1.0.    INTRODUCTION TO LIVING ORGANISMS

Living organisms are all forms of life that carry out basic life processes. They include plants, animals, fungi, protists, and bacteria. These organisms are made up of cells, which are the basic units of life. They interact with their environment, adapt to changes, and maintain internal balance (homeostasis). Understanding living organisms is key to studying biology, the science of life.

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          1.1    CHARACTERISTICS OF LIVING THINGS

MOVEMENT – Ability of an organism to move its whole body or part of its body from one place to another.

RESPIRATION – Process of releasing energy from food.

NUTRITION – Taking in food for energy and growth.

IRRITABILITY/SENSITIVITY – Response to stimuli.

GROWTH – Irreversible increase in size,dry mass and complexity.

EXCRETION – Removal of metabolic waste from the body.

REPRODUCTION – Ability to produce.

DEATH – End of life processes.

ADAPTATION – Ability to adjust to the environment.

COMPETITION – Ability of organisms to struggle for the necessities of life in order to survive in their various environments.

The characteristics of living things can be easily remembered with the acronym MR NIGER DAC.

          1.2     CLASSIFICATIONS OF LIVING THINGS

Classification of living things began with Aristotle, later improved by Carolus Linnaeus in 1758 using a formal taxonomic system. The father of taxonomy, Carolus Linnaeus, developed the modern system of naming and classifying organisms called Binomial Nomenclature.

Living things were classified based on:

• Kingdom
• Phylum (for animals) – Division (for plants)
• Class
• Order
• Family
• Genus
• Species

BASED ON KINGDOMS

• Kingdom Monera (e.g., bacteria)
• Kingdom Protista (e.g., amoeba)
• Kingdom Fungi (e.g., mushrooms)
• Kingdom Plantae (e.g., flowering plants)

— Under this classification, they are further divided into;

• Bryophytes – Includes mosses and liverworts. They lack true roots, stems and leaves and reproduce via spores.
• Pteridophytes – They are divided into two;
• Gymnosperms – Seed-producing plants that do not produce flowers or fruits (e.g., conifers like pine trees).
• Angiosperms – Flowering plants that produce seeds enclosed in fruits. They are divided into:
• Monocotyledons – One seed leaf (e.g. grasses, lilies, maize, cereals).
• Dicotyledons – Two seed leaves (e.g., roses, sunflowers, bean, cowpea).
• Thallophyta (e.g,green,brown,red algae).
• Tracheophyta (e.g vascular plants).

●   Kingdom Animalia (e.g., mammals, birds).

          1.3     INTRODUCTION TO CELL

A cell is defined as the smallest, basic unit of life that is responsible for all of life’s processes. It can also be defined as a mass of cytoplasm that is bound externally by a cell membrane.

Every cell consists of cytoplasm enclosed within a membrane.  A living thing, whether made of one cell (like bacteria) or many cells (like a human), is called an organism.

          1.4     CELL STRUCTURES AND THEIR FUNCTIONS

Cell structures are the different parts or components within a cell that carry out specific functions necessary for the cell’s survival and proper functioning. These structures are also known as Cell Organelles.

They include:

• Cell Membrane – Plays a great role in selective Absorption of materials and protection of the cell.
• Nucleus – Contains hereditary information as it houses the DNA.
• Mitochondria – Powerhouse of the cell,where respiration occurs.
• Chloroplasts – Site of photosynthesis in plant cells.
• Cell wall – It provides protection,shape and mechanical support for the cell.
• Lysosomes – They are sites for respiratory enzymes.
• Ribosomes – They are responsible for protein synthesis.
• Centrioles – They are important in cell division and also serve as the basal body.
• Endoplasmic Reticulum – They aid the transport of materials within the cytoplasm.
• Vacuole – It contains cell sap which acts as an osmoregulator to remove excess water.
• Starch Granules – They store starch for the cell.
• Golgi bodies – They function in synthesis and packaging.
• Nucleolus – They produce ribosomes for protein synthesis.

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          1.5    FORMS IN WHICH LIVING CELL EXIST

• Single or Free Living Organisms – These are organisms that exist independently and are composed of a single cell. Examples include amoeba, paramecium.
• Filamentous Forms – Some organisms exist in long, thread-like structures called filaments e.g. spirogyra.
• Colony Forms – In this form, groups of cells live together but can function independently. Examples include volvox that form colonies.

          1.6     UNICELLULAR & MULTICELLULAR ORGANISMS

UNICELLULAR ORGANISMS – These are made of a single cell that carries out all life processes. Examples: Bacteria, Amoeba, Paramecium, Euglena

MULTICELLULAR ORGANISMS – These consist of many specialized cells working together. Examples: Plants, animals, humans etc. in multicellular organisms, some of these specialized cells include:

• Red blood cells (carry oxygen).
• Nerve cells (transmit signals).
• Muscle cells (contract and enable movement).
• White blood cells (defend against disease) etc.

MOVEMENT MECHANISMS IN UNICELLULAR ORGANISMS

• Flagella – Long, whip-like structures that propel organisms (e.g., euglena).
• Cilia – Short, hair-like projections that beat in waves for movement (e.g., paramecium).
• Pseudopodia – Temporary extensions of the cell that allow crawling and feeding (e.g., amoeba).

      1.7     PLANT AND ANIMAL CELL

CHARACTERISTICS OF PLANT CELLS

• Cell Wall – Plant cells have a rigid cell wall made of cellulose that provides structure and support.
• Chloroplasts – These organelles contain chlorophyll and are responsible for photosynthesis, allowing plants to convert sunlight into energy.
• Large Central Vacuole – Plant cells typically have a large central vacuole that stores water, nutrients, and waste products, helping maintain turgor pressure.
• Definite Shape – Plant cells usually have a fixed, rectangular shape due to the presence of the cell wall.

CHARACTERISTICS OF ANIMAL CELLS

• No Cell Wall – Animal cells do not have a cell wall; instead, they have a flexible plasma membrane that allows for a variety of shapes.
• Centrioles – These structures are involved in cell division and help organise the mitotic spindle.
• Lysosomes – Animal cells contain lysosomes that contain digestive enzymes for breaking down waste materials and cellular debris.
• Small Vacuoles – Animal cells have smaller vacuoles compared to plant cells, which are used for storage and transport.
• Irregular Shape – Animal cells can vary in shape and size, often appearing round or irregular.

1.8     BRIEF HISTORY OF CELL

• 1665 – Robert Hooke (Father of Cell) – First person to observe cells. He looked at cork under a microscope and saw tiny box-like structures, which he called “cells.”
• 1674 – Anton van Leeuwenhoek – Improved the microscope and became the first to observe living cells (like bacteria and protozoa), which he called “animalcules.”
• 1838 – Matthias Schleiden – A botanist who concluded that all plants are made of cells.
• 1839 – Theodor Schwann – A zoologist who stated that all animals are also made of cells. Together with Schleiden, they proposed the early version of the cell theory.
• 1855 – Rudolf Virchow – Added the third part of the cell theory: “All cells arise from pre-existing cells.”

CELL THEORY

• All living things are made up of cells.
• The cell is the basic unit of structure and function in living organisms.
• All cells arise from pre-existing cells.

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          1.9     LEVEL OF ORGANIZATION

Level of organization refers to the hierarchy of complex biological structures and systems that make up living organisms. It shows how simple components combine to form more complex systems.

In biology, the common levels of organization are:

• Cell – The basic unit of life.
• Tissue – A group of similar cells working together.
• Organ – Different tissues working together to perform a function.
• System – A group of organs that work together (e.g., digestive system).
• Organism – A complete living being.