Introduction: the cell
Bodies of human and various animals, plants, bacteria, fungi etc. are made of cells. The cells of various groups of organisms may significantly differ from each other; for example, bacterial cells versus animal cells.
On another hand, the various cells of the same kind of animal or plant, and even inside the same organism belong to different types of tissues. Cells of the same tissue play the same functions. For example, there are epithelial, connective, muscular, neuronal tissues etc. Organs in the body are made of the cells of different tissues which work together allowing organs and, ultimately, the entire body to be alive and active.
The cell is a smallest structural unit of living organisms (1). How small are they? For example, the biggest human cell is ovum, a female reproductive cell, achieves 100 μm (it’s one tenth of millimeter), but most human and animal cells are usually smaller, around 10-20 μm or less. For example, the diameter of white blood cells is between 7-10 μm.
The cell is a hallmark of life. For example, viruses are not made of cells, and cannot be considered as a regular form of life. However, even the viruses could not be exist without living cells of live organisms! The viruses are specialized “molecular parasites”; each group of viruses target the cells of particular type where they can reproduce their genome and protein envelope by using the infected cell’s machinery.
All necessary activities of the living body are possible due to activities of the cells. On the other hand, all components of the cells have special properties and are responsible for the cell’s functioning. Let’s take a closer look at. How the animal and human cells are build or organized? The cells consist of nucleus with nucleolus (sometimes, there are two or more nuclei), cytoplasm and cytoskeleton, all are surrounded by plasma membrane. The cytoplasm consists of protoplasm also called cytosol (liquid part of cytoplasm or intracellular liquid), organelles and inclusion bodies (metabolic or ingested substances). The cytosol occupies about a half of space of the cell; this is a place where the proteins are synthesized and most metabolic reactions happen.
What are cell organelles?
Organelles are tiny subcellular (intracellular) bodies of the cell. Like organs inside the animal or human body, organelles locate and work inside the cell. The animal cell organelles include constantly presenting in the cell structures endoplasmic reticulum, ribosomes, Golgi apparatus, centrosome, mitochondria, endosomes, lysosomes, peroxisomes, fibrils (in some cells). Other structures of the cells (plasma membrane, nucleus with nucleolus and components of cytoskeleton like tubulin microtubules, actin filaments (microfilaments) and intermediate filaments), play important roles in the cell but do not belong to the cell organelles. Inclusion bodies (metabolic or ingested substances) are also not organelles.
It’s interesting that inner spaces of endoplasmic reticulum, Golgi apparatus, endosomes and lysosome are connected with each other and also with outer space of the cell with the use of transport vesicles which bud off from one organelle and merge to another and move molecules between that organelles and out.
Endoplasmic reticulum and its functions
Approximately a half of all membranes in the cell belongs to the organelle called endoplasmic reticulum (ER). Plenty of organelles of another type called ribosomes locate on the surface of ER facing to the cytosol. The ER “occupied” by ribosomes is called by “rough” ER. It is responsible for the synthesis of proteins. There is also smooth ER which has no or a lack of ribosomes on its surface. ER is also a place where lipids are synthesized for all the cell.
ER is an organelle which provide alongside with the Golgi apparatus and microtubules transport of proteins. For example, digestive enzymes produced in the secretory cells of pancreatic gland are transported via endoplasmic reticulum and then by Golgi apparatus from the site of protein formation (ribosomes localized on rough endoplasmic reticulum or directly in cytosol) to the site of Golgi apparatus where secretory vesicles form for following transportation via cytoplasm and excretion of the enzymes outside the cell by merging plasma membrane. The endoplasmic reticulum and Golgi apparatus are also employed by the cell for transportation for proteins to the intracellular localization and the use.
ER is “a factory” for synthesis of proteins and lipids, building parts for constructing various organelles in the cell. This organelle has a large surface area of its membrane where plenty enzymes localize and participate in production of proteins and lipids. Newly synthesized proteins should come first from cytosol. The special signal peptide mediates this transfer.
Ribosomes and their functions
Ribosomes are organelles which are responsible for synthesis of complex membrane proteins and soluble proteins which are needed for building of or the use by other organelles or secretion outside the cell. (Not too much info I am providing here, please look somewhere else if you need).
Mitochondria and their functions
Mitochondrion (pl. mi·to·chon·dri·a) is an organelle with bean-alike body composed of double membrane. The function of mitochondria is a production of energy for the cell. Generally, the lower the numbers of mitochondria in the cell, the larger size of organelles (2).
Mitochondrion is a “power plant” of the cell. Mitochondria employ two complex and important processes of electron transport and oxidative phosphorylation to produce the special molecule called АТР (a·den·o·sine tri·phos·phate). The АТР molecule is used for depositing and the storage of the energy in the cell which consumed for all cellular needs/activities.
To run these processes, each mitochondrion in the cell has four compartments: matrix, inner membrane, intermembrane space, and outer membrane. Each compartment is made of its specific proteins. Mitochondria have their own DNA (own genome) and even other organelles like ribosomes! Interestingly, mitochondrial proteins are coded by both genomes: the own genome of mitochondria and the cellular genome.
Like mitochondria, other organelles chloroplasts of plant cells produce a major amount of ATP consumed in biosynthetic reactions of the cells.
Peroxisomes and their functions
Peroxisomes (also called microbodies), are small spherical organelles enclosed by a single membrane. Their diameter usually ranges from 0.15 to 0.25 μm in most cells, but in liver cells, peroxisomes are larger: from 0.5 to 1.5 μm. While the peroxisomes do not have their own genome (unlike mitochondria which have one), they still capable to replicate in the cell by using a genetic information stored in the cell’s genome.
Peroxisomes produce oxygen peroxide (which they employ for oxidation), and generate catalase an enzyme which they use for degradation excess of oxygen peroxide in the cell.
Peroxisomes present in all eukaryotic cells. The functions of these organelles are different in the cells of various types (tissues).
Peroxisomes, like mitochondria, are responsible for utilization of oxygen in the cell. Peroxisomes also produce toxic intermediates from oxygen. In liver cells, peroxisomes produce three enzymes: 2 oxidases and catalase.
It is believed that peroxisomes and mitochondria have a unique membrane receptor which mediates selective transport of proteins into the organelles from cytosol.
There is a hypothesis that suggests that peroxisomes in modern organisms have been originated from the ancient organelle which was necessary to protect the cells from oxygen’s toxicity (2). Ancient peroxisomes decreased oxygen concentration in the cells and used oxygen’s activity in oxidative reactions. Appearance of mitochondria and the reaction of oxidation phosphorylation made a role of peroxisomes less significant in the modern organisms, because peroxisomes just utilize oxygen without production of energy of ATP molecules.
The name of peroxisome is due to its enzymatic ability to produce hydrogen peroxide (Н2О2). Peroxisomes, particularly in the cells of liver and kidneys, hydrogen peroxide to oxidize many toxic substrates like phenol, formaldehyde, alcohol etc.
Golgi apparatus and its functions
Golgi apparatus (Golgi complex or Golgi body) is an organelle network of membranes, fibrils and granules, it usually locates near the cell nucleus and often near the centrosome. It is built by flattened membrane tanks (called Golgi cisternae) resembling a stack of plates. Each stack is made of 4-6 cisternae with a diameter around 1μm. The cell may have one large or multiple small stacks of Golgi cisternae. Other structures belonging to the Golgi complex are tiny membrane bubbles (Golgi bubbles) located near Golgi cisternae. Their role is transportation of proteins and lipids from Golgi apparatus and between Golgi cisternae.
Golgi apparatus receives a variety of proteins and lipids from ER, modifies and transports them to various sites inside the cell.
Golgi apparatus is responsible for assemble and modification of the complex proteins called proteoglycans by attaching sugars and other chemicals to the side chains of proteins in the presence of special enzymes (2, 3). Some proteoglycans are secreted by the cell as a part of protective mucus, other are employed in the structure of plasma membrane.
Golgi apparatus has two surfaces: one is mature (trans-surface) and the second is developing (cis-surface). Cis-surface of the Golgi complex is tightly linked with ER. Trans-surface of the Golgi complex expands and produces a network or reticulum of tubes (so called Golgi network)
Lysosomes and their functions
Lysosomes are organelles that contain digestive enzymes which destroy old (“wear-out”) organelles of the cell. Moreover, lysosomes also destroy various particles and molecules ingested or “eaten” by the cell (endocytosis). However, before utilization by lysosomes, these particles and molecules are processed by another kind of organelle called endosome.
Types and classifications.
Animals, plant, fungi etc. belong to the large group of organisms called eukaryotes. Eukaryotic organisms are made of the cells (eukaryotic cells) where nucleus is separated by own double membrane from the cytoplasm. In contrast, bacteria belong to prokaryotes, their cell nuclei have no membrane, and their genome is not separated from the bacterial cytoplasm.
References
The American Heritage Dictionary of the English Language. 2nd Ed. Boston: HM, 1982.
B.Alberts etc. Molecular Biology of the Cell. New York :Garland Pub, 1989
Hassel J. R.. Kimura J. H., Hascal V. C. Proteoglycan core protein families Annu Rev. Biochem., 55, 539-567, 1986.
Koolman, J., Röhm, K.H Color Atlas of Biochemistry. Thieme, 2011.