Lecture about Transport of different substances across cell membrane. Continued from last two lectures. Lecture delivered on 26-Jan-2018 to First Year MBBS students in Bannu Medical College, Bannu.

1. TRANSPORT ACROSS
MEMBRANE
BY: Dr. Tehmas Ahmad Khan
Demonstrator-Biochemistry Department

2. • All biological membranes allows transport of certain products inside and outside of
cell.
• For transport Certain integral proteins are required that are Classified as sunder:
• Ion Channels
• Ionophores
• Water Channels (Aquaporin)
• Gap Junction

3. ION CHANNELS
• Pore like structures allowing transport of ions into or outside of cell.
• All channels have gates, and are controlled by opening and closing
• Types of Gates
• Ligand gated channels
• Voltage gated channels

4. PROPERTIES OF ION CHANNELS
• Composed of transmembrane protein subunits.
• Highly selective.
• Well regulated by presence of “gates”.
• Two main types of gates: Ligand-gated and voltage gated.
• Activities are affected by certain drugs.
• Mutations of genes encoding transmembrane proteins can cause diseases.

5. IONOPHORES
• Ionophores contain hydrophilic centers that bind specific ions and are surrounded
by peripheral hydrophobic regions.
• Two Types:
• Mobile ion carriers
• Channel formers

6. WATER CHANNELS AND GAP
JUNCTIONS
• Water Channels are specialized channel proteins that only allow water inside cell.
• Mutation leads to problems in Aquaporin-2 that leads to decreased water
absorption and lead to nephrogenic diabetes insipidus.
• Gap Junctions
• specialized regions on their membranes for intercellular communications which are in
close proximity.

7. TYPES OF TRANSPORT
MECHANISMS

8. PASSIVE OR SIMPLE DIFFUSION
• The solute passes from higher concentration to lower concentration till equilibrium
is reached.
• The process neither requires any carrier protein nor energy.
• Slowest among all.

9. FACTORS AFFECTING NET
DIFFUSION
• Concentration gradient
• Electrical potential
• Hydrostatic pressure gradient
• Temperature
• Permeability coefficient

10. FACILITATED DIFFUSION
• Movement of molecules from higher concentration to lower concentration via a
carrier protein
• The rate of diffusion is faster than simple diffusion
• Example: D-fructose is absorbed from intestine by facilitated diffusion.

11. ACTIVE TRANSPORT
• Against the concentration gradient and electrical gradient.
• Requires energy
• 40 per cent of the total energy requirement in a cell is utilised for active transport
system
• Requires the mediation of specific carrier or transport proteins.

12. TYPES OF TRANSPORT IN ACTIVE
TRANSPORT
• Uniport system Transport single ion in or out of cell
• Co-transport system moves two or more molecules
• Symport carries the two solutes in the same direction.
• Antiport carries the two solutes in the opposite direction.

13. TRANSPORT OF MACRO
MOLECULES
• For transport of big molecules that cannot move into or out of cell via any of the
previous mentioned methods.
• It includes
• Proteins
• Hormones
• immunoglobulins
• low density lipoproteins (LDL)
• viruses

14. EXOCYTOSIS
• Substances created and packaged by Golgi complexes are moved out of cells via
Exocytosis.
• Also called reverse pinocytosis
• Mechanism: Vesicles gets attached to the cell membrane  local influx of Ca++ 
leads to temporary membrane rupture  Substances released outside  vesicles
membrane than integrates with the cell membrane.

15. ENDOCYTOSIS
• Endocytic vesicles are formed when segments of plasma membrane invaginates
enclosing a minute volume of extracellular fluid (ECF) and its contents.
• These vesicles then fuses with lysosomes or other organelles.
• Factors required for endocytosis.
• ATP
• Ca++
• Contractile elements

16. PHAGOCYTOSIS
• Phagocytosis means To engulf something.
• Large particles like Bacteria, viruses and cell debris are engulfed like this.
• Cell extends its pseudopodia around particles and then forms a vesicle like
structure that is called phagopolysome that later attaches to lysosome and is
called phagolysosome.
• This mechanism needs O2 and is called respiratory Burst.

17. PINOCYTOSIS
• Cellular uptake of fluid and fluid contents.
• Fluid phase pinocytosis: Nonselective process in which uptake of a solute by
formation of small vesicles is simply proportionate to its concentration in the
surrounding extracellular fluid (ECF).
• Receptor mediated absorptive pinocytosis: 2 per cent of the external surface of
plasma membrane are covered with receptors. Coming in contact specific fluid
composition lead to invagination of cell membrane into pits that have calithrin
molecule that upon contact signals the actin and myosin filaments to contact and
the invaginated vesicle becomes endosome.

18. THANKS A LOT
• Resources used:
• Textbook of Medical Biochemistry by M.N.CHATTERJEA
• Basic Medical Biochemistry by Lieberman and MARK
• Textbook of biochemistry for medical students by Sreekumari and DM Vasudevan