Cellular Evolution: From Atoms to Complex Life Forms
The Origins of Life: A Cellular Journey
We are mammals, but our biological history began long before millions of years.
The Big Bang occurred approximately 20 billion years ago, forming galaxies and planets. It took one hundred thousand years for the universe to cool after so many explosions!
Our home, the Earth, is about 4 billion years old.
Inorganic molecules began interacting with each other. Gas, water, solids, lightning, and thunderstorms accelerated chemical reactions with their energy, leading to the formation of organic molecules. About 30 of these molecules are considered ancestors, including 20 amino acids (the building blocks of proteins), some basic fatty acids, glucose (a sugar), and the base of acetylcholine, a neurotransmitter important in all living creatures with nervous systems.
The ocean waters, the source of organic molecules for life, were changing. The concentration of solutes and solvents was not initially ideal. About 3 billion years ago, molecules began to come together and multiply. Proteins appeared that could accelerate chemical reactions, replicate themselves, and manage protein synthesis.
Molecules formed disciplined layered membranes, folding and locking with other protein molecules, water, and salt, leading to the appearance of the first cell!
Thus, the Monera (bacteria) appeared in the primeval sea with a basic structure: a lipoprotein membrane, cell cytoplasm or cytosol with proteins and salts, and a small DNA molecule (about one millimeter long) forming a single chromosome with few genes. (Human DNA from every cell, if stretched, would grow to more than 1 meter long!). This DNA provided the information for self-renewal and division, creating billions of tiny living creatures no larger than 1 micron (thousandth of a millimeter) in diameter. To understand how small this is, consider the metric system: dividing by 10 repeatedly, 1 Å (Angstrom) is 1,000 times smaller than a micron and 1 million times smaller than a point on paper (measuring one tenth of a millimeter). A micron is 1 Æ bacteria (diameter).
Nature took 1 to 2 billion years to transform that basic cell without a nucleus (with its DNA dispersed in the cytoplasm, a prokaryotic cell) into a larger, more complex cell with a nucleus, longer chromosomes, and more genes (segments of DNA encoding more proteins). This was a huge step! All in one cell: the eukaryotic cell (Eu = true, Carion = nucleus). First, these cells were alone (protists), then grouped to form tissues, organs, and systems, giving rise to the diversity of life: Fungi, plants, and animals. Whittaker proposed five kingdoms: 1) Monera, 2) Protists, 3) Fungi, 4) Plants, and 5) Animals. However, there are two empires: prokaryotes and eukaryotes.
Bacteria were discovered to not fully meet the conditions of eukaryotes or prokaryotes, but rather an intermediate form: the Archaebacteria found at the sea bottom. This led to a rethinking of classifications, and today we speak of three domains: Eukaryota, Archaea, and Prokaryotes.
There are several theories about this, but the most compelling is that proposed by Lynn Margulis, based on DNA in mitochondria (intracellular organelles in eukaryotes located in the cytoplasm and nucleus). Mitochondria colonized the cytoplasm of larger prokaryotes, learned to live in symbiosis, and gave up part of their DNA, resulting in a longer chromosome. This led to the nuclear structure typical of eukaryotes, and bacteria became small mitochondria. This is the theory of “endosymbionts”, which explains two billion years of early development in our bodies. We begin, then, at the most basic level: the atomic-molecular level.
Atomic and Molecular Level
The molecules that make up our cells and their components are made from atoms: simple, small, uncomplicated, and very functional. This may be the reason for the great versatility and potential for rapid change and adaptation of living beings.
Carbon is one such atom. While not the most abundant, its spatial structure of electron orbitals (similar to a pyramid with four vertices) allows it to easily combine with other atoms, including other carbon atoms, forming a molecular skeleton suitable for long chains or closed and resistant rings. Oxygen, versatile and easy to combine with other atoms (like to form molecular oxygen, essential for aerobic life), is the most abundant element on Earth and in its atmosphere or biosphere. Hydrogen, the simplest atom, is the most agile in transferring energy contained in its single electron orbital and combines with oxygen to form water, “the vital element.” Nitrogen is also key, forming part of structural molecules (proteins) and the ancestral memory of genes (nucleic acids), as well as in energy storage (ATP).
Inorganic molecules, such as chlorine, sodium, iron, potassium, copper, cobalt, nickel, cadmium, phosphorus, and sulfur, also play a role in living organisms.
Smaller atoms form smaller molecules that can move freely and form covalent bonds with water and each other, dissolving easily. If needed, nature will make them gain or lose electrons. Redox processes in fat and insoluble molecules will not pass through cell membranes, but water-soluble molecules will be able to leave bodies and be renewed after the entry of other molecules from other bodies and species millions of years before. Nothing is lost, only transformed!
Some atoms, such as hydrogen, can lose their electron orbital and become protons. Certain molecules can become dangerous to others, leading to acidosis and alkalosis, which organisms must monitor to survive.
Organization of Membranes and Cells
We have described atoms and molecules and their different complexities. Small molecules, with diameters not exceeding 10 Angstroms, usually consist of two, three, or four atoms. They often form crystals and are easily dissolved in water. Water behaves as a dipole and can interact with these molecules through diffusion, forming true solutions. This also allows them to cross cell membranes through pores (not more than 8 Å DEAE), entering and exiting cells. These pores are formed by larger molecules (more than 10 Å DEAE, generally no larger than 100 Å), which are proteins. These proteins are generally compatible with water, but their size makes them more unstable, forming colloidal solutions. Examples include albumin and globulins. Albumin is responsible for the power of attraction called osmosis, while globulins are our “missile attack” in defense of our body (immunity), called antibodies that react to foreign molecules.
Larger molecules (more than 100 Å) have nonpolar and polar structures. They are partially or completely incompatible with water and only allow similar substances to pass through. These are lipids, commonly called “fat,” which cannot form true or false solutions with water but form small droplets that float in it. To mix with water (not dissolve), they need the help of detergents, forming small micro-droplets interspersed among water molecules. Lipids have a strong attraction between molecules (surface tension) and are responsible for the formation of two layers of molecules, which share protein “tubes” with low sugar content and complex structure. This “wall” separates the cell from the surrounding environment (extracellular and intracellular behavior).
Thus, small, polarized, or ionized molecules enter or leave through protein “pores,” “channels,” or “tubes,” often by a “pump” (the famous sodium-potassium-chloride-calcium pump, inactive during electrical nerve extension).
Large molecules will not pass unless they “stick” to the membrane, which invaginates (sinks), surrounding and incorporating them into the cell to be used or destroyed if foreign. This mechanism is called pinocytosis or phagocytosis, depending on whether the particle is liquid or solid.
As we can see, three out of four organic chemicals make up cell membranes of all living things: bacteria, fungi, plants, and animals. The fourth substance is nucleic acids: DNA and RNA, “smart” molecules capable of self-reproduction.
Organelles (small molecular structures responsible for specific functions) are often made up of membranes. Even the cell nucleus, which houses nucleic acids, is surrounded by a nuclear membrane similar to the cellular or cytoplasmic membrane.
Now, notice that we have “surrounded” the cell with its membrane. Inside is the cytoplasm or cytosol, a “jelly” that is sixty percent water, and the organelles. Despite the cell being shaped like a “ball of gelatin,” it is not a “balloon” of heavy water. Inside is a fibrous skeleton, “strands” or “threads” of a protein called actin (the same as part of the contractile apparatus of muscle cells or myocytes). These strands extend in all directions from a point on the inner surface of the membrane to the opposite surface, like the Earth’s axis from pole to pole.
Floating in this sea of water and protein are the organelles, each with specific functions:
Mitochondria: These structures, which can number in the thousands in cells, are responsible for “processing” fuels (fats, sugars, and proteins), “breaking down” atoms into molecules, and “freeing” energy. This vital force attracts orbital electrons into the nucleus of atoms, which is then stored in molecular “batteries” such as ATP (adenosine triphosphate) and PC (creatine phosphate). These phosphate-rich molecules are used for mobile consumption, but these “accumulators” can only “drop” energy for a few seconds (10″ to 30″) of intense cellular activity. Beyond that, the cell must “catabolize” (exergonic catabolism is the energy-releasing molecular break, the entire process called metabolism) fuels to “anabolize” (anabolism is synthesis with energy storage or part of endergonic cell metabolism).
Mitochondrial membranes consist of two layers: external and internal. The internal membrane is “folded,” forming “ridges” that support cytochromes, which are respiratory enzymes.
These proteins speed up chemical reactions (processes that would take hours last seconds). They are biological catalysts.
These enzymes act in the presence of oxygen (aerobically). This gas, abundant in the atmosphere, is a powerful “acceptor” of hydrogen and electrons once they have released their energy (proton acceptors). If chemical processes do not use oxygen (anaerobic), the enzymes, many of them “folded” inside the cell membrane, will release energy to catabolize fuels like glucose, but with much less energy efficiency. Anabolic cellular life will not last beyond 5 minutes (exceptionally 7 to 10).
This explains the urgency of medical rescue teams in cases of cardiac and respiratory arrest: to restore a power source to resume aerobic rather than anaerobic processes, rather than keeping the victim “alive and vegging out” while their heart and lungs have stopped.
Mitochondria are a major step in the evolution of life, taking a billion years to move from a prokaryotic cell (bacteria) that can live without oxygen or with very little, to a eukaryotic cell.
Ribosomes are evolutionarily ancient and, like mitochondria, are found in the kingdom of Monera (bacteria), but are smaller.
Reticular System:
These organelles are “tubular” and connected like a fabric, pattern, or network that stores proteins synthesized on ribosomes. Some ribosomes are attached to the tubular wall or membrane, and this lattice is called granular. In the bare lattice, there are agranular ribosomes. These are the stores of products made by the cell, chemical messengers called cytokines and hormones that travel through the blood or mucus. Examples include digestive enzymes produced in the digestive tract or in accessory glands like the liver and pancreas.
This pipe system is usually connected with the nuclear membrane pores, allowing for faster cellular manufacturing: the “order” from the “design center” in the nucleus goes directly to the “assembly plant” (ribosomes) and storage grid.
Golgi cell (not to be confused with the Golgi system of the tendon)
This is also a tubular membrane system, but a little larger and more shallow, which stores large secretory molecules, often in chemical stock lipo-protein (not hormones whose chemical nucleus is cholesterol).
Fat vacuoles (liposomes):
Within the cytoplasm of any cell, vesicles similar to phagosomes and lysosomes have a lipoprotein envelope membrane (like the cell membrane). They store micro-drops of lipids (fats), which are large molecules with low density (ratio: mass/volume) less than water, causing them to float. These molecules consist of long chains of carbon, such as triglycerides, which release more energy than all others (sugars or carbohydrates and proteins) when catabolized in the presence of oxygen within the mitochondria.
In the body, fats act like gas-oil in a truck: not fast, but a fuel that can deliver energy for long distances in time and distance.
Granules or glycogen granules Reserve:
Within the cytoplasm, especially in myocytes or muscle fibers, another fuel is stored: glucose, a carbohydrate (hydrate carbon or sugar). It is stored as polymerized granules of 300-400 Å, because as a monomer (6 carbons), it would “escape” through the tubular pores in the cell membrane. This is a quick fuel, like naphtha, but gives less power and is exhausted in a minute and a half of intense work, causing the tiredness or fatigue we experience regularly.
Centrioles:
These organelles appear only during cell division.
They are in pairs and placed at each pole of the cell. Microfilaments appear, ranging from one to the other “like a clothesline.” On these filaments, like “clothes pins on a tender,” the chromosomes move, carrying DNA fragments of heredity transmitted from cell to cell.
Cells divide to multiply. After copying all their genetic material (nuclear DNA), a process called “replication” (actually a mirror), the large double-stranded molecule or double helix is fragmented into pieces called chromosomes. These chromosomes then move to either cell pole, led by centrioles, and march on “color zones” (threads). The cell splits in two, creating two new cells (a stem cell and a daughter cell). If we all come from the union of egg and sperm, each with half the genetic information, millions of cell divisions are needed to create a new living being, because we carry millions of cells, each with the total information.
The number of cell divisions or mitoses per day is variable. It is very intense during growth (infancy), slows down a little at the end of adolescence, and stabilizes in adulthood. While the number of mitoses is permanent, mitosis in each cell is not. There are periods when a cell does not divide, but its neighbors do, creating a balance between those that die and those that are born defective. The latter are phagocytosed or “swallowed” by immune cells.
If the “brake” on cell division fails, there will be many more cells, and very immature.
If the “cleaning squad” also fails, atypical or “deformed” cells cannot be eliminated from the body.
In both cases, these cells accumulate in certain organs, producing a “lump” or “tumor.” If they are very immature and/or small, they can invade blood vessels and “colonize” other distant organs (metastasis). This is the cell theory and the theory of cancer immunity. To treat it, gene therapy, immune-suppressing drugs, or stimulants of immunity are used, along with the removal of the tumor (surgical treatment). The tumor takes up space, “chokes” normal cells, and deprives them of nutrients.
Medicine is full of examples of chemicals that can cause “changes” in genes (mutations) that lead to cancer. Exposure to other toxins can depress immunity, causing severe infections by “opportunistic” germs and cancer.
We now turn to the development of a new being from a “stem cell” (mother and father).
Science helps us understand what happens next, until birth, which we call Embryology.
After fertilization, cells (daughters, granddaughters, etc.) are selected for locations in three layers:
a) Ectoderm: The most superficial layer, which results in the skin (epidermis) and the central and peripheral nervous system (neural ectoderm and general ectoderm).
b) Mesoderm: The middle layer, which gives rise to the myocardium, bones, skeletal muscles, joints, kidneys, and gonads.
c) Endoderm: The innermost layer, lining the tubular structures that connect the environment inside the body (respiratory and digestive epithelium). Its cells produce a gelatinous substance to trap molecules or cells that are strange and remove them: mucus. This layer is called the respiratory and digestive mucosa.
Embryology allows us to understand how toxic substances that alter the genes of these three types of cells and/or their expression (genotype and phenotype) can cause skin diseases, nervous system disorders, rheumatism, and diseases of the pleura, pericardium, and peritoneum. If symptoms occur during embryonic development (pregnancy), they are called “birth defects.”
As can be seen, cells in a complex organism like mammals are not alone or dispersed but grouped to form tissues.
They need to be suspended in a compartment that surrounds them: the extracellular matrix. The role of this tissue will be liquid, semi-solid, or semi-solid like jelly, with a web of fibers or threads that serve as a skeleton and support for the cells. Thus, we have climbed a new level of biological organization.