MICROORGANISMS AND BEYOND
4.2 THE BASICS OF THE CELL
The smallest unit able to sustain life is called a cell. A cell can be simply defined as a container of small and large molecules that are essential for the survival of an organism. Unicellular organisms, such as algae, bacteria, and protozoa, contain all
BOx 4.1 NOT ALL MICROORGANISMS ARE GOOD FOR YOU Not all cells and microorganisms are helpful to humankind. Many foodborne ill-nesses are caused by the ingestion of harmful microbes. According to the Center for Disease Control, one in six Americans (nearly 48 million people) will get sick from food poisoning in their lifetime, and 3000 will die of foodborne diseases. Although molds, viruses, and parasites all pose a potential risk of food contamination, some strains of bacteria are the most common culprits. One strain of bacteria, Salmonella, is responsible for a quarter of foodborne illnesses and impacts 1.2 million cases annually in the United States. Most of the transmission comes from food, water, or contact with infected poultry. Raw or unpasteurized milk and milk products pose a special risk of Salmonella illness. Escherichia coli O157 is a strain of bacteria that lives in the intestines of some cattle, swine, and deer. Improperly cooked ground meat that is contaminated by O157 causes hemorrhagic diarrhea and intense abdominal cramps, even resulting in death in extreme cases. Heating milk to 161°F/72°C for 20 s is enough to kill most harmful bacteria including Salmonella and E. coli O157 often found in milk. Another common form of food poisoning is from the bacterium Clostridium botulinum. This bacterium, commonly found in soil, produces several neurotoxins that act to stop the nerves from signaling, caus-ing paralysis. Although botulism food poisoncaus-ing is rare, the proteins that produce the neurotoxins are commonly used for cosmetic reasons to paralyze nerves (Botox injections). How can you reduce your chances of getting food poisoning? Wash fresh fruits and vegetables (even the outer parts of the food that you do not eat, like watermelon rinds and orange peels). Keep your food preparation and eating areas clean. Cook foods, as most cells and microbes do not survive in high cooking temperatures due to disruption of the cell membrane and protein denaturation.
the molecular machinery within a single cell that is necessary for their survival. When cells evolved to live as a collection or group, some organisms became, by definition, multicellular and develop cells that have distinct functions. Although the behavior and biology of the cells in uni‐ and multicellular organisms vary dramatically, several char-acteristics are common among cells. Cells are organized into specialized compartments called organelles. Every cell needs a barrier called a cell membrane or a cell wall that allows specific molecules into or out of the cell. A cell needs to import or make its own food for energy and the creation of new molecules. Common for both plant and animal cells is the thin outer membrane called the plasma membrane. Made of a combination of lipids (phospholipids and cholesterol) and proteins (associated at the surface and those that go all the way through the membrane—transmembrane proteins), the plasma membrane regulates the traffic of molecules entering and leaving the cell. Flavor mole-cules bind to transmembrane proteins found in specialized taste bud cells, signaling to other integral membrane proteins that a flavor molecule is in the food. The region of cells between the nucleus and plasma membrane is called the cytoplasm. Within the cytoplasm are various organelles and proteins. Enzymes, the cell’s catalysts, are responsible for the breakdown and synthesis of molecules in a cell. A diverse and dynamic range of other molecules including carbohydrates, small molecules, nucleo-tides (DNA and RNA), and 20,000–30,000 different proteins also play a key role in the function of the cell. Importantly, residing within DNA is the genetic blueprint or hered-itary information of the cell, which is passed from cell to cell during cell division.
Organisms are classified into two groups, prokaryotes and eukaryotes, based upon the characteristics of their cell or cells. The earliest forms of life are the unicellular prokaryote organisms. The distinguishing cellular feature of prokaryotes is the absence of a nucleus, an organized compartment/organelle that stores the cell’s DNA.
Bacteria are a species of prokaryotes. Plant, fungi, and animal cells contain a nucleus and are thus eukaryotes. Not only do eukaryotes possess a nucleus, but they also contain several types of specialized organelles or structures including mitochondria, chloroplasts, nucleus, and endoplasmic reticulum.
4.2.1 Plant Cells
Plants are multicellular organisms with specialized cells (Fig. 4.1). These have both mitochondria and chloroplasts to process energy for the plant cell. In addition to nuclei, endoplasmic reticulum, and other eukaryotic organelles, plant cells have a rigid structurally complex cell wall made of a number of large and small molecules.
The cell wall is a particularly important part of food and cooking. The plant cell wall is a mixture of complicated carbohydrates and proteins giving plants rigid support.
Specific to food and cooking are some very interesting components. The cellulose, hemicellulose, and pectin are complex carbohydrate polymers that add fiber to food.
Cellulose and hemicellulose serve as thickeners and emulsifiers in some recipes. The pectin in fruit is used to solidify jellies under acidic conditions. About 10% of the plant cell wall is composed of polyphenolic compounds called lignins. Phenols are organic ring structures with the ─OH (hydroxyl) functional group attached. Lignins are a complex set of diverse compounds found in plants and algae (Fig. 4.2). A special
subfamily of polyphenols called lignin (a mixture of polyphenols) gives food and drink an astringent flavor. When oxidized by plant cell enzymes some polyphenols become brown, taste bitter and help plants fight infection.
4.2.2 Animal Cells
Animal cells do not have a plant cell wall or chloroplasts and are highly diverse depend-ing on the tissue each cell comes from. Most animal cells will have nucleus, mitochon-drion, Golgi, and other components. For food and cooking we focus on meat. Muscle cells have a slightly different nomenclature that is described later in this book (Fig. 4.3).
Perhaps you are thinking, how do these cells come together to make a functioning organism or a plant or animal tissue that might be cooked or eaten? Cells do not naturally
Nucleus
The location of the cell’s genetic material called DNA
Cell membrane
Thin, balloon-like layer constructed of amphiphilic phospholipids and proteins. Water and other molecules
can pass through the membrane Amyloplasts
Hold starch granules for storage.
Plant storage cells have many amyloplasts (e.g., cells in a potato)
Chloroplast
Filled with green chlorophyll.
Chlorophyll is a molecule that captures the energy of light so the plant can make glucose in a process called photosynthesis
Vacuole
Contains mostly water, but also enzymes, sugars, proteins, and other small molecules. Can swell with water to fill 90% of the cell’s volume
Cell wall
Strong, rigid layer that surrounds the cell membrane. Provides structure to plant cell
Endoplasmic reticulum
Golgi
Mitochondrion
FIGURE 4.1 The plant cell.
HC
C C H
CH
OH Basic phenol structure
General lignin structure HC
HC
HO
HO
HO HO
HO HO
HO OH
OH
OH
OH OH
OH OH O
O
O
O O
O
O O
O O OMe
OMe
OMe
OMe
OMe OMe
MeO
MeO MeO
Lignin Lignin
FIGURE 4.2 Polyphenols. Lignin is a highly diverse and modified polymer of phenol.
Lignin is a major component of plant cell walls and in our food and drink.
Lysosome Golgi
apparatus
Ribosomes
Membrane
Cytoplasm
Rough endoplasmic reticulum
Perixosome
Nucleus Mitochondrion
Centriole
FIGURE 4.3 The animal cell.
“stick” together. Groups of cells organized into plant and animal tissue are embedded in a jelly-like substance that acts as “cellular glue.” The material surrounding the cells is a mixture of carbohydrates and proteins that are collectively called the extracellular matrix. This matrix is the mortar that holds the cells together into a cohesive mass. As in a tissue, some of these extracellular matrix components are useful in a variety of foods.
Collagen, a protein found in the extracellular matrix, is used as a thickener in sauces and is the main component of gelatin and the gelatin dessert, JELL‐O. Plant extracellular matrix components, such as pectin, are used in cooking fruit pies.
4.2.3 Yeast Cells
Yeast cells are single‐cell eukaryote cells in the fungi kingdom containing organelles such as mitochondria and an enclosed nucleus, but no cell wall. Found in nearly every environment, yeast is very diverse with many strains. Yeast is terribly important in food, beverages, baking, and cooking (Fig. 4.4). Baking bread requires the metabolism of sugars into CO2 gas to give bread its rise. Because of its mitochondria, yeast can use a number of different food sources and convert carbohydrates, fats, and proteins to a range of final products including carbon dioxide, ethanol, and acetic acid. Under oxygen‐rich conditions, yeast will produce a mixture mostly of CO2 and some ethanol. Grown in an oxygen‐depleted environment, yeast will switch to producing primarily ethanol.
4.2.4 Bacteria Cells
Bacteria comprise a broad class of prokaryotic single‐cell organisms without a nuclear envelope and many of the organelles of eukaryotes (Fig. 4.5). Bacteria are an amazingly diverse class of microorganisms, some of which are involved in human FIGURE 4.4 Yeast cells. Budding yeast cells (Saccharomyces cerevisiae) used for baking and brewing.
health and in disease. Throughout this chapter we discuss bacteria as part of the preparation of food. Bacteria that produce and can live in acidic conditions are used to make cheese, and some of the same bacteria are encouraged to grow on meat as it cures and ages. In this case, the bacterium helps produce acid that inhibits unhealthy bacterial growth. Lactic acid‐producing bacteria are used to produce yogurt, sauer-kraut, and fermented meats and sausages. Some winemakers will use bacteria to encourage new flavors including a green apple malonate flavor (malolactic fermen-tation) to wines. Yet others will encourage bacterial growth in wine to produce wine vinegar.
Having a better understanding of the nature of a cell and how a cell is organized will help you better understand ingredients and steps in cooking and baking.