To be safe, one may infer that if anything can reproduce, it is alive or a living entity. Birds, insects, animals, trees, and humans are all examples of living things since they share characteristics such as feeding, breathing, reproduction, growth, and development, and so on. In fact, science now knows that plants also evolve over time.
Living things are made up of cells, the building blocks of life. All living things are composed of proteins, carbohydrates, lipids, and nucleic acids. Organic molecules are derived from carbon, nitrogen, hydrogen, oxygen, phosphorus, sulfur, and other elements. The most abundant element in the universe is hydrogen, which makes up more than 75% of the mass of the stars and galaxies. Carbon makes up more than 95% of the mass of the stars and planets. Oxygen is the third most abundant element in the universe after hydrogen and helium. It makes up more than 20% of the mass of the stars and planets.
In addition to these three primary elements, living things must also include water because they cannot function without it. Some organisms need water for survival while others can survive for a short time without water. For example, bacteria can live for several hours without water but not without nutrients such as sugars and proteins. Animals require water for many different processes including respiration (breathing), digestion, lubrication, and temperature regulation. Humans are a form of animal; therefore, we need water to survive.
Living things are categorized into two broad groups: organic and inorganic substances. Inorganic substances include minerals and other chemical compounds that do not contain carbon. Organic substances are made up of molecules containing carbon atoms bound together by strong covalent bonds. Living organisms can be divided even further into categories based on their metabolic processes: aerobic or anaerobic, uni-cellular or multi-cellular, and so on.
All living things need oxygen to breathe and consume energy from nutrients in the food they eat. Without oxygen, organic materials decompose very quickly. Plants take up oxygen from the air and use the energy from sunlight to convert water and carbon dioxide into glucose and other carbohydrates which provide their energy needs throughout the day. Animals consume plants and other animals and therefore must get oxygen from another source. They do this by breaking down fat into fatty acids and then combining these with oxygen molecules to form triglycerides for storage until needed or conversion back into proteins and new cells.
In addition to oxygen, animals need water to survive.
To be considered a living entity, an object must have eaten, breathed, and reproduced at some point. Even if it is not alive, a dead animal or plant is considered a living creature. Plants (for example, trees, ferns, and mosses), animals (for example, mammals, birds, insects, reptiles, and amphibians), and fungus (for example, mushrooms) all belong to the kingdom Plantae, Animalia, and Fungi, respectively.
Living things are composed of cells that reproduce by division. Cells from one organism can grow into other organisms by transplantation. This process is called grafting. Seeds from one species will usually not grow into plants of another species because they come from different parents. However, this can happen with certain plants such as peas and beans when grown in soil with other seeds. Animals are also able to reproduce by grafting: A male bee will sometimes take its pollen sack and stick it into the body cavity of a flower of a different species to pollinate the flower. In this case, the bee is acting like a grafted plant.
You may have heard that plants "breathe" with each step they take. Actually, this is true only for photosynthetic plants, which are those that use the energy of sunlight to produce food molecules using carbon dioxide and water. All the other types of plants, such as algae, do not need to breathe because they cannot live without light exposure.
The word "living creature" refers to objects that are currently or were previously alive. Anything that has never been alive is referred to as a non-living thing. To be regarded as life, anything must grow and develop, consume energy, reproduce, be composed of cells, respond to its environment, and adapt. All living organisms share these properties to one degree or another.
Non-living things cannot grow or reproduce. They may contain organic molecules, but they are not composed of any elements that can combine to form other substances. Non-living objects include inanimate objects such as rocks and ice, which cannot grow or reproduce, and artifacts that have been created by humans. Living organisms can be divided into two categories based on their metabolism: aerobic organisms use oxygen for their metabolic processes, while anaerobic organisms do not need oxygen for their metabolism.
Living organisms are made up of cells that are organelles within the cell that help control chemical reactions involved in metabolism and reproduction. Cells are the basic unit of life; without them, there is no living thing. Cells are the building blocks of all living organisms. Some cells are very small, like bacteria, while others are much larger, like human cells. Bacteria are only visible under the microscope; human cells are too large to see with the naked eye. In between these two extremes are cells that are visible with the naked eye, such as those found in plants.
While many objects satisfy one or more of these characteristics, a live entity must satisfy all of them. Living organisms come in all shapes and sizes; they range from small bacteria that can only reach 10-12 µm in size, to elephants that weigh over 5 million kg and measure over 6 m tall.
Non-living things are also found in great abundance around us. The earth is filled with millions of different species of plants and animals. Some are large and strong, such as tigers and sharks; others are tiny and fragile, such as bacteria and insects. All living things die, but since they can reproduce themselves, their genetic material is passed on to future generations. This means that non-living things must also be able to reproduce themselves in order for their kind to survive.
Non-living things cannot produce new molecules. When they die, their components such as proteins and DNA are broken down by enzymes into simple substances that can be reused by other organisms. These decomposition processes are called decay. Decayed matter is called compost and it makes a useful fertilizer.
Non-living things cannot think or feel pain. Living organisms possess the ability to think and feel pain. Scientists have proven this fact by testing animals in laboratories and observing their reactions when injured.
Reproduction is a process that can only occur in living beings. As a result, reproduction occurs as a distinguishing feature of living species. Organisms must be alive for their cells to divide and form new individuals.
In biology, life is defined as the fundamental quality of being alive. Living things are made up of cells which contain the chemical components necessary for survival. Cells from single-celled organisms such as bacteria or algae to those from more complex organisms like humans share certain common properties: they cannot survive for very long without water or oxygen, for example; they need nutrients to grow and reproduce; they cannot move around or defend themselves; etc.
Cells take on these characteristics because they can only perform specific functions within the organism. For example, muscles cells are able to contract and expand, allowing an animal to move its body parts. But without a central nervous system to control them, muscles cells would have no way of communicating with each other and would die after a few minutes.
Every cell contains the complete set of genetic instructions needed to make a living thing. These genes are passed down from parent cells to offspring cells during division. Genes are the physical basis of heredity, or the ability to pass on physical traits to future generations.