Both living and nonliving things are made of atoms, molecules, and compounds. How are living and nonliving things different? The discovery of the cell was an important step toward answering this question.
THE DISCOVERY OF CELLS
All living things are made up of one or more cells. A cell is the smallest unit that can carry on all of the processes of life. Beginning in the 17th century, curious naturalists were able to use microscopes to study objects too small to be seen with the unaided eye. Their studies led them to propose the cellular basis of life.
Hooke
In 1665, English scientist Robert Hooke studied nature by using an early light microscope. A light microscope is an instrument that uses optical lenses to magnify objects by bending light rays. Hooke looked at a thin slice of cork from the bark of a cork oak tree. “I could exceedingly plainly perceive it to be all perforated and porous,” Hooke wrote. He described “a great many little boxes” that reminded him of the cubicles or “cells” where monks live. When Hooke focused his microscope on the cells of tree stems, roots, and ferns, he found that each had similar little boxes. The drawings that Hooke made of the cells he saw. The “little boxes” that Hooke observed were the remains of dead plant cells, such as the cork cells.
Leeuwenhoek
The first person to observe living cells was a Dutch trader named Anton van Leeuwenhoek. Leeuwenhoek made microscopes that were simple and tiny, but he ground lenses so precisely that the magnification was 10 times that of Hooke’s instruments. In 1673, Leeuwenhoek was able to observe a previously unseen world of microorganisms. He observed cells with green stripes from an alga of the genus Spirogyra and bell-shaped cells on stalks of a protist of the genus Vorticella . Leeuwenhoek called these organisms animalcules. We now call them protists.
THE CELL THEORY
Although Hooke and Leeuwenhoek were the first to report observing cells, the importance of this observation was not realized until about 150 years later. At this time, biologists began to organize information about cells into a unified understanding. In 1838, the German botanist Matthias Schleiden concluded that all plants were composed of cells. The next year, the German zoologist Theodor Schwann concluded the same thing for animals. And finally, in his study of human diseases, the German physician Rudolf Virchow (1821–1902) noted that all cells come from other cells. These three observations were combined to form a basic theory about the cellular nature of life. The cell theory has three essential parts, which are summarized below.
All living organisms are composed of one or more cells. Cells are the basic units of structure and function in an organism. Cells come only from the reproduction of existing cells.
Developments in Cell Biology
The discovery of cells and the development of the cell theory happened at the beginning of a revolutionary time in the history of science. Before the invention of the microscope, many questions about what makes up living and nonliving things could not be answered. Once cells could be observed, these questions could be explored. Scientists could then turn their attention to finding out how cells function.
The Cellular Basis of Life
Microscopes helped biologists clarify our definition of life. All living things share several basic characteristics. All living things consist of organized parts, obtain energy from their surroundings, perform chemical reactions, change with time, respond to their environments, and reproduce.
In addition, living things must be able to separate their relatively constant internal environment from the ever-changing external environment. The ability to maintain a constant internal environment, called homeostasis, will be discussed later. Finally, all living things share a common history. All cells share characteristics that indicate that cells are related to other living things.
1. Describe the major contributions of Hooke and Leeuwenhoek to cell biology.
2. Identify the advance that enabled Leeuwenhoek to view the first living cells.
3. Describe the research that led to the development of the cell theory.
4.State the three fundamental parts of the cell theory.
5. List three major events in the history of cell biology.
6. Name eight characteristics that all living things share.
7. Applying Concepts If you could go back in time, how would you explain the cell theory to someone who had never heard of cells?
8. Making Calculations
A biologist photographs a cell in a microscope magnified at 40 times. The cell in the photo is 2 mm in diameter. What is the true diameter of the cell in micrometers (μm)?
9. Justifying Conclusions
If organisms exist on other planets, would they consist of cells? Defend your answer.
Cells come in a variety of shapes and sizes that suit their diverse functions. There are at least 200 types of cells, ranging from flat cells to branching cells to round cells to rectangular cells.
CELL DIVERSITY
Cells of different organisms and even cells within the same organism are very diverse in terms of shape, size, and internal organization. One theme that occurs again and again throughout biology is that form follows function. In other words, a cell’s function influences its physical features.
Cell Shape
The diversity in cell shapes reflects the different functions of cells. . The long extensions that reach out in various directions from the nerve cell allow the cell to send and receive nerve impulses. The flat, platelike shape of skin cells suits their function of covering and protecting the surface of the body. As shown below, a cell’s shape can be simple or complex depending on the function of the cell. Each cell has a shape that has evolved to allow the cell to perform its function effectively.
Cells have various shapes. (a) Nerve cells have long extensions. (b) Skin cells are flat and platelike. (c) Egg cells are spherical. (d) Some bacteria are rod shaped. (e) Some plant cells are rectangular.
Cell Size
Cells differ not only in their shape but also in their size. A few types of cells are large enough to be seen by the unaided human eye. For example, the nerve cells that extend from a giraffe’s spinal cord to its foot can be 2 m (about 6 1/2 ft) long. A human egg cell is about the size of the period at the end of this sentence. Most cells, however, are only 10 to 50 μm in diameter, or about 1/500 the size of the period at the end of this sentence.
The size of a cell is limited by the relationship of the cell’s outer surface area to its volume, or its surface area–to-volume ratio. As a cell grows, its volume increases much faster than its surface area does. This trend is important because the materials needed by a cell (such as nutrients and oxygen) and the wastes produced by a cell (such as carbon dioxide) must pass into and out of the cell through its surface. If a cell were to become very large, the volume would increase much more than the surface area. Therefore, the surface area would not allow materials to enter or leave the cell quickly enough to meet the cell’s needs. As a result, most cells are microscopic in size.
Comparing Surface Cells
Materials microscope, prepared slides of plant (dicot) stem and animal (human) skin, pencil, paper Procedure Examine slides by using medium magnification . Observe and draw the surface cells of the plant stem and the animal skin.
Analysis How do the surface cells of each organism differ from the cells beneath the surface cells? What is the function of the surface cells? Explain how surface cells are suited to their function based on their shape.
Small cells can exchange substances more readily than large cells because small objects have a higher surface area–to-volume ratio.
BASIC PARTS OF A CELL
Despite the diversity among cells, three basic features are common to all cell types. All cells have an outer boundary, an interior substance, and a control region.
Plasma Membrane
The cell’s outer boundary, called the plasma membrane (or the cell membrane), covers a cell’s surface and acts as a barrier between the inside and the outside of a cell. All materials enter or exit through the plasma membrane. The surface of a plasma membrane is important.
Cytoplasm
The region of the cell that is within the plasma membrane and that includes the fluid, the cytoskeleton, and all of the organelles except the nucleus is called the cytoplasm. The part of the cytoplasm that includes molecules and small particles, such as ribosomes, but not membrane-bound organelles is the cytosol. About 20 percent of the cytosol is made up of protein.
Control Center
Cells carry coded information in the form of DNA for regulating their functions and reproducing themselves. The DNA in some types of cells floats freely inside the cell. Other cells have a membrane- bound organelle that contains a cell’s DNA. This membranebound structure is called the nucleus. Most of the functions of a eukaryotic cell are controlled by the cell’s nucleus. The nucleus is often the most prominent structure within a eukaryotic cell. It maintains its shape with the help of a protein skeleton called the nuclear matrix. The nucleus of a typical animal cell is siginificant.
A prokaryotic cell lacks a membranebound nucleus and membrane-bound organelles. Most prokaryotic cells are much smaller than eukaryotic cells are.
TWO BASIC TYPES OF CELLS
Fossil evidence suggests that the earliest cells on Earth were simple cells similar to some present-day bacteria. As cells evolved, they differentiated into two major types: prokaryotes and eukaryotes.
Prokaryotes
Prokaryotes (proh-KAR-ee-OHTS) are organisms that lack a membranebound nucleus and membrane-bound organelles. Although prokaryotic cells lack a nucleus, their genetic information—in the form of DNA—is often concentrated in a part of the cell called the nucleoid. Prokaryotes are divided into two domains: Bacteria and Archaea (ahr-KEE-uh). The domain Bacteria includes organisms that are similar to the first cellular life-forms. The domain Archaea includes organisms that are thought to be more closely related to eukaryotic cells found in all other kingdoms of life.
A white blood cell (eukaryotic) changes shape as it attacks purplestained bacterial cells that are much smaller (prokaryotic).
Eukaryotes
Organisms made up of one or more cells that have a nucleus and membrane-bound organelles are called eukaryotes (yoo-KAR-ee-OHTS). Eukaryotic cells also have a variety of subcellular structures called organelles, well-defined, intracellular bodies that perform specific functions for the cell. Many organelles are surrounded by a membrane. The organelles carry out cellular processes just as a person’s pancreas, heart, and other organs carry out a person’s life processes. Eukaryotic cells are generally much larger than prokaryotic cells which shows a white blood cell (eukaryote) destroying tiny bacterial cells (prokaryotes).
In a multicellular eukaryotic organisms, cells organize into tissues. Tissues organize into organs. Organs are part of organ systems, in which organs work together to perform body functions
CELLULAR ORGANIZATION
Over time, cells began to form groups that functioned together. Some cells retained the ability to live outside a group. Others became dependent on each other for survival.
Colonies
A colonial organism is a collection of genetically identical cells that live together in a connected group. Colonial organisms are not truly multicellular because few cell activities are coordinated.
True Multicellularity
As organisms evolved, their cells became more specialized and eventually were unable to survive independently. Groups of cells took on specific roles within the organism. A group of similar cells and their products that carry out a specific function is called a tissue. Groups of tissues that perform a particular job in an organism are called organs. An organ system is a group of organs that accomplish related tasks. The stomach and liver are organs that are part of the digestive system. Finally, several organ systems combine to make up an organism. This hierarchical organization found in multicellular organisms is important.
1. Describe the relationship between a cell’s shape and its function.
2. Explain the factor that limits cell size.
3. Identify and describe three basic parts of a cell.
4. Summarize the differences between prokaryotic cells and eukaryotic cells.
5. List four levels of organization that combine to form an organism.
6. Making Calculations If a cube-shaped cell grew from 1 cm per side to 3 cm per side, how much would its volume change?
7. Forming Reasoned Opinions Why do you think there are three basic structures common to all cell types? Support your answer.
8. Analyzing Processes How are the functions of prokaryotic cells controlled without a nucleus?
Tags: BASIC PARTS OF A CELL, BASIC TYPES OF CELLS, CELL DIVERSITY, Cell Shape, Cell Size, CELL THEORY, Cellular Basis of Life, CELLULAR ORGANIZATION, Comparing Surface Cells, Control Center, Cytoplasm, Developments in Cell Biology, DISCOVERY OF CELLS, Eukaryotes, Plasma Membrane, Prokaryotes, rectangular cells, round cells, True Multicellularity