In the immediate environment we live, we can easily identify living and non-living things. Very common living things include such microscopic forms as bacteria, amoeba, hydra, spirogyra and chlamydomonas. Larger animals such as snails, various forms of insects, fish, goats, man and trees of various sizes are also included. These living things share some common characteristics that relate them intimately with one another and to their
In the same manner, the environment also has non-living things.
The characteristics common to living things which distinguish them from non-living things.
Living things move. While movement is so obvious in animals as they move their bodies from one place to another (locomotion), probably in search of food, shelter, or mates, or even as a means of escape, defence and offence, at other times, movement in animals may only involve certain body parts e.g. churning of bowels, knee jerks, etc. Part movement of the body is also seen in sedentary animals e.g. hydra. In plants, however, movement is restricted to certain body parts such as the shoot and roots. Such movement is usually slow, and often in response to certain stimuli such as light, water, gravity or even chemicals.
The difference in movement between plants and animals is due to the fact that animals have an organised nervous system and can transmit impulses in a defined order to bring about certain actions. Plants on the other hand, depend only on their auxins/hormones to produce certain movements which are often slow.
Organisms feed for the purpose of generating energy to carry out all biological activities, for the replacement of worn-out body tissues (repair services) and for growth.
Green plants produce their food through the process of photosynthesis (see equation below). By this process, plants absorb simple inorganic substances such as water and carbon (IV) oxide or carbon dioxide, and in the presence of sunlight (or any other sources of light) to manufacture large complex food molecules called carbohydrates. Other food substances are built up by incorporation of dissolved minerals into carbohydrate molecules. The food manufactured is always in excess and usually stored.
(The process of photosynthesis, the factors responsible for it, and its significance shall be considered in more details later.)
Since this mode of nutrition is restrained to green plants only, it is described as holophytic (Greek; Holos, Wholy, phyton, plant), or autotrophic.
Animals, on the other hand, cannot manufacture their food, but have to depend directly or indirectly on plants for their food. This form of nutrition is described as holozoic (Greek, Holos, Wholy, zoon, animals), or heterotrophic.
Generally, animals take in complex food substances, often in solid form, which are later broken into simpler forms during the process of digestion.
For their daily bodily activities, living things require energy. This is obtained during respiration, a process by which the food consumed by animals is broken down in oxygen to release energy.
In some organisms, particularly the simple forms, oxygen simply diffuses through their body surfaces. The higher organisms on the other hand have specialised structures that handle oxygen that enters the body, the carbon dioxide and water vapour that arise from tissue respiration. This process may be summarised as follows:
The reactants here are: carbohydrate food, oxygen, while the products are carbon dioxide, water and energy. The details of this process shall be studied later.
Simply defined, growth is an increase in size, volume or complexity. AII living things grow. It results from the food eaten by living things and also from the repairs of worn-out body tissues. It also results from cell multiplication in multicellular organisms.
In animals, growth takes place in all parts of the body and is said to be intercellular, plants grow at the root and shoot apex (apices). Plant growth arises from division of cells of their apical meristem.
This kind of growth is called apical growth. When plants grow in girth, the form of growth is described as secondary growth. Apart from growth being a measure of physiological activity of the organism, growth rings have been used to determine the age of certain organisms, e.g. snails, fishes and plants.
As a result of the numerous biochemical (building up and breaking down) processes, that occur within the bodies of living things, waste substances are produced. These include both liquids, semi-liquids and gases which if allowed to accumulate, will poison or even kill the organism. The waste, therefore, is either stored away in harmless form, or eliminated from the body through specialised structures in a process called excretion.
Examples of excretory products include carbon (iv) oxide, water vapour, ammonia, urea and sweat, while excretory surfaces include the lungs, skin, liver, gills, leaf surface.
Irritability/response to stimulus
The survival of an organism depends on its ability to respond to all changes around it. The changes/ stimuli include: touch, pain, light, heat, chemical, cold, smell and sound.
The complexity of responses to change around the organism varies as we move from the smallest and simplest organisms to the largest and more complex forms. Moreover, animal responses to changes are more precise, predictable and rapid, while that of plants is rather slow, prolonged and directional.
When they mature, organisms reproduce their own kind. The new offspring usually resemble their parents. Sometimes, reproduction involves two different parents. These parents produce specialised sex cells which fuse to form new individuals. This is sexual reproduction.
On the other hand, single parents may just divide several times to give rise to new individuals, without involving any specialised sex cells. This is asexual reproduction. Reproduction is an important phenomenon.
It ensures the continuity of the species. It enhances survival, and produces the medium through which the characteristics of parents are transferred from one generation to another.
Ageing and death
Ageing is the physical and chemical changes that occur in living organisms, which become noticeable with time. These changes generally slow down the overall metabolism of the organism concerned. For example, the active life of the organism is slowed down/hampered, rate of food capture or even the utilisation of food is slow, circulation is slow, growth ceases, leading to the cessation of life, and the organism dies.
An examination of non-living things shows that they do not exhibit the characteristics listed and discussed above for living organisms. Non-living things may perform a few, but not all the characteristics listed above. For example, viruses tend to be the link between living and non-living things. While they can reproduce, within a suitable medium, they cannot carry out any function of life.
Differences between plants and animals
Having treated the characteristics of living things earlier, the differences between plants and animals become fairly obvious as discussed below:
(i) Appearance and Structure: The appearance and structure of plants follow a well defined pattern. There is a shoot and root system of variable forms, with cells that have a rigid, non-living cellulose cell walls. Plants have large vacuoles usually found at the center of the cell. Animals on the other hand have compact bodies and more or less invariable body forms. Animal cells are generally smaller in size, more numerous and are usually enclosed in a delicate cell membrane. Animals have fewer or no vacuoles that are usually in the cytoplasm.
(ii) Body organs: While most plant organs are externally located, those of animals are fewer and located internally within the viscera, where they are protected from the fluctuating influences of pressure.
(iii) Movement: Most plants are firmly attached to the soil and show movement only at certain parts (shoot and roots) in the form of growth. However, plant-like organisms e.g. euglena and chlamydomonas which have flagellum, show active movement in aqueous environments. Generally, however, animals are mobile and move from one place to another. They possess locomotory structures for such purpose, for
example, pseudopodia in amoeba, cilia in paramecium, tentacles in hydra.
(iv) Feeding/nutrition: Plants engage in self-nourishment, utilising simple inorganic substances of the environment, such as carbon dioxide, water, dissolved minerals, chlorophyll, solar energy to manufacture complex food substances in the process of photosynthesis. Plants are, therefore, holophytic/autotrophic in nature. During this process, plants release oxygen to the environment. Animals are themselves, non self-nourishing and depend directly or indirectly on plants for their food. Since animals do not contain chlorophyll, they cannot photosynthesise. They are, therefore, holozoic in nature.
(v) Growth: Plant growth is external and usually restricted to the shoot and root apexes and as such described as apical growth. Plant growth continues throughout life. Animal growth on the other hand is extended to all parts of the body and is said to be intercellular. Growth in animals is limited.
(vi) Excretion: The main waste products in plants are water, carbon dioxide and oxygen (arising from photosynthesis); usually. These products (except oxygen) are stored away in certain cells until the plant dies. In animals however, waste products include water, carbon dioxide and ammonia. These are removed from the body through special excretory organs.
(vii) Response /irritability/sensitivity: Plant reaction to stimuli is slow, usually spreading over a long time no matter the intensity of the stimulus. This may be explained by the fact that plants do not have a nervous system, but depend mostly on hormones for their reactivity. Animals on the other hand have either the diffused or elaborate nervous system with specialised sense organs or organelles. Their reaction to stimuli is, therefore, quick and sometimes automatic.
(viii) Respiration: Both plants and animals respire both day and night, taking in oxygen and giving out carbon dioxide and water vapour. Plants, however, take in carbon dioxide in the day time and give out oxygen to complement the process of photosynthesis.
The place of euglena
We cannot complete this section without looking at euglena, which is an intermediate organism sharing both plant and animal characteristics.
(i) Possession of chlorophyll (chloroplast) and hence photosynthesise under light. Its feeding is, therefore, holophytic.
(ii) Pyrenoids are present. These are storage sites for starch like carbohydrates.
(iii) Paramylum is present. (Paramylum is starch or food, stored as starch granules.)
The animal characteristics
(i) Possession of a flexible and easily deformable outer skin membrane called pellicle.
(ii) There is an eye spot(stigma) that makes it sensitive to light.
(iii) Has a funnel-like gullet/pharynx that leads to a reservoir for taking in and storing food respectively.
(iv) There is a flagellum used for movement.
(v) To assist movement also are myomeres or myonemes
(vi) Contractile vacuole is also present and takes care of the regulation of the internal environment.
(vii) In the absence of light, the organism is holozoic.
These combined characteristics place euglena in a unique group, the Protista alongside other unicellular organism of its nature.
Classification of living things
Classification of living organisms into various groups or taxonomy, constitutes the science of taxonomy or systematics. The science developed out of the need to classify in an easily recognisable manner, known numerous diverse an complex living organisms. Classification is simply based on an arrangement of living things into groups of varying degrees for the purpose of easy recognition. The principles of grouping organisms take into consideration, recognisable structural characteristics and complexities common to them, natural relationships, methods of obtaining food, and their responses to various stimuli.
The scheme of classification that is in common use is the one developed by Carolus Linnaeus, actual name: Carl Von Linne(1707- 1778) a Swedish naturalist of the 18th century. By this scheme, all living organisms may be conveniently placed into two kingdoms: plant and animal kingdoms, which may further be split into major sub-groups called phyla (singular: phylum), while phyla themselves are divided into smaller sub-groups as classes, then order, then families, then general and finally species. Each of these sub-groups contains progressively fewer and fewer kinds of organism, less complex than the former. The taxonomic levels may be summarised as shown below:
For plants, however, the sub-groups, phylum may be substituted with the alternative of division. The classification of plants is also based on their structures, complexity and life histories. The following examples may be useful: Taxonomic classification of man:
Kingdom – Animalia
Phylum – Chordata
Sub-phylum – Vertebrate
Class – Mammalia
Order – Primates
Family – Hominidae
General – Homo
Species – Sapiens
The scientific name for man is Homo Sapiens. Taxonomic classification of rice:
Kingdom – Plantae
Division (phylum) – Tracheophyta
Class – Angiospermae
Order – Graminales
Family – Graminacea
General – Oriza
Species – Sativom
The scientific name for rice is Oriza sativum.
At a glance, the reasons for the classification of plants and animals into their various groups may be summarised as follows:
(i) Classification provides ready information about the major groups of each.
(ii) It is easy to deduce evolutionary relationships between the organisms.
(iii) Today, classification of living organisms into their respective groups constitutes the branch of biology called taxonomy or systematics.
(iv) It is also easy to show at a glance, the unity in diversities of living organisms. At a glance, the main groups of living organisms and the main features of each group may be presented as follows:
a. Plant classification: Plants include both minute unicellular, simple multicellular, higher and complex plants.
b. Schizophyta: These are bacteria, they are non-green unicellular forms. They lack a nucleus and reproduce rapidly by asexual means. These organisms considered by some authorities as too primitive to include in Algae.
c. Thallophyta: Plants in this group have simple bodies; they lack roots, stems and leaves and may reproduce both by asexual and sexual means.
d. Algae: May be green, red, brown, blue green or golden in colour; generally, green without differentiation into roots, stems and leaves. They are mostly found in water. Examples include spirogyra, chlamydomonas, etc.
e. Fungi: Simple, non-green plants, non-photosynthetic; consist of strands. Some are parasite and could cause various diseases. Examples include mucor, mushroom, toadstool, yeast, etc. Fungi are non-photosynthetic as they lack chlorophyll.
Algae and fungi may co-exist to form lichens.
Bryophyta: These are small, multicellular, non-vascular and 6’on-flowering plants. They reproduce by spores and are found in damp places. Examples include mosses and liverworts.
Pteridophyta: These are non-flowering, but vascular plants with true roots, stems and frond-like leaves. Their reproductive spores are found under leaf surface. They occur commonly in moist places. Examples include various ferns and horsetails.
Spermatophyta: These are seed producing plants: They are green and vascular. They exist as:
a. Gymnosperms (Conifers): Generally large, non-flowering vascular plants with cones. They have true roots, stems and needle shaped or scale-like leaves. They withstand both dry and cold climates. Examples include pines and cedar.
b. Angiosperms: Are flowering plants of wide range, ranging from small herbs to very massive trees with well developed roots, stems and leaves.
Angiosperms are of two types including:
(i) Monocotyledons: Plants with one seed leaf, scattered vascular bundles, and petals arranged in groups or multiples of three. Veins on leaves are parallel. Roots are fibrous, e.g. oil palm trees, coconut, grasses of various types and pineapple.
(ii) Dicotyledons: These are plants with two seed leaves; vascular bundles are arranged. The petals are arranged in groups or multiples of four or five. Veins on leaves are reticulate (branched in a net like manner). These plants have tap-roots with other massive tuberous roots in larger forms. Examples include mango, hibiscus, kola plant, beans, melon, etc.
For the same purposes listed above, animals are also classified into related groups according to similarities in structure and complexities. The kingdom animalia may be broadly classified into two groups. These are the invertebrates and the vertebrates.
These are those animals that do not have a backbone (vertebral column). About 95 percent of all known animals are invertebrates and are simple, unicellular or multicellular forms.
Invertebrate phyla include the following:
(i) Protozoa: These are simple, microscopic and unicellular animals. They live freely in water or as parasites in other organisms. Examples include amoeba, paramecium, plasmodium, etc.
(ii) Coelenterata: These are simple many-celled (multicellular) animals with two body layers and tentacles with sting cells (nematocysts). The sting cells are used for food capture and defence. Animals in this group are aquatic and exhibit bilateral symmetry. Examples include hydra, sea anemones, jelly fish.
(iii) Platyhelminthes: These are elongated, thin and flat bodied animals. They have one body opening, (the mouth) through which both food and waste pass. Their bodies are bilaterally symmetrical. They are free living or parasitic and occur as hermaphrodites. Examples include the tapeworm (Tenea sp), liver flukes (Facial hepatica), blood flukes (Sebi stosoma sp).
(iv) Nematoda: These are animals with long, round or cylindrical bodies, pointed at both ends. A mouth and an anus are present. The sexes are separate and occur as free living or parasites. Examples include Ascaris sp, hook worm, guinea worm and filaria worm.
(v) Annelida: These are the true worms with elongated, cylindrical, ringed or segmented bodies with three body layers (triploblastic). Annelids have two body openings: a mouth and anus. The excretory, nervous, reproductive and circulatory systems are well developed. Examples include the earthworm, leeches, ringworm, tube worms, etc.
(vi) Mollusca: Are the soft-bodied animals, usually covered with a calcareous external skeleton (shell). In some, however, the shell is internal. Astout and fleshy muscular foot is usually present with a head, bearing tentacles. There are two body openings, a mouth and an anus. Most mollusca respire by means of a highly vascular mantle cavity. This range of animals are of high economic importance and include snails, clams, slugs, octopuses, squids, etc.
(vii) Arthropoda (derived from Greek): Arthron, meaning jointed and podos, meaning legged, the arthropods are animals with jointed appendages, a segmented body and a covering of skeleton. Due to their variable body forms and sizes, and other forms of adaptations, the arthropods are the most successful animals with the widest distribution on earth. Examples include insects, millipedes, centipedes, crayfish, crabs, spiders, etc.
(viii) Echinodermata: The echinoderms are completely marine animals with bilateral symmetry at the larval stage, but are radially symmetrical at the adult stage. They have a water vascular system, tube feet with suckers on their tips for movement and attachment. Most forms have a calcareous exoskeleton. Examples include star fish, sea urchin, sea cucumber, brittle star, sea lily, etc.
Vertebrates (Phylum Chordata)
These are the animals with a vertebral column (back bone) and jointed skeleton. They are bilaterally symmetrical, and include some of the most advanced animals. Their only phylum, chordata, may be subdivided into the following classes:
(i) Pisces: These are fishes, by far the most primitive chordates whose lives are restricted to water. Their bodies are covered with scales. They move by means of fins, and possess a lateral line for the detection of vibration in water. They breathe by means of gills, while their shapes are streamlined, to ease movement in water. Fishes have the same set of teeth (homodont) and are cold-blooded poikiiotherms. Fishes are either bony or cartilaginous, and include such examples as tilapia, cat fish, mudfish and dog fish, ray fish (trigon) and sharks. For ease of further identification, bony fishes have opercula (a fold of tissue covering gill slits), terminally located mouth and eyelid scales, whereas, cartilaginous fishes have a ventral mouth and placoid scales. Opercula are absent.
(ii) Amphibia: These are cold-blooded poikilothermal tetrapods that live on both land and water; on land for their active life and in water for reproduction. The eggs develop into fish-like tadpoles that breathe by means of gills, while adults breathe through lungs. Amphibians have soft moist skins without scales and, like fishes, have homodont dentition. Under their skin are poison glands, that aid body defence. Examples include: toads, frog, newts, salamanders, etc.
(iii) Reptilia: These are cold-blooded tetrapods with dry, waterproof skins covered with scales or bony plates. They live mainly on land, although, some of them live in water. They breathe by means of lungs, while fertilisation is internal. The eggs are large, leathery (or soft-shelled) and cleidoics i.e., enclosed with a tough shell protecting embryo. They have homodont dentition except in few cases like the snakes, where some of the teeth are specialised for injecting poisons from the poison glands. Examples include: lizards, snakes, tortoise, crocodiles, etc.
(iv) Aves: These usually possess a loose but efficient body covered with feathers, wings and a hard horny bill/beak for feeding. The wings are for flight. They are warm blooded homeotherms with a high metabolic rate. The legs are covered with scales and they lay shelled eggs after internal fertilisation. Birds are both terrestrial and aboreal and show a high level of parental care. Examples include the domestic fowl, duck, turkey, craw, sun-birds, etc.
(v) Mammalia: These are warm blooded animals with body covered with hair, the skin is glandular. They possess external ears (pinnae), mammary glands (for the production of milk, and have a diaphragm (elastic muscular fibre) separating the thorax from the abdomen. They give birth to their young ones live, i.e, they are viviparous except for monotremes that lay eggs, e.g. duckbilled platypus. Mammals possess different set of teeth, i.e., heterodont dentition. They are warm blooded homeotherms. This class consists of the most advanced animals with the most developed brains for thinking and memory. They have the capacity to manipulate and some have adopted an orthograde (erect) posture. Examples include man, bat, cat, cow, and monkey.
Classification of non-living things
In the same way that we have classified living things, we can also classify non-living things. For instance, the scheme for the classification of matter is as shown below:
Objects may be classified into either magnetic or non-magnetic, conductors or non-conductors, hard or soft; liquids into acids, bases and those with neutral characteristics.
Another substance that may be grouped variously is the soil which is either sand, clay, loam, humus, mineral matter, fine gravel, coarse gravel, etc. If we consider water bodies, we may have some of the following groups: marine (salt) water, fresh or inland water, brackish water, rivers, streams, springs, lakes or pond water, etc., each type, having its characteristic features.