Osmosis - Biology Double Award

Osmosis:

Osmosis is the diffusion (movement) of water from a dilute solution to a more concentrated solution through a selectively permeable membrane

What is the difference between Osmosis and Active Transport?

Active Transport requires energy

How does Osmosis work?

When a solute dissolves in water it does so because the water molecules are attracted to the solute and cluster around it. This reduces the freedom of movement of the water molecules.

How does water move in pure water?

Freely moving and exerting pressure. This is called water potential.

What happens when you add sucrose and salt to pure water?

This reduces the freedom of movement of water molecules, so adding some solute lowers the water potential

Look at this image:

Hypotonic = freely moving molecules

Hypertonic = very few freely moving molecules

Describe the effect of Osmosis in plant cells:

• When plant cells are in contact with dilute solutions (which have a higher water potential) they gain water by Osmosis

• The vacuole fills up with water and swells eventually the cell gains so much water that the cell wall pushes back on the contents of the cell

• The cell wall prevents excessive water uptake and so prevents the cell from bursting

Describe Turgid, Flaccid, Plasmolysed:

1. Turgid -

   • Reached max water intake

   • Cell membrane pushes against the cell wall

   • Sign of healthy plant

   • Vacuole expands

2. Flaccid -

   • When plant is droopy it is a sign the cells are turning flaccid

   • Vacuole is smaller

   • Less sturdy

3. Plasmolysed -

   • When plant is unsalvable it is a sign the cells are Plasmolysed

   • Cell membrane tears away from the cell wall

   • Shrivelled vacuole

   • The space between the cell wall and the cell membrane is filled with the external sugar solution because the cell wall is fully permeable

Describe the effect of Osmosis in animal cells:

• If the Animal Cell is placed in a solution with higher water potential the Animal Cell will continue to swell as it doesn’t have a cell wall to prevent excessive water uptake. If the RBC absorbs to much water it bursts (lysis)

• If the Animal Cell is placed in a solution with lower water potential the Animal Cell will lose water by Osmosis and become shrivelled (Crenation)

• Isotonic solutions are solutions with the same water potential as the Animal Cell

Why do plants need water?

1. Support -

   • Plants don’t have a skeleton so they rely on the turgidity of the cells to keep them upright

2. Transport -

   • Mineral ions dissolve in water in the soil and are transported throughout the plant within the xylem

3. Photosynthesis -

   • Water is a reactant in the process of photosynthesis; it combines with CO2 to produce glucose. The rate of photosynthesis will be limited if the water is in short supply.

What does the waxy cuticle do for water in plants?

1. Prevents water loss by evaporation

2. Provides defence against microbial infection

Transpiration:

The evaporation of water off the spongy mesophyll surfaces surfaces followed by diffusion through the air spaces and out of the stomata

Explain the stages of Transpiration:

1. Water evaporates off the spongy mesopyll

2. The water molecules then diffuse through the air spaces and out of the open stomata

What is the transpiration rate?

The amount of water lost from a leaf or a plant in a set period of time

Name the factors that affect the Transpiration Rate:

1. Temperature

   • Increasing the temp increases the rate of evaporation of water from the surface of the spongy mesophyll cells

2. Humidity

   • In humid air, there is a lot of moisture. This reduces the diffusion gradient, decreasing transpiration

3. Wind Speed

   • In windy conditions, moisture filled air can’t accumulate on the underside of the leaf. This increases the diffusion gradient, increasing transpiration

4. Surface Area

   • Increasing leaf surface, increases the number of stomata therefore increasing transpiration. Most stomata are on the lower leaf surface, Vaseline can be used to block stomata in transpiration experiments

5. Light

   • Stomata close in the dark (as CO2 isn’t needed for photosynthesis) Therefore, transpiration stops at night

What items can you use to affect Transpiration Rate?

Heater - Temp Increases

Fan - Wind Speed

Plastic bag over shoot - Humidity

Removal of leaves - Decrease SA

Cut shoot at angle - Increase SA

Explain the apparatus of the bubble potometer:

1. The potometer is used to measure the rate of transpiration by timing how far the air bubble travels along the capillary tube towards the shoot in a set period of time

2. It actually measures the rate of water uptake and assumes this is equal to the rate of water loss. This is an invalid assumption as the plant uses a lot of the water it takes up for the photosynthesis and turgidity of the cells

3. The reservoir is used to reset the air bubble to zero so more measurements can be taken

4. It has to be set up under water and all the joints sealed with grease (Vaseline) to make sure its air tight

Explain the weight potometer experiment:

1. Weigh plant and pot at start of the experiment

2. Leave for 24hrs

3. Reweigh

4. Repeat using heat, fan, or plastic bag to mimic temp, wind or humidity

5. Work out the rate of transpiration which is:

   • Amount of water lost/24hours

Explain the bubble potometer experiment:

1. Set up the bubble potometer under water

2. Remove the capillary tube from the beaker to introduce an air bubble. Set the bubble to zero using reservoir

3. Measure the distance moved by the air bubble in a 30min period

   • Distance moved/time

4. Use the reservoir to reset the bubble to zero

5. Repeat the experiment using a fan

6. The bubble will move further towards the shoot

7. Use the same shoot in both experiments, for the same length of time at the same temp and humidity (fair test)

8. Repeat the experiment 3 times in both conditions to improve reliability