Active Transport

Topics 2.6 and 2.9

Active Transport Overview

• Requires free energy (ATP) to move molecules

Active Transport: Membrane Pumps

• Uses membrane proteins embedded in the membrane to move solutes against their gradients

• Allows cells to maintain concentration gradients that differ from their surroundings

• Some pumps establish and maintain membrane potentials (voltage)

• Examples: sodium-potassium pump, proton pump

Factors Affecting Cellular Transport

• Chemical driving force

• Electrical driving force

• Electrochemical driving force (combination)

Sodium-Potassium Pump(video link – please watch – 2:26)

• Pumps sodium ions (Na+) out of the cell and potassium ions (K+) into the cell

• Regulates osmolarity of cytosol by controlling the solute concentration inside the cell

• Maintains and restores the resting potential of cells, such as neurons, cardiac and kidney

• Primary pump in animal cells

Nerve Cell Signaling(video link – please watch – 2:52)

Proton Pump(video link – please watch – 1:04)

• Moves protons (H+) against the concentration gradient in order to generate a proton gradient across the membrane

• Used in the synthesis of ATP during cellular respiration

• Causes changes in pH

Active Transport: Bulk Transport

• Involves changes in the membrane structure

• Transports macromolecules (proteins, polysaccharides, etc.) by packaging the materials into vesicles

Exocytosis

• Cell secretes large molecules by fusion of the vesicles with the cell membrane

Exocytosis Example

• Release of neurotransmitters between neurons

Endocytosis• Region of the cell membrane sinks inward and

pinches off to form a vesicle to bring extracellular material into the cell

Endocytosis: Phagocytosis

• Cells engulf large particles or other cells using pseudopods

• Example: white blood cell consuming bacteria, ameba consuming food

Endocytosis: Pinocytosis

• Droplets of extracellular fluid are taken into the cell in small vesicles

• Example: Uptake of nutrients by the microvilliof the small intestine

Receptor-Mediated Endocytosis

• Enables cells to acquire specific substances from extracellular fluid

• Molecules bind to receptors

• Example: cholesterol uptake from the blood

Review of Cellular Transport(video link – please watch – 4:44)