The cell, being the basic structural, functional, and biological unit of all living organisms; is the smallest unit of life. They are also considered as the “building blocks of life”.
The human body contains around 30 to 40 trillion cells. The majority of the cells in the human body are red blood cells which make up over 80 percent, but they are responsible for only around 4 percent of total body mass because they only measure 8 micrometres in diameter.
The average size of a fat cell is approximately 100 micrometres contributing nearly 19 percent of body mass and under 0.2 percent to the total cell number. This discovery of cell is mainly credited to Robert Hooke.
Over a century later, there were many schools of thought about cells among scientists. Most debates elaborate on the characteristics of cellular regeneration, and the knowledge of cells as a fundamental unit of life. Cell theory was ultimately articulated in 1839 which is credited to Matthias Schleiden and Theodor Schwann. Although other scientists like Rudolf Virchow also contributed to it.
(Read More – Cell Culture-How to Grow cells in a Lab?)
- 1 Functions Of a Cell
- 2 Types Of Cells
- 3 Cellular Organization
- 3.1 The Plasma Membrane- Outer Layer of the Cell
- 3.2 Endoplasmic Reticulum
- 3.3 The Golgi Apparatus
- 3.4 Lysosomes
- 3.5 Peroxisomes and Vacuoles
- 3.6 Mitochondria- The Powerhouses of the Cell
- 3.7 The Cytoskeleton helps in the maintenance of the Cell’s Shape
- 3.8 Centrioles
- 3.9 Cilia and Flagella
- 3.10 Related
Functions Of a Cell
- Organisms are made of cells where some specific cells meant for structural support, although this being a typical function, is carried out by almost all the cells.
- Cells require nutrients for various biochemical processes which further lead to the production of waste, E.g. Oxygen, carbon dioxide and ethanol like smaller molecules pass across the cell membrane through simple diffusion which is regulated by a concentration gradient. The process doesn’t utilize ATPs, and it’s called passive transport. Whereas larger molecules like proteins pass across the cell membrane by a process called active transport.
- Metabolic reactions can be either catabolic or anabolic. Energy production by breaking down molecules is termed catabolism, whereas anabolic reactions utilize energy to make complex substances from simpler ones.
- Reproduction being a vital process can be carried out by cells through the processes of mitosis and meiosis. Mitosis includes the division of cells to form new cells, which are termed asexual reproduction. Meiosis occurs in gametes or reproductive cells. It is termed sexual reproduction.
Types Of Cells
- Prokaryotes are organisms with no cell nucleus. They are organisms whose cells are arranged in complex structures with internal membranes and cytoskeleton. The most distinctive membrane-bound structure is the nucleus; thus, the name was given prokaryotes.
- The most fundamental difference is that eukaryotes organisms have (Eu) true nucleus containing their DNA. The genetic material present in prokaryotes is not found out to be membrane-bound.
- In eukaryotes, there are mitochondria and chloroplasts which perform various metabolic processes and are considered to have originated from endosymbiotic bacteria.
- The cell walls in prokaryotes are usually produced of peptidoglycan to those of eukaryotes. Many of eukaryote does not have a cell wall.
- Prokaryotes are smaller in size as compared to eukaryotic cells. Prokaryotes have a higher metabolic rate, a higher growth rate, and shorter generation time.
- Prokaryotes contain only a single loop of stable chromosomal DNA present in an area called a nucleoid, whereas eukaryote DNA occurs on tightly bound chromosomes. Although some of the eukaryotes have satellite DNA structures termed plasmids, which is considered as a key prokaryote feature; characteristic genes in prokaryotes are present on plasmids.
- Prokaryotes have very compressed genomes than eukaryotes; because prokaryote genes lack introns and large non-coding regions are present in between the genes. Prokaryote genes are also expressed as operons (groups) rather than individually, which is usually seen in eukaryotes.
The human cell has a plasma membrane, nucleus, and cytoplasm.
- The plasma membrane that environs the cell and keeps it integral regulates the passing of molecules across the cell. It is a phospholipid bilayer that is considered to be semipermeable because it permits specific molecules to pass in the cell.
- The nucleus is centrally situated which can be microscopically observed. The nucleus comprises the chromosomes and is the control centre for the metabolic operative functions and structural features of the cell. The nucleolus is an area inside the nucleus. The cytoplasm is the area between the nucleus and the plasma membrane.
- The cytoplasm is a semifluid medium that contains water and other types of dissolved molecules. The semifluid nature of cytoplasm comes from the presence of proteins. The cytoplasm comprises various organelles that are smaller in size and with a specific function. Organelles make compartments in the cell for keeping the various cellular activities classified from one another.
Cells have a cytoskeleton which is basically a network of interconnected filaments and microtubules that are present in the cytoplasm. Elements of the cytoskeleton maintain cell shape and also permit the cell and its contents to mobilize. (cilia and flagella)
The Plasma Membrane- Outer Layer of the Cell
Cells are enclosed by an outer plasma membrane. The plasma membrane parts the inside of the cell, which is termed the cytoplasm. Plasma membrane integrity is essential; it is a phospholipid bilayer that contains attached (peripheral) or embedded (integral) proteins.
The phospholipid molecule has a head that is polar in nature and tails that are non-polar. Polar heads being charged are hydrophilic (water-loving) and facing towards the watery environment.
The non-polar tails are hydrophobic (fearing water) and face towards an area with no water. When phospholipids are positioned in water, they form a spherical bilayer due to the chemical properties of the heads and the tails. At body temperature, the phospholipid bilayer is in a liquid state.
The fluid-mosaic model proposes that the protein molecules have a varying pattern within the fluid phospholipid bilayer. Our plasma membranes also comprise cholesterol molecules that stabilize the phospholipid bilayer and prevent a drastic reduction in fluidity at lower temperatures.
Sugars chains are attached to the outer surfaces of some of the glycoprotein and glycolipid. These specific carbohydrate chains tag the cell as fitting to a particular individual. (which can be a problem if being rejected in an organ transplant).
Some glycoproteins have a certain confirmation that permits them to serve as a receptor. Some integral proteins form channels through which certain molecules can pass across cells and aids in transport processes.
- The Nucleus of a Cell
The nucleus is a significant structure in human cells. It is of main importance because it stores the genetic information which gives characteristics of the body and their metabolic functions. Every cell comprises a copy of genetic information, but each cell type has certain genes expressed and others repressed.
Activated DNA, with mRNA, controls protein synthesis. When one observes the nucleus through an electron micrograph, chromatin can be seen which contains DNA and some protein as well as some RNA. Chromatin undergoes coiling and turn into chromosomes. Chromatin is an immersed nucleoplasm.
The nucleus is distinct from the cytoplasm by the nuclear envelope, which is continuous with the endoplasmic reticulum. The nuclear envelope has nuclear pores to allow the passage of the protein into the nucleus and ribosomal subunits out of the nucleus.
The double membrane of the nuclear envelope surrounds and contains cellular DNA and serves as a protection to genetic information contained within.
Ribosomes are made up of two subunits, one being large and one being small; each subunit has its own set of proteins and rRNA. Protein synthesis takes place at the ribosomes.
Ribosomes are found free within the cytoplasm either singly or inform of polyribosomes (group). Proteins produced by ribosomes attached to the endoplasmic reticulum may be secreted from the cell further.
- Endomembrane System
Endomembrane system comprises:
1- Nuclear Envelope,
2- Endoplasmic Reticulum,
3- Golgi Apparatus,
4- Lysosomes, and
The Endoplasmic Reticulum (ER) is a complex system made up of membranous channels and saccules that are flattened and continuous with the outer membrane of the nuclear envelope. Rough ER is covered with ribosomes facing the cytoplasm.
Smooth ER, being continuous with rough ER, does not have attached ribosomes. Smooth ER synthesizes the phospholipids that are present in membranes.
The Golgi Apparatus
The Golgi apparatus is named after Camillo Golgi, the one who discovered its presence in 1898. The Golgi apparatus comprises a stack of 3-20 slightly curved saccules. In animal cells, one side of the stack is facing toward the ER, and the other side is facing towards the plasma membrane. Vesicles are seen at the edges of the saccules.
The Golgi apparatus contains certain enzymes that alter proteins and lipids. The vesicles that exit from the Golgi apparatus move to other parts inside the cell. Some of the vesicles go on to the plasma membrane where they release their contents. Other vesicles that leave the Golgi apparatus are lysosomes.
Lysosomes are membranous sacs formed by the Golgi apparatus, which comprises hydrolytic digestive enzymes. Sometimes when the larger molecules are brought into a cell and when a lysosome fuses with these, its contents are digested by lysosomal enzymes into simpler subunits which then enter the cytoplasm.
Some parts of a cell are digested by its own lysosome which is termed as auto-digestion. It is important during the development process. E.g. The fingers of a human embryo are webbed at the initial phase, but they are freed from one another due to lysosomal action. Tay-Sachs disease is a metabolic disorder involving inactive lysosomal enzyme in nerve cells.
Peroxisomes and Vacuoles
Peroxisomes are not really a part of the endo-membranous system, but they are very much similar to lysosomes in structure. They are vesicles like lysosomes, that contain enzymes. Peroxisome enzymes serve to detoxify drugs, alcohol, and other potential toxins. The liver and kidneys have large numbers of peroxisomes as these organs help to cleanse the blood. Peroxisomes also break down the fatty acids so that fats can be metabolized. Vacuoles are present within human cells.
Mitochondria- The Powerhouses of the Cell
The size and shape of mitochondria can vary although they are bounded by a double membrane. The inner membrane is creased to form little parts called cristae, which project into the matrix which is filled with a gel-like fluid.
Mitochondria are called the powerhouses of the cell as the mitochondria convert the chemical energy from carbohydrates into the chemical energy of ATP molecules. For this process, mitochondria utilize oxygen and give off carbon dioxide and water. Thus, it’s right to say that mitochondria carry on cellular respiration.
Parts of digested carbohydrate, protein, and lipid go in the mitochondrial matrix. The matrix contains enzymes to metabolize fragments to carbon dioxide and water. The energy released is used for ATP production that occurs at the cristae.
The Cytoskeleton helps in the maintenance of the Cell’s Shape
Filamentous protein structures form a cytoskeleton which helps in the maintenance of the cell’s shape and anchors the organelles. The cytoskeleton involves microtubules, intermediate filaments, and actin filament.
Microtubules are hollow cylinder-like structure whose wall is made up of 13 longitudinal rows of the globular protein tubulin. Microtubules can assemble and disassemble. Its assembly is regulated by the centrosome, near the nucleus. The centrosome contains the centrioles.
Microtubules help to maintain the shape of the cell and acts as tracks along which organelles move. E.g. During cell division, microtubules form spindle fibres that assist the movement of chromosomes.
Intermediate filaments are tough and resist stress and often form cell-to-cell junctions. Actin filaments are long and very thin, occurring in bundles.
Centrioles are short cylinders. Each cell has a pair of centrioles in the centrosome, and the members of each pair of centrioles are at right angles to each other. Before the cell division, the centrioles duplicate, and the members of the new pair are also at the right angles.
During cell division, the pairs of centrioles get separated so that each daughter cell gets one centrosome. Centrioles are involved in the formation of the spindle apparatus during the process of cell division.
A single centriole forms the anchor point for each individual cilium or flagellum. Basal bodies carry out the formation of both cilia and flagella.
Cilia and Flagella
Cilia and flagella are projections. Cilia are shorter as compared to flagella. Cells that have cilia or flagella are capable of self-movement or moving material along the surface. E.g. sperm cells move by means of flagella. The cells that line the respiratory tract are ciliated.
These cilia sweep off debris trapped within mucus back up the throat. Within a woman’s uterine tubes, ciliated cells are moving the ovum toward the uterus.