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water movement, transport across the plasma membrane (passive transport…
transport across the plasma membrane
active transport
uses energy to drive substances against their concentration or electrochemical gradients
Mediated transport:
moves materials with the help of a transport protein
:smiley:
Carrier mediated transport: mode of action
The substrate to be transported directly interacts with the transporter protein
Because the transporter undergoes a conformational change transport rates are slower than those obtained for channel
:smiley: Carrier mediated transport: properties
These transport proteins have properties similar to enzymes
They exhibit: Specificity, Inhibition, Competition, Saturation (transport maximum)
Transport proteins do not catalyze chemical reactions they mediate transport across the cell membrane at a faster than normal rate
Mediated transport can be passive (facilitated) or active
An energy requiring process that moves molecules and ions against their concentration or electrochemical gradients
the difference between primary and secondary active transport
Primary active transport
energy is directly derived from the hydrolysis of ATP
A typical cell uses30% of its energy (ATP) on primary active transport
Primary active transporters: Na pump
The Na pump maintains a low concentration of Na+and a high concentration of K+in the cytosol
This difference in ion concentrations is important for:
•Maintain resting membrane potential
•Electrical excitability
•Contraction of muscle
•Maintenance of steady state cell volume
•Uptake of nutrients via secondary active transporters
•Maintenance of intracellular pH by secondary active transporters
Because Na and K are continually leaking back into the cell down their respective gradients the pump works continuously –known as the pump-leak hypothesis
Primary active transporters: Na/KATPase
•3 Na +ions removed from cell as 2 K+brought into cell.
•Therefore the pump generates a nett current and is electrogenic. Other examples:Ca/K ATPase (Muscle SR), H/K ATPase (stomach)
Secondary active transport:
energy stored in an ionic concentration gradient is used to drive the active transport of a molecule against its gradient
Uses energy stored in an ion gradients created by primary active transporters to move other substances against their own concentration gradient
Thus these transporters indirectly use the energy obtained by hydrolysis of ATP
Cells have many secondary active transporters which are powered by the Na+gradient initially established by the Na pump
passive transport
diffusion through the lipid bilayer:
Important for absorption of nutrients
excretion of wastes
Nonpolar, hydrophobic molecules
oxygen, carbon dioxide, nitrogen, fatty acids, steroids, small alcohols, ammonia and fat-soluble vitamins (A, E, D and K)
moves substances down their concentration or electrochemical gradients with only their kinetic energy
Non-mediated transport:
does not directly use a transport protein
Diffusion through ion channels
The channel forms a water filled pore that shields the ions from the hydrophobic core of the lipid bilayer
Ions do not bind to channel pore. Therefore transport is very rapid
-> so can generate quite large flow of ions
-> generating currents
Properties of channels: ionic selectivity
Specific amino acids lining the pore determine the selectivity of the channel to ions
By being selective to a particular ion, the channel can harness the energy stored in the different ion gradients
Properties of channels: gating
Channels contain gates that control opening and closing of the pore
Different stimuli can control (gate) channel opening and closing
Stimuli include: voltage, ligand binding, cell volume (stretch), pH, phosphorylation
Properties of ion channels: electrical current
The diffusion of over1 million ions per second through a channel generates a measurable current (~10-12 amp)
The current flowing through an individual channel can be recorded using the patch clamp technique
Current fluctuations represent the opening and closing of single ion channels
The current fluctuations represent the conformational changes in channel structure that are associated with channel gating
Facilitated diffusion of glucose
1) Glucose binds to transport protein (GLUT)
2) Transport protein changes shape. Glucose moves across cell membrane (but only down the concentration gradient)
3) Kinase enzyme reduces glucose concentration inside the cell by transforming glucose into glucose-6-phosphate
•Conversion of glucose maintains concentration gradient for glucose entry
The properties driving water movement across cell membranes
