3 Movement into and out of cells (3.1 and 3.2)

3.1 Diffusion and Osmosis 

1. Describe the role of water as a solvent in organisms with reference to digestion, excretion and transport 

Water is important for all living organisms, many substances can dissolve in it(it is a solvent). This makes it incredibly useful and essential for all life on Earth 

Water is important as a solvent in the following situations within organisms: 

• Dissolved substances can be easily transported around organisms, for example, xylem and phloem of plants and dissolved food molecules in the blood 

• Digested food molecules are in the alimentary canal but need to be moved to cells all over the body – without water as a solvent, this would not be able to happen. 

• Toxic substances such as urea and substances over requirements, such as salts, can dissolve in water, which makes them easy to remove from the body in urine. 

• Water is also an important part of the cytoplasm and plays a role in ensuring that metabolic reactions can happen as necessary in cells 

2. Understand that the energy for diffusion and osmosis comes from the kinetic energy of the random movement of molecules and ions 

3. Understand diffusion as the net movement of molecules or ions from a region of their higher concentration to a region of their lower concentration (i.e. down a concentration gradient), a result of their random movement 

4. Investigate the factors that influence diffusion, limited to: 

Surface Area:

• The bigger the cell or structure is, the smaller its surface area to volume ratio is, slowing down the rate at which substances can move across its surface. 

• Many cells which are adapted for diffusion have increased surface area in some way – for example, root hair cells in plants(which absorb water and mineral ions)and cells lining the ileum in animals(which absorb the products of digestion) 

Temperature:

• The higher the temperature, the faster molecules move as they have more energy. 

• This results in more collisions against the cell membrane and therefore a faster rate of movement across them 

Concentration gradient:

• The greater the difference in concentration on either side of the membrane, the faster the movement across it will occur 

• This is because on the side with the higher concentration, more random collisions against the membrane will occur

Distance:

• The smaller the distance molecules have to travel, the faster transport will occur. 

• This is why blood capillaries and alveoli have walls which are only one cell thick, ensuring the rate of diffusion across them is as fast as possible  

5. Understand osmosis as the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane 

6. Understand that plants are supported by the pressure of water inside the cells pressing outwards on the cell wall 

• When water moves into a plant cell, the vacuole gets bigger, pushing the cell membrane against the cell wall. 

• Water entering the cell by osmosis makes the cell rigid and firm. 

• This is important for plants as the effect of all the cells in a plant being firm is to provide support and strength for the plant, making the plant stand upright with its leaves held out to catch sunlight. 

• The pressure created by the cell wall stops too much water from entering and prevents the cell from bursting. 

• If plants do not receive enough water the cells cannot remain rigid and firm (turgid), and the plant wilts 

7. Describe the effects of osmosis on plant and animal tissues and explain the importance of water potential gradient and osmosis in the uptake and loss of water 

• Turgid plant cells are full of water and contain a high turgor pressure(the pressure of the cytoplasm pushing against the cell wall). 

• This pressure prevents any more water from entering the cell by osmosis, even if it is in a solution that has a higher water potential than inside the cytoplasm of the cells. 

• This prevents the plant cells from taking in too much water and bursting. 

• Plant roots are surrounded by soil water, and the cytoplasm of root cells has a lower water potential than the soil water. This means water will move across the cell membrane of root hair cells into the root by osmosis. 

• The water moves across the root from cell to cell by osmosis until it reaches the xylem. 

• Once they enter the xylem, they are transported away from the root by the transpiration stream, helping to maintain a concentration gradient between the root cells and the xylem vessels 

Animal Tissues:

• Animal cells also lose and gain water as a result of osmosis 

• As animal cells do not have a supporting cell wall, the results on the cells are more severe. 

• If an animal cell is placed into a strong sugar solution (with a lower water potential than the cell), it will lose water by osmosis and become crenated (shrivelled up). 

• If an animal cell is placed into distilled water(with a higher water potential than the cell), it will gain water by osmosis and, as it has no cell wall to create turgor pressure, will continue to do so until the cell membrane is stretched too far and it bursts 

8. Investigate and explain the effects on plant tissues of immersing them in solutions of different concentrations, using the terms turgid, turgor pressure, plasmolysis flaccid 

• When water moves into a plant cell, the vacuole gets bigger, pushing the cell membrane against the cell wall. 

• Water entering the cell by osmosis makes the cell rigid and firm. 

• This is important for plants as the effect of all the cells in a plant being firm is to provide support and strength for the plant, making the plant stand upright with its leaves held out to catch sunlight. 

• The pressure created by the cell wall stops too much water from entering and prevents the cell from bursting. 

• If plants do not receive enough water, the cells cannot remain rigid and firm (turgid), and the plant wilts 

9. Investigate osmosis using materials such as dialysis tubing 

• The most common osmosis practical involves cutting cylinders of potato and placing them into distilled water and sucrose solutions of increasing concentration 

• The potato cylinders are weighed before being placed into the solutions. 

• They are left in the solutions for 20 – 30 minutes and then removed, dried to remove excess liquid and reweighed. 

• The potato cylinder in the distilled water will have increased its mass the most, as there is a greater concentration gradient in this tube between the distilled water(high water potential)and the potato cells(lower water potential) 

• This means more water molecules will move into the potato cells by osmosis, pushing the cell membrane against the cell wall and so increasing the turgor pressure in the cells, which makes them turgid – the potato cylinders will feel hard 

• The potato cylinder in the strongest sucrose concentration will have decreased its mass the most, as there is a greater concentration gradient in this tube between the potato cells(higher water potential)and the sucrose solution (lower water potential). 

• This means more water molecules will move out of the potato cells by osmosis, making them flaccid and decreasing the mass of the cylinder – the potato cylinders will feel floppy 

• If looked at underneath the microscope, cells from this potato cylinder might be plasmolysed, meaning the cell membrane has pulled away from the cell wall 

• If a potato cylinder has not increased or decreased in mass, it means there was no overall net movement of water into or out of the potato cells 

• This is because the solution that the cylinder was in was the same concentration as the solution found in the cytoplasm of the potato cells, so there was no concentration gradient 

3.2 Active transport 

1. Understand active transport as the movement of molecules or ions into or out of a cell through the cell membrane, from a region of their lower concentration to a region of their higher concentration (i.e. against a concentration gradient), using energy released during respiration 

2. Explain the importance of active transport in ion uptake by root hair cells:

• Active transport in root hair cells is vital for absorbing essential ions like potassium and nitrate from the soil. It enables the cells to take up these ions against their concentration gradient, ensuring plants receive the necessary nutrients for growth. This process is energy-intensive but crucial for maintaining proper ion balance and supporting plant health in various environmental conditions. 

Credits: Notes compiled by Manahil Naeem of Karachi Grammar School