Organisms have the potential to carry out the life processes of nutrition, movement, growth, reproduction, respiration, sensitivity and excretion

The following characteristics are those that most biologists accept as being common to all living things. It is true that they may not always be displayed but even the most inactive of organisms has the potential to carry out all these functions. It is equally true that there are times in the life cycles of some organisms where all these functions appear to be suspended as is the case with seed producing organisms (Lotus seeds have been grown after being stored for 160 years).

Movement

Living things move in a directed and controlled way, moving of their own accord. Non-living things only move if they are pushed or pulled by something else. The majority of animals usually move their whole bodies often supported by specialized organs such as fins, wings and legs. These are called locomotors organs moving the animal from place to place.

Plant movement is not locomotors and does not generally involve moving the whole body. Leaves turning towards the light or shoots growing upwards whatever the orientation of the rest of the plant are examples of how plants move. These movements are generally very slow and not always obvious.

Growth

Living things grow. Most animals grow until they reach maturity and then remain at a constant size while plants usually continue to increase in size throughout their life span. It is important to recognize that growth is a permanent increase in measurable features such as volume, mass and length. Cells increase in number by dividing in a process called mitosis (making genetically exact copies). As the soft tissues increase, so there will be associated increase in size of skeletal support tissue such as bone, shell and wood.

When maturity is reached in animal’s cell division continues only at a level to maintain consistent size and to repair loss through damage. Putting on weight as a result of over-eating is not considered to be biological growth in this context

Reproduction

Living things are able to reproduce themselves. If organisms fail to do this, populations will diminish and disappear as their members die from old age, disease, accidents, predation, etc. It is a fundamental law of biology that other living things can only produce living things; every living organism owes its existence to the reproductive activities of other organisms.

This is contrary to the misconceived ideas of spontaneous generation, which some people held in the past. The notion that cockroaches were formed out of crumbs on the bakery floor, that mould was formed out of decaying bread and that rotting sacks of grain turned into mice are examples of how spontaneous generation was thought to operate. Today, these ideas are discredited but they still often provide the stimulus for works of dramatic fiction.

Respiration

Living things respire. Respiration is a complex sequence of chemical reactions, which result in the release of energy from food. There are two types of respiratory process.

Aerobic respiration

Carried out by the vast majority of organisms, this involves oxygen. The by-products of the reaction are water and carbon dioxide both of which are eliminated as waste products. Oxygen is obtained from the air or water using organs designed to optimize gaseous exchange. These include the stomata in plants (small, size regulated pores), spiracles in arthropods, gills in fish and lungs in mammals. The uptake of oxygen and simultaneous elimination of carbon dioxide and water is commonly referred to as breathing. It is important to distinguish between breathing and respiration. It is tempting; particularly with younger children to use the well used term breathing as an all-embracing description of the respiratory process. However, this is not correct and could lead to the reinforcement of misconceptions.

Anaerobic respiration

When oxygen levels are at a low level, it is possible for some simpler organisms and parts of more complex ones to release energy from food without oxygen. This is a far less efficient process but a necessary alternative in some cases. The by-products of anaerobic respiration are different to aerobic. In humans, oxygen starved muscle cells will respire anaerobically under stress such as heavy physical activity. The by-product of this is lactic acid and it is this that causes the puffed out feeling. Yeast cells respire anaerobically in sugar solution producing alcohol as the by-product

Sensitivity

Living things are sensitive to their environment. This means that they detect and respond to events in the world around them. Simple uni-cellular organisms such as Amoeba have limited sensitivity, while higher organisms such as mammals are more sensitive and can react to very small changes in light, sound, touch, taste, smell, temperature, etc.

In higher animals specific organs are developed for the purpose of detecting stimuli. The organization of light sensitive cells into eyes of varying complexity from one species to another is an example.

Plants do not have sensory organs as such but there are clearly certain regions of their bodies such as the shoot tip that are sensitive to light, gravity, water and various chemicals.

Excretion

Living things excrete. Excretion is the removal from the body of waste products which result from normal life processes. Waste products such as carbon dioxide must be removed. If they are allowed to accumulate they cause poisoning which slows down vital chemical reactions. When excretory organs such as kidneys are damaged, the organism quickly displays symptoms of poisoning and death is rapid unless treated.

Excretion should not be confused with egestion, which is the removal from the body of substances with no food value that have passed unused through the digestive systems.

Feeding

Living things feed. Food is the material from which organisms through respiration obtain the energy required to sustain life and carry out all the other defining functions of living things. Food also provides the raw materials for growth and repair. The study of food and feeding is called nutrition.

There are two types of nutrition:

Autotrophic organisms make their own food by a process called photosynthesis. Green plants, for example, manufacture sugar and starch from carbon dioxide and water using the energy of sunlight to drive the necessary chemical reactions

Heterotrophic nutrition
Heterotrophic organisms obtain their food from the bodies of other organisms.

This is done in various ways.

Herbivores such as cattle, tortoises and sparrows eat plants.
Carnivores such as lions, crocodiles, sharks and kestrels eat the flesh of other animals.

Omnivores such as humans can eat both plants and animals.

Saprophytes such as many types of fungi and bacteria, obtain their food in liquid form from the remains of dead organisms. This feeding manifests itself as the process called decay.

Parasites such as tapeworms and mosquitoes live on or in another living organism (called the host) from which they obtain food.