Modern cell theory, based on the work of scientists Theodor Schwann and Matthias Schleiden in 1839, states that all living things are made up of microscopic, functional units called cells. Although nearly 200 years old and modified through the years, cell theory is still considered one of the most basic and important principles of biology. Modern cell theory includes a few basic assertions or statements of fact.
TL;DR (Too Long; Didn't Read)
The modern version of cell theory serves as the basis of all biology. The theory states that all living organisms consist of cells. This theory also asserts that cells: are functional and structural units of living things, formed by pre-existing cells via cell division, contain hereditary information, have the same basic chemical composition and the energy flow of all living creatures occurs within these structural units.
Cells Make Up All Life
First and foremost, modern cell theory states that all living organisms consist of cells. From microscopic, single-celled organisms to enormous elephants, from trees and shrubs to human beings, all living organisms, regardless of scale or species, consist of cells. Life is defined as organisms which grow, respond to stimuli, adapt and reproduce. Without cells, none of these most basic life functions would be possible.
Cells Are Functional, Structural Units
Cells represent more than the solid building blocks of living creatures. Cells provide structure, but they also bestow functions necessary to maintain life. The second tenant of modern cell theory states that cells are functional and structural units.
Some microscopic organisms contain just a single cell. But most living creatures are made up of a vast multitude of cells. For example, the average human being is usually composed of around 50 trillion cells.
Cells Are Formed by Division
Under this theory, all cells are created by division. This means that cells cannot simply appear from nowhere, nor do they appear in an erratic, unorganized way. The cells of all living life forms replicate by dividing in two.
Cell division consists of five phases known as mitosis. During prophase, chromosomes, containing the cell's DNA, gather in the cell's nucleus. In prometaphase, the nucleus around the chromosomes breaks down, and proteins reach across the cell. In metaphase, chromosomes break down into chromatids -- pieces of chromosome which contain DNA -- and align in the cell's center as it begins to lengthen. In anaphase, chromatids are pulled to opposite ends of the cell, and cells begin to divide in two. In telophase, a nucleus forms around each group of chromatids, and the cell splits entirely in two. Two new cells now exist with identical chromosomes and DNA.
Without cell division, life forms could not continuously shed and replace certain parts of themselves the way human beings shed skin cells, nor would they be able to grow or create reproductive cells.
Cells Contain Hereditary Information
Hereditary information consists of all genetic information passed from one generation of a living creature to the next. Hereditary information, for example, ensures that a pregnant cat only gives birth to other cats, as opposed to some other kind of animal. It also guarantees that the kittens will be similar in some way to their parents. This could mean that they will have almost identical fur color, eye color, body size or personalities.
Hereditary information is stored inside the cells of every living being. This information is found inside a cell's nucleus within the chromosomes containing DNA. Made of long chains of amino acids, DNA forms the blueprint for all life. The specific map of amino acids in each living individual is the genome. Without the hereditary information stored in cells, living organisms of any size would not be able to pass on their traits to the next generation.
Cells Are Chemically the Same
As different as some animals and plants may seem from one another, all cells in all living creatures contain the same basic chemical composition. Though certain types of cells, like reproductive cells, may differ slightly from skin cells, for example, all cells are mostly made up of the elements carbon, nitrogen, oxygen and hydrogen. In fact, 95.6 percent of every living organism's weight consists of these four elements.
Energy Flow Occurs Within Cells
Energy flow refers to any process in which energy gets transferred from an outside source to the inside of a living creature's body, or any process by which energy is transferred between a living being's cells. Inside the human brain, information is transferred from cell to cell via electrical impulses. This electricity provides an example of energy flow. Another key example is the process by which organisms convert food into usable energy. When an animal eats food, its cells absorb the nutrients inside of the food, and break them down into energy used to fuel its growth and maintain its basic functions.
Plant cells convert sunlight into usable energy, via a process called photosynthesis. Plastids, tiny organelles inside of plant cells, help to convert the sunlight into nutrition to fuel the plant's body.
The discovery of cells marked the beginning of a whole new chapter in scientific study in 1839. Though modern cell theory is based on tenants which were first recorded nearly 200 years ago, it still stands as the basis for all modern biological study.