Concept of Living things and Cells

Science, Biology, and Life

Science is a process by which we can find out about our world, while biology is the science that deals with the study of living things. Living and non-living things are the two main constituents of the world.

Characteristics of Living Things 

  • Structure: All living organisms are composed of cells. These can be unicellular, as in Amoeba, or multicellular, as in man. 
  • Form and Sizes: Living organisms have definite shapes and sizes. Hence a chicken in Nigeria has the same structure and size as another chicken in Britain.
  • Respiration: Living things breathe oxygen to break down food and release energy for other life processes.
  • Nutrition: Living things take in food from which energy can be obtained through respiration.
  • Movement: All living things move either by themselves, as in animals, or by wind, water, or man, as in plants. 
  • Growth: As living things take in food, they increase in size and complexity by incorporating the digested food products into their bodies.
  • Excretion: All living organisms must have the means to remove or eliminate unwanted waste products of metabolism from the body.
  • Secretion: Different parts of the body of living organisms produce useful liquid substances, e.g. In humans, the salivary glands produce saliva, which contains an enzyme, ptyalin, while in plants, the stem and root apices of maize plants produce plant hormones and auxins.
  • Reproduction: Organisms on attaining sexual maturity reproduce their kind. This can be asexual, as in the binary fission of Amoeba, or sexual, as in human beings.
  • Irritability: This is the ability to live things to respond to stimulus. Part of the body of an organism can respond to push, as in phototropism in green plants, or the whole body of an organism responding to a stimulus, as in taxism in animals or some plants.

Classification of Living Things

All living things are generally classified into two major kingdoms: plant and animal kingdoms. Recent classification divides living organisms into two super kingdoms, prokaryote, and eukaryote. 

Superkingdom Prokaryotes contain only one kingdom: Moncra c.g blue-green algae — Nostoc, bacteria — Bacilli. Superkingdom Eukaryote is subdivided into four kingdoms:

  1. Protista c.g. Unicellular green algae — Euglena 
  2. Protozoa — Amoeba 
  3. Plantae c.g. higher plants — Elias guineas (oil palm tree)
  4. Fungi c.g. toadstool, Amania sp
  5. Animalia c.g. worm, Ascaris sp, toad, Bufo sp 

Kingdom Plantae

  1. Algae c.g. seaweeds, Spirogyra 
  2. Moses c.g. Octoblepharum 
  3. Ferns, e.g., Lygodium 
  4. Gymnosperms c.g. Cycads, Encephalarios 
  5. Angiosperms c.g maize, Zea mays 

Kingdom Animalia


  1. Porifera c.g. sponges
  2. Cnidarians, e.g., Ilydra 
  3. Flatworms, e.g., tapeworm, Taenia 
  4. Roundworms c.g. Ascaris 
  5. Annelida, e.g., earthworm, Libyodrilus 
  6. Arthropods e g. insects: Anopheles; crayfish
  7. Mollusca, e.g., snail, Archachatina 
  8. Echinodermata, e.g., starfish, Asterias 


Protochordates, e.g., Amphioxus 


  1. Pisces c.g. fishes: Tdapia 
  2. Amphibia, e.g., toads: Bufo 
  3. Reptilia c.g. lizards: Agama 
  4. Aves, e.g., birds, Gallus 
  5. Mammalia c.g. rat, man, Rattus, Homo 

Organization of life

Levels of Organization

(i) Cell is the smallest fundamental building unit of die body of living organisms, either plants or animals. The body of a man or of a flowering plant is made up of numerous cells. However, single-celled organisms such as Amoeba are made up of only a single cell.

(ii) Tissue is a group of cells that have similar structures and perform a particular function, e.g., bone, phloem, and the blood of man.

(iii) Organ consists of several tissues grouped together to make a functional unit. e.g., stem, leaf, eye, heart.

(iv) System is senes of organs whose functions are coordinated to produce an effective action in an organism, e.g., The circulatory system of man comprises die heart, arteries, veins, and capillaries.

(v) Organisms result from the efficient coordination of the organs and systems to produce an individual capable of separate existence and able to perpetuate its own kind, e.g., man, goat, mango plant. In unicellular organisms such as Amoeba, their Organization involves not organs but organelles, which are all within a single cell.

 The complexity of Organization in higher organisms 

According to Lamlad, In higher organisms, the variety of tissues and organs found within them are responsible for their efficiency. The specialization of constituent organs in the process of expelling nitrogenous wastes in man. Such as, the kidney’s tubules make the excretory system highly efficient during fi Iteration and re-absorption.

Higher organisms have been successful because they have been able to adapt to changes within their environment. Such changes can be advantageous, but where such changes are too drastic, they may be disadvantageous, e.g., when the environment of a rabbit becomes Hooded over a long period of time or when die habitat of a forest free suddenly becomes dry and less humid, these may be lethal to the organisms.

Cell Structure and Functions of Cell Components

The Cell Structure

A cell is a ‘single unit’ of a living organism. Since both plants and animals differ from one another, the cells that compose the body of a plant or an animal must be different from each other in certain respects.

Plant cells are of different types and sizes. Most plant cells have regular outlines and fixed shapes. They may be cylindrical, polygonal, or rectangular in shape. The ultrastructures of a typical plant cell and an animal cell are shown in the lamlad biology textbook.

Animal cells, like plant cells, are of different shapes and sizes. They may be spherical, rectangular, spindle-shaped, cuboidal, or irregular in shape, as shown in lamlad biology.

Similarities and differences between plant and animal cells


  • They both have cell membranes
  • They have nuclei. 
  • They have cytoplasm.
  • They both have cytoplasm with high proportions of water mixed with chemical compounds. 
  • They both undertake cell division.


Plant cells are usually larger than animal cells.

Plant cells have cellulose cell walls which are absent in animal cells. 

Plant cells have definite shapes, but animal cells have none.

Plant cells contain conspicuous vacuoles of varying sizes with cell sap, but these are usually absent in animal cells and, where present, are usually small and many. 

Plant cells contain plastids, including chlorophyll, but these are absent in animal cells. 

Plant cells have a more watery protoplasm than the protoplasm of animal cells.

Plant cells have carbohydrates stored as starch granules but as glycogen in animal cells. 

 Forms in Which Living Cells Exist

Living cells are made up of one or more cells. In many cases, a single cell is a living organism. The Amoeba found in freshwater is an example. It is a cell of about 300 pm across, and it houses the machinery with which it carries out all the functions of life. It feeds itself and exchanges matter and energy with its environment. It responds to stimuli in its environment. It grows and reproduces. It moves by means of finger-like projections of the body wall called pseudopodia.

Staphylococcus is a spherical cell about 1um in diameter. It is far too small to be seen without the high magnification of a microscope. Nevertheless, it contains within its single cell all the machinery of life.

Mycoplasma is even smaller and can only be seen with the aid of an electron microscope. They are still living things.

Chlamydomonas, which is a microscopic green alga found in freshwater, is a single-celled organism. Within the single cell are found all organelles required to carry out the various functions of life. It reproduces asexually by producing eight or sixteen daughter cells and sexually by the fusion of two individuals called gametes to form a zygote. From the zygote (zygospore), four individuals are produced. 

Euglena viridis is another microscopic organism found in freshwaters. The spindle-shaped body is broad in the middle. The presence of a gullet, flagellum, cyc-spot (photo tactic), contractile vacuole, pellicle, and myonemes often qualifies Euglena as an animal. Equally, the presence of stellate chloroplast and paramylum granules also qualifies it as a plant. It is, therefore, capable of independent existence.

Among the flagellated green algae are some colonial forms like Volvox. It is a colonial form because it is made up of clusters of independent cells. The cells of Volvox have lost their ability to live independently, i.e., they have become specialized. No longer. Every cell carries out all life’s functions.

Some are responsible for reproduction, while others are unable to reproduce themselves and take care of photosynthesis and locomotion. This division of labor among cells is called differentiation.

Spirogyra is a simple multicellular filamentous green alga. Il consists of a single row of cylindrical cells joined end to end. All the cells in the filament are identical. Spirogyra is able to manufacture its known food with the help of spiral chloroplasts. Starch grains normally accumulate around the pyrenoids when exposed to light. 

Each filament shows no differentiation of any cell for a particular function. Each cell is capable of carrying out its own life processes. Thus each cell is physiologically independent, but they are not mobile. Like Chlamydomonas and Euglena, Amoeba and paramecium caudatum are free-living unicellular microorganisms.

Paramecium, as a unicellular organism, has to cany out within one cell all die functions that in a multicellular animal are performed by differentiated cells, tissues, and organs. The macronucleus exercises control over metabolic functions, including growth, whilst the micronucleus is necessary for sexual reproduction. 

The cilia are responsible for locomotion, and the contractile vacuoles for osmoregulation. The trichocysts are used for defense. Food particles swept into the oral groove by ciliary action are taken up into a food vacuole formed at the base of the cytopharynx or gullet. Food vacuoles circulate through the cytoplasm before discharging undigested remains to the exterior through the cell anus called cytoproct.

Most other living things are built from many cells. Two easily studied examples are the epithelial cells that line the inner surfaces of checks and red blood cells. These cells are different from the cells of single-celled organisms like/write in that they are not capable of living an independent existence, i.e., they cannot carry out all the functions of the living organism. Each cell is dependent on others to carry out the functions that it cannot perform by itself.

Functions of Cell Components

Despite the diversity of cell types, there are certain structural and functional features that are common to most cells. 

The cell membrane (plasma membrane)

This is the outer limiting membrane surrounding the cytoplasm. It is a thin, flexible, and selectively permeable cell wall of plant cells. It is only recognizable when such cells arc plasmolyzed.

Cell Wall

Only very rarely is the cell membrane the outer surface of the cell. Usually, some types of exterior coating present. In algae and in all plants, the exterior coating is made of polysaccharide cellulose. This forms a rigid cellulose cell wall characteristic of plant cells. Rigid walls are also found around the cells of bacteria, where they are protein in nature, and in fungi, where the wall is of chitin.

The Nucleus

The cell nucleus is bounded by a pair of membranes that contain pores. Tire pores permit materials to pass into and out of (be the nucleus to the cytoplasm. Within the nuclear membrane is a semi-fluid medium in which the chromosomes are suspended. A nucleolus is usually found in the nucleus. 

The Cytoplasm

This term describes everything within the cell except the nucleus. The extraordinary complexity of structures is found in the cytoplasmic region. There are elaborate patterns of membranes and membrane-bounded compartments within the cytoplasm. These clearly defined structures are called organelles. Each organelle carries out a particular function as follows:

(i) The Mitochondria: They are spherical or sausage shaped bodies found in various members in the cytoplasm of cells. Active cells like liver cells may contain over a thousand of them. They contain enzymes that carry out the oxidation of food substances, and they

synthesize ATP (adenosine triphosphate), the energy of the cell.

(ii) The Chloroplasts: They are formed only in the cells of plants and certain algae. In plant cells, the chloroplasts occur as disc-shaped structures, with as many as up to 50 of them per plant cell. Chlorophyll traps the energy of sunlight and enables it to be used for the synthesis of food. Thus chloroplast is the seat of photosynthesis.

(iii) The Ribosomes: They are among the smallest structures suspended in the cytoplasm. They are roughly spherical, and they are the site of protein synthesis.

(iv) The Endoplasmic Reticulum: This is an elaborate system of membranes within the cell. Evenly split ribosomes are found adhering to the surface of some and hence are called the rough endoplasmic reticulum (RER). Where they are absent, they are called smooths endoplasmic reticulum (SER).

(v) The Golgi Apparatus: This is a stack of flat, membrane-bounded sacs. Proteins synthesized by the RER are transferred to the Golgi apparatus. Additional carbohydrates may be added to them. These protein-filled sacs then migrate to the surface of the cell and discharge their contents to the outside. The Golgi apparatus is also the site where the synthesis of polysaccharides takes place.

(vi) Lysosomes: They are roughly spherical structures bounded by a single membrane. They are produced by the Golgi apparatus and are filled with proteins. These proteins are made up of many enzymes. Lysosomes play an important role in the death of cells because the 

enzymes in them bring about autolysis.

(vii) Vacuoles: They arc cytoplasmic organelles filled with liquid. Food materials or wastes may be found inside vacuoles. A young plant cell contains many small vacuoles, but as the cell matures, these unite to form a large central vacuole. Dissolved food molecules, waste materials, and pigments may be found in them.

(vii) Centrioles: Animal cells and cells of other microorganisms and lower plants contain two centrioles. Just before a cell divides, its centrioles duplicate, and one pair migrates to the opposite side of the nucleus. Spindle fibers then form between them, i.c. They play a role in the formation of spindle fibers during cell division. 

Richard Maxwell

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