In this section, you will explore the following questions:

  • What are the major structures of prokaryotic cells?
  • What limits the size of a cell?

Connection for AP® Courses

According to the cell theory, all living organisms, from bacteria to humans, are composed of cells, the smallest units of living matter. Often too small to be seen without a microscope, cells come in all sizes and shapes, and their small size allows for a large surface area-to-volume ratio that enables a more efficient exchange of nutrients and wastes with the environment.

There are three basic types of cells: archaea, bacteria, and eukaryotes. Both archaea and bacteria are classified as prokaryotes, whereas cells of animals, plants, fungi, and protists are eukaryotes. Archaea are a unique group of organisms and likely evolved in the harsh conditions of early Earth and are still prevalent today in extreme environments, such as hot springs and polar regions. All cells share features that reflect their evolution from a common ancestor; these features are 1) a plasma membrane that separates the cell from its environment; 2) cytoplasm comprising the jelly-like cytosol inside the cell; 3) ribosomes that are important for the synthesis of proteins, and 4) DNA to store and transmit hereditary information.

Prokaryotes may also have a cell wall that acts as an extra layer of protection against the external environment. The term “prokaryote” means “before nucleus,” and prokaryotes do not have nuclei. Rather, their DNA exists as a single circular chromosome in the central part of the cell called the nucleoid. Some bacterial cells also have circular DNA plasmids that often carry genes for resistance to antibiotics (Chapter 17). Other common prokaryotic cell features include flagella and pili.

The content presented in this section supports the learning objectives outlined in Big Idea 1 and Big Idea 2 of the AP® Biology Curriculum Framework. The AP® Learning Objectives merge essential knowledge content with one or more of the seven Science Practices. These objectives provide a transparent foundation for the AP® Biology course, along with inquiry-based laboratory experiences, instructional activities, and AP® exam questions.

Big Idea 1 The process of evolution drives the diversity and unity of life.
Enduring Understanding 1.D The origin of living systems is explained by natural processes.
Essential Knowledge 1.D.2 Scientific evidence from many different disciplines supports models of the origin of life.
Science Practice 4.1 The student can justify the selection of the kind of data needed to answer a particular scientific question.
Learning Objective 1.32 The student is able to justify the selection of geological, physical, chemical, and biological data that reveal early Earth conditions.
Essential Knowledge 2.A.3 Organisms must exchange matter with the environment to grow, reproduce and maintain organization.
Science Practice 2.2 The student can apply mathematical routines to quantities that describe natural phenomena.
Learning Objective 2.6 The student is able to use calculated surface area-to-volume ratios to predict which cell(s) might eliminate wastes or procure nutrients faster by diffusion.
Essential Knowledge 2.A.3 Organisms must exchange matter with the environment to grow, reproduce and maintain organization.
Science Practice 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices.
Learning Objective 2.7 The student will be able to explain how cell sizes and shapes affect the overall rate of nutrient intake and the rate of waste elimination.

Cells fall into one of two broad categories: prokaryotic and eukaryotic. Only the predominantly single-celled organisms of the domains Bacteria and Archaea are classified as prokaryotes (pro- = “before”; -kary- = “nucleus”). Cells of animals, plants, fungi, and protists are all eukaryotes (eu- = “true”) and have a nucleus.

Components of Prokaryotic Cells

All cells share four common components: 1) a plasma membrane, an outer covering that separates the cell’s interior from its surrounding environment; 2) cytoplasm, consisting of a jelly-like cytosol within the cell in which other cellular components are found; 3) DNA, the genetic material of the cell; and 4) ribosomes, which synthesize proteins. However, prokaryotes differ from eukaryotic cells in several ways.

prokaryote is a simple, single-celled (unicellular) organism that lacks a nucleus, or any other membrane-bound organelle. We will shortly come to see that this is significantly different in eukaryotes. Prokaryotic DNA is found in a central part of the cell: the nucleoid(Figure 4.5).

In this illustration, the prokaryotic cell has an oval shape. The circular chromosome is concentrated in a region called the nucleoid. The fluid inside the cell is called the cytoplasm. Ribosomes, depicted as small circles, float in the cytoplasm. The cytoplasm is encased by a plasma membrane, which in turn is encased by a cell wall. A capsule surrounds the cell wall. The bacterium depicted has a flagellum protruding from one narrow end. Pili are small protrusions that project from the capsule in all directions.
Figure 4.5 This figure shows the generalized structure of a prokaryotic cell. All prokaryotes have chromosomal DNA localized in a nucleoid, ribosomes, a cell membrane, and a cell wall. The other structures shown are present in some, but not all, bacteria.

EVERYDAY CONNECTION FOR AP® COURSES

While the Earth is approximately 4.6 billion years old, the earliest fossil evidence for life are of microbial mats that date back to 3.5 billion years.

What type of evidence for life was most likely found in a 3.5 billion year old rock?
  1. Scientists found bones buried in the rock that resemble bones of living animals.
  2. Dead cells buried in the rock superficially resemble living prokaryotic cells.
  3. The fossil superficially resembles living microbial mats that exist today.
  4. Scientists found fossilized prokaryotic cells in the rock that are able to grow and divide.

Most prokaryotes have a peptidoglycan cell wall and many have a polysaccharide capsule (Figure 4.5). The cell wall acts as an extra layer of protection, helps the cell maintain its shape, and prevents dehydration. The capsule enables the cell to attach to surfaces in its environment. Some prokaryotes have flagella, pili, or fimbriae. Flagella are used for locomotion. Pili are used to exchange genetic material during a type of reproduction called conjugation. Fimbriae are used by bacteria to attach to a host cell.

CAREER CONNECTION

Microbiologist

The most effective action anyone can take to prevent the spread of contagious illnesses is to wash his or her hands. Why? Because microbes (organisms so tiny that they can only be seen with microscopes) are ubiquitous. They live on doorknobs, money, your hands, and many other surfaces. If someone sneezes into his hand and touches a doorknob, and afterwards you touch that same doorknob, the microbes from the sneezer’s mucus are now on your hands. If you touch your hands to your mouth, nose, or eyes, those microbes can enter your body and could make you sick.

However, not all microbes (also called microorganisms) cause disease; most are actually beneficial. You have microbes in your gut that make vitamin K.

Microbiologists are scientists who study microbes. Microbiologists can pursue a number of careers. Not only do they work in the food industry, they are also employed in the veterinary and medical fields. They can work in the pharmaceutical sector, serving key roles in research and development by identifying new sources of antibiotics that could be used to treat bacterial infections.

Environmental microbiologists may look for new ways to use specially selected or genetically engineered microbes for the removal of pollutants from soil or groundwater, as well as hazardous elements from contaminated sites. These uses of microbes are called bioremediation technologies. Microbiologists can also work in the field of bioinformatics, providing specialized knowledge and insight for the design, development, and specificity of computer models of, for example, bacterial epidemics.

Cell Size

At 0.1 to 5.0 μm in diameter, prokaryotic cells are significantly smaller than eukaryotic cells, which have diameters ranging from 10 to 100 μm (Figure 4.6). The small size of prokaryotes allows ions and organic molecules that enter them to quickly diffuse to other parts of the cell. Similarly, any wastes produced within a prokaryotic cell can quickly diffuse out. This is not the case in eukaryotic cells, which have developed different structural adaptations to enhance intracellular transport.

Part a: Relative sizes on a logarithmic scale, from 0.1 nm to 1 m, are shown. Objects are shown from smallest to largest. The smallest object shown, an atom, is about 1 nm in size. The next largest objects shown are lipids and proteins; these molecules are between 1 and 10 nm. Bacteria are about 100 nm, and mitochondria are about 1 greek mu m. Plant and animal cells are both between 10 and 100 greek mu m. A human egg is between 100 greek mu m and 1 mm. A frog egg is about 1 mm, A chicken egg and an ostrich egg are both between 10 and 100 mm, but a chicken egg is larger. For comparison, a human is approximately 1 m tall.
Figure 4.6 This figure shows relative sizes of microbes on a logarithmic scale (recall that each unit of increase in a logarithmic scale represents a 10-fold increase in the quantity being measured).
 

Small size, in general, is necessary for all cells, whether prokaryotic or eukaryotic. Let’s examine why that is so. First, we’ll consider the area and volume of a typical cell. Not all cells are spherical in shape, but most tend to approximate a sphere. You may remember from your high school geometry course that the formula for the surface area of a sphere is 4πr2, while the formula for its volume is 4πr3/3. Thus, as the radius of a cell increases, its surface area increases as the square of its radius, but its volume increases as the cube of its radius (much more rapidly). Therefore, as a cell increases in size, its surface area-to-volume ratio decreases. This same principle would apply if the cell had the shape of a cube (see Figure 4.7). If the cell grows too large, the plasma membrane will not have sufficient surface area to support the rate of diffusion required for the increased volume. In other words, as a cell grows, it becomes less efficient. One way to become more efficient is to divide; another way is to develop organelles that perform specific tasks. These adaptations lead to the development of more sophisticated cells called eukaryotic cells.

Besides the volume of the cell, the size of the cell is also important for survival. As mentioned before, most cells are approximately spherical in shape. This is because a sphere is the shape with the largest surface area-to-volume ratio. As nutrients diffuse into the cell, a sphere is the shape where nutrients would have to travel the least distance to reach the center. This is important because nutrients and wastes are always exchanged at the periphery of the cell. The shorter the distance these nutrients and wastes have to travel, the faster the exchange of these molecules are.


VISUAL CONNECTION

On the left, a sphere 1 mm in diameter is encased in a box of the same width. On the right, the same sphere is encased in a box 2 mm in diameter.
Figure 4.7Notice that as a cell increases in size, its surface area-to-volume ratio decreases. When there is insufficient surface area to support a cell’s increasing volume, a cell will either divide or die. The cell on the left has a volume of 1 mm3 and a surface area of 6 mm2, with a surface area-to-volume ratio of 6 to 1, whereas the cell on the right has a volume of 8 mm3 and a surface area of 24 mm2, with a surface area-to-volume ratio of 3 to 1.
On average, prokaryotic cells are smaller than eukaryotic cells. What are some advantages to small cell size? What are some advantages to large cell size?
  1. Small, prokaryotic cells do not expend energy in intracellular transport of substances. Larger eukaryotic cells have organelles, which enable them to produce complex substances.
  2. Small, prokaryotic cells easily escape the spontaneous changes in environmental conditions. Large, eukaryotic cells have complex mechanisms to cope with such changes.
  3. Small, prokaryotic cells divide at a higher rate. Large, eukaryotic cells show division with genetic variations.
  4. Small, prokaryotic cells have fewer phospholipids in their membrane. Large, eukaryotic cells have more transport proteins in their phospholipid bilayer, supporting efficient transport of molecules.

 SCIENCE PRACTICE CONNECTION FOR AP® COURSES

ACTIVITY

Create an annotated diagram to explain how approximately 300 million alveoli in a human lung increases surface area for gas exchange to the size of a tennis court. Use the diagram to explain how the cellular structures of alveoli, capillaries, and red blood cells allow for rapid diffusion of O2 and CO2 among them.

THINK ABOUT IT

Which of the following cells would likely exchange nutrients and wastes with its environment more efficiently: a spherical cell with a diameter of 5 μm or a cubed-shaped cell with a side length of 7μm? Provide a quantitative justification for your answer based on surface area-to-volume ratios.