Membrane transport active transport,passive transport

1. 5.3 , 5.4 Active and Passive
transport
PREPARED BY: Ms. HEMAXI ZALA

3. Transport of small molecules
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There are two types of membrane transport
mechanism used for transport of small
molecules:
Passive transport
Simple diffusion
Facilitated diffusion
2. Active transport
ATP driven active transport
Ion driven active transport

4. PASSIVE TRANSPORT

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The facilitated diffusion of a molecule across a biological
membrane is dependent on specific integral protein called,
uniporters.
The molecule bind to the protein on one side of the
membrane, the protein then undergoes a conformational
change, transport the molecule across the membrane and
then releases it on the other side.
Molecule transported across membrane in this way
include hydrophilic molecule such as glucose, other sugars
and aminoacids.
The transport proteins are capable of being saturated,
display michaelis-menten type binding kinetics, and are
influenced by temperature, Ph and inhibitor molecule in a
similar manner to enzymes.

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An example of facilitated diffusion is the uptake of glucose into
erythrocytes by the glucose transporter.
The erythrocyte glucose transporter an integral membrane
protein of mass 45kDa that is asymmetrically orientated in the
plasma membrane.
This uniporter protein structure transverses the membrane with
12 α helices which form a central pore through which the glucose
molecule is passed upon conformational changes in the protein.

9. ACTIVE TRANSPORT
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The active transport of molecule requires
an input of metabolic energy.
This can be derived either from direct
coupling to the hydrolysis of ATP or by
coupling to the movement of an ion down
its concentration gradient.

10. ATP- driven active transport
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The energy required for the transport of the molecule across
the membrane is derived from the coupled hydrolysis of ATP.
For example, the movement of Na+ and k+ ions by Na+/
k+ATPase.
All cells maintain a high internal concentration of k+ and a
low internal concentration of Na+.
The resulting Na+/k+ gradient across the plasma membrane
is important for the active transport of certain molecules, and
the maintenance of the membrane electrical potential.
The movement across the membrane of Na+,k+,ca+2 and H+,
as well as a number of other molecules, Is directly coupled to
the hydrolysis of ATP.

11. STRUCTURE AND ACTION OF THE
Na+/k+ATPase.
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The Na+/k+ATPase is an integral membrane protein consisting
of 110kDa α and 55kDa β subunits.
The functional unit is either a heterotetramer(α2β2) or more
likely a heterodimer(αβ).
Upon hydrolysis of one molecule of ATP to ADP and pi, the
protein undergoes a conformational change and three Na+ ions
are pumped out of the cell across the plasma membrane and two
k+ ions are pumped in the opposite direction into the cell.
Both ions are being moved up their concentration gradient across
the membrane; hence the requirement for an input of energy.
No transport occurs unless ATP is hydrolyzed, and no ATP is
hydrolyzed if there is no Na+ and k+ to transport.

13. Ion driven active transport
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The movement of the molecule to be transported
across the membrane is coupled to the
movement of an ion, usually either Na+ or H+.
The energy for the movement of the molecule
across the membrane against its concentration
gradient comes from the movement of the ion
down its concentration gradient.

17. Glucose transport into intestinal
epithelial cells
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The cells lining the lumen of the intestine are
polarized, that is they have two distinct sides or
domain which have different lipid and protein
compositions.
The apical or brush border membrane facing the
lumen is highly folded into microvilli to increase
the surface area available for the absorption of
nutrients.
The rest of the plasma membrane, the basolateral
surface, is in contact with neighboring cells and
the blood capillaries.

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Movement between adjacent epithelial cells is prevented by the
formation of light junctions around the cells near the apical domain.
Thus any nutrient molecule in the lumen of the intestine have to pass
through the cytosol of the epithelial cells in order to enter the blood.
Glucose are transported across the apical membrane from a relatively
low concentration in the lumen of the intestine to a relatively high
concentration in the cytosol of the epithelial cells by a glucose/Na+
symporter protein.
This is a form of ion driven active transport, the energy for the
movement of glucose against its concentration gradient comes from
the movement of Na+ down its concentration gradient.
The blood flowing through the capillaries on the basolateral side of
the epithelial cell maintains a concentration gradient of glucose
across this membrane allowing the glucose to move out of the cell by
facilitated difffusion through a glucose transporter which is similar to
the erythrocyte glucose transporter.
The relatively low concentration of Na+ inside the epithelial cell is
maintained by Na+/k+ ATPase on the basolateral membrane, an
example of ATP-driven active transport.

20. Ionophores
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What are Ionophores?
Ionophores are lipid-soluble molecules,
usually synthesized by microorganisms to
transport ions through a lipid bilayer of
cell membrane.

21. Ion Transport
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1. ion carriers
Ion carriers can transfer ions from a
hydrophilic medium, such as water, into a
hydrophobic medium, i.e a biological
membrane, where the ions typically would not
be soluble.
They can do this by binding to particular ions
and acting as a mobile carrier, escorting them
through the hydrophobic environment of cell
membranes, or they can form ion channels.

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2.Ion channel
Ion channels form pores in membranes
through which ions can pass. There are several
types of ion channels, each regulated by
various mechanisms designed to allow only
certain ions to flow into and out of a cell and
only at certain times.