Cell Transport Mechanisms

Transport: Diffusion, Osmosis, and Active Transport

Diffusion

• Involves the movement of molecules, such as smelling cooked food or perfume and diluting juice in water.
• Diffusion is the net movement of particles from an area of high concentration to an area of low concentration, down a concentration gradient, until equilibrium is reached.
• The tube containing starch solution is placed in a beaker of water. The tube (visking tubing) represents the cell membrane and is partially permeable.

Diffusion in Real Life

• Body cells need oxygen and dissolved food molecules.
• These substances cross boundaries within the body to get to where they are needed.
• Oxygen and dissolved food molecules must diffuse into and out of the blood for transportation around the body.

Diffusion In and Out of Cells

• Oxygen and dissolved food molecules are transported to the body’s cells in the bloodstream.
• Cells use food and oxygen for respiration, producing carbon dioxide as a waste product.
• Carbon dioxide is removed from the cell to prevent poisoning.
• Not everything can move into and out of the cell by diffusion.

Osmosis

• Osmosis is a special type of diffusion that only involves the movement of water molecules.
• It occurs across a semipermeable membrane with tiny holes that allow water molecules to pass through, but not larger molecules.
• Water's polar nature allows it to interact with other polar molecules.

Osmosis Demonstration

• A bag made from a semipermeable membrane is tied to a glass tube and filled with a strong sugar solution.
• This bag is placed in a weak sugar solution, causing water molecules from the weak solution to move into the bag.
• The volume of liquid in the semipermeable bag increases, and the liquid rises up the glass tube until it stops.

Osmosis Definition

• Movement of free water molecules from an area of high water concentration (dilute solution) to an area of low water concentration (concentrated solution) across a partially permeable membrane until equilibrium is reached.
• A partially permeable membrane has tiny holes, allowing small molecules to pass through but not large molecules (e.g., cell membrane).

Comparisons: Diffusion and Osmosis

• Similarities:
  • Both involve movement from high to low concentrations until equilibrium is reached.
  • Both are passive processes and don't need energy to happen.
• Differences:
  • Osmosis requires a partially permeable membrane, while diffusion does not.
  • Osmosis is specific to the movement of water molecules, while diffusion involves gas or dissolved molecules.

Effects of Osmosis on Cells

• Solutions are described as hypotonic, hypertonic, and isotonic (relative terms).

Plant Cells

• Plants have a vacuole and a cell wall, providing advantages over animal cells.
• In an isotonic solution, everything stays the same.
• In a hypotonic solution, the cell wall stops the cell from rupturing, and the vacuole swells.
• In a hypertonic solution, the cells will be flaccid and the cell wall will be left trying to hold the cell shape.

Animal Cells

• A concentrated salt solution (low concentration of water) causes water to leave the cell by osmosis, making it flaccid.
• Pure water (or a less concentrated solution) causes the cell to gain volume and expand, potentially bursting because it has no vacuole for storage and no cell wall to resist the expansion.

Active Transport

• Substances can be absorbed against a concentration gradient using energy; this is called active transport.

Concentration Gradient

• Moving down a concentration gradient allows particles to move until equilibrium is reached.
• Moving up or against a concentration gradient requires energy.

Examples of Active Transport

• Plants: Absorption of ions into root hair cells.
• Animals: Absorption of small amounts of sugar into villi.
• Active transport uses the energy from respiration.

Active Transport Mechanism

• Involves carrier proteins on the cell membrane.

Comparisons of Active and Passive Transport

Cell Transport

• Facilitated diffusion is a special type of diffusion.