Diffusion and Osmosis

Diffusion and Osmosis Chapter 3, Section 4 Of your textbook

Passive Transport • Particles are constantly in motion, colliding and scattering. • This motion is random. • Does not require the cell to use ATP / energy • The membrane is still semi-permeable, so only certain substances can cross (in or out) by passive transport.

Passive Transport • A concentrationgradient is the difference in the concentration of a substance from one location to another • When there is a concentration gradient, the NET movement is DOWN their concentration gradient - from areas of higher concentration to areas of lower concentration ANIMATION

Diffusion • Movement of molecules in a fluid or gas from an area of higher concentration to an area of lower concentration ANIMATION

Equilibrium • Diffusion continues until the solution is at equilibrium (no concentration gradient). • Molecules still move, but there is no NET movement.

Facilitated Diffusion • Some small, polar molecules cannot easily diffuse across a membrane. • They need the help of transport proteins.

Facilitated Diffusion • The process of diffusion being “helped” by transport proteins is called facilitateddiffusion. • Still passive transport because molecules are still moving down their concentration gradient • Requires no ATP / energy • ANIMATION

Transport Proteins • In facilitated diffusion, transport proteins, pierce the cell membrane and allow openings for molecules to pass. • These proteins are considered integral proteins because they are fully embedded in the membrane.

Transport Proteins • There are many types of transport proteins. • Most only allow certain ions or molecules to pass. • Some are simple channels or tunnels and some are more complex, shape-changing proteins.

Osmosis • The diffusion of water across a semipermeable membrane is called osmosis. • In a solution, there are water molecules and dissolved particles (the solute). • The more dissolved particles there are, the lower the concentration of water molecules. ANIMATION

Solutions • Water is considered the solvent. • The substance(s) dissolved in water is / are the solute(s). • Together, solvent + solute  solution.

Comparing Solutions • A solution may be desribed as isotonic, hypertonic or hypotonic relative to another solution • These are comparisons; they require a point of reference (ie, my hair is shorter… …than it was last year). • The comparison in biology is usually to the inside of a cell.

Isotonic • A solution is isotonic to a cell if it has the same concentration of dissolved particles as the cell. • This means the water concentration is also the same. • Water molecules move into and out of the cell at an equal rate in an isotonic solution. • The cell size remains the same.

Hypertonic • A hypertonic solution has a higher concentration of dissolved particles than a cell. • This means the water concentration is lower than that of the cell. • Thus, water flows out of the cell – so, the cell will shrivel and eventually die. ANIMATION

Hypotonic • A hypotonic solution has a lower concentration of dissolved particles than a cell. • Therefore the water concentration is higher than that of the cell. • Thus, water diffuses into the cell – causing the cell to expand and potentially burst.

Impact on Cells • In an isotonic solution (center), water enters / exits red blood cells at equal rates. • In a hypertonic solution (like salt water – right), water rushes out and the cell shrivels. • In a hypotonic solution (like distilled water – left), water rushes in and the cell swells / bursts (lysis). Video clips: RBC in isotonic solution RBC in hypertonic solution RBC in hypotonic solution

Adaptations - Plants • Plant cells use the cell wall to prevent bursting. At center, the plant cell is in an isotonic solution. Water moves in / out at equal rates (no NET movement) At left, the plant cell is in a hypotonic solution. Water rushes in, filling the vacuole. This cell is turgid / has high turgor pressure. At right, the plant cell is in a hypertonic solution. Water rushes out of the cell, draining the vacuole. This is called plasmolysis. Video: Elodea in isotonic / hyper / hypo

Adaptations - Protists • Paramecia live in freshwater • This makes paramecia hypertonic to their surroundings • Water is constantly rushing into the paramecium • So the paramecium uses a contractile vacuole to pump the water back out (and prevent bursting) Video: The contractile vacuole in action

Diffusion and Osmosis