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Active Transport, Summaries of Human Genetics

University of the Sciences in PhiladelphiaHuman Genetics

In primary active transport, the energy is derived directly from breakdown of adenosine triphosphate (ATP) or from some other high-energy phosphate.

Typology: Summaries

2021/2022

Uploaded on 09/27/2022

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What is a carrier protein?
•A carrier protein spans the thickness of the plasma membrane and
changes its conformation so that specific binding sites within the
carrier are alternately exposed to the ECF and ICF.
•Carrier-mediated transport systems display 3 characteristics:
1. Specificity: e.g. glucose cannot bind to amino acid carriers and
vice versa.
2. Saturation: A limited no. of carrier binding sites are available
within a particular plasma membrane for a specific substance.
Thus, there is a limit to the amount of substance a carrier can
transport across the membrane in a given time. This is called
Transport Maximum (Tm).
3. Competition: Several different substances are competing for the
same carrier site.
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What is a carrier protein?

  • A carrier protein spans the thickness of the plasma membrane and changes its conformation so that specific binding sites within the carrier are alternately exposed to the ECF and ICF.
  • Carrier-mediated transport systems display 3 characteristics: 1. Specificity: e.g. glucose cannot bind to amino acid carriers and vice versa. 2. Saturation: A limited no. of carrier binding sites are available within a particular plasma membrane for a specific substance. Thus, there is a limit to the amount of substance a carrier can transport across the membrane in a given time. This is called Transport Maximum (Tm). 3. Competition: Several different substances are competing for the same carrier site.

ACTIVE TRANSPORT

Definition:

Active transport is a carrier-mediated transport

wherein molecules and ions are moved against

their concentration gradient across a membrane

and requires expenditure of energy.

The primary active transport carriers are termed

as pumps.

Active transport is divided into 2 types according

to the source of the energy used.

Types of Active Transport:

Active transport is divided into 2 types depending on the source of energy used: In primary active transport , the energy is derived directly from breakdown of adenosine triphosphate (ATP) or from some other high-energy phosphate compound. In secondary active transport , the energy is derived secondarily from energy stored in the form of an ion concentration gradient between the two sides of a cell membrane, created originally by primary active transport. Thus, energy is used but it is “secondhand” energy and NOT directly derived from ATP. In both instances, transport depends on carrier proteins. However, in active transport, the carrier protein functions differently from the carrier in facilitated diffusion because it is capable of imparting energy to the transported substance to move it against the electrochemical gradient by acting as an enzyme and breaking down the ATP itself.

Primary Active Transport

  • In primary active transport, energy in the form of ATP is required to change the affinity of the carrier protein binding site when it is exposed on opposite sides of plasma membrane.
  • The carrier protein also acts as an enzyme that has ATPase activity, which means it splits the terminal phosphate from an ATP molecule to yield ADP and inorganic phosphate plus free energy. Examples:
  1. Sodium-Potassium Pump.
  2. Transport of Hydrogen ions: occurs at 2 places in the human body:
    • in the gastric glands of the stomach
    • In the kidneys

Na-K PUMP:

  • The carrier protein in the pump has 2 subunits; a larger α subunit and a smaller β subunit. They perform the following functions: 1. 3 receptor sites for binding Na ions on the portion of the protein that protrudes to the inside of the cell. 2. 2 receptor sites for potassium ions on the outside.
  1. The inside portion of this protein near the sodium binding site has ATPase activity.

FUNCTIONS OF SODIUM-POTASSIUM PUMP:

1. Control the Volume of each cell: It helps regulate cell

volume by controlling the concentrations of solutes inside the cell and thus minimizing osmotic effect that would induce swelling or shrinking of the cell. If the pump stops, the increased Na concentrations within the cell will promote the osmotic inflow of water, damaging the cells.

2. Electrogenic nature of the pump: It establishes Na and K

concentration gradients across the plasma membrane of all cells; these gradients are critically important in the ability of nerve and muscle cells to generate electrical signals essential to their functioning.

3. Energy used for Secondary active transport: The steep Na

gradient is used to provide energy for secondary active transport.

Co-Transport/ Symport

CO-TRANSPORT OR SYMPORT:

  • The carrier protein has two binding sites: one for the solute being moved against its concentration gradient and one for Na.
  • Sites: intestinal and kidney cells
  • INTESTINAL CELLS: more Na+ is present in the ECF (in the intestinal lumen) than inside the cells (because Na-K pump moves the Na out of the cell keeping its intracellular conc. low).
  • Because of this conc. difference, more Na binds to the carrier protein in the ECF.
  • Binding of Na increases the affinity of the protein for Glucose which is present in low conc. In the ECF.
  • When both Na and Glucose are attached to the carrier protein, it undergoes a conformational change and opens to the inside of the cell.
  • Both Na & glucose are released to the inside of the cell: Na as there is low conc. & glucose as carrier proteins affinity for it decreases as Na is released.
  • The released Na is quickly pumped out by the Na-K pump, keeping the levels of intracellular Na low.
  • Thus, Na has been moved down its “downhill” while glucose is moved “uphill”.

SECONDARY ACTIVE TRANSPORT

CO-TRANSPORT

  • Symport
  • Na moves downhill
  • Molecule to be co- transported moved in the same direction as Na, i.e. to the inside of the cell.
  • E.g. Na with glucose and amino acids.
  • Site: intestinal lumen and renal tubules of kidney. COUNTER TRANSPORT - Anti-port - Na moves downhill - Molecule to be counter- transported moves in the opposite direction to Na, i.e. to the outside of the cell. - E.g. Na with Calcium and Hydrogen ions. - Site: Na-Ca counter transport in almost all cells of the body and Na-H + in the proximal tubules of the kidney.

REVIEW:

MATCH:

  • Diffusion
  • Osmosis
  • Carrier-mediated transport
  • Facilitated Diffusion
  • Primary active transport
  • Secondary active transport
    1. A passive transport process which can be saturated at high substrate conc.
    2. Depends on a solute conc. to drive the movement of solvent across the plasma membrane.
    3. This uses ATP breakdown to move the substance from low to high conc.
    4. This uses a conc. Gradient for one substance to drive the transport of another.