Membrane transport systems allow molecules to pass through cell membranes. There are two main types - passive transport, which moves molecules down concentration gradients without energy, and active transport, which moves molecules against gradients by using cellular energy. Passive transport includes diffusion, facilitated diffusion, and osmosis. Active transport involves transmembrane proteins that act as pumps powered by ATP.

1. CHAPTER TWO
Membrane Transport System
Topic: Membrane Transport System
Outline:
• Membrane transport
• Membrane Transport Proteins
• Classes of carrier proteins
• Types of transport
• Passive transport
• Active transport

2. Teaching and learning methods: Lectures, visual aid, interactive forms, assignment and
group discussion.
At the end of this lecture student should be able to answer following questions.
• What do you mean by Membrane transport?
• Explain the Membrane Transport Proteins.
• Explain the Carrier proteins and ion channel.
• Differentiate between Passive and active transport.
Reference books:
1. Lehninger Principles of Biochemistry, 6th Edition- Nelson DL & Cox MM, Chapter 11;
page: 389-395
2. Bacterial Metabolism, 2nd Edition- Gottschalk G; Chapter five; page: 104-109

3. Membrane transport
• In cellular biology the term membrane transport refers to the
collection of mechanisms that regulate the passage of solutes such
as ions and small molecules through biological membranes, which
are lipid bilayers that contain proteins embedded in them.
• The regulation of passage through the membrane is due to
selective membrane permeability - a characteristic of biological
membranes which allows them to separate substances of distinct
chemical nature. In other words, they can be permeable to certain
substances but not to others.
• The movements of most solutes through the membrane are
mediated by membrane transport proteins which are specialized
to varying degrees in the transport of specific molecules.

4. Figure-1: Fluid mosaic model for membrane structure.

5. Membrane Transport Proteins
• Many molecules must move back and forth from inside and
outside of the cell.
• Most cannot pass through without the assistance of proteins in
the membrane bilayer.
• Each cell has membrane has a specific set of proteins depending
on the cell.
2 kinds of proteins involved
1. Carrier proteins
2. Ion channels

6. 1. CARRIER PROTEINS
 Carrier proteins facilitate the diffusion of different molecules
 Carrier proteins are integral/intrinsic membrane proteins; they exist
within and span the membrane across which they transport substances.
 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

7. Classes of carrier proteins:
Uniport (facilitated diffusion) carriers mediate to transport a single
solute.
Symport (cotransport) carriers bind two dissimilar solutes
(substrates) & transport them together across a membrane.
Transport of the two solutes is obligatorily coupled.
Antiport (exchange diffusion) carriers exchange one solute for
another across a membrane.

8. 2. ION CHANNELS:
 Channel proteins are involved in the
movement of ions, small molecules, or
macromolecules (such as another
protein) across a biological membrane
 Formed by proteins with a central pore
that is lined with charged groups.
 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

9. Impermeable Membranes
• Ions and hydrophilic molecules cannot
easily pass through the hydrophobic
membrane.
• Small and hydrophobic molecules can
easily pass through the hydrophobic
membrane.

10. Types of transport
A. Passive transport
— Simple diffusion
— Facilitated diffusion
— Osmosis
Active transport
A. Primary active transport
B. Secondary active transport
Passive transport
a) Simple Diffusion
— No energy required
— Down concentration gradient
— Molecules equally distribute across available area by type
— Non-polar molecules (steroids, O2, CO2)

12. b) Facilitated diffusion
—No energy required
—Down concentration gradient
—Molecules equally distribute but cross membrane with the help of
a channel protein or carrier protein.

13. a) Osmosis:
—Movement of water across cell membrane
—Water crosses cell membranes via special channels called
aquaporins
—Solutes are impermeable to cell membrane.
—Moves into/out of cell until solute concentration is balanced.

15. •Hypotonic solution – fewer solutes in solution
•Isotonic solution – equal solutes in solution
•Hypertonic solution – more solutes in solution

17.  Active transport
 Active transport is the movement of molecules across a cell membrane in the
direction against their concentration gradient, i.e. moving from an area of
lower concentration to an area of higher concentration.
 Active transport is usually associated with accumulating high concentrations
of molecules that the cell needs, such as ions, glucose and amino acids.
 If the process uses chemical energy, such as from adenosine
triphosphate (ATP), it is termed primary active transport.
 Secondary active transport involves the use of an electrochemical.
 Active transport uses cellular energy, unlike passive transport, which does
not use cellular energy.
 Examples of active transport include the uptake of glucose in the intestines in
humans and the uptake of mineral ions into root hair cells of plants.

19.  Primary active transport
— Primary active transport, also called direct active transport, directly uses
metabolic energy to transport molecules across a membrane.
— Most of the enzymes that perform this type of transport are transmembrane
ATPases. Other sources of energy for Primary active transport
are redox energy and photon energy (light).
— An example of primary active transport using Redox energy is the
mitochondrial electron transport chain that uses the reduction energy
of NADH to move protons across the inner mitochondrial membrane against
their concentration gradient.
— An example of primary active transport using light energy are the proteins
involved in photosynthesis that use the energy of photons to create a proton
gradient across the thylakoid membrane and also to create reduction power
in the form of NADPH.

20. Secondary active transport
—In secondary active transport, also known as coupled transport or co-
transport, energy is used to transport molecules across a membrane;
however, in contrast to primary active transport, there is no direct coupling
of ATP; instead it relies upon the electrochemical potential difference created
by pumping ions in/out of the cell.
—Permitting one ion or molecule to move down an electrochemical gradient,
but possibly against the concentration gradient where it is more concentrated
to that where it is less concentrated increases entropy and can serve as a
source of energy for metabolism (e.g. in ATP synthase).

21.  Process
There are two types of active transport: primary and secondary. In primary active transport,
specialized trans-membrane proteins recognize the presence of a substance that needs to be
transported and serve as pumps, powered by the chemical energy ATP, to carry the desired
biochemical across. In secondary active transport, pore-forming proteins form channels in the
cell membrane and force the biochemical across using an electromagnetic gradient. Often, this
energy is gained by simultaneously moving another substance down the concentration
gradient
Example of primary active
transport, where energy from
hydrolysis of ATP is directly
coupled to the movement of
a specific substance across a
membrane independent of
any other species.

22. Active transport:
Na+K+ Pump (Na+K+ATPase)
3 Na+ out
2 K+ in

23. Active Transport Passive Transport
Definition
Active Transport uses ATP to pump molecules
AGAINST/UP the concentration gradient. Transport
occurs from a low concentration of solute to high
concentration of solute. Requires cellular energy.
Movement of molecules DOWN the concentration
gradient. It goes from high to low concentration, in
order to maintain equilibrium in the cells. Does not
require cellular energy.
Types of
Transport
Endocytosis, cell membrane/sodium-potassium pump &
exocytosis
Diffusion, facilitated diffusion, and osmosis.
Types of
Particles
Transported
Proteins, ions, large cells, complex sugars. Anything soluble (meaning able to dissolve) in lipids,
small monosaccharides, water, oxygen, carbon dioxide,
sex hormones, etc.
Examples
Phagocytosis, pinocytosis, sodium/potassium pump,
secretion of a substance into the bloodstream (process is
opposite of phagocytosis & pinocytosis)
Diffusion, osmosis, and facilitated diffusion.
Importance
In eukaryotic cells, amino acids, sugars and lipids need to
enter the cell by protein pumps, which require active
transport. These items either cannot diffuse or diffuse
too slowly for survival.
It maintains equilibrium in the cell. Wastes (carbon
dioxide, water, etc.) diffuse out and are excreted;
nutrients and oxygen diffuse in to be used by the cell.
Functions
Transports molecules through the cell membrane against
the concentration gradient so more of the substance is
inside the cell (i.e. a nutrient) or outside the cell (i.e. a
waste) than normal. Disrupts equilibrium established by
diffusion.
Maintains dynamic equilibrium of water, gases,
nutrients, wastes, etc. between cells and extracellular
fluid; allows for small nutrients and gases to enter/exit.
No NET diffusion/osmosis after equilibrium is
established.