## Common Characteristics of Living Organisms This section outlines the key characteristics that define life. We'll explore these characteristics in detail, highlighting differences between unicellular and multicellular organisms. ### 1. Cellular Structure All living things are made of cells! All known organisms are composed of cells, the fundamental units of life. Organisms are classified into two groups based on their cellular structure: * **Prokaryotic Cells:** These are simpler cells lacking a membrane-bound nucleus and other organelles. Their genetic material (DNA) is circular and located in the cytoplasm. Examples include bacteria and archaea. * **Key Features:** Cell wall (usually present), no membrane-bound organelles, circular DNA in cytoplasm. * **Eukaryotic Cells:** These are more complex cells possessing a membrane-bound nucleus containing linear DNA, and various other membrane-bound organelles like mitochondria and chloroplasts. Examples include protists, fungi, plants, and animals. * **Key Features:** Cell wall (usually absent), membrane-bound organelles (mitochondria, chloroplasts, etc.), linear DNA within the nucleus. Compare a bacterium (prokaryotic) to a plant cell (eukaryotic). The bacterium is much smaller and simpler, lacking the complex internal structures found in the plant cell, such as chloroplasts and a defined nucleus. Confusing prokaryotic and eukaryotic cells. Remember the key difference lies in the presence or absence of a membrane-bound nucleus and other organelles. **Beyond Cell Type:** Organisms are also classified by cell number: * **Unicellular:** Organisms consisting of a single cell (e.g., amoeba, paramecium, bacteria). * **Multicellular:** Organisms composed of many cells (e.g., plants, animals). ### 2. Levels of Organization Life is organized hierarchically: * **Atom:** The fundamental unit of a chemical element. * **Molecule:** A group of atoms bonded together. * **Organelle:** A specialized structure within a cell performing specific functions (e.g., mitochondria, ribosomes). * **Cell:** The basic unit of life. * **Tissue:** A group of similar cells working together (e.g., muscle tissue, nervous tissue). * **Organ:** A group of different tissues working together (e.g., heart, liver). * **System:** A group of organs working together (e.g., digestive system, circulatory system). * **Organism:** A complete living thing. Visualize this hierarchy as a pyramid, with atoms at the base and the organism at the apex. **Unicellular vs. Multicellular Organization:** * **Unicellular organisms:** Exhibit organization up to the cell level (atom-molecule-organelle-cell). * **Multicellular organisms:** Exhibit organization from atom to organism (atom-molecule-organelle-cell-tissue-organ-system-organism). ### 3. Nutrition All living organisms require nutrients for growth, energy, repair, and metabolic processes. Organisms are categorized by their feeding type: * **Autotrophs (Producers):** Synthesize their own food, typically through photosynthesis (e.g., plants, some algae). * **Heterotrophs (Consumers):** Obtain food by consuming other organisms (e.g., animals, fungi). * **Mixotrophs (Autoheterotrophs):** Can obtain energy through both autotrophic and heterotrophic means (e.g., Euglena, some carnivorous plants). ### 4. Energy Production and Consumption Life requires energy to maintain functions like movement, reproduction, and growth. * **ATP (Adenosine Triphosphate):** The primary energy-carrying molecule in cells. * **Cellular Respiration:** The process of breaking down nutrients to release energy, which is then stored in ATP. * **Energy Utilization:** Organisms use the energy stored in ATP to power their life processes. ### 5. Metabolism Metabolism encompasses all chemical reactions within a cell. It's divided into two categories: * **Anabolism (Synthesis):** Building complex molecules from simpler ones (e.g., protein synthesis from amino acids, photosynthesis). * $n \text{ (amino acid)} \rightarrow \text{protein} + \text{water}$ * $n \text{ (glucose)} \rightarrow \text{polysaccharide} + \text{water}$ * **Catabolism (Breakdown):** Breaking down complex molecules into simpler ones (e.g., digestion, cellular respiration). * $\text{protein} + \text{water} \rightarrow n \text{ (amino acid)}$ ### 6. Homeostasis Homeostasis is the maintenance of a stable internal environment despite external changes. This includes regulating temperature, pH, and other vital parameters. Humans maintain a body temperature of around 37°C through sweating (cooling) or shivering (warming). ### 7. Excretion Excretion is the removal of waste products from an organism. This is crucial for maintaining homeostasis. * **Multicellular organisms:** Utilize various systems (e.g., sweating, urination, respiration) for waste removal. * **Unicellular organisms:** Often excrete waste directly through their cell membrane. * **Plants:** Excrete waste through transpiration (water loss from leaves) and leaf fall. ### 8. Growth and Development Growth involves an increase in size or cell number. Development refers to the progression through different life stages and the maturation of structures. * **Unicellular growth:** Increase in cytoplasm and organelles. * **Multicellular growth:** Increase in cell number through cell division. ### 9. Nucleic Acids Living organisms possess nucleic acids: * **DNA (Deoxyribonucleic Acid):** The genetic material carrying hereditary information and controlling cellular processes. * **RNA (Ribonucleic Acid):** Involved in protein synthesis. These characteristics are interconnected and work together to maintain life.