Vacuoles, Practice Problems and FAQs

Getting groceries to home is a monthly process in most homes. You might have gone grocery shopping with your parents. You may go to a nearby store to buy things, or to a supermarket to get more things in one go. Whatever be the mode of purchase, there are chances of not getting some things which we need. So what do we usually do? We ask the workers for the thing we need and they check and tell us, whether it is available or not. Where do they check if they need extra items? They go and check in the store room, where they unload and store all their inventory.

Fig: Store room

So now think of the living cell and compare it with a supermarket. All the materials needed for the body are provided by the cells. But there should be some backups for future use. Just like the store house in a supermarket, most of the living cells have a similar space, where the cell stores materials. It is known as vacuole. Sometimes the digestion of materials also takes place in the vacuoles. It is also a member of the endomembrane system.

Vacuoles are clearly visible in the plant cells and are less common in animal cells. There are many types of vacuoles and they do different jobs too. So in this article we are going to discuss more about the vacuoles.

Cell and endomembrane system

We know that the cell is the structural and functional unit of life. According to the presence or absence of a well defined nucleus there are two types of cells. They are prokaryotic and eukaryotic cells. A prokaryotic cell does not have a well defined nucleus and a eukaryotic cell has a well defined nucleus. The structure of a prokaryotic cell is simpler than the eukaryotic cell, because it lacks all the cell organelles that are membrane bound. An example of a prokaryotic cell is a bacterial cell.

Fig: Prokaryotic and eukaryotic cells

The cells of protists, fungi, plants and animals are eukaryotic in nature. Along with the membrane bound nucleus, they are composed of membrane bound organelles. Endoplasmic reticulum, mitochondria, Golgi bodies, vacuoles, lysosomes etc are some of those. All the membranous components of a cell are in close coordination with each other and we can call them an endomembrane system. The cell organelles that are involved in the endomembrane system are nucleus, endoplasmic reticulum (ER), Golgi complex, lysosomes and vacuoles.

Fig: Endomembrane system

Vacuole

Among the different cell organelles of the endomembrane system, vacuole is one organ which is prominent in the plant cells, but not in the animal cells. Vacuoles are sac-like membrane bound spaces present in the cytoplasm that are fluid filled and devoid of cytoplasm. The shape of the vacuole depends on the constituents present in them. Vacuoles are found among protists, fungi, plants and animals.

Biogenesis of vacuoles

There are different conflicting opinions about the biogenesis of vacuoles. Some scientists suggest that vacuoles originated from the golgi bodies and ER, through the fusion of vesicles formed by these organelles. Another view is that the vesicles, formed by the invagination of the plasma membrane and also by the tubular extension of the outer nuclear membrane, fuse to form vacuoles.

Types of vacuoles

There are four types of vacuoles depending upon their contents and functions. They are as follows:

Sap vacuoles
Contractile vacuoles
Food vacuoles
Gas vacuoles

Sap vacuoles

The fluid filled vacuoles found in both animal and plant cells are called sap vacuoled. These are smaller in size in animal cells than in plant cells. Mature plant cells have a single, large, central sap vacuole. The smaller vacuoles in young plant cells fuse together to form single large vacuoles in mature plant cells.

Structure of sap vacuole

The structure of a sap vacuole is very simple. They have major two parts:

Tonoplast
Cell sap

Fig: Sap vacuole in a plant cell

Tonoplast

It is the membrane lining of the vacuole that separates the inner components of vacuole from the outer cytoplasm. The major constituent of tonoplast is the phospholipids bilayer embedded with proteins. The tonoplast is semi permeable in nature and has ion channels and transport proteins or carrier proteins to facilitate transport of materials in and out of the vacuole.

Cell sap

The watery fluid along with some constituents filled inside the tonoplast is called cell sap. The cell sap composition is entirely different from that of the cytoplasm. The components of sap vacuoles are as follows:

Minerals
Sugars
Amino Acids
Proteins
Esters
Anthocyanins (water soluble pigments)
Crystalline deposits or waste products

Functions of sap vacuoles

The major functions of sap vacuole are as follows:

The storage of nutrients and mineral salts and their concentration.
Maintenance of osmotic pressure in cells. This helps to maintain the turgidity and water absorption.
The colours of fruits and flowers are due to the presence of water soluble pigments in sap vacuoles. Examples for pigments are anthocyanin and anthoxanthins. Anthocyanins provide red, purple and blue colours and anthoxanthin provides ivory to deep yellow to the plant parts. This colouration is helpful to attract the pollinators like insects and birds and also helpful in seed dispersal by animals.
Alkaloids and tannins are stored in sap vacuoles, which protects the plants from herbivores.
Hydrolytic enzymes are present in some plant vacuoles that act as lysosomes.
Sap vacuoles can store and concentrate waste products.
Storage of tannins and latex.
The rapid exchange of solutes and gases between the cytoplasm and adjoining fluids are facilitated by the large central vacuole that spreads the cytoplasm into a thin layer.

Contractile vacuoles

The vacuoles that enlarge by accumulating fluids and collapse by throwing them out of the cell are called contractile vacuoles. They are found in protozoan and algal cells. They are entirely absent in animal cells. A few feeding canals are connected to the contractile vacuole.

Fig: Contractile vacuoles in Amoeba

The function of feeding canals is to obtain water with or without waste products from surrounding cytoplasm and pour it into the vacuoles. As a result the vacuoles swell up. This process is called diastole. Then the enlarged vacuole will move to the surface of the plasma membrane and fuse with the membrane. This helps them to discharge the contents into the exterior part of the cell. This process is called systole.

Fig: Diastole and systole

The major functions of a contractile vacuole are as follows:

They take part in osmoregulation and excretion.
Contractile vacuoles in the freshwater organisms, helps them to throw out the extra water from their body and prevent them from bursting.

Food vacuoles

Food vacuoles are formed by the fusion of phagosomes and lysosomes. A phagosome is a vesicle formed by the fusion of cell membranes around a food particle and lysosomes are the cell organelles that have digestive enzymes in them. They are usually seen in the cells of protists, several lower animals and phagocytes of higher animals.

Fig: Food vacuole

Gas vacuoles

A group of submicroscopic vesicles, each containing an individual protein membrane around it is called a gas vacuole. They are also known as pseudo vacuoles. They are usually seen in prokaryotic cells. These vesicles contain metabolic gases. The major functions of a gas vacuole are as follows:

To store gases
To regulate the buoyancy of the cell
To provide mechanical support
To protect the cell from harmful radiations

Functions of vacuole

The major functions of a vacuole are as follows:

Storage of materials
Generation of turgor or hydrostatic pressure to maintain the cell shape
Endocytosis and exocytosis
Defence
Recycling

Storage

The materials that are stored in the vacuoles include salts, minerals, proteins and pigments within the cell. The cell sap consists of additional protons which can move into and out of the vacuole. This will help to maintain or stabilise the acidic environment inside the cell. The force created during the movement of protons promotes the displacement of nutrients inside and outside the vacuole. A large number of lipids are also stored in the vacuoles. The stored materials are used by the cytoplasm when required.

Turgor or hydrostatic pressure

Tonoplast is a partially permeable membrane and it allows the exchange of water between cytoplasm and vacuole sap. A pressure potential is generated when there is an osmotic entry of water into the cytoplasm from outside and also to the cell sap from the cytoplasm. This pressure potential is called turgor pressure or hydrostatic pressure. This will push the protoplast against the cell wall and the cell wall will exert an equal and opposite counter pressure called the wall pressure. The form and mechanical strength of the succulent parts of a plant like leaves, flowers, young stems etc. is maintained by the balanced interaction between the turgor pressure and wall pressure.

Endocytosis and exocytosis

The two fundamental processes in the intracellular digestion are endocytosis and exocytosis, which is done by the vacuoles. The process of uptaking of food materials is called endocytosis and the process of excretion of digested food materials are called exocytosis. The intracellular digestion is done using the hydrolytic enzymes present in the vacuole.

Fig: Endocytosis and exocytosis

Defence

Vacuoles provide protection against several factors. Vacuoles can eliminate the harmful substances from the body. It also helps the cell to attack the bacteria entering it. With the help of lysosomes, vacuoles can protect the cell against the bacteria, viruses and other microbes. It also protects the cell from shrinking or bursting by maintaining the osmotic balance of the cell.

The accumulation of tannins, waste and secondary metabolites in the vacuoles acts as an astringent. This will help the plants from the attack of herbivores.

Recycling

The destruction and recycling of disintegrated proteins is a function of vacuoles. The recycled proteins are then used for building up the cells. This is usually observed in plant cells. Hence the maintenance of balance between the degradation and production of cellular components are also considered as the function of vacuoles.

Vacuoles of different organisms

Most of the eukaryotic organisms have vacuoles in their cells. Different organisms have different types of vacuoles and they do different functions.

Vacuole of protozoa

The single celled organisms like freshwater protozoans contain food vacuoles and water filled contractile vacuoles that lie in the cytoplasm. The major functions of vacuoles present in protozoa are ingestion, storage of food, digestion, excretion and water expulsion.

Vacuole of plant

Most mature plant cells have a single large vacuole. Large vacuoles are formed by the fusion of smaller ones during the course of growth and development. These vacuoles occupy more than 80 percent of the plant cell. The presence of a vacuole is the reason why the plant nucleus is pushed to one side of the cell.

Fig: Structure of vacuole of a typical plant cell

This vacuole is delimited by a semi or partially permeable membrane called tonoplast. The fluid that fills the vacuole is called sap or vacuole sap. The major constituents of plant cell sap are sugars, mineral salts, organic acids, proteins, pigments, oxygen, carbon dioxide, wastes etc. The main function of plant vacuoles is to maintain osmotic pressure of the cell.

The plant vacuoles also consist of secondary metabolites such as tannins and many other biological pigments. This helps in protection of the cell from self toxicity.

Vacuole of fungi

The vacuoles of fungi are the same as that of plants. The major functions of vacuoles of a fungi are regulation of osmotic pressure, maintaining ion concentration, storage of amino acids, maintenance of cell homeostasis, and degradative procedures.

Vacuole of animals

The size of animal vacuoles are smaller than that of plants. Animal cells may either not have vacuoles or may have a few very small vacuoles. The vacuoles are highly reduced in animals because they use most of their energy for various metabolic activities and do not need to store much food as they can move around to gather food. Vacuoles in animals are generally used to store waste materials and eliminate those. The major functions of animal vacuoles are cell protection, storage, recycling, endocytosis and exocytosis.

Practice Problems

1. Which of the following statements are correct regarding the vacuoles in different organisms?

I) The freshwater protozoans contain vacuoles that are filled with water.

II) Large plants vacuoles are formed by the fusion of smaller ones during the course of growth and development.

III) Regulation of osmotic pressure, maintaining ion concentration, storage of amino acids are some of the functions of vacuoles of fungi.

IV) The size of animal vacuoles are larger than that of plants.

I, II, III, IV
I, II, III
I and III
IV

Solution: Vacuoles are sac-like membrane bound spaces present in the cytoplasm that are fluid filled and devoid of cytoplasm. The shape of the vacuole depends on the constituents present in them. They are found among protists, plants and animals. Most of the eukaryotic organisms have vacuoles in their cells. Different organisms have different types of vacuoles and they do different functions. The vacuoles of single celled organisms like freshwater protozoans are filled with water and are called contractile vacuoles. These vacuoles help in water expulsion by taking in excess water and then coming in contact with the cell membrane to contract and spill their content to the outside.

Mature plant cells generally have a single large vacuole. Large vacuoles are formed by the fusion of smaller ones during the course of growth and development. These vacuoles occupy more than 80 percent of the plant cell. The vacuoles of fungi are the same as that of plants. The major functions of vacuoles of a fungi are regulation of osmotic pressure, maintaining ion concentration, storage of amino acids, maintenance of cell homeostasis, and degradative procedures. The size of animal vacuoles are smaller than that of plants. The major functions of animal vacuoles are cell protection, storage, recycling, endocytosis and exocytosis. Hence the correct option is b.

2. What are the components of a plant cell vacuole?

Only Water
Metabolic gases
Water and dissolved substances
Cytoplasm

Solution: The sac-like membrane bound space present in the cytoplasm are called vacuoles. It consists of an empty space filled with fluid and some other constituents. 90 percent of a plant cell is occupied by the vacuole. The major components of a vacuole are water, sap, excretory products and other materials that are not useful to the cell. The cell sap consists of additional protons which can move into and out of the vacuole. This will help to maintain or stabilise the acidic environment inside the cell. Hence the correct option is c.

3. Which of the following cells has a vacuole that occupies ninety percent of the cell?

Bacterium
Amoeba
Onion cell
Virus

Solution: Plants have large and numerous vacuoles. Large vacuoles are formed by the fusion of smaller ones during the course of growth and development. These vacuoles occupy around 90 percent of the plant cell. Mature plant cells like parenchyma and collenchyma cells have exceptionally large central vacuoles. This vacuole is delimited by a semi or partially permeable membrane called tonoplast. The fluid that fills the vacuole is called sap or vacuole sap. The major constituents of a plant sap are sugars, mineral salts, organic acids, proteins, pigments, oxygen, carbon dioxide, wastes etc. since bacterium is a prokaryote, it lacks vacuole. In amoeba the vacuole is very small, which helps in osmoregulation. The virus is acellular. Hence the correct option is c.

4. Which of the following organelles contains cell sap?

Ribosomes
ER
Mitochondria
Vacuole

Solution: The sac-like membrane bound spaces present in the cytoplasm are called vacuoles. It consists of an empty space filled with fluid and some other constituents. The structure of a eukaryotic sap vacuole is very simple. They have major two parts that are the tonoplast and cell sap. Tonoplast is the membrane lining of the vacuole that separates the inner components of vacuole from the outer cytoplasm. The watery fluid along with some constituents filled inside the tonoplast is called cell sap. The cell sap composition is entirely different from that of the cytoplasm. Hence the correct option is d.

FAQs

1. What is the importance of a vacuole in a rubber tree?

Answer: Vacuoles contain accumulations of latex as milky emulsions. Some cells called laticifers are specialised for this function. The latex of the rubber tree (Hevea brasiliensis) contains the enzymes and the intermediates for rubber synthesis. Natural rubber is one of the important polymers which is used to make many useful products like tyres, medical devices, toys, cloths etc.

2. Who discovered the vacuoles?

Answer: Spallanzani was the first to observe vacuoles in protozoa but mistook them to be respiratory organelles. Felix Dujardin was the one who reported the presence of vacuoles in Infusoria, which includes the protozoans, unicellular algae, ciliates etc and named them so. He found it as some aqueous spaces and later named them vacuoles. He regarded them as a characteristic feature of a living organism.

3. Are vacuoles present in blood cells?

Answer: Yes, vacuoles are present in the red blood cells. Their frequency of occurrence was studied by transmission electron microscopy. About 13 percent of the cell sections were composed of small vacuoles. Those vacuoles have a mean diameter of 130+/-72 nm. The approximate estimation of the number of the vacuoles in each red blood cell is 20.

4. Is it possible for a plant cell to live without vacuole?

Answer: No. A plant cell can not survive without vacuole. If vacuoles are absent, then the functions of vacuoles like water storage and maintenance of structure will not be possible. Because of this, the death of the plant cells will happen eventually.

YOUTUBE LINK: https://www.youtube.com/watch?v=-NOY_k8iN9A&t=2893s (57:43-59:33)