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Chemistry: Diffusion, Osmosis, Active transport, Exchange surface

1. Diffusion

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. This continues until the particles are evenly distributed. It is a passive process, meaning it does not require energy.

  • Factors affecting the rate of diffusion:

    • Concentration gradient: A steeper gradient means a faster rate.
    • Temperature: Higher temperature means faster particle movement, thus a faster rate.
    • Surface area: A larger surface area allows more particles to diffuse at once.
    • Distance: Shorter diffusion distance means a faster rate.

  • Examples of diffusion in biology:

    • Oxygen moving from the alveoli into the blood.
    • Carbon dioxide moving from the blood into the alveoli.
    • Digested food molecules (e.g., glucose) moving from the small intestine into the blood.
    • Urea moving from liver cells into the blood plasma.

2. Osmosis

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) across a partially permeable membrane. It is a specific type of diffusion for water.

  • Partially permeable membrane (also known as a selectively permeable membrane): A membrane that allows small molecules (like water) to pass through but restricts the passage of larger molecules (like dissolved solutes).

  • Effects of osmosis on cells:

    • Animal cells (e.g., red blood cells):
    • In a dilute solution (higher water potential outside): Water moves into the cell, causing it to swell and potentially burst (lysis).
    • In a concentrated solution (lower water potential outside): Water moves out of the cell, causing it to shrink and crenate.
    • In an isotonic solution (same water potential): No net movement of water; cell remains normal.
    • Plant cells:
    • In a dilute solution (higher water potential outside): Water moves into the cell. The cell swells, and the cell membrane presses against the cell wall, making the cell turgid. The cell wall prevents bursting.
    • In a concentrated solution (lower water potential outside): Water moves out of the cell (plasmolysis). The cell membrane pulls away from the cell wall, and the cell becomes flaccid or plasmolysed (severe water loss).
    • In an isotonic solution: No net movement of water; cell is flaccid but not plasmolysed.

3. Active Transport

Active transport is the movement of particles across a cell membrane against their concentration gradient (from an area of lower concentration to an area of higher concentration). This process requires energy, usually supplied by respiration (in the form of ATP).

  • Key features:

    • Requires energy (ATP).
    • Moves substances against a concentration gradient.
    • Involves specific carrier proteins in the cell membrane.

  • Examples of active transport in biology:

    • Absorption of mineral ions by plant root hair cells from dilute soil solution.
    • Absorption of glucose in the small intestine when its concentration in the gut is lower than in the blood.
    • Reabsorption of glucose and amino acids by kidney tubules.

4. Exchange Surfaces

Organisms have specialised exchange surfaces to efficiently move substances into and out of their bodies. Efficient exchange surfaces share several key characteristics:

  • Characteristics of efficient exchange surfaces:

    • Large surface area: Provides ample space for diffusion (e.g., folded alveoli in lungs, villi in the small intestine).
    • Thin barrier: Typically only one cell thick, ensuring a short diffusion path (e.g., alveolar and capillary walls).
    • Good blood supply: Maintains a steep concentration gradient by continuously transporting absorbed substances away and bringing new substances (e.g., capillaries surrounding alveoli and villi).
    • Ventilation/Maintaining a concentration gradient: For gaseous exchange, a good supply of fresh air ensures a high concentration of oxygen and a low concentration of carbon dioxide (e.g., breathing in the lungs).

  • Examples of exchange surfaces:

    • Lungs (alveoli): Gas exchange (oxygen in, carbon dioxide out).
    • Small intestine (villi and microvilli): Absorption of digested nutrients.
    • Fish gills (lamellae): Gas exchange in aquatic animals.
    • Plant leaves (stomata and air spaces): Gas exchange for photosynthesis and respiration.

5. Practical Investigations

5.1 Potato Osmosis Practical

This experiment investigates the effect of different concentrations of sugar (sucrose) or salt solutions on the mass of potato cylinders due to osmosis.

  • Procedure:

    1. Cut several potato cylinders (e.g., using a cork borer) of equal length and ensure they are all dried to remove excess surface water.
    2. Measure the initial mass of each potato cylinder using a digital balance (\pm 0.01g) and record it.
    3. Prepare a series of different sucrose solution concentrations (e.g., 0 M, 0.2 M, 0.4 M, 0.6 M, 0.8 M, 1.0 M) along with distilled water (0 M being the control).
    4. Place one potato cylinder into each labelled test tube containing a specific concentration of sucrose solution.
    5. Leave the potato cylinders submerged for a set period (e.g., 24 hours).
    6. Remove the potato cylinders, blot them gently to remove excess surface solution, and measure their final mass.
    7. Record the final mass for each cylinder.

  • Calculations and Analysis:

    • Calculating percentage change in mass: This is more reliable than absolute change as it accounts for initial differences in potato cylinder size.
    • \text{Percentage change in mass} = \frac{\text{(Final mass} - \text{Initial mass)}}{\text{Initial mass}} \times 100
    • Graphing results: Plot a graph of percentage change in mass (y-axis) against the concentration of sucrose solution (x-axis).
    • Interpreting the graph:
    • Where the line crosses the x-axis (0% change in mass), this indicates the point at which there was no net movement of water, meaning the water potential inside the potato cells was isotonic to the sucrose solution concentration at that point. This gives an estimate of the water potential of the potato tissue.
    • If the potato gains mass (positive percentage change), water moved into the cells (external solution was more dilute).
    • If the potato loses mass (negative percentage change), water moved out of the cells (external solution was more concentrated).