Cell Physiology and Diffusion | Form One - High School Biology

Osmosis is a process by which solvent molecules move from a region of high concentration (dilute solution) to a region of low concentration (concentrated solution) through a semi permeable membrane.
Osmosis can be described as a special type of diffusion since it involves movement of solvent (water) particles from a region of high concentration to a region of low concentration.

Requirements: 500cm³ beaker, visking tubing, a piece of thread, glass rod, concentrated sugar solution, 500 cm³ distilled water.
The procedure could be followed for this experiment;

1. Into the beaker, put 350 cm³ of the distilled water.
2. Dip the visking tubing in water to moisten it. Open the visking tubing and tie one end with the thread provided.
3. Half fill the visking tubing with the sugar solution provided and then tie the open end ofthe tubing. Ensure no sugar solution spills out of the tubing.
4. Immerse the visking tubing into the distilled water in the beaker and suspend it using the glass rod provided.
5. Leave the set up for about 30 minutes.
6. Record your observations.
7. Explain the observations made.

Observations:
- The visking tubing became swollen indicating that its cell contents increased. The amount of water in the beaker decreased. This implies that water moved from the beaker into the visking tubing.

Explanation:
- The visking tubing contains both sugar and water molecules.
- The beaker contains a higher concentration of water molecules than the visking tubing.
- The water molecules diffused from the beaker (where they are highly concentrated) into the visking tubing (where they are lowly concentrated).
- Even though there is a higher concentration of sugar molecules in the visking tubing, they were not able to diffuse out of the visking tubing due to their large molecular sizes. The visking tubing is semi permeable.

Other than visking tubing, dialysis tubing or cellophane are also other semi permeable membranes that can be used in this experiment.

- When two separate solutions are separated by a semi permeable membrane, there will be movement of water molecules from their region of high concentration (dilute solution) to a region of low concentration (the highly concentrated solution) across the semi permeable membrane.
- The semi permeable membrane does not allow movement of solute particles across it.
- The movement of the water molecules continues until the separate solutions have the same concentrations.
- Solutions with the same concentrations are referred to as isotonic solutions. The solutions are said to be isotonic to each other.
- A lowly concentrated solution (dilute solution) is referred to as a hypotonic solution. A hypotonic solution has less of the solute molecules but more of the solvent molecules.
- A highly concentrated solution with more of the solute particles but less of the solvent particles is referred to as a hypertonic solution. - When isotonic solutions are separated with a semi permeable membrane, there will be no net movement of solvent molecules to any of the solutions since they have the same concentration of solvent molecules.

Osmotic Pressure

- When a concentrated solution is separated from distilled water by a semi permeable membrane, the concentrated solution will develop a force with which it draws water through the semi permeable membrane from the distilled water.
- Osmotic pressure refers to the force with which a concentrated solution draws water to itself.
- An osmometer is an instrument used to measure the osmotic pressure.

Osmotic Potential

- This is a measure of the pressure a solution would develop to withdraw water molecules from pure water when separated by a semi permeable membrane.

- As discussed earlier, the cell membrane is semi permeable. Let us discuss what would happen if an animal cell say red blood cell is placed in solutions of varying concentrations.

1. Red blood cell in hypotonic solution e.g. distilled water
   - Distilled water has a higher concentration of water molecules compared to the red blood cell cytoplasm.
   - When a red blood cell is placed in a hypotonic solution, water will move into the cell through osmosis.
   - The cell will swell and burst. Swelling of red blood cell when placed in a hypotonic solution is referred to as haemolysis. The cell is said to be haemolysed.
2. Red blood cell in hypertonic solution
   - A hypertonic solution has a low concentration of water molecules compared to the red blood cell cytoplasm. Water will, therefore, be drawn out of the cell into the hypertonic solution.
   - The cell will shrink and become small. The cell is said to be crenated.
   - The process by which animal cells shrink and become smaller when placed in hypertonic solutions is referred to as crenation.
3. Red blood cell in isotonic solution
   - When placed in an isotonic solution, the cell remains unchanged.
   - This is because there will be no net inflow or outflow of water between the cell and the solution.

NOTE: When the cell becomes haemolysed or crenated, its functioning is impaired.
- This implies that the body fluids and blood plasma surrounding the cells must be kept at the same concentration as the animal cells. This will prevent bursting or shrinking of the cells that would otherwise impair their physiology.

- Water relations in plant cells differ with that in animal cells.
- A plant cell has both a cellulose cell wall and cell membrane.
- The centre of the cell contains vacuole with sap. The sap is a solution of salts and sugars and is bound by a membrane, the tonoplast.
- The cell membrane and tonoplast are semi permeable while the cellulose cell wall is fully permeable.

1. Plant cell in hypotonic solution e.g. distilled water
   - If a plant cell is placed in water or hypotonic solution, the cell will draw water from the hypotonic solution through osmosis causing the cell to distend.
   - The cellulose cell wall is rigid and does not allow plant cells to burst as in the case of animal cells.
   - As the cell gains more water, the vacuole enlarges and exerts an outward an outward pressure on the cell wall called turgor pressure.
   - The turgor pressure increases as more water is taken into the vacuole causing the cell to stretch until the cell cannot stretch any more. The cell becomes firm and is said to be turgid.
   - Turgor pressure is the outward pressure that the cell cytoplasm exerts on the cell wall as it gains more water through osmosis.
   - When the cell wall is being stretched towards the outside, it will develop a resistant pressure to stretching that is equal and opposite to turgor pressure called wall pressure.
2. A plant cell in a hypertonic solution
   - When placed in a hypertonic solution, the plant cell will lose water to the solution through osmosis.
   - As the water moves out of the cell, the cell starts to shrink, becomes less rigid or flabby and is said to be flaccid.
   - It the cell loses more water, its contents reduce in size and the plasma membrane pulls away from the cell wall towards the centre.
   - The process through which plant cells lose water, shrink and become flaccid is called plasmolysis.
   - Plasmolysis can be reversed when a flaccid cell is placed in distilled water in a process called deplasmolysis.

Wilting

- Plants always lose water to the atmosphere through transpiration and evaporation.
- Simultaneously, the plant cells lose water and draw more from the soil.
- Wilting is a phenomenon that occurs when plant cells lose more water than they draw from the soil making the plant cells to lose their turgor pressure and droop.
- At night, plants always recover from wilting since stomata are closed and water loss through evapotranspiration is significantly reduced.
- Where water supply from the soil is inadequate, the plants may fail to recover from wilting and instead undergo permanent wilting.

• Absorption of water from the soil
  - The root hair cell of plants absorbs water from the soil through osmosis. Osmosis also helps in distribution and movement of water from the roots to other parts of the plant.
• Osmosis plays an important role in support in herbaceous plants and young seedlings.
  - When the cells of these plants take in water through osmosis, the cells become firm or rigid and thus gain support.
• Osmosis plays a role in opening and closing of stomata in plants
  - The guard cells surrounding the stomata synthesize glucose through photosynthesis in the presence of light.
  - As glucose accumulates in the guard cells, the osmotic pressure of the guard cells increase making them to draw water from adjacent cells through osmosis.
  - When the guard cells become turgid, they bulge outwards leading to opening of the stomata.
  - Opening of the stomata is crucial as it allows for gaseous exchange in plants.
  - At night, there is no glucose synthesis. The glucose available in the guard cells is respired on leading to reduction of glucose and consequently reduction in osmotic pressure.
  - The guard cells lose turgidity and close the stomata.
• Osmosis also plays a role in feeding in insectivorous plants
  - These plants live on nitrogen deficient soils and trap insects from whence they obtain the nutrients.
  - These plants possess special structures that suddenly change their turgor pressure when disturbed.
  - The change in turgor pressure enables the special structures to rapidly close thereby trapping the insects.
• Osmosis also plays a role in osmoregulation in animals.
  - In kidney tubules of animals, water is withdrawn from the tubules into the body cells through osmosis through the tubular walls.
  - This enables animals to maintain the osmotic pressure of the body fluids.

• Concentration of solutions and concentration gradient.
  - Osmosis is greater when the separated solutions have a greater difference in osmotic pressure.
  - In summary, the greater the concentration gradient, the greater the rate of osmosis and vice versa.
• Temperature
  An increase in temperature would increase the rate of osmosis as it increases the energy content of the molecules.
• Thickness of the membranes
  - The thicker the membrane the lower the rate of osmosis while the rate of osmosis is greater through thinner membranes.

- Active transport refers to the process through which substances are moved across the cell membrane and against a concentration gradient.
- Diffusion and osmosis alone do not account for movement of substances in and out of the cells.
- In particular, there are some mineral salts that occur at low concentrations in the soil water than in the cell sap. Some of these mineral salts cannot be absorbed by the plants through diffusion.
- A mechanism that would move them into the cells against the concentration gradient will be useful.
- Active transport requires energy. This is unlike diffusion and osmosis that only depend on concentration gradient for them to take place.
- It is postulated that there are protein carrier molecules on the cell membrane that aid in the moving these substances across the membrane.
- These carrier molecules combine with the substances being transported across the membrane and then move them from one side of the membrane to the other side.
- Cellular intake of solutes is largely through active transport.

- Active transport is important in living things in that:

• It helps in re-absorption of sugars and some salts by the kidney to the bloodstream.
• It helps in absorption of some mineral salts from the soil by roots.
• Absorption of digested food from alimentary canal of animals into the bloodstream.
• It leads to accumulation of substances into the body to offset osmotic imbalance in arid and saline environments.
• It plays a role in excretion of waste products from body cells.

• Oxygen concentration
  - Oxygen is required in respiration process that yields energy for active transport.
  - Under low oxygen concentration, the rate of respiration will be low hence there will be production of little energy leading to low rate of active transport.
  - Increase in oxygen concentration translates into a higher energy production leading to high rate of active transport.
• Change in pH
  - Change in pH affects the respiratory process which is enzyme controlled.
  - Respiratory enzymes require optimum pH for their efficient activity.
  - Extreme pH conditions will increase lower the rate of active transport since the enzymes controlling respiration will be denatured.
• Glucose concentration
  - Glucose is the chief respiratory substrate.
  - At low glucose concentration, there will be less production of energy leading to decreased rate of active transport.
  - Rate of active transport increases with increase in glucose concentration due to increase in the rate of energy production.
• Temperature
  - Temperature affects the enzyme controlled respiration process.
  - At low temperatures, the enzymes are inactive hence the rate of respiration will be low resulting into low rate of active transport since there will be less production of energy.
  - An increase in temperature increases the rate of respiration since the enzymes become more activated.
  - At temperatures beyond 40 degrees Celsius, the enzymes become denatured, respiration stops and so does active transport.
• Presence of metabolic inhibitors e.g. cyanide.
  - These are substances which act as metabolic poisons.
  - They stop the rate of respiration leading to production of no energy. Active transport is, thus, stopped.