What Is Life? The Seven Characteristics of Life

Biologists have identified seven characteristics that all organisms share.

There are characteristics that all living organisms share. While it may seem obvious that a rock is not alive and a person is, there are questions regarding other things, such as viruses. The following are seven characteristics that are used to define life.

Cells and Organization

Cells are the basic unit of life. The smallest organism consists of one cell. Within that cell is the cellular equivalent of organs, called organelles. Organelles are made of molecules or macromolecules such as carbohydrates, lipids, proteins, and nucleic acids. These organelles serve different functions including transferring ribonucleic acid (RNA), making adenosine triphosphate (ATP) for energy, or transferring molecules in and out of the cell.

Similar cells combine to form tissues, such as the pericardium, the membrane around the heart. Tissues give rise to organs, like the liver, heart, or skin. Unlike the single-celled organism, complex organisms consist of a combination of cells, tissues, and organs.


Organisms must acquire and use energy in order to maintain their complex, living systems. Cells metabolize molecules, such as nutrients, to produce energy and use it for other processes, like replicating deoxyribonucleic acid (DNA).

Some organisms, including plants and some bacteria, can convert light energy to produce nutrients. This is called photosynthesis. Other organisms, such as animals, must consume other organisms to meet their energy needs.

Environmental Adaptation

Organisms must be able to adapt to their changing environments. For example, during the season when the sun is directly overhead, plants will grow upward, but they will grow more toward the side during those seasons when the sun is lower in the sky. Horses shed their coats in the spring and summer to allow for easier cooling during the warmer weather. They acquire a thick coat for the winter months when it is necessary to keep warm air close to the skin.


Organisms must maintain an internal environment that is conducive to cell metabolism. For example, humans must maintain a specific body temperature. When they are cold, they will shiver, a response that causes the muscles to produce more heat. When they are too warm, the body will produce sweat which carries heat away from the body when it evaporates. The degree to which this must be done varies among organisms.

Growth and Development

All organisms undergo a process of growth and development. Single-celled organisms start as smaller cells that grow. The contents of the cell may become more diverse and complex. More complex organisms, such as humans, start out as a single cell that divides to create more cells. These new cells differentiate to develop into specialized cells that form different tissues and organs.


All living organisms have DNA, which is genetic material containing the information and instructions for forming an organism. All organisms die, and must be able to pass this genetic material on to create more organisms. This is done through reproduction. Single-celled organisms may divide in half and grow. Other organisms reproduce sexually, a process that requires another organism with which to share its DNA.


The book, Biology, by Robert J. Brooker, et al., defines biological evolution as “the phenomenon that populations of organisms change over the course of many generations.” This can include the beak length and shape of a particular species of bird that is better suited to acquiring certain kinds of food in a particular environment.

Living organisms share these seven characteristics, but that does not make them the same. The ability of the individual to adapt and the population to evolve allows for the diverse number of species throughout the world. This diversity allows organisms to live within the various environments of the earth.


About Amnesic Shellfish Poisoning

Amnesic shellfish poisoning, also known as ASP, is a rare disease caused by demoic acid that is usually found in razor clams, mussels and dungeness crab but it can be found in other shellfish as well. It is caused by a reddish brown plant that is found in saltwater. On the plant is a diatom called Nitzchia pungens. The toxins from this plant contaminate the shellfish in high doses, and when consumed by humans it can cause serious problems.

ASP was first discovered in 1987. Four people died in Canada after eating shellfish and many others got sick, some of which developed forms of amnesia. In the early 1990’s pelicans on California’s coast near Monterey became crazed and when tested were found to have large amounts of demoic acid in their system. ASP is one of four shellfish syndromes. The other three are Paralytic, Neurologic and Diarrheal.

What Are Diatoms and Domoic Acid?

Diatoms are unicellular organisms that are often found attached to filamentous algae. There are two major groups of diatoms, pennates and centric. Pennates are pen shaped diatoms and centric diatoms are cylinder shaped. Pennates are most likely to be found in freshwater, while the centric can usually be found in marine water. They are basically the brown slimy stuff found on rocks, wood, seaweed etc… that shellfish feed off of.

Domoic acid is a neurotoxin that naturally occurs in marine life and is found in diatoms. The condition caused by this disease is also sometimes called domoic acid poisoning or DAP. DAP can affect all mammals. Domoic acid has been found on the East coast, West coast and in the Gulf of Mexico.

Cause, Symptoms and Treatment of ASP

Shellfish eat the diatoms and during the filtering process retain some of the demoic acid. When there are large levels of algae in the water, shellfish secret less of the toxins, therefore making them more toxic. When humans consume the infected shellfish they get sick. This is a very rare disease, but it can cause:

  • Diarrhea
  • Severe abdominal pain
  • Vomiting
  • Nausea
  • Heart palpitations
  • Severe headaches
  • Permanent short-term memory loss
  • Coma
  • Death

Symptoms usually occur within 24 hours of consumption of infected shellfish. There is no antidote for demoic acid. Patients that develop symptoms after eating shellfish should seek medical attention immediately even though there is very little that can be done.

There is no way to tell if the shellfish is infected with demoic acid. There is no taste or odor associated with the contaminated seafood. State departments of health issue warning when unsafe levels of marine toxins have been found. ASP is usually present in late summer and early fall making that the best time to avoid shellfish consumption if there has been a large algae bloom on the coast.

The Eustachian Tubes: Maintaining Pressure in the Ear

Eustachian tubes, which are also sometimes known as the auditory or pharyngotympanic tubes, are small cartilage and bone canals. The canals connect the middle ear, which is an air-filled cavity in the internal structure of the ear, and the nasopharynx, the lower nasal cavity behind the mouth. This connection allows air movement into and out of the ear, maintaining the pressure inside the ear canal at the external atmospheric pressure.

Ear Pressure and Hearing

The function of the ear is to hear by converting sound waves to electrical impulses. Sound waves travel through the air and into the ear canal, where they proceed through the middle ear to vibrate the eardrum on the far end. The sound waves are then transmitted to the bones and structures of the inner ear, which are then responsible for transmitting the signals to the brain. Maintaining atmospheric pressure within the middle ear allows the sound waves to proceed unchanged.

Eustachian Tubes and Altitude

When the atmospheric pressure changes, there is sometimes a sensation in the ear as the pressure within the Eustachian tubes adjusts. When driving up a mountain or flying in an airplane, a popping sensation is often felt, hearing is impaired for a short period of time, or a popping sound is heard as the pressure adjusts. Swallowing, chewing, or yawning pulls on the neck muscles and can help the Eustachian tubes open, releasing the pressure in the ears.

Ear Drainage and the Nasal Cavity

The Eustachian tubes also allow mucus produced by the lining of the middle ear to drain. The mucus drains from the ear to the nasopharynx and then to the stomach, similar to the sinuses. The mucus is present to protect the ear from foreign material, similar to mucus in the nose and throat. Drainage prevents the ear from becoming clogged with this helpful material, which can affect hearing and increase the risk of ear infections.

In the case of respiratory illness, material can make its way from the nasal cavity to the ear through the auditory tubes. Blockage of the tube or tubes from a throat infection can result in an ear ache and potential spread of the infection to the inner ear.

Eustachian Tube Problems in Children

Children often have a more horizontally placed tube and may experience blocked drainage due to the anatomic position, resulting in a greater occurrence of ear infections. Doctors sometimes insert synthetic tubes to help keep the auditory tubes open and facilitate appropriate air and fluid exchange.

What Eustachian Tubes Do Not Do

The Eustachian tubes do not maintain balance per se; this is accomplished by hair cells in the inner ear. The pressure in the ear, though, can affect the function of the ear structures, so the auditory tubes play a role in maintaining the integrity of the system that maintains balance. Ear infections are known to affect equilibrium because of their clogging effect in the ear.

Similarly, swimmer’s ear is mistakenly attributed to problems with the Eustachian tubes. Water can become trapped in the ear canal and lead to an ear ache and dulled hearing, followed by inflammation and infection. The blockage prevents proper air movement to the ear drum, but is not a clogging of the auditory tubes.

Functions and Structure of Ribosomes: Small Organelles that Carry Out the Process of Translation

Ribosomes are small organelles made of RNA and protein that carry out the important work of translating mRNA templates into proteins.

Ribosome Structure – Subunits of RNA and Protein

Ribosomes in eukaryotes are made up of two subunits, a large subunit, called 60-S, and a small subunit, named 40-S. In prokaryotes, the subunits are 50-S and 30-S.

These two subunits are made in the nucleus and join together in the cytoplasm to create the ribosome whenever mRNA is present and proteins need to be made. The two subunits join together, hook onto the mRNA and start protein synthesis. During the production of proteins, the larger subunit binds to tRNA and amino acids and the small subunit binds to the mRNA template. When the ribosome finishes reading the mRNA and making the protein, the two subunits break apart again.

Function of Ribosomes – Protein Construction

The function of ribosomes is to make proteins in a process called protein synthesis. The ribosomes combine amino acids, the building blocks of proteins, in the order specified by a messenger RNA (mRNA) template.

As the ribosome moves along the mRNA and reads the sequence, amino acids are attached to and organized by transfer RNA (tRNA), a special type of RNA that can bind to both the ribosome and amino acids.

Location of Ribosomes in the Cell – Free and Attached Ribosomes

Ribosomes come in two types, free ribosomes and membrane bound ribosomes, which can be found in different places within the cell and carry out slightly different versions of protein synthesis.

Free ribosomes are found floating in the cytosol of the cell, the liquid that fills the cell interior. Free ribosomes can move around in the cytosol and they generally make proteins for use inside the cell.

Membrane bound ribosomes, also called attached ribosomes, attach to the endoplasmic reticulum, creating rough endoplasmic reticulum, RER. These rough ER ribosomes make proteins that will be exported for use outside the cell or used in cell membranes. The proteins generated by ribosomes in the rough ER travel into the ER and are then packaged for transport to the plasma membrane to be incorporated there or sent outside the cell.

Ribosomes are capable of chaning between one type and another. Free ribosomes can become membrane-bound ribosomes and vice-versa depending on what the cell needs at any given time.

It is important for students of biology to understand the function and structure of ribosomes, since these important organelles carry out the steps of protein synthesis that create all of the proteins in the body. Without ribosomes, there would be no protein construction and no work could ever get done inside or out of the cell.

Small Sand Dwellers and Filter Feeders: Eight Unusual Animal Phyla Recognised by Specialists

Large filter feeders (some whales, bivalve molluscs, and barnacles for example) are well known, but the smaller ones are often obscure. Life between grains of sand or mud implies tiny size, which means that these animals are also unfamiliar – rarely seen without the aid of a microscope.

Rotifers, Micrognathozoa, and Gnathostomulids

  • The 100 or so species in the phylum Gnathostomulida are small marine animals. Most are less than 1 mm long, and they scrape food off sand or mud grains, especially in places where there is little oxygen. Gnathostomulid mouthparts suggest that they are related to the rotifers.
  • Micrognathozoa have their own phylum – even though only one species is known so far. Limnognathia maerski was discovered in Greenland in 1994, living in spring water. The animal is minute, less than one tenth of a millimeter long, and therefore one of the smallest known. Very little is known about this phylum, but it is thought to be close to the Rotifera.
  • Rotifers (Phylum Rotifera) are common in fresh water. There are over 2,000 described species of rotifer, and they were discovered early in the dawn of microscopy (by John Harris in 1696). They are able to survive long periods dehydrated, rather like the tardigrades, and this explains how they can get from pond to pond – as bits of ‘dust’ blown in on the wind. Most are less than a millimeter long, and they feed by filtering small particles (fish waste, dead bacteria, algae etc.) at the prodigious rate of 100,000 times their own volume per hour. They are ecologically important for this reason – helping to keep fresh water clean.

(Look at a ‘Gallery of Rotifer Images’).

Gastrotrichs and Kinorhynchs

  • Gastrotrichs are very small and short-lived (most only living for a few days). They typically live between sand grains as part of the ‘meiofauna’. There are about 700 species in this phylum, and it is not at all clear which other phyla are closest to them – body form suggests one set of possibilities, genetic studies another.
  • The 150 or so kinorhynchs, or ‘Mud Dragons’, also live in mud and sand, where they eat diatoms. Like the gastrotrichs it is unclear how this phylum fits in, in the evolutionary sense.

Brachiopods, Bryozoans, and Xenoturbellids

  • Brachiopods are often large animals with two shell valves, much like bivalve molluscs, but they are not closely related to them (or any of the Phylum Mollusca). 99% of all brachiopods are extinct, and those that remain are sessile filter feeders. They usually attach to the substrate by means of a long stalk.
  • Bryozoans (or ectoprocts), with over 8,000 living species, usually built tough calcareous casings for their colonies. This group of small individuals is often called a ‘sea mat’, and it is permanently attached to a rock or large plant. The individuals filter seawater. It is thought that the bryozoans are most closely related to the brachiopods.
  • The phylum Xenoturbellida consists of two known species. They are worm-like and were once thought to be related to the molluscs (because molecular studies indicated the presence of molluscan DNA), but it is now thought that this DNA gets into the animals because they either eat, or parasitise, molluscs. They have a very simple body plan, and very little is known about them.

The animals in seven of these phyla are too small for most people to notice, and the brachiopods (the eighth phylum) will normally be encountered as fossils. As fossils the brachiopods can easily be mistaken for bivalve molluscs.