osmosis

• What is osmosis in biological systems?

  Movement of water molecules only
• What does the term water potential (ψ) describe?

  Tendency of water to move concentrations
• How does osmosis occur through a membrane?

  Water moves from high to low water potential
• What is the water potential of pure water?

  Zero
• What is osmosis?

  The passage of water from a region of higher water potential to a region of lower water potential, through a partially permeable membrane
• Why is water potential (ψ) used to describe the tendency of water molecules to move?

  Because it describes the tendency of water molecules to move from a high to low concentration
• Why does adding a solute like sugar to water lower the water potential?

  Because it reduces the proportion of free water molecules that can move about
• What is the relationship between water potential (ψ), solute potential (ψs), and pressure potential (ψp) in plant cells?

  ψcell = ψs + ψp
• How does the solute potential (ψs) affect the water potential of a plant cell?

  The presence of solutes in the vacuole lowers the cell's water potential
• What are the key factors that affect water potential (ψ) in plant cells?

  • Solute potential (ψs): Lowered by presence of solutes, making water potential more negative
  • Pressure potential (ψp): Increased by hydrostatic pressure from water entering the cell, making water potential more positive
  • Overall water potential (ψcell) = ψs + ψp
• How does the direction of water movement by osmosis depend on the water potential gradient?

  • Water moves from a region of higher water potential (less negative) to a region of lower water potential (more negative)
  • This is because water has a greater tendency to leave the region of higher water potential
• What do plant cells need to maintain turgor?

  A hypotonic external medium
• How can the internal concentration of a cell be calculated?

  Using potato cylinders
• What happens to potato cylinders in different sucrose concentrations?

  They undergo changes in mass due to osmosis
• What is the significance of ψp, ψw, and ψs in potato cells?

  • ψp = pressure potential
  • ψw = water potential
  • ψs = solute potential
• What is the water potential when potato cells are fully turgid?

  ψp = 0
• What does it mean when ψw = ψs in potato cells?

  Water potential is equal to solute potential
• What occurs at the point of incipient plasmolysis?


  ψw = ψs
• What happens to potato cells when they are fully plasmolysed?


  ψw is greater than ψs of the external medium
• Why does an increase in mass occur at 0.2 M sucrose concentration?

  Water enters potato cells by osmosis
• What causes no change in mass at the point of incipient plasmolysis?

  Water potential either side is equal
• What happens to potato cells at a sucrose concentration of 1.0 M?

  They decrease in mass due to osmosis
• Why does mass decrease at 1.0 M sucrose concentration?

  Water leaves potato cells by osmosis
• What is the direction of water movement during osmosis in potato cells?

  From high to low water potential
• What type of membrane do potato cells have that affects osmosis?

  A partially permeable membrane
• How does osmosis affect the mass of potato cylinders in different sucrose concentrations?

  Mass increases in hypotonic, decreases in hypertonic
• What are the effects of different sucrose concentrations on potato cylinders?

  • 0.2 M: Increase in mass
  • 0.3 M: No change in mass (incipient plasmolysis)
  • 1.0 M: Decrease in mass