# 1.1 CELLS - THE BASIC UNIT OF LIFE A cell is defined as the *basic structural and functional unit* of all living organisms. It is the smallest unit and fundamental building block of life. All organisms are composed of one or more cells. The shape and size of cells are often related to the specific function they perform. --- ### Classification by Cell Number Organisms can be classified based on the number of cells they contain. | **Type** | **Definition** | **Examples** | | :--- | :--- | :--- | | **Unicellular** | Organisms made of a single cell. | *Amoeba*, *Paramecium*, *Euglena* | | **Multicellular** | Organisms made of many cells. | Plants, Animals | --- ### Properties of Life A single cell exhibits all the basic properties necessary for life. These are often remembered by the acronym **MR. H GREN**. * **Movement**: Living things show movement, which can be external (changing location) or internal (movement of substances within the organism). * **Respiration**: The chemical processes in which living things use substances from their environment (like glucose) to release energy for life processes. * **Homeostasis**: The ability of living things to maintain a stable and constant internal environment, despite changes in the external environment. * **Growth**: The ability of living things to increase in size or change shape. * **Reproduction**: The process by which living things produce offspring, either sexually or asexually, to ensure the continuation of the species. * **Excretion**: The removal of metabolic waste products from the body to prevent them from becoming toxic. * **Nutrition**: The intake and use of materials and gases from the environment for energy, growth, and repair. --- ### Cell Theory The development of the cell theory was a collective effort of many scientists. It provides the foundation for understanding biological systems. * **Robert Hooke (1665):** First observed "cells" in cork. * **Theodor Schwann (1839):** Concluded that all animals are made of cells. * **Rudolf Virchow (1855):** Proposed *Omnis cellula e cellula*, meaning all cells arise from pre-existing cells. #### Modern Cell Theory Tenets Modern biology has added further depth to the classical theory: 1. Energy flow (metabolism and biochemistry) occurs within cells. 2. Cells contain hereditary information (DNA) which is passed from cell to cell during cell division. 3. All cells are basically the same in chemical composition in organisms of similar species. --- ### Exceptions to Cell Theory While the cell theory applies to the vast majority of life, there are notable exceptions: * **Viruses:** They are acellular (not made of cells) and cannot reproduce outside a host cell. * **Atypical Cells:** Mature mammalian Red Blood Cells (RBCs) lack a nucleus, and sieve tube cells in plants also lose their nuclei at maturity. Fungal hyphae can be coenocytic (multinucleated without cell walls). --- ### Structure-Function Relationship The diverse shapes and sizes of cells are directly related to the specific roles they play within an organism. For example, nerve cells are long to transmit signals over distances, while RBCs are biconcave to maximize surface area for oxygen transport.