RAVEN

CHAPTER 4: CELL STRUCTURE

WHERE DOES IT ALL FIT IN?

The information in Chapter 4 provides details about prokaryotic and eukaryotic cells. It puts the chemistry of the earlier chapters into a picture of the cell. Emphasis is placed on eukaryotic cells because of their complexity and great diversity. Many terms are introduced in the chapter and show be reinforced as much as possible during coverage of cell in this and other chapters dealing with cell concepts. Chapter 5 is a critical reference when covering the chapters later in the text that describe cell transport, metabolism, cell replication, and genetics.

SYNOPSIS

All life is composed of cells, individually or as components of multicellular organisms. With certain exceptions, cells are very small. Substances diffuse more rapidly in a small cell, enhancing both its metabolism and its communication with other cells and with its environment. As a cell’s size increases, its volume increases at a much greater rate than its surface area. A cell’s survival depends on its surface, where all molecules enter and exit. If there is too little surface to support the workings of the interior, the cell will die.

Many of the structural differences between prokaryotes and eukaryotes are visible at the level of the light microscope; the presence of the nucleus in eukaryotes, for example. The nuclear material in prokaryotes is a single, circular strand of DNA, unencumbered by either proteins or a surrounding membrane and is difficult to see at the same scale. Observation with an electron microscope reveals details about the cytoskeleton and internal membrane systems of the eukaryotes, both absent in prokaryotes. Different kinds of microscopes and different staining procedures can be used to obtain a desired image.

On a biochemical level, all of the reactions of a prokaryote, including those associated with ribosomes, occur openly in its cytoplasm, bounded only by invaginations of the plasma membrane. The reactions in a eukaryote are compartmentalized by the endoplasmic reticulum (ER) and by various membrane-bound organelles. Among these organelles are the nucleus, the Golgi apparatus, lysosomes, and microbodies (peroxisomes and glyoxysomes). The smooth and rough ER differ in appearance and function.

Rough endoplasmic reticulum possesses ribosomes while smooth endoplasmic reticulum lacks them. Various chemical products are synthesized on the rough endoplasmic reticulum, channeled into the Golgi bodies, and packaged into microbodies and lysosomes. Smooth ER contains embedded enzymes and is involved in carbohydrate and lipid synthesis and detoxification.

Some eukaryotic organelles contain DNA, notable among these are the cell’s powerhouses, the mitochondria and the chloroplasts. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, provides a framework to anchor the organelles and give a cell its shape. Microtubules move organelles within a cell assisted by kinesin and dynein proteins. They also provide the characteristic 9+2 arrangement of eukaryotic flagella, cilia, and centrioles. Plant, fungi, and some protists have special adaptations that are lacking in other cells. Plant cells have a large central vacuole which serves as a storage compartment and helps increase the cell’s surface-to-volume ratio. Plants cells and some protists have strong, rigid cell walls composed of cellulose whereas fungi have chitin in their cell walls. Animal cells lack cell walls but have their cytoskeleton linked to the extracellular matrix.

LEARNING OUTCOMES

·  Explain the cell theory.

·  Describe the factors that limit cell size.

·  Categorize structural and functional similarities in cells.

·  Describe the organization of prokaryotic cells.

·  Distinguish between bacterial and archaeal cell types.

·  Compare the organization of eukaryotic and prokaryotic cells.

·  Discuss the role of the nucleus in eukaryotic cells.

·  Describe the role of ribosomes in protein synthesis.

·  Identify the different parts of the endomembrane system.

·  Contrast the different functions of internal membranes and compartments.

·  Evaluate the importance of each step in the protein processing pathway.

·  Describe the structure of mitochondria and chloroplasts.

·  Compare the function of mitochondria and chloroplasts.

·  Explain the probable origin of mitochondria and chloroplasts.

·  Contrast the structure and function of different fibers in the cytoskeleton.

·  Illustrate the role of microtubules in intracellular transport.

·  Describe how cells move.

·  Identify the different cytoskeletal elements involved in cell movement.

·  Classify the elements of extracellular matrix in animal cells.

·  Differentiate between types of cell junctions.

·  Describe the roles of surface proteins.

COMMON STUDENT MISCONCEPTIONS

There is ample evidence in the educational literature that student misconceptions of information will inhibit the learning of concepts related to the misinformation. The following concepts covered in Chapter 4 are commonly the subject of student misconceptions. This information on “bioliteracy” was collected from faculty and the science education literature.

·  Cells are within an organism but do not make up the body

·  All cells are spherical

·  Cells are 98% water in chemical composition

·  The cytoplasm is similar to pure water

·  All animal cells have every type of organelle

·  Mitochondria are built by the cell

·  Mitochondria produced all of the cell’s ATP

·  Prokaryotes produce ATP in mitochondria

·  All cells have a nucleus

·  Plant cells do not have mitochondria

·  Smooth ER and rough ER are separate structures

·  The Golgi body is an independent organelle unrelated to the ER

·  All cells of multicellular organisms carry out the same tasks

·  Plant cells lack protein whereas animal cells are high in protein

·  Cell walls are the same as plasma membranes

·  Cell membranes are solid

INSTRUCTIONAL STRATEGY PRESENTATION ASSISTANCE

There is a lot of unfamiliar vocabulary in this chapter. Etymology of new words may be helpful.

Introduce the plasma membrane carefully. The next two chapters delve into its importance within and between cells.

Most students are familiar with simple cell theory. It may be appropriate to discuss Redi’s and Pasteur’s experiments refuting spontaneous generation at this point, when discussing cells originating from other cells. Students have some difficulty with the differences between prokaryotes and eukaryotes, therefore stress nuclear organization and membrane compartmentalization as definitive characteristics.

Many texts don’t present much in the way of why cells are the size they are. This is an important concept elaborated on later in this text, in relation to why most animals and some plants have specific size limitations.

HIGHER LEVEL ASSESSMENT

Higher level assessment measures a student’s ability to use terms and concepts learned from the lecture and the textbook. A complete understanding of biology content provides students with the tools to synthesize new hypotheses and knowledge using the facts they have learned. The following table provides examples of assessing a student’s ability to apply, analyze, synthesize, and evaluate information from Chapter 5.

Application / ·  Have students predict which organelles would become more active it the cell was induced to produce more protein secretions.
·  Ask students explain which organelles would be damaged if a drug that kills prokaryotes were to end a person’s cells.
·  Ask students explain a why certain invasive organisms that cause disease in humans produce chemicals that block in integrins.
Analysis / ·  Ask students to explain the implications of a disease that disrupts the function of the Golgi apparatus.
·  Ask students to explain the survival advantages of an infectious agent that has the ability to disable lysosomes.
·  Have students determine some of the consequences of research showing that nicotine reorganizes cytoskeleton structure.
Synthesis / ·  Ask students hypothesize the characteristics of an endosymbiont that purportedly aids feeding in an organism that has no digestive system.
·  Have students determine the best classification of a cell that has chloroplasts and flagella, but has no cell wall or central vacuole.
·  Ask students to explain why some cells in an organism can have 100 times more mitochondria than other cells.
Evaluation / ·  Ask students to debate the pros and cons of using mechanical devices to replace lost body functions once carried out by cells.
·  Ask students to evaluate the consequences of a health product claiming to stimulate activity of the mitochondria.
·  Have students discuss the benefits and risks of producing agricultural animals that have chloroplasts expressed in their skin cells.

VISUAL RESOURCES

  1. A large, clear plastic bag is a reasonable facsimile of a cell’s plasma membrane. A prokaryote can be represented by a bag with various objects inside to represent their metabolic processes. A large bag with the objects inside of smaller bags represents a eukaryote and its compartmentalizing membrane-bound organelles. One could further place the bag in a box to represent the cell wall of a plant.
  1. Fisher educational division sells a superb yet simple model of a lipid bilayer (and it is inexpensive). A saturated salt solution is the cell interior, mineral oil the exterior (the oil floats on the salt solution). Styrofoam balls are one side of the membrane (they float on the oil), plastic balls are the other side (they stay at the salt/oil interface). Plastic rods or straws into the balls serve as the lipid tails.
  1. One may want to discuss cell fractionation and gradient centrifugation in relationship to isolating the various cell parts so they can be studied. A short description of various kinds of microscopes (especially the rationale behind why electron microscopes resolve much smaller structures) might be helpful, although this is frequently discussed in the laboratory setting. To a great extent, much of the material in this chapter is supported by laboratory activities where the students can observe many of the larger structures first-hand.
  1. Most students mistakenly associate chloroplasts with plants and mitochondria with only animals. Stress that mitochondria are present in virtually all eukaryotes. Remember that these two organelles will be visited again when cell metabolism is discussed. Students may as well learn their structure now as opposed to later when they will be attempting to understand the biochemistry too.
  1. Electron micrographs are a must for this material. They can be difficult for students to interpret, therefore accompanying line drawings that simplify the micrograph are beneficial. Or use markers to outline and/or colorize particular structures on either photographs or transparencies.

IN-CLASS CONCEPTUAL DEMONSTRATIONS

A. Organelle Role Playing

Introduction

Critical thinking can be done in many ways to encourage student retention of biology concepts. Students can be asked to role model different organelles to learn the function of the organelle and its interconnections with other organelles. This demonstration can be performed in the classroom or outdoors.

Materials

·  Student volunteers to represent one organelle

o  Cell membrane

o  Nucleus

o  Chloroplast

o  Mitochondrion

o  Rough ER

o  Smooth ER

o  Golgi apparatus

o  Lysosome

·  Colored markers

·  8 ½” X 11” sheets of white paper labeled with the name of one organelle

o  Cell membrane

o  Nucleus

o  Chloroplast

o  Mitochondrion

o  Rough ER

o  Smooth ER

o  Golgi apparatus

o  Lysosome

Procedure & Inquiry

  1. Ask the selected students to quickly design a labeled sign representing their organelle
  2. Assign each student to an organelle and hand them the appropriate label.
  3. Then ask the student to give a two-minute description of their relationship to the other organelles, for example I take ______from the nucleus and provide the Golgi apparatus with ______.
  4. The other organelles are free to discuss any interrelationships not mentioned by the other organelles.
  5. Ask the class assess the accuracy of the information discussed by the organelles.
  6. Then ask the class to do a “Survivor-style” vote to determine which organelle could be “voted out” without outright killing the cell.

A. Cytoskeleton in Action

Introduction

Seeing the activity of an organelle by demonstrating a living model is an excellent means of reinforcing the learning of cell structure. Elodea is a simple to use model for demonstrating cytoskeleton activity. Students can envision the nature and complexity of the cytoskeleton arrangement by tracking the path of chloroplasts traveling around the elodea leaf cytoskeleton.

Materials

·  Microscope attached to a video camera

·  LCD projector linked to the video camera

·  Fresh elodea in 300C water

·  Slide with cover slip

·  1 cigarette soaked overnight in 5 ml 30% ethanol solution

Procedure & Inquiry

  1. Prepare a wet mount of one elodea leave
  2. Project a leaf under high magnification for viewing by the class
  3. Ask to students to observe and discuss any pattern of movement by the chloroplasts
  4. Ask the class to hypothesize why the chloroplasts are being moved around the by cytoskeleton and what factors, such as direction of sunlight irradiation, would affect chloroplast movement
  5. Tell the class you are going to add cigarette extract to the elodea
  6. Then add on drop of the cigarette solution onto the slide while it is still on the stage
  7. Ask the students to observe what happens (the chloroplasts will stop moving)
  8. Have the class hypothesize the reason for the loss of chloroplast movement

LABORATORY IDEAS

  1. Cell Chemistry
  2. Have students use the microscope to investigate the chemistry of living cells.
  3. Tell the class that you want students to explore the chemistry of cell components. They should be asked to draw and record in writing their observation of the cells being investigated.
  4. Provide students with the following materials:
  5. Microscope
  6. 8 microscope slides and cover slips
  7. Scalpels
  8. Forceps
  9. Toothpicks
  10. Dimethyl Sulfoxide (DMSO)
  11. Potassium Iodide (IKI) or Lugol’s solution
  12. Concentrated Sudan IV solution
  13. Biuret reagent
  14. Hematoxylin stain
  15. Droppers
  16. Potato
  17. Explain to students the different molecules that are detected using the various stains:
  18. Potassium Iodide - starch
  19. Sudan IV solution - lipids
  20. Biuret reagent - proteins
  21. Hematoxylin stain – nucleic acids
  22. Tell students that they are going to compare the chemical composition of plant and animal cells. They are going to do this using by potato cells and human cheek cells as models.
  23. Have students carry out the following procedure:
  24. Prepare 4 wet mounts of thin potato slivers
  25. Add 1 drop DMSO to all slides
  26. Add stains:
  27. 1 drop of potassium Iodide
  28. 2 drops of sudan IV solution
  29. 2 drops of Biuret reagent
  30. 2 drops of hematoxylin stain
  31. Students should let slides sit for 1 minute and then observe under the microscope.
  32. Prepare 4 wet mounts of cheek cells
  33. Carefully remove cheek cells with toothpick
  34. Add 1 drop DMSO to all slides
  35. Add stains:
  36. 1 drop of potassium Iodide
  37. 2 drops of sudan IV solution
  38. 2 drops of Biuret reagent
  39. 2 drops of hematoxylin stain
  40. Students should let slides sit for 1 minute and then observe under the microscope.
  41. Ask the students to note any differences in molecular composition between the animal and plant cells.
  42. Use proper safety precautions and dispose of reagents appropriately according to the MSDS.
  1. Detection of Catalosomes
  2. Catalosomes of one of many small vacuoles that carry out specific cell functions.
  3. Students can investigate the presence of catalosomes using a simple hydrogen peroxide test for the presence of catalase.
  4. Provide students with the following materials:
  5. Microscope
  6. 2 microscope slides and cover slips
  7. Scalpels
  8. Forceps
  9. Medical grade hydrogen peroxide
  10. Droppers
  11. Dimethyl Sulfoxide (DMSO)
  12. Potato
  13. Elodea leaves
  14. Onion roots
  15. Explain to students they will be looking for organelles called catalosomes and detecting them by looking at metabolic activity of the organelle.
  16. Then describe that catalase is able to convert hydrogen peroxide into water and oxygen gas. Ask them to hypothesize how they could determine if the reaction is taking place and to what degree it is occurring.
  17. Instruct students to carry out the following procedure:
  18. Prepare a wet mounts of potato cells
  19. Add 1 drop DMSO to all slides
  20. Add hydrogen peroxide and immediately view under the microscope
  21. Prepare a wet mounts of elodea cells
  22. Add 1 drop DMSO to all slides
  23. Add hydrogen peroxide and immediately view under the microscope
  24. Prepare a wet mounts of onion root cells
  25. Add 1 drop DMSO to all slides
  26. Add hydrogen peroxide and immediately view under the microscope
  27. Ask students to describe where the catalosomes appear to be located in the particular cells and explain any differences seen in catalosome activity between the different specimens.
  28. Use proper safety precautions and dispose of reagents appropriately according to the MSDS.

LEARNING THROUGH SERVICE