THE EUKARYOTIC CELL

Eukaryotic cells are typically 1000 to 10,000 times larger in volume than most bacteria and archaea. Although some live independent lives as single-celled organisms, such as amoebae and yeasts (Figure 1–16), others live in multicellular assemblies. As we have seen, all of the more complex multicellular organisms—including plants, animals, and fungi such as mushrooms—are formed from eukaryotic cells.

A micrograph shows Saccharomyces cerevisiae. — **Figure 1–16** **Yeasts are simple, free-living eukaryotes.** The cells shown in this micrograph belong to the species of yeast, *Saccharomyces cerevisiae*, used to make dough rise and turn malted barley juice into beer. As can be seen in this image, the cells reproduce by growing a bud and then dividing asymmetrically into a large mother cell and a small daughter cell; for this reason, they are called budding yeast.

By definition, all eukaryotic cells have a nucleus. But possession of a nucleus goes hand-in-hand with possession of a variety of other organelles, most of which are membrane-enclosed and common to all eukaryotic organisms but absent in prokaryotes. In this section, we take a look at the main organelles found in eukaryotic cells from the point of view of their functions, and we consider how they came to serve the roles they have in the life of the eukaryotic cell. Unraveling the roles these organelles play in the life of a eukaryotic cell is one of the crowning achievements of biological exploration, and it laid the foundation for understanding the biology of our own cells—and how these systems become impaired by aging and disease.

The Nucleus Is the Information Store of the Cell

The nucleus is typically the most prominent organelle in a eukaryotic cell (Figure 1–17). It is enclosed within two concentric membranes that form the nuclear envelope, and it contains molecules of DNA—extremely long polymers that encode the genetic information of the organism. In the light microscope, these giant DNA molecules become visible as individual chromosomes when they become more compact before a cell divides into two daughter cells (Figure 1–18). Bacteria and archaea also store their genetic information in the form of DNA; however, these cells do not keep their DNA corralled inside a nuclear envelope, segregated from the rest of the cell contents.

Schematic A shows an animal cell and micrograph B shows the nucleus of a mammalian cell. — **Figure 1–17** **The nucleus contains most of the DNA in a eukaryotic cell.** (A) This drawing of a typical animal cell shows its extensive system of membrane-enclosed organelles. The nucleus is colored *brown*, the nuclear envelope is *green*, and the cytoplasm (the interior of the cell outside the nucleus) is *white.* (B) An electron micrograph of the nucleus in a mammalian cell. Individual chromosomes are not visible because at this stage of the cell-division cycle, the DNA molecules are dispersed as fine threads throughout the nucleus. (B, by permission of E.L. Bearer and Daniel S. Friend.)

Three micrographs depict the condensation of chromosomes during cell division. — **Figure 1–18** **Chromosomes become visible when a cell is about to divide.** As a eukaryotic cell prepares to divide, its DNA molecules become progressively more compacted (condensed), forming wormlike chromosomes that can be distinguished in the light microscope (see also Figure 1–5). The photographs here show three successive steps in this chromosome condensation process in a cultured cell from a newt’s lung; note that in the last micrograph on the right, the nuclear envelope has broken down in preparation for the chromosomes to move into position for cell division. (Courtesy of Conly L. Rieder, Albany, New York.) (Dynamic Figure) In an early fly embryo (*below*), nuclei divide without accompanying cell division, producing a cell that has thousands of nuclei. These nuclei divide synchronously, their chromosomes (*green*) condensing and being pulled into daughter nuclei by a cytoskeletal machine (*red*) called the mitotic spindle. (Courtesy of William Sullivan.) For another view of the mitotic spindle, see Figure 1–29.

Mitochondria Generate Usable Energy from Food Molecules

Mitochondria (singular, mitochondrion) are present in essentially all eukaryotic cells, and they are among the most conspicuous organelles present in the cytoplasm (see Figure 1–8B). When seen with an electron microscope, individual mitochondria are found to be enclosed in two separate membranes, with the inner membrane formed into folds that project into the interior of the organelle (Figure 1–19).

A micrograph depicts the structure of a mitochondrion, diameter approximately 1 micrometer. — **Figure 1–19** **Mitochondria have a distinctive internal structure.** (A) An electron micrograph of a cross section of a mitochondrion reveals the extensive infolding of the inner membrane. (B) This three-dimensional representation of the arrangement of the mitochondrial membranes shows the smooth outer membrane (*gray*) and the highly convoluted inner membrane (*red*). The inner membrane contains most of the proteins responsible for energy production in eukaryotic cells; it is highly folded to provide a large surface area for this activity. (C) In this schematic cell, the innermost compartment of the mitochondrion is colored *orange*. (A, courtesy of Daniel S. Friend, by permission of E.L. Bearer.)

A schematic depicts the structure of a mitochondrion. — **Figure 1–19** **Mitochondria have a distinctive internal structure.** (A) An electron micrograph of a cross section of a mitochondrion reveals the extensive infolding of the inner membrane. (B) This three-dimensional representation of the arrangement of the mitochondrial membranes shows the smooth outer membrane (*gray*) and the highly convoluted inner membrane (*red*). The inner membrane contains most of the proteins responsible for energy production in eukaryotic cells; it is highly folded to provide a large surface area for this activity. (C) In this schematic cell, the innermost compartment of the mitochondrion is colored *orange*. (A, courtesy of Daniel S. Friend, by permission of E.L. Bearer.)

Microscopic examination by itself, however, gives little indication of what mitochondria actually do for the cell. Their function was discovered by breaking open cells and then spinning the soup of cell fragments in a centrifuge; this treatment separates organelles and other subcellular structures according to their size and density. Purified mitochondria were then tested to see what chemical processes they could perform. This approach revealed that mitochondria are generators of chemical energy for the cell. They harness the energy from the oxidation of food molecules, such as sugars, to produce adenosine triphosphate, or ATP—the basic chemical fuel that powers most of the cell’s activities. Because the mitochondrion consumes oxygen and releases CO₂ in the course of this activity, the entire process is called cell respiration—essentially, breathing at the level of the cell. Without mitochondria, eukaryotes such as animals, fungi, and plants would be unable to use oxygen to extract the energy they need from the food molecules that nourish them, a process we consider in detail in Chapter 14.

Mitochondria contain their own DNA and reproduce by dividing. Their strong resemblance—both in appearance and in DNA sequence—to modern-day bacteria provides persuasive evidence that mitochondria evolved from an aerobic bacterium that was engulfed by an anaerobic ancestor of present-day eukaryotic cells. This event created a symbiotic relationship that provided both partner cells with metabolic support, enabling them to survive and reproduce. The observations of the Asgard archaeon in close association with its prokaryotic partner cells strongly suggest that the original capturing cell was an archaeon (Figure 1–20).

An illustration demonstrates the process of an archaeon engulfing a bacteria. — **Figure 1–20** **Mitochondria are thought to have evolved from engulfed bacteria.** It is extremely likely that mitochondria evolved from aerobic bacteria that were captured and subsequently engulfed by an ancient, anaerobic archaeal cell. According to this model, the surface protrusions of an ancient, Asgard-like archaeon expanded and surrounded an aerobic bacterium, fostering a symbiotic relationship between the two cells. Over time, the expanded protrusions fused with one another, trapping the bacterium as an endosymbiont within the body of the archaeon, where it initially remained enclosed by an internal membrane derived from the archaeal plasma membrane. At some point, the symbiont escaped from this archaeal membrane and entered the cell’s cytosol, where it eventually evolved into a mitochondrion containing DNA and surrounding membranes derived from the engulfed bacterium. As indicated, the archaeal plasma membrane that folded inward during the process of protrusion, expansion, and fusion is proposed to have formed both the nuclear envelope and the internal membrane-enclosed organelles, such as the endoplasmic reticulum. (Adapted from H. Imachi et al., *Nature* 577:519–525, 2020 and from D.A. Baum and B. Baum, *BMC Biol.* 12:76–92, 2014.)

Chloroplasts Capture Energy from Sunlight

Chloroplasts are large, green organelles that are found in the cells of plants and algae, but not in the cells of animals or fungi. These organelles have an even more complex structure than mitochondria: in addition to their two surrounding membranes, they possess internal stacks of membranes containing the green pigment chlorophyll (Figure 1–21).

Micrograph A shows a leaf cell with chloroplasts. Schematic B shows a chloroplast. — **Figure 1–21** **Chloroplasts in plant cells capture the energy of sunlight.** (A) Seen in this light micrograph, a single cell isolated from a leaf of a flowering plant has many green chloroplasts. (B) A drawing of a single chloroplast, showing the inner and outer membranes, as well as the highly folded system of internal membranes containing the green chlorophyll molecules that absorb light energy. (A, courtesy of Preeti Dahiya.)

Chloroplasts carry out photosynthesis—trapping the energy of sunlight in their chlorophyll molecules and using this energy to drive the manufacture of energy-rich sugar molecules. In the process, they release oxygen as a molecular by-product. Plant cells can later extract this stored chemical energy when they need it, just as animal cells do: by oxidizing these sugars and their breakdown products, mainly in the mitochondria. Chloroplasts thus enable plants to get their energy directly from sunlight. They also allow plants to produce the food molecules—and the oxygen—that mitochondria use to generate chemical energy in the form of ATP. How these organelles work together is discussed in Chapter 14.

Like mitochondria, chloroplasts contain their own DNA, reproduce by dividing in two, and are thought to have evolved from bacteria—in this case, from photosynthetic bacteria that were engulfed by eukaryotic-like cells that had already acquired mitochondria (Figure 1–22).

A chart shows the evolution of eukaryotes. — **Figure 1–22** **Eukaryotes contain a combination of genomes that originally derived from archaea and bacteria.** The bacterial and archaean lineages diverged from one another very early in the evolution of life on Earth. Some time later, archaeal cells are thought to have acquired mitochondria by establishing symbiotic relationships with aerobic bacteria; later still, a subset of these single-celled eukaryotes acquired chloroplasts. Mitochondria are essentially the same in plants, animals, and fungi, and therefore were presumably acquired before these lines diverged about 1.5 billion years ago.

Internal Membranes Create Intracellular Compartments with Different Functions

Nuclei, mitochondria, and chloroplasts are not the only membrane-enclosed organelles inside eukaryotic cells. The cytoplasm contains a profusion of other organelles that are surrounded by single membranes (see Figure 1–8A). Most of these structures are involved with the cell’s ability to import raw materials and to export both useful substances and waste products that are produced by the cell (a topic we discuss in detail in Chapter 12).

The endoplasmic reticulum (ER) is an irregular maze of interconnected spaces enclosed by a membrane (Figure 1–23). It is the site where most cell-membrane components, as well as materials destined for export from the cell, are made. This organelle is especially extensive in cells that are specialized to secrete large amounts of protein. Stacks of flattened, membrane-enclosed sacs constitute the Golgi apparatus (Figure 1–24), which modifies and packages molecules made in the ER that are destined to be either secreted from the cell or transported to another cell compartment. Lysosomes are small, irregularly shaped organelles in which intracellular digestion occurs, releasing nutrients from ingested food particles into the cytosol and breaking down unwanted molecules for either recycling within the cell or excretion from the cell. Indeed, many of the large and small molecules within the cell are constantly being broken down and remade. Peroxisomes are small, membrane-enclosed vesicles that provide a sequestered environment for a variety of reactions in which hydrogen peroxide is used to inactivate toxic molecules. Membranes also form many types of small transport vesicles that ferry materials between one membrane-enclosed organelle and another. All of these membrane-enclosed organelles are highlighted in Figure 1–25A.

Schematic A shows the location of the endoplasmic reticulum within a cell. Micrograph B shows the endoplasmic reticulum. — **Figure 1–23** **The endoplasmic reticulum produces many of the components of a eukaryotic cell.** (A) Schematic diagram of an animal cell shows the endoplasmic reticulum (ER) in *green.* (B) Electron micrograph of a thin section of a mammalian pancreatic cell, which is specialized for protein secretion, shows a small part of the ER, which is vast in this cell type. Note that the ER is continuous with the membranes of the nuclear envelope. The *black* particles studding the region of the ER (and nuclear envelope) shown here are ribosomes, structures that translate RNAs into proteins. Because of its appearance, ribosome-coated ER is often called “rough ER” to distinguish it from the “smooth ER,” which does not have ribosomes bound to it. (B, courtesy of Lelio Orci.)

Schematics A and B shows the somewhat consolidated Golgi apparatus in an animal cell. Micrograph C shows the Golgi apparatus. — **Figure 1–24** **The Golgi apparatus is composed of a stack of flattened, membrane-enclosed discs.** (A) Schematic diagram of an animal cell with the Golgi apparatus colored *red.* (B) More realistic drawing of the Golgi apparatus. Some of the vesicles seen nearby have pinched off from the Golgi stack; others are destined to fuse with it. Only one stack is shown here, but several can be present in a cell. (C) Electron micrograph that shows the Golgi apparatus in a typical animal cell. (C, courtesy of Brij J. Gupta.)

A schematic shows the various membrane enclosed organelles inside a cell. A schematic shows the cytosol. — **Figure 1–25** **Membrane-enclosed organelles are distributed throughout the eukaryotic cell cytoplasm.** (A) The various types of membrane-enclosed organelles, shown in different colors, are each specialized to perform a different function. (B) The part of the cytoplasm that fills the space outside of these organelles is called the cytosol (colored *blue*).

A schematic shows a eukaryotic cell undergoing endocytosis and exocytosis. — **Figure 1–26** **Eukaryotic cells engage in continual endocytosis and exocytosis across their plasma membrane.** They import extracellular materials by endocytosis and secrete intracellular materials by exocytosis. Endocytosed material is first delivered to membrane-enclosed organelles called endosomes (discussed in Chapter 15).

A continual exchange of materials takes place between the endoplasmic reticulum, the Golgi apparatus, the lysosomes, the plasma membrane, and the outside of the cell. The exchange is mediated by transport vesicles that pinch off from the membrane of one organelle and fuse with another, like tiny soap bubbles that bud from and combine with other bubbles. At the surface of the cell, for example, portions of the plasma membrane tuck inward and pinch off to form vesicles that carry material captured from the external medium into the cell—a process called endocytosis (Figure 1–26). Animal cells can engulf very large particles, or even entire foreign cells, by endocytosis. In the reverse process, called exocytosis, vesicles from inside the cell fuse with the plasma membrane and release their contents into the external medium (see Figure 1–26); most of the hormones and signal molecules that allow cells to communicate with one another are secreted from cells by exocytosis. How membrane-enclosed organelles move proteins and other molecules from place to place inside the eukaryotic cell is discussed in detail in Chapter 15.

The Cytosol Is a Concentrated Aqueous Gel of Large and Small Molecules

If we were to strip the plasma membrane from a eukaryotic cell and remove all of its membrane-enclosed organelles—including the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, chloroplasts, and so on—we would be left with the cytosol (Figure 1–25B). In other words, the cytosol is the part of the cytoplasm that is not contained within intracellular membranes. In most cells, the cytosol is the largest single compartment. It contains a host of large and small molecules, crowded together so closely that it behaves more like a water-based gel than a liquid solution (Figure 1–27). The cytosol is the site of many of the chemical reactions that are fundamental to the cell’s existence. The early steps in the breakdown of nutrient molecules take place in the cytosol, for example, and it is here that most proteins are made by ribosomes.

An illustration of the cytosol of Escherichia coli contains tightly packed organelles. — **Figure 1–27** **The cytosol is extremely crowded.** This atomically detailed model of the cytosol of *E. coli* is based on the sizes and concentrations of 50 of the most abundant large molecules present in the bacterium. RNAs, proteins, and ribosomes are shown in different colors (Movie 1.2). (From S.R. McGuffee and A.H. Elcock, *PLoS Comput. Biol.* 6:e1000694, 2010.)

The Cytoskeleton Is Responsible for Directed Cell Movements

The cytosol is not merely a thin, structureless soup of chemicals and organelles. Using an electron microscope, one can see that in eukaryotic cells the cytosol is crisscrossed by long, fine filaments. Frequently, the filaments are seen to be anchored at one end to the plasma membrane or to radiate out from a central site adjacent to the nucleus. This system of protein filaments, called the cytoskeleton, is composed of three major filament types (Figure 1–28). The thinnest of these filaments are the actin filaments; they are abundant in all eukaryotic cells but occur in especially large numbers inside muscle cells, where they serve as a central part of the machinery responsible for muscle contraction. The thickest filaments in the cytosol are called microtubules (see Figure 1–7B), because they have the form of minute hollow tubes; in dividing cells, they become reorganized into a spectacular array that helps pull the duplicated chromosomes apart and distribute them equally to the two daughter cells (Figure 1–29). Intermediate in thickness between actin filaments and microtubules are the intermediate filaments, which serve to strengthen most animal cells. These three types of filaments, together with other proteins that attach to them, form a system of girders, ropes, and motors that gives the cell its mechanical strength, controls its shape, and drives and guides its movements (Movie 1.3 and Movie 1.4).

A micrograph shows actin filaments as a network of protein filaments that extend from the cell nucleus to the cell membrane. A micrograph shows microtubules as a network of protein filaments that extend from the cell nucleus to the cell membrane. A micrograph shows intermediate filaments as a network of protein filaments in a cell. — **Figure 1–28** **The cytoskeleton is a network of protein filaments that can be seen crisscrossing the cytoplasm of eukaryotic cells.** The three major types of filaments can be detected using different fluorescent stains. Shown here are (A) actin filaments, (B) microtubules, and (C) intermediate filaments. (A, Molecular Expressions at Florida State University; B, courtesy of Nancy Kedersha; C, courtesy of Clive Lloyd.)

A micrograph and a schematic depict a dividing cell with duplicated chromosomes attached to the spindle of the microtubules. — **Figure 1–29** **Microtubules help segregate the chromosomes in a dividing animal cell.** A transmission electron micrograph and schematic drawing show duplicated chromosomes attached to the microtubules of a mitotic spindle (discussed in Chapter 18). When a cell divides, its nuclear envelope breaks down (see Figure 1–18) and its DNA condenses into visible chromosomes, each of which has duplicated to form a pair of conjoined chromosomes that will ultimately be pulled apart into separate daughter cells by the spindle microtubules. See also Panel 1–1, pp. 12–13. (Photomicrograph courtesy of Conly L. Rieder, Albany, New York.)

QUESTION 1–5

Suggest a reason why it might have been advantageous for eukaryotic cells to have evolved elaborate internal membrane systems that allow them to import substances from the outside, as shown in Figure 1–26.

Because the cytoskeleton governs the internal organization of the cell as well as its external features, it is as necessary to a plant cell—which is boxed in by a tough cell wall—as it is to an animal cell that freely bends, stretches, swims, or crawls. In a plant cell, for example, organelles such as mitochondria are driven in a constant stream around the cell interior along cytoskeletal tracks (Movie 1.5). And animal cells and plant cells alike depend on the cytoskeleton to separate their internal components into two daughter cells during cell division (see Figure 1–29).

The cytoskeleton’s role in cell division may be its most ancient function. Even bacteria contain proteins that are distantly related to those that form the cytoskeletal elements involved in eukaryotic cell division, showing that this class of proteins was present in the very earliest cells; in bacteria, these proteins also form filaments that play a part in cell division. We examine the cytoskeleton in detail in Chapter 17, discuss its role in cell division in Chapter 18, and review how it responds to signals from outside the cell in Chapter 16.

The Cytosol Is Far from Static

The cell interior is in constant motion. The cytoskeleton is a dynamic jungle of protein ropes that are continually being strung together and taken apart; its filaments can assemble and then disappear in a matter of minutes. Motor proteins use the energy stored in molecules of ATP to trundle along these tracks and cables, carrying organelles and proteins throughout the cytoplasm, and racing across the width of the cell in seconds. In addition, the large and small molecules that fill every free space in the cell are knocked to and fro by random thermal motion, constantly colliding with one another and with other structures in the cell’s crowded cytosol.

Of course, neither the bustling nature of the cell’s interior nor the details of cell structure were appreciated when scientists first peered at cells in a microscope; our knowledge of cell structure accumulated slowly.

A few of the key discoveries are listed in Table 1–1. In addition, Panel 1–2 (p. 27) summarizes the main differences between animal, plant, and bacterial cells.

TABLE 1–1 HISTORICAL LANDMARKS IN DETERMINING CELL STRUCTURE
1665	Hooke uses a primitive microscope to describe small chambers in sections of cork that he calls “cells”
1674	van Leeuwenhoek reports his discovery of protozoa. Nine years later, he sees bacteria for the first time
1833	Brown publishes his microscopic observations of orchids, clearly describing the cell nucleus
1839	Schleiden and Schwann propose the cell theory, stating that the nucleated cell is the universal building block of plant and animal tissues
1857	von Kölliker describes mitochondria in muscle cells
1879	Flemming describes with great clarity chromosome behavior during mitosis in animal cells
1881	Ramón y Cajal and other histologists develop staining methods that reveal the structure of nerve cells and the organization of neural tissue
1898	Golgi first sees and describes the Golgi apparatus by staining cells with silver nitrate
1902	Boveri links chromosomes and heredity by observing chromosome behavior during sexual reproduction
1952	Palade, Porter, and Sjöstrand develop methods of electron microscopy that enable many intracellular structures to be seen for the first time. In one of the first applications of these techniques, Huxley shows that muscle contains arrays of protein filaments—the first evidence of a cytoskeleton
1957	Robertson describes the bilayer structure of the cell membrane, seen for the first time in the electron microscope
1960	Kendrew describes the first detailed protein structure (sperm whale myoglobin) to a resolution of 0.2 nm, using x-ray crystallography. Perutz proposes a lower-resolution structure for hemoglobin
1965	de Duve and colleagues use a cell-fractionation technique to separate peroxisomes, mitochondria, and lysosomes from a preparation of rat liver
1968	Petráňn and collaborators make the first confocal microscope
1970	Frye and Edidin use fluorescent antibodies to show that plasma membrane molecules can diffuse in the plane of the membrane, indicating that cell membranes are fluid
1974	Lazarides and Weber use fluorescent antibodies to stain the cytoskeleton
1994	Chalfie and collaborators introduce green fluorescent protein (GFP) as a marker to follow the behavior of proteins in living cells
1990s–2000s	Betzig, Hell, and Moerner develop techniques for superresolution fluorescence microscopy that allow observation of biological molecules too small to be resolved by conventional light or fluorescence microscopy

PANEL 1–2 CELL ARCHITECTURE

A 3 panel illustration shows an animal cell, a bacterial cell, and a plant cell.

Many Eukaryotes Live as Solitary Cells

Although all multicellular organisms are eukaryotes, many eukaryotic species lead a more solitary life. Such free-living, motile, unicellular eukaryotes are called protozoans. These cells may not inhabit the range of extreme environments that are colonized by bacteria and archaea, but they do adopt an impressive variety of behaviors and appearances. Didinium, for example, is a large, carnivorous protozoan with a diameter of about 150 μm—roughly 10 times that of the average human cell. It has a globular body encircled by two fringes of cilia, and its front end is flattened except for a single protrusion rather like a snout (Figure 1–30A). Didinium uses its beating cilia to swim at high speed, and when the organism encounters a suitable prey, usually another type of protozoan, it releases numerous small, paralyzing darts from its snout region. Didinium then attaches to and devours the other cell, inverting like a hollow ball to engulf its victim, which can be almost as large as the protozoan itself (Figure 1–30B).

A micrograph shows a Didinium ingesting another protozoan, a Paramecium. — (B)

Not all protozoans are predators. They can be photosynthetic or carnivorous, motile or sedentary. Their anatomy is often elaborate, occasionally ostentatious, and it includes such structures as sensory bristles, photoreceptors, sinuously beating cilia, stalklike appendages, mouthparts, stinging darts, and musclelike contractile bundles. Although they are single cells, protozoans can be as intricate and versatile as many multicellular organisms (Figure 1–31). Much can be learned about fundamental cell biology from studies of these fascinating life-forms, including the origins of multicellularity and the mysteries of our own evolutionary past.

Seven schematics of diverse protozoans labeled from A through G. — **Figure 1–31** **An assortment of protozoans illustrates the enormous variety within this class of single-celled eukaryotes.** These drawings are done to different scales, but in each case the scale bar represents 10 μm. The organisms in (A), (C), and (G) are ciliates; (B) is a heliozoan; (D) is an amoeba; (E) is a dinoflagellate; and (F) is a euglenoid. To see the latter in action, watch Movie 1.6. Because these organisms can be seen only with the aid of a microscope, they—like bacteria and archaea—are also referred to as microorganisms. (From M.A. Sleigh, *The Biology of Protozoa*. London: Edward Arnold, 1973. With permission from Edward Arnold.)

nucleus: In biology, refers to the prominent, rounded structure that contains the DNA of a eukaryotic cell. In chemistry, refers to the dense, positively charged center of an atom.
chromosome: Long, threadlike structure composed of DNA and proteins that carries the genetic information of an organism; becomes visible as a distinct entity when a plant or animal cell prepares to divide.
chloroplast: Specialized organelle in algae and plants that contains chlorophyll and serves as the site for photosynthesis.
photosynthesis: The process by which plants, algae, and some bacteria use the energy of sunlight to drive the synthesis of organic molecules from carbon dioxide and water.
endoplasmic reticulum (ER): Labyrinthine membrane-enclosed compartment in the cytoplasm of eukaryotic cells where lipids and proteins are made.
Golgi apparatus: Membrane-enclosed organelle in eukaryotic cells that modifies the proteins and lipids made in the endoplasmic reticulum and sorts them for transport to other sites.
cytosol: Contents of the main compartment of the cytoplasm, excluding membrane-enclosed organelles such as the endoplasmic reticulum and mitochondria. Also used to refer to the cell fraction remaining after membranes, cytoskeletal components, and other organelles have been removed.
cytoskeleton: System of protein filaments in the cytoplasm of a eukaryotic cell that gives the cell shape and the capacity for directed movement. Its most abundant components are actin filaments, microtubules, and intermediate filaments.
protozoan: A free-living, nonphotosynthetic, single-celled, motile eukaryote.
mitochondrion: Membrane-enclosed organelle, about the size of a bacterium, that carries out oxidative phosphorylation and produces most of the ATP in eukaryotic cells.