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Chapter 1

Biochemistry: What You Need to Know and Why

IN THIS CHAPTER

Understanding the importance of biochemistry

Looking at the parts and functions of animal cells

Seeing the differences between animal and plant cells

If you’re enrolled in a biochemistry course, you may want to skip this chapter and go right to the specific chapter(s) in which we discuss the material you’re having trouble with. But if you’re thinking about taking a course in biochemistry or just want to explore an area that you know little about, keep reading. This chapter gives you basic information about cell types and cell parts, which are extremely important in biochemistry.

Sometimes you can get lost in the technical stuff and forget about the big picture. This chapter sets the stage for the details.

Why Biochemistry?

We suppose the flippant answer to the question “Why biochemistry?” is “Why not?” or “Because it’s required.”

That first response isn’t a bad answer, actually. Look around. See all the living or once living things around you? The processes that allow them to grow, multiply, age, and die are all biochemical in nature. Sometimes we sit back and marvel at the complexity of life, fascinated by the myriad chemical reactions that are taking place right now within our own bodies and the ways in which these biochemical reactions work together so we can sit and contemplate them.

When John learned about the minor structural difference between starch and cellulose, he remembers thinking, “Just that little difference in the one linkage between those units is basically the difference between a potato and a tree.” That fact made him want to learn more, to delve into the complexity of the chemistry of living things, to try to understand. We encourage you to step back from the details occasionally and marvel at the complexity and beauty of life.

What Is Biochemistry and Where Does It Take Place?

Biochemistry is the chemistry of living organisms. Biochemists study the chemical reactions that occur at the molecular level of organisms. Biochemistry is normally listed as a separate field of chemistry. However, in some schools it’s part of biology, and in others it’s separate from both chemistry and biology.

Biochemistry really combines aspects of all the fields of chemistry. Because carbon is the element of life, organic chemistry (the study of carbon-based compounds) plays a large part in biochemistry. Many times, biochemists study how fast reactions occur — that’s an example of physical chemistry. Often, metals are incorporated into biochemical structures (such as iron in hemoglobin) — that’s inorganic chemistry. Biochemists use sophisticated instrumentation to determine amounts and structures — that’s analytical chemistry. And biochemistry is also similar to molecular biology; both fields study living systems at the molecular level, but biochemists concentrate on the chemical reactions that occur.

Biochemists may study individual electron transport within the cell, or they may study the processes involved in digestion. If it’s alive, biochemists study it.

Types of Living Cells

All living organisms contain cells. A cell is not unlike a prison cell. The working apparatus of the cell is imprisoned within the bars — known as the cell membrane. Just as a prison inmate can still communicate with the outside world, so can the cell’s contents. The prisoner must be fed, so nutrients must be able to enter every living cell. The cell has a sanitary system for the elimination of waste. And, just as inmates may work to provide materials for society outside the prison, a cell may produce materials for life outside the cell.

Cells come in two types: prokaryotes and eukaryotes. (Viruses also bear some similarities to cells, but these similarities are limited. In fact, many scientists don’t consider viruses to be living things.) Prokaryotic cells are the simplest type of cells. Many one-celled organisms are prokaryotes.

The simplest way to distinguish between these two types of cells is that a prokaryotic cell contains no well-defined nucleus, whereas the opposite is true for a eukaryotic cell.

Prokaryotes

Prokaryotes are mostly bacteria. Besides the lack of a nucleus, a prokaryotic cell has few well-defined structures. The prison cell’s structure has three components: a cell wall, an outer membrane, and a plasma membrane. This structure allows a controlled passage of material into and out of the cell. The materials necessary for proper functioning of the cell float about inside it, in a soup known as the cytoplasm. Figure 1-1 depicts a simplified version of a prokaryotic cell.

FIGURE 1-1: Simplified prokaryotic cell.

Eukaryotes

Eukaryotes are animals, plants, fungi, and protists (any organism that isn’t a plant, animal, or fungus). Many are unicellular organisms, like most algae, while other types of algae are multicellular. You consist mostly of eukaryote cells. In addition to having a nucleus, eukaryotic cells have a number of membrane-enclosed components known as organelles. Eukaryotic organisms may be either unicellular or multicellular. In general, eukaryotic cells contain much more genetic material than prokaryotic cells.

Animal Cells and How They Work

All animal cells (which, as the preceding section explains, are eukaryotic cells) have a number of components, most of which are considered to be organelles. These components, and a few others, are also present in plant cells (see the section “A Brief Look at Plant Cells,” later in this chapter). Figure 1-2 illustrates a simplified animal cell.

FIGURE 1-2: Simplified illustration of an animal cell.

The primary components of animal cells include

 Plasma membrane: This structure separates the material inside the cell from everything outside the cell. The plasma or cytoplasm is the fluid inside the cell. For the sake of the cell’s health, this fluid shouldn’t leak out. However, necessary materials must be able to enter through the membrane, and other materials, including waste, must be able to exit through the membrane. (Imagine what a cesspool that cell would become if the waste products couldn’t get out!) Transport through the membrane may be active or passive. Active transport requires that a price be paid for a ticket to enter (or leave) the cell. The cost of the ticket is energy. Passive transport doesn’t require a ticket. Passive transport methods include diffusion, osmosis, and filtration.

 Centrioles: These structures behave as the cell’s train conductors. They organize structural components of the cell like microtubules, which help move the cell’s parts during cell division.

 Endoplasmic reticulum: The cell can be thought of as a smoothly running factory. The endoplasmic reticulum is the main part of the cell factory. This structure has two basic regions, known as the rough endoplasmic reticulum (the factory assembly line for protein production), which contains ribosomes, and the smooth endoplasmic reticulum, which does not. (You can find out more about ribosomes and their function later in this list.) The rough endoplasmic reticulum, through the ribosomes, is the factory’s assembly line. The smooth endoplasmic reticulum is more like the shipping department, which ships the products of the reactions that occur within the cell to the Golgi apparatus.

 Golgi apparatus: This structure serves as the cell’s postal system. It looks a bit like a maze, and within it, materials produced by the cell are packaged in vesicles — small, membrane-enclosed sacs. The vesicles are then mailed to other organelles or to the cell membrane for export. The cell membrane contains customs officers (called channels) that allow secretion of the contents from the cell. Secreted substances are then available for other cells or organs.

 Lysosomes: These are the cell’s landfills. They contain digestive enzymes that break down substances that may harm the cell (Chapter 6 has a lot more about enzymes). The products of this digestion may then safely move out of the lysosomes and back into the cell. Lysosomes also digest no-longer functioning (dead) organelles. This slightly disturbing process, called autodigestion, is really part of the cell digesting itself. (We’ve never gotten that hungry!)

 Mitochondria: These structures are the cell’s power plants, where the cell produces energy. Mitochondria (singular mitochondrion) use food, primarily the carbohydrate glucose, to produce energy, which comes mainly from breaking down adenosine triphosphate (or ATP, to which we have dedicated Chapter 13).

 Nucleus/nucleolus: Each cell has a nucleus and, inside it, a nucleolus. These two regions serve as the cell’s control center and are the root from which all future generations originate. A double layer known as the nuclear membrane surrounds the nucleus. Usually the nucleus contains a mass of material called chromatin. If the cell is entering a stage leading to reproducing itself through cell division, the chromatin separates into chromosomes.In addition to conveying genetic information to future generations, the nucleus produces two important molecules for the interpretation of this information. These molecules are messenger ribonucleic acid (mRNA) and transfer ribonucleic acid (tRNA). The nucleolus produces a third type of ribonucleic acid known as ribosomal ribonucleic acid (rRNA). (Chapter 9 is all about nucleic acids.)

 Ribosomes: These components contain protein and ribonucleic acid subunits. In the ribosomes, amino acids are assembled into proteins. Many of these proteins are enzymes, which are part of nearly every process that occurs in the organism. (Part II of this book is devoted to amino acids, proteins, and enzymes.)

 Small vacuoles: Also known as simply vacuoles, these structures serve a variety of functions, including storage and transport of materials. The cell may later use these stored materials, or if the cell no longer needs these materials, they are simply waste.

A Brief Look at Plant Cells

Plant cells contain the same components as animal cells, plus a cell wall, a large vacuole, and, in the case of green plants, chloroplasts. Figure 1-3 illustrates a typical plant cell.

The cell wall is composed of cellulose. Cellulose, like starch, is a polymer of glucose. The cell wall provides structure and rigidity.

The large vacuole serves as a warehouse for large starch molecules. Glucose, which is produced by photosynthesis, is converted to starch, a polymer of glucose. At some later time, this starch is available as an energy source. (Chapter 7 talks a lot more about glucose and other carbohydrates.)

FIGURE 1-3: Simplified illustration of a plant cell.

Chloroplasts, present in green plants, are specialized chemical factories. These are the sites of photosynthesis, in which chlorophyll absorbs sunlight and uses this energy to combine carbon dioxide and water to produce glucose and release oxygen gas.

The green color of many plant leaves is due to the magnesium-containing compound chlorophyll.