Organelles: Functions and Examples of Cell Organelles

Organelles of a cell play important functions in the daily activities of the eukaryotes or prokaryotes. This article is aimed at discussing what an organelle is, the types and functions of organelles, structure, and examples of organelles. It will also look at the list of organelles (plant and animal organelles) and the difference between prokaryotic and eukaryotic organelles.

Definition of organelles in biology

In biology, organelles are defined as specialized subunits of a cell (either eukaryotic or prokaryotic) that have a defined role.

When cells make organelles, they face three significant obstacles. The first is the development and maturation of the organelle’s basic building pieces. The membrane, membrane-bound macromolecules, and the cytoskeletal machinery that shapes the organelle are all part of this first obstacle.

Secondly, proteins, amino acids, lipids, carbohydrates, or their monomers, as well as co-factors, enzymes, and signaling molecules, must all be present in the organelle. These chemicals must be delivered into these subcellular compartments in a precise and frequently active manner.

Finally, organelles must be maintained throughout the cell’s life cycle and accurately segregated during cell division. To fulfill these objectives, cells adopt a variety of tactics from all throughout the living world.

Depending on their principal function in the body, different cell types often have a predominance of a specific organelle. Cells in the parenchyma of leaves, for example, are densely packed with chloroplasts, whereas cells in the root are frequently devoid of this organelle. A rapidly shifting vacuole could be found in an active single-celled creature like a paramecium. Protein secreting cells usually feature a well-developed Golgi network as well as a prominent rough endoplasmic reticulum.

Types of organelles

• Based on membrane binding
• Based on specific tasks in the cell

Based on membrane binding

1. Membrane-bound organelles
2. Non-membrane-bound organelles

Membrane bound organelles

• The nucleus
• Endoplasmic reticulum
• Golgi apparatus
• Mitochondria
• Plastids
• Lysosomes
• Vacuoles

Eukaryotic membrane bound organelles are cellular structures that are connected by a biological membrane. The membrane is normally made up of a single or double layer of lipids with interspersed proteins.

Nucleus

The nucleus is an organelle that regulates gene expression and is responsible for maintaining the integrity of DNA as well as controlling cellular activities such as metabolism, growth, and reproduction. Because of its relatively large size and typically round shape, the nucleus is one of the most visible structures in a cell.

A nuclear envelope, which is a lipid bilayer perforated with nuclear pores, holds the nucleus together. However, some cells do not have a nucleus like at maturity, red blood cells lose their nucleus to provide a greater affinity for respiratory gases such as oxygen.

Endoplasmic reticulum

The endoplasmic reticulum (ER) is a double-membranous protein producing organelle that is solely accountable for protein and lipid synthesis, carbohydrate metabolism, drug detoxification, and intracellular transport.

There are two kinds of ER: rough ER and smooth ER. The surface of the rough ER is studded with ribosomes, whereas the surface of the smooth ER is devoid of bound ribosomes. Both types are made up of labyrinthine, interconnected flattened sacs or tubules that run through the cytoplasm and may stretch to the plasma membrane.

Golgi apparatus

The Golgi apparatus is a double-membraned organelle that is involved in glycosylation, molecule packaging for secretion, lipid transport within the cell, and the formation of lysosomes. It consists of membrane-bound stacks.

Mitochondria

Mitochondria are double-membrane-bound spherical or rod-shaped organelles that contain their own genome, making them semi-autonomous. They are primarily responsible for the production of ATP via cellular respiration.

Plastids

Plastids are double-membraned organelles found in photosynthetic cells like plant cells. Plastids are classified into three types namely chloroplasts, chromoplasts, and leucoplasts. Chloroplasts are photosynthesis-involved plastids that contain green pigment. Chromoplasts are plastids that contain pigments other than green. Leucoplasts are pigment-less plastids that play a role in food storage.

Lysosomes

Lysosomes are cytoplasmic single-membrane entities that contain a variety of digestive enzymes. They have a single membrane and are primarily involved in digestion and the removal of surplus or worn-out organelles, food particles, and engulfed viruses or bacteria.

Vacuoles

Vacuoles are membrane-bound vesicles found in the cytoplasm of cells, particularly plants. They help with structural support, intracellular secretion, excretion, storage, and digestion.

Endosomes

Endosomes are membrane-bound cytoplasmic organelles that transport endocytosed molecules to the lysosome.

Non membrane bound organelles

1. Ribosomes
2. Microtubules
3. Actin filaments
4. Intermediate filaments
5. Centrioles
6. Cytoskeleton

Non membrane bound organelles are cytoplasmic organelles that are not membrane-bound but perform specialized functions.

Ribosomes

Ribosomes synthesize proteins as polypeptide chains, whether free in the cytosol or associated with the rER. This happens as a result of RNA translation. Ribosomes specifically bind to messenger RNA, abbreviated mRNA.

The ribosome reads a series of nucleotide bases in three-base groups known as codons. The start codon is the first codon read and each codon after the start codon represents a different amino acid, which is then carried to the ribosome by transfer RNA, abbreviated tRNA. The amino acid-carrying tRNA is bound to the ribosome’s A site. In the P site, the amino acid is linked to the amino acid that comes before it.

A peptide bond is a bond between two amino acids in a polypeptide chain. After forming the peptide bond, the ribosome moves to the next three nucleotide bases on the mRNA strand and repeats the process until a stop codon is reached.

Microtubules

Microtubules are in charge of the movement of organelles and other structures such as lysosomes and mitochondria. These are elongated, nonbranching polymers composed of -tubulin and -tubulin dimers that are made up of approximately thirteen circular dimeric tubulins. Tubulin dimers are all arranged in a specific pattern to have the same orientation and microtubules are therefore considered polar because of this orientation, with a plus and minus end. At the plus end, growth occurs and at the minus end, there is no growth.

Actin filaments

Actin filaments are found in nearly all cell types and they are formed by a helical arrangement of smaller molecules, their structure is similar to that of microtubules. Actin filaments are thinner and more flexible than microtubules and it is required for a variety of cell functions. For example, they serve as anchors in the movement of membrane proteins and serve as the core of microvilli.

Intermediate filaments

From the name, the intermediate filaments have a size that is between that of microtubules and actin filaments. The intermediate filaments are made up of a rod domain with globular domains at both ends and the rod domain is made up of coiled-coil dimers formed by a pair of helical monomers twisted around each other.

The subunits of intermediate filaments serve a structural function in the cell even though they vary by tissue. In the nucleus, cytosol, and extracellular environment, they primarily form a linked continuum of filaments and they play an important role in the creation of cell-to-cell and cell-to-extracellular matrix junctions.

Centrioles

Centrioles are structural organelles that are made up of 9 microtubule triplets arranged in cylinders and they are responsible for the formation of basal bodies (Basal bodies serve as the foundation for flagella and cilia) and mitotic spindles (Mitotic spindles play a role in chromosome separation during cell division). Centrioles are also responsible for the determination of the location of mitotic spindles during anaphase.

Cytoskeleton

The skeleton system of cells is known as the cytoskeleton. Its network extends all the way inside the cells and the cytoskeleton is a dynamic network made up of protein filaments that are linked together such as actin filaments, intermediate filaments, and microtubules. These are the three main components that make up a cytoskeleton.

When a portion of the cytoskeleton contracts or extends, the cells deform and can change shape and movement. The cytoskeleton also functions as a highway system within the cytosol where motor proteins can transport cargos as they walk along the cytoskeleton. This intracellular transportation system can move a wide range of intracellular cargoes, including proteins, RNAs, vesicles, and even entire organelles.

Based on specific tasks in the cell

• Autophagosomes
• Chromosomes
• Sister chromatids
• Centrosomes

These types of organelles are usually temporary because the cell only needs them at a certain stage of the life cycle.

Autophagosomes

Autophagosome is a temporary organelle for autophagy, and autophagy (also known as “self-eating“) is a process in which cells recycle some of their existing proteins and organelles due to nutrient scarcity.

Damaged proteins and organelles will be labeled with “garbage tags.” The cell recognizes the tags and collects the recyclable materials in autophagosomes. Autophagosomes transport waste from the cell to lysosomes for degradation. It also carries out mitophagy which is a type of autophagy that is used to degrade bad mitochondria.

Chromosomes

When cells prepare for cell division, each DNA thread is organized into a much more compact structure known as a “chromosome.” Each human cell contains 23 pairs of chromosomes (1-22, and X or Y). A chromosome is formed by wrapping DNA around histone proteins to form a nucleosome, which is a core complex.

Sister chromatids

After DNA duplication, sister chromatids are X-shaped chromosomes that remain attached at a centromeric region (centromere). During mitosis, sister chromatids will be separated into two identical chromosomes.

Centrosomes

Centrosomes are organelles that appear only during mitosis and function as the primary microtubule organizing center (MTOC). There are two centrosomes in each cell. When mitosis begins, they move to the opposite positions of the cells.

Microtubules protrude from the centrosome and connect to the centromeres of sister chromatids. Both centromeres retrieve their microtubules simultaneously in order to separate the sister chromatids and move into new cells.

Organelles function

1. Nutrition
2. Transport
3. Digestion
4. ATP Generation
5. Control and Regulation

Nutrition

Unicellular organisms ingest food as large particles from their surroundings and digest it within their cells. This necessitates the presence of organelles such as food vacuoles, phagosomes, and lysosomes to transport digestive enzymes.

The digestive system in most large animals is in charge of food ingestion and breakdown into monomeric units such as glucose and amino acids. The enzymes required for this process are produced on the rough endoplasmic reticulum and secreted via the Golgi network. The circulatory system transports digested food to each cell, which then allows passive transport or uses energy to actively take up nutrients.

Transport

Intracellular transport is frequently managed by crisscrossing cytoskeletal filaments that function as corridors. These filaments form a network of pathways that allow organelles to be positioned and materials to be transported. They are assisted in this activity by motor proteins, which typically have two domains, one to interact with the cargo and the other to navigate the cytoskeletal filament.

Most neurons, for example, have a long axon that conducts electrical impulses along its length and synaptic vesicles containing neurotransmitters frequently populate the end of an axon and are required for electrical signal transmission from one neuron to the next at the synapse. Microtubules create a path for the components of these vesicles to follow as they move towards the synapse. Their movement is powered by motor proteins known as kinesins.

Other materials, such as enzymes or peptide hormones, are transported through the Trans Golgi Network to be used by the cell or exocytosed.

The most dramatic intracellular transport events occur during cell division. Through complex and tightly regulated cellular machinery, chromosomes are accurately segregated and transported to opposing poles of the cell.

Digestion

The primary structures involved in intracellular digestion are lysosomes. They consist of a number of hydrolytic enzymes that are initiated by the organelles’ acidic pH. These enzymes are synthesized in the cytoplasm in inactive forms before being transported into the organelle via transmembrane channels.

For bulk digestion, lysosomes can fuse with other organelles such as phagosomes. For example, when pathogenic microorganisms are ingested by immune system cells and destroyed by the action of powerful hydrolytic enzymes. This combination plays a key role in immunity.

ATP Generation

Mitochondria are frequently used by heterotrophs for aerobic respiration and ATP generation. Autotrophs use solar radiation or other chemical processes to create high-energy bonds in ATP. Both membranous organelles are important in the production of ATP.

Control and Regulation

To maintain homeostasis, large, complex organisms require the use of a nervous and an endocrine system. The nucleus is the most important organelle for control and regulation within a cell and cells receive information about their surroundings via complex signaling cascades, which frequently result in changes to their RNA or protein content. As a result, the nuclear environment is strictly regulated, and material import and export via the nuclear envelope is a critical process for the cell.

The nuclear membrane contains a number of special structures known as nuclear pores, and transport proteins known as importins and exportins to mediate macromolecule entry and exit. These macromolecules can upregulate a specific gene, influence RNA splicing, signal the start of cell division, or even initiate the process of apoptosis.

Organelles examples

Almost every cell in eukaryotic organisms possesses organelles with the exception of the mammalian red blood cell. Some of the commonly seen organelles include mitochondria, plastids (autotrophs), endoplasmic reticula, Golgi apparatus, lysosomes, and vacuoles. Synaptic vesicles are also found in some special cells, such as neurons and these structures are all membrane-bound.

Macromolecular complexes such as ribosomes, spliceosomes, centrioles, and centrosomes are not surrounded by a membrane but are vital organelles in most cells, performing vital functions such as cytoskeleton organization, protein synthesis, and RNA processing.

Bacteria have organelles that are both protein-bound and lipid-bound. These can be made of a simple monolayer membrane (as in carboxysomes) or a bilayer membrane (magnetosomes).

List of plant cell organelles and their functions

Plant cell organelles	Functions of plant cell organelles
Cell wall	Provision of tensile strength and resistance to mechanical and osmotic stress.
Cell membrane	They keep toxic substances out of the cell and they contain receptors and channels that allow specific molecules that mediate cellular and extracellular activities, such as ions, nutrients, wastes, and metabolic products, to pass between organelles and between the cell and the outside environment, and they separate vital but incompatible metabolic processes conducted within organelles.
Nucleus	The nucleus directs and regulates the cell’s activities (for example, growth and metabolism) and houses the genes, which contain hereditary information.
Plastids	Plastids are in charge of food production and storage.
Central vacuole	Serves as a reservoir, waste dump, storage region, and even a means of keeping the cell in shape.
Golgi apparatus	Assists in the processing and packaging of proteins and lipid molecules, particularly proteins destined for cell export.
Ribosomes (protein producing organelles)Help with message decoding and peptide bond formation (protein synthesis)MitochondriaThe most important functions of mitochondria are to produce the cell’s energy currency, ATP (i.e., phosphorylation of ADP), through respiration, and to regulate cellular metabolism.
Lysosome	They degrade extraneous or worn-out cell parts. They could be used to eliminate invading viruses and bacteria.

List of animal cell organelles and their functions

Animal cell organelles	Functions of animal cell organelles
Cell membrane	Building and defining the inside and outside of a cell.
Nuclear membrane	Aid in the separation of chromosomes from the rest of the components of the cell.
Nucleus	Storage of DNA.
Centrosome	Provision of microtubules and structure for the cell
Lysosome (Cell Vesicles)	Destruction of protein.
Cytoplasm	Provision of a platform upon which other organelles operate in the cell.
Golgi Apparatus	Transportation and exportation of protein.
Mitochondrion	Production of energy.
Ribosome	Synthesizing of protein.
Endoplasmic Reticulum (ER)	Protein production and modification.
Vacuole	Regulation and storage of water.
Nucleopore	This is a channel that transports nucleic acids and proteins into and out of the nucleus of the cell.

Structure of organelles

Cell organelles	Structure of organelles
Cell membrane	It is made up of lipid- and protein-based double membranes and it can be found in both plant and animal cells.
Centrosomes	Centrioles make up this structure, which is exclusively present in animal cells.
ChloroplastsThese are organelles of the plant cell and they contain chlorophyll which is a green-colored pigment.
Cytoplasm	A jelly-like fluid made up of water, dissolved nutrients, and cell waste products.
Endoplasmic Reticulum	This is a network of membranous tubules that is found within a cell’s cytoplasm. This organelle is also involved in the production of protein in the cell.
Golgi apparatus	These are organelles that are membrane-bound and sac-like and are found in the cytoplasm of eukaryotic cells.
Lysosomes	These are small, circular single membrane bound organelles that are filled with digestive enzymes.
Mitochondria	This is the “Power House of the Cell” which is an oval-shaped, membrane-bound organelle. In eukaryotes, mitochondria are the organelles primarily involved in the generation of ATP (energy).
Nucleus	This is a large, double-membrane-bound organelle that contains all of the genetic information in the cell.
Peroxisome	A cytoplasmic membrane-bound cellular organelle that contains the reducing enzyme.
PlastidsThese are organelles of the plant cell with two membranes and are classified into three types namely, Leucoplast, chromoplasts, and chloroplast.
Ribosomes	These are organelles that do not have a membrane and can be found floating freely in the cytoplasm or embedded within the endoplasmic reticulum. They are known to be protein-producing organelles.
Vacuoles	These are fluid-filled, membrane-bound organelles found in the cytoplasm.

Eukaryotic organelles Vs prokaryotic organelles

Many organelles are found in eukaryotic cells, including the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, and chloroplast (plastid). However, not all of these organelles are found in a single cell or organism. For example, the chloroplast is abundant in plant cells but not in animal cells. According to the endosymbiotic theory, there are organelles that have their own DNA apart from the nucleus and are thought to have evolved from endosymbiotic bacteria. The organelles in question are mitochondria and plastids.

Prokaryotes, which were previously thought to lack organelles, have recently been discovered to have their own type of “organelles.” These prokaryotic cell organelles are referred to as proteinaceous micro-compartments rather than true organelles.

Carboxysome (a protein-shell compartment in some bacteria for carbon fixation), chlorosome (a light-harvesting complex in green sulfur bacteria), magnetosome (found in magnetotactic bacteria), and thylakoid are some examples of these proteinaceous micro-compartments (in some cyanobacteria).

FAQ on organelles