Dr Asma Jabeen
Assistant professor
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Define active transport and describe its general
mechanism.
Identify the types of active transport(primary and
secondary) and contrast the differences in between.
Give examples for the substances using active
transport as a method of transport
Compare and contrast passive diffusion, facilitated
diffusion and active transport
Brief the mechanism of vesicular transport(exocytosis
& endocytosis)
Active transport
When a cell membrane moves molecules or ions “uphill”
against a concentration gradient (or uphill against an
electrical or pressure gradient), the process is called
Active transport.
Types:
 Primary active transport
 Secondary active transport
Substances that use active transport mechanism
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Sodium ions
Potassium ions
Calcium ions
Iron ions
Hydrogen ions
Chloride ions
Iodide ions
Urate ions
Several sugars and amino acids
Role of Carrier proteins
 In active transport, the transport depends on carrier
Proteins that penetrate through the cell membrane
 The carrier protein is capable of imparting energy to
the transported substance to move it against the
electrochemical gradient
Primary active transport
The energy for the process is derived directly from the
breakdown of adenosine triphosphate (ATP) or of some
other high energy phosphate compound.
Examples:
 Na K pump
 Primary active transport of Calcium
 Primary active transport of Hydrogen ions
Sodium Potassium Pump
Na-K pump…..General Characteristics
Carrier protein with 2 globular subunits:
Alpha and beta
Alpha subunit (100,000 mol. Wt.)
(larger)
1.Has 3 receptor sites for Na on inside
2.Has 2 receptor sites for K on outside
3.Inside portion has ATPase activity
Beta subunit ( 55,000 mol. Wt.)
(smaller)
Anchors the protein complex in the
lipid membrane.
How the Na K pump works?
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Binding of three sodium inside and two potassium
outside activates ATPase function, cleaves ATP , splitting
it to ADP and liberates high energy phosphate bond of energy.
 The liberated energy then cause a chemical and conformational
change in protein carrier molecule, extruding the three sodium
out and two potassium to inside.
Reverse function of Na K Pump
If the electrochemical gradients for Na and K are
experimentally increased enough,,so that the energy stored in
Their gradients is greater than chemical energy of ATP
hydrolysis, ions will move down their conc gradient and ATP
will be formed from ADP and phosphate.
Significance of sodium potassium pump
 For some cells such as electrically active nerve cells,
60 to 70% of the cell’s energy requirement may be
devoted to pumping sodium out of cell and K into the
cell
 This pump performs a continual surveillance role in
maintaining normal cell volume
Significance of sodium potassium pump
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By the movement of sodium and potassium ions,
a net of one positive charge is moved from the
Interior of the cell to the exterior for each pump
cycle creating negativity on the inside.
It is said to be Electrogenic because it creates an
electrical potential across the cell membrane
Primary active transport of Calcium ions
Calcium ions are normally maintained at extremely low
Concentration in the cytosol of virtually all cells in the
Body…about 10,000 times less than that in ECF.
This is achieved by two primary pumps:
 One is in cell membrane, pumps calcium to outside
 Other one pumps calcium into one of the vesicular
organelles such as SR of muscle, mitochondria of all
cells
 The carrier protein acts as an ATPase enzyme
Primary active transport of Hydrogen ions:
At two places in the body, primary active transport
Of hydrogen ions is important:
1. In the gastric glands of the stomach
2. In the late distal tubules and cortical collecting ducts
of the kidneys
Energetics of Primary active transport
The energy required is proportional to the logarithm of
The degree that the substance is concentrated:
Energy(in calories per osmole) = 1400 log C1/C2
The energy required to concentrate a substance 100 folds
is twice as compared to concentrating a substance 10
times
Secondary Active Transport
The concentration gradient developed due to diffusion
of sodium is a storehouse of energy..This diffusion energy
of sodium can pull other substances along with sodium
through the cell membrane.
Types of Active transport:
 Co transport
 Counter transport
Co- Transport
 The carrier serves as an attachment point for both the
Sodium ions and the substance to be transported.
 Once attached, the energy gradient for sodium causes
both the sodium ions and other substance be transported
together to interior of cell
 e.g. Co transport of glucose and amino acids along
with sodium
Co- Transport
Na+
AA
Na+ gluc
Na+ 2 HCO3-
Counter transport
The type of active transport in which the sodium ions
move in one direction and other ions in the opposite
direction
 Sodium - hydrogen counter transport
 Sodium – calcium counter transport
Counter Transport
Na+
Na+
Ca2+
Na+/HCO3-
H+
Cl-/H+
Passive diffusion, facilitated diffusion & Active transport
• Occurs down a concn.
gradient
• No mediator or involves
a “channel” or “carrier”
• No additional energy
Occurs against a concn.
gradient
• Involves a “carrier”
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• Requires ENERGY
Vesicular Transport
Ingestion by the cell -Endocytosis
Very large particles enter the cell by a specialized function
Of the cell membrane called endocytosis.
 Pinocytosis
 Phagocytosis
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Pinocytosis means ingestion of minute particles that
form vesicles of extracellular fluid and particulate
constituents inside the cell cytoplasm
Phagosytosis means ingestion of large particles, such
as bacteria, whole cells or portions of degenerating
tissue
Receptor-mingestediated
Ingestion by the cell -Endocytosis
endocytosis:
• Molecules attach to
cell-surface receptors
concentrated in clathrincoated pits
• Receptor binding
induces invagination
• Also ATP-dependent
and involves recruitment
of actin and myosin
Figure 2-11
Digestion of Substances
Endocytosisin
Pinocytotic or Phagocytic
Vesicles
Figure 2-12
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
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
Define active transport and describe its general
mechanism.
Identify the types of active transport(primary and
secondary) and contrast the differences in between.
Give examples for the substances using active
transport as a method of transport
Compare and contrast passive diffusion, facilitated
diffusion and active transport
Brief the mechanism of vesicular transport(exocytosis
& Endocytosis)
Thank You