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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