3.3 - Movement of Substances Across a Plasma Membrane in Living Organisms

Passive and Active Transport in Living Organisms

Passive Transport

• Gaseous exchange between an alveolus and a blood capillary by simple diffusion.
• Reabsorption of water through the renal tubule in the kidney by osmosis.
• Absorption of water by a plant root hair cell by osmosis.
• Absorption of fructose molecules in the villus by facilitated diffusion.

Active Transport

• Absorption of glucose and amino acids in the villus.
• Reabsorption of glucose through the renal tubule in the kidney.
• Transport of sucrose from a leaf to phloem tissue.
• Absorption of mineral ions by a plant root hair cell.

Isotonic, Hypotonic and Hypertonic Solutions

Isotonic Solution

• Two solutions with the same solute concentration are isotonic to each other.
• There is no net movement of water between them.

Hypotonic Solution

• Has a lower solute concentration and higher water potential than the solution compared with it.
• Water diffuses from the hypotonic solution to the other solution by osmosis.

Hypertonic Solution

• Has a higher solute concentration and lower water potential than the solution compared with it.
• Water diffuses from the other solution to the hypertonic solution by osmosis.

Effects on Animal Cells

In a Hypotonic Solution

• Water diffuses into a red blood cell by osmosis.
• The cell swells and bursts because its plasma membrane is too thin to withstand the osmotic pressure.
• Bursting of a red blood cell is haemolysis.

In an Isotonic Solution

• Water diffuses into and out of the cell at the same rate.
• There is no net movement of water; the cell maintains its normal shape.

In a Hypertonic Solution

• Water diffuses out of a red blood cell by osmosis.
• The cell shrinks and undergoes crenation.

Experiment: Effects of Different Solutions on Animal Cells

• Problem: What are the effects of hypotonic, hypertonic and isotonic solutions on animal cells?
• Hypotheses: A hypotonic solution causes cells to burst; a hypertonic solution causes cells to shrink; an isotonic solution leaves cells normal.
• Manipulated variable: surrounding-solution concentration. Responding variable: condition of chicken blood cells. Fixed variables: surrounding temperature and cell type.
• Observe fresh chicken red blood cells and cells placed in distilled water, 0.15 M sodium chloride and 0.50 M sodium chloride under a light microscope.
• Distilled water causes haemolysis; 0.15 M sodium chloride maintains the normal shape; 0.50 M sodium chloride causes crenation.

Effects on Plant Cells

In a Hypotonic Solution

• Water diffuses into the vacuole by osmosis.
• The vacuole expands and pushes the cytoplasm and plasma membrane against the cell wall; the cell becomes turgid.
• The cell does not burst because the cell wall is rigid and strong.
• Turgor pressure supports and maintains cell shape.
• Turgid guard cells swell so that stomata remain open for photosynthesis.

In an Isotonic Solution

• Cell sap and extracellular solution have the same water potential.
• Water moves into and out of the cell at the same rate; the cell becomes flaccid.

In a Hypertonic Solution

• Water diffuses out of the vacuole by osmosis.
• The vacuole and cytoplasm shrink; the plasma membrane pulls away from the cell wall.
• This process is plasmolysis; leaves and stems bend downwards and wilt.
• A plasmolysed cell returned immediately to a hypotonic solution regains turgidity by deplasmolysis.

Experiment: Effects of Different Solutions on Plant Cells

• Problem: What are the effects of different solution concentrations on plant cells?
• Hypotheses: A hypotonic solution makes cells turgid; a hypertonic solution causes plasmolysis; an isotonic solution makes cells flaccid.
• Manipulated variable: surrounding-solution concentration. Responding variable: condition of plant cells. Fixed variable: plant-cell type.
• Observe onion scale-leaf epidermal cells mounted in distilled water, 0.50 M sucrose and 1.0 M sucrose under a light microscope.
• Flow excess distilled water across cells exposed to 1.0 M sucrose to observe deplasmolysis.