Osmosis: Striking a Balance

Maintaining A Balance Cells are surrounded by watery solutions and are filled by watery solutions. Cells are surrounded by watery solutions and are filled by watery solutions. A solution is a mixture in which one or more substances (Solutes) are dissolved in another substance (Solvent) A solution is a mixture in which one or more substances (Solutes) are dissolved in another substance (Solvent) The concentration of a solute is important to organisms. The concentration of a solute is important to organisms. Organisms cannot live unless the concentration of dissolved substances stays within a narrow range. Organisms cannot live unless the concentration of dissolved substances stays within a narrow range.

Balance and Homeostasis This balance of conditions is called Homeostasis This balance of conditions is called Homeostasis The movement of water across the membrane helps to balance the concentrations of dissolved substances in a cell The movement of water across the membrane helps to balance the concentrations of dissolved substances in a cell The diffusion of water across a selectively permeable membrane is called osmosis The diffusion of water across a selectively permeable membrane is called osmosis

Osmosis Like other molecules the movement of water is controlled by differences in concentration (a concentration gradient) Like other molecules the movement of water is controlled by differences in concentration (a concentration gradient) Water will move in the direction where there is a high concentration of solute (hence a lower concentration of water) Water will move in the direction where there is a high concentration of solute (hence a lower concentration of water) A simple rule to remember is SALT SUCKS A simple rule to remember is SALT SUCKS Salt is a solute, when it is concentrated inside or outside a cell, it will draw water in its direction Salt is a solute, when it is concentrated inside or outside a cell, it will draw water in its direction

Tonicity The measurement of solute concentration on either side of a membrane is known as Tonicity The measurement of solute concentration on either side of a membrane is known as Tonicity The tonicity of a cell’s environment affects its rate of osmosis. The tonicity of a cell’s environment affects its rate of osmosis.

Isotonic In an isotonic solution, the concentration of solute is the same on both sides of the membrane (inside the cell and outside). In an isotonic solution, the concentration of solute is the same on both sides of the membrane (inside the cell and outside). A cell placed in an isotonic solution neither gains or loses water. A cell placed in an isotonic solution neither gains or loses water. Most cells in the body are in an isotonic solution. Most cells in the body are in an isotonic solution. Why? Why?

Red Blood Cells Plant Cells

Hypertonicity A hypertonic solution is one that has a high solute (Less water) concentration. A hypertonic solution is one that has a high solute (Less water) concentration. Cells in a hypertonic solution will lose water. Cells in a hypertonic solution will lose water.

Hypertonicity As a result of a difference in structure, plant and animal cells respond differently Animal Cells shrink crenate Plant cells, maintain shape because of their cell wall, the cell membrane breaks away from the cell wall resulting in Plasmolyis

Red Blood Cells Plant Cells

Hypotonicty A hypotonic solution is one that has less solute (more water). A hypotonic solution is one that has less solute (more water). When placed in a hypotonic solution, cells tend to gain water and swell When placed in a hypotonic solution, cells tend to gain water and swell

Hypotonicty If an animal cell swells too much it will burst. This is called Lysis. If an animal cell swells too much it will burst. This is called Lysis. Plant cells rarely burst because of their cell wall. As water fills the cell the membrane is pushed up against the wall (turgor pressure) making the plant cell turgid. Plant cells rarely burst because of their cell wall. As water fills the cell the membrane is pushed up against the wall (turgor pressure) making the plant cell turgid.

Red Blood Cells Plant Cells

Hypotonicity Many single cell organisms live in hypotonic solution so they may be in danger of constantly bursting. Many single cell organisms live in hypotonic solution so they may be in danger of constantly bursting. To help these organisms they have a contractile vacuole to pump water out of the cell. To help these organisms they have a contractile vacuole to pump water out of the cell.

Questions How can we determine which solution is isotonic to a given set of cells? How can we determine which solution is isotonic to a given set of cells? Is the concentration the same for all cells? Is the concentration the same for all cells?