What Makes a Cell the Building Block of Life?

A cell is the smallest unit of life that can carry out all the basic functions necessary for living things. Cells are called the building blocks of life because every living organism is made up of one or more cells. In this article, you will learn how cells were discovered, what the cell theory states, how to tell unicellular from multicellular organisms, and the structure and differences between plant and animal cells, as per the ICSE Class 6 Biology syllabus.

Robert Hooke discovered cells when he observed a thin slice of cork under a microscope and saw tiny box-like structures, which he called 'cells'. This discovery led to the development of cell theory. The main points of cell theory are: all living organisms are made up of cells, the cell is the structural and functional unit of life, and new cells arise from pre-existing cells by cell division. This means that everything living, from the smallest bacterium to the largest tree, is built from cells, and all the life processes happen within these cells. For example, when you look at a leaf under a microscope, you can see many small cells making up the leaf tissue. A common misconception is that only plants have cells, but in reality, both plants and animals are made of cells.

Unicellular organisms are made up of only one cell, which performs all the functions needed for life. Examples include Amoeba, Paramecium, and bacteria. Multicellular organisms have many cells that work together, with each cell often having a special job. Examples are humans, mango trees, and dogs. Cells also come in many shapes and sizes. For example, nerve cells are long and branched to carry messages, red blood cells are round and flat to carry oxygen, and the ostrich egg is a single cell that is large and visible to the naked eye. In real life, when you eat an egg, the yellow yolk and the white are both parts of one giant cell. Many students think all cells look the same, but their shape and size depend on their function.

A plant cell is usually rectangular, while an animal cell is more rounded. Both types have a cell membrane, cytoplasm, and nucleus, but plant cells also have a cell wall, large vacuole, and plastids (like chloroplasts). The cell wall gives shape and protection to plant cells. The cell membrane controls what enters and leaves the cell. The cytoplasm is a jelly-like substance where cell activities happen. The nucleus acts as the control centre and contains the nuclear membrane, nucleolus, and chromatin threads (which hold genetic material). Vacuoles store water and other substances; they are larger in plant cells. Plastids, such as chloroplasts, help make food in plants. Mitochondria are the 'powerhouses' of the cell, releasing energy from food. Protoplasm refers to the living part of the cell, including the cytoplasm and nucleus. For example, the green colour in leaves is due to chloroplasts in plant cells. Many students forget to include the cell wall and chloroplasts when drawing a plant cell.

Plant cells have a cell wall, large vacuole, and plastids (like chloroplasts), while animal cells do not. Animal cells usually have smaller vacuoles and lack plastids. The levels of organisation in a multicellular organism are: cell (basic unit, e.g. muscle cell), tissue (group of similar cells, e.g. muscle tissue), organ (group of tissues, e.g. heart), organ system (group of organs, e.g. circulatory system), and organism (the whole living thing, e.g. a human). For example, in your body, blood is a tissue made of blood cells, which together form organs like the heart, which is part of the circulatory system, making up the whole organism. Students often confuse tissues and organs, but a tissue is made of similar cells, while an organ is made of different tissues working together.