Practical 6
What is this practical about?
This practical is about osmosis or water movement through cells, specifically chicken eggs. This explores the concept of hyper, hypo, and isotonic solutions. Then you will design an experiment to investigate the movement of water in and out of plant tissues and how plant cells are affected by solutions with different solute concentrations.
6.1: Effect of different solute concentrations on animal cells.
Examining waterflow through the egg membrane.
6.2: Effect of different solute concentrations on plant tissues.
Make a qualitative and quantitative observations.
Can plants be altered by placing them in different solutions?
The science behind the practical
Hyper: too much, hypo: too little, iso: the same as
When we refer to something as “tonic” we’re talking about the strength of the solution. In this practical, we talk about the solution strength relative to the concentration of solution inside the cell.
Hypertonic solution: solution that is stronger than the solution inside a cell (i.e contains a higher concentration of solutes such as salts, sugars, etc.). A cell placed in a hypertonmic solution will tend to lose water as osmosis will cause water flow from the cell to outside.
Hypertonic solution: solution that is weaker than the wolution in the cell (and thus contains lower concentrations of sugar, salt or other solutes than the cell). De-ionised water is as weak as you can get, so it is a hypotonic solution. A cell placed in a hypotonic solution will tend to gain water from the solution because of osmosis.
An isotonic solution is one that has the same concentration as the cell. Flows of water will be equal in both directions (inot and out of the cell) so there will be no net change in cell water content.
IN ANIMAL CELLS:
Lysed: when water is entering the cell (placed in hypotonic)
Normal: isotonic water movement
Shriveled: when water is leaving the cell (placed in hypertonic)
IN PLANT CELLS:
Turgid (normal): water entering the cell (placed in hypotonic)
Flaccid: isotonic water movement
Shriveled (plasmolyzed): water loss from the cell due to hypertonic surroundings, causing the cytoplasm to pull away from the cell wall. (placed in hypertonic)
Diffusion is the phenomenon of movement down a concentration gradient. Molecules move randomly in gases and solutions, constantly changing speed and direction. This random motion means two separate groups of molecules will tend to mix when placed together. This naturally results in diffusion.
Osmosis is the diffusion of water molecules.
Osmosis in plant vs animals
Osmosis affects plants and animal cells differently because plant cells have a strong cell wall and animal cells do not.
Plants use osmosis to give them rigidity—> rainwater is hypotonic to plant cells and water enters the cells, making them swell and push against the cell walls. The cells are said to be turgid. This rurgidity gives non-woody plants their strength. This is why wilting plants can stand up again after being given water.
Plant cells are most healthy in a hypotonic environment. In an isotonic or mildly hypertonic environment, a plance cell will become flaccid.
Animal cells do not have a cell wall. This means that an animal cell will easily burst if its placed in a hypotonic solution. This is called lysis of the cell and it kills the cell. Animal cells are most healthy in an isotonic environment.
Neither plant nor animal cells do well in a hypertonic environment. Animal cells will shrivel and ultimately die. In plant cells, the membrane pulls away from the cell wall in a process called plasmolysis, which is usually lethal.
Glossary
Amphipathic – This describes a molecule that contains both polar (i.e. hydrophilic) and nonpolar (i.e. hydrophobic) parts. The hydrophilic part tends to be surrounded by the aqueous environment and their hydrophobic part is usually shielded from the water. E.g. in lipid bilayers.)
Aquaporin - A protein occurring in cell membranes that functions as a water channel. Aquaporins increase permeability of the membrane to water and can thus accelerate the process of osmosis. They are integral to the plasma membranes of red blood cells and the renal collecting ducts in animal cells. in plants they occur in the cell membrane and vacuole membrane (tonoplast).
Decalcification – This refers to the process of removing calcium from tissues.
Diffusion - The random movement of particles (e.g. molecules or ions) from an area of high concentration to an area of low concentration until an even distribution of particles (i.e. uniform concentration) is obtained. Small molecules and ions (such as oxygen and Na+ ) can diffuse across a plasma membrane.
Dynamic equilibrium - A state of balance between continuing processes.
Flaccid – This describes plant cells that have become soft and less rigid than normal because the cytoplasm within its cells has shrunk and contracted away from the cell walls because of the loss of water
Hydrophilic - Having an affinity for water.
Hypertonic – (Of a solution) having a concentration such that it gains water by osmosis across a membrane
from some other specified solution. When the membrane and the other solution are unspecified, they are
taken to be the cell membranes and the interior of cells respectively. Hypotonic - (Of a solution) having a
concentration such that it loses water by osmosis across a membrane to some specified other solution. When
the membrane and the other solution are unspecified, they are taken to be the cell membranes and interior
of cells respectively.
Isotonic - Describing solutions that have the same osmotic pressure. Lysis - The disintegration of a cell by
rupture of the cell wall or membrane.
Osmosis - The net movement of solvent molecules from a dilute (hypotonic) solution to a more concentrated
(hypertonic) solution through a differentially/selectively permeable membrane. The distribution of water in
living organisms is dependent to a large extent on osmosis, water entering and leaving the cells through their
partially permeable cell membranes. The pressure required to stop the flow of pure water into a solution
across a partially permeable membrane is a characteristic of the solution and is called the osmotic pressure.
Plasmolysis - The loss of water by osmosis from a plant cell to the extent that the cytoplasm shrinks away
from the cell wall. This happens when the cell is placed in a solution that has a higher solute concentration
than that of the cell sap, i.e. a hypertonic solution.
Qualitative observations – These observations are more subjective and involve the use of the five sensory
organs and how they function to examine attributes (e.g. sight, smell, taste, touch, and hearing).
Quantitative observations – These observations are objective and usually involve the collection of data
based on their numerical attributes. The primary focus is on numbers and values.
Solute - The substance dissolved in a solvent in forming a solution.
Solution - A homogeneous mixture of a liquid (the solvent) with a gas or solid (the solute). In a solution, the
molecules of the solute are discrete and mixed with the molecules of solvent. There is usually some interaction
between the solvent and solute molecules.
Solvent - A liquid that dissolves another substance or substances to form a solution.
Tonicity - The effective osmotic pressure of a solution relative to another solution from which it is separated
by a membrane permeable only to water molecules. It is determined by the concentration of non-penetrating
solutes in the solution relative to that in the other solution. Hence, the tonicity of a solution surrounding a cell
determines whether the solution will cause water to flow in or out of the cell across the plasma membrane. A
hypertonic solution will cause water to flow out of the cell, whereas a hypotonic solution will cause water to
flow into the cell. If the solution is isotonic, there will be no net flow of water across the cell membrane.
Turgor – (turgid) The condition in a plant cell or other cell with a semirigid cell wall (i.e. cells of archaea,
bacteria, fungi, and some protists) when the force causing water to enter the cell by osmosis is balanced by
the pressure of the fluid inside the cell pushing against the cell wall— i.e. the turgor pressure. A state of
turgor thus prevents further water from entering the cell. Turgidity assists in maintaining the rigidity of plants;
a decrease in turgidity leads to wilting.
Wilting - The condition that arises in plants when more water is lost by evaporation than is absorbed from
the soil. This causes the cells to lose their turgor and the plant structure droops. Plants can normally recover
from wilting if water is added to the soil, but permanent wilting and possible death can result once the cells
are plasmolysed
Hydrophobic - Lacking affinity for water.