Biology (4BI1)
Topic 6 of 18Pearson EdExcel

Movement of Substances

How cells transport vital materials across their partially permeable membranes.

All living organisms are made of cells, and for these cells to function, they must be able to move substances in and out. This exchange is controlled by the cell surface membrane, which is partially permeable (or selectively permeable). This means it allows small molecules like water, oxygen, and carbon dioxide to pass through freely but regulates the passage of larger molecules like glucose and ions. The three primary mechanisms for this movement are diffusion, osmosis, and active transport.


### Diffusion


Diffusion is the net movement of particles (molecules or ions) from a region of their higher concentration to a region of their lower concentration. This movement occurs down a concentration gradient and is a passive process, meaning it does not require the cell to expend any metabolic energy. The particles move randomly, but the overall effect is that they spread out until they are evenly distributed.


Key examples in biological systems include:

  • The movement of oxygen from the alveoli in the lungs into the red blood cells.
  • The movement of carbon dioxide from respiring body tissues into the bloodstream.
  • The absorption of digested food molecules like amino acids from the small intestine into the blood.

    • The rate of diffusion is influenced by several factors:

  • Concentration Gradient: The steeper the gradient (i.e., the bigger the difference in concentration), the faster the rate of diffusion.
  • Temperature: Higher temperatures increase the kinetic energy of particles, causing them to move faster and thus increasing the rate of diffusion.
  • Surface Area: A larger surface area allows for more particles to cross the membrane at any given time, increasing the rate. Structures like the alveoli in the lungs and villi in the small intestine are adapted to have a large surface area.
  • Diffusion Distance: A shorter distance for particles to travel results in a faster rate of diffusion. This is why capillaries and alveoli have walls that are only one cell thick.

    • ### Osmosis


    Osmosis is a special type of diffusion specifically concerning water molecules. It is defined as the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.


    Water potential (Ψ) is a measure of the concentration of free water molecules. Pure water has the highest possible water potential, defined as zero. Adding solutes (like salt or sugar) to water lowers its water potential, making the value negative. Therefore, water will always move from a less negative water potential to a more negative water potential.


    Osmosis has different effects on animal and plant cells:

  • Animal Cells: An animal cell (e.g., a red blood cell) placed in pure water (higher water potential) will gain water, swell, and eventually burst, a process called lysis. If placed in a concentrated salt solution (lower water potential), it will lose water, shrink, and become shrivelled or crenated.
  • Plant Cells: A plant cell placed in pure water will also gain water, but its strong cellulose cell wall prevents it from bursting. The cell swells and becomes firm or turgid. This turgor pressure is vital for supporting non-woody plants. If a plant cell is placed in a concentrated solution, it loses water, and the cytoplasm and vacuole shrink, pulling the cell membrane away from the cell wall. This condition is known as plasmolysis, and the cell is described as flaccid or plasmolysed.

    • ### Active Transport


    Sometimes, a cell needs to move substances from a low concentration to a high concentration, i.e., against a concentration gradient. This process cannot happen passively and requires energy. Active transport is the movement of particles against their concentration gradient, using energy released from respiration.


    The energy is supplied in the form of ATP (adenosine triphosphate). This process is carried out by carrier proteins embedded in the cell membrane. These proteins bind to a specific molecule or ion, change shape using energy from ATP, and carry the substance across the membrane.


    Key examples of active transport include:

  • The uptake of mineral ions (e.g., nitrates) from the soil by plant root hair cells. The concentration of these ions is often higher inside the root cell than in the soil water.
  • The absorption of glucose from the small intestine into the blood, even when the concentration of glucose in the blood is already higher than in the intestine.

    • In summary, while diffusion and osmosis are passive mechanisms that allow substances to move down a concentration gradient, active transport is an energy-dependent process essential for moving substances against a gradient, ensuring cells can accumulate the necessary molecules for life.

    Key Points to Remember

    • 1Diffusion is the passive net movement of particles from a region of higher concentration to lower concentration.
    • 2Osmosis is the net movement of water molecules across a partially permeable membrane, from a higher to a lower water potential.
    • 3Active transport moves substances against a concentration gradient and requires energy in the form of ATP.
    • 4Diffusion and osmosis are passive processes that do not require metabolic energy from the cell.
    • 5The rate of diffusion is affected by the concentration gradient, temperature, surface area, and diffusion distance.
    • 6Plant cells become turgid in solutions with high water potential and plasmolysed in solutions with low water potential.
    • 7Animal cells undergo lysis (bursting) in hypotonic solutions and crenation (shrinking) in hypertonic solutions.
    • 8Active transport relies on carrier proteins in the cell membrane to 'pump' molecules or ions.

    Pakistan Example

    Salinity Stress and Crop Yield in Sindh

    In many agricultural areas of Pakistan, particularly in the lower Indus basin in Sindh, soil salinity is a major problem. Continuous irrigation can lead to the accumulation of salts in the topsoil. This high concentration of salt creates a very low (highly negative) **water potential** in the soil. For crops like cotton and wheat, this presents a significant challenge. The water potential gradient between the plant's root hair cells and the surrounding soil water is reduced, or even reversed. As a result, the plant cannot absorb sufficient water through **osmosis**. In severe cases, water may even move out of the roots into the soil, causing the plant cells to become **flaccid** and then **plasmolysed**. This leads to wilting, stunted growth, and drastically reduced crop yields, impacting the livelihood of farmers and the national economy.

    Quick Revision Infographic

    Biology — Quick Revision

    Movement of Substances

    Key Concepts

    1Diffusion is the passive net movement of particles from a region of higher concentration to lower concentration.
    2Osmosis is the net movement of water molecules across a partially permeable membrane, from a higher to a lower water potential.
    3Active transport moves substances against a concentration gradient and requires energy in the form of ATP.
    4Diffusion and osmosis are passive processes that do not require metabolic energy from the cell.
    5The rate of diffusion is affected by the concentration gradient, temperature, surface area, and diffusion distance.
    6Plant cells become turgid in solutions with high water potential and plasmolysed in solutions with low water potential.
    Pakistan Example

    Salinity Stress and Crop Yield in Sindh

    In many agricultural areas of Pakistan, particularly in the lower Indus basin in Sindh, soil salinity is a major problem. Continuous irrigation can lead to the accumulation of salts in the topsoil. This high concentration of salt creates a very low (highly negative) **water potential** in the soil. For crops like cotton and wheat, this presents a significant challenge. The water potential gradient between the plant's root hair cells and the surrounding soil water is reduced, or even reversed. As a result, the plant cannot absorb sufficient water through **osmosis**. In severe cases, water may even move out of the roots into the soil, causing the plant cells to become **flaccid** and then **plasmolysed**. This leads to wilting, stunted growth, and drastically reduced crop yields, impacting the livelihood of farmers and the national economy.

    SeekhoAsaan.com — Free RevisionMovement of Substances Infographic
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