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Cell organelles, Practice Problems and FAQs

Everyone likes music and some will have their own genre of music. Most of us will have our own favourite musicians and sometime we will follow some music band too. Now just think about your favourite music band and count the number of musicians you can find in a band. You can find singers, guitarists, drummers, flautists, etc. When they work together, with proper coordination, then only the music band will work properly and then only the music will be good.

Just like this, we also have some team members inside our cell that work together as a unit, for the proper functioning of our body. They are called cell organelles. The improper functioning of a single cell organelle can affect our body very badly. For example, the nucleus is one of the important cell organelle, which helps in the division of cells and and control all the activities of the cell. So what happens if the nucleus is not working properly? We know that it is the DNA or genetic material which helps in the synthesis of proteins and this process is important for the proper functioning of the body. Hence the entire biological process of the body will get affected. Likewise, each of the cell organelle has their own importance in the cell.

All the living organisms are made up of cells. So what do you think, are all the cells the same or not? It is not. There are different kinds of cells and the cell organelles present in them also differ. In this article we are going to discuss more about the cell organelles of different cells.


                                Fig: Different types of cells

Table of contents

  • Levels of organisation
  • Cell organelles
  • Cell organelles in prokaryotes
  • Cell organelles in eukaryotes
  • Endomembrane system
  • Practice Problems
  • FAQs

Levels of organisation

All living organisms are made of tiny compartments called cells. It is the basis of all life on the Earth. Hence it is known as the fundamental unit of life. Cells are the smallest possible entity of any living organism. Different cells come together to form tissues, which form organs and organ systems, which in turn forms an organism. The various structures in our body are made up of cells and hence, anything less than a complete structure of a cell does not ensure independent living.


                                  Fig: Levels of organisation

Classification of organisms on the basis of nature of nucleus

We can classify the organisms into two groups on the basis of the nature of the nucleus and they are the prokaryotes and the eukaryotes. The basis for this classification is the presence of a membrane bound nucleus.

Prokaryotes

They do not have a membrane bound nucleus, instead they have a portion in their cell called nucleoid, which contains all their genetic material. Prokaryotes also have a limited number of cell organelles, but these are non-membrane bound.

Eukaryotes

In eukaryotes, the cells possess a well developed membrane bound nucleus. Along with the nucleus, eukaryotic cells also have membrane bound cell organelles, which are helping the cells for the proper functioning.


                                            Fig: Structure of prokaryotic and eukaryotic cell

Cell organelles

Cell organelles are the cellular components of a cell. These can be both membrane bound and non-membrane bound. These are normally present within the cells and have particular structure and function. Some cell organelles can be seen outside the cell, but attached to the cell membrane. Most of the cell organelles coordinate and function efficiently for the normal functioning of the cell. The various functions of the cell organelles are the locomotion, cell division, energy production, protein synthesis etc.

Major cell organelles

The major cell organelles of a cell are as follows:

  • Cell wall
  • Cell membrane
  • Cytoplasm
  • Nucleus
  • Endoplasmic reticulum
  • Golgi apparatus
  • Ribosomes
  • Mitochondria
  • Lysosome
  • Vacuoles
  • Plastid
  • Cytoskeletal structure
  • Centrosome
  • Cilia
  • Flagella

Classification of cell organelles

The cell organelles can be classified into three types on the basis of the presence or absence of membrane. They are as follows:

  • Non-membrane bound cell organelles
  • Single membrane bound cell organelles
  • Double membrane bound cell organelles

Non-membrane bound cell organelles

Organelles without membrane can be seen in both prokaryotic and eukaryotic cells. The cell organelles without membrane include cell wall, ribosomes and cytoskeleton.

Single membrane bound cell organelles

Those cell organelles that have only one membrane are called single membrane bound organelles and these are present only in eukaryotic cells. The single membrane bound organelles include vacuole, lysosome, Golgi apparatus and endoplasmic reticulum.

Double membrane bound cell organelles

Those cell organelles which have inner and outer membrane are called double membrane bound cell organelles. These are also present in eukaryotic cells only. The double membrane bound cell organelles are nucleus, mitochondria and chloroplast.

Prokaryotes

The primitive cells that are present on the Earth are the prokaryotic cells. These cells are small and they divide rapidly. Their genetic material is termed as the primitive nucleus, since it is not surrounded by a membrane. The examples of prokaryotic cells are bacteria, blue green algae (cyanobacteria), Mycoplasma, Spirochaetes etc. Since the prokaryotic cells are unicellular, their body itself is a cell.

Major cell organelles in prokaryotes

The cell organelles that are present in the prokaryotes are all non membrane bound and they are as follows:

  • Cell envelope
  • Nucleoid
  • Plasmid
  • Ribosome
  • Mesosomes
  • Cytoskeleton
  • Flagella


                                                   Fig: Structure of a bacterial cell

Cell envelope

Prokaryotic cell envelope is a complex, multilayered structure that protects the organisms from external pressures. Cell envelope consists of three parts and they perform distinct functions but act as a single protective unit. The components of cell envelope are as follows:

  • Glycocalyx
  • Cell wall
  • Cell membrane


                                                 Fig: Components of cell envelope

Glycocalyx

The outermost layer of the cell envelope is called glycocalyx. It is made up of mainly carbohydrates along with proteins and lipids. The major function of a glycocalyx is to evade immune cells of the host. The bacterial glycocalyx has been defined as polysaccharide components lying outside the outer membrane of gram-negative cells or the peptidoglycan layer of gram-positive cells. Glycocalyx can be of two types and they are as follows:

  • Slime layer - It is a loose sheath which protects the cell from loss of water and nutrients.
  • Capsule - It is a thick and tough sheath which makes the cell sticky.


                           Fig: Glycocalyx of bacteria

Cell wall

The layer present just below the glycocalyx which provides shape and support to the prokaryotic cell is called the cell wall. The main composition of the cell wall is peptidoglycan which is a polymer. It contains polysaccharides and amino acids. The alternating residues of the polymer, peptidoglycan are N- acetylglucosamine (NAG) and N- acetyl muramic acid (NAM). Teichoic acid is also a component in some bacteria.


                                Fig: Cell wall of bacteria

The only prokaryotic cell that lacks the cell wall is Mycoplasma. If we consider the cell walls of a Gram positive and Gram negative bacteria, the cell wall composition is different.


                                                      Fig: Mycoplasma

Cell wall of Gram positive bacteria

It has a single layered cell wall which is mostly composed of a thick peptidoglycan layer. The stain during the Gram staining process is trapped by the thick layer of highly cross linked peptidoglycan. In this cell wall teichoic acid is also present. They appear purple-brown coloured in Gram staining.


                            Fig: Cell wall of Gram positive bacteria

Cell wall of Gram negative bacteria

It has a thinner layer of cell wall which is covered by a single layer of lipoproteins and lipopolysaccharides. Teichoic acid is absent in the cell wall, but porins are present. They appear pink coloured in Gram staining.


                   Fig: Cell wall of Gram negative bacteria

Cell membrane

The innermost layer of the cell envelope is called the cell membrane. It is also called plasma membrane. The region between the plasma membrane and outer membrane of the Gram negative bacteria or cell wall of Fungi and Gram positive bacteria is called the periplasmic space.


                          Fig: Cell membrane of bacterial cell

The structure of plasma membrane is the same in both prokaryotic and eukaryotic cells. The cell membrane is selectively permeable to some molecules present on either side of it.


                      Fig: Structure of Cell membrane

Cell membranes are quasi (partly) fluid in nature. The fluid nature is due to non-rigid fatty acid chains and they appear mosaic mostly due to the presence of proteins. Hence the structure of the cell membrane was explained by Singer and Nicolson through a fluid mosaic model. This quasi-fluid nature of the cell membrane is important for the cell growth, formation of intercellular junctions, secretions, endocytosis and cell division. The components of a cell membrane are phospholipids, proteins, cholesterol and carbohydrates. The phospholipid bilayer has a polar or hydrophilic head and nonpolar or hydrophobic tail. Head is towards the outer side of the bilayer and tail is towards the inner side of the bilayer.


                                                    Fig: Lipid bilayer

Nucleoid and plasmid

There is no well-defined nucleus in prokaryotic cells. The genetic material is basically naked i.e not enveloped by a nuclear membrane. Hence it is termed as nucleoid. The naked genetic material is not associated with proteins, lipids or any other molecule. The genetic material can be chromosomal and extra chromosomal.

Chromosomal genetic material

The chromosomal genetic material is called a nucleoid which is linear and coiled. They are not associated with histones (naked DNA) and occur as single copies.

Extrachromosomal genetic material

The extrachromosomal genetic material is called plasmid. They are small, circular double stranded DNA. They also confer unique phenotypic characters and occur as multiple copies. Plasmids are responsible for the antibiotic resistance in some bacteria.


                                   Fig: Nucleoid and plasmids

Ribosome

It is a cell organelle without cell membrane and made up of RNA and proteins. It is the site of protein synthesis. Prokaryotic ribosome is a 70S type. It consists of one small subunit and one large subunit. The small subunit is 30S and the large subunit is 50S. Both these subunits come together at the time of protein synthesis else they are present as deserted units. During the synthesis of proteins, a chain of polysomes or polyribosomes are formed. Polysomes are a group of ribosomes on the mRNA. They appear like beads on strings in polysomes.


                                                Fig: Prokaryotic ribosomes

Mesosomes

The special membranous invaginations of a prokaryotic cell membrane are called mesosomes. These are formed of vesicles, tubules and lamellae. Vesicle is a structure consisting of cytoplasm enclosed by a lipid bilayer, tubule is a tube-like structure and lamellae is thin layer, membrane, or plate of tissue.

Major functions of a mesosome

The major functions of a mesosome are as follows:

  • They play a role in cell wall formation.
  • DNA replication and distribution to daughter cells.
  • Contains enzymes for aerobic respiration.
  • Secretion of glycocalyx.
  • Increases surface area of plasma membrane.


                                                   Fig: Mesosome

Chromatophores

In some prokaryotes like cyanobacteria (eg: Nostoc), there are other membranous extensions into the cytoplasm called the chromatophores which contain pigments.


                            Fig: Chromatophores in cyanobacteria

Cytoskeleton

The collective name for the structural filaments in prokaryotes are called prokaryotic cytoskeleton. The analogues of most of the cytoskeletal proteins found in eukaryotic cells are found in a prokaryotic cell. Along with this some other unique cytoskeletal proteins are also present. The first identified cytoskeletal element is FtsZ, which is similar to the actin-myosin contractile ring. The major functions of a prokaryotic cytoskeleton are cell division, protection, shape determination and polarity determination.

Flagella

Appendages are the structures attached to the cell membrane. Flagella is an appendage which is formed by the extension of the cell membrane. It is present in the motile bacteria and it is the longest portion of the prokaryotic cell body. Flagellin is the protein present in the flagella. So the major function of a flagellum is to help the bacteria in motility. The bacterial flagellum is composed of three parts and they are as follows:

  • Filament - The longest part extends from the cell surface.
  • Hook - It is a curved and tubular structure.
  • Basal body - It is the most complex part of the flagella.

The basal body is present in the cell membrane and the hook and filament extend out from the basal body beyond the cell wall.


                                                                Fig: Flagella

Cell organelles in eukaryotes

Plant cells and animal cells are the eukaryotic cells. But there are some differences in the cell organelles present in plant and animal cells. The cell organelles are present according to their metabolic functions.

Cell organelles in plants

A plant cell is a rectangular cell which is composed of non membranous bound, single membranous bound and double membranous bound cell organelles. The presence of an additional cell wall around the plasma membrane and a large vacuole in the cytoplasm are the two major features that distinguishes a plant cell from an animal cell. All the cell organelles are dispersed in the cytoplasm of a plant cell.


                                             Fig: Structure of a plant cell

Non-membrane bound cell organelles of a plant cell

The cell organelles that are not bound by a membrane in a plant cell are as follows:

  • Cell wall
  • Cell membrane
  • Ribosomes
  • Cytoskeleton
Cell wall

The outermost covering of a plant cell is called a cell wall. It is composed of cellulose, hemicellulose and pectin. The plant cell wall is composed of three main layers and they are the middle lamella, primary cell wall and secondary cell wall. The major function of a plant cell wall is to protect the plant cell from mechanical stress. The particular structure of a plant cell is also because of the presence of a cell wall.


                    Fig: Structure of plant cell wall

Cell membrane

The layer present just below or internal to the cell wall is the cell membrane. It is a semipermeable membrane made up of lipid bilayer. The structure of the plant cell membrane is similar to the prokaryotic cell membrane. The components of a cell membrane are phospholipids, proteins, cholesterol and carbohydrates. The major function of a cell membrane is the regulation of the entry and exit of the substances in the cell.


            Fig: Structure of the cell membrane

Ribosomes

The smallest non-membrane cell organelle is called the ribosome. RNA and proteins are the major components of ribosomes. The major function of the ribosomes is the synthesis of proteins. They have two subunits, one small and one large subunit. In plant cells, the ribosomal units are of 80S type. The large subunit is 60S and small subunit is 40S.


                    Fig: Structure of ribosome

Cytoskeleton

Cytoskeleton is the collective name used for three organelles and they are microtubules, microfilaments and intermediate filaments. They are made up of actin proteins and they can contract. Maintaining the shape of the cell is the main function of the cytoskeleton. Internal organisation, division and movements of the cells are also controlled by cytoskeleton.


                         Fig: Structure of cytoskeleton

Single membrane bound cell organelles of a plant cell

The single membrane bound cell organelles of a plant cell are as follows:

  • Vacuole
  • Golgi apparatus
  • Endoplasmic reticulum
  • Microbodies
Vacuole

More than 80 percent of the plant cell is composed of a central vacuole. It is a single membrane bound cell organelle. The membranous covering is called tonoplast. Inside the tonoplast, there is a cell sap which stores water, food, minerals, salts etc. Turgidity of the cell is also maintained by the vacuole. There are four types of vacuoles in a plant cell and they are as follows:

  • Contractile vacuole - It contracts to help in removing the excess ions and water from the cell to balance water flow into the cell.
  • Sap vacuole - It facilitates the rapid exchange between the surrounding environment and the cytoplasm.
  • Food vacuole - It possesses digestive enzymes for the breakdown of the nutrients.
  • Gas vacuole - It helps in the storage of gases and also provides mechanical strength and buoyancy to the cell.


                    Fig: Structure of vacuole of a typical plant cell

Golgi apparatus

The single membrane bound, sac-like organelle seen near the nucleus are called Golgi apparatus. They have a smooth and flat layered structure. The stack of flattened sacs are called cisternae. For the purpose of transportation of substances from the Golgi apparatus to the internal or external part of the cell, vesicles are formed from the cisternae. Vesicles can fuse with the membrane of a cell or other cell organelles. Vesicles and cisternae are together called a Golgi apparatus. In plant cells, the Golgi apparatus is called dictyosomes and are dispersed in the cytoplasm of the cell.


                      Fig: Structure of the Golgi apparatus

Endoplasmic reticulum

The double membrane bound cell organelle which forms a network of small tubular structures in the cytoplasm is called endoplasmic reticulum. It forms as an extension to the nuclear membrane. The three membrane bound channels of endoplasmic reticulum are cisternae, tubules and vesicles. They are interconnected. The cisternae and tubules are hollow structures, and they have a space inside them called lumen. There are two types of endoplasmic reticulum. They are the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER).

RER

When the ribosomes are present on the membrane of endoplasmic reticulum, it is called rough endoplasmic reticulum. It mainly helps in the synthesis of proteins.

SER

When the ribosomes are absent on the membrane of endoplasmic reticulum, it is called smooth endoplasmic reticulum. The major function of the smooth endoplasmic reticulum is lipid metabolism. It helps in the synthesis and storage of glycogen, fat and sterols.


                              Fig.: Endoplasmic reticulum

Microbodies

Microbodies are the single membrane bound cell organelles which are the vesicles with various enzymes. They are also called cytosomes. The major three types of microbodies in a plant cell are as follows:

  • Spherosomes - These are mainly involved in the synthesis and storage of lipids.
  • Glyoxysomes - These participate normally in the conversion of lipids to sugar during the germination of oilseeds.
  • Peroxisomes - These play an important role in the metabolism, signalling and reactive oxygen species detoxification.


                         Fig: Structure of peroxisomes

Double membrane bound organelles of a plant cell

The double membrane bound cell organelles of a plant cell are as follows:

  • Nucleus
  • Plastids
  • Mitochondria
Nucleus

The double membrane bound cell organelle which is called the brain of a cell is the nucleus. It stores the hereditary materials and also regulates the cell division, metabolism and growth of the plant cell. In plant cells the nucleus will be positioned in one side of the cell, because of the presence of the large vacuole in the cytoplasm. The number of nucleus inside the cell varies and according to the number of nucleus the cells can be called enucleate, mononucleate, binucleate and multinucleate. The outer covering of a nucleus is called the nuclear envelope and it is composed of numerous pores called nuclear pores. The transparent semifluid substance inside the nuclear envelope is called nucleoplasm or nuclear sap. Nucleolus is the dense spherical substance in the nucleoplasm. It is composed of genetic material.


                                  Fig: Structure of nucleus

Plastids

Plastids are the double membrane bound cell organelle which can store the substances needed for a plant cell. It possesses an inner and outer membrane. The space between the two membranes is called peri plastidial space. They are called semi autonomous cell organelles, because they have their own DNA. Some plastids can provide colour to the plants and also help in the photosynthesis. The chloroplast, leucoplast and chromoplast are the examples of plastids.

  • Chloroplast - It is the plastid which helps the plants to carry out photosynthesis.
  • Leucoplast - It is colourless and they store starch and oil.
  • Chromoplast - It helps in the synthesis and storage of carotenoid pigments in flowers, fruits and leaves and gives them their colour.


                      GIF: Chloroplast

Mitochondria

The powerhouse of a cell is called the mitochondria. It has an inner membrane, and an outer membrane. The space between two membranes is called the peri mitochondrial space. The foldings of the inner mitochondrial membrane is called cristae. The major function of a mitochondria is providing energy to the cell. They provide energy in the form of ATP by breaking down carbohydrates and sugar molecules.


                     Fig: Structure of the mitochondria

Cell organelles in animals

Animal cells are small and have an erratic structure when compared to the plant cell. They have many organelles that are similar to the plant cells, since they are eukaryotic in origin. The cell organelles are dispersed in the cytoplasm and enclosed inside the cell membrane. Non membrane bound organelles, single membrane bound organelles and double membrane bound organelles are also present in animal cells.


                                     Fig: Structure of animal cell

The similar cell organelles of plants and animals are cell membrane, nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, ribosomes, and cytoskeleton. Microbodies are presnet in both but they are different in both plant cells and animal cells. Cell wall, central vacuole and plastids are absent in animal cells. Centrosomes are mostly exclusive to animal cells, but can be seen in some lower group of plants. Lysosomes can be only seen in animal cells.

Non-membrane bound cell organelles of an animal cell

The non-membrane bound cell organelles of an animal cell are as follows:

  • Cell membrane
  • Ribosomes
  • Cytoskeleton
  • Centrosomes

The structure and functions of cell membrane, ribosomes and cytoskeleton has already been explained in the non-membrane bound cell organelles of a plant cell.

Centrosomes

Centrosomes are the non membrane bound cell organelle which is located near the nucleus. A pair of centrioles are called diplosomes. They are seen during the non-dividing phase of a cell. They are arranged inside a specialised cytoplasm called the centrospheres or kinoplasm, in a perpendicular manner. The major function of centrosomes is to pull the chromosomes to both the ends of a cell during cell division. They do this by the formation of spindle fibres.


                                               Fig: Structure of centrosome

Single membrane bound cell organelles of an animal cell

  • Endoplasmic reticulum
  • Golgi apparatus
  • Lysosomes
  • Microbodies

Structure and functions of Golgi apparatus and endoplasmic reticulum have already been discussed in the single membrane bound cell organelles of a plant cell.

Lysosomes

The single membrane bound cell organelles that are formed from the Golgi apparatus for the intracellular digestion are called lysosomes. These are formed as the buds of vesicles from the Golgi apparatus. Hydrolytic enzymes are present in the lysosomes. The major functions of lysosomes are the repair of the cell membrane, breaking down of macromolecules, digestion of foreign substances such as viruses, bacteria etc. During the starvation of cells or damage of cells, the digestion of cells is also controlled by the lysosomes. Hence they are called as the suicidal bags of the cells.


                                                            Fig: Lysosome

Microbodies

Among the three types of microbodies such as spherosomes, glyoxysomes and peroxisomes, only two are present in the animal cells. They are peroxisomes and glyoxysomes. Sphaerosomes are absent in animal cells. The major function of peroxisomes is to break down the long chain fatty acids and the function of glyoxysomes is to convert the fats into carbohydrates.


                                     Fig: Structure of peroxisomes

Double membrane bound cell organelles of an animal cell

  • Nucleus
  • Mitochondria

Structure and functions of nucleus and mitochondria has already been discussed in the double membrane bound cell organelles of a plant cell.

Cell organelles present outside the cell membrane of a eukaryotic cell

The two cell organelles which can be seen outside the eukaryotic cell are cilia and flagella.

Cilia and flagella

The hairy outgrowths from the cell membrane are called cilia and flagella. They are normally seen as the extensions of the cell membrane, just like in the prokaryotic cell and hence they are covered by the cell membrane. Cilia is not found in prokaryotic cells. Their Major function is locomotion and sensation in animal cells. The size of cilia is smaller than flagella. The number of cilia is more than the flagella. The major locations of cilia are epithelial cells of several internal organs like lungs, trachea, digestive system etc. An example of the cells with flagella in animals are sperms.


                                    Fig: Examples of cilia and flagella in animals

Endomembrane system

In eukaryotic cells, a system of organelles are present which can function in close coordination with each other. This is known as the endomembrane system. The cell organelles includes in the endomembrane system are as follows:

  • Nuclear envelope
  • Endoplasmic reticulum
  • Golgi complex
  • Lysosomes
  • Vesicles

The major function of an endomembrane system is to modify, package and transport lipids and proteins.


                                            Fig: Endomembrane system

Practice Problems

1. Which of the following cell organelles can be found outside the cell membrane of a prokaryotic cell?

  1. Cilia
  2. Flagella
  3. Both cilia and flagella
  4. Plasmid

Solution: Cell organelles are the cellular components of a cell. They can be both membrane bound and non-membrane bound. They are present within the cells and have particular structure and function. Some organelles can be seen outside the cell, but attached to the cell membrane. The hairy outgrowths from the cell membrane are called cilia and flagella. They are normally seen as the extensions of the cell membrane, just like in the prokaryotic cell and hence they are covered by the cell membrane. Cilia is not found in prokaryotic cells. Flagella is an appendage which is formed by the extension of the cell membrane of a prokaryotic cell. It is present in the motile bacteria and it is the longest portion of the prokaryotic cell body. It is made up of a protein called flagellin. The extrachromosomal genetic material is called plasmid. These are small, double stranded circular DNA. They also confer unique phenotypic characters to the cell and occur as multiple copies. Hence the correct option is b.

2. Which of the following are the single membrane bound organelles of a plant cell?

  1. Vacuole
  2. Centrosomes
  3. Ribosomes
  4. Mitochondria

Solution: Cell organelles are the cellular components of a cell. They can be both membrane bound and non-membrane bound. They are present within the cells and have particular structure and function. There are some cell organelles which are non-membrane bound too. Organelles without membrane can be seen in both prokaryotic and eukaryotic cells. The non membrane bound organelles of a plant cell are ribosomes, cytoskeleton and centrosomes. Those cell organelles that have only one membrane are called single membrane bound organelles and they are present only in eukaryotic cells. The single membrane bound organelles of a plant cell are vacuole, Golgi apparatus, endoplasmic reticulum, and microbodies. Those cell organelles which have inner and outer membrane are called double membrane bound cell organelles. They are also present in eukaryotic cells only. The double membrane bound organelles of a plant cell are nucleus, plastids and mitochondria. Hence the correct option is a.

3. Which of the following cell organelles are present in animal cells but not in plant cells?

  1. Plastids
  2. Microbodies
  3. Lysosomes
  4. Cell membrane

Solution: Animal cells are small and have an erratic structure when compared to the plant cell. They have many cell organelles that are similar to the plant cells, since they are eukaryotic in origin. The cell organelles are dispersed in the cytoplasm and enclosed inside the cell membrane. Non membrane bound organelles, single membrane bound organelles and double membrane bound organelles are also present in animal cells. The similar cell organelles of plants and animals are cell membrane, nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, ribosomes, and cytoskeleton. Microbodies are presnet in both but they are different in both plant cells and animal cells. Cell wall, central vacuole and plastids are absent in animal cells. Centrosomes are mostly exclusive to animal cells, but can be seen in some lower group of plants. Lysosomes can be only seen in animal cells.The single membrane bound cell organelles that are formed from the Golgi apparatus for the intracellular digestion are called lysosomes. These are formed as the buds of vesicles from the Golgi apparatus. Hydrolytic enzymes are present in the lysosomes. Hence the correct option is c.

4. Which of the following statements are false about the cell organelles of prokaryotic cells?

I) All the cell organelles of a prokaryotic cell are non membrane bound.
II) Glycoclayx, cell wall and cell membrane are the three parts of a cell envelope.
III) The cell wall of gram positive and gram negative bacteria are the same.
IV) There is no well-defined nucleus for prokaryotic cells.
V) Prokaryotic ribosome is 80S type.

  1. I, II, III, IV, V
  2. III and V
  3. I and V
  4. II, III and V

Solution: The cell organelles that are present in the prokaryotes are all non membrane bound and they are cell envelope, nucleoid, plasmids, ribosomes, mesosomes, cytoskeleton and flagella. Prokaryotic cell envelope is a complex, multilayered structure that protects the organisms from external pressures. The components of cell envelope are glycocalyx, cell wall and cell membrane. The layer just below the glycocalyx which provides shape and support to the prokaryotic cell is called the cell wall. For the gram positive and gram negative bacteria, the cell wall composition is different. Gram positive bacteria have a single layered cell wall. It is mostly composed of a thick peptidoglycan layer. Gram negative bacteria have a thinner layer of cell wall. It is covered by a single layer of lipoproteins and lipopolysaccharides. There is no well-defined nucleus in prokaryotic cells. The genetic material is basically naked i.e not enveloped by a nuclear membrane. Hence it is termed as nucleoid. Ribosome is a cell organelle without cell membrane made up of RNA and proteins. It is the site of protein synthesis. Prokaryotic ribosome is a 70S type. It consists of one small subunit and one large subunit. The small subunit is 30S and the large subunit is 50S. Both these subunits come together at the time of protein synthesis else they are present as deserted units. Hence the correct option is b.

FAQs

1. What is a protocell?
Answer:
The synthetic form of cellular life is called a protocell. It is constructed from the phospholipid vesicles. The purpose of protocell is to understand the emergence of life form from a non living chemical network. The evolution of genomically encoded advantageous functions has not yet occurred in the protocell and it is the major difference of protocell from a true cell.

2. What is endosymbiotic theory?
Answer:
The evolution of eukaryotic cells from the prokaryotic organisms is explained by the endosymbiotic theory, or symbiogenesis. According to this theory the cell organelles such as mitochondria and chloroplasts are descended from the former free living prokaryotes. When the first aerobic chemoautotroph gets engulfed by the prokaryotic cells, the indigestion of that chemoautotroph leads to the formation of mitochondria, which can generate energy. Chloroplast is formed when the prokaryotic cells engulf the cyanobacterium.


                                           Fig: Evolution of first plant cell

3. What are hydrogenosomes?
Answer:
A membrane enclosed organelle seen in some anaerobic flagellates, ciliates and fungi are called hydrogenosomes. They are evolved from protomitochondria and they produce hydrangea and ATP in the absence of oxygen or in anaerobic condition. It was discovered by D. G. Lindmark and M. Muller in 1973.

4. What is a cytosol?
Answer:
The intracellular fluid or the fluid inside the cell is called cytosol or cytoplasmic matrix or ground plasm. Cytosol can be explained as the liquid matrix surrounding the cell organelles. Membranes separate the cytosol into compartments. Many metabolic reactions of a prokaryotic cell and eukaryotic cell are taking place in the cytosol. The difference between cytosol and cytoplasm is that cytosol is the fluid of cytoplasm (excluding the contents inside the organelles) and the entire content inside the cell is the cytoplasm (excluding the contents of the nucleus).

YOUTUBE LINK: https://www.youtube.com/watch?v=-NOY_k8iN9A&t=2893s

Related Topics

Prokaryotic cells: Cell envelopes, Cell membrane invagination (Mesosome), Practice Problems and FAQs

Prokaryotic cells: Cell appendages, Ribosomes, Inclusion bodies, Staining properties, Practice problems and FAQs

Cell membrane, History of development of the fluid mosaic model, Practice Problems and FAQs

Cell wall, Practice Problems and FAQs

Mitochondria: Structure, Functions, Practice problems and FAQs

Plastids: Chloroplasts, Chromoplasts, Leucoplasts Comparison between Mitochondria and Chloroplasts, Practice problems and FAQs

Ribosomes, Microbodies, Practice problems and FAQs

Eukaryotic cells: Cytoskeleton, Cilia, Flagella, Centrosome and centrioles - ultrastructure and function

Nucleus, Practice Problems and FAQs

Endoplasmic reticulum: Occurrence, Types, Functions, Practice Problems and FAQs

Vacuoles, Practice Problems and FAQs

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