Osmosis

Key Features of Osmosis:

1. Passive Process: Osmosis is passive, meaning it does not require energy.
2. Water Only: Osmosis involves the movement of water molecules only.
3. Semi-Permeable Membrane: The membrane allows water to pass through but blocks larger molecules like sugars or proteins.

Selectively Permeable

• Water can move freely across cell membranes.
• Cells need water as it takes part in some chemical reactions and provides a medium for metabolism to occur.
• The movement of salts, glucose, and amino acids into and out of cells is strictly regulated.
  • An imbalance of these molecules in the body can harm or kill cells.
  • The cell membrane is selectively permeable, meaning it controls the movement of these substances.

Osmosis in Plant Cells

• Plant cells are surrounded by a strong cellulose wall.
• The cell wall is fully permeable, meaning both large and small molecules can pass in or out of it.
• Inside the cell wall is the cell membrane, which is semi-permeable.

Plant Cells in a Solution Less Concentrated Than Their Cytoplasm

• The cytoplasm of a plant cell contains salts, starch, sugars, proteins, and other molecules dissolved in water.
• If a plant cell is placed in distilled water or a solution that is less concentrated than its cytoplasm, water will move into the cell by osmosis.
• Water passes into the vacuole, causing it to swell. This makes the cell membrane push against the cell wall.
• The cell becomes much firmer due to turgor pressure and is described as turgid.

Plant Cells in a Solution More Concentrated Than Their Cytoplasm

• If a plant cell is placed in a highly concentrated solution, water will leave the vacuole and the cell due to osmosis.
• This causes the cell to lose turgor pressure, making it weaker. The cell is described as plasmolysed.
• A loss of turgor pressure leads to the plant wilting.
• This can be reversed by placing the plant cells in a less concentrated solution, such as tap water, to allow water to move back into the cells.

Osmosis in Animal Cells

• Animals do not have a cell wall.
• They have a cell membrane, which is semi-permeable.

Animal Cells in a Solution Less Concentrated Than Their Cytoplasm

• If animal cells are placed in a very dilute solution, water will move into the cells by osmosis.
• The cells will expand as water enters.
• As animal cells do not have a cell wall to provide support, they will eventually burst if too much water enters.

Animal Cells in a Solution More Concentrated Than Their Cytoplasm

• If animal cells are placed in a very concentrated solution, water will leave the cells by osmosis.
• This causes the cells to lose shape, shrink, and become dehydrated.

Osmosis and Food Preservation

Osmosis is used to prevent the growth of microorganisms like bacteria and fungi on food. This helps to:

• Prevent harmful toxins from forming.
• Slow down food decay, keeping food fresh for longer.

How It Works:

1. Salty Solutions:

• Fish and meat are soaked in a very salty solution.
• Water moves out of the cells of any microorganisms by osmosis.
• This causes the microorganisms to die, preserving the food.

2. Sugar Solutions:

• A concentrated sugar solution works in the same way.
• For example, in jam, the high sugar concentration removes water from bacteria or fungi, killing them. By removing water through osmosis, microorganisms cannot survive or grow, making this an effective method of food preservation.