In unicellular (single-celled) organisms, the single cell performs all life functions. It functions independently. However, multicellular (many celled) organisms have various levels of organization within them. Individual cells may perform specific functions and also work together for the good of the entire organism. The cells become dependent on one another.
KEY POINTS
The atom is the smallest and most fundamental unit of matter, and the bonding of at least two atoms or more form molecules.
The simplest level of organization for living things is a single organelle, which is composed of aggregates of macromolecules.
The biological levels of organization of living things arranged from the simplest to most complex are: atom, molecule, organelle, cells, tissues, organs, organ systems, organisms
Living things are highly organized and structured, following a hierarchy that can be examined on a scale from small to large. The atom is the smallest and most fundamental unit of matter. It consists of a nucleus surrounded by electrons. Atoms form molecules which are chemical structures consisting of at least two atoms held together by one or more chemical bonds.
From Organelles to BiospheresMacromolecules can form aggregates within a cell that are surrounded by membranes; these are called organelles. Organelles are small structures that exist within cells. Examples of these include: mitochondria and chloroplast, which carry out indispensable functions. Mitochondria produce energy to power the cell, while chloroplasts enable green plants to utilize the energy in sunlight to make sugars.
All living things are made of cells, and the cell itself is the smallest fundamental unit of structure and function in living organisms. (This requirement is why viruses are not considered living: they are not made of cells. To make new viruses, they have to invade and hijack the reproductive mechanism of a living cell; only then can they obtain the materials they need to reproduce. ) Some organisms consist of a single cell and others are multicellular.
Cells are classified as prokaryotic or eukaryotic. Prokaryotes are single-celled or colonial organisms that do not have membrane-bound nuclei; in contrast, the cells of eukaryotes do have membrane-bound organelles and a membrane-bound nucleus. In larger organisms, cells combine to make tissues, which are groups of similar cells carrying out similar or related functions. Organs are collections of tissues grouped together performing a common function. Organs are present not only in animals but also in plants. An organ system is a higher level of organization that consists of functionally related organs. Mammals have many organ systems. For instance, the circulatory system transports blood through the body and to and from the lungs; it includes organs such as the heart and blood vessels. Furthermore, organisms are individual living entities. For example, each tree in a forest is an organism. Single-celled prokaryotes and single-celled eukaryotes are also considered organisms and are typically referred to as microorganisms.
Atom
Molecule
Organelle
Cell
Tissue
Organ
Organ System
Organism
Practice Problems
1. What is the basic unit of structure and function in living things? a. Cell b. Tissue c. Organ d. Organ system
2. A living thing that is made up of a group of organ systems is a(n)? a. Cell b. Organ c. Tissue d. Organism
3. Cells are to tissues as tissues are to _______. a. Organs b. Bacteria c. Organ Systems d. Organisms
4. Blood, which is made up of a group of special cells, is an example of which of the following? a. Cell b. Tissue c. Organ system d. Organism e. Organ
5. A human's skeleton is an example of which of the following? a. Organ System b. Organism c. Organ d. Cell e. Tissue
6. An animal's digestive system represents which of the following levels of organization? a. Cellular system b. Organ system c. Tissues d. Organs
7. The stomach is an example of which of the following? a. Cell b. Tissue c. Organism d. Organ e. Organ system
8. Tissues that work together to carry out a specific job are called a(n) ______. a. Organism b. Cell c. Organ system d. Organ