Characteristics of Living Organisms — Edexcel Linear - OL Revision Notes & Topical Questions

To be classed as living, an organism must carry out all the life processes, remembered by MRS C GREN: Movement, Respiration, Sensitivity, Control, Growth, Reproduction, Excretion, Nutrition. If something does not carry out all of these it is dead or non-living; viruses are good examples of non-living particles.

Organisms must obtain food for energy and materials for growth. Plants make their own food by photosynthesis using sunlight, carbon dioxide and water, producing glucose and oxygen. Animals (and fungi) consume food; they digest large complex molecules into simpler ones, which are absorbed into the blood and carried to the cells for energy, growth and repair.

Respiration is a chemical process in all living organisms that releases energy from glucose, either in the presence of oxygen (aerobic) or in its absence (anaerobic).

Chemical reactions inside cells are metabolic reactions; they produce waste products, some toxic, which must be removed. Excretion is the removal of toxic materials and waste products of metabolism.

Egestion is different: faeces are the remains of food that was never absorbed, not a product of cell reactions.

Sensitivity is the ability to detect and respond to stimuli, which gives the best chances of survival.

Movement is an action causing a change of position or place; movement from place to place is locomotion. Plants cannot move from place to place but can change orientation, as sunflowers do tracking the sun.

Control means keeping the internal environment within limits (homeostasis). Thermoregulation holds human body temperature at about 37 °C (sweating, vasodilation); osmoregulation controls water levels; plants use transpiration to cool themselves, losing heat as water evaporates from stomata.

Reproduction produces more of the same kind of organism and is vital for the survival of the species.

The eight characteristics depend on one another.

Asexual reproduction in bacteria involves creating exact copies of the parent cell; only one parent is needed, so the offspring are clones. Growth is a permanent increase in size. In animals an individual grows larger between the zygote and adult stage with changes in proportion or shape; in plants an individual keeps growing throughout life as new shoots, leaves and branches form year after year.

All living organisms can be grouped by shared features into plants, animals, fungi, protoctists and prokaryotes. The first four are eukaryotes: their cells have a distinct membrane-bound nucleus, and they may be single-celled or multicellular. Prokaryotes are always single-celled with no nucleus; their DNA is a single circular chromosome in the cytoplasm and their cells are much smaller.

Be ready to recall these features and complete a comparison table for three groups. Fungi are usually multicellular (Mucor) but some are single-celled (yeast); multicellular fungi are made of hyphae with many nuclei organised into a mycelium. Examples are moulds, mushrooms and yeasts.

The protoctists are a very diverse group whose features differ from animals, plants and fungi (examples: amoeba, Paramecium, Plasmodium, Chlorella).

Asexual reproduction in bacteria is fastunder good conditions a population can double about every twenty minutes, producing a large number of identical cells.

When answering, remember the orderstate the group, state the feature, then give the example, such as “fungi have chitin cell walls and feed on dead organic matter, for example mushrooms and moulds.” Take care not to mix fungi and plants, as both have cell walls but the materials are different.

A pathogen is an organism (or infectious agent) that causes disease in another organism, its host. Groups of organisms that include pathogens are bacteria, fungi, protoctists and viruses. Most members of these groups do not cause disease; the term applies only to those that do. Match each named disease to its pathogen group rather than assuming every infection is bacterial.

Viruses are non-living particles; they cannot carry out all the life processes. The features of viruses are:

Because they have no cytoplasm or membranes, viruses cannot respire, feed or grow by themselves. To reproduce, a virus enters a host cell and uses the host’s machinery to make new virus particles.

A pathogen causes disease by invading the host and reproducing, then damaging cells or releasing toxins.

Prevention depends on the pathogenhygiene and clean water limit bacterial and protoctist infections, controlling mosquitoes limits malaria, and vaccination protects against many viral diseases.

In a data question you may be given a list of features and asked to decide whether the infectious agent is a bacterium or a virus.