Biology Midterm
How does the body store extra calories as fat?
Acetyl CoA molecules can be diverted and combined to form fats.
Explain the most important finding of this study, and describe the evidence the researchers used to arrive at their conclusion.
After observing that the slug cells contained an identical copy of the psbO gene, whereas the chloroplast did not contain the gene, the researchers concluded that the slug's own cells provided the necessary photosynthetic proteins and that the chloroplasts were not independent of the slug.
What are two examples of fermentation pathways?
Alcohol fermentation and lactic acid fermentation are two examples.
Explain the fact that species as diverse as humans and yeasts use the same biochemical pathways to extract energy from nutrient molecules.
All species share a common ancestor and therefore share many of the same biochemical pathways
What do cells do with the ATP they generate in respiration?
ATP is the source of energy to power most of the chemical reactions that take place in the cell.
What are some examples of alternative electron acceptors used in anaerobic respiration?
Alternative electron acceptors in anaerobic respiration include nitrate, sulfate, and carbon dioxide.
What color would plants be if they absorbed all wavelengths of visible light? Why?
If plants absorbed all wavelengths of visible light, they would appear black because no light would be reflected.
Why do the reactions of respiration occur step-by-step instead of all at once?
If the reactions of respiration occurred all at once, the sudden release of heat energy would harm or destroy cells.
How is substrate-level phosphorylation different from chemiosmotic phosphorylation?
Substrate-level phosphorylation uses an enzyme to directly transfer a phosphate group from a donor molecule to ADP, forming a molecule of ATP. Chemiosmosis uses an electron transport chain to create a gradient of H+ ions; ATP synthase uses the potential energy in the gradient to add a phosphate group to ADP.
Why do all organisms need ATP?
All organisms need ATP to provide the potential chemical energy that powers the chemical reactions that occur in their cells.
Health-food stores sell a product called "pyruvate plus,F which supposedly boosts energy. Why is this product unnecessary? What would be a much less expensive substitute that would accomplish the same thing?
"Pyruvate plusF is unnecessary because cells make their own pyruvate. Glucose (or another simple sugar) would be a cheaper alternative that would accomplish the same thing, because glycolysis breaks glucose into two pyruvate molecules.
Over the past decades, the CO2 concentration in the atmosphere has increased. a. Predict the effect of increasing carbon dioxide concentrations on photorespiration. b. Scientists suggest that increasing CO2 concentrations are leading to higher average global temperatures. If temperatures are increasing, does this change your answer to part (a)?
(a) Increasing CO2 levels should decrease the rate of photorespiration. (b) Increasing temperatures should increase the rate of photorespiration.
How many ATP molecules per glucose does fermentation produce?
A fermenting organism produces 2 ATP molecules per glucose molecule.
What are the three general ways to generate ATP from food, and which organisms use each pathway?
Anaerobic respiration: microorganisms; Aerobic respiration: plants, animals, and bacteria in O2 rich environments; Fermentation:
When vegetables and flowers are grown in greenhouses in the winter, their growth rate greatly increases if the CO2 concentration is raised to two or three times the level in the natural environment. What is the biological basis for the increased rate of growth?
Assuming that carbon (and not some other essential element) limits growth, the increased availability of CO2 means more carbon can be incorporated into glucose, which in turn contributes to plant growth.
How is an autotroph different from a heterotroph?
Autotrophs can make their own organic compounds, but heterotrophs have to get their carbon by consuming other organic compounds
How are breathing and cellular respiration similar? How are they different?
Both involve gas exchange by taking in oxygen and releasing carbon dioxide. They differ in where the process takes place. In breathing, the exchange takes place in the alveoli of the lungs; in cellular respiration, it takes place in the cells themselves.
How is cellular respiration related to breathing?
Breathing inhales air to bring in the needed O2 for aerobic respiration and exhales air to remove the CO2 created in aerobic respiration.
Describe the relationship among the chloroplast, stroma, grana, and thylakoids.
Chloroplasts are organelles that contain the other structures or substances. Inside the chloroplast, the stroma is a fluid-filled space that surrounds grana, which are stacks of thylakoids.
How many ATP, NADH, CO2, FADH2, and H2O molecules are produced at each stage?
Converting glucose to 2 pyruvates produces 2 ATP and 2 H2O. Shuttling 2 pyruvates into the mitochondrion produces 2 NADH. The 2 Acetyl CoA enter the Krebs Cycle and produce 2 ATP, 6 NADH and 2 FADH2. From all these products, the electron transport chain can produce up to 36 ATP.
Cite a reaction or pathway that occurs in each of the following locations: cytoplasm mitochondrial matrix inner mitochondrial membrane intermembrane compartment
Cytoplasm: glycolysis; (b) Mitochondrial matrix: Krebs cycle; (c) Inner mitochondrial membrane: electron transport chain; (d) Intermembrane compartment: ATP synthase dissipates the high concentration of protons in this compartment
At which points do digested polysaccharides, proteins, and fats enter the energy pathways?
Digested polysaccharides enter at glycolysis. Proteins enter at glycolysis, the transition reaction and the Krebs cycle. Fats enter at both glycolysis and the Kreb's cycle.
What happens to the CO2 and H2O waste products?
Excess carbon dioxide and water leave the cell through the plasma membrane; water is used throughout the organism, and CO2 is returned to the external environment.
At what point does O2 enter the energy pathways of aerobic respiration? What is the role of O2? Why does respiration stop if a person cannot breathe? What happens if cellular respiration stops?
O2 enters the energy pathways at the electron transport chain; it is the final electron acceptor. Cellular respiration requires oxygen to proceed. If an individual cannot breathe, then his or her cells will not receive the oxygen necessary to complete cellular respiration. Also, CO2 levels will rise in the blood if CO2 is not eliminated through breathing. The cessation of cellular respiration deprives the cell of energy, eventually killing the cell.
Where do the O2 and glucose used in respiration come from?
Oxygen is provided from the environment and enters the cell through the plasma membrane. Glucose is provided from digesting other organisms or from photosynthesis.
What conditions maximize photorespiration?
Photorespiration is more likely when plants close their stomata to minimize water loss, letting CO2 levels fall while O2 levels rise.
How do chemiosmotic phosphorylation and substrate-level phosphorylation each generate ATP? In which pathways does each occur?
Substrate level phosphorylation generates ATP when a high-energy donor molecule physically transfers a phosphate group to
Why must the first metabolic pathways have been anaerobic?
The first metabolic pathway must have been anaerobic because O2-producing photosynthesis by cyanobacteria dates to 3.5 billion years ago. Life has been on Earth at least a billion years longer than that.
What are the three main components of sunlight?
The three main components of sunlight are high-energy radiation such as ultraviolet radiation; visible light; and low-energy radiation such as infrared radiation.
What is the net gain of ATP and NADH for each glucose molecule undergoing glycolysis?
There is a net gain of 2 ATPs and 2 NADHs for each glucose molecule that undergoes glycolysis.
How do NADH and FADH2 power ATP formation?
These molecules deliver energy-rich electrons to the electron transport chain to power the concentration of hydrogen ions (also delivered by NADH and FADH2). Those concentrated hydrogen ions are used to power the addition of a phosphate group to ADP.
a) Thylakoid membrane: internal membrane in a chloroplast; contains the photosystems that capture light energy
internal membrane in a chloroplast; contains the photosystems that capture light energy
b) Chloroplast:
organelle of photosynthesis in plants and algae
c) Reaction center:
the chlorophyll a molecule that actually uses the energy in photosynthesis reactions.
How do electrons pass from photosystem II to photosystem I?
Electrons pass from photosystem II to photosystem I in the electron transport chain. Light strikes antenna pigments in photosystem I; these pigments transfer the energy to the reaction center chlorophyll molecule of photosystem I. The reaction center chlorophyll releases two energized electrons, which are replaced by the electrons from photosystem II.
Define these terms and arrange them from smallest to largest: a. thylakoid membrane b. chloroplast c. reaction center d. photosystem e. electron transport chain
From smallest to largest: reaction center; photosystem; electron transport chain; thylakoid membrane; chloroplast.
What occurs in each of the three stages of cellular respiration?
Glycolysis splits glucose into two pyruvate molecules and produces ATP and some NADH for later use. The Kreb's cycle releases CO2 and produces additional ATP and NADH, as well as FADH2. The electron transport chain uses energy stored in the electron carriers NADH and FADH2 to create a gradient of hydrogen ions across the inner membrane of the mitochondrion. ATP synthase uses this gradient to phosphorylate ADP to form ATP.
Explain how to arrive at the estimate that each glucose molecule theoretically yields 36 ATPs.
Here is how you estimate that each glucose molecule yields 36 ATPs. Glycolysis yields 2 NADH and 2 ATP; the formation of acetyl CoA yields 2 NADH; the Krebs cycle yields 6 NADH, 2 ATP, 2 FADH2. When the NADH and FADH2 from glycolysis and the Krebs cycle contribute their electrons to the electron transport chain; they yield a total of 34 ATP (derived from NADH and FADH2). Once 2 ATP are subtracted (the "costF of moving NADH from glycolysis to the intermembrane space), the net yield is 36 ATP per molecule of glucose.
Where in the cell does each of stage of respiration occur? (note: preceding question contains error; found in confirming pages)
In a eukaryotic cell, respiration takes place in many different places. Glycolysis takes place in the cytoplasm where glucose is converted into pyruvate. Also, in the cytoplasm, pyruvate is converted to Acetyl CoA, which enters the matrix of the mitochondria. Within the mitochondrial matrix, the Kreb's cycle reactions take place; the results of the Kreb's cycle are carbon dioxide, NADH, and FADH2 molecules. The electron transport chain uses electrons from NADH and FADH2 to pump hydrogen ions across the inner membrane of the mitochondria to the intermembrane compartment. From that compartment, hydrogen returns to the mitochondrial matrix though the ATP pump, producing ATP from ADP. ATP is then shuttled throughout the cell.
How does photorespiration counter photosynthesis?
In photorespiration, the rubisco enzyme uses O2 instead of CO2. The resulting chemical reaction liberates CO2 that has already been fixed, and thus counters photosynthesis.
How does photorespiration counteract photosynthesis?
In photorespiration, the rubisco enzyme uses O2 instead of CO2. The resulting chemical reaction liberates CO2 that has already been fixed, and thus counters photosynthesis.
What happens in photosystem I?
In photosystem I, energy from sunlight energizes a pair of electrons in the reaction center (replaced by a pair of electrons from photosystem II) and these are passed to molecules of NADP+ to reduce them to NADPH. This molecule carries the electrons (and potential energy) to the carbon reactions of photosynthesis.
Describe the events in photosystem II, beginning with light and ending with the production of ATP.
In photosystem II, light strikes a photosynthetic pigment; energy is absorbed; energy bounces to the chlorophyll molecule in the reaction center, which releases two energized electrons; the electrons are replaced by two electrons stripped from a water molecule, forming oxygen gas (O2) and two hydrogen ions (H+); as the energized electrons move along the proteins of the electron transport chain of photosystem II, hydrogen ions build up in the space within the thylakoid, forming a reservoir of potential energy; as ATP synthase moves hydrogen ions back to the stroma, the released energy bonds a phosphate group onto ADP, forming ATP.
Of the many groups of photosynthetic bacteria, only cyanobacteria use chlorophyll a. How does this observation support the hypothesis that cyanobacteria gave rise to the chloroplasts of today's plants and algae?
In plants, the main photosynthetic pigment is chlorophyll a. Out of all of the existing photosynthetic bacteria, the only possible source of this pigment is cyanobacteria.
A chemical works as a disinfectant by poking holes in bacterial cell membranes. Why would this stop the cells from making ATP? Why would the inability to make ATP kill a cell?
In prokaryotes, the production of ATP occurs at the cell membrane. A membrane with holes could not maintain the H+ gradient necessary to produce ATP. The inability to produce ATP would kill a cell because ATP is the energy source that drives its processes.
What is the role of O2 in the electron transport chain?
In the electron transport chain, O2 functions as the final electron acceptor. O2 binds to electrons that have traveled to the end of the transport chain, which allows subsequent electrons to move along the pathway.
What happens in each of the two main stages of photosynthesis?
In the first stage of photosynthesis (light reactions), light energy is captured by pigments and converted to the chemical energy of ATP and NADPH. In the second stage of photosynthesis (carbon reactions), the energy of ATP and the electrons in NADPH are used to make glucose from CO2.
How does it benefit a photosynthetic organism to have more than one type of pigment?
Multiple pigments allow a photosynthetic organism to absorb energy from a broader range of wavelengths of light.
Determine whether each of the following molecules is involved in the light reactions, the carbon reactions, or both and explain how: O2, CO2, carbohydrate, chlorophyll a, photons, NADPH, ATP, H2O.
O2 is produced by the light reactions. CO2 is fixed during the carbon reactions and provides the carbon source for sugar. Carbohydrates are the product of the carbon reactions. Chlorophyll a is used in the light reactions to absorb light energy. Photons of light provide the energy input for the light reactions. NADPH is the electron carrier produced in the light reactions, which provides the source of electrons for the reduction of CO2 in the carbon reactions. ATP is the chemical energy produced in the light reactions and used in the carbon reactions for the synthesis of PGAL. H2O provides the replacement electrons for photosystem II and the H+ for chemiosmotic phosphorylation in the light reactions.
Why is photosynthesis essential to life on Earth?
Photosynthesis is essential to life on Earth because almost all life on the planet ultimately depends on it for a food source.
What is photosynthesis? Describe the reactants and products in words and in chemical symbols.
Photosynthesis is the chemical process in which plants, algae, and some microorganisms convert solar energy into chemical energy. A photosynthetic cell uses light energy to form glucose and oxygen gas from carbon dioxide and water. The chemical equation is:
Photosynthesis takes place in plants, algae, and some microbes. How does it affect a meat-eating animal?
Photosynthesis produces the plant tissue that herbivores eat. A meat-eating animal that eats the herbivore therefore relies indirectly on photosynthesis. Photosynthesis also produces O2, which meat-eating animals require.
How can plants release more O2 in photosynthesis than they consume in respiration?
Plants can release more O2 in photosynthesis than they consume in respiration because they do not respire all of the glucose they produce. For example, plants may store glucose as starch or cellulose.
How are photosynthesis, glycolysis, and cellular respiration interrelated?
Respiration and photosynthesis each generate products required as the starting materials for the other. Photosynthesis uses CO2 and H2O to generate glucose and O2; glycolysis splits the glucose and passes the products to the reactions of cellular respiration, which uses the glucose and O2 to generate CO2 and H2O.
What are the roles of rubisco, RuBP, ATP, and NADPH in the Calvin cycle?
Rubisco is an enzyme used to fix CO2 by combining it with RuBP, which is a five-carbon sugar. ATP provides the necessary energy and NADPH the necessary electrons for this reaction.
In 1941, biologists exposed photosynthesizing cells to water containing a heavy oxygen isotope, designated 18O. The "labeled" isotope appears in the O2 gas released in photosynthesis, showing that the oxygen came from the water. Where would the 18O have ended up if the researchers had used 18O-labeled CO2 instead of H2O?
The 18O that was used in the CO2 should become a part of the glucose (C6H12O6) molecule since it was fixed in the carbon reaction.
How is the CAM pathway like C4 metabolism, and how is it different?
The CAM pathway is like C4 metabolism since it occurs in plants from a hot and dry climate, and malate is used during the fixation of CO2. CAM plants differ from C4 plants as they open their stomata at night and CO2 diffuses into spaces within leaves. Mesophyll cells then store the carbon dioxide in a 4-carbon molecule within a vacuole. In the daytime, when CAM plants minimize water loss by keeping their stomata closed, CO2 moves from vacuoles and enters the Calvin cycle.
Why is the Calvin cycle also called the C3 pathway?
The Calvin cycle is called the C3 pathway because the first stable compound produced is the three-carbon molecule PGA.
How does the Krebs cycle generate CO2, ATP, NADH, and FADH2?
The Krebs cycle generates CO2, ATP, NADH, and FADH2 as it rearranges and oxidizes citrate through several intermediate molecules. The energy and electrons derived from these chemical reactions are stored in ATP, NADH, and FADH2. A molecule of CO2 is released in two of these chemical reactions.
One of the first investigators to explore photosynthesis was Flemish physician and alchemist Jan van Helmont. In the early 1600s, he grew willow trees in weighed amounts of soil, applied known amounts of water, and noted that in 5 years the trees gained more than 45 kg, but the soil had lost only a little weight. Because he had applied large amounts of water, van Helmont concluded (incorrectly) that plants grew solely by absorbing water. What is the actual source of the added biomass? Explain your answer.
The actual source of the added biomass was attained through carbon fixation into plant material.
How does the reaction center chlorophyll interact with the antenna pigments in a photosystem?
The antenna pigments capture light energy and send it to the reaction center chlorophyll, which uses it for the reactions of photosynthesis.
Suppose you hold one group of active beetles at 20°C and another group at 30°C. After several hours, you place each beetle in a device that measures how far the animal can fly at 20°C. Which group of beetles do you predict will fly farther?
The beetles at 30oC used less energy initially and so have more energy to use for flight; therefore, they will fly
How are the electrons from photosystem II replaced?
The boosted electrons lost from the reaction center in photosystem II are replaced by electrons stripped from a water molecule.
What is the product of the carbon reactions?
The carbon reactions produce the three-carbon molecule PGAL.
Which respiratory reactions occur in each part of the mitochondrion?
The electron transport chain is located on the inner mitochondrial membrane. The Krebs cycle occurs in the mitochondrial matrix. (In prokaryotes, the electron transport chain is embedded in the cell's outer membrane, and the Krebs cycle occurs in the cytoplasm).
Birds and mammals are endotherms: "they maintain a constant internal body temperature no matter whether the environment is cold or hot (within limits, of course). An endotherm that gets too cold will increase its metabolic rate to generate heat. An ectothermic animal such as a reptile, on the other hand, allows its body temperature to fluctuate with the environment. If you own a pet rat and a pet snake of equal weight, which will require more food and why?
The endotherm (mouse) would require more food because it has to maintain a constant body temperature. Since it must increase its metabolic rate to maintain that temperature in a cold environment, it will need more food to provide energy.
How did the origin of photosynthesis alter Earth's atmosphere and the evolution of life?
The evolution of photosynthesis filled the atmosphere with O2 gas giving an evolutionary advantage to those organisms that could use O2 in respiration to produce the most energy. The oxygen also helped produce the ozone layer, which decreased damage from the sun and lead to an explosion in life's diversity.
The researchers also looked for the psbO gene in pufferfish (a vertebrate animal) and slime molds (a nonphotosynthetic protist). The gene was absent in both species. How was this finding important to the interpretation of the results of this study? The pufferfish and slime molds served as control groups.
The fact that the psbO gene was found in the sea slugs, but not these other eukaryotic organisms, strengthened the conclusion that the gene was present as a result of horizontal gene transfer as opposed to vertical gene transfer from some ancestral species.
What happens to the glucose that plants produce?
The glucose that plants produce in photosynthesis is used as fuel for the plant's own cellular respiration, which generates the ATP that powers biochemical reactions and allows the plant to grow. The glucose is also used to produce other chemical compounds (including amino acids) and to produce cellulose for plant cell walls. Excess glucose is stored as starch or sucrose.
Describe how a C4 plant minimizes photorespiration.
The leaves of C4 plants have a distinctive arrangement of mesophyll cells and bundle sheath cells. Each vein is surrounded by a concentric ring of bundle-sheath cells that, in turn, are surrounded by a concentric ring of mesophyll cells. In mesophyll cells, CO2 is converted to a 4-carbon compound, oxaloacetate, which is reduced to malate. The malate then moves to the bundle sheath cells, where the CO2 is released to the Calvin cycle. The high CO2 concentration and low O2 concentration in the bundle sheath cells reduces the effects of photorespiration.
Where in the chloroplast do the light reactions and the carbon reactions occur?
The light reactions occur in the thylakoid membrane of a chloroplast, and the carbon reactions occur in the stroma of a chloroplast.
What is the relationship between the light reactions and the carbon reactions?
The light reactions provide the energy and electrons for the carbon reactions (in the form of ATP and NADPH). The carbon reactions use the energy and electrons from the light reactions to reduce CO2, forming organic molecules (such as glucose).
What is the overall equation for cellular respiration?
The overall equation that describes cellular respiration is:
In a properly functioning mitochondrion, is the pH in the matrix lower than, higher than, or the same as the pH in the intermembrane compartment? If you add one or more poisons described in this chapter's Apply It Now box, does your answer change?
The pH is higher in the matrix since the hydrogens are concentrated in the intermembrane compartment. If, however, you add one of the electron transport inhibitors or DNP then the electron transport chain would cease and the hydrogen ions in the intermembrane space would equilibrate with the matrix creating equal pH. If you added oligomycin then the hydrogen ions would stay concentrated, and the pH would still be higher in the matrix.
What are the parts of a mitochondrion?
The parts of a mitochondrion are an outer membrane that envelops the mitochondrion; a highly folded inner membrane; an intermembrane compartment between the two membranes; and an inner fluid called the mitochondrial matrix.
One of the classic experiments in photosynthesis occurred in 1771, when Joseph Priestley found that if he placed a mouse in an enclosed container with a lit candle, the mouse would die. But if he also added a plant to the container, the mouse could live. Priestley concluded that plants "purify" air, allowing animals to breathe. What is the biological basis for this observation?
The plant released O2 as a product of photosynthesis; in turn, the mouse used the O2 in aerobic respiration.
How does aerobic respiration yield so much ATP from each glucose molecule, compared with glycolysis alone?
The remaining H+ ions in glucose are removed and transported by NADH and FADH2 to the electron transport chain. The accumulation of hydrogen ions by the electron transport chain powers the production of ATP.
What are the starting materials of glycolysis?
The starting materials of glycolysis are glucose and 2 molecules of ATP.
A seed is a plant embryo packaged with food supply. Soaking a seed in water prompts the embryo to beigin respiring, metabolizing its food supply to fuel its growth. Suppose that Anna has 50 soaked seeds. She boils half of them, killing their embryos, and lets them return to room temperature. She then places the dead seeds in one container and live seeds in another. If she later measures the temperature in the two containers, will they be different? Explain your answer.
The temperatures would be higher in the container that has living seeds. Heat is a byproduct of metabolic activity.
How does the actual ATP yield compare to the theoretical yield?
The theoretical yield is 36 ATP, but some protons leak across the membrane of the mitochondria and energy is used to move pyruvate and ADP across the mitochondrial membrane which would reduce the actual ATP yield to about 30 ATP per molecule of glucose.
Pyruvate contains three carbon atoms; an acetyl group has only two. What happens to the other carbon atom?
The third carbon atom from pyruvate is released as CO2.
Ben decides to bake bread. The recipe says to dissolve yeast in a mixture of sugar and hot water. Shortly after he does so, the mixture begins to bubble. What is happening? How would the outcome change if Ben forgets to add the sugar?
The yeast is using the glucose in the sugar as a fuel for fermentation. One byproduct of alcohol fermentation is CO2 gas, which makes the mixture bubble. Without the sugar there would be no fermentation and no bubbling.
A student runs 5 kilometers each afternoon at a slow, leisurely pace. One day, she runs 2 km as fast as she can. Afterward she is winded and feels pain in her chest and leg muscles. She thought she was in great shape! What, in terms of energy metabolism, has she experienced?
When she increased her pace, her muscles depleted the available O2 and used lactic acid fermentation to generate ATP instead. The resulting buildup of lactic acid caused the soreness.
Describe the energy pathways that are available for cells living in the absence of O2.
Without O2, cells can use anaerobic respiration, in which an inorganic molecule (not O2) is used as the final electron acceptor. In fermentation an organic molecule is used as the acceptor (acetaldehyde or pyruvate itself).
e) Electron transport chain:
a group of aligned proteins that shuttle electrons, releasing energy with each step; an electron transport chain links the two photosystems.
d) Photosystem:
a unit consisting of chlorophyll a aggregated with other pigments molecules and proteins that anchor the entire complex in the membrane.