04/10/2010
Four types of transport
Transport Across Cell Membranes
1.
Getting things in and out of cells
2.
3.
4
4.
Part Two
Simple diffusion
Facilitated diffusion
Osmosis
Active transport
Active transport
Passive
transport
Active transport

Moves molecules AGAINST their concentration gradients
◦ Imagine biking uphill vs. Biking downhill
ACTIVE TRANSPORT

Input of energy is required
Direct active transport

ATP provides the energy input
DIRECT
Active
transport
ATP‐driven
Symport
INDIRECT
ion‐driven
Antiport
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04/10/2010
Adenosine triphosphate
Adenosine triphosphate (ATP)

Primary source of free energy in living cells.

Composed of:
p

PO4 groups are negatively charged 
unstable

Hydrolyzing the bond between the 2nd
and 3rd PO4 4 releases free energy
gy
◦ adenine
◦ ribose
◦ 3 phosphate (PO4) groups
Electrochemical potentials
Example: Sodium/Potassium Pump
[K+] is very high inside the cell and low outside.
[Na+] is the opposite ‐ high outside, low inside.
The maintains these concentration gradients


Chemical potential – Concentration gradient
Electrical potential – Charge gradient
Since ions are charged, if a chemical potential exists, so does an electrical one
Each time the pump works, it moves:
◦3 Na+ ions out
◦2 K+ ions in
For charged molecules, this potential determines the direction of movement.
Ex. 2 – K+ constantly diffuses out
Direction of gradient
Chemical
Ex. 1 – Na+ constantly enters cells Electrical
Net
ECF
+
Direction of gradient
Chemical
Electrical
-
Net
ICF
ECF
+
ICF
The interior of the cell membrane is always more negative than the exterior
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04/10/2010
Indirect active transport

Energy is provided by ions

Ions move along their electrochemical gradients, through membrane pumps

Electrochemical gradients of these ions are
are maintained by ATP pumps (direct maintained by ATP pumps (direct
active transport)

The pumps harness the kinetic energy The
pumps harness the kinetic energy
released, and use it to transport other molecules against their concentration gradients
DIRECT
Active
transport
ATP‐driven
Symport
INDIRECT
ion‐driven
Antiport
Symport & & Antiport
Antiport

Indirect active transport aka cotransport

Types:
Symport
Antiport
•Ion and molecule travel in same •Opposite directions: One goes in, direction
the other goes out
aa. symport
b. antiport
Symport: Example
Symport
: Example
Antiport: Example
Antiport
: Example
Na+/glucose transporter
Ca2+ transporter
◦ Na+ travels inwards
◦ Glucose is transported in at the same time, against its concentration gradient
◦ Driving ion (usu. Na+) travels in
◦ Ca2+ is pumped out
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SUMMARY
Ion‐driven transport can be classified according to the relative direction of molecule transport: Membrane transport can be:
1. Passive (no E input required)
2. Active
Active transport can be powered by:
a. ATP b. Ionic electrochemical gradients
i. same (symport)
ii. Opposite (antiport)
Passive
Passive
Direct
MEMBRANE
TRANSPORT
Direct
MEMBRANE
TRANSPORT
ATP‐driven
Active
Symport
Active
Indirect
ion‐driven
ATP‐driven
Symport
Indirect
ion‐driven
Antiport
Antiport
Endocytosis

For getting large molecules in and out...

ENDOCYTOSIS & EXOCYTOSIS
& EXOCYTOSIS
Membrane folds in (“invaginates”)  Desired molecule is captured
 Invagination deepens  Membrane pinches off to form a vesicle/vacuole

Endo – in
Cell membrane engulfs molecules from outside
Endocytosis

Pinocytosis – “Cell drinking”
◦ Liquids are captured, along with any solutes

Phagocytosis – Solids are captured
Solids are captured
◦ If selective uptake is required, it involves interactions between membrane receptors and vesicle tags.
 Proteins, cholesterol
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04/10/2010
Exocytosis



Exo – out
Outbound substances are packed into vesicles
Vesicle membrane fuses with cell membrane –
expels contents.
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