167 Chapter 4 Answers: Cells

4.2 Discovery of Cells and Cell Theory: Review Questions and Answers

1. Describe cells.  Cells are the basic units of structure and function of living things, and they are the smallest units that can carry out the processes of life.
2. Explain how cells were discovered. 
3. The first cells from an organism (cork) were observed by Hooke in the 1600s. Soon after, microscopist van Leeuwenhoek observed many other living cells.
4. Outline the development of cell theory.  In the early 1800s, Schwann and Schleiden theorized that cells are the basic building blocks of all living things. Around 1850, Virchow saw cells dividing and added that living cells arise only from other living cells. These ideas led to cell theory, which states that all organisms are made of cells, all life functions occur in cells, and all cells come from other cell.
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6. Identify the structures shared by all cells.  A plasma membrane, cytoplasm and genetic material.
7. Proteins are made on ribosomes.
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9. Robert Hooke sketched what looked like honeycombs — or repeated circular or square units — when he observed plant cells under a microscope.
   1. What is each unit? A cell.
   2. Of the shared parts of all cells, what makes up the outer surface of each unit? The plasma membrane. In plants, this is additionally covered by a cell wall, but that is not a common part of all cells.
   3. Of the shared parts of all cells, what makes up the inside of each unit? The cytoplasm

1. Explain why most cells are very small. Cells are usually very small so they do not have too much volume for their surface area to handle passing all the needed materials into and out of the cell. A larger cell has greater needs for materials transport, and at the same time has less transport capacity because of its relatively smaller surface area.
2. Discuss variations in the form and function of cells. Cells with different functions often have different shapes to help them carry out their functions. For example, nerve cells transmit messages to and from other cells and they have multiple projections that let them communicate with many other cells.
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5. Do human cells have organelles? Explain your answer. Yes, human cells have organelles because they are eukaryotes.
6. Which are usually larger – prokaryotic or eukaryotic cells? What do you think this means for their relative ability to take in needed substances and release wastes? Discuss your answer. Answers will vary. Sample answer: Eukaryotic cells are usually larger. In general, larger cells are less efficient at transporting substances across their membranes than smaller cells. However, having organelles with various functions might help the eukaryotic cells be more efficient.
7. DNA in eukaryotes is enclosed within the nuclear membrane.
8. Name three different types of cells in humans. Answers will vary but may include: nerve cells, sperm cells, and white blood cells.
9. Which organelle provides energy in eukaryotic cells? Mitochondria.
10. What is a function of a vesicle in a cell? To store substances.

1. What are the general functions of the plasma membrane? The plasma membrane forms a barrier between the cytoplasm inside the cell and the environment outside the cell. It protects and supports the cell and controls everything that enters and leaves the cell.
2. Describe the phospholipid bilayer of the plasma membrane. The phospholipids in the plasma membrane are arranged in two layers, called a phospholipid bilayer. The water-fearing (hydrophobic) tails of the phospholipid molecules are on the interior of the membrane, and the water-loving (hydrophilic) heads of the phospholipid molecules are on the exterior of the membrane. This arrangement allows hydrophobic but not hydrophilic molecules to pass through the membrane.
3. Identify other molecules in the plasma membrane. State their functions. Other molecules in the plasma membrane include the steroid cholesterol, which helps the plasma membrane keeps its shape, and transport proteins, which help other substances pass through the plasma membrane.
4. Why do some cells have plasma membrane extensions, like flagella and cilia? Plasma membrane extensions allow cells to have functions such as movement. For example, a single-celled organism might have a flagellum to help it move through water, while human airway cells are lined with cilia that move in a sweeping motion to clear the airways of mucus and particles.
5. Explain why hydrophilic molecules cannot easily pass through the cell membrane. What type of molecule in the cell membrane might help hydrophilic molecules pass through it? Hydrophilic molecules cannot easily pass through the cell membrane because they are repelled by (or “fear”) the hydrophobic inside of the membrane.
6. Which part of a phospholipid molecule in the plasma membrane is made of fatty acid chains? Is this part hydrophobic or hydrophilic? The tails of the phospholipid molecules in the plasma membrane are made of fatty acid chains. They are hydrophobic.
7. The two layers of phospholipids in the plasma membrane are called a phospholipid bilayer.
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9. Steroid hormones can pass directly through cell membranes. Why do you think this is the case? Steroid hormones are lipids and are hydrophobic, so they can pass directly through cell membranes.
10. Some antibiotics work by making holes in the plasma membrane of bacterial cells. How do you think this kills the cells? Answers may vary. Sample answer: When antibiotics make holes in the plasma membrane of a bacterial cell, this may cause the contents of the cell to leak out which kills the cell. This would also prevent the cell from keeping out harmful substances, which could also kill the cell.
11. What is the name of the long, whip-like extensions of the plasma membrane that helps some single-celled organisms move? Flagella

1. Describe the composition of cytoplasm.  Draw a picture of a cell, including the basic components required to be considered a cell, and the organelles you have learned about in this section. Cytoplasm consists of a watery liquid called cytosol, which contains many dissolved substances and within which cell structures are suspended. In eukaryotic cells, the structures include a cell nucleus and other organelles.
2. What are some of the functions of cytoplasm? Functions of the cytoplasm include helping to give the cell shape and to hold cell structures such as organelles, and providing a site for many of the biochemical reactions that take place inside the cell.
3. Outline the structure and functions of the cytoskeleton. The cytoskeleton is a protein framework, or scaffolding, that crisscrosses the cytoplasm inside a cell. Its main functions are to give the cell structure and to keep cell structures, such as organelles, in place.
4. Is the cytoplasm made of cells? Why or why not? No, the cytoplasm is not made of cells because it makes up the interior of cells – it is not composed of cells itself.
5. Name two types of cytoskeletal structures. Microfilaments, intermediate filaments and microtubules.
6. In the picture of the different cytoskeletal structures above (Figure 4.5.2), what do you notice about these different structures? Answers will vary. Sample answer: I notice that the different cytoskeletal structures are located in different regions of the cells. The red-labeled structures are around the outside edges of the cells, while the green-labeled structures are contained in the more interior portion of the cells surrounding the cell nuclei.
7. Describe one example of a metabolic process that happens in the cytosol. Answers will vary. Sample answer: Enzymes dissolved in cytosol break down larger molecules into smaller products that can then be used by organelles of the cell.
8. In eukaryotic cells, all of the material inside of the cell, but outside of the nucleus is called the cytoplasm.
9. What is the liquid part of cytoplasm called? Cytosol.
10. What chemical substance composes most of the cytosol? Water. Cytosol is 80% water.
11. When yeast cells deprived of nutrients go dormant, their cytoplasm assumes a solid state. What effect do you think a solid cytoplasm would have on normal cellular processes? Explain your answer. Answers will vary. Sample answer: A solid cytoplasm would prevent the free-flow of nutrients and wastes through the cell. Also, many molecules would have trouble encountering and reacting with each other in this solid state, so this would impede biochemical reactions.

1. What is an organelle? An organelle is a structure within the cytoplasm of eukaryotic cells that is enclosed within a membrane (except in the case of ribosomes) and performs a specific job.
2. Describe the structure and function of the nucleus. The nucleus is the largest organelle in a eukaryotic cell. It is surrounded by a membrane, called the nuclear envelope, which has pores that allow large proteins and RNA molecules to pass through. Inside the nuclear envelope is a watery substance called nucleoplasm, most of the cell’s DNA, which makes up chromosomes, and a structure called a nucleolus that is involved in the assembly of ribosomes. Because it contains all of an organism’s genes and regulates their expression, the nucleus acts as the control center of the cell.
3. Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins. The nucleus contains the genetic code in its DNA molecules. The code is carried from DNA in the nucleus to ribosomes in the cytoplasm where the code is used to synthesize proteins. The rough endoplasmic reticulum (RER) is studded with ribosomes and provides a framework for ribosomes to synthesize proteins. Bits of its membrane pinch off to form vesicles, which carry the proteins away from the RER. The Golgi apparatus processes the proteins and prepares them for use both inside and outside the cell. It packages the proteins it receives from the RER, labels them, and sends them on to their destinations.
4. Why are mitochondria referred to as the “power plants of the cell”? Mitochondria are referred to as the power plants of the cell because their function is to make energy available to the cell. They use energy from organic compounds such as glucose to make molecules of ATP (adenosine triphosphate), an energy-carrying molecule that is used almost universally inside cells for energy.
5. What roles are played by vesicles and vacuoles? Roles played by vesicles and vacuoles include storing and transporting materials, providing chambers for biochemical reactions, and breaking down foreign matter, dead cells, and poisons.
6. Why do all cells need ribosomes — even prokaryotic cells that lack a nucleus and other cell organelles? All cells, even prokaryotic cells, need ribosomes in order to synthesize proteins, which all living things require for basic life processes such as catalyzing biochemical reactions and transporting other substances.
7. Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory? Endosymbiotic theory says that mitochondria were once free-living prokaryotic organisms that infected (or were engulfed by) larger prokaryotic cells. The two organisms then evolved a symbiotic relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra energy from the smaller prokaryotes. Eventually, the smaller prokaryotes became permanent guests of the larger cells, as organelles inside them. One piece of evidence that supports this theory is that mitochondria contain their own DNA.
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1. What is the main difference between passive and active transport?  The main difference between passive and active transport is energy. Passive transport occurs without any input of energy from the cell, whereas active transport cannot occur without the input of energy.
2. Summarize three different ways that passive transport can occur. Give an example of a substance that is transported in each way. Three different ways that passive transport can occur are simple diffusion, osmosis, and facilitated diffusion. Simple diffusion is the movement of a substance due to a difference in concentration without any help from other molecules. Oxygen molecules are transported across the cell membrane in this way. Osmosis is a special case of simple diffusion across a membrane. It applies only to water molecules. Facilitated diffusion is the movement of a substance across a membrane due to a difference in concentration with the help of transport proteins, such as channel proteins or carrier proteins. Large or hydrophilic molecules and charged ions are transported in this way.
3. Explain how transport across the plasma membrane is related to homeostasis of the cell. 
4. Homeostasis is the maintenance of stable conditions inside a cell. Homeostasis requires constant adjustments because conditions are always changing both inside and outside the cell. By moving substances into and out of the cell, transport across the plasma membrane keeps conditions within normal ranges inside the cell, thus playing an important role in the cell’s homeostasis.
5. In general, why can only very small, hydrophobic molecules cross the cell membrane by simple diffusion?  Generally only very small, hydrophobic molecules can cross the cell membrane by simple diffusion because large molecules have trouble physically passing through the cell membrane and hydrophilic molecules can’t pass through the hydrophobic interior of the lipid bilayer without assistance.
6. Explain how facilitated diffusion assists with osmosis in cells. Define osmosis and facilitated diffusion in your answer. Osmosis is the diffusion of water molecules across a membrane. In cells, water molecules diffuse across the plasma membrane with the help of transport proteins. Diffusion with the help of transport proteins is called facilitated diffusion. Therefore facilitated diffusion assists in osmosis in cells by allowing water to diffuse across the membrane.
7. Imagine a hypothetical cell with a higher concentration of glucose inside the cell than outside. Answer the following questions about this cell, assuming all transport across the membrane is passive, not active.
   • Can the glucose simply diffuse across the cell membrane? Why or why not? Glucose would flow out of the cell due to diffusion because the concentration of glucose is higher inside of the cell. Molecules move via diffusion from an area of higher concentration to an area of lower concentration.
   • Assuming that there are glucose transport proteins in the cell membrane, which way would glucose flow — into or out of the cell? Explain your answer. No, because if the concentration of glucose is the same inside and outside of the cell, there is no force of diffusion moving the molecules. A few molecules might move back and forth randomly, but there would be no net movement in one direction or the other.
   • If the concentration of glucose was equal inside and outside of the cell, do you think there would be a net flow of glucose across the cell membrane in one direction or the other? Explain your answer.  No, because a concentration gradient and/or transport proteins are required for glucose to cross the membrane.
8. What are the similarities and differences between channel proteins and carrier proteins? Channel proteins and carrier proteins are both transport proteins in the cell membrane that allow the movement of substances across the membrane. However, they do this in different ways. Channel proteins form tiny pores in the membrane where substances can pass through, while carrier proteins bind to specific molecules, changing their shape in the process, to pass the molecules through the membrane.
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1. Define active transport. Active transport is transport of substances across a plasma membrane that requires energy often because the substances are moving from an area of lower concentration to an area of higher concentration or because they are large molecules.
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3. What is the sodium-potassium pump? Why is it so important? The sodium-potassium pump is a mechanism of active transport that moves sodium ions out of the cell where they are less concentrated and potassium ions into the cell where they are more concentrated, using energy from ATP and carrier proteins in the plasma membrane. The sodium-potassium pump is so important because it is needed to pump the two kinds of ions against their concentration gradients and maintain membrane potential across the plasma membrane. Maintaining this potential is necessary for many normal functions, including the transmission of nerve impulses and the contraction of muscles.
4. The drawing below shows the fluid inside and outside of a cell. The dots represent molecules of a substance needed by the cell. Explain which type of transport — active or passive — is needed to move the molecules into the cell. Active transport is needed to move the molecules into the cell because the molecules are more concentrated inside than outside the cell so energy is needed for the molecules to cross the plasma membrane in this direction.
5. What are the similarities and differences between phagocytosis and pinocytosis? Phagocytosis and pinocytosis are both types of endocytosis mediated by vesicles, meaning that the vesicles are taking a substance into the cell. The difference is that phagocytosis is the taking in of whole cells or other solid particles and pinocytosis is the taking in of fluid.
6. What is the functional significance of the shape change of the carrier protein in the sodium-potassium pump after the sodium ions bind? When the sodium-potassium pump changes shape after binding to sodium, it causes the protein to pump the sodium ions out of the cell. This then allows potassium ions to bind with the pump, which then get pumped into the cell.
7. A potentially deadly poison derived from plants called ouabain blocks the sodium-potassium pump and prevents it from working. What do you think this does to the sodium and potassium balance in cells? Explain your answer. If the sodium-potassium pump is blocked with ouabain, it would cause a build up of sodium ions inside the cell and a decrease in potassium ions inside the cell because the pump wouldn’t be doing its normal job of transporting sodium ions out of the cell and bringing potassium ions into the cell.

1. Define energy. Energy is defined in science as the ability to do work.
2. Why do living things need energy? Living things need energy to carry out life processes. Energy is required to break down and build up molecules and to transport many molecules across plasma membranes.
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4. Compare and contrast the two basic ways that organisms get energy. The two basic ways that organisms obtain energy is by making their own food or by consuming other organisms for food. Plants, algae, and some bacteria make food in the form of glucose by photosynthesis. Organisms that make food are called autotrophs or producers. Organisms that get food by consuming other organisms are called heterotrophs or consumers.
5. Describe the roles and relationships of the energy molecules glucose and ATP. Glucose stores chemical energy in a concentrated, stable form. Glucose is transported in the blood and taken up by cells, but it contains too much energy for biochemical reactions within cells. ATP stores less chemical energy but contains just the right amount to provide energy for most cellular processes. The cells of organisms obtain ATP by breaking down glucose in the process of cellular respiration.
6. Summarize how energy flows through living things. The flow of energy through living things begins with photosynthesis, which stores energy from light in the chemical bonds of glucose. By breaking the chemical bonds in glucose, cells release the stored energy and make the ATP they need via cellular respiration. The energy of ATP is used to carry out the work of cells, but some of it is lost as heat, so there must be a constant input of energy in living things.
7. Why does the transformation of ATP to ADP release energy? ATP has three phosphate groups. When it loses one phosphate group, it becomes ADP. There is energy stored in the chemical bond between the phosphate group and the ADP molecule, so when that bond is broken, energy is released.

1. What is the purpose of cellular respiration? Provide a concise summary of the process. The purpose of cellular respiration is to break down glucose, release energy, and form molecules of ATP, which is the energy-carrying molecule that cells use to power biochemical processes. The process of cellular respiration involves glucose and oxygen reacting to form carbon dioxide, water, and chemical energy (in ATP) in a complex, three-stage process.
2. State what happens during glycolysis. During glycolysis, a glucose molecule is split into two molecules of pyruvate in the cytoplasm of the cell. This stage results in a net gain of two molecules of ATP.
3. Describe the structure of a mitochondrion. A mitochondrion is an organelle that has an inner and outer membrane, separated by an intermembrane space. The space enclosed by the inner membrane is called the matrix.
4. What molecule is present at both the beginning and end of the Krebs cycle?  Oxaloacetate.
5. What happens during the electron transport stage of cellular respiration? During the electron transport stage of cellular respiration, high-energy electrons are released from NADH and FADH2 (from the first two stages) as they move down electron-transport chains on the inner membrane of the mitochondrion. Some of the energy of the electrons is used to pump hydrogen ions across the inner membrane, from the matrix into the intermembrane space. This creates an electrochemical gradient that causes ions to flow back across the membrane into the matrix. The compound called ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. The compound also acts as an enzyme to catalyze the formation of ATP from ADP and phosphate. Water is formed from the “spent” electrons that were transported down the electron-transport chain when they combined with oxygen.
6. How many molecules of ATP can be produced from one molecule of glucose during all three stages of cellular respiration combined? During all three stages of cellular respiration combined, as many as 38 molecules of ATP can be produced from just one molecule of glucose, although typically 36 are produced.
7. Do plants undergo cellular respiration? Why or why not? Yes, plants undergo cellular respiration. All living things undergo cellular respiration, including autotrophs such as plants.
8. Explain why the process of cellular respiration described in this section is considered aerobic. This process of cellular respiration is considered aerobic because it uses oxygen.
9. Name three energy-carrying molecules involved in cellular respiration. Glucose, oxygen, water and carbon dioxide – ATP, NADH and FADH₂ .
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11. Which stage of aerobic cellular respiration produces the most ATP? Stage III, electron transport.
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1. Explain the primary difference between aerobic cellular respiration and anaerobic respiration. Aerobic respiration needs oxygen to occur, while anaerobic does not.
2. What is fermentation? The main difference between aerobic cellular respiration and anaerobic respiration is that the former requires oxygen whereas the latter does not.
3. Compare and contrast alcoholic and lactic acid fermentation. Fermentation is an important way of making ATP without oxygen (anaerobic). Both alcoholic and lactic acid fermentation are anaerobic processes in which no oxygen is required. Both start with glycolysis, with is the first and only anaerobic stage of aerobic cellular respiration, in which two molecules of ATP are produced from each molecule of glucose. Alcoholic fermentation produces an alcohol (such as ethanol) and carbon dioxide. Lactic acid fermentation produces lactic acid and no carbon dioxide. Alcoholic fermentation is carried out by yeasts and some bacteria. Lactic acid fermentation is undertaken by some bacteria, including those in yogurt, as well as human muscle cells when they are being used for intense short-duration activity.
4. Identify the major pros and the major cons of anaerobic respiration relative to aerobic cellular respiration. The main pro of anaerobic respiration relative to cellular respiration is its speed. The main con of anaerobic respiration is the small amount of ATP it produces. Whereas cellular respiration produces up to 38 molecules of ATP per molecule of glucose, anaerobic respiration produces only two molecules of ATP per molecule of glucose.
5. What process is shared between aerobic cellular respiration and anaerobic respiration? Describe the process briefly. Why can this process happen in anaerobic respiration, as well as aerobic respiration? Glycolysis, which is the breakdown of glucose to products including pyruvic acid and energy stored in ATP molecules.
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7. What is the reactant (or starting material)common to aerobic respiration and both types of fermentation?  Glucose.

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2. Explain why cell division is more complex in eukaryotic than prokaryotic cells.  Cell division is more complex in eukaryotic than prokaryotic cells because eukaryotic cells are more complex. Prokaryotic cells have a single circular chromosome, no nucleus, and few other organelles. Eukaryotic cells, in contrast, have multiple chromosomes contained within a nucleus and many other organelles. All of these cell parts must be duplicated and then separated when a eukaryotic cell divides.
3. Using a technique called flow cytometry, scientists can distinguish between cells with the normal amount of DNA and those that contain twice the normal amount of DNA as they go through the cell cycle. Which phases of the cell cycle will have cells with twice the amount of DNA? Explain your answer.  The phases of the cell cycle with cells with twice the normal amount of DNA are towards the end of S phase (when the DNA is done being replicated – i.e. doubled), G2 (which follows S), and M (mitotic phase, before the cell splits in two during cytokinesis). Once the cell splits into two during cytokinesis, the daughter cells will have the normal amount of DNA again.
4. What were scientists trying to do when they took tumor cells from Henrietta Lacks? Why did they specifically use tumor cells to try to achieve their goal? Scientists were trying to grow human cells in the lab for use in medical testing but were not having success. They specifically used tumor cells from Henrietta Lacks because tumor cells have uncontrolled growth, so they thought they would be more likely to survive and divide in the lab.

1. Describe the different forms that DNA takes before and during cell division in a eukaryotic cell. At the start of cell division, the DNA in a eukaryotic cell takes the form of a grainy material called chromatin. After DNA replicates and the cell is about to undergo mitosis, the DNA condenses and coils into the X-shaped form of chromosomes. Each chromosome consists of identical sister chromatids, which are joined together at a region called a centromere.
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3. Identify the four phases of mitosis in an animal cell, and summarize what happens during each phase. The four phases of mitosis are prophase, metaphase, anaphase, and telophase. During prophase, DNA condenses into chromosomes, the nuclear envelope breaks down, the centrioles separate and begin to move to opposite poles of the cell, and a spindle starts to form between the centrioles. During metaphase, spindle fibres attach to the centromere of each pair of sister chromatids, and the sister chromatids line up at the equator of the cell. During anaphase, sister chromatids separate, centromeres divide, and sister chromatids are pulled toward opposite poles of the cell by the shortening of the spindle fibres. During telophase, the chromosomes begin to uncoil and form chromatin, the spindle breaks down, and new nuclear envelopes form.
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5. Explain what happens during cytokinesis in an animal cell. During cytokinesis in an animal cell, the plasma membrane of the parent cell pinches inward along the cell’s equator until two daughter cells form.
6. What do you think would happen if the sister chromatids of one of the chromosomes did not separate during mitosis? Answers will vary. Sample answer: If the sister chromatids of one of the chromosomes did not separate during mitosis, those two chromatids would travel together into one of the daughter cells. This would result in one daughter cell having a double version of that chromosome and the other daughter cell missing that chromosome entirely.
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5. Briefly explain how the energy in the food you eat gets there, and how it provides energy for your neurons in the form necessary to power this process. Answers will vary. Sample answer: Energy stored in the food you eat ultimately comes from the sun and is stored in chemical bonds through photosynthesis in molecules such as glucose and more complex molecules that can be broken down to glucose in your body. Glucose is further broken down in the processes of aerobic cellular respiration and anaerobic respiration, both of which produce ATP. This ATP can then be used as an energy source for processes such as the active transport of sodium and potassium ions.
6. Explain why the inside of the plasma membrane — the side that faces the cytoplasm of the cell — must be hydrophilic. Answers will vary. Sample answer: The cytoplasm is mostly water, so the inside of the plasma membrane must be hydrophilic (“water loving”).
7. Explain the relationships between interphase, mitosis, and cytokinesis. Answers will vary. Sample answer: Interphase, mitosis, and cytokinesis are all parts of cell division in eukaryotic cells. During interphase, the cell prepares to divide by replicating DNA and organelles, and by producing needed proteins. Mitosis then comes next and is the actual dividing of the nucleus into two. Cytokinesis then follows and refers to the splitting of the cytoplasm so that two identical daughter cells, each with their own nucleus, are formed.