In Investigation Five, we will discuss higher orders of organization above the level of individual cells. As we shall see, cells organize into tissues, which in turn can organize with different types of tissues to form organs and then organ systems, like the muscle system, or the nervous system, and so on.

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Cells are found in living organisms and contain many different structures. What are some structures that you would expect to find in a cell? Students should indicate that they would expect to find a cell membrane and cytoplasm in a cell. They may suggest that some but not all cells may also contain a nucleus.

How would you describe the size of most cells? Can cells be seen with the unassisted eye? Students should indicate that most cells are microscopic and cannot be viewed with the unassisted eye. A microscope is needed to view most cells. As an example, students may recall that their cheek cells were approximately 0.0015 cm in length.

Note: In general, the cells of most organisms are microscopic. However, there are some exceptions such as the giant squid axon and birds eggs which can be viewed with the unassisted eye.

One way to classify cells is to categorize them as eukaryotic or prokaryotic. Describe the difference between a eukaryotic and prokaryotic cell. Students should indicate that a eukaryotic cell is one that contains a clearly defined nucleus, a nucleus that is separated by a membrane from the cytoplasm. A prokaryotic cell does not contain a clearly defined nucleus.

Do the parts of a cell have a function? What do the structures of the parts of a cell tell you about their functions? Students should suggest that the parts of a cell do have a function. Some of the functions of the different parts of a cell can be inferred from their structures. For example, the cell membrane surrounds a cell. It separates the inside from the outside of a cell. Therefore, it is likely that the cell membrane helps to control substances that can enter and exit a cell.

The organelles in a cell have functions. What would be examples of some functions of a cell? Students should indicate that cells have functions. For instance, cheek cells line the mouth, protecting the blood vessels and muscles of the mouth from food, enzymes, and bacteria; and the cells of an Elodea leaf perform photosynthesis, providing food for the plant.

Where would you expect to find cells? Can you give some examples of living organisms that have cells? Student answers may vary. Students should suggest that cells can be found in living organisms such as onion plants, Elodea plants, and humans.

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This is the first of three slides that focus on the relationship between structure and function. Students recently learned that the shape of a convex lens in an objective lens of a compound microscope determines how an image will look when viewed through it.

Increased curvature of the lens (structure) results in a greater magnification of the image (function).

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This slide and the next slide explain the relationship between structure and function in another example.

A Swiss engineer called George de Mestral was walking his dog in the Alps. When he returned home, he found that numerous plant seed burrs were tangled in his dog’s fur (A). He found that he could pull the burrs free from the dog but when examined under a hand lens, he saw that the burr was actually tangled in fur (B).

When burrs (C) are examined closely in a microscope (D), they are found to have tiny hooks on the end of their barbs. Thus, the microscopic structure of the burr helps to explain its function. In this case, the function of reversibly sticking to animal fur is to help the plant spread its seeds in the burr far from the plant so that the species can spread.

However, de Mestral was not finished. As an engineer, he thought that one surface reversibly binding to another might have useful applications as a fastener. And so, he used these ideas to invent…

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Velcro!

This slide shows a close-up electron microscope image of a small section of Velcro. Notice how the hook structures on the top surface interact with the fuzzy surface on the bottom. Further, notice how similar the Velcro microscopic image looks to the microscopic image of the burr on the previous slide.

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With this slide, we return to the subject of cells. Notice how we can extend a discussion of structure and function by comparing the structures in cells (organelles) to their functions.

Notice that the same structures that exist in both plant and animal cells perform similar functions (cell membrane and nucleus in this example).

Also notice that, if a specific organelle is missing from a cell (no chloroplasts in animal cells), that function is also missing from that cell (photosynthesis in this case).

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Before continuing further with the Investigation, assess students’ prior knowledge of the terms tissue, organ, and organism. If students are unfamiliar with these terms or are unsure of how to define these structures, direct them to locate the terms tissue and organ in their Scientist’s Glossary. Discuss examples of organisms such as humans, cats, trees, plants, reptiles, and birds.

Cell: the basic unit of organization of living organisms. Single-cell organisms live independently while other types of cells are organized into tissues.

Tissue: a collection of cells that form the structures of a plant or animal and work together to perform a function.

Organ: One of the parts of the body that performs a function. Different tissues combine to form organs. The brain, heart, and stomach are examples of organs.

Organism: an individual animal, plant, or single-celled life form. 

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Use an analogy to facilitate student comprehension of the relationship between cells,  tissues,  organs, and organisms.    Divide students into groups of four.   Explain that you would like them to brainstorm and think of an analogy that describes the relationship between cells, tissues,  organs, and organisms.  If necessary,   discuss with students what is meant by the term analogy. 

To help students, share an example of an analogy:

Organism = Book 

Organs = Chapters 

Tissues = Pages 

Cells = Words

Students should record their examples in Problem 1 of their Scientist Data Record.

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This slide simply shows the four different major types of tissue in a human (or another animal) body.

• Connective Tissue
• Epithelial Tissue
• Nervous Tissue
• Muscle Tissue

Each section of the slide illustrates the various levels of organization of tissue and organs, as well as organ systems.

Next, tell students that in Investigation Five they will observe slides of four different specimens. The slides are of different tissues, structures, and organs of plants and animals.

Explain that several of the specimens they will view differ from those that they observed in Investigations Two through Four in the way in which they were prepared.

Ask students: Did you prepare the fingerprint, cheek cell, and onion specimens in Investigations Two, Three, and Four the same way? Students should indicate that different procedures were used for specimen preparation in Investigations Two, Three, and Four.

Ask students: Can you name different ways in which you prepared a specimen for analysis? Students should indicate that a  paper with a  fingerprint was placed onto the slide in Investigation Two. In Investigations Three and Four, a sample of cheek cells and onion skin was placed in water on a slide.

Help students recall and classify three different ways in which they prepared a specimen:  whole-mount,  smear  (type of wet mount),  and section  (type of wet mount). The fingerprint specimen in  Investigation  Two was prepared as a whole mount:  the entire piece of paper with the fingerprint was placed onto the slide.  The cheek cell specimen was prepared as a smear, a type of wet mount: a sample was taken from the cheek and smeared onto the water on the slide. The onion specimen was prepared as another type of wet mount: a slice or section. A piece of onion skin was removed from the whole onion and placed onto the water on the slide.

Ask students:  Why was it necessary to prepare a  piece of the onion for analysis and smear of the cheek cells?  Why could the  “whole mount”  procedure not be used for these two specimens? Students should suggest that a  whole-mount procedure involves placing the entire structure on a slide. This would not be possible with the cheek or a whole onion. Neither would fit on a slide or under the objectives of the microscope.

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With this slide, we begin to prepare for Investigation 5 lab. In the lab, students will examine four  prepared slides that were prepared by three different procedures:

• Smear: (human blood)
• Whole Mount: (Elodea leaf) Similar to a wet mount preparation that students prepared in Investigation 4.
• Cross-Section: (Pinus [pine] stem) and human colon (large intestine)

SLIDE MICRO5-pre-10 illustrates how blood samples are prepared for microscopic examination. The micrograph (photograph taken through a microscope) in the background shows nicely smeared red blood cells (erythrocytes) as well as two darkly stained white blood cells (leukocytes).

NOTE THAT THE PRELAB EXERCISE INVOLVING MODELING OF CROSS-SECTIONS AND LONGITUDINAL SECTIONS IS OMITTED FROM THIS PRELAB. THIS INVOLVES STUDENT SDR PAGES 76 (BEGINNING WITH PROBLEM 2) THROUGH 80.

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This slide shows the process used to prepare a wet mount. The whole mount of an Elodea leaf that students will examine in Investigation Five lab is prepared in a similar manner except that the specimen is dry and permanently fixed between the slide and coverslip.

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This slide shows how cross-section slides are prepared. while it shows the cross-cutting (sectioning) of a plant stem, in reality, most cross-sections are prepared with the use of an instrument called a microtome, that can cut extremely (one cell think) sections. A microtome is shown below:

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This final slide simply asks students to consider three questions as they examine their prepared sides in the Lab:

How are the cells of different plants and animals similar? How are they different?

What is the relationship between the arrangement of cells and the structure and function of a tissue, organ, organ system, or organism?

How do you think cross-sectional and longitudinal sectioning might affect a specimen’s appearance?

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