Transport system in cell membrane

1. Transport across the
membrane

3. Transport Across the
Cell Membrane
 Maintains homeostasis of cell
 Membrane is selectively permeable –
some things can pass through but
others can’t
 3 types:Passive transport, Active
transport and Bulk transport

4. Types of Transport

5. 1. Passive Transport
• Passive = no energy req’d
• Move through membrane due to
differences in concentration gradient
• 3 different types
1. Diffusion
2. Osmosis
3. Facilitated diffusion

6. 1. Diffusion
 Movement of molecules from an area
of high concentration to an area of low
concentration across a concentration
gradient
 Used transport small molecules like
CO2 H2O, and O2

8. Some major examples of
diffusion in biology
• Gas exchange at the alveoli — oxygen from air
to blood, carbon dioxide from blood to air.
• Gas exchange for photosynthesis — carbon
dioxide from air to leaf, oxygen from leaf to air.
• Gas exchange for respiration — oxygen from
blood to tissue cells, carbon dioxide in opposite
direction.
•Transfer of transmitter substance —
acetylcholine from presynaptic to postsynaptic

9. 2. Osmosis
 Diffusion of water from a region of
high concentration to a region of low
concentration
 Water can diffuse into or out of a cell,
it depends on the concentration on
either side of the cell membrane

10. Some major examples of
osmosis
• Absorption of water by plant roots.
• Re-absorption of water by the proximal and
distal convoluted tubules of the nephron.
• Re-absorption of tissue fluid into the venule
ends of the blood capillaries.
• Absorption of water by the alimentary canal —
stomach, small intestine and the colon.

11. 3 Types of Osmosis
1. Hypotonic
 Contain a low concentration
of solute relative to another
solution.
 Cell is placed in a hypotonic
solution, the water diffuses
into the cell, causing the cell
to swell and possibly explode.

12. 2. Hypertonic
 Contain a high concentration
of solute relative to another
solution.
 When cell is placed in a
hypertonic solution, water
diffuses out of the cell,
causing the cell to shrivel.

13. 3. Isotonic
 Contain the same
concentration of solute as
an another solution.
 When cell placed in an
isotonic solution, the water
diffuses into and out of the
cell at the same rate.
 Fluid that surrounds the
body cells is isotonic.

15. 3. Facilitated diffusion

16. Key features
 how glucose/charged ions moves into
cells
 passive – does not require energy
 uses
◦ Carrier proteins (Transporters or permeases)
◦ Channel proteins (Hydrophilic Ion channels
or porins)
 solute molecules “combine” with carrier
proteins in the membrane.
 carrier molecules speed (or facilitate) the
passage of the solute molecules across
the membrane.

17. Spot the difference!

18. 2 kinds of proteins involved:
1. CARRIER PROTEINS
 bind to a specific type of diffusing
molecule.
 have a highly specific hydrophilic region to
which the solute molecule binds.
 binding cause the protein to undergo a
change in shape that moves the solute
across the bilayer and release it on the
other side

20. Carrier proteins

21. 2. Hydrophilic transmembrane
channels
 Ion channels
 Porins
 Aquaporins

22. ION CHANNELS
 formed by proteins with
a central pore that is
lined with hydrophilic
amino acid side chains.
 help the diffusion of
charged particles such
as Ca2+, Na+, K+, HCO3-
and Cl ions.
 Some channels are
gated and allow cells to
regulate the flow of ions
from one cell to another.

23.  Ligand gated channels
◦ "nicotinic" Acetylcholine receptor
◦ ionotropic glutamate-gated receptors
◦ ATP-gated P2X receptors
◦ anion-permeable γ-aminobutyric acid-
gated GABAA receptor
 Voltage gated channels
◦ Voltage gated calcium channels
◦ Voltage gated potassium channels
◦ Volgate gated sodium channels
 Mechanosensitive channels

27. Porins
 Compared to ion channels, the pores
found in outer membranes of
mitochondria, chloroplasts and many
bacteria are some what larger and
much less specific.
 Formed by multipass transmembrane
proteins

28. Porins

29. Aquaporins (AQPs)

30. Factors affecting Rate of
Diffusion
1. Concentration Difference
 happens ONLY when a concentration
gradient is present and solute travels
ALONG (down) a concentration gradient
2. Saturation
 there are only a limited number of carrier
molecules per unit area of membrane.
 rate of movement reaches a max. when all
carrier molecules are fully loaded with solute
molecules

31. 2. Active Transport
• The transport of molecules or ions
across a membrane by carrier proteins
against a concentration gradient.

32.  Requires energy
 Involves carrier proteins in the
membrane.
 Hydrolysis of ATP releases the energy
required for active transport.
 Cells involved in active transport have a
large number of mitochondria to provide
the ATP required

33. Spot the difference

35. Major examples of Active Transport
 Re-absorption of glucose, amino acids
and salts by the proximal convoluted
tubule of the nephron in the kidney.
 Sodium/potassium pump in cell
membranes (especially nerve cells)

37. Active transport involves the movement of materials against a
concentration gradient and requires an expenditure of energy
This energy may be harnessed by one of two means:
The direct hydrolysis of ATP (primary (direct) active transport)
By coupling with the transport of another molecule moving along
its electrochemical gradient (secondary (indirect) active
transport)
The coupled transport of two distinct molecules is called co-
transport (the movement of a single molecule is called uniport)
If the two molecules are transported in the same direction it is
called symport
If the two molecules are transported in opposite directions it is
called antiport

39. Sodium-Potassium Pump
 ex. of active transport
 Exists in most cell membranes.
 Actively removes sodium ions from the cell while
actively accumulating potassium ions into them
from their surroundings

40. Proton Pump

41. Glucose crosses the intestinal epithelium

42. ATP-driven pumps

44. ER calcium pump is a P-type transporter

45. Na-K ATPase is also a P-type pump

46. ABC transporters are the largest family of membrane
transport proteins
REM: MDR

47.  ABC transporters
 MDR transporter in
cancer cells

49. 3. Bulk Transport
 used for materials (large) to enter via
passive or active transport
 vesicles created by folding of cell
membrane onto itself to either engulf or
expel materials
 2 types: endocytosis & exocytosis

51. Endocytosis
 the transport of large particles into the
cell in vesicles formed by folding in of
the cell surface membrane
 3 types:
1. Pinocytosis (cell drinking)
• intake of small droplet of
extracellular fluid along
with solute particles
• occurs in all cells often

52. 2. Phagocytosis (cell eating)
 intake of large droplet of extracellular fluid
including particulate matter (bacteria or
organic matter)
 occurs only in specialised cells like
amoeba or macrophages (bacteria fighting
immune cells)

53. 3. Receptor-assisted
endocytosis
 involves intake of specific molecules
that attach to special protiens in cell
membrane that serve as receptors
 have a unique shape that fit only to
one specific molecule
 ex. animal cells use this to bring
cholesterol into cell

55. Exocytosis
 the reverse process and is used to secrete
proteins, e.g digestive enzymes, out of the
cells.
 vesicle forms inside cell moves to
membrane and empties contents outside of
cell
 ex. pancreas secretes insulin