Osmosis
Introduction
• Osmosis is a crucial process in cellular function, involving the
movement of free water molecules.
Definition: Osmosis is the net movement of free water
molecules through a partially permeable membrane,
following a water potential gradient.
Water Potential Gradient:
1. Water Potential: The water potential of a solution
measures the concentration of free water molecules
(concentration of free water molecules not associated
with solutes)
2. Higher Water Potential: More free water molecules
present.
3. Movement: Water moves from an area of high water
potential (more free water) to low water potential (fewer
free water molecules).
• Partially permeable membranes allow certain molecules to
pass through but restrict others.
• In cells, the cell surface membrane and nuclear membrane are
examples.
Direction of Water Movement:
• If the external solution has a higher water potential than
the cell's interior, water enters the cell.
• Conversely, if the external solution has a lower water
potential (higher solute concentration), water exits the cell.
• The osmotic concentration of a solution concerns only those
solutes that have an osmotic effect.
• Many large insoluble molecules found in the cytoplasm, for
example starch and lipids, do not affect the movement of water
and so are ignored when considering osmotic concentration.
• Only soluble particles are considered, including the big plasma
proteins such as albumin and fibrinogen.
• In animal cells, uncontrolled water movement into the cell from
dilute solutions must be avoided to prevent cell swelling and
bursting.
• The balance of water movement is vital for maintaining cell
integrity and function.
Modelling Osmosis:
• A model cell can be created using an artificial membrane
that's permeable to water but not to other substances like
sucrose.
• Experiments with such models demonstrate water movement.
• For instance, using Benedict's test for non-reducing sugars
helps identify regions with sucrose presence.
Osmotic Concentrations in Solutions:
• The osmotic concentration of a solution considers only
solutes that affect osmosis.
• Large molecules in cell cytoplasm that don't influence
water movement are excluded from osmotic concentration
calculations.
Types of Solutions Based on
Osmotic Concentration:
1. Isotonic Solution: The osmotic concentration of solutes is
the same as that in the cell's cytoplasm. In this environment,
there's no net movement of water into or out of the cell.
2. Hypotonic Solution: Lower osmotic concentration of solutes
compared to the cell's cytoplasm. Water tends to move into
the cell, potentially causing it to swell.
3. Hypertonic Solution: Higher osmotic concentration of
solutes than in the cell's cytoplasm. Water tends to move out
of the cell, possibly leading to cell shrinkage.
Osmosis in animal cells
• Animal cells are sensitive to osmosis, likened to 'fragile balloons
filled with jelly.'
• Cell Bursting: Excessive water intake causes cells to burst due to
the lack of a rigid cell wall.
• Cell Shrinking: When too much water leaves, cells shrivel as the
cytoplasm becomes concentrated, disrupting internal structures
and chemical reactions.
Osmosis in plant cells
• Plant cells have a similar internal structure to animal cells but are
encased within a rigid 'box' of cellulose cell walls.
• Cellulose Wall Function: Prevents bursting by exerting inward
pressure, known as pressure potential, balancing the osmotic
force.
• Turgor State: When water enters a plant cell in a hypotonic
environment, it swells until the pressure potential equals the
osmotic force, leading to a rigid state called turgor, crucial for
plant structure and support.
• Osmosis Observation: Plant cells are commonly used for
osmosis studies due to their visibility under light microscopes
and the pronounced changes they exhibit during osmosis.
• Plasmolysis: In hypertonic solutions, plant cells lose turgor,
and the cell membrane may detach from the cell wall
(incipient plasmolysis). If water loss continues, complete
plasmolysis occurs where the cell's contents shrink
significantly.
• Cell Shape and Size: Despite these changes, the overall size
and shape of plant cells remain relatively constant due to the
rigidity of the cell wall
