CIE IGCSE Biology 3.1 Diffusion, Osmosis & Active Transport

3.1 Diffusion, Osmosis & Active Transport

3.1.1 Diffusion in Biology

• Diffusion:

  • Movement of molecules from a region of higher concentration to a region of lower concentration.
  • Molecules move down a concentration gradient due to their random movement.

• Diffusion across the cell membrane:

  • Cells are surrounded by a partially permeable membrane.
  • The cell membrane restricts the free movement of molecules; some molecules cross easily, others with difficulty or not at all.
  • Selection is based on the size of the molecules.

• Importance of diffusion for living organisms:

  • Obtaining requirements.
  • Getting rid of waste products.
  • Carrying out gas exchange for respiration.

Examples of diffusion in living organisms

• Plants require oxygen for respiration at all times.
• Plants require carbon dioxide for photosynthesis when conditions are suitable (e.g., enough light and a suitable temperature).

Energy for diffusion

• All particles move randomly (Brownian motion).
• Energy for diffusion comes from the kinetic energy of the random movement of molecules and ions.

3.1.2 Factors that Influence Diffusion

• Surface area to volume ratio:

  • The bigger a cell or structure is, the smaller its surface area to volume ratio.
  • This slows down the rate at which substances can move across its surface.
  • Cells adapted for diffusion have increased surface area (e.g., root hair cells, cells lining the ileum).

• Distance:

  • The smaller the distance, the faster the transport.
  • Blood capillaries and alveoli have walls that are one cell thick to ensure fast diffusion.

• Temperature:

  • The higher the temperature, the faster molecules move due to increased energy.
  • This results in more collisions against the cell membrane and a faster rate of movement across it.

• Concentration Gradient:

  • The greater the difference in concentration on either side of the membrane, the faster the movement.
  • More random collisions occur against the membrane on the side with the higher concentration.

3.1.3 Water

• Water as a Solvent:

  • Water is important for all living organisms because many substances can dissolve in it.

• Importance within organisms:

  • Dissolved substances can be easily transported around organisms (e.g., xylem and phloem of plants, dissolved food molecules in the blood).
  • Digested food molecules need to be moved to cells all over the body.
  • Toxic substances (e.g., urea) and substances in excess (e.g., salts) can dissolve in water for easy removal from the body in urine.
  • Water is an important part of the cytoplasm and plays a role in ensuring metabolic reactions can happen in cells.

3.1.4 Osmosis

• Osmosis:
  • Diffusion of water molecules from a dilute solution (high concentration of water) to a more concentrated solution (low concentration of water) across a partially permeable membrane.
  • Water moves down its concentration gradient.
  • The cell membrane is partially permeable, allowing small molecules (like water) through but not larger molecules (like solute molecules).

3.1.5 Osmosis Experiments

• Immersing plant cells in solutions of different concentrations:

  • Cut cylinders of root vegetables (e.g., potato or radish) and place them into distilled water and sucrose solutions of increasing concentration.
  • Weigh the cylinders before placing them into the solutions.
  • Leave them in the solutions for 20-30 minutes, then remove, dry, and reweigh.

• If the plant tissue gains mass:

  • Water moved into the plant tissue from the surrounding solution by osmosis.
  • The solution is more dilute than the plant tissue.

• If the plant tissue loses mass:

  • Water moved out of the plant tissue into the surrounding solution by osmosis.
  • The solution is more concentrated than the plant tissue.

• If there is no overall change in mass:

  • There is no net movement of water, and the concentration in both the plant tissue and the solution are equal.

• Investigating osmosis using dialysis tubing:

  • Dialysis tubing is a non-living partially permeable membrane made from cellulose.
  • Pores are small enough to prevent passage of large molecules (e.g., sucrose) but allow smaller molecules (e.g., glucose and water) to pass through.

• Demonstration:
  1. Filling a section of dialysis tubing with concentrated sucrose solution.
  2. Suspending the tubing in a boiling tube of water for a set period of time.
  3. Noting whether the water level outside the tubing decreases as water moves into the tubing via osmosis.

• Water moves from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane.

3.1.6 Osmosis in Animals & Plants

• Osmosis in Plant Tissues:

  • When water moves into a plant cell, the vacuole gets bigger, pushing the cell membrane against the cell wall.
  • Water entering the cell by osmosis makes the cell rigid and firm (turgid).
  • This is important for support and strength, allowing the plant to stand upright and leaves to catch sunlight.
  • The pressure created by the cell wall prevents too much water from entering and prevents the cell from bursting.
  • If plants do not receive enough water, the cells cannot remain rigid and firm (turgid), and the plant wilts.

• Osmosis: Extended:

  • Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane.
  • A dilute solution has a high water potential, and a concentrated solution has a low water potential.

• Plant cells in solutions of different concentrations:

  • When plant cells are placed in a solution that has a higher water potential (dilute solution) than inside the cells (e.g., distilled water), water moves into the plant cells via osmosis.
  • Water molecules push the cell membrane against the cell wall, increasing the turgor pressure in the cells, which makes them turgid.
  • When plant cells are placed in a concentrated solution (with a lower water potential than inside the cells), water molecules will move out of the plant cells by osmosis, making them flaccid.
  • If plant cells become flaccid, it can negatively affect the plant's ability to support itself.
  • If looked at underneath the microscope, the plant cells might be plasmolysed, meaning the cell membrane has pulled away from the cell wall.

• Animal cells in solutions of different concentrations:

  • Animal cells also lose and gain water as a result of osmosis.
  • As animal cells do not have a supporting cell wall, the results on the cell are more severe.
  • If an animal cell is placed into a strong sugar solution (with a lower water potential than the cell), it will lose water by osmosis and become crenated (shrivelled up).
  • If an animal cell is placed into distilled water (with a higher water potential than the cell), it will gain water by osmosis and, as it has no cell wall to create turgor pressure, will continue to do so until the cell membrane is stretched too far and it bursts.

3.1.7 Active Transport

• Active Transport:
  • Movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration.

3.1.8 Proteins & Active Transport

• Importance of Active Transport:

  • Energy is needed because particles are being moved against a concentration gradient, in the opposite direction from which they would naturally move (by diffusion).

• Vital processes:
  * Uptake of glucose by epithelial cells in the villi of the small intestine and by kidney tubules in the nephron.
  * Uptake of ions from soil water by root hair cells in plants.

• Protein Carriers:

  • Active transport works by using carrier proteins embedded in the cell membrane to pick up specific molecules and take them through the cell membrane against their concentration gradient.
  • Substance combines with carrier protein molecule in the cell membrane.
  • Carrier transports substances across membrane using energy from respiration to give them the kinetic energy needed to change shape and move the substance through the cell membrane.
  • Substance released into cell