Organisms are living things. All living things share six important characteristics. All living things have a cellular organization, contain similar chemicals, use energy, respond to their surroundings, grow and develop, and reproduce.
A cell is the basic unit of structure and function in an organism. Unicellular, or single-celled, organisms include bacteria, the most numerous organisms on Earth. Multicellular organisms are composed of many cells that are specialized.
Cells are composed of chemicals. The most abundant chemical in cells is water. Another chemical called carbohydrate is a cell's energy source. Proteins and lipids are the building materials of cells. Nucleic acids are the genetic materials that direct the cell's activities. Also, the cells of organisms use energy to grow and repair injured parts.
A change in an organism's surroundings that causes the organism to react is called a stimulus. An organism reacts to a stimulus with a response-an action or change in behavior.
Living things grow and develop. Growth is the process of becoming larger. Development is the process of change that occurs during an organism's life to produce a more complex organism.
Another characteristic of organisms is the ability to reproduce, or produce offspring that are similar to the parents. Living things arise from living things through reproduction.
People once believed the mistaken idea that living things arise from nonliving sources-an idea called spontaneous generation. Controlled experiments helped disprove spontaneous generation. In a controlled experiment, a scientist carries out two tests that are identical in every respect except for one factor, called the variable.
All organisms need four things to stay alive. All living things must satisfy their basic needs for water, food, living space, and stable internal conditions.
Organisms that make their own food are called autotrophs. Organisms that cannot make their own food are called heterotrophs. Heterotrophs consume autotrophs or other heterotrophs. All organisms need food, water, and living space. Because space on Earth is limited, some organisms compete for food and space.
Because conditions in their surroundings can change, organisms must be able to keep the conditions inside their bodies constant. The maintenance of stable internal conditions despite changes in surroundings is called homeostasis.
Classiﬁcation is the process of grouping things based on their similarities. Biologists use classiﬁcation to organize living things into groups so that the organisms are easier to study. The scientiﬁc study of how living things are classiﬁed is called taxonomy.
The Swedish scientist Carolus Linnaeus created a naming system for organisms called binomial nomenclature, in which each organism is given a twopart name. The ﬁrst part of an organism's scientiﬁc name is its genus. A genus is a classiﬁcation grouping that contains similar, closely related organisms. The second part of an organism's scientiﬁc name sets each species apart from one another in the genus. A species is a group of similar organisms that can mate and produce offspring that can also mate and reproduce.
Modern biologists classify organisms into eight levels: domain, kingdom, phylum, class, order, family, genus, and species. Organisms are grouped by their shared characteristics. The more classiﬁcation levels that two organisms share, the more characteristics they have in common.
Today, a three-domain system of classiﬁcation is commonly used. The three domains are Bacteria, Archaea, and Eukarya. Within the domains are kingdoms. Organisms are placed into domains and kingdoms based on their cell type, their ability to make food, and the number of cells in their bodies.
Members of the domain Bacteria are prokaryotes. Prokaryotes are organisms whose cells lack a nucleus. A nucleus is a dense area in a cell that contains nucleic acids-the chemical instructions that direct the cell's activities. In prokaryotes, nucleic acids are scattered throughout the cell.
Like bacteria, members of the domain Archaea are unicellular prokaryotes. Some archaea are autotrophs while others are heterotrophs. Although bacteria and archaea are similar, they are placed in different domains because there are important differences in the structure and chemical makeup of their cells.
Members of the domain Eukarya are eukaryotes-organisms with cells that contain nuclei. Scientists classify organisms in the domain Eukarya into one of four kingdoms: protists, fungi, plants, or animals.
Protists can be autotrophs or heterotrophs, unicellular or multicellular. Most fungi are multicellular eukaryotes. All fungi are heterotrophs. Most feed by absorbing nutrients from dead or decaying organisms.
Plants are all multicellular eukaryotes. In general, plants are autotrophs and feed almost all of the heterotrophs on land. All animals are multicellular eukaryotes. All animals are heterotrophs.
All living things are made of cells. Cells are the basic units of structure and function in living things. Most cells are too small to be seen with the naked eye.
The invention of the microscope made it possible for people to discover and learn about cells. A microscope is an instrument that makes small objects look larger. Some microscopes do this by using lenses to focus light. A simple light microscope contains only one lens. A light microscope that has more than one lens is called a compound microscope.
One of the ﬁrst people to observe cells was Robert Hooke. In 1663, Hooke observed the structure of a thin slice of cork using a compound microscope he had built himself. At about the same time, Anton van Leeuwenhoek built simple microscopes and used them to observe tiny objects. Leeuwenhoek called the single-celled organisms he saw animalcules.
In 1838, Matthias Schleiden concluded that all plants are made of cells. The next year, Theodor Schwann concluded that all animals are also made of cells. In 1855, Rudolf Virchow proposed that new cells are formed only from existing cells. Schleiden, Schwann, Virchow, and others helped develop the cell theory. The cell theory states: All living things are composed of cells; cells are the basic unit of structure and function in living things; all cells are produced from other cells.
For a microscope to be useful, it must combine two important properties-magniﬁcation and resolution. Magniﬁcation is the ability to make things look larger than they are. The lenses in light microscopes magnify an object by bending the light that passes through them. A lens that magniﬁes is thicker in the center than at the edges and is called a convex lens. Because a compound microscope uses more than one lens, it can magnify an object more than a simple microscope. The total magniﬁcation of a compound microscope is equal to the magniﬁcations of the two lenses multiplied together. The ability to clearly distinguish the individual parts of an object is called resolution. Resolution is another term for the sharpness of an image.
Since the 1930s, scientists have developed different types of electron microscopes. Electron microscopes use a beam of electrons instead of light to produce a magniﬁed image. Because they use tiny electrons to produce images, the resolution of electron microscopes is much better than the resolution of light microscopes.
A cell is very small. Inside a cell are even smaller structures called organelles, which carry out speciﬁc functions within the cell. The cell wall is a rigid layer of nonliving material that surrounds the cells of plants and some other organisms. A plant's cell wall helps to protect and support the cell. The cell wall is made of a strong, ﬂexible material called cellulose, and many materials can pass through it.
In cells that do not have cell walls, the cell membrane is the outside boundary that separates the cell from its environment. All cells have cell membranes. In cells with cell walls, the cell membrane is located just inside the cell wall. The cell membrane controls what substances come into and out of a cell.
The nucleus is a large, oval structure that acts as the "brain” of the cell. You can think of the nucleus as the cell's control center, directing all of the cell's activities. The nucleus is surrounded by a protective membrane called the nuclear envelope. Materials pass in and out of the nucleus through small openings, or pores, in the nuclear envelope. Inside the nucleus, strands of chromation contain genetic material. Also, the nucleolus is the place where ribosomes are made.
The cytoplasm is the region between the cell membrane and the nucleus. Many cell organelles are found in the cytoplasm. The mitochondria are known as the "powerhouses” of the cell because they convert energy in food molecules to energy the cell can use to carry out its functions. Passageways called the endoplasmic reticulum carry proteins and other materials from one part of the cell to another. Small, grainlike bodies called ribosomes function as factories to produce proteins. Collections of sacs and tubes called Golgi bodies receive proteins and other newly formed materials from the endoplasmic reticulum, package them, and distribute them to other parts of the cell. The Golgi bodies release materials outside the cell. In plants and some other organisms, large, green structures called chloroplasts capture energy from sunlight and use it to produce food for the cell. Large water-ﬁlled sacs called vacuoles are the storage areas of cells. A vacuole stores food and other materials needed by the cell. Small, round structures called lysosomes contain chemicals that break down certain materials in the cell.
Plants and animals contain many cells. In a many-celled organism, the cells are often quite different from each other and are specialized to perform speciﬁc functions. In many-celled organisms, cells are often organized into tissues, organs, and organ systems.
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