Diffusion, osmosis and mass flow

1. It facilitates the removal of waste materials from the organism's body, the excess of which could harm an organism.
2. It ensures that essential materials like oxygen, nutrients, water, hormones and mineral salts are supplied to the cells and tissues as required.
3. It enables essential substances to move from one part of the body to another. For example, food manufactured by photosynthesis in plant leaves is transported from leaves to other organs of the plant for use or storage.

Diffusion is the movement of ions or molecules from the region of higher concentration to the region of lower concentration, without involving any permeable membrane. A difference in concentration of a substance between two regions is known as concentration gradient.

Diagram showing diffusion

Experimental set-up to demonstrate diffusion process

This is the movement of water molecules from a region of higher concentration to a region of lower concentration through a semi-permeable membrane.

A partially permeable membrane is a membrane that allows small particles such as water molecules to pass through it, but not larger particles such as sugar molecules and ions from salts. Examples of semi-permeable membranes are cell membranes and a pig's bladder.

Osmosis occurs when water moves down its concentration gradient across the semi-permeable membrane.

Therefore, for osmosis to take place there must be:

1. Two solutions with different concentrations; and
2. A partially permeable membrane to separate them.

Movement of water molecules during osmosis

Effects of osmosis on animal cells

Osmosis, the movement of water across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration, has significant effects on animal cells. These effects depend on the type of solution (hypotonic, hypertonic, or isotonic) the cells are exposed to:

Animal cells in a hypotonic solution

Definition: A hypotonic solution has a lower solute concentration than the cytoplasm of the cell.

Effect on cells:

1. Water enters the cell by osmosis.
2. The cell swells as its internal pressure increases.
3. If the cell lacks mechanisms to expel the excess water, it bursts due to excessive pressure, a process called haemolysis (especially observed in red blood cells).

Animal cells in a hypertonic solution

Definition: A hypertonic solution has a higher solute concentration than the cytoplasm of the cell.

Effect on cells:

1. Water leaves the cell by osmosis.
2. The cell shrinks and becomes shriveled as it loses water, a process called crenation.

Animal cells in an isotonic solution

Definition: An isotonic solution has the same solute concentration as the cytoplasm of the cell.

Effect on cells:

1. There is no net movement of water.
2. Water moves in and out of the cell at an equal rate.
3. The cell maintains its normal shape and size, ensuring proper functioning.

Key observations in red blood cells

1. In hypotonic solution: Red blood cells swell and may burst (haemolysis).
2. In hypertonic solution: Red blood cells shrink and develop a wrinkled appearance (crenation).
3. In isotonic solution: Red blood cells maintain their biconcave shape, as osmotic pressure is balanced with the surrounding plasma.

Biological importance

Maintaining osmotic balance:

1. Osmoregulation is crucial for animal cells to prevent excessive water gain or loss.
2. Specialized mechanisms like contractile vacuoles (in some organisms) or the regulation of solute concentration in body fluids help maintain this balance.

Implications in medicine:

Intravenous (IV) fluids must be isotonic to avoid damage to red blood cells and other tissues.

Adaptations:

Animals living in freshwater (hypotonic environments) and marine environments (hypertonic environments) have evolved specific adaptations to manage osmotic stress.

Osmosis is important for the reabsorption water in the colon and the kidneys. This help to maintain the body's water balance.

In an isotonic solution, plant cells neither lose nor gain water. In a hypotonic solution cells absorb water, causing the cell membrane to push against the cell wall. The cell is to be turgid. It does not burst because membrane exerts pressure on the cell wall restricts additional intake of water. Turgid plants to maintain their shape.

In a hypertonic solution, plant cells lose water this causes the vacuole to shrink and their cell membrane to pull away from wall, making the cell flaccid. Such a cell is to be plasmolyzed and the process plasmolysis.

If a plasmolyzed cell is placed in a hypotonic solution, it absorbs water and becomes turgid.

Osmosis is important for the absorption of water by plant roots. Opening and closing of stomata also depend on osmosis. When guard cells absorb water the stomata open and when they lose water the stomata close.

Unicellular organisms that live in fresh water, for example amoeba and euglena, are hypertonic to surrounding so water enters the organisms by osmosis. These organisms have a contractile vacuole. The contractile vacuole collects the excess water and removes it from the cell. This prevents the cells from bursting

Water balance in amoeba through contractile vacuole

Mass flow is the movement of materials in large quantities and across a long distance in the body of an organism due to differences in pressure between the two regions. The process of mass flow moves materials in higher plants and animals.

Important processes that involve diffusion are:

1. Gaseous exchange in the lungs of animals and in the leaves of plants
2. Absorption of digested food in the ileum the process of diffusion
3. Removal of west materials from cells
4. Absorption of nutrients and oxygen into cells

Living organisms

Diffusion

Diffusion is the passive movement of molecules from a region of high concentration to a region of low concentration. It is essential for transporting materials within organisms:

In animals:

1. Exchange of gases in the lungs occurs through diffusion. Oxygen diffuses from alveoli into the blood, while carbon dioxide diffuses from the blood into the alveoli.
2. Nutrients and waste products are transported at the cellular level by diffusion.

In plants:

1. Carbon dioxide diffuses into the leaf for photosynthesis, and oxygen diffuses out as a by-product.
2. Diffusion facilitates the movement of water vapor out of the leaf during transpiration.

Osmosis

Osmosis is the movement of water across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration. Osmosis plays critical roles in maintaining life processes:

In animals:

1. Nutrient transport: Nutrients are transported into cells, and waste materials are expelled through osmosis.
2. Homeostasis: Osmosis helps maintain the fluid balance within cells, ensuring they do not swell or shrink excessively.
3. Excretion: The kidneys rely on osmosis to purify the blood by removing waste materials and regulating water levels in the body.

In plants:

1. Water absorption: Plant roots absorb water from the soil via osmosis.
2. Turgor pressure: Osmosis maintains the turgidity of plant cells, which is necessary for structural support and growth.
3. Transpiration stream: Osmosis drives the movement of water from the roots to the leaves, enabling photosynthesis and nutrient transport.

Mass flow

Mass flow refers to the bulk movement of materials in response to a pressure gradient. It is a critical process for long-distance transport within organisms:

1. In animals:
   1. Blood circulation involves mass flow, as the heart pumps blood to transport oxygen, nutrients, hormones, and waste products throughout the body.
   2. The lymphatic system also relies on mass flow to transport lymph and maintain immune responses.
2. In plants:
   1. The xylem conducts water and dissolved minerals from the roots to the leaves through mass flow, driven by transpiration and root pressure.
   2. The phloem transports sugars and other organic molecules produced during photosynthesis from the leaves to other parts of the plant via mass flow.

Biological significance

Transport of essential substances: These processes ensure that oxygen, nutrients, and water reach cells while waste products are removed efficiently.

1. Homeostasis: Osmosis and diffusion help maintain the internal environment's stability, regulating water and solute levels.
2. Energy efficiency: Diffusion and osmosis are passive processes that do not require energy, reducing the organism's metabolic burden.
3. Support and growth in plants: Osmosis provides the turgor pressure required for plant structure, while mass flow supports the distribution of nutrients and water.