Ribosome, mitochondria and lysosome

This slide deal about ribosome, mitochondria, and lysosome in detail.

1. Ribosome, Mitochondria and Lysosome Prakash Dhakal M. Sc. Public Health Microbiology Tribhuvan University, Nepal

2. Ribosome Ribosome is a complex molecular machine found within all living cells i.e. Prokaryotic and Eukaryotic cells. Ribosomes are the protein builders or the protein synthesizers of the cell. In Prokaryotes, the ribosomes are found freely floating in the cytoplasm. Eukaryotic ribosomes may be either floating around the cytoplasm or attached to endoplasmic reticulum . Endoplasmic reticulum with attached ribosome is called rough endoplasmic reticulum . Ribosomes are remarkably abundant in cells. A single Eukaryotic cell may contain as many as 10 million ribosomes. In prokaryote, ribosomes may number as many as 15,000  constituting as much as one-quarter of the cell’s total mass. 

3. The size of the ribosome within the cell varies depending upon the cell type and on factors such as whether the cell is resting or replicating. Ribosomes were first observed by Romanian Cell Biologist George Emil Palade using an electron microscope in mid 1950s . The term “ribosome” was proposed by scientist Richard B. Roberts in 1958. Nobel prize was distributed to different scientists for the discovery and detail study of ribosome.

4. Prokaryotic ribosome Eukaryotic ribosome

5. Ribosome is largely made up of specialized RNA known as  ribosomal RNA (rRNA) as well as dozens of distinct proteins . The ribosomal proteins and rRNAs are arranged into two distinct ribosomal pieces of different size, known generally as the large and small subunit of the ribosome. The larger subunit is larger in size than the smaller subunit. Prokaryotic ribosomes are around 20 nm (200 Å) in diameter and are composed of 60% rRNA and 40% ribosomal proteins.  Eukaryotic ribosomes are between 25 to 30 nm (250–300 Å) in diameter with an rRNA to protein ratio that is close to 1. The ribosomal subunits of Prokaryotes and Eukaryotes are quite similar.

6. The unit of measurement used to describe the ribosomal subunits and the rRNA fragments is the  Svedberg unit , a measure of the rate of sedimentation in centrifugation rather than size. This accounts for why fragment names do not add up: for example, Prokaryotic 70S ribosomes are made of 50S and 30S subunits. Prokaryotes have 70S  ribosomes, each consisting of a small ( 30S ) and a large ( 50S ) subunit. Their small subunit has a  16S  RNA subunit bound to 21 proteins. The large subunit is composed of a  5S  RNA subunit and a 23S RNA subunit with 31 proteins. Eukaryotes have 80S ribosomes, each consisting of a small ( 40S ) and large ( 60S ) subunit. Their 40S subunit has an  18S  RNA and 33 proteins. The large subunit is composed of a  5S RNA ,  28S RNA,   5.8S RNA  subunits and 46 proteins.

7. Functions They assemble amino acids to form specific proteins, proteins are essential to carry out cellular activities.  The process of production of proteins, the deoxyribonucleic acid produces mRNA by the process of DNA transcription.  The genetic message from the mRNA is translated into proteins during DNA translation.  The sequences of protein assembly during protein synthesis are specified in the mRNA. The mRNA is synthesized in the nucleus and is transported to the cytoplasm for further process of protein synthesis. In the cytoplasm, the two subunits of ribosomes are bound around the polymers of mRNA; proteins are then synthesized with the help of transfer RNA.  The proteins that are synthesized by the ribosomes present in the cytoplasm are used in the  cytoplasm itself. The proteins produced by the bound ribosomes are transported outside the cell.

8. Mitochondria Mitochondria is a double membrane bound organelle found in all eukaryotic organisms . They are known as the power house of the cell . The word mitochondria is derived from the Greek word ( mitos -- "thread", and   " chondrion -- "granule). They are rod shaped organelles and are commonly between 0.75 and 3  μm  in diameter but vary considerably in size and structure. They are only visible when stained. They are freely floating through out the cell. The number of mitochondria in a cell can vary widely by organism, tissue and cell type. For e.g. red blood cell have no mitochondria whereas liver cells can have more than 2000. Muscle cells need a lot of energy so they have loads of mitochondria. Neurons don’t need many.

9. Although most of a cell’s DNA is contained in the nucleus, the mitochondria has its own DNA, so also called as “ semi- autonomous organelles ”. Structure A mitochondrion contains outer and inner membranes. These are made of  phospholipid  double layers and proteins. There are five distinct compartments within the mitochondrion. They are: the outer mitochondrial membrane, the intermembrane space (the space between the outer and inner membranes), the inner mitochondrial membrane, the cristae (formed by infoldings of the inner membrane), and the matrix (space within the inner membrane)

10. Matrix

11. Outer membrane : The outer mitochondrial membrane encloses the entire organelle and is about 60 to 75  Å thick . It has a protein to phospholipid ratio about 1:1 by weight. It contains large numbers of integral membrane proteins called  porins . These porins allow molecules of 5000  daltons  or less in molecular weight to  diffuse freely from one side of the membrane to the other. The outer membrane also contains various enzymes. Intermembrane space: The  intermembrane space is the space between the outer membrane and the inner membrane. It is also known as perimitochondrial space.

12. Since the outer membrane is freely permeable to small molecules, the concentrations of small molecules, such as ions and sugars, in the intermembrane space is the same as in the cytoplasm. 3) Inner membrane: It has a very high protein to phospholipid ratio (more than 3:1 by weight). The inner membrane is rich in protein and it contains about 1/5 of the total protein in a mitochondrion. In addition, the inner membrane is rich in an unusual phospholipid called Cardiolipin . Unlike the outer membrane, the inner membrane doesn’t contains porins and is highly impermeable to all molecules .

13. Almost all ions and molecules require special membrane transporters to enter or exit the inner membrane. 4) Cristae : The inner mitochondrial membrane contains numerous compartments of cristae , which expand the surface area of the inner mitochondrial membrane , enhancing its ability to produce ATP. For e.g. for liver mitochondria, the area of the inner membrane is about five times as large as the outer membrane. This ratio is variable and mitochondria from cells that have greater demand of ATP, such as muscle cells contains even more cristae . Cristae contains small round bodies known as F 1 particles or oxysomes .

14. 5) Matrix The matrix is the space enclosed by the inner membrane. It contains about 2/3 of the total protein in a mitochondrion. The matrix is important in the production of ATP with the aid of ATP synthase enzyme contained in the inner membrane. It contains highly concentrated mixture of hundreds of enzymes, ribosomes, tRNA and DNA. Since due to presence of DNA, it can manufacture their own RNAs and proteins.

15. Functions The most prominent function of mitochondria is to produce energy to the cell in the form of ATP. It also regulates the metabolic activity of the cell.

16. Lysosome Lysosomes are single membrane organelles found in nearly all types of eukaryotic cells. But RBC (erythrocytes) does not contains lysosomes . They are spherical vesicles which contain hydrolytic enzymes that break down virtually all kinds of bio-molecules (proteins, nucleic acids, carbohydrates and lipids). The lysosomes acts as the waste disposal system of the cell by digesting unwanted materials in the cytoplasm, both from outside and inside of the cell. They are also responsible for destroying the cell after its death. They were discovered by Belgian biologist Christian de Duve, he received Nobel prize for that in 1974. Lysosomes contains more than 60 different enzymes. These enzymes are synthesized in the rough endoplasmic reticulum.

17. The size of lysosome varies from 0.1 to 1.2 µm with a pH range of 4.5 to 5.0 . In their simplest form, lysosomes are visualized as dense spherical vacuoles. The outer surface is formed by a single membrane, a phospholipid bilayer that can fuse with some other membrane bound organelles. The enzymes contained within lysosome are known collectively as acid hydrolases and work best at in acidic environments . i.e. at low pH. Enzymes are enclosed by membrane. If they were not enclosed, the enzymes contained within lysosomes could cause damage to other structures within the cell. The rest of the cell is therefore protected by these enzymes being isolated within a membrane.

18. Functions Releases enzymes outside of the cell ( exocytosis ). Breakdown-- “digestion” of materials from inside the cell ( autophagy ). Breakdown-- “digestion” of materials from outside the cell ( heterophagy ). Completely breakdown cells that have died ( autolysis ). Diseases Lysosomes are responsible for a group of genetically inherited disorders called Lysosomal Storage Diseases (LSD) . They are a type of inborn errors of metabolism caused by one of the enzymes. There is no medical treatment to cure LSD. These genetic defects are related to several neurodegenerative disorders, cancer, cardiovascular diseases, and ageing-related diseases. In some disease conditions lysosomes begin to break down living cells.