Living organism :

.An individual living thing, such as an animal or a plant , is called an organism. The term ‘living organism’ is usually used to describe something which displays all the characteristics of living things.

Characteristics of living things:

There are seven activities which make organisms different from non-living things. These are the seven characteristics of living organisms.

1 Nutrition:

Living things take in materials from their surroundings that they use for growth or to provide energy. Nutrition is the process by which organisms obtain energy and raw materials from nutrients such as proteins, carbohydrates and fats.

2 Respiration:

Respiration is the release of energy from food substances in all living cells. Living things break down food within their cells to release energy for carrying out the following processes.

3 Movement

All living things move. It is very obvious that a leopard moves but what about the thorn tree it sits in? Plants too move in various different ways. The movement may be so slow that it is very difficult to see.

4 Excretion

All living things excrete. As a result of the many chemical reactions occurring in cells, they have to get rid of waste products which might poison the cells. Excretion is defined as the removal of toxic materials, the waste products of metabolism and substances in excess from the body of an organism.

5 Growth:

Growth is seen in all living things. It involves using food to produce new cells. The permanent increase in cell number and size is called growth.

6 Reproduction :

All living organisms have the ability to produce offspring.

7 Sensitivity :

All living things are able to sense and respond to stimuli around them such as light, temperature, water, gravity and chemical substances.

Variety of living organisms:

Living organisms share characteristics such as the ability to move and reproduce. There are different types of living organisms including plants, animals, fungi, bacteria and protoctists.

Animals

Animals are multicellular organisms – they consist of many cells that work together. Examples of animals include mammals (such as humans) and insects (such as houseflies and mosquitoes).

Animal cell structure

The main parts of an animal cell are the nucleus, cell membrane and cytoplasm

Animal cells:

do not have cell walls
do not contain chloroplasts, so animals cannot carry out photosynthesis
may store carbohydrate as glycogen

Animals usually have nerves or nervous systems for coordination, and they are able to move from place to place.

Plants

Plants are also multicellular organisms – they consist of many cells that work together. Examples include cereals (such as maize), and peas and beans.

Plant cell structure

Plant cells contain the same parts as animal cells. They also have some additional ones:

chloroplasts
cell wall made of cellulose
permanent vacuole

Other features of plants

Plant cells contain chloroplasts so plants can carry out photosynthesis. They store carbohydrates as starch or sucrose.

Fungi

Mushrooms, toadstools and moulds (such as Mucor) are multicellular fungi. Yeast is an example of a single-celled fungus.

Fungal cell structure

Fungal cells have a cell wall made of chitin (remember that plant cell walls are made of cellulose). A yeast cell

Athlete’s foot, caused by a fungus

Some fungi are pathogens, for example the fungal infection which causes athlete’s foot.

Fungal structure

A multicellular fungus showing rounded spore cases and spores and thread-like hyphae

Multicellular fungi, such as Mucor, are organised into a mycelium - which is made from thread-like structures called hyphae.

The hyphae contain many nuclei.

Fungal nutrition

Fungi cannot carry out photosynthesis. Instead they use saprotrophic nutrition. They secrete enzymes onto their food so that digestion happens outside the fungal cells. They then absorb the digested organic products.

Fungal cells may store carbohydrate as glycogen (remember that plant cells store carbohydrate as starch).

Bacteria

Rod-shaped Escherichia coli (E.coli) bacteria, commonly found in the intestines of animals

Bacteria are microscopic single-celled organisms. Examples of bacteria include:

Lactobacillus bulgaricus (a rod-shaped bacterium used to make yoghurt from milk)
Pneumococcus (a spherical bacterium that acts as the pathogen that causes pneumonia)

Bacterial cell structure

Bacterial cells have a cell wall made of polysaccharides and proteins. They do not have a nucleus, but instead they have a circular chromosome of DNA. They may also have small extra circles of DNA called plasmids.

Other features of bacteria

Some bacteria can carry out photosynthesis, but most bacteria feed from other organisms (living or dead).

Some bacteria are pathogens, for example Pneumococcus (which causes pneumonia).

Protoctists

Protoctists are microscopic single-celled organisms.

Some protoctists, such as Amoeba, have features like an animal cell. Others, such as Chlorella, have chloroplasts and are more like plants.

Some protoctists are pathogens. For example, Plasmodiumis, the pathogen that causes malaria.

Viruses

Viruses are very small particles capable of infecting every type of living organism. They are parasitic and can only reproduce inside living cells. For example:

the tobacco mosaic virus – this stops chloroplasts forming in tobacco plants and causes the tobacco leaves to become discoloured
the influenza virus – this causes flu
HIV (human immunodeficiency virus) – this causes AIDS

Virus structure

Virus particles have a variety of shapes. They do not have a cellular structure. Instead, they have a core of genetic material surrounded by a protein coat. Their genetic material can be DNA or RNA, but not both.