Particles move across membranes by active transport.
Application: Structure and function of sodium–potassium pumps for active transport and potassium channels for facilitated diffusion in axons.
In the previous IB biology article, passive transport is introduced. The diffusion relies on kinetic energy to move the molecules down the concentration gradient. Actually, molecules can go against the gradient by active transport.
- In reality, many essential molecules are in lower concentration in the external environment than in the cytoplasm. It is impossible for us to depend on the diffusion solely to take up these molecules. In such case, active transport plays a major role.
- Active transport = molecule moves against the concentration gradient
- carrier protein is needed to bring the molecule into the cytoplasm
- carrier protein is specific to certain targets
- unlike the carrier protein mention in the facilitated diffusion, it requires energy
- the driving force is no longer the free kinetic energy from the concentration gradient, a new source of energy is required by breaking down the ATP
- ATP is like a battery, it is produced from mitochondria (like a battery factory)
- ATP gives energy once it breaks down into ADP + Pi (ATP --> ADP + Pi)
Active Transport = molecules move against the concentration gradient with the expense of energy
- sodium-potassium pump is a carrier protein
- it maintains the concentration of sodium (Na+) and potassium (K+) ions in the cell and extracellular fluid
- extracellular fluid: low K+, high Na+
- cell cytoplasm: high K+, low Na+
- the cell is likely to accumulate the K+ and remove Na+ but it goes against the conc. gradient
- thus, sodium-potassium pump is required to go against the conc. gradient
- 3 Na+ are pump out, 2 K+ are pump in by the sodium-potassium pump
- ATP --> ADP + Pi to releases energy to drive the pump
In this IB biology topic, you have to know:
- differences between diffusion and active transport
- carrier proteins in facilitated diffusion and active transport are different in the expense of energy
- sodium-potassium pump 3 sodium out and 2 potassium in to build the conc. gradient
~That is the end of the topic~
1. Socratic Q&A, https://socratic.org/questions/how-do-molecules-move-across-the-plasma-membrane