Table of contents
Learning Objectives
By the end of this section, you should be able to:
- Identify all cell organelles and their respective functions.
Introduction
In the previous blog post, we discussed cell and the different types of cell which are prokaryotic cell and Eukaryotic cell. Today we will be discussing the different cell organelles and their functions.
What are cell organelles?
Organelles means Little Organs.
Systematically, cell organelles are little organs or specialised structures that perform important cellular function. Cell organelles help the cell do different work.
In practice, cell is like a Company where production takes place, and the organelles are the respective workers working within the company.
Each organelle has its own purpose which is very specific, that is, A Will not do B's job.
The Cell Organelles
Plasma membrane
Plasma membrane can also be called Cell membrane.
Plasma membrane is the outer lining of an eukaryotic cell. This membrane separates and protects a cell from its environment, and is made mostly of a double layer of proteins and Lipids. It is a complex structure arranged in a double-layered sheet known as Lipid bilayer.
A plasma membrane or cell membrane regulates what enters and leaves the cell (like a fence with gates). It also provides protection and support for the cell.
In prokaryotes, plasma membrane is usually referred to as cell membrane.
Cell Wall
Cell wall is a rigid structure composed mainly of Cellulose (carbohydrate fiber) and it provides support and protect for cell.
Cell wall is not present in an animal cell, it is present in plant cells and all Prokaryotic cells, and it is what gives these cells their definite shape.
In practice, cell wall is like an additional protection outside a cell (like another barricade).
Cytoskeleton
Cytoskeleton acts to organize and maintain a Cell's shape. It is responsible for the anchor of organelles in place. It also aids the uptake of external materials by cells (endocytosis) and movement of cells.
Cytoplasm
Cytoplasm is a large fluid-filled space and it is filled with a liquid sometimes called cytosol. In prokaryotes, this space is relatively free of components, while in Eukaryotes, the cytoplasm is where all of a Cell's organelles reside.
Cytosol contains dissolved nutrients, and moves materials around a cell through a process called cytoplasmic streaming. Cytoplasm also contains many salts and it is an excellent conductor of electricity.
Nucleus
Nucleus is the largest organelle found in an eukaryotic cell. Nucleus is known as the control center of a cell because it controls different activities within a cell. It is like 'the brain of a cell' and it is surrounded by a double-layered membrane called Nuclear Membrane or Nuclear envelope.
Nucleus is spheroid in shape. Nuclear membrane or Nuclear envelope contain thousands of several holes called Nuclear pores. These pores let messages in and out of the Nucleus. The inner fluid in a Nucleus is called Nucleoplasm.
Nucleus essentially houses a Cell's chromosomes and is the place where almost all DNA replication and RNA synthesis occur.
During processing, DNA is transcribed, or synthesized, into a special RNA, called mRNA.
This mRNA is then transported out of the nucleus, where it is translated into specific protein molecules.
In prokaryotes, DNA processing takes place in the cytoplasm; because they lack a nucleus.
Nucleolus
Nucleolus is a smaller structure found within the nucleus of an eukaryotic cell. Nucleolus is a dense knot of chromatin and it basically produces and assemble Ribosomes that are needed by a cell. There can be more than one nucleolus in a cell.
Ribosome
Ribosome is the protein manufacturing structure in a cell. It is made up of rRNA and proteins and it is responsible for processing the genetic instructions that are carried by mRNA.
Ribosome consists of 2 interlocking parts called subunits.
One large subunit and one small subunit, and they both have different functions during protein synthesis.
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| Ribosome |
The process of converting a mRNA's genetic code into the exact sequence of amino acids that make up a protein is called Translation.
Protein synthesis is extremely important to all cells, and therefore a large number of ribosomes (upto hundreds or thousands) are found throughout a cell.
Ribosomes are found in two forms:
- Free ribosomes (floaters): They float freely in the cytoplasm.
- Ribosomes connected to Rough Endoplasmic Reticulum.
Ribosomes are found in both prokaryotes and eukaryotes.
Mitochondria (Singular: mitochondrion)
Mitochondria are self replicating organelles and they are found in various shapes and sizes in the cytoplasm of all eukaryotic cells. They are known as the Power-House of cells because this is where all the energy required by a cell is generated.
Mitochondria contain their own genome (genetic make up) that is separate and different from the Nuclear Genome of a cell.
Mitochondria have an outer membrane which surrounds the entire organelle, and an inner membrane which projects inwards and forms folds. These inward folds are called Cristae. The numbers and shapes of Cristae in different mitochondria vary, depending on the tissue and the organism in which they are found.
Mitochondria also has their own fluid-filled space which serves like the cytoplasm for mitochondria and it is called a Matrix.
How does a Mitochondria work? | How does cell generate energy?
Mitochondria play a major role in the generation of energy required by a cell, and this process involves a number of complex pathways which I am going to break down.
Most of the food we eat contain complex sugars (disaccharides, polysaccharides) like starch, etc.
Our bodies cannot make use of these complex sugars (starch), therefore it has to be broken down.
The complex sugar is broken down during digestion process until it reaches its simplest and absorbable form which is Glucose (simple sugar) C6H12O6.
Glucose is then absorbed in the small intestine via villi, and transported to various cells via cardiovascular system.
Glucose enters a cell through special molecules found in a cell's (plasma) membrane called Glucose Transporters.
Once it (glucose) is inside a cell; it needs to be transformed into energy. This is carried out via two different pathways:
The first is called Glycolysis
And the second is called Kreb's cycle or Citric acid cycle.
Glycolysis
Glycolysis, which is the first pathway during transformation of glucose into energy, occurs in the cytoplasm of a cell.
When glucose enters a cytoplasm, it is broken down into 2 molecules of a chemical called PYRUVIC ACID (or pyruvate) by addition of two phosphate groups from ATP (Adenosine triphosphate) and series of enzymes.
This means that Glycolysis is a series of reaction which involves the break down of a glucose molecule (removing the hydrogen atoms by phosphorylation) forming Pyruvic acid, and this occurs without oxygen (this makes it to be called Anaerobic metabolism, anything Anaerobic means oxygen is not involved).
Consequently, only a total of four (4) ATP molecules are made from one molecule of glucose via this pathway.
2 ATPs were consumed in the process, this makes it a net of two (2) ATPs acquired by Glycolysis.
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| Glycolysis. Photo_Credit: Khanacademy.org |
In prokaryotes like Bacteria, Glycolysis is the only method that is used for converting inorganic molecules to energy.
To read more on glycolysis, enter Here
Kreb's Cycle (Citric Acid cycle)
Kreb's cycle, also known as Citric acid cycle or Tricarboxylic acid cycle, is the second pathway and it starts immediately glycolysis is complete.
This reaction takes place in the Mitochondrial matrix.
Here, Pyruvate is converted through series of enzymatic processes, into AcetylCoenzyme A or acetylcoA, which is joined to a four–carbon molecule called OAA (Oxaloacetic acid).
Once these two molecules are joined, they make a six–carbon molecule called Citric acid. Citric acid is then broken down and modified.
As these processes occur, hydrogen ions and carbon molecules are produced. Carbon molecules are used to make carbon(IV) oxide while Hydrogen ions are picked up by NAD (Nicotinamide adenine dinucleotide) and FAD (Flavin adenine dinucleotide) which are transformed into NADH and FADH respectively.
Eventually, the complete process produces four–carbon Oxaloacetic Acid again, ending up where it started off.
It is called Kreb's cycle because of this, OAA (Oxaloacetic acid) is used up and reproduced.
At the end of the day, a total of 24–28 molecules of ATP (Adenosine triphosphate which is the form of cells' energy) is gotten from one glucose molecule that was converted to pyruvate.
This process goes on and on, because in two minutes, a human body system expends an approximate ATP of about 1 billion.
Chloroplasts are similar to mitochondria but are found only in plants. Both organelles are surrounded by a double membrane with an intermembrane space; both have their own DNA and are involved in energy metabolism; and both have reticulations, or many foldings, filling their inner spaces. Chloroplasts convert light energy from the sun into ATP through a process called photosynthesis.
Endoplasmic Reticulum
Endo- = inside.
Plasma = fluid.
Reticulate = repeated patterns.
Endoplasmic reticulum serves like a connecting tube for transportation of molecules from one structure to another. They serve as a transport network for molecules that need to be transferred to and fro specific destinations, unlike other molecules that move freely in the cytoplasm. All endoplasmic reticulum are connected to the Nuclear membrane.
Endoplasmic Reticulum are of two types:
- Rough Endoplasmic Reticulum (RER)
- Smooth Endoplasmic Reticulum (SER)
Rough Endoplasmic Reticulum
If you recall, it was previously said that some Ribosomes are attached to Endoplasmic Reticulum, these are called Rough Endoplasmic Reticulum. The attachment of ribosomes to an endoplasmic reticulum is what gave the name Rough Endoplasmic Reticulum.
When these Ribosomes translate mRNA into proteins, the proteins either stay in the rough endoplasmic reticulum or they are taken completely out of the cell to where they're needed.
Smooth Endoplasmic Reticulum
A smooth Endoplasmic reticulum, just as the name implies, is smooth. There are no Ribosomes that attach to them, and they serve as collection centres for proteins that are synthesized in Rough Endoplasmic reticulum. They also transport and store lipids and detoxifying drugs.
Golgi body
Golgi body has several names:
Golgi Apparatus.
Golgi Body.
Golgi Complex.
Golgi apparatus was discovered by Italian Mr Camillo Golgi in 1898.
Golgi body is composed of Flattened stack membranes like a "pancake stack" and its function is to collect proteins from Endoplasmic reticulum for further processing, packaging and transport out of the cell. Lysosomes are also produced in the Golgi body.
Lysosomes and Peroxisomes
Lysosomes and peroxisomes are often referred to as the garbage disposal system of a cell.
Both organelles are somewhat spherical, bound by a single membrane, and rich in digestive enzymes (=naturally occurring proteins that speed up biochemical processes).
Lysosomes contain more than three dozen enzymes for degrading proteins, nucleic acids, and certain sugars called polysaccharides.
All of these enzymes work best at a low pH, reducing the risk that these enzymes might digest the cell perhaps they somehow escape from the lysosome.
One function of lysosomes is to digest foreign bacteria that invade a cell. Other functions include helping to recycle receptor proteins and other membrane components and degrading worn out organelles such as mitochondria.
Lysosomes also help repair damage to plasma membrane by serving as a membrane patch, sealing the wound.
Peroxisomes function is to get rid of toxic substances, such as hydrogen peroxide, or other metabolites.
High numbers of peroxisomes are found in the liver, where toxic by–products are known to accumulate.
All of the enzymes found in a peroxisome are imported from the cytosol. Each enzyme that is transferred to a peroxisome has a special sequence at one end of the protein, called a PTS or Peroxisomal Targeting Signal, that allows the protein to be taken into the peroxisome, where they then function to relieve the cell of toxic substances.
Peroxisomes often resemble a lysosome, however, peroxisomes are self replicating, whereas lysosomes are formed in the Golgi complex.
Peroxisomes also have membrane proteins that are critical for various functions, such as for importing proteins into their interiors and to divide into daughter cells.
Chromosomes
Chromosomes are proteinous structures found within the Nucleus of a cell. Inside a chromosome is a cell's DNA which stores genetic/hereditary information.
When a cell is at rest, DNA is known as Chromatin.
Chromosomes are made up of two chromatids joined together at the center called the centromere. All chromosomes have a short upper section which is known as p-arm and a long lower section known as q-arm.
Centrioles
Centrioles, also known as centrosome, are made of microtubules of contractile proteins. These tubules are in a 9 × 3 arrangement as seen in the image above.
Centrioles are arranged at right angles to each other OUTSIDE of a cell's nucleus, not in the cytoplasm but in the Centrosome matrix.
Centrioles produce spindle fibers during cell division. As cell division begins, centrioles move towards opposite ends of the cell. Once positioned, centrioles will produce spindle fibers that will aid cell division.
Cilia (singular: Cilium)
Cilia are short hair-like structures which are basically used for fast movement by a cell.
Flagella (singular: flagellum)
Flagella are long whip-like structures which are basically used for movement by a cell.
Vacuoles
Vacuoles are spaces filled with important substances like food or water. Vacuoles can also be said to be the stomach of unicellular organisms. However, vacuoles are primarily storage centres inside a cell.
Where do Viruses fit?
Viruses are not classified as cells and therefore are neither unicellular nor multicellular organisms. Most people do not even classify viruses as "living" because they lack a metabolic system and are dependent on the host cells that they infect to reproduce. Viruses have genomes that consist of either DNA or RNA, and there are examples of viruses that are either double-stranded or single-stranded. Importantly, their genomes code not only for the proteins needed to package its genetic material but for those proteins needed by the virus to reproduce during its infective cycle.
Other minor cell organelles you should know
Glycosome
Glycosomes are membrane-bound intracellular organelles which contain all the enzymes necessary for glycolysis, glycerol metabolism and fixation of CO2. It is responsible for storing glycogen for a Cell's main energy source.
Chromatin
Chromatin refers to a combination of DNA and proteins that form the chromosomes, found in the cells of humans and other higher organisms. Many of the proteins—namely, histones—package the massive amount of DNA in a genome, into a highly compact form that can fit in a cell's nucleus. The primary function is to package long DNA molecules into more compact, denser structures.
MicroVilli
Microvilli are thin finger-like membrane protrusions that are found on the surface of a wide variety of cell types, including intestinal epithelial cells, dendritic cells, and neurons. They are supported by micro filaments. Their function is to increase surface area of a cell.
Microtubule
Microtubules are narrow, hollow tube-like structure found in the cytoplasm. Microtubules help support the shape of a cell. They also help chromosomes move during cell division and help small structures called cell organelles to move inside the cell.
Microfilament
Microfilaments, also called actin filaments, are polymers of the protein 'actin' and are part of a cell’s cytoskeleton. Their functions include cytokinesis, amoeboid movement and cell motility in general, changes in cell shape, endocytosis and exocytosis, cell contractility and mechanical stability.
Intermediate filament
Intermediate filaments are important components of the cell's cytoskeletal system. They may stabilize organelles, like the nucleus, or they may be involved in specialized junctions.
Centrosome matrix
Centrosome matrix, also known as Pericentriolar material is a network of small fibers that surrounds the centrioles in cells.
Summary
📌 Cell organelles are little organs or specialised structures that performs important cellular function. Cell organelles help the cell do different work.
📌 The Plasma membrane is the outer lining of an eukaryotic cell.
📌 Cell wall is a rigid structure composed mainly of Cellulose (carbohydrate fiber) and it provides support and protect for the cell.
📌 The cytoskeleton acts to organize and maintain the Cell's shape.
📌 The cytoplasm is a large fluid-filled space and it is sometimes called the cytosol.
📌 The Nucleus is known as the control center of the cell because it controls different activities within the cell. It is like 'the brain of the cell' and it is surrounded by a double-layered membrane called the Nuclear Membrane or Nuclear envelope.
📌 Nucleolus is a dense knot of chromatin and it essentially makes and assemble Ribosomes needed by the cell.
📌 Ribosome is the protein manufacturing structure in a cell. It is made of rRNA and proteins and it is responsible for processing the genetic instructions carried by a mRNA.
📌 Mitochondria are self replicating organelles and they are found in various shapes (mostly kidney bean shape) and sizes in the cytoplasm of all eukaryotic cells. They are known as the Power-House of cells because this is where all the energy required by a cell is generated.
📌 The endoplasmic reticulum serves like a connecting wire for transportation of molecules from one structure to another.
📌 The presence of ribosomes on an endoplasmic reticulum is what gave the name Rough Endoplasmic Reticulum.
📌 The Golgi body is composed of Flattened stack membranes like a "pancake stack" and its function is to collect the proteins from Endoplasmic reticulum for further processing, packaging and transport out of the cell.
📌 Lysosomes and peroxisomes are often referred to as the garbage disposal system of a cell.
📌 Chromosomes are thread-like structures found within the Nucleus of a cell. Inside a chromosome is a cell's DNA which stores genetic/hereditary information.
📌 When a cell is at rest, DNA is known as Chromatin.
📌 Centrioles produce spindle fibers during cell division.
📌 Cilia are short hair-like structures which are basically used for fast movement of a cell.
📌 Flagella are long whip-like structures which are basically used for movement of a cell.
📌 Vacuoles are basically storage center inside the cell. A virus is not classified as "living" because they lack a metabolic system and are dependent on the host cells that they infect to reproduce.
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Test Questions
1.
What part of a Cell's subunit is responsible for disposal of waste, maintaining its shape or integrity and replicating itself?
This quiz has been created using the tool FunaabExplains Quiz Generator
