ACTIVITY: THE CELL THEORY



ACTIVITY: THE CELL THEORY

History of the Cell Theory

Our present knowledge of the cell represents the work of many scientists. Today, thousands of biologists are extending our understanding of the details of cell structure and function. However, there are a few scientists who are particularly notable for their outstanding contributions to the early development of the cell theory.

Questions

Use the names from the following list to help fill in the blanks in the statements below.

Robert Brown Matthias Schleiden

Felix Dujardin Theodor Schwann

Henri Dutrochet Rudolf Virchow

Robert Hooke

1. The earliest known examination of cells took place in 1665, when ______________________ an English scientist, studied some thin sections of cork under a crude microscope. He observed many small cavities surrounded by walls. He named these tiny units ______________________.

2. In 1824, the French scientist ______________________ came to the conclusion that both plant and animal tissues were always made up of cells.

3. In 1831, _____________________ observed the cells of an orchid leaf and noted a dark circular structure near the cell. He called this structure the ______________________________.

4. In 1835, the French biologist ______________________ studied the living cell and noted that it contained a material that is today called protoplasm.

5. In 1839, _________________________ and _________________________ also came to the conclusion that all plant and animal tissues are made up of cells.

6. In 1855, the German biologist ___________________________, as a result of observing cells undergoing division, stated that all cells come from pre-existing cells.

CELL STRUCTURE

All cells are enclosed by a cell membrane. Within the membrane are the nucleus and the cytoplasm, which consists of all the material outside the nucleus and inside the cell membrane. Within the cytoplasm are organized structures that perform specific functions. These structures are called organelles.

THE GENERALIZED CELL

Cells vary greatly in the details of their form and in the special functions they perform. However, most cells have certain features in common. The diagram below represents a “generalized” animal cell. It is not a drawing of any particular type of cell, but it does show the organelles that are usually present in the cells of animals. Typical plant cells are somewhat different from animal cells, and some of the differences will be mentioned as we proceed.

Questions

1. Use the following list to label the structures indicated in the drawing of the generalized animal cell.

Cell membrane

Nucleus

Nuclear membrane

Nucleolus

Chromosome

Centriole

Mitochondrion

Lysosome

Endoplasmic reticulum

Golgi body

Vacuole

2. Fill in the names of the structures whose functions are listed below.

|STRUCTURE |FUNCTION |

| |Cell respiration |

| |Protein synthesis |

| |Contains hereditary information |

| |Storage of water, undigested food, and/or waste products |

| |Active in movement of the chromosomes during cell division |

| |Storage of digestive enzymes |

| |Transport within the cytoplasm |

| |Packages secretions |

CELL MEMBRANE

The cell, or plasma, membrane surrounds the cell. It plays an active role in determining which substances enter and which substances leave the cell. Because some substances can pass freely through the cell membrane and others cannot, the membrane is said to be selectively permeable, or semipermeable. The permeability of the plasma membrane varies from one cell type to another and from time to time in the same type of cell, depending on the state of metabolic activity. The cell membrane is composed of lipids and proteins.

Questions

1. Describe the functions of the cell membrane.

2. The cell membrane is composed of __________________ and ____________________.

3. Why is the cell membrane described as “semipermeable”?

NUCLEUS

The nucleus is the control center for all cell functions. It is surrounded by a semipermeable membrane. Within the nucleus are the chromosomes and at least one nucleolus. The nucleolus is a site of RNA synthesis, and it may also be responsible for the production of ribosomes.

Questions

1. The control center of the cell is the _______________________.

2. What are the functions of the nucleolus?

CYTOPLASM

The material in the cell outside the nucleus is called the cytoplasm. Although it contains thousands of substances, it consists mainly of water. Within the cytoplasm are the various organelles of the cell. The cytoplasm provides the environment in which the organelles carry on the life processes of the cell.

Questions

1. What is the main function of the cytoplasm?

2. The cytoplasm consists mainly of _______________________.

MITOCHONDRIA

Mitochondria are slipper-shaped organelles found in the cytoplasm. They are enclosed by a double membrane, whose inner layer is highly folded so that it has a large surface area. Most stages of cell respiration occur in the mitochondria. The energy released during respiration is stored in the form of high-energy chemical bonds in molecules of ATP.

Questions

1. What is the function of mitochondria?

2. In what substance is the energy released during cell respiration stored? __________________

3. What is the advantage of the folding of the inner membrane of the mitochondrion?

RIBOSOMES

Ribosomes are small, dense granules found free in the cytoplasm and the nucleus and lining the membranes of some endoplasmic reticulum. Ribosomes are composed mainly of RNA. They are the centers of protein synthesis in the cell.

Questions

1. Where are ribosomes located in the cell?

2. What is the function of ribosomes?

3. What happens to amino acids at the ribosomes?

ENDOPLASMIC RETICULUM

The endoplasmic reticulum is a membrane-bounded system of channels or tubes through which materials are transported within the cell. The membranes of the endoplasmic reticulum may also serve as sites of biochemical reactions. There are two types of endoplasmic reticulum – smooth and rough. The rough, or granular, appearance of some endoplasmic reticulum is due to the presence of ribosomes on the endoplasmic reticulum membranes. Rough endoplasmic reticulum is found mainly in cells involved in protein synthesis. Smooth endoplasmic reticulum, which has no ribosomes on its membrane, is found mainly in cells involved in synthesis of non-protein substances.

Questions

1. What are the functions of endoplasmic reticulum?

2. The two types of endoplasmic reticulum are _________________ and __________________.

3. In what types of cells are each found?

GOLGI BODIES

The Golgi body is made up of a series of membrane-enclosed sacs, and it is usually found near the nucleus. This organelle is associated with the production of lysosomes and with the synthesis of various secretions.

Questions

1. What are the functions of the Golgi body?

2. Where is the Golgi body generally located in the cell?

LYSOSOMES

Lysosomes are “packages” or sacs, of digestive enzymes. They keep the enzymes separated from the rest of the cell contests until they are needed. Lysosomes, which are found mainly in animal cells, are egg-shaped structures enclosed by a membrane.

Questions

1. What is the function of lysosomes?

2. Where in the cell are lysosomes produced?

VACUOLES

Vacuoles are membrane-enclosed structures that are generally filled with water containing various dissolved substances. Vacuoles in animal cells are usually small. Large vacuoles are often present in protests and in plant cells. In the protests there are food vacuoles in which food is digested so that it can be used by the cell. In some fresh-water protozoa there are water vacuoles that remove excess water from the cell and discharge it back into the environment. Much of the inside of a typical green plant cell is filled with a large vacuole. The pressure created by the vacuole helps to maintain the rigid structure of the cell and of the plant.

Questions

1. In protozoans such as the amoeba digestion occurs within __________________________.

2. How is excess water removed from fresh-water protozoa?

3. What is the function of the large vacuole present in the cells of green plants?

CENTROSOMES

Centrosomes are small organelles found just outside the nucleus in animal cells. Within the centrosome are two small structures called centrioles, which are necessary for the movement of chromosomes during cell division. Centrioles are also associated with the formation of cilia and flagella in a wide variety of cell types.

Questions

1. Centrosomes are found only in the cells of ______________________.

2. What are the functions of centrioles?

CELL WALLS

Cell walls are structures found just outside the cell membrane in plant cells. The cell wall is considered to be a “nonliving” part of the cell, since it does not take part in any of the life functions of the cell. The cell wall is made up chiefly of cellulose, which is relatively rigid, and provides support for the cell.

Questions

1. What are the functions of the cell wall?

2. What are cell walls composed of?

CHLOROPLASTS

Chloroplasts, found only in plant cells and in some protests, contain the green pigment chlorophyll, which carries on the process of photosynthesis. In photosynthesis, light energy is used for the manufacture of food.

Questions

1. The source of energy for photosynthesis is ______________________.

2. The most important pigment in chloroplasts is ______________________.

3. Label the parts indicated in the drawing below of a generalized plant cell.

4. What are three structural differences between a typical animal cell and a typical green plant cell?

REVIEW EXERCISES

A. Using the vocabulary terms in the following list, fill in the blanks in the statements below.

Cell membrane gel

Cellulose Golgi body

Cell wall lysosomes

Centrosomes mitochondria

Chlorophyll nucleolus

Chloroplasts nucleus

Chromosomes ribosomes

Cytoplasm sol

Endoplasmic reticulum vacuole

1. Most steps in the process of cell respiration take place within the cytoplasmic organelles called _________________________.

2. Sacs of digestive enzymes found only in the cytoplasm of animal cells are called ______________________.

3. In animal cells centrioles are found within _________________________.

4. The relatively rigid structure that supports and gives shape to the plant cell is the _________________________, which is made of _________________________.

5. In plant cells, the green pigment __________________ is found with ___________________.

6. The cytoplasmic organelles involved in protein synthesis are the _______________________.

7. The site of RNA synthesis within the nucleus is the _________________________.

8. The material that fills the area between the nucleus and the cell membrane is the _________________________.

9. The membrane-bounded system of channels through which materials are transported within the cell is the _________________________.

10. The organelle consisting of a series of membrane-enclosed sacs usually found near the nucleus is the _________________________.

11. The inside of a green plant cell may be almost completely filled with a large _____________________.

12. The control center of the cell is the _________________________.

13. The passage of materials into and out of the cell is controlled by the ___________________.

14. Hereditary information is contained in the _________________________ in the nucleus.

15. The cytoplasm can change from a watery _________________________ phase to a more viscous _________________________ phase.

B. In the answer space for each question, write the letter of the choice that best completes the statement.

_____ 1. Cell membranes are (a) semipermeable (b) impermeable (c) permeable (d) non-permeable

_____ 2. Transport in mammals involves absorption and (a) circulation (b) absorption (c) transpiration (d) assimilation

_____ 3. Which organelle is associated with the process of synthesis? (a) ribosome (b) mito-chondria (c) cell membrane (d) vacuole

_____ 4. The organelle that controls passage of material into and out of the cell is the (a) chloroplast (b) nucleus (c) cytoplasm (d) cell membrane

_____ 5. The cell wall is composed of (a) glycogen (b) proteins (c) cellulose (d) fatty acids

_____ 6. Lysosomes contain (a) glycogen (b) digestive enzymes (c) hormones (d) fats

_____ 7. Lysosomes are found (a) only in protozoa (b) only in animal cells (c) only in plant cells (d) only in brown algae

_____ 8. The nucleolus is the site of (a) DNA synthesis (b) RNA synthesis (c) glucose breakdown (d) glycogen synthesis

_____ 9. The chromatin network if found in the (a) cytoplasm (b) nucleus (c) endoplasmic reticulum (d) centrosomes

_____ 10. The nuclear membrane is (a) semipermeable (b) impermeable (c) permeable (d) nonpermeable

ACTIVITY: PASSAGE OF MATERIALS THROUGH MEMBRANES

Transport is the life process by which needed materials are brought to the cells of an organism and wastes are carried away from the cells. Transport may be described in two parts. One involves the passage of materials through the cell membrane, both into and out of the cell. The second is the circulation of materials to and from the cells.

Question

1. What is accomplished by the life process of transport?

DIFFUSION

The particles of every substance (atoms, molecules, or ions) are in constant, random motion. In liquids and gases, this random motion results in frequent collisions. As the particles rebound from these collisions, they tend to move apart. Where a substance is highly concen-trated, collisions of its particles are more frequent than where the substance is less concentrated. As a result, its particles tend to spread away from regions of higher concentration and toward regions of lower concentration. This spread of a substance from a region of higher concentration to a region of lower concentration is called diffusion. The difference in concentration from one region to another is called the concentration gradient of the substance.

Questions

1. What is diffusion?

2. What is a concentration gradient?

OSMOSIS

Osmosis is the diffusion of water through a membrane. The concentration of water is highest in pure water. Then other substances are dissolved in the water, the concentration of the water decreases as the concentration of the dissolved substances increases. Water will therefore diffuse from a solution that has a higher concentration of water to one that has a lower concentration of water.

Questions

1. What is osmosis?

2. Would a cell lose or gain water when placed in a solution with a salt concentration greater than that of the cell cytoplasm?

3. Would a cell lose or gain water when placed in a solution with a salt concentration less than that of the cell cytoplasm?

PASSIVE TRANSPORT

The cell membrane in semipermeable – that is, some substances can pass through it freely, while others cannot. The movement of substances that can pass freely through a membrane depends only on the concentration gradient for that substance. Thus the movement of such a substance requires no expenditure of energy by the cell. This is called passive transport.

Questions

1. What is passive transport?

2. What factor determines the direction of net movement of molecules in passive transport?

ACTIVE TRANSPORT

Living cells have the capacity to move certain molecules and ions through the cell membrane against the normal direction of diffusion. That is, these substances can be moved from an area of lower concentration to an area of higher concentration. This process, called active transport, requires the expenditure of energy by the cell.

Questions

1. The paramecium is a fresh water protozoan. The salt content of its cytoplasm is greater than that of the surrounding medium.

a. Does water tend to enter or leave the paramecium? Is this process of passive or active transport?

b. How does the paramecium expel water? Is this process of passive or active transport? Explain.

2. Where does the energy for active transport come from?

PINOCYTOSIS

Some molecules that cannot pass through the cell membrane by simple diffusion enter the cell by pinocytosis – a type of active transport. When the molecules involved come in contact with the cell membrane, the membrane folds inward, forming a pouch, or pinocytic vesicle. The vesicle breaks off, forming a vacuole in the cell cytoplasm, and the cell membrane closes over.

Questions

1. What is pinocytosis?

2. What types of substances are taken into the cell by pinocytosis?

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Taken from: Learning Program for Biology by William D. Schraer; 1978; Sebco Publishing

Taken from: Learning Program for Biology by William D. Schraer; 1978; Sebco Publishing

Taken from: Learning Program for Biology by William D. Schraer; 1978; Sebco Publishing

Taken from: Learning Program for Biology by William D. Schraer; 1978; Sebco Publishing

Taken from: Learning Program for Biology by William D. Schraer; 1978; Sebco Publishing

Taken from: Learning Program for Biology by William D. Schraer; 1978; Sebco Publishing

Taken from: Learning Program for Biology by William D. Schraer; 1978; Sebco Publishing

Taken from: Learning Program for Biology by William D. Schraer; 1978; Sebco Publishing

Taken from: Learning Program for Biology by William D. Schraer; 1978; Sebco Publishing

Taken from: Learning Program for Biology by William D. Schraer; 1978; Sebco Publishing

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