What is a cell?
What is a prokaryote?
What is a eukaryote?
The Animal Kingdom is first subdivided into groups called phyla (singular: phylum). The phyla are divided into "classes," the classes into "orders," and so on until we get to the smallest division called species.
The ranking of the different subdivisions is shown in the examples of four different animals in the following chart, Examples of the Scientific Classification System. This method of classifying animals is used by scientists. The genus and species names are italicized, and the two names together are the scientific name of the animal. For example, the scientific name of the house fly is Musca domestica.
There are 15 phyla of animals, and each phylum contains animals that have a combination of characteristics that animals in other phyla do not have. The phylum Arthropoda are the animals with jointed legs, segmented bodies, and a tough or hard outer covering that also serves as their skeleton (exoskeleton). Insects have these characteristics, so they belong to Phylum Arthropoda, but so do millipedes, spiders, ticks, crabs, lobsters, and crayfish, which are not insects.
The class Insecta, or insects, are the Arthropoda that have three pairs of legs, a segmented body divided into three regions (head, thorax, and abdomen), one pair of antennae and, usually, wings. Other Arthropoda classes have more than three pairs of legs and only one or two body regions, and they never have wings. Other common classes of Arthropoda are Crustacea (such as sowbugs, crayfish, crabs), Diplopoda (millipedes), Chilopoda (centipedes), and Arachnida (such as spiders, ticks, mites, scorpions).
All animals are eukaryotes.
All living organisms can be sorted into one of two groups depending on the basic structure of their cells. These two groups are called the prokaryotes and the eukaryotes.
Prokaryotes and eukaryotes differ from one another in many fundamental ways but the differences are difficult to see. Cells are tiny and you must use a microscope to view their internal structure. But once magnified, the differences between prokaryotes and eukaryotes becomes more apparent. The internal structure of a prokaryote cell is less complex than that of a eukaryote. Prokaryotes have no nucleus, eukaryotes have a nucleus. In eukaryotes, the nucleus holds the cell's genetic material within a membrane. In prokaryotes, genetic material is located in a central region of the cell and is called a nucleoid, it is not enclosed in a membrane.
And the differences do not stop at the nucleus. There are numerous other differences between prokaryotes and eukaryotes:
* Eukaryotes have linear strands of genetic material (DNA); prokaryotes have circular DNA.
* Eukaryotic DNA is attached to proteins (called histones) and is organized into chromosomes; prokaryotic DNA is not associated with proteins nor is it organized in chromosomes.
* Eukaryotic cells have numerous membrane-bound structures within the cell called organelles. These organelles perform various functions within the cell. Prokaryotes lack any such membrane-bound structures.
* Eukaryotic cells are more complex, structured, and larger than prokaryotic cells.
Despite these differences, eukaryotes and prokaryotes share many characteristics and are therefore believed to be evolutionarily related. Scientists believe that the more complex eukaryotes evolved from the earlier prokaryotes.
So all animals are eukaryotes. That is, they are made up of complex cell that have membrane-bound nuclei and organelles. Their DNA is linear and organized into chromosomes. Keep in mind though, that the reverse is not true—not all eukaryotes are animals. Plants, fungi, and protists—in addition to animals—are eukaryotes.
All animals are heterotrophs.
All living things need carbon to support the basic processes of life such as growth, development, and reproduction. There are two ways an organism can get the carbon they need:
* An organism can assimilate carbon from their environment in the form of carbon dioxide (from the atmosphere) or inorganic compounds. They can ingest other organisms and thus obtain carbon from the organic materials that make up the other organism. The term autotroph describes such organisms
* An organism can obtain the carbon they need by getting it from other living organisms that are made up of carbon-containing organic molecules. The term heterotroph describes this type of organism, one that uses organic materials as a source of energy for growth and development.
Green plants are autotrophs. They take energy from the sun and use it along with carbon available in the atmosphere in the form of carbon dioxide to produce sugar, a simple organic compound. In contrast, all animals are heterotrophs. They ingest plants and other organisms as a way to get their carbon the energy they need to live.
Animals are multicellular. Most animals (with the exception of sponges) have bodies that are differentiated into tissues.
Animals' bodies are made up of multiple cells. In most animals, those cells are organized into different tissues that perform different functions. During the development of an animal, cells differentiate so they can perform specific functions. Groups of cells with similar specializations that form a common function are referred to as a tissue. One exception to this is sponges (Phylum Porifera) which have do not posses tissues (they don't have muscles or nerves).
Most animals are capable of movement.
Unlike plants, which are fixed to the substrate in which they grow, most animals are motile (capable of movement). One exception is again sponges, which are sedentary organisms.
Most animals undergo sexual reproduction although a small number are capable of asexual reproduction.
Most animals reproduce sexually via egg and sperm. This means genetic information is crossed between individuals and variability between parents and offspring is maintained. There are a few animals that are capable of asexual (non sexual) reproduction. These include .
Most animals are diploid.
The terms diploid and haploid are used to describe how many copies of genetic material are contained within a cell. In diploid cells, there are two full sets of the cell's genetic material, in haploid cells, there is only one full set of the cell's genetic material. Most animals are made up of diploid cells.
Animals do not possess rigid cell walls (plants do have rigid cell walls).
In plants, the cell wall provides structural support and protection for the cell. Some bacteria also possess cell walls. Animals do not have cell walls.
Scientific Classification System
Examples of the Scientific Classification Systems
Kindom |
Animal |
Animal |
Animal |
Animal |
Phylum |
Arthropoda |
Arthropoda |
Chordata |
Mollusca |
Class |
Insecta |
Arachnida |
Mammalia |
Gastropoda |
Order |
Diptera |
Acarina |
Carnivora |
Pulmonata |
Family |
Muscidae |
Ixodidae |
Felidae |
Limacidae |
Genus |
Musca |
Dermacentor |
Felis |
Argiolimax |
Species |
domestica |
variabilis |
domestica |
reticulatus |
Common Name |
(house fly) |
(dog tick) |
(house cat) |
(gray garden slug) |
What is a cell?
Put most simply, a cell is the basic unit of a cellular system, which is kind of backward, since its saying that a cell is part of something made out of cells! Another way of looking at it is as though a cell were a brick, and the body is an enormous wall or a house - made up of cells. Unfortunately this falls down again, because a brick is a relatively simple structure, whereas a cell can be very complicated!
The image represents what could be described as a typical eukaryotic cell, containing the basic features of all cells.
When it is made, every human cell contains its own living instructions, which it received from the cell that it came from - kind of like having a bit of your parents hanging about with you all the time, telling you what to do. This is in the form of DNA that can be found in the nucleus.
What is a prokaryote?
A prokaryote is a kind of really simple cell, because it doesn't have a nucleus, or many of the other membrane-bound organelles which you'll usually find in a cell.
In fact, prokaryotes are usually unicellular - i.e. they exist as just one cell on their own - unlike eukaryotes, which are sometimes unicellular, but often multicellular.
They still have to perform certain functions - including producing proteins - so there are certain key features:
A nucleoid region, containing the DNA needed to make proteins, but importantly not arranged in a nucleus;
Ribosomes, which is where the process of translation takes place;
A membrane, to keep everything together;
There are also features which are exclusive to prokaryotes.
The most important piece of information when considering prokaryotes is that they don't have the membrane-bound organelles you find in eukaryotes. The word pro-karyote literally means 'before the nucleus', and if you subscribe to the theory of evolution, the suggestion is that prokaryotes came about before the nucleus had 'evolved'.
It is generally considered that prokaryotes are bacteria, and that's about it - in actual fact there are other organisms involved, but bacteria make up the greatest proportion of prokaryotic cells.
What is a eukaryote?
Eu-karyote means 'true-nucleus', to the extent that, unlike prokaryotes, eukaryotic cells contain a nucleus. They also contain many other organelles which are not found in the prokaryote.
Eukaryotes can be either multicellular or unicellular - that is, the total number of cells in a particular eukaryote can vary. Humans have millions of cells, because we are relatively large organisms, but there are organisms which are much smaller.
The important feature of a eukaryote is the nucleus which you will invariably find in its cells (though of course humans are eukaryotes, yet their mature red blood cells do not contain a nucleus!) and this is a very important distinction to make.
A typical eukaryotic cell will contain a whole range of organelles, including the ribosomes and membrane found in prokaryotes. They will also have mitochondria, which produce energy in the form of ATP; rough endoplasmic reticulum, which - coated in ribosomes as they are - help to produce proteins; smooth endoplasmic reticulum, which produce certain types of hormones; vesicles, which are involved in import and export of substances; and more!
Importantly eukaryotes have a nucleus and other organelles. They are the cells which you find in plants and animals (including humans!) and their complexity reflects an incredibly efficient, well-structured design.
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