The cell is the basic unit of life. Though it may not appear so in larger, more complex organisms, life begins and ends with the cell. Every single living thing is a collection of anywhere from one to trillions of cells, from a microscopic henitalium bacterium to the honey fungus spread over 2.4 miles of Oregon’s Blue Mountains.
French biologist Édouard Chatton was the first to characterize the distinction between the prokaryote and the eukaryote. He coined the terms in his 1925 paper Pansporella perplex: Reflections on the Biology and Phylogeny of the Protozoa, calling them prokaryota and eukaryota, the two empires of life. The primary difference between the two is the lack or presence of a true nucleus and membrane bound organelles.
The word eukaryote comes from the old Greek eu, meaning true, and karyon, meaning kernel. Unlike prokaryotes, eukaryotes contain true nuclei. They also have a variety of membrane bound organelles, and may exist as either single cell organisms or multicellular organisms. There are four kingdoms of eukaryotes.
The word protista comes from the old Greek protistos, meaning the very first. Protists are single celled eukaryotes, and are believed to be the first eukaryotes to have evolved. This kingdom is unique among the four kingdoms. They can be either single cell or multi cell, but do not form specialized tissue. Protista is the most diverse of the four eukaryotic kingdoms, so diverse that it is often defined as any eukaryote that isn’t a plant, animal, or fungus. Kingdom Protista is further broken down based on their similarities to either plant, animal, or fungal cells.
Protozoa – Protozoans are similar to animals, in that they have the ability to move and lack the ability to produce their own food. However, while animals are multicellular organisms, and are found in both moist and dry areas across Earth, all protozoans are single celled and exist in moist and wet environments. They are further categorized into the four phyla based on how they move: sacordinians, zooflagellates, ciliaphorans, and sporozoans.
Algae – Algae are plant-like organisms that can be either single or multi celled. Much like plants, they produce their own food and release oxygen. Algae are further broken down into green algae, brown algae, golden algae, and fire algae.
Slime Molds – Saprophytic protists, or slime molds, are single celled organisms that share many similar traits with the molds from Kingdom Fungi. They are found in moist soils, decaying plant matter, and trees. Under favorable conditions, they form multicellular colonies. When the conditions worsen, they differentiate into individual fruiting bodies bearing spores at their tips. These spores possess cell walls that help them survive in the less than favorable conditions, and can disperse by air currents to relocate.
Kingdom Fungi consists of single celled yeasts and molds and of many multicellular fungi that produce fruiting bodies, such as mushrooms and puffballs. While some form of early fungus probably existed before even the earliest of plants or animals, fungi were not recognized as different from plants until 1969. While not the most common fungi, mushrooms are probably the most commonly recognized. Have a look at a mushroom, and it’s easy to see why they were considered plants for so long. They sprout up out of the ground or other plants, and they don’t move around and eat other organisms or mate.
However, fungi are not plants. Much like plants, fungal cells do have cell walls. However, these cell walls contain chitin. Chitin is a long-chain polymer of a glucose derivative. It can be found in the exoskeletons of some mollusks and insects, and well as in various other body parts of some other animals. It cannot, however, be found in the cell walls of any cell other than a fungal cell. Fungi also do not produce their own food, as plants do. Instead, they must find food to consume.
Given this, fungi might begin to seem more like animals than plants. However, fungi certainly don’t belong in that kingdom either. As stated above, fungal cells do have cell walls. Animal cells do not. Also, while animals move around and ingest food, fungi are stationary organisms that are incapable of hunting or grazing. They spread by either growing or sending out spores. When a fungus encounters food, it grows into the food source and excretes digestive enzymes into it. Then, it absorbs the dissolved nutrients through a process that is quite similar to how plants absorb nutrients from the soil.
Fungi are the primary decomposers in any ecosystem. In the circle of life, plants produce the food that animals eat. Eventually, all animals and plants die. Fungi then digest the dead organic matter, or in the case of parasitic fungi, the living organic matter, and break down the nutrients stored within.
Much like the algae of Kingdom Protista, plant cells have cell walls made of cellulose. With the notable exception of parasitic plants that leach nutrients from others, plants also produce their own food through the process of photosynthesis. Unlike algae, all plants are multicellular, and even in photosynthetic plants, not all cells are capable of photosynthesis. Plant cells are unique from any other type of cell in that each has a water-filled vacuole.
While perhaps not immediately apparent, plants contain organs and organ systems. Different types of plant cells join together to form specialized tissues, which then join to form organs. Each leaf, stem, and root is a separate organ. The leaves together, the root system, and all of the parts of the stem are all organ systems. Plants can reproduce both asexually and sexually. Flowers are pretty to look at and often pleasant to smell, but their primary purpose is not to provide a pleasing experience to humans. Flowers are the reproductive organ systems of many plants. They include one or more carpel, each of which contains an ovary. Atop the ovary is a style and a stigma, which receive and funnel pollen toward the ovary for fertilization. Stamens contain anthers and filaments. The anther produces pollen, which is the male reproductive cell. Some plants produce what are called perfect flowers, which are flowers that contain both male and female organs. Others produce imperfect flowers, which are either male or female. Kingdom Plantae is further divided into four distinct categories.
Bryophyta – These are non-vascular land plants that contain no conducting tissues. Bryophytes, such as mosses and liverworts, are small and typically grow close to the ground. They reproduce through spores, and are non-flowering plants.
Pteridophytae – These plants are seedless vascular plants like horsetails. Their reproduction process is also carried out by spores.
Angiosperm – Angiosperms are flowering plants that develop seeds. They develop their seeds within an ovary, which itself is embedded in a flower. After fertilization, the flower falls off and the ovary bulges to become a fruit. Inside the fruit, the seeds that will eventually become new angiosperms develop.
Gymnosperm – Gymnosperms are non-flowering plants that produce seeds that aren’t enclosed in a fruit. The largest and easiest group of Gymnosperms to recognize is the conifer group. Conifers, such as pine trees, develop exposed seeds on a cone.
The final eukaryotic kingdom is Kingdom Animalia, more commonly known as animals. The word animal comes from the Latin animalis, which means having breath. As this suggests, all animals breathe. While animals are not the only organisms that undergo cellular respiration, most other organisms that do are single celled. In order to get the required oxygen to every cell in an animal’s body, the animal needs advanced circulatory and respiratory systems. While the organ systems are at least a little different from one species to the next, the general idea is the same. Animals breathe in, and the oxygen is dissolved into the blood and sent to every cell in the body. Each cell takes oxygen and expels carbon dioxide, which is then carried through the blood and expelled from the body.
Animal cells don’t have cell walls like plants or fungi. They do, however, form specialized tissues. These tissues form organs, which form the organ systems that work together as the individual body. Eleven major organ systems are present in animals, though they aren’t all the same in each animal, and some animals lack one or more altogether.
- Respiratory System – The respiratory system exchanges gases between the lungs or gills and the outside environment. This moves oxygen from the external environment to the internal environment so that individual cells can perform cellular respiration. It also removes the carbon dioxide waste and maintains the pH of the blood.
- Circulatory System – The circulatory system transports oxygen and nutrients to cells and carbon dioxide and other waste molecules away from cells. It also acts as the transport system for the endocrine and immune systems.
- Endocrine System – The endocrine system works closely with the nervous system to control the activity of internal organs as well as to coordinate long range responses to external stimuli. The endocrine system secretes hormones into the circulatory system that regulate body metabolism, growth, and reproduction.
- Nervous System – The nervous system coordinates and controls the actions of the internal organs and body systems. Memory, learning, emotions, and conscious thought are also controlled by the nervous system.
- Immune System – The immune, or lymphatic system defends the internal environment from invading microorganisms and viruses, as well as from cancerous cells.
- Digestive System – The digestive system digests food into nutrient molecules by breaking it down both mechanically and chemically until it is small enough to pass through the blood stream and into cells.
- Excretory System – The excretory system regulates the volume of internal body fluids and eliminates metabolic wastes from the internal environment.
- Skin, or Integument System – This system is the outer layer of protection for the body. It seals the body to prevent both water loss and invasion by foreign bodies. It also shields the body’s internal tissues from environmental exposure.
- Skeletal System – The skeletal system provides support and protection, as well as attachment points for muscles. It provides a rigid framework for movement and protects other organs. It also stores minerals and produces blood cells.
- Muscular System – The muscular system facilitates movement and locomotion. It produces body movements and heat, maintains posture, and supports the body.
- Reproductive System – The reproductive system is mostly controlled by the endocrine system, and is responsible for the survival and perpetuation of the species. Elements of this system produce hormones that control and aid in sexual development, and the organs of this system produce the gamete cells that combine in the female system to produce offspring.
There are 36 recognized phyla of Kingdom Animalia, but in general, animals are divided into two groups based on the absence or presence of a spine. Invertebrates do not have spines and vertebrates do. With the exception of arthropods like insects and spiders, the animals most familiar to most people are mammals, birds, fish, reptiles, and amphibians – all of which are vertebrates. However, we vertebrates make up less than 5% of all animals! The vast majority of animals are actually invertebrates, and most of them are aquatic and rarely seen by most people.
The cell is the basic unit of life, and there are two types of cells. Eukaryotic cells have nuclei and membrane bound organelles, while prokaryotic cells do not. Eukaryotes evolved after prokaryotes, and can either be single or multi celled organisms. Domain Eukaryota is divided into four kingdoms. Kingdom Protista contains the primitive protists that are thought to be the first eukaryotes and the evolutionary predecessors to the other three kingdoms. Fungi are the primary decomposers of the ecosystem and break down organic matter to return primary nutrients to the soil. Plants are non-moving organisms that produce their own food through photosynthesis and release oxygen into the environment. Animals breathe in oxygen and exhale carbon dioxide, which is needed by plants for photosynthesis. Animals can’t make their own food, and so have to either graze or hunt to consume other organisms.