bio2: The human cell (v1.0)

This essay describes human cells and some of its most important parts and functions.

The cell is the basic structural and functional unit of all forms of life. Every cell consists of cytoplasm enclosed within a membrane, and contains many macromolecules such as proteins, DNA and RNA, as well as many small molecules of nutrients and metabolites. Will primarily focus on cytoplasm, nucleus and mitochondria. 

The main components of the cytoplasm are the cytosol (a gel-like substance), the organelles (the cell's internal sub-structures), and various cytoplasmic inclusions. The cytoplasm is about 80% water and is usually colorless. The metabolism of sugars to release energy and create ATP through glycolysis occurs in the cytoplasm. Many cellular activities take place within the cytoplasm, such as many metabolic pathways, including glycolysis and photosynthesis (in plants) and processes such as cell division. Many cellular functions are dependent on the permeability of the cytoplasm. An example of such a function is cell signaling, a process which is dependent on the manner in which signaling molecules are allowed to diffuse across the cell.  While small signaling molecules like calcium ions are able to diffuse with ease, larger molecules and subcellular structures often require aid in moving through the cytoplasm.   

The cell nucleus is a membrane bound organelle found in cells. Cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells have no nuclei. The main structures making up the nucleus are the nuclear envelope, a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm; and the nuclear matrix, a network within the nucleus that adds mechanical support. DNA is housed in the nucleus and RNA is transcribed from DNA.

Nucleotides are organic molecules composed of a nitrogenous base, a pentose sugar and a phosphate. They serve as units of the nucleic acid polymers DNA and RNA, both of which are essential biomolecules within all life-forms on Earth. Nucleotides are obtained in the diet and are also synthesized from common nutrients by the liver. Nucleotides are composed of three subunit molecules: a nucleobase, a five-carbon sugar (ribose or deoxyribose), and a phosphate group consisting of one to three phosphates. The four nucleobases in DNA are guanine, adenine, cytosine and thymine.  In RNA, uracil is used in place of thymine. DNA and RNA are chains of nucleotides.

The cell nucleus contains nearly all of the cell's genome. Nuclear DNA is often organized into multiple chromosomes – long strands of DNA dotted with various proteins, that protect and organize the DNA. The genes within these chromosomes are structured in such a way to promote cell function. The nucleus maintains the integrity of genes and controls the activities of the cell by regulating gene expression. Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, proteins or non-coding RNA, and ultimately affect a phenotype (the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment).

The essay here drills down more into genetics and inheritance and evolution all driven by DNA: https://jaykasi.blogspot.com/2022/04/fundamentals-of-evolution-genetics-and.html

Mitochondria are small, often between 0.75 and 3 micrometers and are not visible under the microscope unless they are stained. Unlike other organelles (miniature organs within the cell), they have two membranes, an outer one and an inner one. Each membrane has different functions.

Mitochondria are split into different compartments or regions, each of which carries out distinct roles.

Some of the major regions include the:

·        Outer membrane: Small molecules can pass freely through the outer membrane. This outer portion includes proteins called porins, which form channels that allow proteins to cross. The outer membrane also hosts a number of enzymes with a wide variety of functions.

·        Intermembrane space: This is the area between the inner and outer membranes.

·        Inner membrane: This membrane holds proteins that have several roles. Because there are no porins in the inner membrane, it is impermeable to most molecules. Molecules can only cross the inner membrane in special membrane transporters. The inner membrane is where most ATP is created.

·        Cristae: These are the folds of the inner membrane. They increase the surface area of the membrane, therefore increasing the space available for chemical reactions.

·        Matrix: This is the space within the inner membrane. Containing hundreds of enzymes, it is important in the production of ATP. Mitochondrial DNA is housed here (see below).

Different cell types have different numbers of mitochondria. For instance, mature red blood cells have none at all, whereas liver cells can have more than 2,000. Cells with a high demand for energy tend to have greater numbers of mitochondria. Around 40 percent of the cytoplasm in heart muscle cells is taken up by mitochondria. Most of the ATP in cells for energy comes from reactions in the mitochondria.

Although most of our DNA is kept in the nucleus of each cell, mitochondria have their own set of DNA. Interestingly, mitochondrial DNA (mtDNA) is more like bacterial DNA. One theory is that millions of years back, bacteria representing the mitochondria formed a symbiotic relationship with the cell. The mtDNA holds the instructions for a number of proteins. The human genome stored in the nuclei of our cells contains around 3.3 billion base pairs, whereas mtDNA consists of less than 17,000.  During reproduction, half of a child’s DNA comes from their father and half from their mother. However, the child always receives their mtDNA from their mother. Because of this, mtDNA has proven very useful for tracing genetic lines. For instance, mtDNA analyses have concluded that humans may have originated in Africa relatively recently, around 200,000 years ago, descended from a common ancestor, known as mitochondrial Eve.