Diffusion and Osmosis

Diffusion

• Diffusion is the process where particles spread out due to their movement
• Particles diffuse from a high concentration area to a low concentration area
• Particles move "down the concentration gradient" until equilibrium is reached
• At equilibrium, the concentration of particles is the same everywhere
• Higher temperatures speed up diffusion, as particles have more kinetic energy

Cell Membranes and Permeability

• Cell membranes are semi-permeable, allowing some particles to pass through but not others
• Water molecules can typically pass through cell membranes
• Larger molecules like glucose and salts usually cannot pass through

Osmosis

• Nutrients are dissolved in water within the body, forming solutions
• A low concentration of glucose in a cell implies a high concentration of water
• Water moves in/out of cells down the concentration gradient, this is known as Osmosis
• Osmosis occurs because glucose molecules are too large to fit through the membrane
• Water moves to dilute the higher concentration, balancing the concentrations inside/outside cell

Impacts of Water Concentration

• Water enters cells with higher glucose concentrations, diluting the concentration
• Overwatering plants lowers the concentration of nutrients outside root hair cells
• Excess water moves into the root hair cells, causing them to become turgid and potentially burst
• Drinking excessive water can lead to water moving into cells, causing them to swell
• Dehydration occurs when too little water is consumed, and water moves out of cells
• Kidneys regulate water levels in the body, preventing extreme cases of over/under hydration

Potato Cylinder Experiment

• Potatoes are used to demostrate Osmosis in a practical experiment
• Equal-sized cylinders are bored out of a potato, removing any skin
• Potato cylinders are weighed to find initial mass.
• The cylinders are placed in different concentration solutions (e.g., glucose) for a set amount of time
• The cylinders are removed, dried, and weighed again to determine the final mass
• The change in mass is calculated (final mass - initial mass) and converted to percentage change
• A graph of percentage change in mass vs. solution concentration is plotted
• The independent variable is the concentration of solution.
• The dependent variable is the percentage change in mass of potato cylinders
• Controlled variables are: size/shape of cylinders, temperature, solution volume, and time left
• The point where the line crosses the x-axis indicates the glucose concentration inside the potato cells
• Mass increase = water moved into cells due to higher internal glucose concentration
• Mass decrease = water moved out of cells due to higher external glucose concentration

Active Transport

• Salts and sugars enter cells through active transport due to cell membranes being semi-permeable
• Active transport uses carrier proteins in the cell membrane to move molecules in/out of the cell
• Active transport requires energy, unlike diffusion and osmosis
• Active transport can move particles against the concentration gradient (low to high concentration)
• Plants use active transport to absorb minerals from the soil into root hair cells
• Active transport helps to reach already higher concentrations of minerals inside the cells