Cell Respiration Essay Question

The following is a comprehensive list of essay questions that have been asked on past AP exams. The questions are organized according to units.

Unit 1 (Basic Chemistry and Water)

1.  The unique properties (characteristics) of water make life possible on Earth. Select three properties of water and:

    1. for each property, identify and define the property and explain it in terms of the physical/chemical nature of water.
    2. for each property, describe one example of how the property affects the functioning of living organisms.

Unit 2 (Organic Chemistry, Biochemistry, and Metabolism)

2.  Describe the chemical composition and configuration of enzymes and discuss the factors that modify enzyme structure and/or function.

3.  After an enzyme is mixed with its substrate, the amount of product formed is determined at 10-second intervals for 1 minute. Data from this experiment are shown below:

    Time (sec)








    Product formed (mg)








    Draw a graph of these data and answer the following questions.

    1. What is the initial rate of this enzymatic reaction?
    2. What is the rate after 50 seconds? Why is it different from the initial rate?
    3. What would be the effect on product formation if the enzyme where heated to a temperature of 100° C for 10 minutes before repeating the experiment? Why?
    4. How might altering the substrate concentration affect the rate of the reaction? Why?
    5. How might altering the pH affect the rate of the reaction? Why?

4.  Enzymes are biological catalysts.

    1. Relate the chemical structure of an enzyme to its specificity and catalytic activity.
    2. Design a quantitative experiment to investigate the influence of pH or temperature on the activity of an enzyme.
    3. Describe what information concerning the structure of an enzyme could be inferred from your experiments.

Unit 3 (Cell Structure and Function, Cell division)

5.  Describe the fluid-mosaic model of a plasma membrane. Discuss the role of the membrane in the movement of materials through it by each of the following processes:

    1. Active transport
    2. Passive transport

6.  Describe the structure of a eukaryotic plant cell. Indicate the ways in which a nonphotosynthetic prokaryotic cell would differ in structure from this generalized eukaryotic plant cell.

7.  Discuss the process of cell division in animals. Include a description of mitosis and cytokinesis, and of the other phases of the cell cycle. Do Not include meiosis.


8.  A laboratory assistant prepared solution of 0.8 M, 0.6 M, 0.4 M, and 0.2 M sucrose, but forgot to label them. After realizing the error, the assistant randomly labeled the flasks containing these four unknown solutions as flask A, flask B, flask C, and flask D.

    Design an experiment, based on the principles of diffusion and osmosis, that the assistant could use to determine which of the flasks contains each of the four unknown solutions. Include in your answer (a) a description of how you would set up and perform the experiment: (b) the results you would expect from your experiments: and (c) an explanation of those results based on the principles involved. (Be sure to clearly state the principles addressed in your discussion.)

9.  Cells transport substances across their membranes. Choose THREE of the following four types of cellular transport.

    • Osmosis
    • Active Transport
    • Facilitated Diffusion
    • Endocytosis/exocytosis

For each of the three transport types you choose,

    1. Describe the transport process and explain how the organization of cell membranes functions in the movement of specific molecules across membranes; and
    2. Explain the significance of each type of transport to a specific cell (you may use difference cell types as examples.)

Unit 4 (Photosynthesis and Cellular Respiration)

10.  Describe the similarities and differences between the biochemical pathways of aerobic respiration and photosynthesis in eukaryotic cells. Include in your discussion the major reactions, the end products, and energy transfers.

11.  The rate of photosynthesis may vary with changes that occur in environmental temperature, wavelength of light, and light intensity. Using a photosynthetic organism of your choice, choose only ONE of the three variables (temperature, wavelength of light, or light intensity) and for this variable

    • design a scientific experiment to determine the effect of the variable on the rate of photosynthesis for the organism;
    • explain how you would measure the rate of photosynthesis in your experiment;
    • describe the results you would expect. Explain why you would expect these results.

12.  Describe the light reactions of photosynthesis and, for both a C3 and a C4 plant, trace the path of a carbon dioxide molecule from the point at which it enters a plant to its incorporation into a glucose molecule. Include leaf anatomy and biochemical pathways in your discussion of each type of plant.

13.  Explain what occurs during the Krebs (citric acid) cycle and electron transport by describing the following:

    1. The location of the Krebs cycle and electron transport chain in mitochondria.
    2. The cyclic nature of the reactions in the Krebs cycle.
    3. The production of ATP and reduced coenzymes during the cycle.
    4. The chemiosmotic production of ATP during electron transport.

14.  Membranes are important structural features of cells.

    1. Describe how membrane structure is related to the transport of materials across the membrane.
    2. Describe the role of membranes in the synthesis of ATP in either cellular respiration or photosynthesis.

15. Energy transfer occurs in all cellular activities. For 3 of the following 5 processes involving energy transfer, explain how each functions in the cell and give an example. Explain how ATP is involved in each example you choose.

        • cellular movement
        • active transport
        • synthesis of molecules
        • chemiosmosis
        • fermentation

16. The results below are measurements of cumulative oxygen consumption by germinating and dry seeds. Gas volume measurements were corrected for changes in temperature and pressure.

    Cumulative Oxygen Consumed (mL)

    Time (minutes)






    22° C Germinating Seeds






    Dry Seeds






    10° C Germinating Seeds






    Dry Seeds






    1. Using the graph paper provided, plot the results for the germinating seeds at 22° C and at 10° C.
    2. Calculate function the rate of oxygen consumption for the germinating seeds at 22° C, using the time interval between 10 and 20 minutes.
    3. Account for the differences in oxygen consumption observed between:
      1. germinating seeds at 22° C and at 10° C
      2. germinating seeds and dry seeds
    4. Describe the essential features of an experimental apparatus that could be used to measure oxygen consumption by a small organism. Explain why each of these features is necessary.

    Unit 5 (Meiosis, Mendelian Genetics, DNA Replication)

17.  State the conclusions reached by Mendel in his work on the inheritance of characteristics. Explain how each of the following deviates from these conclusions.

    1. Autosomal linkage.
    2. Sex-linked (X-linked) inheritance.
    3. Polygenic (multiple-gene) inheritance.

18.  Experiments by the following scientists provided critical information concerning DNA. Describe each classical experiment and indicate how it provided evidence for the chemical nature of the gene.

    1. Hershey and Chase- bacteriophage replication
    2. Griffith and Avery, MacLeod and McCarty- bacterial transformation
    3. Meselson and Stahl- DNA replication in bacteria

19.  Discuss Mendel’s laws of segregation and independent assortment. Explain how the events of meiosis I account for the observations that led Mendel to formulate these laws.

20.  An organism is heterozygous at two genetic loci on different chromosomes.

    1. Explain how these alleles are transmitted by the process of mitosis to daughter cells.
    2. Explain how these alleles are distributed by the process of meiosis to gametes.
    3. Explain how the behavior of these two pairs of homologous chromosomes during meiosis provides the physical basis for Mendel’s two laws of inheritance.

    Labeled diagrams that are explained in your answer may be useful.

    Unit 6 (Protein Synthesis, Gene Expression, DNA Technology)

21.  A portion of specific DNA molecule consists of the following sequence of nucleotide triplets.


    This DNA sequence codes for the following short polypeptide.

    methionine – leucine – glutamic acid – proline – arginine

    Describe the steps in the synthesis of this polypeptide. What would be the effect of a deletion or an addition in one of the DNA nucleotides? What would be the effects of a substitution in one of the nucleotides?

22.  Describe the operon hypothesis and discuss how it explains the control of messenger RNA production and the regulation of protein synthesis in bacterial cells.

23.  Scientists seeking to determine which molecule is responsible for the transmission of characteristics from one generation to the next knew that the molecule must (1) copy itself precisely, (2) be stable but able to be changed, and (3) be complex enough to determine the organism’s phenotype.

  • Explain how DNA meets each of the three criteria stated above.
  • Select one of the criteria stated above and describe experimental evidence used to determine that DNA is the hereditary material.


24.  Describe the biochemical composition, structure, and replication of DNA. Discuss how recombinant DNA techniques may be used to correct a point mutation.

25.  Describe the production and processing of a protein that will be exported from a eukaryotic cell. Begin with the separation of the messenger RNA from the DNA template and end with the release of the protein at the plasma membrane.

26.  Describe the steps of protein synthesis, beginning with the attachment of a messenger RNA molecule to the small subunit of a ribosome and ending generalized with the release of the polypeptide from the ribosome. Include in your answer a discussion of how the different types of RNA function in this process.

27.  The diagram below shows a segment of DNA with a total length of 4,900 base pairs. The arrows indicate reaction sites for two restriction enzymes (enzyme X and enzyme Y).

    1. Explain how the principles of gel electrophoresis allow for the separation of DNA fragments.
    2. Describe the results you would expect from the electrophoresis separation of fragments from the following treatments of the DNA segment above. Assume that the digestions occurred under appropriate conditions and went to completion.
      1. DNA digested with only enzyme X
      2. DNA digested with only enzyme Y
      3. DNA digested with enzyme X and enzyme Y combined
      4. Undigested DNA
    3. Explain both of the following.
      1. The mechanism of action of restriction enzymes.
      2. The different results you would expect if a mutation occurred at the recognition site for enzyme Y.

28.  By using the techniques of genetic engineering, scientists are able to modify genetic materials so that a particular gene of interest from one cell can be incorporated into a different cell.

        • Describe a procedure by which this can be done.
        • Explain the purpose of each step of your procedure.
        • Describe how you could determine whether the gene was successfully incorporated.
        • Describe an example of how gene transfer and incorporation have been used in biomedical or commercial applications.

29.  Assume that a particular genetic condition in a mammalian species causes an inability to digest starch. This disorder occurs with equal frequency in males and females. In most cases, neither parent of affected offspring has the condition.

    1. Describe the most probable pattern of inheritance for this condition. Explain your reasoning. Include in your discussion a sample cross(es) sufficient to verify your proposed pattern.
    2. Explain how a mutation could cause this inability to digest starch.
    3. Describe how modern techniques of molecular biology could be used to determine whether the mutant allele is present in a given individual.

    Unit 7 (Evolution, Population Genetics, Speciation)

29.  Describe the special relationship between the two terms in each of the following pairs.

    1. Convergent evolution of organisms and Australia.
    2. Blood groups and genetic drift.
    3. Birds of prey and DDT.

30.  Describe the modern theory of evolution and discuss how it is supported by evidence from two of the following areas.

    1. population genetics
    2. molecular biology
    3. comparative anatomy and embryology

31.  Describe the process of speciation. Include in your discussion the factors that may contribute to the maintenance of genetic isolation.

32.  Do the following with reference to the Hardy-Weinberg model.

    1. Indicate the conditions under which allelic frequencies (p and q) remain constant from one generation to the next.
    2. Calculate, showing all work, the frequencies of the alleles and the frequencies of the genotypes in a population of 100,000 rabbits, of which 25,000 are white and 75,000 are agouti. (In rabbits the white color is due to a recessive allele, w, and the agouti is due to a dominant all, W.)
    3. If the homozygous dominant condition were to become lethal, what would happen to the allelic and genotypic frequencies in the rabbit population after two generations?

33.  Evolution is one of the major unifying themes of modern biology.

    1. Explain the mechanisms that lead to evolutionary change.
    2. Describe how scientists use each of the following as evidence for evolution.
      1. Bacterial resistance to antibodies.
      2. Comparative biochemistry.
      3. The fossil record.

34.  Genetic variation is the raw material for evolution.

    1. Explain three cellular and/or molecular mechanisms that introduce variation into the gene pool of a plant or animal population.
    2. Explain the evolutionary mechanisms that can change the composition of the gene pool.

35.  In a laboratory population of diploid, sexually reproducing organisms a certain trait is studied. This trait is determined by a single autosomal gene and is expressed as two phenotypes. A new population was created by crossing 51 pure breeding (homozygous) dominant individuals with 49 pure breeding (homozygous) individuals. After four generations, the following results were obtained.

    Number of Individuals

























    1. Identify an organism that might have been used to perform this experiment, and explain why this organism is a good choice for conducting this experiment.
    2. On the basis of the data, propose a hypothesis that explains the change in phenotypic frequency between generation 1 and generation 3.
    3. Is there evidence indicating whether or not this population is in Hardy-Weinberg equilibrium? Explain.

 Unit 8 (Chemical Evolution, Prokaryotes, Eukaryote Evolution, Protista)

36.  Scientists recently have proposed a reorganization of the phylogenetic system of classification to include the domain, a new taxonomic category higher (more inclusive) than the Kingdom category, as shown in the following diagram.

Universal Ancestor

Domain Bacteria             Domain Archaea Domain Eukarya

(Eubacteria)             (Archaebacteria) (Eukaryotes)


    • describe how this classification scheme presents different conclusions about the relationships among living organisms than those presented by the previous five-kingdom system of classification
    • describe three kinds of evidence that were used to develop the taxonomic scheme above, and explain how this evidence was used. The evidence may be structural, physiological, molecular, and/or genetic.
    • Describe
    • four of the characteristics of the universal ancestor.

Unit 9 (Introduction to Plants, Fungi, Invertebrates)

37.  In the life cycles of a fern and a flowering plant, compare and contrast each of the following:

    1. The gametophyte generation.
    2. Sperm transport and fertilization.
    3. Embryo protection.

38.  Describe the differences between the terms in each of the following pairs.

    1. Coelomate versus acoelomate body plan.
    2. Protostome versus deuterostome development.
    3. Radial versus bilateral symmetry.
    4. Explain how each of these pairs of features was important in constructing the phylogenetic tree shown below. Use specific examples from the tree in your discussion.

    Unit 10 (Vertebrates, Basic Animal Structure and Function)

39.  Select two of the following three pairs and discuss the evolutionary relationships between the two members of each pair you have chosen. In your discussion include structural adaptations and the functional significance.

    Pair A: green algae—vascular plants

    Pair B: prokaryotes—eukaryotes

    Pair C: amphibians—reptiles

    Unit 11 (Animal Nutrition, Circulation, Respiration, Immune System)

40.  Describe the structure of a mammalian respiratory system. Include in your discussion the mechanisms of inspiration and expiration.

41.  Describe the processes of fat and protein digestion and product absorption as they occur in the human stomach and small intestine. Include a discussion of the enzymatic reactions involved.

42.  Describe the following mechanisms of response to foreign materials in the human body.

    1. The antigen-antibody response to a skin graft from another person.
    2. The reactions of the body leading to inflammation of a wound infected by bacteria.

43.  Discuss the processes of exchange of O2 and CO2 that occur at the alveoli and muscle cells of mammals. Include in your answer a description of the transport of these gases in the blood.

44.  Many physioligical changes occur during exercise.

    1. Design a controlled experiment to test the hypothesis that an exercise session causes short-term increases in heart rat and breathing rate in humans.
    2. Explain how at least three organ systems are affected by this increased physical activity and discuss interactions among these systems.

45.  The graph below shows the response of the human immune system to exposure to an antigen. Use this graph to answer part a and part b of this question.

    1. Describe the events that occur during period I as the immune system responds to the initial exposure to the antigen.
    2. Describe the events that occur during period II following a second exposure to the same antigen.
    3. Explain how infection by the AIDS virus (HIV) affects the function of both T and B lymphocytes.

    Unit 12 (Homeostasis, Reproduction, Development)

47.  Discuss the processes of cleavage, gastrulation, and neurulation in the frog embryo; tell what each process accomplishes. Describe an experiment that illustrates the importance of induction in development.

48.  The evolutionary success of organisms depends on reproduction. Some groups of organisms reproduce asexually, some reproduce sexually, while others reproduce both sexually and asexually.

    1. Using THREE difference organisms, give an example of one organism that reproduces sexually, one that reproduces asexually, and one that reproduces BOTH sexually and asexually. For each organism given as an example, describe two reproductive adaptations. These adaptations may be behavioral, structural, and/or functional.
    2. What environmental conditions would favor sexual reproduction? Explain. What environmental conditions would favor asexual reproduction? Explain.

    Unit 13 (Endocrine System, Nervous System, Sensory and Motor Mechanisms)

49.  Discuss the sources and actions of each of the following pairs of hormones in humans and describe the feedback mechanisms that control their release.

    1. Insulin—glucagon
    2. Parathyroid hormone—calcitonin
    3. Thyrotropin (TSH)—thyroxine (T4)

50.  Beginning at the presynaptic membrane of the neuromuscular junction, describe the physical and biochemical events involved in the contraction of a skeletal muscle fiber. Include the structure of the fiber in your discussion.

52.  Describe the negative and positive feedback loops, and discuss how feedback mechanisms regulate each of the following.

    1. The menstrual cycle in nonpregnant human female.
    2. Blood glucose levels in humans.

53.  Discuss how cellular structures, including the plasma membrane, specialized endoplasmic reticulum, cytoskeletal elements, and mitochondria, function together in the contraction of skeletal muscle cells.

54.  Structure and function are related in the various organ systems of animals. Select two of the following four organ systems in vertebrates:

    • respiratory
    • digestive
    • excretory
    • nervous

For each of the two systems you choose, discuss the structure and function of two adaptations that aid in the transport or exchange of molecules (or ions). Be sure to relate structure to function in each example.

Unit 14 (Plant Structure and Function)

55. Relate the structure of an angiosperm leaf to each of the following:

    1. Adaptations for photosynthesis and food storage.
    2. Adaptations for food translocation and water transport.
    3. Specialized adaptations to a desert environment.

56.  Define the following plant responses and explain the mechanism of control for each. Cite experimental evidence as part of your discussion.

    1. Phototropism
    2. Photoperiodism

57.  Describe the structure of a bean seed and discuss its germination to the seedling stage. Include in your essay hormonal controls, structural changes, and tissue differentiation.

58.  Describe the effects of plant hormones on plant growth and development. Design an experiment to demonstrate the effect of one of these plant hormones on plant growth and development.

59.  Trace the pathway in a flowering plant as the water moves from the soil through the tissues of the root, stem, and leaves to the atmosphere. Explain the mechanisms involved in conducting water through these tissues.

60.  Discuss the adaptations that have enabled flowering plants to overcome the following problems associated with life on land.

    1. The absence of an aquatic environment for reproduction.
    2. The absence of an aquatic environment to support the plant body.
    3. Dehydration of the plant.

61.  A group of students designed an experiment to measure transpiration rates in a particular species of herbaceous plant. Plants were divided into four groups and were exposed to the following conditions.

    Group I-Room conditions (light, low humidity, 20° C, and little air movement.)
    Group II-Room conditions with increased humidity.
    Group III-Room conditions with increased air movement (fan)
    Group IV-Room conditions with additional light

    The cumulative water loss due to transpiration of water from each plant was measured at 10-minute intervals for 30 minutes. Water loss was expressed as milliliters of water per square centimeter of leaf surface area. The data for all plants in Group I (room conditions) were averaged. The average cumulative water loss by the plants in Group I is presented in the table below.

    Average Cumulative Water Loss by the Plants in Group I

    Time (minutes)

    Average Cumulative Water Loss (milliliter H2O centimeter2)


    3.5 x 10-4


    7.7 x 10-4


    10.6 x 10-4

    1. Construct and label a graph using the data for Group I. Using the same set of axes, draw and label three additional lines representing the results that you would predict for Groups II, III, and IV.
    2. Explain how biological and physical processes are responsible for the difference between each of your predictions and the data for Group I.
    3. Explain how the concept of water potential is used to account for the movement of water from the plant stem to the atmosphere during transpiration.

62.  Numerous environmental variables influence plant growth. Three students each planted a seedling of the same genetic variety in the same type of container with equal amounts of soil from the same source. Their goal was to maximize their seedling’s growth by manipulating environmental conditions. Their data are shown below.

    Plant Seedling Mass (grams)
    Day 1Day 30
    Student A424
    Student B535
    Student C464
    1. Identify three different environmental variables that could account for differences in the mass of seedlings at day 30. Then choose one of these variables and design an experiment to test the hypothesis that your variable affects growth of these seedlings.
    2. Discuss the results you would expect if your hypothesis is correct. Then provide a physiological explanation for the effect of your variable on plant growth.

    Unit 15 (Ecology)

63.  Define and explain the role of each of the following in social behavior.

    1. Territoriality.
    2. Dominance hierarchies.
    3. Courtship behavior.

64.  Describe the trophic levels in a typical ecosystem. Discuss the flow of energy through the ecosystem, the relationship between the different trophic levels, and the factors that limit the number of trophic levels.

65.  Describe and give an example of each of the following. Include in your discussion the selection advantage of each.

    1. Pheromones.
    2. Mimicry.
    3. Stereotyped behavior (instinct).

66.  Describe the process of ecological succession from a pioneer community to a climax community. Include in your answer a discussion of species diversity and interactions, accumulation of biomass, and energy flow.

67.  Describe releasers, imprinting, and communications, as each of these terms relates to animal behavior. You may include in your answer a discussion of the classical studies of Niko Tinbergen, Konrad Lorenz, and Karl von Frisch.

68.  Describe the biogeochemical cycles of carbon and nitrogen. Trace these elements from the point of their release from a decaying animal to their incorporation into a living animal.

69.  Using an example for each, discuss the following ecological concepts.

    1. Succession
    2. Energy flow between trophic levels.
    3. Limiting factors.
    4. Carrying capacity.

70.  Living organisms play an important role in the recycling of many elements within an ecosystem. Discuss how various types of organisms and their biochemical reactions contribute to the recycling of either carbon or nitrogen in an ecosystem. Include in your answer one way in which human activity has an impact in the nutrient cycle you have chosen.

71.  Survival depends on the ability of an organism to respond to changes in its environment. Some plants flower in response to changes in day length. Some mammals may run or fight when frightened. For both of these examples, describe the physiological mechanisms involved in the response.

72.  Interdependence in nature is illustrated by the transfer of energy through trophic levels. The diagram below depicts the transfer of energy in a food web of an Arctic lake located in Alaska (J )

    1. Choosing organisms from four different trophic levels of this food web as examples, explain how energy is obtained at each trophic level.
    2. Describe the efficiency of energy transfer between trophic levels and discuss how the amount of energy available at each trophic level affects the structure of the ecosystem.
    3. If the cells in the dead terrestrial plant material that washed into the lake contained a commercially produced toxin, what would be the likely effects of this toxin on this food web? Explain.





    4 p.m.


    8 p.m.




For the data above, provide information on each of the following.

    • Summarize the pattern.
    • Identify THREE physiological or environmental variables that could cause the slugs to vary their distance from each other.
    • Explain how each variable could bring about the observed pattern of distribution.

Choose ONE of the variables that you identified and design a controlled experiment to test your hypothetical explanation. Describe results that would support or refute your hypothesis.

Cumulative Essays

74.  Describe how the following adaptations have increased the evolutionary success of the organisms that possess them. Include in your discussion the structure and function related to each adaptation.

    1. C4 metabolism
    2. Amniotic egg
    3. Four-chambered heart
    4. Pollen

75.  Describe the anatomical and functional similarities and difference within each of the following pairs of structures.

    1. Artery—vein
    2. Small intestine—colon
    3. Skeletal muscle—cardiac muscle
    4. Anterior pituitary—posterior pituitary

76.  Discuss how each of the following has contributed to the evolutionary success of the organisms in which they are found.

    1. seeds
    2. mammalian placenta
    3. diploidy

77.  Angiosperms (flowering plants) and vertebrates obtain nutrients from their environment in different ways.

    1. Discuss the type of nutrition and the nutritional requirements of angiosperms and vertebrates.
    2. Describe 2 structural adaptations in angiosperms for obtaining nutrients from the environment. Relate structure to function.
    3. Interdependence in nature is evident in symbiosis. Explain tow symbiotic relationships that aid in nutrient uptake, using examples from angiosperms and/or vertebrates. (Both examples may be angiosperms, both may be vertebrates, or one may be from each group.

78.  The problem of survival of animals on land are very different from those of survival of animals in an aquatic environment. Describe four problems associated with animal survival in terrestrial environments but not in aquatic environments. For each problem, explain an evolutionary solution.

79.  The survival of organisms depends on regulatory mechanisms at various levels. Choose THREE from the following examples. Explain how each is regulated.

    • The expression of a gene.
    • The activity of an enzyme.
    • The cell cycle.
    • The internal water balance of a plant.
    • The density of a population.

80.  Photosynthesis and cellular respiration recycle oxygen in ecosystems. Respond to TWO (and only two) of the following:

    1. Explain how the metabolic processes of cellular respiration and photosynthesis recycle oxygen.
    2. Discuss the structural adaptations that function in oxygen exchange between each of the following organisms and its environment: a plant; an insect; a fish.
    3. Trace a molecule of O2 from the environment to a muscle cell in a vertebrate of your choice.

81.  Biological recognition is important in many processes at the molecular, cellular, tissue, and organismal levels. Select three of the following, and for each of the three that you have chose, explain how the process of recognition occurs and give an example of each.

    1. Organisms recognize others as members of their own species.
    2. Neurotransmitters are recognized in the synapse.
    3. Antigens trigger antibody response.
    4. Nucleic acids are complementary.
    5. Target cells respond to specific hormones.

82.  Communication occurs among the cells in a multicellular organism. Choose THREE of the following examples of cell-to-cell communication, and for each example, describe the communication that occurs and the types of responses that result from this communication.

  • communication between two plant cells
  • communication between two immune-system cells
  • communication either between a neuron and another neuron, or between a neuron and a muscle cell
  • communication between a specific endocrine-gland cell and its target cell


2. What compound is phosphorylated for ATP formation? What is the resulting compound when ATP releases energy?

ATP, or adenosine triphosphate, is formed after the binding of one phosphate molecule (phosphorylation) to one ADP (adenosine diphosphate) molecule. This is a process that stores energy in the produced ATP molecule.

When ATP provides energy to the cellular metabolism, it releases one of its phosphate ions and ADP reappears.

ADP can also release more phosphate ions and generate AMP (adenosine monophosphate) or even non-phosphorylated adenosine. Adenosine production from ATP is used in tissues that need an urgent supply oxygen, such as in the heart during a myocardial infarction (heart attack). This is because adenosine creates a local vasodilator effect, thus providing faster vasodilation than other physiological methods.

Aerobic and Anaerobic Cell Respiration

3. What are the types of cell respiration?

There are two types of cell respiration: aerobic cell respiration, a reaction with the participation of molecular oxygen (O₂); and anaerobic cell respiration, without the participation of molecular oxygen and which uses other inorganic molecules as an oxidant instead. There are several varieties of anaerobic cell respiration. The main one is fermentation.


4. Under which conditions do aerobic cells use fermentation?

Some cells that usually obtain energy from aerobic cellular respiration can use fermentation when oxygen is not available.

There are bacteria and fungi that, in the absence of oxygen, use their anaerobic metabolic capability for energy supply. Muscle cells also use fermentation when oxygen is scarce.

5. What is the difference between aerobic and anaerobic organisms?

Aerobic organisms are those whose cells do not survive without oxygen, since they depend on aerobic cell respiration to obtain energy for ATP production. Anaerobic organisms are those that live or can live in environments devoid of oxygen.

6. What is the difference between facultative anaerobes and obligate anaerobes?

Facultative anaerobes, like the fungi Saccharomyces cerevisiae, a brewing yeast, can survive in oxygen-poor environments by using fermentation. However, when oxygen is available, these organisms use aerobic respiration.

Obligate anaerobes are those that cannot survive when oxygen is present. Some fungi, some bacteria (like the agent of botulism Clostridium botulinum, and the agent of tetanus, Clostridium tetani) and some protozoans are examples of obligate anaerobes.

7. What are the two types of fermentation? What are their chemical equations?

The two main types of fermentation are alcoholic fermentation and lactic fermentation.

In alcoholic fermentation, pyruvic acid, an intermediate molecule, is converted into ethanol with the release of carbon dioxide. The equation for alcoholic fermentation is as follows:

C₆H₁₂O₆ + 2 ADP + P --> 2 C₂H₅OH + 2 CO₂ + 2 ATP

In lactic fermentation, pyruvic acid is transformed into lactic acid and there is no production of carbon dioxide. The equation for lactic fermentation is:

C₆H₁₂O₆ + 2 ADP + P --> 2 C₃H₅OOH + 2 ATP

8. In general, what are the reagents and products of fermentation?

In fermentation, glucose (sugar) is broken down into pyruvic acid (each glucose molecule forms two pyruvic acid molecules). In this process, two molecules of ATP are produced.

Depending on to the type of fermentation, pyruvic acid can produce ethanol and carbon dioxide (in alcoholic fermentation) or lactic acid (in lactic fermentation). There are other varieties of fermentation in which pyruvic acid can generate acetic acid (acetic fermentation), propionic acid, isopropanol (also a type of alcohol), etc. The type of fermentation depends on the species of the involved organisms.

9. Why are alcoholic fermenting organisms and not lactic fermenting organisms used in the cooking of cakes and breads?

Fermentation causes cakes and breads to grow. This is accomplished by the release of carbon dioxide in alcoholic fermentation, as the gas passes through the dough and makes it grow. In lactic fermentation, there is no release of carbon dioxide and the desired result would not be obtained.

10. What substance causes the acidic flavor of fermented milk?

Some bacteria ferment milk lactose via lactic fermentation, producing lactic acid. This product is responsible for the acidic flavor of yogurt, curds and milk. 

11. How can knowledge pf fermentation explain the origin of muscle cramps and pains after intense physical exercise?

A typical fermentation process due to oxygen scarcity happens in muscle tissue. Under intense use, muscles require too much energy (ATP) and consume much more oxygen to produce that energy. High consumption leads to oxygen scarcity and the muscle cells begin to use lactic fermentation to satisfy their energy needs. In this situation, muscle pain, cramps and fatigue are due to the lactic acid released by fermentation.

Aerobic Respiration

12. How many ATP molecules are produced for each glucose molecule used in fermentation? How many ATP molecules are produced for each glucose molecule used in aerobic respiration?

In fermentation, two ATP molecules are produced from one glucose molecule. In aerobic respiration, a much more productive process, 36 ATP molecules are made from one glucose molecule.

13. What cell organelle is specialized in aerobic respiration?

The cell organelles specialized in aerobic respiration are the mitochondria. 

14. What are the main components of the mitochondrion structure?

Mitochondria are organelles enclosed by two lipid membranes. The inner membrane invaginates to the interior of the organelle, forming cristae and delimiting an internal space known as the mitochondrial matrix. 

15. What are the three phases of cell respiration?

The three phases of aerobic cell respiration are glycolysis, the Krebs cycle and the respiratory chain (also known as the electron transport chain).


16. What is glycolysis? What are the products of this process?

Glycolysis, the first stage of aerobic cell respiration, is a process in which glucose is broken down to form two pyruvic acid molecules along with the formation of two ATP and two NADH molecules.

Glycolysis is a complex reaction that involves the formation of several intermediate molecules until pyruvic acid molecules are made. Although two ATP molecules are consumed in the reaction, four molecules of ATP are also produced; therefore, a positive balance of two ATP molecules is obtained. Two NADH molecules are also produced. In glycolysis, the 6-carbon structure of glucose is broken down and two organic chains of three carbons each are produced. These chains produce two pyruvic acid molecules. 

  • Cell Respiration Review - Image Diversity: glycolysis

17. Does glycolysis occur within the mitochondria?

Glycolysis happens in the cytosol and not within the mitochondria. Pyruvic acid molecules later enter the mitochondria to participate in the next phase of aerobic cell respiration.

18. How many ATP molecules are made after glycolysis?

Glycolysis is a process similar to the breaking down of glucose in fermentation. It produces (a final balance of) two molecules of ATP for each glucose broken down. 

19. What is NAD? What is the role of the NAD molecule in glycolysis?

NAD (nicotinamide adenine dinucleotide) is a hydrogen acceptor and necessary reductant (to receive hydrogen) in some reactions, in which it is reduced and converted into NADH₂. During glycolysis, two NAD molecules gain hydrogen ions released after an intermediate reaction, thus forming NADH₂. 

The Krebs Cycle

20. What happens to the pyruvic acid molecules made by glycolysis during aerobic respiration? What is the sequence of reactions that follows?

The pyruvic acid molecules made in the cytosol via glycolysis enter the mitochondria.

Within the mitochondria, each pyruvic acid molecule is converted into one molecule of acetyl-CoA (acetyl coenzyme A), releasing one carbon dioxide molecule. The Krebs Cycle (also known as the citric acid cycle), the second stage of aerobic respiration, then begins.

21. What is the official name of pyruvic acid?

Pyruvic acid is 2-oxopropanoic acid. Therefore, it is composed of three linearly bound carbon atoms with one extremity forming the organic acid function (COOH) and the middle carbon atom binding to an oxygen atom by double bond.

22. Why can it be said that each glucose molecule moves the Krebs cycle twice?

Each glucose molecule “cycles” the Krebs cycle twice because, after glycolysis, each used glucose molecule has generated two pyruvic acid molecules and each pyruvic acid molecule is converted in a 1:1 proportion into acetyl CoA. Each acetyl CoA molecule then goes through a Krebs cycle.

23. Why is the Krebs cycle also called the final common pathway of the breaking down of organic compounds?

The Krebs cycle is called the final common pathway of the breaking down of organic compounds because it is also possible to generate acetyl CoA from the breaking down of lipids and proteins. Since acetyl CoA is the substrate that triggers the Krebs cycle, this process is called the final common pathway because it is activated by other organic molecules (lipids and proteins) and not only by glucose.

The body uses its energy reserves of fat and protein to cycle the Krebs cycle when experiencing malnutrition or when there is no glucose available for the cells. 

24. What are the final energy products of each round of the Krebs cycle? Where is the majority of the useful energy found at the end of Krebs cycle?

After each round of the Krebs cycle, two carbon dioxide molecules, eight protons (hydrogen ions) captured by NAD and FAD (also a hydrogen acceptor) and one ATP molecule are produced.

During the Krebs cycle, acetyl CoA is broken down. At the end, the useful energy is incorporated into hydrogen atoms transported by FADH₂ and NADH₂ molecules. 

25. How many carbon dioxide molecules are released after each cycle of the Krebs cycle? For a single glucose molecule, how many carbon dioxide molecules were already released by aerobic respiration up to that point?

Each round of the Krebs cycle releases two carbon dioxide molecules.

At the end of the cycle, all carbon atoms from the original glucose molecule broken down  during glycolysis are already released, incorporated in carbon dioxide molecules. That occurs because, for each glucose, two pyruvic acid molecules were made during glycolysis. Each of these two pyruvic acid molecules is then converted into acetyl CoA with the release of one carbon dioxide molecule (two in total). Since each of the two produced acetyl CoA molecules cycles the Krebs cycle once, the initial glucose molecule triggers two rounds of the Krebs cycle and, as a result, four other carbon dioxide molecules are produced.

All of the six carbons atoms of the glucose molecule are then incorporated into six carbon dioxide molecules (two made during acetyl CoA formation and four during the two cycles of the Krebs cycle).

The Respiratory Chain

26. Where in the mitochondria does the process called the respiratory chain occur? Which products of the Krebs cycle are used in that final phase of aerobic respiration?

The respiratory chain, or the electron transport chain, is performed by protein systems located in the inner membrane of the mitochondria. Energized electrons of hydrogen atoms transported by NADH₂ and FADH₂ are the products of the preceding phases which are used in the respiratory chain.

27. What are cytochromes?

Cytochromes are proteins of the internal mitochondrial membrane that are specialized in electron transfer and which participate in the respiratory chain. Energized electrons released by the hydrogen donors NADH₂ and FADH₂ (then reconverted into NAD and FAD) pass through a sequence of cytochromes, losing energy each time. The energy is then used in the synthesis of ATP. 

28. During what part of the respiratory chain do electrons from FADH₂ and NADH₂ passing through cytochromes release energy for ATP synthesis? What is this ATP synthesis called?

FADH₂ and NADH₂ are oxidized into FAD and NAD and release hydrogen ions and highly energized electrons at the beginning of the respiratory chain.

The energy lost by electrons passing through the cytochromes is used to pump protons (hydrogen ions) out of the inner mitochondrial membrane (to the region between the inner and the outer membranes of the mitochondrion). The concentration gradient of hydrogen between the inner and the outer spaces separated by the inner membrane forces protons (hydrogen ions) to return to the mitochondrial matrix (the region inside the inner membrane). However, that return is only possible if the hydrogen ions pass through an enzymatic complex called ATP synthetase, which is embedded in the inner membrane. During that passage, the ATP synthetase phosphorylates ADP and ATP molecules are produced.

Hydrogen released in the mitochondrion then bonds with oxygen to form water. As a reaction that depends on oxygen, this type of ATP synthesis is called oxidative phosphorylation.

29. Until the Krebs cycle, aerobic respiration can be described without mentioning oxygen, the chemical element after which the reaction gets its name. In which part of the process does this chemical element take part? What is its importance?

Oxygen is involved in aerobic respiration in its final phase, the respiratory chain. It is of fundamental importance because it is responsible for the maintenance of the hydrogen concentration gradient between the spaces separated by the inner mitochondrial membrane. This gradient promotes the functioning of ATP synthetase and therefore the phosphorylation of ADP to form ATP. In the space inside the inner membrane, oxygen binds to free hydrogens atoms to form water. This hydrogen consumption maintains the hydrogen gradient and the traffic of protons through the ATP synthetase.

The entire aerobic respiration process takes place to make ATP synthetase work. Aerobic organisms, like us humans, for example, need to breathe oxygen to maintain the hydrogen concentration gradient and to keep ATP synthetase working.

30. What is the effect of the poison cyanide on aerobic respiration?

Cyanide is a poison that inhibits the last cytochrome of the respiratory chain, interrupting ATP formation and thus causing the cell to die.

31. What is anoxia?

Anoxia is a situation in which there is no available oxygen in the cell. Without oxygen, the respiratory chain stops, there is no ATP production, the cell does not obtain energy and it dies.

Anoxia can be caused, for example, by pulmonary insufficiency (drowning, extensive pulmonary injuries, etc.), by obstructions, stoppages and deficiencies in tissue circulation (atherosclerosis of the coronary arteries that irrigate the myocardium, tourniquets, cardiac arrest), by hemolysis (lysis of red blood cell) or hemoglobin diseases (anemia, fetal erythroblastosis), etc. 

The Efficiency of Aerobic Respiration

32. How many ATP molecules are produced after in aerobic respiration and what is the net energy gain of the process?

After aerobic respiration, 38 ATP molecules are produced from the consumption of one glucose molecule (but two of these ATP molecules are consumed by glycolysis). The net gain of the process is then 36 ATP molecules per glucose molecule. 

33. What is the general equation for aerobic respiration (also representing ADP and phosphate)?

The general equation for aerobic respiration is:

C₆H₁₂O₆ + 6 O₂ + 36 ADP + 36 P --> 6 CO₂ + 6 H₂O + 36 ATP 

34. Why can the consumption of molecular oxygen indicate the metabolic rate of aerobic organisms?

The consumption of molecular oxygen (O₂) has a direct relationship with metabolic rate of aerobic cells and therefore the metabolic rate of organisms. Cells with a greater metabolic activity require more energy and this energy comes from ATP molecules. As ATP production is required, the intensity of aerobic cell respiration is also higher and more oxygen is consumed.

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