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Ribosomes, Microbodies, Practice problems and FAQs

Ribosomes, Microbodies, Practice problems and FAQs

Did you know that the human body produces around 100,000 different proteins and enzymes with different combinations of just 20 amino acids? Have you ever tried to know where in the body are the proteins made or who are responsible for the synthesis of proteins?

Do you remember the phrase ‘protein factories of the cell’? Do you remember which cell organelle was this phrase used for? It’s the ribosome.

Ribosomes are non membrane-bound organelles which are present in both prokaryotic and eukaryotic cells. Do you think that ribosomes in prokaryotes and eukaryotes are similar? Well, they are not and we will discuss them in this article.

Eukaryotic cells also have some membrane bound organelles which hold various oxidative enzymes depending on the type of the cell. These organelles are known as microbodies. In this article we will also look into the three different types of microbodies in eukaryotic cells.

Table of contents

  • Ribosomes
  • Microbodies
  • Practice problems
  • FAQs

Ribosomes

Ribosomes are non membranous organelles which are composed of RNA and proteins. These can be observed under electron microscope only and were first discovered in plant cells by Robinson and Brown in 1953. George Palade was the first to identify them in animal cells in 1955, using an electron microscope. In 1955, Palade named these organelles as ribosomes.



                                                           Fig: Ribosome

Location

Ribosomes are found in both prokaryotes and eukaryotes, except for mature sperms and RBCs. In prokaryotic cells, ribosomes are found free in cytoplasm whereas in eukaryotes, they are found attached to the surface of rough endoplasmic reticulum (RER) as well as free in cytoplasm. 

Eukaryotic cells also possess ribosomes in the mitochondrial matrix and in the stroma of plastids.

Ribosomes are found in larger numbers in cells which are actively involved in protein synthesis, for example, liver cells, pancreatic cells, meristematic plant cells, etc.

Types of ribosomes

Ribosomes are of two types: 70 S and 80 S. S here represents the Svedberg unit which is the sedimentation constant and is a measure of the size of a particle depending on how fast a particle sediments in a centrifuge. 



                                  GIF: Centrifuge

70S ribosomes are found in prokaryotic cells and in the mitochondrial matrix and stroma of plastids in eukaryotic cells. 80 S ribosomes are found in the cytoplasm or on the surface of RER of eukaryotic cells.



                                     Fig: Different types of ribosomes

Shape and size

Ribosomes are oblate spheroidal structures that show uniformity in shape, irrespective of the type of ribosome. Prokaryotic ribosomes are around 17-21 nm in diameter and 20-29 nm in length whereas eukaryotic ones are around 20-24 nm in diameter and 30-34 nm in length.

Structural organisation

Both types of ribosomes (70S and 80S) are similar in structure. Each ribosome contains two subunits- one larger and one smaller. The 70S ribosome is made up of a smaller 30S and a larger 50S subunit. The 80S ribosome is made up of a smaller 40S and larger 60S subunit.



                      Fig: Structural organisation of different types of ribosomes

These subunits float separately in cytoplasm and join only at the time of protein synthesis. A cleft exists between the two subunits and polypeptide chains are synthesised in this cleft on the mRNA. In eukaryotic cells, a channel extends from this cleft, through the larger subunit into the lumen of the endoplasmic reticulum. Polypeptide chains, once synthesised, are pushed into the lumen of ER. 

The larger subunits have a P site (Peptidyl site) for the attachment of the tRNA which carries the peptide chain, an A site (amino acid site) for the attachment of the charged tRNA with activated amino acid and an E site for the the free tRNA after it has transferred the peptide chain to the P site.

Chemical composition

Chemical composition of ribosomes involves highly folded rRNA molecules and many attached proteins. They also contain a little lipid and metallic ions like Mg 2+. 

In the 70S ribosome the RNA comprises 65% of the ribosomal composition and proteins comprise 35%. In 80S, RNA content is 45% and proteins is around 55%.

Functions of ribosome

The main function of ribosomes is protein synthesis, hence they are termed as ‘protein factories’ of the cell. Ribosomes also provide sites for the attachment of tRNA and mRNA which participate in protein synthesis. Sometimes the same mRNA strand has multiple ribosomes reading it at the same time. This structure is called a polysome.



                        Fig: Protein synthesis by ribosomes

Microbodies

Microbodies are observed only under electron microscopes. These are single membrane-bound vesicles which are not a part of the endomembrane system and are present in the cytoplasm of a variety of eukaryotic cells. Prokaryotic cells do not possess microbodies. 

Microbodies contain a variety of oxidative enzymes and are mainly involved in detoxification of peroxides, in photorespiration in plants and oxidative metabolism.

Microbodies are of three different types - 

  1. Peroxisomes
  2. Spherosomes
  3. Glyoxysomes



                                                    Fig: Types of microbodies

Peroxisomes

The term peroxisome was coined by de Duve in 1969 for the microbodies which consist of the enzymes that are responsible for the synthesis of peroxide and produce hydrogen peroxide during their metabolic activities. Peroxisomes were first discovered in the liver and kidney cells of rats.

Location

They are found in the cytoplasm of both plant and animal cells. Peroxisomes lie in close association with the endoplasmic reticulum, mitochondria and chloroplasts.

Structural organisation

The peroxisomes have an average diameter of around 0.5 to 1 micron. They have a granular matrix enclosed within a single phospholipid membrane. The matrix consists of a central crystalline or fibrous core known as a nucleoid. The oxidative enzymes found in peroxisomes are peroxidase, catalase, b-amino acid oxidase, uric acid oxidase, D-amino acid oxidase, b-hydroxy acid oxidase. 



                                                        Fig: Structure of peroxisomes

Functions

They help in photorespiration in plant cells, lipid metabolism in animal cells and catalyse condensation reactions, oxidative removal of amine groups from amino acids, detoxification of alcohol and oxidation of toxic substances.

Glyoxysomes

Glyoxysomes are similar to peroxisomes but these contain enzymes of the glyoxylate cycle in addition to the peroxisomal enzymes. Glyoxysomes were first described by R. W. Breidenbach in 1967.

Location

These are mainly present in plant tissues. Germinating seeds of castor, peanuts, water melon, etc. generally have a large number of glyoxysomes.

Structural organisation

Glyoxysomes are single membrane bound organelles which have a matrix with a crystalloid core. Enzymes present within glyoxysomes are isocitrate lyase, malate synthetase and many enzymes of the TCA (Krebs’) cycle.



                                       Fig: Glyoxysomes

Function

The enzymes in the glyoxysomes participate in the glyoxylate cycle which helps in the synthesis of carbohydrates from lipids stored in the endosperm of germinating seeds.

Spherosomes

Spherosomes are spherical or ovoid particles that are mostly involved in lipid and protein metabolism. These were first discovered by Perner in 1953.

Location

Spherosomes are found in most plant cells and arise from the endoplasmic reticulum.



                              Fig: Spherosomes in plant cells

Structural organisation

They are spherical or ovoid particles which are bound by a half unit membrane made up of a single layer of phospholipid molecules. The phospholipids are arranged in a way such that their hydrophilic polar heads face the outer environment and the hydrophobic nonpolar tails are towards the inner side.

Function

Spherosomes are associated with the synthesis and storage of lipids. Due to the presence of hydrolytic enzymes, spherosomes of maize root tip cells and tobacco endosperm tissue also perform a limited variety of hydrolytic functions.

Practice problems

Q 1. Which of the following is/are the function(s) of peroxisomes?

I. Catalase activity
II. Carrying out the glyoxylate cycle
III. Degradation of very long-chain fatty acids
IV. Produce dihydrogen

a. I and II
b. II and IV
c. III and IV
d. I and III

Answer: Peroxisomes possess the catalase enzyme which can break down hydrogen peroxide to water and oxygen. They also help in lipid metabolism by breaking down the long chain fatty acids.

Thus, the correct option is d.

Q 2. Glyoxysomes are involved in 

a. Conversion of fatty acids to lipids
b. Conversion of fatty acids to carbohydrates
c. Conversion of amino acids to lipids
d. Conversion of amino acids to proteins

Answer: Glyoxysomes are single membrane bound organelles which contain enzymes of the glyoxylate cycle. During seed germination, these enzymes perform the glyoxylate cycle to convert lipids stored in the endosperm into carbohydrates.

Thus, the correct option is b.

Q 3. ‘S’ in 50S ribosomal subunit represents the

a. Diameter of the particle
b. Length of the particle
c. Sedimentation coefficient of the particle
d. Surface area of the particle

Answer: ‘S’ in 50S stands for the Svedberg unit which represents the sedimentation coefficient of the particle and is a measure of how fast the particle sediments in a centrifuge based on their particle size.

Thus, the correct option is c.

Q 4. Mitochondrial ribosomes in eukaryotes are made up of 

a. 30S and 50S subunits
b. 30S and 60S subunits
c. 40S and 60S subunits
d. 40S and 50S subunits

Answer: Eukaryotic cells possess 70S ribosomes in the mitochondrial matrix which are composed of 30S and 50S subunits. Thus, the correct option is a.

FAQs

Q 1. Are glyoxysomes always present in all plant cells?
Answer: Glyoxysomes are temporary structures that appear only during brief periods in the plant cells of certain beans and nuts which have stored fat in their endosperms. During seed germination, the glyoxysomes help in conversion of lipids into carbohydrates and disintegrate thereafter.

Q 2. Why do peroxisomal disorders affect the nervous system?
Answer: Peroxisomes play an important role in the formation of plasmalogen which is a phospholipid that is abundantly present in the myelin sheath of neurons. Peroxisomal disorders due to improper functioning of peroxisomes or lack of specific peroxisomal enzyme activities. This affects plasmalogen synthesis which in turn affects the nervous system.

Q 3. Are spherosomes considered as plant lysosomes?
Answer: Spherosomes are present only in plant cells and are considered to be lysosome equivalents in plant cells such as root tip cells of maize, endosperm cells of tobacco, etc. They consist of hydrolytic enzymes such as acid phosphatases and hence help in hydrolytic reactions, as lysosomes do in animal cells.

Q 4. How many ribosomes are generally present in a cell?
Answer: The number of ribosomes in an eukaryotic cell which is actively synthesising proteins can be as high as 10 million. Prokaryotic cells such as Escherichia coli possess 10,000 to 30,000 ribosomes.

Related Topics

Eukaryotic cells 

Endomembrane system: Endoplasmic reticulum, Golgi bodies, Lysosomes, Vacuoles 

mitochondria 

Plastids 

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