LITMIR.BIZ

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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.

Chapter 2

      Seems So Basic: Water Chemistry and pH

      IN THIS CHAPTER

      

Understanding the roles and properties of water

      

Exploring the differences between acids and bases

      

Controlling pH with buffers

      Water is one of the most important substances on Earth. People swim, bathe, boat, and fish in it. It carries waste from people’s homes and is used in the generation of electrical power. Humans drink it in a variety of forms: pure water, soft drinks, tea, coffee, beer, and so on. Water, in one form or another, moderates the temperature of the Earth and of the human body.

      In the area of biochemistry, water is also one of the lead actors. The human body is about 70 percent water. Water plays a role in the transport of material to and from cells. And many, many aqueous solutions take part in the biochemical reactions in the body.

      In this chapter, we examine the water molecule’s structure and properties. We explain how water behaves as a solvent. We also look at the properties of acids and bases, and the equilibria that they may undergo. Finally, we discuss the pH scale and buffers, including the infamous Henderson-Hasselbalch equation. So sit back, grab a glass of water (or your favorite water-based beverage), and dive in!

The Fundamentals of H₂O

      Water is essential to life; in fact, human beings are essentially big sacks of water. Water accounts for 60 to 95 percent of living human cells, and 55 percent of the water in the human body is in intracellular fluids. The remaining 45 percent (extracellular) is divided among the following:

       Plasma (8 percent)

       Interstitial (between cells) and lymph (22 percent)

       Connective tissue, cartilage, and bone (15 percent)

      Water also is necessary as a solvent for the multitude of biochemical reactions that occur in the body:

       Water acts as a transport medium across membranes, carrying substances into and out of cells.

       Water

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