The very existence of cells represents a notable element of standardization in Nature’s design. ~ Steven Vogel
Prokaryotes & Eukaryotes
▫ The earliest living cells were prokaryotes. Most single-celled microbes are prokaryotes, including bacteria and archaea.
▫ Eukaryotes had a chimeric origin: formed from a host archaeon incorporating a bacterium. Eukaryotic evolution was an incremental step from the cooperative exchanges and intimate relationships common among prokaryotic microbes.
▫ Prokaryotes have compartmentalized functioning but lack the formality of the organelles which eukaryotes employ: specialized, membrane-enclosed cell structures. Though eukaryotic cells have a more elaborated structure, cellular functioning of prokaryotes and eukaryotes is similar.
▫ Cell form conveys a speed-versus-sophistication tradeoff. Prokaryotic cells have a greater surface-area-to-volume ratio than eukaryotes, which affords prokaryotes faster metabolism, higher growth rate, and faster generation time.
Eukaryotic Cell Types
▫ There are 3 basic eukaryotic cell types: germline, stem and somatic. The cells which comprise the body parts of a multicellular organism are somatic.
Germline cells are the special cells of sexual reproduction, producing gametes. In animals, the gametes are eggs and sperm cells. Plant germ cells produce ovules and pollen.
Stem cells are generic cells that differentiate into a specialized cell type, including somatic cells, or generate more stem cells. Somatic cells may sometimes act as stem cells.
▫ Prokaryotes replicate via binary fission, a form of asexual reproduction.
▫ Multicellular eukaryotic cell division varies by cell type. Stem cells and somatic cells undergo mitosis to create new cells. Germline cells divide during meiosis.
▫ Metabolism has 2 contexts: for an entire organism, and at the cellular level. For an organism, metabolism is the process of creating usable energy by digesting nutrients. Likewise, cellular metabolism – catabolism – is the controlled process of breaking down organic matter to harvest energy via cellular respiration.
▫ Cellular respiration converts nutrients into ATP. There are 2 cellular respiration techniques: anaerobic and aerobic. Early life did not have an ample supply of oxygen, and so had to employ anaerobic respiration, which is much less efficient; photosynthesis excepted. Aerobic respiration evolved in non-photosynthetic life as quickly as atmospheric oxygen became available.
▫ ATP is the chemical coin of metabolism, used to store and transport chemical energy within cells. ATP is also employed in communication within and between cells.
▫ Biosynthesis is the process of biological construction: conversion of substrates into more complex products. A substrate is the material upon which enzymes act to create a product. Common biosynthetic products include carbohydrates, proteins, vitamins, and lipids.
▫ A metabolic pathway is a series of chemical reactions occurring within a cell, typically with an intended bioproduct. Metabolic rate is the speed at which a pathway transpires.
▫ Anabolism and catabolism are complementary metabolic pathways: anabolism builds matter (biosynthesis), catabolism breaks it down.
▫ There are as many metabolic pathways as there are products, either catabolic or anabolic. A cell may employ several hundred pathways, but only a couple dozen are critical to cell functioning. The critical pathways are identical in most life forms.
Organization & Functioning
▫ From the molecular level on up, there is a natural tendency for biological systems to optimize self-assembly and responsiveness by operating near peak biological phase transition points. Such efficiency is exemplary of coherence in Nature.
▫ Cells are aware of the condition of their organelles (cellular proprioception), and control organelle size and function as needed to meet cellular needs, given available resources.
▫ Cells retain memories, not only of themselves, but also their neighborhood.
▫ Based upon inputs from the cells they service, intercellular networks employ optimal algorithms to make quick decisions, which they then disseminate to their employers.
▫ When cells need to migrate, they discern the proper direction and proceed according to plan. Cell migration is commonly coordinated with other cells.
▫ Healing illustrates cell intelligence. Perhaps the most astonishing example of cell intelligence is embryogenesis.
▫ Ultimately, multicellular life relies upon cells knowing just what to do and when to do it, with intricate coordination among themselves.