OVERVIEW OF A TYPICAL CELL
The body of an average sized adult consists of between 50 and 100 trillion cells, including over 200 different cell types. With regard to their general characteristics, human cells fall into two groups: somatic cells and sex cells. Somatic cells (sō-MA-tik; soma, body), also called body cells, include all cells in the body except gametes. Gametes (GAM-ēts; “spouse”), or sex cells, function only in sexual reproduction and include sperm in the male and ova (Ō-vuh; “eggs”) in the female. Considering the variety of cells in the body, it is difficult to portray a “typical” body cell exactly. However, all body cells share certain structural features. This section describes these features, along with a few other characteristics that are present only in certain types of cells (see Figure 4-1).
Figure 4-1. A generalized cell
A typical somatic cell consists of three major parts: the plasma membrane, the cytoplasm, and the nucleus. The plasma membrane (also called the cell membrane) forms the cell’s outer surface and holds the cell’s inner contents intact. The cytoplasm (SĪ-tō-plazm; cyto, cell; plasm, something formed) includes all parts of the cell between the plasma membrane and the nucleus. The nucleus is an example of an organelle, a specialized structure or compartment that performs a particular function inside the cell.
THE CYTOSOL AND INCLUSIONS
Almost two-thirds of the human body consists of fluids, and about 70 percent of this fluid volume is intracellular (inside cells). The cytosol (SĪ-to-sol) is the intracellular fluid between the plasma membrane and the nucleus. In most body cells, the cytosol consists mostly of water and accounts for more than half of the cell’s volume. Since water has a high specific heat (that is, it takes a relatively high amount of heat energy to change its temperature), the watery cytosol helps protect the cell against sudden temperature changes. More importantly, because it is a “universal solvent,” water is a favorable medium for holding the reactants and products of the millions of chemical reactions that occur every second in the cell.
Proteins are abundant in the cytosol and cause it to behave like a colloid. For this reason, the consistency of the cytosol is more like liquid gelatin or raw egg white. The cytosol’s relatively high viscosity (thickness), along with a network of filamentous (thread-like) proteins supports the organelles. Without this support, organelles would tend to “sink” to the bottom of the cell or bounce around when the body moves. With the help of its plasma membrane, a cell maintains concentrations of proteins and other chemicals in its cytosol that differ substantially from the extracellular fluid. The relative concentrations of major ions and groups of organic compounds in the cytosol and extracellular fluids are shown in Table 4-1.
Table 4-1. Relative concentrations of major substances in the cytosol and ECF
| Substance | Intracellular Fluid | Extracellular Fluid |
|---|---|---|
| Bicarbonate ions (HCO3⁻) | Higher | |
| Calcium ions (Ca2+) | Higher | |
| Chloride ions (Cl⁻) | Higher | |
| Glucose | Higher | |
| Oxygen gas (O2) | Higher | |
| Sodium ions (Na⁺) | Higher | |
| Amino Acids | Higher | |
| Carbon dioxide (CO2) | Higher | |
| Lipids | Higher | |
| Magnesium ions (Mg2+) | Higher | |
| Phosphate ions (PO43-) | Higher | |
| Potassium ions (K⁺) | Higher | |
| Proteins | Higher | |
| Sulfate ions (SO42-) | Higher |
In addition to cushioning the organelles, the cytosol is a storage depot. Cellular products may collect in the cytosol into variable-sized clumps called inclusions (in-KLŪ-shunz). The inclusions in most body cells consist of metabolic products or substances that the cells can quickly convert to energy. The four most common inclusions are:
- Fat droplets: consist mainly of triglycerides and may occupy up to 95% of the cytosol in the fat-storing cells called adipocytes (AD-i-pō-sīts; adipo, fat). These cells are abundant just beneath the skin where they cushion and help insulate the body. Between meals, the adipocytes release their energy-rich triglyceride molecules into the blood so other cells can use them as a source of energy.
- Glycogen: a polymer (long chain) of glucose molecules and is the body’s principle carbohydrate reserve. Liver and muscle cells in particular store large amounts of glycogen as inclusions called glycogen granules. During strenuous physical activity, liver cells release the glucose molecules into the blood for other cells to use as an energy source.
- Melanin (MEL-a-nin; melan, black): the dark pigment (coloring agent) that is responsible for most variations in skin and hair color. Its synthesis occurs inside cells called melanocytes (MEL-a-nō-sīts or mel-AN-ō-sīts) which are abundant in the skin, hair roots, and the light-sensitive parts of the eyeball. Melanin absorbs ultraviolet light, protecting deeper tissues from these potentially harmful rays.
- Hemoglobin (HĒ-mō-glō-bin; hemo, blood):the oxygen-carrying compound that is responsible for the color of red blood cells (RBCs). In fact, mature RBCs have no organelles, leaving hemoglobin as the main organic ingredient in their cytosol. Hemoglobin binds to oxygen molecules (O2) as RBCs flow through the lungs and releases O2 as RBCs flow through other tissues.
THE CYTOSKELETON
In addition to inclusions, the cytosol contains a vast network of interlocking filamentous proteins that forms the cell’s cytoskeleton. Similar to the way in which an internal, bony skeleton supports your body, an internal cytoskeleton supports each of your cells. Additionally, the cytoskeleton can change the location or position of organelles in the cytosol, enable certain kinds of cells to move about within the body, and help a cell to divide. In fact, some cell biologists assert that the cytoskeleton’s indispensable role in directing motion is what allows a cell to “do” rather than merely to “be.” The cytoskeleton consists of three types of protein filaments: microfilaments, microtubules, and intermediate filaments (see Figure 4-2).
Figure 4-2. The cytoskeleton
Cytoskeleton Quiz
Microfilaments
Extending everywhere in the cytosol, micro-filaments are the smallest, but most numerous, components of the cytoskeleton. Microfilaments consist of actin molecules (AK-tin; beam), the most common free protein in the cytosol. Numerous globular (G) actin molecules join to form a thin, flexible, helical (coiled) strand that resembles a pearl necklace. What is more, the G-actin molecules line up so regularly that they give the microfilament “endedness,” in which G actin molecules on opposite ends of the filament display different parts of their chemical structure. For this reason, cell biologists refer to one end of the filament as the “plus” end and the other end as the “minus” end. Keep this fact in mind, because endedness is one of the keys to understanding how a cell moves things around in its interior. Some microfilaments form stable and long-lasting structures in the cytosol, but more often a cell repeatedly builds and then dismantles microfilaments as it goes about its business. The most important roles of microfilaments include:
- Shaping and Supporting the Plasma Membrane: Most microfilaments pack densely near the plasma membrane in a region called the cell cortex (KOR-teks, “bark”). Here, the microfilaments crisscross on top of one another and bond to projections of proteins in the plasma membrane to form a web-like pattern. This web of microfilaments gives each type of body cell its characteristic shape, and it helps prevent the plasma membrane from splitting open if stretched.
- Facilitating Cell Crawling: Some body cells use a process called cell crawling to move. The cell thrusts out its plasma membrane in the direction of travel and, at the same time, “reels in” the membrane behind it. To accomplish this feat, the cell adds actin monomers to microfilaments aimed at the plasma membrane. The growing microfilaments act like battering rams that push the plasma membrane forward in little streamers called pseudopodia (sū-dō-PŌ-dē-a; pseudo,false; podia, feet). At the cell’s “tail end,” motor molecules made of the protein myosin(MĪ-ō-sin) form bridges between the plasma membrane and nearby microfilaments. Using energy from ATP, the motor molecules change shape repeatedly, “walking” towards the plus end of the microfilament. As a result, the motor molecules tug the microfilaments toward the interior of the cell, dragging the plasma membrane along. Some white blood cells employ cell crawling to search for invading microorganisms or dead cells to ingest. Then again, truly spectacular examples of cell crawling occur during fetal life, when cells migrate in vast swarms as they form the tissues and organs.
Note:
Some biologists have estimated that neurocytes, the earliest forms of brain cells, crawl a collective one million miles before reaching their final destinations in the brain.
- Moving cell vesicles: A cell’s actin microfilaments also serve as an intracellular railroad. Large substances drawn into a cell, and many of a cell’s products, don’t float about randomly in the cytosol. Instead, a cell packages them in membrane-enclosed sacs, called vesicles. One end of a myosin motor molecule attaches to a vesicle while the other end attaches to a microfilament. By regularly changing its shape, the motor molecule “walks” the cargo-containing vesicle through the cytosol to its destination inside or outside the cell, but this movement is always toward the “plus” end of the microfilament (Figure 4-3)
Figure 4-3. Microfilament
- Completing cell division: When a cell divides, microfilaments in the cell cortex gather in parallel bands around the cell’s “midsection” and cleave the cell in two. During this process, myosin motor molecules attach between adjacent microfilaments in the band and “walk” both filaments past each other. Like slowly tightening a belt around the waist, the microfilaments pull the plasma membrane in towards the interior of the cell. This process continues until the cell pinches in two, forming independent cells.
Microtubules
Now, let’s consider the structure and functions of the largest threads in the cytoskeleton. A cell’s microtubules are large-diameter filaments that extend from an “organizing center” out toward the plasma membrane. Each microtubule consists of a hollow coil of tubulin (TŪ-byoo-lin) molecules. Each tubulin molecule is a globular protein that resembles a pearl in a necklace. The regular arrangement of its molecules gives microtubules “endedness,” just like microfilaments. The cell changes the length of its microtubules by adding or removing tubulin molecules from the “plus” end. Microtubules play three distinct roles in a body cell:
- Positioning cytoplasmic organelles: Your first glimpse at a generalized cell might give you the impression that the cytoplasm is like a “tossed salad” in which organelles are scattered haphazardly. Nothing could be further from the truth! The cell uses microtubules to position its organelles precisely where they are needed most and where they function best. Just as important, the shape and integrity of some of the cell’s most important organelles solely depend on microtubules. If these large cytoskeletal filaments were to disappear, some organelles would collapse like puppets with cut strings. Still others would come apart and drift uselessly within the cytosol. The microtubule-based movement of organelles occurs when tubulin joins forces with two motor molecules of its own: kinesins (ki-NĒ-sinz) and dyneins (DĪ-nēnz). The motor molecules work by sensing the “endedness” of the microtubule strand: kinesins “walk” their load toward the plus end, while dyneins “walk” theirs toward the minus end (Figure 4-4). A cell needs many motor molecules of both varieties to position its largest organelles. The net effect is like moving a large curtain along a curtain rod: the cell can bunch up the “curtain” (organelle), spread it out, or move it away from or toward the cell’s interior.
Figure 4-4. Microtubule
- Moving motile cell extensions: Some body cells have relatively long finger-like or tail-like extensions. Columns of microtubules linked by dynein motor molecules cause these extensions to beat back and forth like tiny whips. Rhythmic beating starts as dynein molecules form cross bridges between microtubules on one side of an extension. The dynein molecules “walk” the adjacent microtubules past one another causing the extension to bend. After a moment, dynein molecules on the opposite side of the extension perform the same walking routine, causing the extension to bend in the reverse direction.
- Facilitating cell reproduction: Before a dividing cell pinches in two, it replicates (makes copies of) its chromosomes. A chromosome is a dense package of DNA and proteins that holds a portion of a cell’s genetic code. After the replication process is complete, each chromosome consists of two “sister” strands of DNA called chromatids (KRŌ-ma-tidz),held together at a site called the centromere (SEN-trō-mēr). Clusters of microtubules called spindle fibers grow towards and attach to the chromatids. One spindle fiber attaches to one chromatid at a site called the kinetochore (ki-NE-tō-kor). Just before the cell divides, each kinetochore begins to dismantle the plus ends of their attached microtubules. In a process that can be compared to a firefighter sliding down the pole at the fire station, the chromosomes aren’t so much “pulled apart” but rather slide away from one another by unraveling the spiral ends of the microtubules that hold them together:
Like a bicycle wheel, microtubules fan out through a body cell in a hub-and-spoke pattern. The hub is the cell’s centrosome (SEN-tro-sōm; centro, center; some, body), the production center for microtubules. A hazy-looking sphere with an indefinite border, a centrosome huddles close to the nucleus in non-dividing cells. A centrosome consists of hundreds of tiny rings of a special type of tubulin protein. The “minus” end of a microtubule emerges from but stays “plugged in” to one of the centrosome’s tubulin rings. In this way, a centrosome controls the number, orientation, and location of all the microtubules that extend into the cytosol.
A centrosome is not an organelle, but it encloses two cylindrical organelles called centrioles (SEN-trē-olz), which align perpendicularly to one another inside a centrosome. Each centriole consists of nine bundles of microtubules, with three microtubules per bundle. Cell biologists still don’t know what centrioles do. They once thought that centrioles made spindle fibers, but this is unlikely, since plant cells lack centrioles but make plenty of spindle fibers. Also, when biologists experimentally plucked out the centrioles from a centrosome, the cell still made spindle fibers at the right time and divided with ease. You might say that centrioles are organelles in search of a job!
Intermediate Filaments
Finally, let’s turn our attention to the third—and toughest—component of the cytoskeleton. As their name implies, intermediate filaments are thicker than microfilaments but thinner than microtubules. These filaments are the strongest parts of the cytoskeleton and their strength helps cells endure mechanical stress. Many cells have intermediate filaments made of keratins (KAIR-i-tinz), a diverse family of proteins. These cells weave their intermediate filaments from small keratin subunits, rather like a rope-maker weaves a braided rope. Two pairs of keratin filaments bundle into a four-filament subunit, then eight subunits twist together to form a finished intermediate filament. Since they lack the “strand-of-beads” design like microfilaments and microtubules, intermediate filaments lack “endedness,” which means they can’t guide cell movement. However, intermediate filaments are exceptionally durable. Your hair and nails are the fused remnants of intermediate filaments that originated inside special skin cells.
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Table 4-2. Cytoskeleton of a Body Cell
| Filament Type | Composition | Distribution | Functions | Motor Molecules |
|---|---|---|---|---|
| Microfilaments | Actin | Concentrated along cell cortex, but present throughout cytosol |
| Myosin |
| Microtubules | Tubulin | Hub-and-spoke pattern extending from centrioles to plasma membrane |
| Kinesin Dynein |
| Intermediate filaments | Varies; usually keratin protein | Hub-and-spoke pattern extending from nucleus to cell junctions |
| None |
Intermediate filaments are a part of most body cells, but they predominate in cells that need strong internal support. Some nerve cells, for example, have long slender extensions called axons that pass electrical-like signals to distant parts of the body. Numerous intermediate filaments within the axons prevent them from snapping like overstretched rubber bands whenever you move your body. Intermediate filaments are also abundant in sheets of cells that form the epidermis, or outer portion of the skin. Intermediate filaments span the cytoplasm from protein plugs called cell junctions that “weld” together the plasma membranes of neighboring cells. If external force stretches tissue cells, the intermediate filaments snap tight like ropes to prevent the cells from rupturing or pulling apart from their neighbors. Table 4-2 provides a summary of the cytoskeletal structures.
THE PROJECTIONS OF A BODY CELL
In addition to the host of proteins and carbohydrates that protrude from the plasma membrane, a body cell may have larger organelle-like projections that serve to (1) increase the cell’s surface area, (2) sweep extracellular substances along the plasma membrane, or (3) propel the cell from place to place. The most important projections of a body cell include its microvilli, cilia, and a flagellum.
Microvilli
The tiniest projections found on certain body cells are microvilli (mī-krō-VIL-ī; villi, shaggy hair). Each of these stubby little cylinders is so small (measured in nanometers or billionths of a meter) that you would need to use a powerful electron microscope to see them clearly. An internal scaffolding of actin microfilaments supports each microvillus, and the entire mass of microvilli gather into a dense patch extending from only one side of a body cell. A patch of microvilli gives a cell a soft, peach-fuzz texture when viewed under a light microscope, a feature that biologists call the cell’s brush border.
Microvilli are useful because they dramatically increase a cell’s surface area. A demonstration you can try at home suggests why a large surface matters. Take a shower and dry off with your favorite towel. After your next shower, dry off using a bed sheet trimmed to match the size of a towel. Which drying device do you think would be more effective? Although the towel and sheet are the same size, the long fiber loops projecting from the towel vastly increase the towel’s surface area, speeding its absorption of water. Similarly, cells that need to take up substances from the extracellular fluid fast usually bear microvilli. Examples of such “absorbent” cells include those lining your intestines. Keep in mind that an individual microvillus is a temporary structure, one that can form and fade away in as little as 15 minutes. Compare this to a snail’s eyes that extend out but can retract quickly whenever the snail senses danger. The short “cycling time” of microfilaments allows a cell to adjust the topography of its plasma membrane quickly to cope with changes in the surrounding environment.
Cilia and Flagella
Although on the surface they resemble microvilli, cilia (SIL-ē-a; small hair) and flagella (fla-JEL-a; whip) are much larger. And, unlike microvilli, which are specialized for absorption, cilia and flagella devote themselves solely to pushing extracellular fluids. A cilium and a flagellum have the same basic design and operate the same way. Both are cylinders made of a circle formed by nine pairs of microtubules, called doublets, with an additional microtubule pair running down the center; this arrangement of microtubules is referred to as a “9 + 2” array (Figure 4-5).
A basal body anchors a cilium or flagellum to the cell cortex and directs the assembly of microtubules. Basal bodies and centrioles are identical organelles. Cell biologists still haven’t figured out how a basal body’s microtubule “triplets” give rise to the microtubule “doublets” in a cilium or flagellum. Recall that dynein motor molecules can form cross-bridges between microtubule pairs and “walk” one pair past the other. This repeated process generates the force that makes a cilium or flagellum alternately bend and straighten.
Although they work the same way, cilia and flagella behave differently and have different functional roles. Cilia project from the surface of tissue cells that face a body cavity, and a patch of ciliated cells work in unison. In an initial thrust called the power stroke, all the cilia sweep against the extracellular fluid in the same direction. During a subsequent recovery stroke, the cilia coil back into a “question-mark like” shape before extending to full length. Both movements in succession generate a forceful ciliary “wave” that can propel fluids and small particles across a tissue surface in a single direction, even against the force of gravity. Think of how spectators at a sports event do “the wave” in the stands, with whole blocks of people standing and throwing their hands in the air then sitting down, to produce a rhythmic wave that “rolls” across the stands. Ciliary movement pushes debris up and out of the respiratory tract, and it is a critical process for moving an ovum (female reproductive cell) through the female reproductive system.
Cilia wave back and forth to push extracellular fluids past a stationary cell, but a flagellum pushes a motile cell through the extracellular fluid. The focus of a flagellum’s purpose in humans is in reproduction. A flagellum forms the tail of a sperm cell, a free-floating male reproductive cell. The flagellum whips about far more forcefully and rapidly than a cilium. And, since a sperm cell is able to move from one place to another, the beating flagellum can propel the sperm through the female reproductive system. The structure of a centriole, basal body, and cilium is shown in Figure 4-5
Figure 4-5. Structure of centrioles and cilia
CYTOPLASMIC ORGANELLES
You’ve been reading up to now about structures that maintain the cell’s physical integrity, act as a biochemical barrier, and move substances from place to place. Now, let’s consider the cytoplasmic structures that perform many of the chemical reactions in the cell—the cytoplasmic organelles. Back when cell biologists and biochemists started “unpacking” the parts of cells, they assigned the newly discovered organelles to one of two categories. Membranous organelles come wrapped in a double-layered membrane (bilayer) of phospholipids, while non-membranous organelles do not have this lipid wrapper. Why do some organelles need a membrane? Recall that one trademark of a cell is its ability to maintain the unique chemical balance of its cytosol. Membranous organelles are a further refinement of this strategy. Just as the cytosol differs chemically from the ECF, the chemical environment in a membranous organelle can differ substantially from that of the cytosol. In fact, some of the chemical reactions that take place within some organelles are so toxic that if leaked into the cytosol they would kill the cell in an instant. Apart from its role as a chemical barrier, the lipid bilayer surrounding membranous organelles serves as a kind of “universal mailing envelope” for intracellular transport. Vesicles that pinch off from the membrane of one organelle move along the cytoskeleton to neighboring organelles or to the plasma membrane.
Nevertheless, as most cell biologists would admit, knowing that a particular organelle has (or lacks) a membrane doesn’t tell us much about what that organelle does for the cell. For this reason, we will conclude the tour of the cell’s cytoplasm by grouping cellular organelles into two broad (but still imperfect) functional categories: (1) organelles that synthesize, modify, or package essential molecules, and (2) organelles that transport or digest organic molecules.
Organelles that Synthesize and Package Products
In Chapter 2, you read that synthesis reactions unite smaller molecules to make larger molecules, consuming energy in the process. Synthesis reactions occur constantly to (1) build and maintain its organelles, membranes, and cytoskeleton; (2) make molecular products for export; and (3) replace the macromolecules it consumes as part of metabolism. Like goods rolling down a factory assembly line, some of a cell’s newly synthesized products require customizing, sorting, and packaging before they are ready for delivery and use elsewhere. The first functional category of organelles, which synthesize, modify, sort, or package a cell’s molecular products, includes ribosomes, mitochondria, endoplasmic reticula, and Golgi complexes.
Ribosomes
Ranked among the cell’s smallest components, ribosomes (RĪ-bō-sōmz) are non-membranous organelles that serve as construction sites for synthesizing proteins. A functional ribosome consists of one large and one small subunit, each of which forms from the union of ribosomal proteins and ribosomal RNA (rRNA) molecules. (Note: Even the proteins that make up part of a ribosome are made at ribosomes!) Ribosomal subunits are given numbers based on their size, which determines their “sedimentation” rate in a liquid. A lowercase “s” is used to denote this rate of sinking. Since the larger subunits sink faster, they are assigned a larger number than the small subunit. The large and small ribosomal subunits in bacteria (also known as prokaryotic cells; prō-kair-ē-OT-ik) are numbered 50s and 30s, respectively, whereas, in cells that have a nucleus (also known as eukaryotic cells; ū-kair-ē-OT-ik) they are 60s and 40s.
The destination of proteins made at a ribosome depends on the location of the ribosome in the cell. Some ribosomes attach to the surfaces of other organelles, becoming bound ribosomes, while other ribosomes move about in the cytosol as free ribosomes. Most proteins made at bound ribosomes leave the cell and function elsewhere in the body. For example, certain cells in the pancreas use bound ribosomes to make the protein insulin, an indispensable hormone that helps most other cells in the body take in glucose from the blood. Most proteins made at free ribosomes remain inside the cell and carry out various cellular functions.
Figure 4-6. Ribosome
Mitochondria
A cell needs a constant supply of ATP to carry out its normal metabolic activities. Adenosine triphosphate (ATP) is a molecular “fuel” that a body cell uses to drive nearly all of its endergonic reactions. Most of this fuel is produced inside mitochondria (mı̄t-ō-KON-drē-a; singular is mitochondrion, mı̄t-ō-KON-drē-un;). Viewed with a microscope, these organelles look like tiny packets filled with threads and grains of sand (mito, thread; chondria, granules). A typical body cell may have between 1000 and 2000 mitochondria, but this number can rise or fall depending on a cell’s need for ATP. In fact, cell biologists sometimes figure out a body cell’s level of physiological activity by counting the number of mitochondria in its cytoplasm. Cells that engage in unusually rapid or complex chemical reactions (e.g., cells in the brain, muscles, and liver) typically are jam-packed with mitochondria.
Mitochondria are extraordinary in that they possess two phospholipid bilayers, one inside the other. The outer membrane is smooth, but the inner membrane has many folds called cristae (KRIS-tē; “crests”). The cristae project into an inner compartment that contains an enzyme-rich gel called the matrix (MA-triks; “womb”). A narrow intermembrane space (also called the outer compartment) separates the inner and outer membranes. This configuration is important because most ATP synthesis occurs along the inner membrane, and the cristae increase the surface area of the membrane that is responsible for ATP synthesis (see Figure 4-7).
Figure 4-7. Mitochondrion
Mitochondria have some curious features that led biologists to speculate that these organelles once led an independent life. Mitochondria have one double helix DNA molecule that is “circular” (without an end—like a rubber band), similar to the DNA in bacteria. What is more, a mitochondrial ribosomes resemble those in bacteria (50s/30s), and a mitochondrion divides inside a cell through binary fission, the same method bacteria use to divide. Hence, biologists suspect that mitochondria were once free-living bacteria that survived after they entered a larger cell. The theory of their origin is known as the endosymbiotic theory (en-dō-sim-b-OT-ik; endo, inside; bio, life) (Figure 4-8). If so, both entities came to benefit from this event–the mitochondrion lives sheltered inside the cell’s stable chemical environment, and in exchange for lodging, it exports astounding numbers of ATP molecules every second into its “host’s” cytoplasm. Mitochondria obtain energy for ATP synthesis by breaking the chemical bonds in molecules derived from glucose and fatty acids. This type of ATP synthesis involves a string of chemical reactions that are collectively known as aerobic (cellular) respiration, a process that requires oxygen. In the absence of oxygen, mitochondria stop producing ATP.
Figure 4-8. Endosymbiotic theory for mitochondrion origin.
Endoplasmic Reticulum
body cell’s largest membranous organelle is its endoplasmic reticulum, or ER (en-dō-PLAZ-mik re-TIK-ū-lum). In fact, the ER’s membrane has a greater surface area than the plasma membrane. As the second part of its name suggests (reticul-, net), the ER is a membranous network that extends from the surface of the nuclear membrane like a pleated curtain. While the endoplasmic reticulum may look haphazardly unfurled in the cytoplasm, microtubules precisely arrange the ER into functionally advantageous shapes. The ER has many roles, but its chief role is being a site for the synthesis of proteins and lipids. Closer inspection of the ER reveals that it has two quite visually distinct parts: (1) the wavy rough ER and, (2) the more tube-like smooth ER.
The rough endoplasmic reticulum is continuous with, and remains positioned close to the cell’s nuclear membrane. The wavy layers of the rough ER’s lipid bilayer enclose cisternae (sis-TER-nā; “reservoirs”), fluid-filled chambers that modify and store newly formed proteins. The rough ER is so-named because of its grainy appearance, which is due to the numerous ribosomes that dot its face like sesame seeds on a bun (Figure 4-9).
Figure 4-9. Endoplasmic reticulum
The ribosomes don’t permanently fasten to the membrane, but attach and release as they start and finish synthesizing a protein. Actually, the newly emerging protein helps to “thumbtack” its ribosome to the rough ER’s outer membrane. In addition, the rough ER often attaches carbohydrates to proteins in a process called glycosylation (glī-kos-i-LĀ-shun), transforming them into glycoproteins. All proteins that a cell exports to the rest of the body originate at the rough ER. Thus, cells that specialize in secreting substances tend to have exceptionally large rough ERs.
Moving out from the nucleus, the folds of the rough ER eventually break into the networks of tubules and sacs known as the smooth endoplasmic reticulum, so named because its surface lacks ribosomes. The smooth ER is the principle construction site for phospholipids and cholesterol. In some specialized cells, however, the smooth ER takes on equally specialized roles. For example, in muscle cells the smooth ER stores vast amounts of calcium ions, which it releases on cue to stimulate muscle contraction. In other cells, the smooth ER is heavily coated with enzymes that can transform substances as they enter or leave the cell. The unusually large smooth ER of liver cells, for example, holds enzymes that detoxify harmful substances in the blood such as drugs, alcohol, or various environmental toxins. After synthesizing and processing proteins and lipids, the ER packages these products into vesicles for transport to other organelles. Most of these vesicles, called transport vesicles, move to the Golgi complex.
Organelle Function Matcher
Golgi Complex
Newly synthesized proteins do not linger in the cisternae of the rough ER for very long. Within minutes, vesicles engulf these proteins, pinch off from the ER, and travel along microtubule railroads toward a Golgi complex (GŌL-jē), also called a Golgi body or Golgi apparatus (Figure 4-10). Perhaps the most curious-looking of all the membranous organelles, the Golgi complex consists of a stack of plate-like sacs. The wide cis face of a Golgi complex points at the rough ER, while its narrower trans face points at the plasma membrane.
Figure 4-10. Golgi complex
Note:
Since vesicles from the ER enter the cis face of the Golgi complex, think of cis as the “come in site.”
Transport vesicles arriving from the ER enter the cis face and pass from plate-to-plate in the direction of the trans face. Ultimately, the Golgi complex packages these substances into large vesicles that move to and fuse with the plasma membrane, or they unite with smaller vesicles that travel to other organelles.
The Golgi complexes are a cell’s protein modifiers, sorters, and packagers. Just as an automobile maker customizes vehicles on an assembly line to meet the preferences of individual buyers, each plate in a Golgi complex sequentially modifies proteins until they reach their “ready-to-use” form. How a Golgi complex does this is a complex story that cell biologists are still unraveling. As vesicles arrive at each plate, they display membrane markers that serve as biochemical “order tickets” that tell enzymes in the plate what to do to the incoming proteins. The Golgi complex modifies individual proteins in a step-by-step fashion, just as a car on an assembly line may have a stick shift installed earlier on the line, and red leather seats installed further along. What is more, the Golgi complex “reads” the membrane markers and markers on the proteins it modifies, in order to sort out and package proteins that are bound for different destinations.
The major kind of “customizing” that the plates of a Golgi complex do is adding carbohydrates (glycosylation) and lipid molecules to proteins. For example, the Golgi complex synthesizes and attaches very large polysaccharides to proteins to form proteoglycans (prō-tē-ō-GLĪ-kanz), substances that give connective tissues such as ligaments and tendons their flexibility and strength. The Golgi complex also attaches carbohydrates to lipid molecules, forming many of the glycolipids that become part of the plasma membrane.
Organelles that Transport and Digest Products
If the organelles just described function asperoxisomes function as shipping containers, recycling centers, and decontamination sites. Simply put, vesicles are containers for hauling molecules on their membranous surfaces or within their fluid-filled interiors. On the other hand, you can distinguish lysosomes and peroxisomes by something unique that happens inside these spherical organelles, which both function as chemical reaction chambers.
Vesicles
You have likely noticed that previous sections mentioned vesicles time and again to this point in our tour of a body cell. That’s to be expected, since vesicles interact directly with a number of membranous organelles and with the plasma membrane. Vesicles (VES-i-klz, “bags”) are small, spherical organelles that transport substances through the cytoplasm, and they originate at certain membranous organelles and at the plasma membrane. Several organelles, including the ER, Golgi complex, and lysosomes, interact with one another through vesicles. Because all of these organelles are “membranous,” and because they work together as a coordinated system to perform vital functions for the cell, cell biologists refer to them collectively as the cell’s endomembrane system. Vesicles function as the “pick-up and delivery trucks” between the endomembrane system and the plasma membrane; coincidently, cell biologists refer to the movement of vesicles within a cell as vesicular traffic.
Just as a steady flow of truck traffic moving products and supplies within a city keeps the city operational, a steady flow of vesicular traffic moving molecular products and supplies within a cell keeps the cell functioning properly. Vesicular traffic runs along two major routes within a cell:
In brief, the secretory pathway involves synthesizing molecular products at the endomembrane system and exporting them from the cell at the plasma membrane. The endocytic pathway involves taking in substances from the extracellular environment at the plasma membrane and transporting them to the endomembrane system for processing. Based on their particular roles in these pathways, cell biologists classify vesicles as transport, secretory, or endocytic.
Transport vesicles carry organic molecules from the ER to the Golgi complex and from the Golgi complex to other organelles and the plasma membrane. Watching transport vesicles develop on the surface of the ER and Golgi complex is sort of like watching someone blow bubbles with bubblegum. After synthesizing proteins on its outer surface and modifying them in its interior, the ER packages its molecular products inside budding transport vesicles. After pinching off of the ER, the transport vesicles latch onto motor molecules that pull them along the cytoskeleton to the cis face of the Golgi complex. After fusing with the cis face membrane, the vesicle releases its contents into the Golgi complex for further processing. After modification, the organic molecules enter transport vesicles that bud off of the trans face of the Golgi complex. Some of the Golgi-generated vesicles fuse with endocytic vesicles, while others fuse with the plasma membrane.
When a transport vesicle fuses with the plasma membrane, some of the vesicle’s contents may spill into the extracellular fluid in a process called secretion (sē-KRĒ-shun). But what prevents a transport vesicle containing contents destined for secretion from fusing with a mitochondrion or some other organelle instead of the plasma membrane? The answer is membrane proteins. Certain proteins that project from the vesicle’s surface function as receptors that recognize specific proteins or other compounds in the plasma membrane. Therefore, vesicles that bump into membranes other than the plasma membrane will not fuse with them because they lack the appropriate protein receptors. On the other hand, transport vesicles that bind with other organelles have receptors that recognize specific proteins on the other organelle’s surface.
Note:
The word secretion can refer to the process by which a vesicle releases a substance into the ECF, or it can refer to the actual substance released from a vesicle into the ECF.
Secretory vesicles (SĒ-kre-tor-ē) are specialized transport vesicles that bud off of the Golgi complex and congregate near the plasma membrane, but secrete their contents only when prompted by a chemical cue in the ECF. (Recall that transport vesicles reaching the plasma membrane do not require such a cue before they can secrete their contents.) This chemical signal is a specific molecule that binds to a receptor molecule on the outer face of the plasma membrane and initiates chemical reactions inside the cell. These chemical reactions cause the secretory vesicles to fuse with the plasma membrane and secrete their contents. Since the body carefully controls the concentrations of the extracellular chemical signals, the release of substances from secretory vesicles is regulated secretion. Only certain kinds of cells, called secretory cells, produce secretory vesicles and release large amounts of one type of secretion. For example, following a meal, secretory cells in your pancreas secrete large amounts of digestive enzymes that help break down the food molecules in your intestine.
In addition to expelling cellular products into the ECF, transport and secretory vesicles supply new lipids and proteins to the plasma membrane. This is important for two reasons. First, molecules in the plasma membrane eventually “wear out” and the cell must replace them. When a vesicle fuses with the plasma membrane, any molecules that are part of the vesicle’s membrane become part of the plasma membrane. Since the transport vesicle folds inside out during the fusion process, any molecules (including glycoproteins and glycolipids) adhering to the inner surface of the vesicle will end up facing the ECF. Conversely, any molecules adhering to the outer surface of the vesicle will end up facing the cytoplasm. Second, the plasma membrane must grow as a cell grows. Before dividing, a cell must grow larger; to grow also, the plasma membrane incorporates membranes of transport vesicles.
Endocytic vesicles (en-dō-SIT-ik) pinch off of the plasma membrane and deliver substances from the ECF into the cell. These vesicles form in a process called endocytosis (en-dō-sī-TŌ-sis), during which a portion of the plasma membrane pinches inward while enclosing substances from the ECF. After moving into the cytoplasm, endocytic vesicles eventually fuse with lysosomes (described next). Endocytic vesicles are common in certain types of white blood cells that ingest and destroy bacteria and viruses.
Lysosomes
Lysosomes (LĪ-sō-sōmz) are highly specialized vesicles that pinch off of the Golgi complex, remain inside the cell, and decompose food molecules and worn-out organelles. Lysosomes contain enzymes called hydrolases (HĪ-drō-lā-sez) that digest large organic molecules, breaking them into smaller molecules by hydrolysis. Since certain byproducts of hydrolysis are reusable, lysosomes function like recycling centers for the cell. Hydrolases work only in an acid environment (about pH 5), which is the case inside of the lysosome. The cytosol’s pH is around 7; therefore, if a few hydrolase molecules leak into the cytosol, they cease to function and usually do not harm the cell.
One of the most important functions of lysosomes is to digest the contents of endocytic vesicles. Some of the large molecules that a cell takes in from the ECF and packages into endocytic vesicles can serve as a source of energy or building blocks for cellular components. However, in order to release this energy or make the building blocks available, the cell must first digest or break down the large molecules. After forming at the plasma membrane, an endocytic vesicle moves through the cytosol and fuses with a lysosome. The lysosome’s hydrolase enzymes mix with the contents of the vesicle, quickly digesting them. Some of the chemical byproducts of this digestion enter the cytosol for use in various metabolic activities, while the unusable materials remain inside the vesicle. Finally, motor molecules attach to the vesicle and transport it to the plasma membrane where the “waste” molecules enter the ECF.
Lysosomes also perform “housekeeping” chores by removing worn-out organelles from the cytoplasm and recycling their reusable molecules. When a cell digests one of its own organelles, the process is called autophagia (aw-tō-FĀ-jē-a; auto, self; phag, eating). Cell biologists have not yet determined how lysosomes identify worn-out organelles and mark them for destruction. However, prior to autophagia, a vesicle derived from the ER envelops the worn-out organelle. Motor molecules then transport the vesicle along the cytoskeleton where it can unite with a lysosome. Just as a salvage yard strips out all the “good stuff” from worn-out automobiles, lysosome enzymes strip out usable molecules from worn-out organelles. Lastly, lysosomes are responsible for autolysis (aw-TOL-i-sis; “self-breaking”) of certain cells. When these cells reach the end of their life span, numerous lysosomes rupture allowing the hydrolase enzymes to spill into the cytosol. As mentioned earlier, a small amount of hydrolase leaking into the cytosol would likely be neutralized by the higher-pH cytosolic liquid; however, having numerous lysosomes rupture can overwhelm the buffering capacity of the cytosol, resulting in the cell being digested from the inside out. Thus, lysosomes have the nickname “suicide bags.”
Peroxisomes
Peroxisomes (per-OX-i-sōmz) are tiny organelles that break down a variety of compounds, including fatty acids, amino acids, and various toxins. In the process, the peroxisome makes hydrogen peroxide (H2O2), which is responsible for the organelle’s name. Although peroxisomes may seem similar to lysosomes, there are several key differences between these organelles. First, lysosomes bud off the Golgi complex, while peroxisomes reproduce by simply pinching in two. Second, peroxisomes contain different enzymes than those found in lysosomes. Free ribosomes synthesize these enzymes, which then enter a peroxisome through carrier proteins located in the peroxisome’s membrane.
The two enzymes that allow peroxisomes to play a unique role inside cells are oxidase and catalase. Oxidase (OKS-si-dās ) removes electrons from (oxidizes) fatty acids and other molecules, causing these molecules to break apart. By-products of fatty acid oxidation leave the peroxisome and can enter the smooth ER, which converts them to cholesterol or other lipids. Oxidase also detoxifies harmful molecules, such as free radicals. Recall that free radicals are molecules that have unpaired electrons, a condition that causes them to steal electrons from other compounds. This action can damage DNA and other vital molecules. Oxidase converts the free radicals to H2O2, but H2O2 is also detrimental to the cell. Fortunately, the peroxisome contains catalase (KAT-a-lās ), an enzyme that converts the H2O2 into water and oxygen: 2H2O2 → 2H2O + O2 . Like oxidase, catalase can detoxify certain harmful compounds, such as formic acid (found in the venom of insect stingers) and alcohol. Hydrogen peroxide in an open wound forms bubbles containing O2 liberated when catalase from damaged cells converts the peroxide into oxygen and water. Table 4-3 summarizes the cytoplasmic organelles.
Table 4-3. Membranous vs. non-membranous organelles and their relationship to their functional categories
| Membranous organelles | Functional category* | General Function |
|---|---|---|
| Mitochondria | 1 | Aerobic ATP production |
| Endoplasmic reticulum | 1 | Lipid synthesis, forms transport vesicles |
| Golgi complex | 1 | Sorting and packaging of molecules for export |
| Vesicles | 2 | Storage, digestion, transport |
| Lysosomes | 2 | Digestion |
| Peroxisomes | 2 | Neutralize toxic substances |
| Non-membranous organelles | ||
| Centrioles | 1 | Unknown, but associated with centrosome, which synthesizes microtubules |
| Ribosomes | 1 | Protein synthesis |
| *1 = Synthesize and/or sort; 2 = Transport and/or digest | ||
Flashcards
Master the fundamental components and processes of the cell.
Glossary
Comprehensive terminology for the Overview of the Cell chapter. Click on any term to see its definition.
