of ion channel

Answer 36

Ex. 1. Antibiotic Gramicidin A

Question 37

Describe structure of ion channel

Answer 37

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.

Question 38

4 types of transport systems based on functional sense according to no. Molecules moved and direction of movement . With example

Answer 38

1. Uniport - Moves one type of molecule bi- directionally
2. 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.

Question 39

What is jeevanee ?

Answer 39

Oral rehydration solution
Used to rehydrate persons who are dehydrated.

Question 40

Composition of jeevanee . With total osmolarity

Answer 40

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

Question 41

3 things required for endocytosis

Answer 41

1. Energy usually from hydrolysis of ATP
2. Ca2+
3. Contractile elements in cell

Question 42

How does jeevanee work

Answer 42

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.

Question 43

What is fluid phase pinocytosis

Answer 43

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.

Question 44

Example of fluid phase pinocytosis

Answer 44

Fibroblasts, for example, internalize their plasma membrane at about one-third the rate of macrophages