Active Transport

Quite often substances need to move against their concentration gradient. Active Transport allows this to happen.

I.e. Glucose and amino acids are pumped out from urine back inside the blood. I.e. Gill cells in fish pump out sodium ions into sea water I.e. Maintaining the pH level with in the cell by pumping protons (H+) out. (40 % of your cell’s energy is used for active transport.)

Active Transport Pumps

We will look at 3 types: Sodium – Potassium Pump Purpose: Transports –Sodium ions (Na+) out of the cell –Potassium ions (K+) into the cell

How does it work? With the energy from 1 ATP molecule, –3Na+ ions are able to move out and –2 K+ ions are able to move in. To get the energy stored from ATP, the pump must ‘hydrolize’ (or break a part) –ATP ADP + Pi –(Adenosine Triphosphate – Adenosine (Dipohsphate) + 1 phosphate)

The Process: 3 Na+ ions bind to the Na+ binding site on cytosolic side of the pump 2 K+ ions bind to the K+ binding site on the extracellular side of the pump This Triggers the Na+/K+ pump to hydrolyze 1 ATP molecule (ATP ADP + Pi) The pump then changes shape allowing 3 Na+ ions to move out and 2 K+ ions to move in

The end result from this main pump helps other protein pumps in the membrane transfer ions and molecules against their concentration gradient by creating an artificial concentration gradient. To achieve this, –Na+ ions must continually be diffusing inside the cell and –K+ ions continually be diffusing out.

Two pumps that depend on this are…. 1. Na+/glucose pump I.e. Found in the lining of small intestine Kidney tubules

Here, glucose needs to move against a large concentration gradient Na+ ions and glucose bind to their specific binding sites of this carrier protein Shape of protein changes Na+ and Glucose move easily through

2. K+/H+ Pump I.e. Found in Stomach lining These protein pumps move H+ out of the cell against its concentration gradient to maintain a normal pH level in the cell.

Endocytosis The process where the cell membrane folds around and traps substances from the extracellular fluid to form a vesicle. There are 3 types:

1. Phagocytosis (Cell ‘eating’) (p. 36) Seen in the macrophages of our immune system Eats up bacteria The cell envelops bacteria and other large particles and then internalizes them.

2. Pinocytosis (‘drinking’) The cell takes up small droplets of extracellular fluid and any material dissolved in it.

3. Receptor-mediated endocytosis (p. 37)

Molecules in the extracellular fluid have proteins on their surface that ‘fit’ with a receptor on the cell membrane. (binding protein) The specific receptor on the cell surface recognizes the molecule by its binding protein or protein ‘tag’(I.e. ‘like a code’) and binds tightly to it. This triggers endocytosis Results in a vesicle carrying the macromolecule

I.e. LDL Cholesterol is not water soluble, thus, Must be carried by a particle called LDL LDL –Droplets covered with a single layer of phospholipids –Has a protein ‘tag’ The protein ‘tag’ binds to the cell surface receptor

Triggers endocytosis Once internalized, the vesicles empty its contents The Membrane with receptors (from the vesicle) returns to the surface Turns inside out so receptors face outside once again. Process is repeated

HIV Is a virus with a binding protein that mimics a specific binding protein required for a receptor on the cell surface It tricks the cell to believe it is a macromolecule needed for the cell

Once inside, the virus uses the cell’s ‘machinery’ to replicate Problem? The binding protein on the virus is constantly changing to fit different receptors on various cell surfaces therefore, the cell does not recognize HIV as a virus.

Exocytosis Reverse of endocytosis A vesicle inside the cell moves toward the surface Fuses with the membrane The contents of the vesicle are secreted out into the extracellular fluid I.e. Specialized cells in the pancreas secrete the hormone insulin through exocytosis