Diffusion vs. Facilitated Diffusion
Diffusion is the passive movement of molecules from an area of higher concentration to an area of lower concentration until equilibrium is reached. It does not require energy (ATP) and occurs directly through a membrane if the substance is permeable.
Facilitated Diffusion is also a passive process, but it involves the use of transport proteins embedded in the cell membrane to help larger or polar molecules that cannot directly diffuse through the lipid bilayer. Like diffusion, facilitated diffusion does not require energy, as it relies on the concentration gradient.
Transport Proteins
A transport protein is a protein that aids in the movement of molecules across a cell membrane. Here are three examples:
- Channel Proteins: These form pores in the membrane that allow specific ions or molecules to pass through, such as potassium channels.
- Carrier Proteins: These bind to specific molecules and change shape to shuttle them across the membrane, such as glucose transporters (GLUT).
- Aquaporins: A specific type of channel protein that facilitates water transport across the cell membrane.
Transporting a Large Molecule into a Cell
If a molecule is very large and needs to cross the cell membrane, it is likely to be transported via endocytosis. This is an active transport process where the cell membrane engulfs the molecule, forming a vesicle that brings the molecule inside the cell. Types of endocytosis include phagocytosis (cell eating) and pinocytosis (cell drinking).
Osmosis and Its Nature
Osmosis is the process by which water molecules move through a selectively permeable membrane from an area of lower solute concentration (higher water concentration) to an area of higher solute concentration (lower water concentration) in an attempt to equalize solute concentrations on both sides of the membrane. Osmosis is a type of passive transport.
Difference Between Osmosis and Diffusion
The main difference between osmosis and diffusion is that diffusion refers to the movement of solute molecules (e.g., glucose or ions) across a membrane, while osmosis specifically refers to the movement of solvent molecules (mainly water) across a selectively permeable membrane. Additionally, osmosis is influenced by the concentration gradient of water, whereas diffusion is influenced by the concentration gradient of the solute.
Effect of Hypotonic Solution on Red Blood Cells
When red blood cells are placed in a hypotonic solution (a solution with a lower solute concentration than the inside of the cells), water will move into the cells via osmosis. This can cause the red blood cells to swell and potentially burst, a process known as lysis.
Effect on a Saltwater Fish in Freshwater
If a saltwater fish is placed in freshwater, it will be unable to osmoregulate properly. The freshwater environment has a lower solute concentration compared to the fish's body fluids. As a result, water will move into the fish's body through osmosis, which can lead to swelling and potential death due to cellular and osmotic stress. Saltwater fish typically excrete small amounts of water and retain salt to maintain their internal environment, but in freshwater, they cannot excrete enough excess water quickly enough.