This document summarizes membrane transport mechanisms, including passive transport processes like simple diffusion, facilitated diffusion, and osmosis as well as active transport processes like primary active transport, secondary active transport, and vesicular transport. Key transport proteins like carrier proteins, ion channels, and pumps are described. Specific examples of transport systems like sodium-potassium pumps, glucose transporters, and exocytosis/endocytosis are provided. The roles of these transport mechanisms in maintaining homeostasis and their implications for certain diseases are also mentioned.

1. TRANSPORT
XIANG ZHUXING

2. Review
 Cell membrane has selective permeability.
 _________CAN across membrane
 _________CANNOT across membrane.
TO MAINTAIN
HOMEOSTASIS,
WHAT TO DO?
TRANSPORT

3. Transport
Small
Molecules
Large
Molecules
Passive
Transport
Active
Transport
Osmosis
Facilitated
Diffusion
Primary
Active
Transport
Secondary
Active
Transport
Carrier
Proteins
Ion
Channels
Exocytosis
Endocytosis
O2; CO2
Glucose,
aa
Na+, K+
Ca2+,Cl−
Na+/K+
Pump
Na+ and
Glucose
Phagocytosis
Pinocytosis
Simple
Diffusion
Diffusion
Filtration

4. Figure 11-4a Molecular Biology of the Cell (© Garland Science 2008)
Overview

5. Passive Transport
No energy required
Movement is due to gradient
differences in concentration, charge
Move to equalize gradient
High moves toward low

7. Simple Diffusion
Does not require energy.
Move from high to low cont.
Until evenly distributed.
Example: O2,CO2,fatty acids, steroids,
glycerol.

8. Facilitated Diffusion
 A type of passive transport that does not
require energy to move molecules.
 Direction depends on conc. gradient.
 Use transport proteins to move molecules
that are large or charged(ions).
 Large polar: glucose and amino acids
 Ions: Na+, Cl-
Carrier proteins & ion channels.

10. Ion Channel Proteins
 Form hydrophilic pores to transport
inorganic ions across the membrane.
 Ion channels are selective and gated

11. Types of Ion Channels
leak channel: always open
Based on stimulus that can open/close
gates:
I. Voltage-gated ion channels
II. Ligand-gated ion channels
III. Mechanically-gated ion channels

13. Voltage-gated Ion Channels
Changes in
membrane
potential
E.g: Na+, K+,
Ca2+,Cl-

14. Ligand-gated Ion Channels
 “Chemically-gated”
 Due to binding of a chemical signal (ligand)

15. Mechanically-gated Channels
Due to mechanical
stimulation.
Sensory receptors
that respond to
vibration, touch,
pressure or stretch

16. Aquaporin
 Water channel proteins
 Embedded in the
membrane.
 Increases the
permeability to water
 No energy is required
(passive)
 Kidney distal and
collecting tubules

17. Carrier Proteins
 A.k.a transporters or permeases
 Bind molecules (glucose, aa)
 Undergo conformational change
 Molecule moves across membrane
“ping” “pong”

19. Cont’d
Carrier-mediated transport exhibits the
properties of specificity, competition,
and saturation.
Saturation: rate of transport limited by
number of carrier proteins.
Blood-brain barrier, GIT mocusa.

21. Classification
Carrier proteins transport either one or
two types of molecules:
Uniport: one type, bidirectionally
Two types (couple)
I. Symport: both in same direction (co-)
II. Antiport: in opposite direction
(counter-)

22. S=Solute

24. The Glucose Transporter Family
 The initial event in the cellular metabolism of glucose is its
transport across the cell membrane into the cytoplasm of the cell.
 This step is performed by the GLUT permeases
 Members of the family include:
 GLUT-1: a uniport carrier found in red blood cells and blood-brain barrier
where it functions in the basal transport of glucose into the cell
 Glucose levels higher in the blood than inside the cells
 Low cellular glucose is maintained by hexokinase which phosphorylates
glucose to glucose-6-phosphate (traps glucose in cell this way, cell lacks
transporter for phosphorylated sugars)
 GLUT-2: a uniport carrier found in hepatocytes, where it functions in
bidirectional glucose transport
 Also found in other cells such as kidney cells, surface absorptive cells and
pancreatic beta cells
 GLUT-3: a uniport carrier found in neurons, basal glucose uptake.
 GLUT-4: a uniport carrier found in adipocytes, skeletal muscle cells, and
cardiac myocytes where it functions in the insulin-stimulation transport
of glucose into the cell
 GLUT-5: a uniport carrier found on surface absorptive cells of the small
intestine mucosa where it functions in the transport of fructose into the cell

25. Active Transport
Molecular movement against gradient
Requires energy
 Function:
1. Maintain the chemical and electrical
charge at rest.
2. Intake of nutrient substances.
3. Remove waste and secretory products.

26. Active Transport
Large molecules(non-lipid soluble), ions
Based on the source of energy used for
transport:
I. Primary active transport
(pump/ATPase)
II. Secondary active transport

27. Primary Active Transport
 Transmembrane carrier protein.
 Use the energy directly from hydrolysis of ATP
 Conformational change
 Examples:
1. Na+/K+ pump
2. Ca2+ pump
3. H+/K+ ATPase

28. Sodium-Potassium Pump

29. Sodium Potassium Pump
 Present in all eukaryotic cells
 Functions:
1. Maintain sodium potassium cont.
difference across the cell membrane.
2. Maintain volume of the cell.
3. Causes negative electrical charge inside the
cell.
4. Active the carrier protein.

30. Inhibition of Na+/K+ Pump
 The pump requires binding of Na+ and K+
and ATP for its operation. If cont. of any
substances is low Dysfunction.
 Metabolic poisons -----ATP
 Cardiac drugs: ouabain and digitalis
 Low temp.
 Less O2

32. Calcium Pump
 Ca2+ is extremely low in
cytosol
 Function:
1. Maintain high cont. in
ECF
2. Helps in storage of Ca2+
in sarcoplasmic reticulum
needed for instant muscle
contraction.

33. Proton Pump
Generates potential and H+ cont. gradient

35. Hydrogen Potassium ATPase
 Gastric glands- parietal cells- HCl secretion-
pumps H+ into the gastric lumen, meantime
K+ inside the cell.
 Renal tubules- intercalated cells in the late
distal tubules and cortical collecting ducts-
secretion of H+ & reabsorption of K+

36. Secondary Active Transport
ATP provides the
energy for transport
indirectly.
Uses energy of an
electrochemical
gradient produced
originally by primary
active transport

37. Sodium-Glucose Symport

38. Sodium-Glucose Symport
Reabsorption of
glucose from
renal tubule
Absorption of
glucose from
small intestine
(next page)

39. Cont’d
GLUT5(Fructose)
(epithelial cell)

40. Other Examples of Cotransport
ECF
Cytosol

42. Counter-Transport
 Two examples:
 Na+-Ca2+ counter-
transport
 Cardiac cells
 Na+-H+ counter-
transport
 Proximal tubule
of kidneys

45. Vesicular Transport
Transport of membrane bounded
substances moving across plasma
membrane.
Require energy.
Classification:
I. Exocytosis
II. Endocytosis

46. Endocytosis
Movement of large material
Particles ; Organisms ; Large molecules
Movement is into cells
 Types of endocytosis
I. Phagocytosis— “Cell eating”
II. Pinocytosis– “Cell drinking”
III. Receptor-mediated endocytosis- specific
particles, recognition.

47. Process of Endocytosis

48. LDL

49. Exocytosis
 Reverse of endocytosis
 Cellular secretion
 Process:
I. Vesicle moves to
cell surface
II. Membrane of
vesicle fuses
III. Materials expelled

50. Cont’d
Substances and processes involved:
I. Secretion of proteins like enzymes,
hormones, antibodies from cells
II. Release of neurotransmitter from
presynaptic neurons
III. Recycling of plasma membrane

51. Associated Diseases
Mutations affecting membrane proteins can
cause diseases:
1. Familial hypercholesterolemia is due to
mutations in the gene encoding the LDL
receptor.
2. Cystic fibrosis is due to mutation in the
gene encoding CFTR, a chloride
transporter.

53. References :
oAlberts B, Johnson A, Lewis J, et al: Molecular Biology of
the Cell,5th ed. Garland Science, 2008.
ohttps://www.slideshare.net/biochem/membrane-
transport-ppt-biochemistry
oVance DE, Vance J (editors): Biochemistry of Lipids,
Lipoproteins and Membranes, 5th ed. Elsevier, 2008.
oSakai, Hideki; Fujii, Takuto; Takeguchi, Noriaki (2016).
"Chapter 13. Proton-Potassium (H+/K+) ATPases:
Properties and Roles in Health and Diseases.
oDoherty GJ, McMahon HT: Mechanisms of endocytosis.
Annu Rev Biochem 2009;78:857.
oLodish H, Berk A, Kaiser CA,Guyton, Arthur C – Hall,
John E. Textbook of Medical Physiology. 11th
edition. 2006. ISBN 0-7216-0240-1.