Vacuole
- A vacuole is a membrane-bound organelle found in all plant and fungal cells, as well as in some protist, animal, and bacterial cells.
- In plant cells, vacuoles are particularly prominent, with a large, fluid-filled central vacuole occupying a significant portion of the cell’s volume.
- Large vacuoles are also present in certain filamentous sulfur bacteria, such as Thioploca, Beggiatoa, and Thiomargarita.
- The function and significance of vacuoles vary depending on the cell type.
- They play a much more prominent role in plant, fungal, and certain protist cells compared to animal and bacterial cells.
- A single cell may contain multiple vacuoles, each enclosed by a membrane called the tonoplast.
- Typically, vacuoles occupy more than 30% of a plant cell’s volume but can range from 5% to 90% based on the cell type.
Structure of Vacuoles
- Vacuoles do not have a fixed shape or size; their structure adapts according to the cell’s needs.
- In young, actively dividing plant cells, vacuoles are small and originate from vesicles derived from the Golgi apparatus.
- As the cell matures, these smaller vacuoles merge to form a single large central vacuole.
- The vacuole is enclosed by a membrane known as the tonoplast, which separates the vacuolar contents from the cytoplasm.
- The tonoplast regulates ion movement and isolates potentially harmful materials. Vacuoles function similarly to lysosomes in animal cells, containing hydrolytic enzymes and storing various substances such as sugars, salts, acids, nitrogenous compounds, and pigments like anthocyanins.
- The pH inside plant vacuoles varies widely, ranging from highly alkaline (pH 9–10) to highly acidic (pH 3) depending on the stored substances.
Types of Vacuoles
- Sap Vacuoles:
- Found in plant and some animal cells.
- In young plant cells, multiple small sap vacuoles exist, which later merge into a large central vacuole.
- The vacuolar sap facilitates rapid exchange between the cytoplasm and the surrounding environment.
- Contractile Vacuoles:
- Found in some freshwater protists and algae.
- Consists of an extensible and collapsible membrane connected to feeding canals (e.g., Paramecium).
- Plays a role in osmoregulation and excretion by expelling excess water.
- Food Vacuoles:
- Present in protozoa, lower animals, and phagocytic cells in higher animals.
- Formed by the fusion of a phagosome with a lysosome.
- Contains digestive enzymes that break down nutrients for absorption.
- Air Vacuoles (Gas Vacuoles):
- Found only in prokaryotic cells.
- Consist of numerous small vesicles surrounded by protein membranes.
- Provide buoyancy, mechanical strength, and protection against harmful radiation.
MCQs on Vacuoles
Basic Concepts
- What is a vacuole?
a) A non-membrane-bound organelle
b) A membrane-bound organelle
c) A type of ribosome
d) A type of mitochondrion
Answer: b) A membrane-bound organelle - Vacuoles are particularly large in which type of cells?
a) Animal cells
b) Bacterial cells
c) Plant cells
d) Red blood cells
Answer: c) Plant cells - Which of the following is the membrane surrounding the vacuole called?
a) Cell membrane
b) Nuclear envelope
c) Tonoplast
d) Plasma membrane
Answer: c) Tonoplast - In plant cells, vacuoles can occupy up to what percentage of the cell’s volume?
a) 10%
b) 30%
c) 50%
d) 90%
Answer: d) 90% - Which of the following is NOT a function of vacuoles?
a) Storage of nutrients
b) Waste disposal
c) Protein synthesis
d) Maintenance of turgor pressure
Answer: c) Protein synthesis
Types of Vacuoles
- Sap vacuoles are primarily found in:
a) Bacteria
b) Animal cells
c) Plant cells
d) Viruses
Answer: c) Plant cells - Contractile vacuoles help in:
a) Food storage
b) Digestion
c) Osmoregulation
d) Photosynthesis
Answer: c) Osmoregulation - Contractile vacuoles are found in:
a) Paramecium
b) Human liver cells
c) Plant roots
d) Mitochondria
Answer: a) Paramecium - Food vacuoles are formed by the fusion of:
a) Ribosomes and mitochondria
b) Golgi apparatus and lysosomes
c) Phagosomes and lysosomes
d) Endoplasmic reticulum and ribosomes
Answer: c) Phagosomes and lysosomes - Air vacuoles (gas vacuoles) are found in:
a) Eukaryotic cells
b) Only in animal cells
c) Only in plant cells
d) Prokaryotic cells
Answer: d) Prokaryotic cells
Functions of Vacuoles
- Vacuoles help maintain:
a) Mitochondrial energy production
b) Cell turgor pressure
c) Nucleic acid synthesis
d) Protein formation
Answer: b) Cell turgor pressure - Which pigment is stored in plant vacuoles?
a) Chlorophyll
b) Haemoglobin
c) Anthocyanin
d) Keratin
Answer: c) Anthocyanin - The pH of vacuolar sap in plant cells can range from:
a) 0-1
b) 5-7
c) 3-10
d) 1-2
Answer: c) 3-10 - What is the role of hydrolytic enzymes in vacuoles?
a) Synthesize proteins
b) Break down molecules
c) Transport oxygen
d) Convert light into energy
Answer: b) Break down molecules - In which type of cells do vacuoles function similarly to lysosomes?
a) Fungal cells
b) Bacterial cells
c) Animal cells
d) Protozoan cells
Answer: a) Fungal cells
Vacuoles in Different Organisms
- Large vacuoles are found in filamentous sulfur bacteria such as:
a) Escherichia coli
b) Staphylococcus aureus
c) Thioploca
d) Mycobacterium
Answer: c) Thioploca - In animal cells, vacuoles are usually:
a) Large and central
b) Small and numerous
c) Absent
d) Thick-walled
Answer: b) Small and numerous - Which organism uses food vacuoles for intracellular digestion?
a) Humans
b) Paramecium
c) Fungi
d) Cyanobacteria
Answer: b) Paramecium - In fungi, vacuoles help in:
a) Photosynthesis
b) Storage and waste degradation
c) DNA replication
d) Cell respiration
Answer: b) Storage and waste degradation - Which type of vacuole is unique to prokaryotic cells?
a) Sap vacuole
b) Contractile vacuole
c) Food vacuole
d) Air vacuole
Answer: d) Air vacuole
Miscellaneous
- The tonoplast is selectively permeable, meaning:
a) It allows all molecules to pass freely
b) It blocks all ions and molecules
c) It controls movement of substances in and out of the vacuole
d) It functions as a rigid structure
Answer: c) It controls movement of substances in and out of the vacuole - The main storage form of nitrogen in plant vacuoles is:
a) Proteins
b) Ammonia
c) Nitrates
d) Uric acid
Answer: c) Nitrates - What happens to the size of the vacuole as a plant cell matures?
a) It decreases
b) It remains the same
c) It increases
d) It disappears
Answer: c) It increases - If the vacuole of a plant cell loses water, the plant will:
a) Become turgid
b) Wilt
c) Grow faster
d) Synthesize more chlorophyll
Answer: b) Wilt - Vacuoles help in detoxification by:
a) Synthesizing proteins
b) Isolating harmful substances
c) Producing ATP
d) Enhancing photosynthesis
Answer: b) Isolating harmful substances