Living things are organisms that possesses or display the characteristics of life or being alive. They include many kinds of organisms, from plants, animals, fungi, and algae that can be easily seen in nature, to numerous small organisms called protozoa, bacteria, and archaea that can only be seen with a microscope can be found in every type of habitat on earth-land and lakes, rivers, and oceans. Although all these creatures are very different from each other, they all have two things in common: they are all descendants from an ancient ancestor, and they are all alive.
EVOLUTION OF LIFE
Most scientists believe that the first creatures on Earth may have evolved within a billion years of the formation of the Earth, which occurred about 4.5 billion years ago. This belief is based on evidence from the fossil record. Fossil remains of microorganisms similar to cyanobacteria (a group of microorganisms formerly known as blue-green algae) have been found in rocks about 3.5 billion years old.
The early earth was very different from today’s earth. The atmosphere is rich in hydrogen, which is essential for subsequent chemical events. According to a scientific hypothesis, a mixture of life-important elements such as carbon, nitrogen, oxygen and hydrogen is concentrated in a warm pool bathed in the ultraviolet rays of the sun.
In this mixture, chemical elements combine in increasingly complex reactions to form organic molecules such as proteins and nucleic acids. When they combine and recombine, these molecules eventually form highly primitive cells that can reproduce themselves. Over millions of years, the process of natural selection has helped to evolve single-shell and multi-shell organisms from an ancient common ancestor.
BASIC NEEDS OF LIVING THINGS
All creatures have certain basic needs. The most fundamental needs of organisms are:
Water; without this important resource, life cannot exist. Many chemical reactions that occur in cells require water. It also helps transport nutrients and eliminate waste.
Nutrients for energy, growth and repair. Each organism has its own way of obtaining nutrition. Some organisms, such as animals and protozoa, get nutrients from ingesting food. Plants and algae make their own food through the process of photosynthesis. Fungi obtain nutrients by decomposing and absorbing rotting organic materials.
Air and light are also the key needs of some creatures. Air is the basic need of most organisms, although some types of microorganisms cannot tolerate oxygen. For plants and other organisms that undergo photosynthesis, light is the basic requirement of life.
Space is another key basic requirement; organisms such as plants and fungi fixed to the matrix need a certain amount of space to grow and thrive. Animals and other creatures that can move need living space as well as territory to find food and companions.
Living things are organisms with functions, or processes, necessary for life. To be classified as a living thing, an organism must be able to accomplish all of these things.
Creatures have the ability to move in some way without outside help. The movement may include the flow of matter within the organism or the external movement of the organism or part of the organism.
The organism responds to the surrounding conditions. For example, green plants grow towards the sun, and certain microorganisms shrink into small balls when they touch them, and humans blink when the light hits their eyes.
All organisms must be able to release energy stored in food molecules through a chemical process called cellular respiration. In aerobic breathing, oxygen is absorbed and carbon dioxide is released. Some types of bacteria and archaea use a type of cellular respiration called anaerobic respiration, in which the action of oxygen is carried out by other reactants.
Creatures need energy to survive. Energy comes from nutrients or food. Green plants, algae, and certain archaea and bacteria can make food from water and carbon dioxide through photosynthesis. Plants called legumes can make proteins by absorbing nitrogen provided by bacteria living in the nodules of the roots of plants. Animals, fungi, protozoa and many archaea and bacteria need to get food from external sources. They do this in different ways, all of which depend on the physical adaptability of the organism. Some animals such as mammals bite into food with their teeth; some insects suck nectar from flowers. Many species of protozoa and bacteria absorb nutrients through the membranes that cover their bodies.
As they roll in the snow, the size of the snowball will grow, and the salt crystals will grow in the salt water as it evaporates. Although these lifeless objects become larger, they do not grow like creatures. Creatures grow by making new parts and materials and changing old parts and materials. This happens when the seeds grow into plants or chicks mature into hens. As humans grow, they add new structures, such as teeth, and change the proportions of others.
When creatures multiply, they produce new creatures. This is true of even the simplest microorganisms, which can multiply by simply dividing into two parts. Each new part is able to move, feed, grow and perform other functions of life. This type of reproduction is called asexual because it can be carried out without a mating partner. In sexually reproducing mammals and birds, for example, a partner is required to reproduce. However, some higher organisms are able to reproduce asexually; some plants are an example of this, as are some reptiles.
All living things are known to produce waste through the process of life. A lot of waste comes from food. The rest is generated by movement, growth, and other life functions. If this waste remains in the organism, it will soon cause disease and death. Therefore, organisms must have a way to deal with waste. The process of removing waste from the body is called excretion.
LIVING THINGS ARE FORMED FROM CELLS
Cells are the cornerstone of the world of life. Various organisms such as bacteria, archaea, algae, fungi, protozoa, animals, and plants are all composed of one or more cells. Cells are made up of components that help organisms eat, breathe, excrete waste, and perform all necessary life functions.
The components are organized, which means that they fit and work together. The activity of the cell is controlled by the genetic material of the cell-its DNA. In eukaryotes, DNA is contained in a membrane-binding structure called the nucleus.
The word eukaryotic originates from the Greek eu (true) and karyon (nuclear. In eukaryotic cells, most specialized tasks, such as obtaining energy from food molecules and producing cell growth materials, occur in many closed bodies called organelles.
In prokaryotes, from Greek pro(before) and karyon. Prokaryotes are thought to have evolved before eukaryotes. Prokaryotes such as cyanobacteria can photosynthesize food; their food-making chlorophyll is dispersed in cells. In eukaryotic photosynthetic organisms, such as plants and algae, chloroplasts contain chloroplasts.
Living things are classified in the Five Kingdoms system, which began to be adopted in the 1970s, separating fungi into their own kingdoms. It also created a kingdom called Monera for all prokaryotes and a kingdom called Protista for all eukaryotes that do not belong to the kingdom of plants, animals or fungi. The three branches, called domains, are archaea, bacteria, and Eukarya. The eukaryotes in this domain contain all eukaryotes, namely protists, fungi, plants and animals.
Bacteria are single-celled prokaryotes(organisms without obvious nuclei or organelles). Almost all bacteria have a rigid cell wall, which contains a substance called peptidoglycan. Typical shapes of bacterial cells include spheres, rods, and spirals. Some bacteria have flagella, and they use it to push themselves. As a group, bacteria are highly diverse. Some bacteria are aerobic, some are anaerobic. Some, such as purple bacteria and cyanobacteria, contain chlorophyll, so you can make your own food.
Purple bacteria swim through flagella. Although they are photosynthetic, the green particles they contain are different forms of chlorophyll from those found in other photosynthetic organisms. Cyanobacteria have no flagella and usually live in chains or clumps covered by jelly-like substances. They contain real chlorophyll and are therefore autotrophic.
However, under certain conditions, they may also obtain food from other sources. Most bacteria are heterotrophic, including an important group of bacteria that decompose substances from dead organisms. Other important bacterial groups include pathogenic bacteria and bacteria that convert nitrogen in the air into compounds that plants can use.
Archaea, like bacteria, is a single-celled prokaryotic organism, and its appearance is similar to that of bacteria. Nevertheless, they are genetically and structurally and biochemically different from bacteria. For example, the cell walls of archaea do not contain peptidoglycan, and archaea processes DNA in a more complex way. Although a large number of archaea live in a variety of habitats, including the ocean and soil, a distinctive feature of some species is that they can thrive in environments that are deadly to other organisms.
In a sense, these habitats are similar to some of the early conditions on Earth, such as boiling hot springs and an oxygen-deficient atmosphere. The ability of archaea to thrive under such extreme conditions suggests that they have adapted to them a long time ago, and the pattern of the genetic code of archaea suggests that these creatures may be one of the earliest life forms on Earth.
Protists are a very diverse group of living things, mainly single-celled organisms, they are eukaryotes—that is, they have real nuclei and organelles-and are not considered to belong to the animal, plant or fungal kingdom. They can live as individual individuals or groups called colonies, and they can be autotrophic or heterotrophic. Under the five-kingdom classification, protists form the Kingdom Protista, and under the three-domain system, most biologists continue to use this classification. Many protists live in the ocean or fresh water. Protozoa are usually divided into animal-like protozoa, most of which are heterotrophic; plant-like algae, which are autotrophic; and fungi Ike mucus mold and water mold, which are saprophytic. Among the better studied protists, there are euglenoids, paramecium, and diatoms. Some protozoa have flagella or cilia to help propel them through their environment. This helps them capture food and escape predators.
The fungal kingdom contains a wide variety of organisms, from yeast to mold and mold to mushrooms and toadstool. Fungi are classified as heterotrophic eukaryotes with cell walls. In addition, all fungi are multicellular. The presence of cell walls in these organisms has inspired biologists to classify them from plants for many years. However, fungi have many characteristics that are not found in plants. Fungi lack chlorophyll and chloroplasts; they cannot synthesize their own food, but must rely on other organisms for nutrition. Many fungi do this through symbiotic relationships with other organisms. (See also lichen. Like animals, fungi must digest food before absorbing food, but unlike animals, fungi digest food in vitro. To do this, fungi secrete enzymes into their surroundings; these enzymes degrade or break down food into small molecules, which are then absorbed by the fungus.
Plants are multicellular eukaryotes and are classified in kingdom plants. Members of the plant kingdom range from simple green vines and mosses to huge complex trees such as sequoia. Almost all plants contain chlorophyll and are autotrophs. Some plants are blood vessels-that is, they have specialized tissues that bring water and nutrients to various parts of the plant. Vascular plants include flowering plants, trees and the most familiar terrestrial plants. Other plants are non-vascular; they lack roots, stems, and leaves, and are usually aquatic. Some terrestrial plants, including mosses and liver plants, are also non-vascular. Terrestrial non-vascular plants are usually small. Their lack of vascular systems limits the amount of nutrients that can be transported to all cells. A few species of plants such as dodder and Indian tube are non-photosynthetic parasites, and others such as Venus-flytrap is photosynthetic but carnivorous-they trap insects as a source of nitrogen and minerals.
Living things are organisms classified in the kingdom of Animalia are multicellular eukaryotes. Because their cells lack chlorophyll, all animals are heterotrophic animals. They have different types of tissue in their bodies and are usually free to move. Animals are sometimes called metazoans, thus distinguishing them from single-celled protozoa. Animals can be divided into two categories: invertebrates and vertebrates. Invertebrates—such as insects, starfish(starfish)and worms-lack backbone. The body tissue of many invertebrates is supported by some kind of external structure, called an exoskeleton. Vertebrates have a backbone. Animals classified as vertebrates include fish; amphibians, such as frogs and salamanders; reptiles, such as snakes and lizards; birds; and mammals, such as dogs, cows, horses, monkeys and humans.