Biological membranes and transport

1. presentationtitle
Biological membranes and transport

3. Introduction
• Membranes define the external boundary of cells and
regulate the molecular traffic across that boundary.
• It separates and protect the cell from the external
environment.
• Plasma membrane also provide a connecting system
between the cell and its environment.
• Membranes are tough but flexible, self sealing, and
selectively permeable to polar solutes.
• Their flexibility permits the shape changes that
accompany cell growth and movements.

4. Composition of membranes
• The membranes are composed of lipids, protein and
carbohydrates.
• The actual composition differ from tissue to tissue.
Among the lipids, amphipathic lipids (containing
hydrophobic and hydrophilic groups) namely
phospholipids, glycolipids and cholesterol are found
in animal membranes.
• Many animal cell membranes have thick coating of
complex polysaccharides referred to as glycocalyx.

6. Structure of membranes
• Fluid mosaic model proposed by Singer and Nicolson
is a more recent and acceptable model for membrane
structure.
• The biological membrane usually have a thickness of
5 – 8 nm.
• A membrane is essentially composed of a lipid
bilayer.
• The hydrophobic (nonpolar) region of the lipids face
each other at core of the bilayer while the hydrophilic
(polar) region face outward.

7. More than lipids…
• In 1972, S.J. Singer & G. Nicolson proposed that
membrane proteins are inserted into the phospholipids
bilayer.
It’s like a fluid
It’s like a mosaic
It’s the fluid mosaic
model

8. Phospholipids

9. • Fatty acid tails – hydrophobic
• Phosphate group head – hydrophilic
• Arranged as a bilayer

10. Membrane proteins
• Membrane proteins are the nanomachines that enable
membranes to send and receive messages and to
transport molecules into and out of cells and
compartments.
• Without membrane proteins the phospholipid
membrane would present an impenetrable barrier and
cells would be unable to communicate with their
neighbors, transport nutrients into the cell or waste
products out of it, or respond to external stimuli.

11. Cont…
• Both unicellular and multicellular organisms need
membrane proteins in order to live. The membrane
proteins that are present in a particular membrane
determine the substances to which it will be
permeable and what signal molecules it can
recognize.

12. Types of membrane proteins
Based on their structure, there are main three types
membrane proteins.
1. Integral membrane protein t that is
permanently anchored or part of membrane.
2. Peripheral membrane protein that is
temporarily attached to lipid bilayer or integral
proteins.

14. Cont….
3. Lipid anchored proteins
These are proteins located on the surface of the cell
membrane that are covalently attached
to lipids embedded within the cell membrane.
These proteins insert and assume a place in the bilayer
structure of the membrane alongside the similar fatty
acid tails.

15. Functions of biological membrane
• Biological membranes carry out functions that are
indispensable for life.
• provide a barrier against the extracellular
environment
• Give shape to the cell, creating a matrix for insertion
of proteins
• storing and transmitting energy
• receiving and amplifying signals

16. Transport
Across
biological
membrane

17. Transport across cell membrane
• Biological membrane regulates the movement into
and out of the cell. The cell membrane is selectively
permeable. It lets some substances pass through
rapidly, and some substances pass through more
slowly, but prevents other substances passing through
it at all.
• Lipid soluble substances, water & urea can easily
pass through the lipid bilayer of the cell membrane

18. • The lipid bilayer of the cell membrane is
impermeable to lipid insoluble substances such as
ions & charged or polar molecules like glucose.
These substances pass through specialized protein
channels, carrier proteins & active pump mechanism.
Large macromolecules are transported through
vesicles.

19. Types of transport
• Passive transport
Diffusion – simple, facilitated
Osmosis
• Active transport
Primary – Secondary
• Vesicular transport
Endocytosis – Exocytosis – Transcytosis

20. No energy needed
• Passive transport
Energy needed
• Active transport

21. Passive transport

22. Diffusion
Simple diffusion
• It is the movement of
ions or molecules from
a region of their high
concentration to a
region of their low
concentration, without
the expenditure of
energy
Facilitated diffusion
• Facilitated diffusion is
the movement of
specific molecules (or
ions) across the plasma
membrane, assisted by a
carrier protein.
• No energy required

23. • Diffusion occurs down a concentration gradient.
• Facilitated diffusion is of two types
1. Channel mediated
2. Carrier mediated
• In channel mediated facilitated
diffusion, ions or small molecules move through a water
filled channel. It functions much like a bridge over a
river that must rise and lower in order to allow boats to
pass. It is of two types
 Leak channel always open
 Gated channel require a stimulus (binding
of an ion) to open.
Difference
in type of
protein
used

24. • Carrier mediated facilitated diffusion is the
movement of small polar molecules across the
membranes by a carrier protein that changes its shape
to pass through. Such as glucose molecules.

26. Osmosis
.
• Osmosis is the movement of water molecules
(solvent) through a selectively permeable membrane/
semi permeable membrane like the cell membrane.
• Water diffuses across a membrane from an area of
high concentration to an area of low concentration.
• Semi-permeable membrane is permeable to water, but
not to the solute i.e., sugar

28. Active transport
• Molecules move against the concentration gradient
(low to high)
• Energy must be provided
• Exhibit saturation kinetics
Active transport is divided into
two types according to the source of the energy used to
cause the transport:
1. Primary active transport
2. Secondary active transport.

29. Primary active transport
• It is also called direct active transport, directly uses
chemical energy such as ATP to transport solutes
across a membrane against their concentration
gradient.
• An example of primary active transport is sodium-
potassium pump which moves sodium ions to the
outside of the cell and potassium ions to the inside of
the cell.

31. Secondary active transport
• Secondary active transport is the transport of two
different molecules across a transport membrane
using energy in other forms than ATP.
• Two types of molecules are transported at once.
• Concentration gradient of driving ion provides the
energy for the transport of driven ion/molecule
against the concentration gradient.
• Moving down its electrochemical gradient is referred
to as the driving ion because it is movement of
this ion that drives the uphill movement of other ion.

32. • Symport and antiport are two types of trans
membrane proteins
• Symport moves molecules in same direction
• Antiport` moves molecules in different
direction.
• Glucose sodium pump co transporter is an example.

34. Vesicular transport
Transport of larger substances such as proteins,
carbohydrates by membranous sac/vesicles
Endocytosis
Cell intakes contents from outside of cell. Plasma
membrane traps the substance by folding inward. The
lipid bilayer then fuses to form a vesicle tthat engulfs
the substance.
Types
1. receptor mediated endocytosis
2. phagocytosis (solids)
3. pinocytosis (liquids)

35. • Phagocytosis target large structure as bacteria and
food particles as white blood cells engulf bacteria
through phagocytosis.
• Pinocytosis is non specific as amino acids, fatty acids
etc
• Receptor mediated endocytosis is specific for
substances recognized by a cell surface receptor.

37. Exocytosis
• Exocytosis is the process by which a large amount of
molecules are released; thus it is a form of bulk
transport. In exocytosis, membrane-bound secretory
vesicles are carried to the cell membrane, and their
contents (i.e., water-soluble molecules) are secreted
into the extracellular environment.