BIO734 Official Handouts

Chapter 1

Cells

In This Chapter:

Introduction · Reproduction · Solved Problems · Introduction

A cell is the smallest unit that exhibits all of the qualities associated with the living state. Cells must obtain energy from an external source to carry on such vital processes as growth, repair, and reproduction. All of the chemical and physical reactions that occur in a cell to support these functions constitute its metabolism. Metabolic reactions are catalyzed by enzymes. Enzymes are protein molecules that accelerate biochemical reactions without being permanently altered or consumed in the process. The structure of each enzyme (or any other protein) is encoded by a segment of a deoxyribonucleic acid (DNA) molecule referred to as a gene. Molecular and cell biology are the sciences that study all life processes within cells and at the molecular level. In doing so, these sciences draw upon knowledge from several scientific disciplines, including biochemistry, cytology, genetics, microbiology, embryology, and evolution.

Structurally, there are two basic kinds of cells: prokaryotic and eukaryotic. Prokaryotic cells, including bacteria and archae, although far from simple, are generally much smaller and less complex structurally than eukaryotic cells. The major difference is that the genetic material (DNA) is not sequestered within a double-membrane structure called a nucleus (see Figure 1-1). In eukaryotes, a complete set of genetic instructions is found on the DNA molecules, which exist as multiple linear structures called chromosomes that are confined within the nucleus. Eukaryotic cells also contain other membrane-bound organelles within their cytoplasm (the region between the nucleus and the plasma membrane). These subcellular structures vary tremendously in structure and function. Most eukaryotic cells have mitochondria, which contain the enzymes and machinery for aerobic respiration and oxidative phosphorylation. Thus, their main function is generation of adenosine triphosphate (ATP), the primary currency of energy exchanges within the cell. This organelle is bounded by a double membrane. The inner membrane, which houses the electron transport chain and the enzymes necessary for ATP synthesis, has numerous foldings called cristae, which protrude into the matrix, or central space. Mitochondria contain their own DNA and ribosomes, but most of their proteins are imported from the cytoplasm.

You Need to Know

Mitochondria are nicknamed the “powerhouses” of the cell because of their role in ATP production.

Chloroplasts contain the photosynthetic systems for utilizing the radiant energy of sunlight and are found only in plants and algae. Photo- synthesis is the process that converts light energy into the chemical bond energy of ATP, which in turn can be used to convert carbon dioxide (CO2) and water (H2O) into carbohydrates. Chloroplasts contain an internal system of membranes called thylakoids, a circular chromosome, and their own ribosomes. The flattened, vesicular thylakoids contain the chlorophyll pigments, the enzymes, and other molecules needed to harness light energy for conversion to chemical energy. Carbon fixation occurs in the stroma, the space between the thylakoids and the inner membrane. Prokaryotic cells lack internal membranes, but photosynthetic bacteria contain invaginations of the plasma membrane called mesosomes. Centrioles, located within the centrosome, are associated with the cell’s polar regions, toward which the chromosomes migrate during cell division, and are found only in animal cells. The endoplasmic reticulum (ER) amplifies the surface area available for specialized biochemical reactions and the synthesis of certain types of proteins. The Golgi complex directs the transport of proteins and other biomolecules to specific locations within the cell. Vacuoles serve as storage compartments for food, water, or other molecules. Enzymes digest materials brought into the cell within lysosomes. Ribosomes function in the manufacture of proteins. The ribosomes in prokaryotes are smaller than those found in the cytoplasm in eukaryotes, but are similar in size and structure to those found in the mitochondria and chloroplasts of eukaryotes. Eukaryotic ribosomes associated with the ER give it a granular appearance, hence the name rough ER.

Remember

Proteins that are:

1. membrane bound
2. secreted
3. compartmentalized

are synthesized on the ER.