A&P 106: Chapter 3 - The Cellular Level

A three-base sequence about midway along the sequence of tRNA that can bond to mRNA is called a(n) __________.

Anticodon A tRNA molecule has a tail that binds a specific amino acid. Roughly midway along its length, the nucleotide chain forms a tight loop that can interact with an mRNA strand. The loop contains three nitrogenous bases that form an anticodon, which will hydrogen bond to a complementary codon on mRNA.

The specific term for a cancer-causing agent is __________.

Carcinogen Cancer-causing agents are known as carcinogens. Examples of carcinogens include mutagens, radiation, chemicals, bacteria, and viruses.

The components of the plasma membrane that can transport solutes through the plasma membrane are __________.

Carrier proteins Carrier proteins bind solutes and transport them across the plasma membranes. Carrier proteins may require ATP to power this transport, but they may also do this transport without expending ATP.

The Golgi apparatus secretes newly synthesized proteins out of the cell by a process called __________.

Exocytosis In exocytosis, a vesicle formed inside the cell fuses with and becomes part of the plasma membrane. When this takes place, the vesicle contents are released into the extracellular environment. The ejected chemicals are secretory products, like hormones or enzymes.

Which organelle works with the rough endoplasmic reticulum to secrete proteins out of the cell?

Golgi apparatus The rough endoplasmic reticulum functions as a combination workshop and shipping warehouse. It is where many newly synthesized proteins are chemically modified and packaged for export to their next destination, the Golgi apparatus. One of the functions of the Golgi apparatus is modifying and packaging secretions, such as hormones and enzymes, for release from the cell.

A typical somatic cell has all of the following parts except __________.

Interstitial fluid Cells are surrounded by a watery medium known as extracellular fluid. The extracellular fluid in most tissues is called interstitial fluid.

Along the length of the DNA strand, information is stored in the sequence of __________.

Nitrogenous bases The information in a cell is stored in DNA as a specific arrangement of nitrogenous bases. The order of these bases determines the genes that are activated. Individual DNA strands are formed by sugar-phosphate linkages that form the backbone, and the double helix of the DNA molecule is stabilized by hydrogen bonds between the nitrogenous bases of each strand. DNA is responsible for the blueprints that will end up as an amino acid chain, the precursor of a protein.

There is evidence that there is a limit to the number of times a cell can divide before it stops dividing. The structures that probably cause this are called __________.

Telomeres The number of cell divisions that a cell and its descendants undergo seems to be regulated at the chromosome level. The structures that do this regulation are telomeres. These telomeres are terminal segments of DNA with their associated proteins. They form caps at the ends of chromosomes and protect the ends of chromosomes during mitosis. But each time a cell divides, some of the telomere structure breaks off and they get shorter. When they get too short, repressor gene activity signals the cell to stop dividing

A codon is defined as __________.

a three-base sequence on mRNA Each DNA triplet corresponds to a sequence of three nucleotides in the mRNA strand. Such a three-base mRNA sequence is called a codon. Codons contain nitrogenous bases that are complementary to those triplets in the DNA template strand.

Ions and other small water-soluble materials cross the plasma membrane only by passing through __________.

A channel Cell communication happens across the cell membrane. Part of the response to this communication is the passage of small water-soluble molecules and other ions through a protein channel. One of the ways that a cell detects a signal is through a ligand binding to its receptor protein. Other membrane-bound proteins can serve as anchors for molecules and cytoskeletal proteins.

The difference between an area of high concentration of solute and an area of low concentration of solute is known as __________.

A concentration gradient When you put a tablespoon of sugar into a glass of iced tea, it gets sweeter as you get closer to the bottom. That is because there is a higher concentration of sugar at the bottom of the glass. This change in the concentration of solute molecules (of sugar) as you get closer to the bottom is called a concentration gradient. If you let the glass sit on the counter, eventually the sugar will totally dissolve and, by reaching equilibrium, be evenly distributed throughout the iced tea. If you stirred the sugar into the iced tea so that it totally dispersed and mixed evenly, then it would no longer have a concentration gradient. This type of gradient is different from a pressure gradient because it utilizes differences in solutes to maintain or achieve homeostasis.

Red blood cells are hemolyzed when the cells are placed in __________.

A hypotonic solution Since humans have up to 25 trillion RBCs floating around a watery medium called plasma, there are specific terms to describe what could happen to them in this solution. If they lyse or break, it is called hemolysis. If they shrivel up like a prune, it is called crenation. Recall that the definition of a hypotonic solution is one with fewer solutes. If an RBC is placed in a hypotonic solution, that means there are more solutes inside the RBC. Water will move into the cell, trying to reach equilibrium between the solutes inside and those outside the cell. This will cause the cell to fill up and hemolyze (burst) like an overfilled water balloon. If the solution is hypertonic, then the water inside the cell will move out because the relative concentration of solute in the cell is higher. With an isotonic solution, there is no net movement either way because the concentrations inside and outside the cell are the same. In a salt solution, it will depend upon the concentration of salt. For instance, seawater is 3% salt, which is considered hypertonic to cells, whereas a solution of 0.9% is the salt concentration in your blood. So this is an isotonic solution.

A sequence of three nitrogenous bases can specify the identity of __________.

A single amino acid Nitrogenous bases are the basic unit of DNA. The sequence of three nitrogenous bases in a row denotes a single amino acid. Each DNA molecule is denoted by a series of covalently bound nitrogenous bases, a specific sequence of which identifies a specific gene, which in turn is ultimately translated into a specific peptide chain.

Which of the following is an example of phagocytosis in the human body? A. A white blood cell engulfing a bacterium B. Expulsion of air from the lungs C. Vacuolar digestion of a solvent D. Expulsion of a specific volume of blood from the left ventricle

A. A white blood cell engulfing a bacterium Phagocytosis is a process closely associated with white blood cells in the immune system. A white blood cell engulfing a bacterium is an example of phagocytosis in the human body. This process is called "cell eating." The white blood cell finds a bacterium and begins engulfing it. Once inside, it uses powerful digestive enzymes to break down and destroy the bacterium. Phagocytosis occurs on the cellular level and is different from vacuolar digestion of a solvent because vacuoles are internal organelles. Expulsion of air from the lungs is called exhalation and actually corresponds to the expulsion of a specific blood volume (stroke volume) from the ventricles during the cardiac cycle.

Which of the following is true of transmembrane potential? A. All of the listed responses are correct. B. Positive and negative charges are separated. C. Changes can trigger muscle contraction. D. It is measured in millivolts.

A. All of the listed responses are correct. The transmembrane potential of a cell is based on the unequal distribution of ions and proteins on either side of the membrane. The membrane acts as a barrier to separate the positive and negative charges. This difference in charge can be measured in millivolts. A negative sign denotes that the charge is greater inside the cell. Changes in the transmembrane potential are very important for cellular responses. For example, these changes can trigger muscles to contract.

Which of the following statements about cancer cells is not accurate? A. Cancer cells eat normal cells in an organ. B. Cancer cells do not resemble normal cells. C. Cancer cells are very metabolically active. D. Cancer cells no longer perform their original function.

A. Cancer cells eat normal cells in an organ Cancer cells do not resemble normal cells. They have a different shape and become abnormally large or small. The cancer cells in tumors are very metabolically active. Their presence causes the formation of new blood vessels to supply their metabolic needs. Malignant cells no longer perform their original function, so organ function deteriorates. They are so metabolically active that they outcompete normal cells for nutrients. So, the normal cells deteriorate.

Which of the following substances do not help promote or stimulate cell division? A. Chalones B. M-phase promoting factor C. Growth Hormone D. Epidermal growth factor

A. Chalones Mitotic rates are genetically controlled, and many different stimuli may be responsible for activating genes that promote cell division. Some may be hormones, peptides, or nutrients derived from food. Chalones are produced by many tissues in the body and have an effect on cells in the immediate area, but they inhibit cell division.

Which of the following statements is most accurate? A. Gene repression results in cellular differentiation. B. Gene repression results in increased stem cell divisions. C. Gene repression results in an increase in the cell's ability to produce different proteins. D. Gene repression results in an increase in the cell's functional capabilities.

A. Gene repression results in cellular differentiation All cells in our bodies, except sperm and eggs, have the same DNA. As cells specialize and become liver cells, or heart cells, or nerve cells, different sets of genes are turned off or repressed. When a gene is functionally eliminated, the cell loses the ability to produce certain proteins, and to perform any function involving that protein. Cells specialize their function in response to gene repression. This specialization is differentiation.

Which of the following is one of the great advantages of moving materials by active transport? A. The process is not dependent on a concentration gradient. B. Receptor sites are not necessary for the process to occur. C. The process has no energy cost. D. Carrier proteins are not necessary.

A. The process is not dependent on a concentration gradient The process is not dependent on a concentration gradient. A major consideration in determining the success of any physiological process is energy cost in terms of ATP requirements. Processes that do not utilize energy, such as diffusion and osmosis, could be considered default mechanisms. Processes that require energy are very important because they enable the possibility of having different conditions. For instance, passive transport is based on concentration gradients, and everything "goes with the flow." However, active transport is not dependent on a concentration gradient since it is powered by ATP. In fact, in many examples, movement of molecules by active transport goes against the concentration gradient. This can be highly advantageous, such as in the case of sodium-potassium exchange pumps. Furthermore, active transport is not constrained by carrier proteins, and receptor sites aren't needed. So although it has an energy cost, the advantages outweigh the loss of energy.

What is the term for the movement of a molecule across a membrane from an area of low concentration to an area of high concentration using a carrier protein and ATP?

Active transport One way that a cell can control its response to environmental cues is by using a transport mechanism to get across a membrane. This transport mechanism needs energy because it goes against the concentration gradient and is called active transport, which can move molecules in both directions. Special cases of active transport that move molecules in only one direction include exocytosis and endocytosis. In exocytosis, bulk amounts of molecules are released from a cell, whereas endocytosis pulls bulk amounts of molecules into a cell. Some molecules that cannot simply diffuse through a membrane use channels, and some of these channels have specific binding sites. In some cases, a channel may have a second binding site. This second site can be used by another molecule to get into the cell using the first site as an access door. This process of using another molecule's channel interaction to get into or out of a cell is called facilitated diffusion.

The genetically controlled death of cells is called __________.

Apoptosis Cells have regulatory and monitoring systems that allow them to detect if there is internal damage or errors in the DNA, termed mutations. If the cell cannot repair itself, a safety mechanism exists to get rid of the damaged cell or keep mutated cells from growing out of control. This safety mechanism is called apoptosis and involves a programmed pathway to cell suicide. During interphase, the cell will prepare for mitosis by duplicating its organelles and synthesizing proteins. Once the chromosomes have been separated in late mitosis, the dividing cell undergoes cytokinesis to complete the formation of the new daughter cells.

How does facilitated diffusion differ from simple diffusion?

Carrier proteins are involved Simple diffusion is the movement of substances across a cell membrane based on following concentration gradients. Carrier-mediated diffusion is similar in that it does not use energy (ATP) and follows concentration gradients. The difference is that a carrier protein is involved. This is very similar to fitting a key into a lock and opening the door. The solute to be transported into the cell will bind to a specific site on the protein. When the solute binds, it is like turning a key: A structural change occurs that is like opening the lock mechanism. At that point, the carrier protein changes shape and forms an opening to the inside of the cell, like a door swinging open to the inside of a room.

The form that DNA exhibits when a cell is not dividing is __________.

Chromatin At intervals along their length, the DNA strands wind around histone proteins, forming a complex known as a nucleosome. In cells that are not dividing, the nucleosomes are loosely coiled within the nucleus, forming a tangle of fine filaments known as chromatin. As the cell is dividing, the coiling becomes tighter, forming chromosomes.

Cytokinesis begins with the indentation of the plasma membrane called the __________.

Cleavage furrow Cytokinesis is the division of the cytoplasm into two daughter cells. Cytokinesis begins with the formation of a cleavage furrow and continues through telophase. This cleavage furrow is an indentation in the plasma membrane and continues until the cytoplasm is completely divided in two.

An injection of a concentrated salt solution into the circulatory system would result in __________.

Crenation of RBCs The physiologic concentration of salt in blood plasma is 0.9%. If a salt solution more concentrated than that is injected into the circulatory system, the RBCs would shrivel up or crenate. This is because the water in the RBCs would move out in an attempt to reach equilibrium. When RBCs swell because they are in a hypotonic solution, it is termed hemolysis if the cells rupture. If the salt solution injected is slightly less than 0.9%, then a slight increase in cellular volume would be observed. Saline injections of 0.9% (isotonic conditions) would have little or no effect on the RBCs.

The cell membrane isolates the cytoplasm from the surrounding fluid environment. Why is this important for cellular communication?

Cytoplasm has a composition different from that of the extracellular fluid, and the differences must be maintained. Cells respond to changes in their external environment. These changes can take the form of ion fluctuations, signaling molecules, osmolarity, etc. If the fluid was the same inside and outside of the cell, then the cell would not be able to detect a change, nor effect a response. Therefore, the composition of the cytoplasm in cells is different from that of the extracellular fluid; and these differences have to be maintained or else everything would be in a static, neutral state and no responses could occur. Additionally, the integrity of the nuclear membrane needs to be maintained in order for the nucleus to perform its functions. Within the cytoplasm are cellular organelles, many of which need to be located in specific regions in order to function correctly. In many cases, it is their orientations to the nucleus that will determine their locations. If the cell membrane were destroyed, the organelles would no longer have their orientations or locations, and they would lose their shapes and/or functions.

Which of the following factors influence(s) diffusion rates? A. distance B. molecular size C. temperature D. All of the above

D. All of the above Diffusion is a mechanism of membrane transport in cells that uses concentration gradients to drive the process. Because it is based on concentration gradients, diffusion doesn't use energy. But the cell needs to have a way to regulate what goes through its membrane; otherwise, if the concentration gradient for a substance was higher outside the cell, then ALL of that substance would flow out. The way a cell regulates diffusion is based on the principle of selective permeability. Even though a cell is selectively permeable to substances, the rate of diffusion across the membrane can be influenced by three factors. Diffusion can increase or decrease based on how close a molecule is to the membrane (distance). Temperature is another factor; the warmer the temperature, the greater the diffusion rate. The last factor is the molecular size. Smaller molecules can diffuse across the membrane much more quickly than larger ones can. If a molecule is too large, a protein channel or other mechanism may be used for diffusion.

A cell that has switched off multiple genes and restricted its ability to perform certain functions is said to be __________.

Differentiated As cells undergo development, they change and become more specialized. This happens because stem cells, which can take many paths, become programmed for one type of organ or tissue. For this to happen, cells must turn off the genes for everything else. When cells turn off these genes to become more specialized, they have differentiated. Cells that cannot differentiate or that lose their special characteristics can become cancer cells. If these cancer cells escape from their tissues of origin and form a tumor elsewhere, they are said to have metastasized.

Whether in embryos or adults, stem cells have the unique capability to develop into a cell with a specific or specialized function. This process is called __________.

Differentiation Humans are estimated to be made up of 75-100 trillion cells, all starting from a single cell. In order to go from one cell to the vast number of tissues, organs, and systems in the human body, subsequent populations of cells have to become specialized. Since the stem cell can become a multitude of cell types, the most efficient way for cells to become specialized is to shut off sequences of genes until only the cell expressing the needed traits remains. This process of shutting off active genes until only the needed characteristics remain is called differentiation.

All transport through the plasma membrane can be classified as __________.

Either active or passive Membrane transport can be put into two categories based on whether or not energy is used. These categories are either active (if energy is used) or passive (if energy isn't used). Two types of active transport are pinocytosis, also known as cell drinking, and phagocytosis, also called cell eating. Passive transport mechanisms for small molecules and ions or water are called diffusion and osmosis, respectively. These types of transport mechanisms determine whether a cell membrane is permeable or impermeable.

The effect of diffusion in body fluids is that it tends to __________.

Eliminate local concentration gradients One major advantage to diffusion besides its being energy-free is that it eliminates local concentration gradients trying to reach equilibrium. This is opposite to what happens with active transport (energy-necessary). Two very important considerations regarding normal cell function are membrane transport and energy consumption. Proper cell function is based on signals from the extracellular fluid being interpreted. Transport across the membrane is one way a cell can receive signals and effect a response. Since membrane transport happens continuously, any energy use can have a huge impact on cell function. One mechanism of energy-free membrane transport is diffusion. Diffusion works by using concentration gradients to its advantage. If there is a higher concentration of a substance on one side of the membrane, diffusion drives the movement of that substance down its gradient to the side with the lower concentration. Think of it like a branch floating on a river being carried along with the current. Active transport tends to increase the concentration of a substance on one side of the membrane. Since diffusion occurs in response to concentration gradients, molecular charges are not as important a factor. Also, regarding the substance, whether it is active or inactive is not taken into consideration if it is freely present in the system.

The process for bringing large volumes of extracellular material into the cell by forming vesicles at the cell surface is known as __________.

Endocytosis Extracellular material can be packaged in vesicles formed at the cell surface and imported into the cell by a process called endocytosis. It is possible to bring large volumes of extracellular material into the cell this way. There are three major types of endocytosis: receptor-mediated endocytosis, pinocytosis, and phagocytosis.

A type of process that moves chemicals through the plasma membrane following their concentration gradient and using a carrier is called __________.

Facilitated diffusion Many essential nutrients, such as glucose and amino acids, are insoluble in membrane lipids and are too large to fit through membrane channels. These substances must get into the cell for normal cellular metabolism. They can be passively transported across the membrane by carrier proteins in a process called facilitated diffusion. This process still makes use of the concentration gradient for those chemicals to move them.

The stage of the cell cycle during which a cell prepares to divide by growing in size is __________.

G1 A cell that is ready to divide begins the G1 phase. During this phase, the cell makes enough mitochondria, cytoskeletal elements, endoplasmic reticula, ribosomes, Golgi membranes, and cytosol for two functional cells. This is necessary because during telophase of mitosis, the cytoplasm will divide in half. This growth is important so the two new cells have enough organelles each to survive.

DNA is made up of a long series of nucleic acids. A predetermined sequence that contains the information to produce a specific protein is called a __________.

Gene DNA is made up of a series of nucleic acids bound together by a sugar backbone as well as hydrogen bonds, with their complementary base pairs, A to T and C to G. Stretched end to end, DNA from one cell would be almost 3 meters in length. This long strand of sequences is divided into functional units that regulate what will be produced. These functional units are called genes. DNA is organized into arrays of sequences called chromosomes. Within the chromosomes, DNA is further organized by wrapping around special proteins called histones. This DNA-histone complex makes up the nucleosome. Ribosomes are important in protein synthesis and contribute to the formation of the histone proteins.

What is the functional unit of heredity?

Gene Traits that are passed on from parents to children are said to be inherited. These heritable features are determined by the genes that are expressed. These functional units originate from DNA, which is then transcribed to mRNA. The mRNA moves from the nucleus into the cytoplasm, where every three nucleotides is a codon that will be translated into an amino acid.

The process of differentiation resulting in the appearance of characteristic cell specializations involves __________.

Gene repression (switching off) From the process of fertilization and the formation of a single-celled zygote, a human being with up to 100 trillion cells will develop. The diversity of cells in the human body is staggering. For example, cardiac muscle cells, neurons, skin cells, etc., all developed from that single cell. In order for these specialized cells to develop, gene repression (switching off) must occur. In gene repression, genes are switched off in a process of "elimination" until the right combination of genes that express a particular type of cell or tissue remains. In many cases, there is an irreversible alteration in the protein structure. A gene that was previously turned on in development and turned off for differentiation—and then turns back on when it shouldn't—is called an oncogene.

The proteins that allow DNA strands to form chromosomes are __________.

Histones DNA strands wind around histone proteins, forming a complex known as a nucleosome. As the cell is dividing, the coiling around these histone proteins becomes tighter, forming chromosomes. When the chromosomes coil so tightly, they become visible. This form of DNA allows easier dividing of the contents of the nucleus during cell division.

The spreading process of a primary tumor is called __________, and the dispersion of malignant cells to establish a secondary tumor is called __________.

Invasion, metastasis Cancers that form solid tumors are categorized based on how much they have infiltrated into the surrounding tissue. In other words, is the tumor still a small, tightly clustered ball of cells, or has it branched out like the limbs of a tree? This branching out process is called invasion and is correlated with tumor aggressiveness. The process of cells from the tumor breaking away, invading other tissues far away from the original tumor, and starting to grow another (secondary) tumor is called metastasis. Generally speaking, metastasis comes after invasion. Cancer cell penetration into tissues is usually associated with cells that metastasize and enter circulation. In contrast, white blood cells are normal cells in circulation that can penetrate into tissues.

Cells in our body are surrounded by extracellular fluid. A solution in which a cell exists that causes it to remain a normal size and shape is called a(n) __________ .

Isotonic Solution The total solute concentration in an aqueous solution is the solution's osmotic concentration. When we need to describe the effect of various osmotic solutions on cells, we use the term tonicity. A solution that does not cause any osmotic flow of water into or out of a cell is called isotonic.

The smooth endoplasmic reticulum (SER) has a variety of functions that center around the synthesis of __________.

Lipids and carbohydrates The endoplasmic reticulum is a tubular network of intracellular membranes that is contiguous with the nuclear membrane. If no ribosomes are associated with it, then the endoplasmic reticulum is referred to as smooth, and its main function is the synthesis of lipids and carbohydrates.

A type of cellular reproduction in which reproductive cells or sex cells are produced is called __________.

Meiosis At fertilization, a single cell exists. At maturity, your body has about 75 trillion cells. This involves a form of cellular reproduction called cell division. Even when development is complete, cell division continues. Somatic cells are produced by a form of cell division called mitosis. During the reproductive life of males and females, sperm and eggs are formed by cell division also. However, the production of sex cells occurs by a different form of cell division called meiosis

The unequal distribution of charges across the cell membrane results in the ______________.

Membrane potential In order for cells to be able to maintain homeostasis, they have to be able to detect and respond to changes in the external environment. One very important way they do this is by exploiting the properties of the cell membrane. Recall that the cell membrane is basically a phospholipid bilayer. The external hydrophilic heads allow the cell to interact with its microenvironment, while the hydrophobic tails insulate the cytoplasm from the extracellular fluid. Substances passing through have to meet specific criteria—e.g., size, charge, etc. This physical barrier acts as a buffer zone between the different charges inside and outside. This net charge difference is called the transmembrane potential. To maintain this difference requires the cell membrane to go against a natural concentration gradient. This is accomplished by generating ATP to power membrane pumps. In some cases, substances can be transported across the membrane through carrier-mediated transport. In this process, an integral carrier protein can bind a specific substance and carry it across the membrane. The triplet code is the number of DNA nucleic acids that correspond to one amino acid. For example, TGT are the nucleic acids that code for the amino acid cysteine.

During which stage of cell division are all the chromosomes lined up in the center of the cell?

Metaphase Metaphase begins as all the chromosomes move to a narrow central zone called the metaphase plate. Metaphase ends when all the chromosomes are aligned in the plane of the metaphase plate.

What is the term for the movement of malignant cells from their site of origin to distant tissues and organs?

Metastasis When a tumor cell breaks off from the primary tumor, moves through the body, and forms another tumor in a different tissue or organ, it is called metastasis.

The cytoskeleton gives a cell strength and rigidity and anchors the position of major organelles. What are the primary components of the cytoskeleton?

Microtubules In order to keep their morphology intact, cells have structural proteins that form a cytoskeleton, which gives cells strength and anchors the organelles. The primary structural components are microtubules.

All of the following are membranous organelles except __________.

Microvilli and centrioles Cellular organelles can be placed into two major categories: They are enclosed within membranes, or they are not enclosed within membranes. Generally, organelles that are not enclosed within membranes are structural. Examples include microvilli and centrioles. Organelles that are membrane-bound, such as the nucleus, endoplasmic reticulum, peroxisomes, and Golgi apparatus, have a common connection because of their membranes. Mitochondria, where energy is produced, have a double membrane.

Most of the energy a cell needs to live is generated by the __________.

Mitochondria Energy in a cell is dispensed with great care. The energy needs of the cell are met by generating it within organelles specialized for this purpose. These organelles are the mitochondria.

What does the formation of a malignant tumor indicate?

Mitotic rates of cells are no longer responding to the normal control mechanisms. Tumor formation occurs in a series of stages. First, the tumor cells may be encapsulated in a connective tissue enclosure. These types of cancers are usually contained and have not infiltrated the surrounding tissue. They may resemble normal cells, but they do not divide much faster. When the mitotic rates of cells are no longer responding to the normal control mechanisms, the tumor is considered malignant. If it is not treated but is allowed to progress, then the tumor cells may break away from the original tumor mass and start a new tumor elsewhere. Metastatic cancers are very difficult to treat.

The passive process in which water moves through the plasma membrane from an area of high water concentration to an area of low water concentration is called __________.

Osmosis Water is so important to the human body that water has a special type of membrane transport all its own. This is called osmosis. All other molecules that move across the cell membrane without needing energy do so through a process called simple diffusion. In some cases, one molecule is too big to go through the membrane, so it hitches a ride with a molecule that has a specific channel to get into or out of a cell. This is called facilitated diffusion. If energy is needed to transport a molecule across a cell membrane, it is called active transport.

The energy-producing process in the mitochondria involves a series of reactions in which __________ is consumed and __________ is generated.

Oxygen, Carbon Dioxide There are two basic methods of producing energy in the form of ATP in humans: either with oxygen (aerobically) or without oxygen (anaerobically). If oxygen is involved, energy production takes place in the mitochondria, one function of which is specifically to use oxygen to generate ATP. When oxygen is used, the reaction is much more efficient, and carbon dioxide is generated as a by-product.

Protein channels within the plasma membrane __________.

Permit water and ion movement at all times The cell membrane is selectively permeable. The molecules that cannot pass through the membrane can enter through protein channels that span the membrane, acting like a tunnel that permits the continuous movement of ions and water. Gated protein channels open and close to regulate ion passage. The smooth endoplasmic reticulum transports only lipids. These types of transport are energy-free. The types of membrane-associated proteins that require ATP as an energy source are ATPase pumps, such as the sodium-potassium pump.

Which organelle absorbs and breaks down fatty acids and other organic compounds?

Peroxisome Lipids are complex molecules that need to be broken down before they can be used in cells. Peroxisomes are organelles filled with degrading enzymes that are responsible for catabolizing fatty acids and other compounds.

The replication of DNA occurs primarily during which part of interphase?

S Phase The nucleus of each cell contains all the genetic information for one particular person or one particular species. So replicating DNA correctly is extremely important. If it is not replicated exactly, then the result will be a loss of that gene's function. S phase is the stage of the cell cycle devoted to ensuring the correct replication of DNA. All the proteins and organelles that will be shared between the daughter cells is duplicated in G1. During G2, centriole replication occurs because these structures will guide the sister chromatids to each side of the dividing cell. Mitosis is the phase during which the duplicated chromosome pairs are actually pulled apart. This is a very short phase because the cell has to get back up and running.

Structurally, the plasma membrane is best described as a __________.

Phospholipid bilayer interspersed with proteins Most human cell membranes are selectively permeable. One of their major characteristics is that water-soluble molecules are rebuffed by the membrane—that is, they can't just move through it. However, water-soluble molecules go through the membrane all the time. The membrane structure is the key to this selectivity. The membrane's ability to repel water-soluble molecules while selectively allowing some of them through comes about because the membrane is made up of a phospholipid bilayer interspersed with proteins that make channels allowing passage of certain molecules. The bilayer also has proteins that identify a molecule as belonging or not. These proteins are interspersed throughout the membrane and are attached to its inside or its outside. Thus the membrane is not just one phospholipid layer with proteins integrated on the periphery. In order to retain its selective nature, it could not be made of a protein layer with phospholipids interspersed because then it would be like a lobster shell or a beetle shell and be completely impermeable.

Most of the surface area of the plasma membrane consists of __________.

Phospholipids The cell membrane has to be made of a structural component that can serve as a barrier between the inside and the outside. This structural component consists of phospholipids. Another important structural molecule that is found in membranes is cholesterol. Channels that go through membranes are made of proteins, and the molecules that are used as recognition factors on the outside of membranes are glycoproteins.

During which phase of the cell cycle does chromatin coil up tightly and chromosomes become visible?

Prophase Prophase begins when the chromosomes coil so tightly they become visible as single structures. As a result of DNA replication during the S-phase, two copies of each chromosome now exist. Each copy, called a chromatid, is connected to its duplicate at the centromere.

The components of the plasma membrane that bind to specific molecules in the extracellular fluid surrounding the cell are the __________.

Receptor proteins Ligands are molecules found outside a cell that bind to and activate receptor proteins. The receptor protein-ligand association is very important for several reasons. For example, only a specific ligand can link to a specific binding site of the receptor protein. Therefore, this binding determines the specificity of a response. In other words, no binding = no response. Carrier proteins are different because although there is a binding site for one substance (like sodium), there is also a binding site for a second molecule (like glucose). So the glucose enters based on whether the sodium binds to its molecular pocket. Enzymes are a type of protein that can speed up a biological reaction without becoming part of it. They are not responsible for membrane transport. Channel proteins don't discriminate among molecules. They act like tunnels that allow molecules into or out of a cell based on size.

The voltage that exists across the cell membrane when cells are not being stimulated in some way is called __________.

Resting membrane potential The cell's interior has a slight negative charge, while the exterior has a slight positive charge. The positive and negative charges are attracted to each other and would normally rush together, but the plasma membrane keeps them apart. When the positive and negative charges are held apart, an electrical force develops, called a membrane potential or transmembrane potential. The membrane potential in an undisturbed or unstimulated cell is called its resting membrane potential.

The passive process in which a fat-soluble molecule moves through the plasma membrane from an area of high concentration to an area of low concentration is called __________.

Simple Diffusion Since so much of cell function relies on communication with the external environment, membrane transport is extremely important, and the cell continuously interprets signals and exchanges solute molecules between the extracellular fluid and the cytosol. If this exchange does not require energy, it is called simple diffusion. Energy production and consumption are very important in cellular systems. Therefore, how a cell uses its energy is strictly regulated. Whenever a cell can perform a process without using energy, it will do so. A special type of diffusion called osmosis involves water. If the transport of molecules across the membrane requires energy, it is called active transport. A type of active transport that brings in bulk molecules by having the membrane bubble inward is called endocytosis.

The primitive or unspecialized cells that can be stimulated to undergo cell division to produce a daughter cell that will specialize are called __________.

Stem cells Special cells called stem cells are relatively unspecialized cells. Their only function is to produce daughter cells. Each time a stem cell divides, one of its daughter cells develops functional specializations while the other prepares for further stem cell divisions. All of us, even though we are adults, have populations of stem cells in us.

The nucleoli are organelles within the nucleus that are responsible for __________.

Synthesizing ribosomal RNA The genetic information for a cell is housed within the nucleus. There, it is organized, synthesized, and replicated. While most organelles are located in the cytoplasm, one organelle is specific to the nucleus: the nucleolus. Most cells have varying numbers of nucleoli depending upon their function because the function of the nucleoli is to synthesize ribosomal RNA, which is critical to protein synthesis. In addition, they assemble the ribosomal subunits used in protein translation. Nucleoli are made up of RNA, enzymes, and histone proteins. These histones also help organize the nuclear material by acting like spools for the threadlike chromatin. This histone-chromatin coiled complex is called the nucleosome.

Why are lysosomes sometimes called "cellular suicide packets"?

The breakdown of lysosomal membranes can destroy a cell. Depending on whether or not a cell is able to repair damage or correct errors in DNA, it will sometimes commit suicide. One way this happens is by the breakdown of the membranes that form the lysosome. Inside are caustic, destructive enzymes that will destroy a cell if they leak into the cytoplasm. This process is called autolysis. However, lysosomes do not function only as suicide sacs; they also fuse with endocytic vesicles and share their contents, which promptly digests the solid materials. If organelles are damaged beyond repair, lysosomes fuse with their membranes and destroy the damaged organelles. Lysosomes are not known for penetrating other cells. However, there is ongoing research attempting to harness their destructive capabilities and target diseased cells elsewhere in the body.

What is the most notable characteristic of the G0 phase of an interphase cell?

The cell is not preparing for mitosis Within the different stages of the cell cycle, G0 can be thought of as the stage during which the cell does normal, daily functions. A skeletal muscle cell contracts, neurons send electrical impulses, bone cells produce bone matrix, etc. When a cell needs to divide, it will enter G1 and begin duplicating the organelles that will be distributed between the new daughter cells. Replicating DNA correctly is so important for the future of the cell that it has its own phase: It is during S phase that the chromosomes are duplicated. In order to do this, a special enzyme called DNA polymerase binds to the nitrogenous bases of the DNA and facilitates the binding of the complementary nitrogenous bases in their correct base-pair sequences. Otherwise, gene expression won't occur or will be wrong.

During osmosis, water will always flow across a membrane toward the solution that has __________.

The higher concentration of solutes Osmosis is very similar in nature to diffusion—it is energy-free and works by eliminating concentration gradients. The difference is that instead of substances (solutes) moving, it is water (a solvent) that moves to the area of the higher concentration of solutes. In some cases, the solute is too big to move across the membrane, so water does the moving. Diffusion and osmosis both continue until equilibrium is reached, which occurs when there is an equal concentration of solutes or solvents across the membrane.

If the template strand of a gene had a nucleotide sequence of TACCGG, what would the sequence of bases in the mRNA be? If that mRNA were used in translation, what would the anticodons in the tRNA be?

The mRNA sequence is AUGGCC; the tRNA sequence is UACCGG. The strand of triplets that specify the sequence of amino acids in a polypeptide is the coding strand. The other strand is the template strand, and it contains the complementary triplets that will be used as a template for mRNA production. The resulting mRNA strand will have a nucleotide sequence identical to the coding strand, but with uracil substituted for thymine. The anticodon is complimentary to the codon.

Which of the following processes produces messenger RNA from the template strand of the DNA?

Transcription The process of producing RNA from a template strand of DNA is known as transcription. Think of it this way: The DNA is master plan that determines which genes will be expressed. This master plan then has to be written down, or transcribed into blueprints to be followed. These blueprints, in the form of RNA, then need to be fabricated, much like a house is built following the blueprint. In cells, the process of fabricating proteins from RNA is called translation, which is similar to translating design plans into a finished product. Replacing small pieces of the cell membrane is an example of a translated product. This continuous process of forming and exchanging membrane segments is known as membrane flow.

The nucleic acid molecules that actually deliver amino acids to the ribosome where translation occurs are the __________.

Transfer RNA molecules The amino acids are provided by transfer RNA (tRNA), a relatively small mobile type of RNA. Each tRNA molecule binds and delivers a specific type of amino acid. More than 20 different kinds of tRNA exist—at least one for each of the 20 amino acids used in protein synthesis. A tRNA molecule has a tail that binds an amino acid.

The process of protein synthesis is called __________.

Translation Proteins are the products of genes that are turned on. This process of producing proteins is called translation. Producing proteins can be compared to car manufacturing. The plans are copied and sent out from the headquarters; the blueprints of the initial design are then sent out to the different auto assembly plants (factories). In the cell, copying the design is the process of DNA replication. The blueprints being sent to the car factories is similar to transcription, in which the RNA, formed from DNA, is sent out for production. The final product, the proteins, are translated from the blueprints. Like any car that doesn't meet the standards, cells that produce abnormal proteins are destroyed. This is called autolysis.

A solution that is hypotonic to cytoplasm has __________.

a solute concentration lower than that of the cytoplasm A solute is a substance that is dissolved in a fluid. The fluid doing the dissolving is called the solvent. If the fluid outside the cell has a solute concentration lower than that of the cytoplasm, the solution is said to be hypotonic (having a low concentration of solutes). If the extracellular fluid has more solutes than are contained in the cytoplasm, it is considered hypertonic (having a high concentration of solutes). If the concentrations of solutes inside and outside the cell are the same, then the solution is isotonic. When water concentration is being measured, then it is termed osmotic concentration.

Cilia function to __________.

move fluids and secretions across the cell surface Cilia are extensions of the plasma membrane that facilitate the movement of fluids and secretions across the cell surface. Although at first glance these extensions appear to increase the surface area of the cell out to the extracellular environment, their function is motility, not absorption. Thus, they are found on epithelial cells, which adhere together very strongly because of tight junctions and desmosomes. Therefore, the cilia on these cells move substances across the cell—rather than move the cell through the surrounding fluid. Within the cilia are microtubules, the same structural proteins that move DNA molecules during cell division.

The four stages of mitosis in correct sequence are __________.

prophase --> metaphase --> anaphase --> telophase The life of a cell can be divided into three major phases: interphase, mitosis, and cytokinesis. Interphase is the phase in which the cell spends most of its time performing all its normal functions and, if necessary, preparing for cell division. Mitosis is the phase in which the duplicated chromosomes of the cell separate into two identical nuclei. This phase is divided into four stages in the following order: prophase, metaphase, anaphase, telophase. Once the DNA has been successfully separated, the cytoplasm and its contents are redistributed, and two new cells—daughter cells—are formed through the process of cytokinesis.

Membranous organelles differ from nonmembranous organelles in that membranous organelles are __________.

surrounded by lipid membranes that isolate them from the cytosol When you put a drop of oil into water, the oil stays together and doesn't disperse throughout the water. That is because water and oil have opposite charges, among other differences. This same principle holds true for membranes. Since the cell membrane is composed of a phospholipid bilayer, it keeps water out. In the cell, membrane-bound organelles are surrounded by the same lipid membranes that keep them isolated from the water-soluble cytosol. That way, important reactions occur inside the cell, and these important functions for normal cell maintenance are not lost. Some of these organelles, such as the endoplasmic reticulum, are found close to the nucleus, while others, such as lysosomes and mitochondria, are found everywhere in the cell. Thus, it is usually only the cell membrane that maintains continuous contact with the cytosol and the extracellular environment.

The process of cytokinesis involves __________.

the even division and redistribution of the proteins and molecules of the cytoplasm During late M phase, the parent cell that is replicating needs to evenly divide and redistribute the proteins and molecules of the cytoplasm. To accomplish this, the process of cytokinesis takes place. Microtubules help guide this distribution process. Following this, the physical separation of the daughter cells occurs during telophase. In order to prepare for this phase, the cell first has to duplicate the DNA, which happens during S phase. In early prophase, spindle fibers form between the centriole pairs to help guide the separation of the sister chromatids. In late telophase, the nuclear contents are reorganized, and the nuclear membrane reforms. This leads to cytokinesis.

Water-soluble ions and molecules cannot enter certain regions of a cell membrane because of __________.

the presence of hydrophobic tails on the interior of the membrane Water-soluble ions and molecules cannot enter certain regions of a cell membrane because of the presence of hydrophobic tails on the interior of the membrane. The phospholipid bilayer is a very elegant cellular barrier. Structurally, you can think of one side as an octopus with only two legs. The heads are on the outside and can interact with water, meaning they are hydrophilic. This makes sense since humans are about two-thirds water. The inner portion is made of hydrophobic tails (or, in our example, legs). These tails from each side interact, helping keep the bilayer together. The tails are also hydrophobic, which means they repel water. Since they are on the inside of the bilayer, the cell is able to interact with its watery environment (hydrophilic heads) while repelling the water-soluble substances (hydrophobic tails) so that they can't come in or be lost going out. But the membrane has mechanisms that allow movement of water substances in and out. For example, channel proteins allow only certain-sized molecules to go through. In specialized cells such as neurons, gated channels open or close when specific ions are present.