CHAPTER 3: The Cellular Level of Organization

Question 1

Name and describe the three main parts of a cell.

Answer 1

the plasma membrane

the cytoplasm (consisting of the cytosol and organelles)

the nucleus

Question 2

Describe the plasma membrane of the cell.

Answer 2

The plasma membrane forms the cell’s flexible outer surface, separating the cell’s internal environment (everything inside the cell) from the external environment (everything outside the cell). It is a selective barrier that regulates the flow of materials into and out of a cell. This selectivity helps establish and maintain the appropriate environment for normal cellular activities. The plasma membrane also plays a key role in communication among cells and between cells and their external environment.

Question 3

Describe the cytoplasm of the cell.

Answer 3

The cytoplasm consists of all the cellular contents between the plasma membrane and the nucleus, and is composed of cytosol and organelles.

Question 4

Describe the nucleus of the cell.

Answer 4

The nucleus is a large organelle that houses most of a cell’s DNA. Within the nucleus, each chromosome, a single molecule of DNA associated with several proteins, contains thousands of hereditary units called genes that control most aspects of cellular structure and function.

Question 5

Identify the parts of a cell on a diagram.

Answer 5

Question 6

Distinguish between cytoplasm and cytosol.

Answer 6

The cytoplasm consists of all the cellular contents between the plasma membrane and the nucleus, including cytosol and organelles.

Cytosol is the fluid portion of cytoplasm, containing water, ions, glucose, amino acids, fatty acids, proteins, lipids, ATP, and waste products. It is the site of many chemical reactions required for a cell’s existence.

Question 7

Identify the 4 functions and the parts of the plasma membrane of the cell.

Answer 7

Question 8

Explain the concept of selective permeability.

Answer 8

Plasma membranes permit some substances to pass more readily than others. This property of membranes is termed selective permeability. The lipid bilayer portion of the plasma membrane is highly permeable to nonpolar molecules such as oxygen (O2), carbon dioxide (CO2), and steroids; moderately permeable to small, uncharged polar molecules, such as water and urea (a waste product from the breakdown of amino acids); and impermeable to ions and large, uncharged polar molecules, such as glucose.

Question 9

Define the electrochemical gradient and describe its components.

Answer 9

The selective permeability of the plasma membrane allows a living cell to maintain different concentrations of certain substances on either side of the plasma membrane. A concentration gradient is a difference in the concentration of a chemical from one place to another, such as from the inside to the outside of the plasma membrane.

The plasma membrane also creates a difference in the distribution of positively and negatively charged ions between the two sides of the plasma membrane. Typically, the inner surface of the plasma membrane is more negatively charged and the outer surface is more positively charged. A difference in electrical charges between two regions constitutes an electrical gradient. Because it occurs across the plasma membrane, this charge difference is termed the membrane potential.

The combined influence of the concentration gradient and the electrical gradient on movement of a particular ion is referred to as its electrochemical gradient.

Question 10

Differentiate between passive and active transport.

Answer 10

Substances generally move across cellular membranes via transport processes that can be classified as passive or active, depending on whether they require cellular energy. In passive processes, a substance moves down its concentration or electrical gradient to cross the membrane using only its own kinetic energy. In active processes, cellular energy is used to drive the substance “uphill” against its concentration or electrical gradient. The cellular energy used is usually in the form of adenosine triphosphate (ATP).

Question 11

Diffusion

Answer 11

In diffusion, if a particular solute is present in high concentration in one area of a solution and in low concentration in another area, solute molecules will diffuse toward the area of lower concentration—they move down their concentration gradient. Diffusion happens faster at higher temperatures, when there is a steeper concentration gradient, when the diffusing molecules are smaller, when there is a larger membrane surface area available, and when there is less distance to cover. Diffusion is a kind of passive transport.

Question 12

Differentiate between simple and facilitated diffusion.

Answer 12

Simple diffusion is a passive process in which substances move freely through the lipid bilayer of the plasma membranes of cells without the help of membrane transport proteins. Solutes that are too polar or highly charged to move through the lipid bilayer by simple diffusion can cross the plasma membrane by a passive process called facilitated diffusion. In this process, an integral membrane protein assists a specific substance across the membrane. The integral membrane protein can be either a membrane channel or a carrier.

Question 13

Osmosis

Answer 13

Osmosis is a type of diffusion in which there is net movement of water through a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration. In an isotonic solution, red blood cells maintain their normal shape; in a hypotonic solution, they swell and undergo hemolysis; in a hypertonic solution, they shrink and undergo crenation.

Question 14

How does water pass through the plasma membrane of the cell during osmosis?

Answer 14

(1) by moving between neighboring phospholipid molecules in the lipid bilayer via simple diffusion.

(2) by moving through aquaporins, or AQPs, integral membrane proteins that function as water channels.

Question 15

Differentiate between the three kinds of tonicity and their effects.

Answer 15

Question 16

Define active transport and identify the two sources of cellular energy that drive this process.

Answer 16

Active transport is considered an active process because energy is required for carrier proteins to move solutes across the membrane against a concentration gradient. Two sources of cellular energy can be used to drive active transport: (1) Energy obtained from hydrolysis of adenosine triphosphate (ATP) is the source in primary active transport; (2) energy stored in an ionic concentration gradient is the source in secondary active transport.

Question 17

Primary Active Transport

Answer 17

In primary active transport, energy derived from hydrolysis of ATP changes the shape of a carrier protein, which “pumps” a substance across a plasma membrane against its concentration gradient. Indeed, carrier proteins that mediate primary active transport are often called pumps. The most prevalent primary active transport mechanism expels sodium ions (Na+) from cells and brings potassium ions (K+) in. Because of the specific ions it moves, this carrier is called the sodium–potassium pump.

Question 18

Secondary Active Transport

Symporters

Antiporters

Answer 18

In secondary active transport, the energy stored in a Na+ or H+ concentration gradient is used to drive other substances across the membrane against their own concentration gradients. Because a Na+ or H+ gradient is established by primary active transport, secondary active transport indirectly uses energy obtained from the hydrolysis of ATP. In secondary active transport, a carrier protein simultaneously binds to Na+ and another substance and then changes its shape so that both substances cross the membrane at the same time. If these transporters move two substances in the same direction they are called symporters; antiporters, by contrast, move two substances in opposite directions across the membrane.

Question 19

Define the three types of endocytosis.

Answer 19

Receptor-mediated endocytosis is a highly selective type of endocytosis by which cells take up specific ligands (ligands are molecules that bind to specific receptors).

Phagocytosis or “cell eating” is a form of endocytosis in which the cell engulfs large solid particles, such as worn-out cells, whole bacteria, or viruses. Only a few body cells, termed phagocytes, are able to carry out phagocytosis. Two main types of phagocytes are macrophages, located in many body tissues, and neutrophils, a type of white blood cell.

Bulk-phase endocytosis, also called pinocytosis or “cell drinking,” a form of endocytosis in which tiny droplets of extracellular fluid are taken up. No receptor proteins are involved; all solutes dissolved in the extracellular fluid are brought into the cell. During bulk-phase endocytosis, the plasma membrane folds inward and forms a vesicle containing a droplet of extracellular fluid.

Question 20

Transcytosis

Answer 20

In transcytosis, vesicles undergo endocytosis on one side of a cell, move across the cell, and then undergo exocytosis on the opposite side.

Question 21

Describe the cytoskeleton and its three kinds of filaments.

Answer 21

The cytoskeleton is a network of protein filaments that extends throughout the cytosol. In the order of their increasing diameter, these structures are microfilaments, intermediate filaments, and microtubules.

Microfilaments are the thinnest elements of the cytoskeleton and have two general functions: They help generate movement and provide mechanical support, including for cell extensions called microvilli, which greatly increase the surface area of the cell and are abundant on cells involved in absorption.

Intermediate filaments are thicker than microfilaments but thinner than microtubules, and are found in parts of cells subject to mechanical stress; they help stabilize the position of organelles such as the nucleus and help attach cells to one another.

Microtubules, the largest of the cytoskeletal components, are long, unbranched hollow tubes that help determine cell shape and also function in the movement of organelles such as secretory vesicles, of chromosomes during cell division, and of specialized cell projections, such as cilia and flagella.

Question 22

Centrosome

Answer 22

The centrosome, or microtubule organizing center, located near the nucleus, consists of two components: a pair of centrioles and the pericentriolar matrix. During cell division, centrosomes replicate so that succeeding generations of cells have the capacity for cell division.

Question 23

Cilia and Flagella.

Answer 23

Microtubules are the dominant components of cilia and flagella, which are motile projections of the cell surface. Cilia move fluids along the cell’s surface, and flagella move the entire cell.

Question 24

Ribosomes.

Answer 24

Ribosomes are the sites of protein synthesis. They contain rRNA (ribosomal RNA).

Question 25

Differentiate between rough ER and smooth ER.

Answer 25

The endoplasmic reticulum (ER) is a network of membranes in the form of flattened sacs or tubules. Rough ER is continuous with the nuclear membrane and is studded with ribosomes, the sites of protein synthesis. Smooth ER extends from the rough ER to form a network of membrane tubules. Unlike rough ER, smooth ER does not have ribosomes on the outer surfaces of its membrane, and does not synthesize proteins, however, smooth ER contains unique enzymes that make it functionally more diverse than rough ER.

Question 26

Lysosomes

Answer 26

Lysosomes are membrane-enclosed vesicles that form from the Golgi complex. They can contain as many as 60 kinds of powerful digestive and hydrolytic enzymes that can break down a wide variety of molecules once lysosomes fuse with vesicles formed during endocytosis. Lysosomal enzymes also help recycle worn-out cell structures. A lysosome can engulf another organelle by autophagy, digest it, and return the digested components to the cytosol for reuse. Lysosomal enzymes may also destroy the entire cell that contains them, a process known as autolysis. Autolysis occurs in some pathological conditions and also is responsible for the tissue deterioration that occurs immediately after death.

Question 27

Peroxisomes

Answer 27

Another group of organelles similar in structure to lysosomes, but smaller, are the peroxisomes. Peroxisomes, also called microbodies, contain several oxidases, enzymes that can oxidize (remove hydrogen atoms from) various organic substances.

Question 28

Proteasomes

Answer 28

A typical body cell contains many thousands of proteasomes, in both the cytosol and the nucleus. Discovered only recently because they are far too small to discern under the light microscope and do not show up well in electron micrographs, proteasomes were so named because they contain myriad proteases, enzymes that cut proteins into small peptides.

Question 29

Mitochondria

Answer 29

Mitochondria consist of a smooth external mitochondrial membrane, an internal mitochondrial membrane containing mitochondrial cristae, and a fluid-filled cavity called the mitochondrial matrix. These so-called powerhouses of the cell produce most of a cell’s ATP and can play an important early role in apoptosis.

Question 30

Describe the structure and function of the nucleus.

Answer 30

The **nucleus** is a spherical or oval-shaped structure that usually is the most prominent feature of a cell. A double membrane called the **nuclear** **envelope** separates the nucleus from the cytoplasm. Both layers of the nuclear envelope are lipid bilayers similar to the plasma membrane. The outer membrane of the nuclear envelope is continuous with *rough ER* and resembles it in structure. Many openings called **nuclear pores** extend through the nuclear envelope. Inside the nucleus are one or more spherical bodies called **nucleoli** that function in producing ribosomes.

Question 31

Proteome

Answer 31

Just as genome means all of the genes in an organism, proteome refers to all of an organism’s proteins.

Question 32

Describe the sequence of events in protein synthesis.

Answer 32

In the process called **gene expression**, a gene’s DNA is used as a template for synthesis of a specific protein. First, in a process aptly named **transcription**, the information encoded in a specific region of DNA is transcribed (copied) to produce a specific molecule of RNA (ribonucleic acid). In a second process, referred to as **translation**, the RNA attaches to a ribosome, where the information contained in RNA is translated into a corresponding sequence of amino acids to form a new protein molecule.

Question 33

How do DNA and RNA store information?

Answer 33

DNA and RNA store genetic information as sets of three *nucleotides*. A sequence of three such nucleotides in DNA is called a **base triplet**. Each DNA base triplet is transcribed as a complementary sequence of three nucleotides, called a **codon**. A given codon specifies a particular amino acid. The **genetic code** is the set of rules that relate the base triplet sequence of DNA to the corresponding codons of RNA and the amino acids they specify.

Question 34

Describe the 3 types of RNA.

Answer 34

**1. Messenger RNA (mRNA)** directs the synthesis of a protein. **2. Ribosomal RNA (rRNA)** joins with ribosomal proteins to make ribosomes. **3. Transfer RNA (tRNA)** binds to an amino acid and holds it in place on a ribosome until it is incorporated into a protein during translation. One end of the tRNA carries a specific amino acid, and the opposite end consists of a triplet of nucleotides called an anticodon. By pairing between complementary bases, the tRNA anticodon attaches to the mRNA codon. Each of the more than 20 different types of tRNA binds to only one of the 20 different amino acids.

Question 35

Describe the process of transcription.

Answer 35

The enzyme **RNA polymerase** catalyzes transcription of DNA. However, the enzyme must be instructed where to start the transcription process and where to end it. Only one of the two DNA strands serves as a template for RNA synthesis. The segment of DNA where transcription begins, a special nucleotide sequence called a **promoter**, is located near the beginning of a gene. Transcription of the DNA strand ends at another special nucleotide sequence called a **terminator**, which specifies the end of the gene. When *RNA polymerase* reaches the terminator, the enzyme detaches from the transcribed RNA molecule and the DNA strand.

Question 36

Introns and Extrons

Answer 36

Not all parts of a gene actually code for parts of a protein. Regions within a gene called **introns** do not code for parts of proteins. They are located between regions called **exons** that do code for segments of a protein.

Question 37

Describe the process of translation.

Answer 37

In the process of **translation**, the nucleotide sequence in an *mRNA molecule* specifies the amino acid sequence of a protein. Ribosomes in the cytoplasm carry out translation. Several ribosomes attached to the same mRNA constitute a **polyribosome**. The simultaneous movement of several ribosomes along the same mRNA molecule permits the translation of one mRNA into several identical proteins at the same time. Translation occurs in the following way: 1. An mRNA molecule binds to the small ribosomal subunit at the mRNA binding site. A special tRNA, called **initiator tRNA**, binds to the start codon (AUG) on mRNA, where translation begins. The tRNA anticodon (UAC) attaches to the mRNA codon (AUG) by pairing between the complementary bases. Besides being the start codon, AUG is also the codon for the amino acid methionine. Thus, methionine is always the first amino acid in a growing polypeptide. 2. Next, the large ribosomal subunit attaches to the small ribosomal subunit–mRNA complex, creating a functional ribosome. The *initiator tRNA*, with its amino acid (methionine), fits into the **P site** of the ribosome. 3. The anticodon of another tRNA with its attached amino acid pairs with the second mRNA codon at the **A site** of the ribosome. 4. A component of the large ribosomal subunit catalyzes the formation of a peptide bond between methionine and the amino acid carried by the tRNA at the A site. 5. Following the formation of the peptide bond, the resulting two peptide protein becomes attached to the tRNA at the A site. 6. After peptide bond formation, the ribosome shifts the mRNA strand by one codon. The tRNA in the P site enters the **E site** and is subsequently released from the ribosome. The tRNA in the A site bearing the two-peptide protein shifts into the P site, allowing another tRNA with its amino acid to bind to a newly exposed codon at the A site. Steps 3 through 6 occur repeatedly, and the protein lengthens progressively. 7. Protein synthesis ends when the ribosome reaches a stop codon at the A site, which causes the completed protein to detach from the final tRNA. In addition, tRNA vacates the P site and the ribosome splits into its large and small subunits.

Question 38

Differentiate between somatic and reproductive cell division.

Answer 38

A **somatic cell** is any cell of the body other than a germ cell. **A germ cell** is a gamete (sperm or oocyte) or any precursor cell destined to become a gamete. In somatic cell division, a cell undergoes a nuclear division called **mitosis** and a cytoplasmic division called **cytokinesis** to produce *two genetically identical cells*, each with the same number and kind of chromosomes as the original cell. Somatic cell division *replaces dead or injured cells and adds new ones during tissue growth*. Reproductive cell division is the mechanism that produces **gametes**, the cells needed to form the next generation of sexually reproducing organisms. This process consists of a special two-step division called **meiosis**, in which the number of chromosomes in the nucleus is reduced by half.

Question 39

Interphase and Mitotic (M) Phase

Answer 39

The cell cycle consists of two major periods: **interphase**, when a cell is not dividing, and the **mitotic (M) phase**, when a cell is dividing.

Question 40

Describe the 3 phases of interphase.

Answer 40

Interphase consists of three phases: G1, S, and G2. The S stands for synthesis of DNA. Because the G phases are periods when there is no activity related to DNA duplication, they are thought of as gaps or interruptions in DNA duplication. Cells that remain in G1 for a very long time, perhaps destined never to divide again, are said to be in the G0 phase. The G1 phase is the interval between the mitotic phase and the S phase. During the S phase, DNA replication occurs. As a result of DNA replication, the two identical cells formed during cell division later in the cell cycle will have the same genetic material. The G2 phase is the interval between the S phase and the mitotic phase.

Question 41

Describe the 4 phases of mitosis.

Answer 41

**Prophase:** During early prophase, the chromatin fibers condense and shorten into chromosomes. Because longitudinal DNA replication took place during the S phase of interphase, each prophase chromosome consists of a pair of identical strands called **chromatids**. A constricted region called a **centromere** holds the chromatid pair together. At the outside of each centromere is a protein complex known as the **kinetochore**. Later in prophase, tubulins in the pericentriolar material of the centrosomes start to form the **mitotic spindle**, a football-shaped assembly of microtubules that attach to the kinetochore. As the microtubules lengthen, they push the centrosomes to the poles (ends) of the cell so that the spindle extends from pole to pole. The mitotic spindle is responsible for the separation of chromatids to opposite poles of the cell. Then, the nucleolus disappears and the nuclearenvelope breaks down. **Metaphase:** During metaphase, the microtubules of the mitotic spindle align the centromeres of the chromatid pairs at the exact center of the mitotic spindle. This plane of alignment of the centromeres is called the **metaphase plate (equatorial plane)**. **Anaphase:** During anaphase, the centromeres split, separating the two members of each chromatid pair, which move toward opposite poles of the cell. Once separated, the chromatids are termed **chromosomes**. As the chromosomes are pulled by the microtubules of the mitotic spindle during anaphase, they appear V-shaped because the centromeres lead the way, dragging the trailing arms of the chromosomes toward the pole. **Telophase:** The final stage of mitosis, telophase, begins after chromosomal movement stops. The identical sets of chromosomes, now at opposite poles of the cell, uncoil and revert to the threadlike chromatin form. A nuclear envelope forms around each chromatin mass, nucleoli reappear in the identical nuclei, and the mitotic spindle breaks up. **Cytokinesis**: As noted earlier, division of a cell’s cytoplasm and organelles into two identical cells is called cytokinesis. This process *usually begins in late anaphase* with the formation of a cleavage furrow, a slight indentation of the plasma membrane, and is completed after telophase.

Question 42

Describe the stages and events of the entire somatic cell cycle.

Answer 42

Question 43

Describe the 4 stages of Meiosis I.

Answer 43

**Meiosis**, the reproductive cell division that occurs in the gonads (ovaries and testes), produces gametes in which the number of chromosomes is reduced by half. Unlike mitosis, which is complete after a single round, meiosis occurs in two successive stages: meiosis I and meiosis II. **MEIOSIS I**, which begins once chromosomal replication is complete, consists of four phases: prophase I, metaphase I, anaphase I, and telophase I. Prophase I is an extended phase in which the chromosomes shorten and thicken, the nuclear envelope and nucleoli disappear, and the mitotic spindle forms. *Two events that are not seen in mitotic prophase occur during prophase I of meiosis.* First, the two sister chromatids of each pair of homologous chromosomes pair off , an event called **synapsis**. The resulting four chromatids form a structure called a **tetrad**. Second, parts of the chromatids of two homologous chromosomes may be exchanged with one another. Such an exchange between parts of nonsister (genetically different) chromatids is called **crossing-over**. This process, among others, permits an exchange of genes between chromatids of homologous chromosomes. Due to crossing-over, the resulting cells are genetically unlike each other and genetically unlike the starting cell that produced them. Crossing-over results in genetic recombination—that is, the formation of new combinations of genes—and accounts for part of the great genetic variation among humans and other organisms that form gametes via meiosis. In **metaphase I**, the tetrads formed by the homologous pairs of chromosomes line up along the metaphase plate of the cell, with homologous chromosomes side by side. During **anaphase I**, the members of each homologous pair of chromosomes separate as they are pulled to opposite poles of the cell by the microtubules attached to the centromeres. The paired chromatids, held by a centromere, remain together. (Recall that during mitotic anaphase, the mentromeres split and the sister chromatids separate.) **Telophase I** and **cytokinesis** of meiosis are similar to telophase and cytokinesis of mitosis. The net effect of meiosis I is that each resulting cell contains the haploid number of chromosomes because it contains only one member of each pair of the homologous chromosomes present in the starting cell.

Question 44

Describe the 4 stages of Meiosis II.

Answer 44

**Meiosis II** also consists of four phases: prophase II, metaphase II, anaphase II, and telophase II. These phases are similar to those that occur during mitosis; the centromeres split, and the sister chromatids separate and move toward opposite poles of the cell. In summary, meiosis I begins with a diploid starting cell and ends with two cells, each with the haploid number of chromosomes. During meiosis II, each of the two haploid cells formed during meiosis I divides; the net result is four haploid gametes that are genetically different from the original diploid starting cell.

Question 45

Compare Mitosis and Meiosis.

Answer 45

Question 46

Glycocalyx

Answer 46

The carbohydrate portions of glycolipid and glycoprotein integral membrane proteins form an extensive sugary coat called the **glycocalyx**. The pattern of carbohydrates in the glycocalyx varies from one cell to another. Therefore, *the glycocalyx acts like a molecular “signature” that enables cells to recognize one another.*

Question 47

What are the factors that affect the diffusion rate of substances across plasma membranes?

Answer 47

(1) Steepness of the concentration gradient (2) Temperature (3) Mass of the diffusing substance (4) Surface area of the plasma membrane (5) Diffusion distance

Question 48

Sodium-Potassium Pump

Answer 48

The most prevalent primary active transport mechanism *expels sodium ions (Na+) from cells and brings potassium ions (K+) in.* Because of the specific ions it moves, this carrier is called the sodium–potassium pump. Because a part of the sodium–potassium pump acts as an ATPase, an enzyme that hydrolyzes ATP, another name for this pump is Na+–K+ ATPase.

Question 49

Where are ribosomes located within the cell?

Answer 49

Some ribosomes are attached to the *outer surface of the nuclear membrane* and to an extensively folded membrane called the *endoplasmic reticulum*. These ribosomes synthesize proteins destined for specific organelles, for insertion in the plasma membrane, or for export from the cell. *Other ribosomes are “free”* or unattached to other cytoplasmic structures. Free ribosomes synthesize proteins used in the cytosol. Ribosomes are also located within *mitochondria*, where they synthesize mitochondrial proteins.

Question 50

What are the various functions of Smooth ER?

Answer 50

Smooth ER lacks ribosomes. It synthesizes fatty acids and steroids; inactivates or detoxifies drugs and other potentially harmful substances; removes phosphate from glucose-6-phosphate; and releases calcium ions that trigger contraction in muscle cells.

Question 51

Golgi Complex

Answer 51

The Golgi complex consists of flattened sacs called cisterns. The entry, medial, and exit regions of the Golgi complex contain different enzymes that permit each to modify, sort, and package proteins for transport in secretory vesicles, membrane vesicles, or transport vesicles to different cellular destinations.

Question 52

Describe how DNA is structured within the nucleus.

Answer 52

Within the nucleus are most of the cell’s hereditary units, called **genes**, which control cellular structure and direct cellular activities. Genes are arranged along **chromosomes**. Each chromosome is a long molecule of DNA that is coiled together with several proteins. This complex of DNA, proteins, and some RNA is called **chromatin**. The total genetic information carried in a cell or an organism is its **genome**. In cells that are not dividing, the chromatin appears as a diffuse, granular mass. Electron micrographs reveal that chromatin has a beads-on-a-string structure. Each bead is a **nucleosome** that consists of double-stranded DNA wrapped twice around a core of eight proteins called **histones**. The string between the beads is called **linker DNA**, which holds adjacent nucleosomes together. In cells that are not dividing, another histone promotes coiling of nucleosomes into a larger-diameter chromatin fiber, which then folds into large loops. Just before cell division takes place, however, the DNA replicates (duplicates) and the loops condense even more, forming a pair of **chromatids**.