Chapter 4 Test Yourself

4-4 What is then term for mRNA formation?

. Messenger RNA (mRNA) formation is known as transcription, which occurs in the nucleus

4-1 What molecules are more likely to diffuse into a cell? What three principles are involved?

. The three principles that enable molecules to diffuse into a cell are: 1. Molecular size: Very small molecules, such as water (H2O), may pass through cellular membrane pores (approximately 0.8 nm in diameter), but larger molecules, such as glucose, cannot. 2. Lipid solubility: Lipid-soluble molecules (e.g., alcohol and steroids) and dissolved gases (e.g., oxygen [O2] and carbon dioxide [CO2]) can pass through the lipid bilayer with ease, whereas other molecules may not. 3. Molecular charge: Ions are small, but their charge prevents easy passage through the membrane pores. Specialized pores called channels selectively allow certain ions to pass through but not others.

4-3 What three factors play a role in the control of cell division?

1. Normal cells stop dividing when they come into contact with surrounding cells. This phenomenon is called contact inhibition. 2. Growth-inhibiting substances may be released from cells when their numbers reach a certain point. 3. A number of checkpoints are reached during cell division when the cell reassesses the division process. These checkpoints occur during the G1 and G2 phases of interphase.

4-2 What is the difference between excretion and secretion? These are both examples of what?

Excretion is the movement of waste products from the intracellular to the extracellular environment, and secretion is the movement of manufactured molecules from the intracellular to the extracellular environment. Both are examples of exocytosis.

4-1 What is an electrolyte?

An electrolyte is a charged particle (an anion or cation) capable of conducting an electric current in solution

4-2 Describe how sodium and potassium enter and exit the cell

Because of the concentration gradients of sodium (Na+) and potassium (K+), potassium tends to diffuse out of the cell and sodium diffuses in. To maintain appropriate levels of intracellular potassium and extracellular sodium, the cell must pump potassium into the cell and pump out sodium. Because diffusion is ongoing, the active transport system must work continuously. The rate of transport depends on the concentration of sodium ions in the cell. ATP is provided by cellular respiration and, with the assistance of the enzyme ATPase, is broken down for use as energy on the inner surface of the cell membrane. The pump can cycle several times using just one molecule of ATP, so that for every molecule of ATP, two potassium ions are moved intracellularly and three sodium ions are moved extracellularly.

4-1 What is another name for hydrostatic pressure in the body?

Blood pressure

4-3 Why is it important for chromatin to coil and form discrete chromosomes before cell division?

DNA is a very long, cumbersome, and delicate molecule. The formation of duplicate, supercoiled chromosomes is essential for life and enables the cell to divide its genetic material equitably for a new generation, without tangling or breaking the long, delicate chains of genetic code.

4-4 When in the cell cycle does DNA replication occur?

DNA replicates during the synthetic (S) phase of interphase.

4-2 How do electrolytes enter the cell?

Electrolytes enter cells via active transport (without the assistance of a concentration gradient).

4-2 What are the principal ions involved in maintaining a cell's resting membrane potential?

Sodium and potassium

4-2 Is there normally a higher concentration of sodium inside or outside the cell? Where is there a higher concentration of potassium?

Sodium is 10 to 20 times higher outside the cell than it is inside. Potassium is 10 to 20 times higher inside the cell than outside.

4-2 When is a membrane process considered active?

The movement of molecules and substances across the cell membrane is considered active when the process requires energy. Energy is stored in molecules such as adenosine triphosphate (ATP). Removal of the terminal phosphate in ATP releases the stored energy needed to drive an active membrane process.

4-4 What are the nucleotides found in DNA? In RNA?

The nucleotides found in DNA are adenine (A), cytosine (C), guanine (G), and thymine (T). The nucleotides found in RNA are adenine (A), cytosine (C), guanine (G), and uracil (U).

4-1 What is diffusion? Is it an active or a passive membrane process?

The passive movement of solute down a concentration gradient

4-3 What is the genetic basis of cellular differentiation?

The position of genes in chromosomes determines the genetic basis of cellular differentiation. Some genes may be located on a region of the chromosome that is available for transcription, whereas other genes may be located inside the molecule and cannot be reached by transcription molecules. We say that one gene is "turned on" while the other gene is "turned off." Genes can be turned off permanently or temporarily. Chromosomes are dynamic in their ability to twist, so that a gene that was once inaccessible on the inside can be moved to the outside of the molecule for use. Differentiation involves the temporary or permanent inhibition of genes that may be active in other cells.

4-2 Describe the three types of endocytosis

The three types of endocytosis are phagocytosis, pinocytosis, and receptor-mediated endocytosis.

4-3 What are the two major periods that comprise the life cycle of the cell?

The two major periods of the cell's life cycle are: interphase, when the cell is growing, maturing, and differentiating; and mitotic phase, when the cell is actively dividing.

4-3 What happens in each of these stages?

1. Prophase: Chromatin coils and condenses into chromosomes that are visible with light microscopy. These chromosomes are composed of two identical chromatids linked together at a constriction in their middle known as the centromere or kinetochore. The cytoplasm becomes more viscous as microtubules from the cytoskeleton are disassembled and the cell becomes round. Two pairs of centrioles form anchors on which new microtubules are constructed, and as the microtubules lengthen, they push the centrioles farther and farther apart. In this way a mitotic spindle is formed that provides the structure and machinery necessary to separate the chromosomes. Because transcription and protein synthesis cannot occur while the DNA is tightly coiled, the appearance of chromosomes marks the cessation of normal synthetic processes. Prophase is thought to conclude with the disintegration of the nuclear envelope. 2. Metaphase: Chromosomes line up in the exact center of the spindle, known as the equator. The chromosomes are evenly spread apart and form what is called the metaphase plate midway between the poles of the cell. The centromere of each chromosome is attached to a single spindle fiber. 3. Anaphase: The centromeres of the chromosomes split apart and each chromatid becomes its own chromosome. The spindle fiber separates, each spindle segment shortens, and the twin chromosomes are pulled away from each another. The chromosomes take on a V shape as they are dragged at their midpoint toward the centrioles at opposite ends of the cell. The cell becomes elongated, and the cytoplasm begins to constrict along the plane of the metaphase plate. Although anaphase is the shortest phase of mitosis and usually lasts only a few minutes, its importance is clear in light of the devastating consequences if an error were to occur in chromosome separation. In anaphase the advantage of separating compact bodies of chromosomes, rather than long thin threads of chromatin, is particularly obvious. 4. Telophase: Begins when chromosomal movement stops. The chromosomes, having reached the poles, begin to unravel, elongate, and return to a diffuse threadlike form (chromatin). A nuclear envelope appears around each new set of chromosomes, and nucleoli appear in each nucleus. The microtubules that made up the spindle in the earlier phases of mitosis disassemble, and a ring of peripheral microfilaments begins to squeeze the cell into two parts. Ultimately, the cell pinches itself in half, dividing the cytoplasm and forming two completely separate daughter cells. The process of cytoplasmic division is called cytokinesis and marks the end of telophase.

4-4 What are codons and what role do they play in transcription?

A codon is a set of three adjacent nucleotides in an mRNA molecule that specifies the incorporation of an amino acid into a peptide or polypeptide chain or that signals the end of peptide synthesis.

4-1 Why do changes in osmolality of body fluids affair an animal's desire to drink and its ability to concentrate or dilute urine?

A hormonal feedback loop helps maintain the osmolality of body fluids within a very narrow range. An increase in the osmolality of blood, for example, stimulates the desire to drink and the release of antidiuretic hormone (ADH) in the pituitary. ADH, in turn, stimulates the kidney to resort water from protourine resulting in the concentration of urine. The opposite happens if the osmolality of blood becomes too low. The release of ADH in the brain is repressed and urine becomes concentrated via active removal of water from protourine in the kidney

4-1 What defines a passive membrane process?

A passive membrane process is one that does not require energy (ATP).

4-1 Give specific examples of both actions and anions

Actions are positively charged and include potassium (K+), magnesium (Mg2+), calcium (Ca2+), and sodium (Na+). Anions are negatively charged and include sulfate (SO42-), hydrogen phosphate (HPO42-), chloride (Cl-), and bicarbonate (HCO3-).

4-1 How is facilitated diffusion different from simple diffusion? What is the limiting factor in the rate of facilitated diffusion?

Facilitated diffusion requires the assistance of an integral protein or carrier protein located in the bilayer. The number of available carrier proteins limits the rate of facilitated diffusion.

4-1 What effect does a hypotonic solution have on a cell?

Facilitated diffusion requires the assistance of an integral protein or carrier protein located in the bilayer. The number of available carrier proteins limits the rate of facilitated diffusion.

4-1 What is the relationship between hydrostatic pressure and filtration?

Filtration is based on a pressure gradient. Liquids may be pushed through a membrane if the pressure on one side of the membrane is greater than that on the other side. The force that pushes a liquid is called hydrostatic pressure.

4-1 List three fluid compartments in the body

Fluid compartments in the body include: intracellular, interstitial, and intravascular

4-1 What passive membrane process causes this affect?

If the extracellular fluid is hypotonic, the inside of the cell contains a higher concentration of solutes than the outside of the cell. In this scenario, water would flow by osmosis into the cell and cause it to swell and possibly burst.

4-4 Compare and contrast the structures of DNA and RNA

In DNA the sugar is deoxyribose, and in RNA the sugar is ribose. In DNA these bases are weakly bonded to nitrogenous bases on an opposing strand. In this way, DNA forms a double-stranded molecule, the basic structure of which is analogous to a twisted ladder in which the vertical poles are composed of alternating sugar and phosphate groups and the horizontal rungs are paired nitrogenous bases. DNA's molecular structure is therefore called the double helix. RNA, however, is a single-stranded molecule that has no opposing strand. The single strand of RNA is similar in structure to each of the strands found in DNA.

4-1 Give specific examples of solutes in the body

Large molecules such as soluble proteins, phospholipids, cholesterol, and triglycerides and also smaller electrolytes such as sulfate, hydrogen phosphate, chloride, bicarbonate, magnesium, calcium, potassium, and sodium

4-2 What is the difference between a symport and an anti port system?

Many active transport systems move more than one substance at a time. If all the substances are moved in the same direction, the system is called a symport system. However, if some substances are moved in one direction and others moved in the opposite direction, the system is called an antiport system.

4-3 Is interphase a time when the cell is resting? Why or why not?

No. Interphase is a period of intense metabolic activity in the cell. Before each cell can divide, a perfect copy of the DNA must be created to pass on to the daughter cells. This replication of DNA occurs during interphase. In addition, all of the enzymes and other proteins needed to drive cell division must be created during interphase.

4-4 What is a nucleotide and how is it structured?

Nucleotides are the building blocks or units of DNA and RNA molecules and are composed of three subunits: a nitrogenous base, a five-carbon sugar, and a phosphate group. DNA and RNA nucleotides are linked to form a "backbone" of alternating sugar and phosphate groups. The nitrogenous bases project out of this backbone.

4-4 Of the thousands of different proteins that a cell could make, how many does it actually produce? Why?

Of approximately 100,000 proteins that a cell could make, it makes only a few hundred because cells have different functions. The number of proteins made depends on the function of the cell.

4-1 What is the relationship between Souter and asmolality

Osmolality is a measurement of the concentrations of solute in fluid. As the concentration of solute increases (or decreases), so too does the osmolality. In other words, fluids that have a high concentration of solute have a high osmolality.

4-1 Which electrolytes are normally more concentrated outside the cell and which ones are more concentrated inside the cell?

Potassium is more concentrated inside the cell, and sodium is more concentrated outside the cell

4-3 What are the four stages of the mitotic phase?

Prophase, metaphase, anaphase, and telophase

4-4 Where does protein synthesis begin?

Protein synthesis begins in the nucleus with transcription and ends in the cytoplasm with translation.

4-1 Give two examples of conditions that result from fluid shifts

Pulmonary edema and cutaneous edema are both caused by abnormal shifts in fluid between compartments

4-4 Can you describe the events that occur in translation?

Translation is the process of making protein and occurs in the cytoplasm with the aid of ribosomes. Protein synthesis can occur either in cytosol, using free-floating ribosomes, or on rough endoplasmic reticulum (rRER), using fixed ribosomes. In the cytosol many free-floating ribosomes attach to a single strand of mRNA at the same time and begin "reading" the genetic sequence on the mRNA. The ribosomes that attach to the strand of mRNA form molecular "docking stations" that enable another molecule, transfer RNA (tRNA), to also bond to the mRNA strand. Each tRNA carries a specific amino acid and has a specific anticodon that correlates with the particular amino acid it carries. The "reading" process involves the bonding of a specific anticodon on the (tRNA to a complimentary codon on the mRNA. At the docking site tRNA molecules bond to the mRNA molecule, bringing the amino acids they carry into close approximation of one another. This allows the amino acids to bond directly to one another. Once the amino acid it carries is bonded to the adjacent amino acid, the tRNA molecule leaves the ribosome to pick up another amino acid. Over time a chain of amino acids, called a polypeptide chain, forms on the ribosome. When the chain has reached its required length, it disconnects from the ribosomal "docking station" and floats out into the cytosol (or if translation occurred on RER, the peptide is taken into the tubular cisternae of the RER for modification). Polypeptides are linked together to form proteins.

4-1 Why do changes in osmolality cause fluid to move from one compartment to another?

Water moves freely between fluid compartments based on changes in osmolality of the fluid in each compartment. Fluid moves toward the compartment with the highest concentration of solute, and therefore the highest osmolality