# Osmosis in Animal & Plant Cells ## Effects of Osmosis on Animal Cells - Animal cells can **lose** and **gain water** as a result of osmosis - Unlike plant cells, animal cells **lack a supporting cell wall** - This makes the consequences of water movement much more **severe** ### Animal Cells Losing Water - If an animal cell is placed in a solution with a **lower water potential** than the cell (e.g. a concentrated salt solution): - Water will **leave** the cell through its partially permeable cell surface membrane by osmosis - The cell will **shrink** and **shrivel up** - This process is called **crenation** (the cell has become **crenated**) - Usually **fatal** for the cell - Crenation occurs when the cell is in a **hypertonic environment** - The solution outside has a **higher solute concentration** than inside the cell

A hypertonic solution has a higher solute concentration (lower water potential) than the cell

### Animal Cells Gaining Water - If an animal cell is placed in **pure water** or a **dilute solution**: - Water will **enter** the cell through its partially permeable cell surface membrane by osmosis - The pure water or dilute solution has a **higher water potential** than the cell - The cell will continue to gain water by osmosis until: - The cell membrane is **stretched too far** - The cell **bursts** (**cytolysis**) - This occurs because there is **no cell wall** to withstand the increased pressure - This is **fatal** for the cell - Lysis occurs when the cell is in a **hypotonic environment** - The solution outside has a **lower solute concentration** than inside the cell

A hypotonic solution has a lower solute concentration (higher water potential) than the cell

A constant water potential must be maintained inside animal bodies to prevent cell death

### Animal Cells in Isotonic Environments - If an animal cell is in an **isotonic environment**: - The solution outside has the **same solute concentration** as inside the cell - Water molecules move into and out of the cell at the **same rate** - **No net movement** of water - **No change** to the cells

An isotonic solution has the same solute concentration (same water potential) as the cell

## Effects of Osmosis on Plant Cells - Like animal cells, plant cells can also **lose** and **gain water** as a result of osmosis - Plant cells **have a supporting cell wall** - The results of water loss or gain are **less severe** than in animal cells ### Plant Cells Losing Water - If a plant cell is placed in a solution with a **lower water potential** than the plant cell (e.g. a concentrated sugar solution): - Water will **leave** the plant cell through its partially permeable cell surface membrane by osmosis - As water leaves the **vacuole** of the plant cell: - The **volume** of the plant cell **decreases** - The **protoplast** gradually **shrinks** - It no longer exerts pressure on the cell wall - As the protoplast continues to shrink: - It begins to **pull away** from the cell wall - This process is known as **plasmolysis** - The plant cell is **plasmolysed**

Plasmolysis is when the protoplast shrinks and pulls away from the cell wall due to water loss

### Plant Cells Gaining Water - If a plant cell is placed in **pure water** or a **dilute solution**: - Water will **enter** the plant cell through its partially permeable cell surface membrane by osmosis - The pure water or dilute solution has a **higher water potential** than the plant cell - As water enters the **vacuole** of the plant cell: - The **volume** of the plant cell **increases** - The expanding **protoplast** (living part inside the cell wall) pushes against the cell wall: - **Pressure builds up** inside the cell - The **inelastic cell wall** prevents the cell from bursting - The pressure created by the cell wall also: - Stops **too much water** from entering - Helps to **prevent** the cell from **bursting** - When a plant cell is fully inflated with water and has become **rigid** and **firm**: - It is described as fully **turgid**

A turgid cell is fully inflated with water, rigid and firm due to turgor pressure

#### Importance of Turgidity - **Turgidity** is important for plants because: - When all cells in a plant are firm, it provides **support** and **strength** for the plant - Makes the plant **stand upright** - Leaves are **held out** to catch sunlight - If plants do **not receive enough water**: - The cells **cannot remain rigid and firm** (turgid) - The plant **wilts**

Plant turgidity replaces the need for a skeleton in providing structural support

## Comparing Water Movement in Plant and Animal Cells | Feature | Plant Cells | Animal Cells | |---------|-------------|--------------| | **Cell membrane** | Present; composed of phospholipid bilayer; partially permeable | Present; composed of phospholipid bilayer; partially permeable | | **Cell wall** | Present; composed of cellulose; fully permeable | Absent | | **Osmosis** | Can occur | Can occur | | **Cell placed in solution with lower water potential** | • Water leaves cell through partially permeable cell surface membrane by osmosis
• Volume of cell decreases
• Protoplast shrinks / pulls away from cell wall
• Cell is plasmolysed | • Water leaves cell through partially permeable cell surface membrane by osmosis
• Volume of cell decreases
• Cell shrinks / shrivels up
• Cell is crenated | | **Cell placed in solution with higher water potential** | • Water enters cell through partially permeable cell surface membrane by osmosis
• Volume of cell increases
• Protoplast expands / pushes against cell wall
• Cell wall withstands increased pressure created
• Pressure increases until cell is rigid and firm (turgid)
• Cell is fully inflated with water and no more can enter | • Water enters cell through partially permeable cell surface membrane by osmosis
• Volume of cell increases
• No cell wall to withstand increased pressure created
• Cell membrane eventually stretched too far
• Cell bursts (cytolysis) |

Animal cells do not plasmolyse because they lack a cell wall; only plant cells plasmolyse

## Key Terminology Summary | Term | Definition | Cell Type | |------|------------|-----------| | **Crenation** | Cell shrinkage and shriveling due to water loss | Animal cells only | | **Cytolysis** | Cell bursting due to excessive water gain | Animal cells only | | **Plasmolysis** | Protoplast shrinking away from cell wall due to water loss | Plant cells only | | **Turgid** | Cell fully inflated with water, rigid and firm | Plant cells only | | **Flaccid** | Cell not fully inflated, lacks firmness | Plant cells only | | **Hypertonic** | Solution has higher solute concentration than the cell | Both | | **Hypotonic** | Solution has lower solute concentration than the cell | Both | | **Isotonic** | Solution has same solute concentration as the cell | Both | ## Additional Details on Plant Cell Membranes and Walls - Plant **cell membranes** are composed of a **phospholipid bilayer** - They are **partially permeable** (only certain molecules can cross) - Plant **cell walls** are made of **cellulose** - They are **freely (fully) permeable** - In a **plasmolysed** cell: - The external solution will be exerting pressure on the protoplast - There is **not an empty space** between the cell wall and protoplast - The solution has entered the freely permeable cell wall