The Living Cell

 The Living Cell

In This Chapter

▶ Finding out what makes cells the basic units of life

▶ Taking a look at the structure of prokaryotic and eukaryotic cells

▶ Discovering how enzymes accelerate reactions

Every living thing has cells. The smallest creatures have only one, yet

they’re as alive as you are. What exactly is a cell? In plain and simple terms, it’s the smallest living piece of an organism — including you. Without cells, you’d be a disorganized blob of chemicals that’d just ooze out into the environment. And that is why the cell is the fundamental unit of life.

You get to explore the purpose and structure of cells in this chapter. And because cells rely on chemical reactions to make things happen, you also find out all about enzymes, which are proteins that help speed up the pace of chemical reactions.

An Overview of Cells

Cells are sacs of fluid that are reinforced by proteins and surrounded by

membranes. Inside the fluid float chemicals and organelles, structures inside

cells that are used during metabolic processes. (Yes, an organism contains

parts that are smaller than a cell, but these structures can’t perform all the functions of life on their own, so they aren’t considered to be alive.)

 A cell is the smallest part of an organism that retains characteristics of the entire organism. For example, a cell can take in fuel, convert it to energy, and eliminate wastes, just like the organism as a whole can. Because cells can perform all the functions of life (as shown in Figure 4-1), the cell is the smallest.

Cells can be categorized in different ways, according to structure or function, or in terms of their evolutionary relationships. In terms of structure, scientists categorize cells based on their internal organization:

 ✓ Prokaryotes don’t have a “true” nucleus in their cells. Nor do they have

organelles. Bacteria and archaea are all prokaryotes.

 ✓ Eukaryotes have a nucleus in their cells that houses their genetic material.

They also have organelles. Plants, animals, algae, and fungi are all eukaryotes.

Peeking at Prokaryotes

Prokaryotes include cells you’ve probably heard of, such as the bacteria E.

coli and Streptococcus (which causes strep throat), the blue-green algae that occasionally cause lake closures, and the live cultures of bacteria in yogurt, as well as some cells you may never have heard of, called archaeans.

Whether you’ve heard of a specific prokaryote or not, you’re likely well aware that bacteria have a pretty bad rap. They seem to make the papers only when they’re causing problems, such as disease. Behind the scenes, though, bacteria are quietly performing many beneficial tasks for people and the rest of life on planet Earth. Why, if bacteria could get some good headlines, those headlines might read a little something like this:

 ✓ Bacteria are used in human food production! Yogurt and cheese are

quite tasty, humans say.

 ✓ Bacteria can clean up our messes! Oil-eating bacteria help save

beaches, and other bacteria help clean up our sewage.

 ✓ Normal body bacteria help prevent disease! Bacteria living on the body

can prevent disease-causing bacteria from moving in.

 ✓ Bacteria are nature’s recyclers! Bacteria release nutrients from dead

matter during decomposition.

 ✓ Bacteria help plants grow! Nitrogen-fixing bacteria can pull nitrogen out

of the air and convert it to a form that plants can use.

The cells of prokaryotes are fairly simple in terms of structure because they

don’t have internal membranes or organelles like eukaryotic cells do (we

cover all the structures present in eukaryotic cells later in this chapter).

The majority of prokaryotic cells share these

characteristics:

 ✓ A plasma membrane forms a barrier around the cell, and a rigid cell wall

outside the plasma membrane provides additional support to the cell.

 ✓ DNA, the genetic material of prokaryotes, is located in the cytoplasm, in

an area called the nucleoid.

 ✓ Ribosomes make proteins in the cytoplasm.

 ✓ Prokaryotes break down food using cellular respiration (which requires

oxygen) and another type of metabolism

called fermentation (which doesn’t require oxygen).