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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Vacuoles and Storage
Vacuoles are membrane-bound organelles found in the cells of plants, fungi and some protists, as well as in some animal cells. They're like the storage rooms of cells, holding various substances including water, nutrients, waste products and sometimes even toxic materials. In this session, we'll explore how these cellular compartments work and why they're so important for cell function.
Key Definitions:
Plant cells typically have one large central vacuole that can occupy up to 90% of the cell's volume. This vacuole is filled with cell sap and plays crucial roles in:
Animal cells contain much smaller vacuoles that are typically temporary structures. They serve different functions:
The tonoplast is the membrane that surrounds the vacuole. It's a type of selectively permeable membrane that controls what enters and leaves the vacuole. This membrane is crucial for maintaining the different conditions inside the vacuole compared to the rest of the cell.
The tonoplast contains transport proteins that allow specific substances to move in and out of the vacuole. These proteins use energy (in the form of ATP) to pump substances against their concentration gradient, allowing the vacuole to store materials at higher concentrations than found in the cytoplasm.
Special proteins called aquaporins in the tonoplast allow water to move in and out of the vacuole. This helps maintain the cell's water balance and contributes to turgor pressure.
The tonoplast contains ion pumps that move minerals like potassium, calcium and sodium into the vacuole for storage. This helps maintain the correct ion balance in the cytoplasm.
The tonoplast helps isolate potentially harmful waste products from the rest of the cell by sequestering them inside the vacuole, protecting the cell from damage.
Vacuoles are versatile storage compartments that can hold a wide variety of substances. Let's explore what they store and why this storage is important for cell function.
Vacuoles store:
This storage function is particularly important for plant survival during periods of nutrient scarcity.
Many plant cell vacuoles contain pigments that give plants their characteristic colours:
These pigments help attract pollinators to flowers and seed dispersers to fruits.
Beyond their storage role, vacuoles play a crucial structural role in plant cells. The large central vacuole pushes the cytoplasm and organelles against the cell wall, creating turgor pressure when the vacuole is filled with water. This pressure:
When plants don't have enough water, the vacuoles shrink, turgor pressure decreases and plants wilt. This is why watering your houseplants perks them up – their vacuoles refill with water, increasing turgor pressure and making the plant stand upright again.
Many plants use their vacuoles as storage sites for defensive compounds that protect against herbivores and pathogens. For example:
These compounds are kept separate from the rest of the cell in vacuoles to prevent them from interfering with normal cellular processes. When an insect or animal damages the plant, these compounds are released, deterring further attack.
Different organisms have evolved specialised vacuoles to suit their particular needs:
Single-celled organisms like Paramecium and Amoeba that live in freshwater environments face a constant challenge: water continuously enters their cells by osmosis because the cell has a higher solute concentration than the surrounding water. To prevent bursting, these organisms have contractile vacuoles that:
These vacuoles can be seen pulsating under a microscope as they fill and empty, sometimes several times per minute.
Some animal cells, particularly white blood cells and protists like amoebas, form temporary food vacuoles during phagocytosis (cell eating). This process involves:
This process is vital for immune function in humans, as white blood cells use it to destroy bacteria and other pathogens.
You can actually observe vacuoles in plant cells using a simple light microscope. One of the easiest ways is to look at the cells of an onion epidermis or Elodea (pondweed) leaf:
In onion cells, you'll see large, clear central vacuoles that take up most of the cell space. In Elodea, you might even observe cytoplasmic streaming - the movement of cytoplasm around the large central vacuole.
If you add a drop of salt solution to the edge of the coverslip, you can observe plasmolysis - the shrinking of the vacuole and pulling away of the cell membrane from the cell wall as water leaves the vacuole by osmosis.
The largest known vacuoles are found in the cells of some desert plants. These plants can survive long periods of drought because their vacuoles can store massive amounts of water when it's available. The saguaro cactus, for example, can expand its diameter by up to 20-25% after heavy rainfall as its vacuoles fill with water. This stored water can then be used during dry periods, allowing the cactus to survive for months without rain.
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