Introduction to Cells. Cytology Part I 3A: The Structure of Cells

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1 Introduction to Cells Cytology Part I 3A: The Structure of Cells

2 Your Goals for this Section -Outline the events that led to the development of the cell theory -State the cell theory -Discuss the relationship between cell size and surface area

3 A Recap of what you know by now: Everything in the world is composed of matter and energy Chemicals (elements) are the basic components of all matter Atoms (the smallest unit of an element) may combine to form molecules Molecules combine to form various cellular structures

4 3A: The Structure of Cells This chapter on cellular structure shows how the chemistry learned in Chapter 2 can be applied to both the structure and function of the many entities that compose a cell.

5 The idea of what cells look like and do is part of what biologists call cell theory The Cell Theory is basically summed up in 3 basic principals: 1. The Cell is the basic unit of structure and organization in organisms 2. All organisms are composed of one or more cells 3. All cells come from preexisting cells

6 How Cell Theory Came to be Cytology, the study of cells, can be traced back over three hundred years to the English scientist Robert Hooke In 1665, he published Micrographia, a report of his observation using a simple compound microscope that could provide a magnification power of approximately thirty times (30x). By contrast, a typical modern high-school microscope may magnify the image of the specimen as much as four hundred times (400x). In Observation XVIII of Micrographia, Hooke described his observation of a thin piece of cork. I could exceedingly plainly perceive it to be all perforated and porous, much like a honey-comb... These pores, or cells, were indeed the first microscopical pores I ever saw, and perhaps, that were ever seen, for I had nor met with any writer or person that had made any mention of them before this.

7 Because the neat rows of little boxes reminded him of the rows of rooms in a monastery, he named the structures cells. Later he studied sections of carrots and ferns and described similar findings. In the specimens that Hooke studied, he saw only the walls of rather uniformly shaped boxes. Even though scientists now know that the boxes are found in vastly different shapes, sizes, and arrangements, they are still called cells. The first person to observe living organisms under a microscope was Anton van Leeuwenhoek in an amateur lens maker who had been intrigued after reading Hooke s Micrographia. However, for the next two hundred years, microscopes were considered to be exotic instruments. It was not until the nineteenth century that the use of microscopes (microscopy) became more commonplace.

8 Gradually, as microscopes improved, scientists began to realize that a cell is not empty but is filled minute structures. Although it had been recognized earlier, Robert Brown described the nucleus in 1833 as a special structure found in cells. His and other studies attached significance to the role of the nucleus during cellular division and ti the nuclei of egg and sperm cells. As more and more scientists studied and shared their findings with other scientists, they recognized the contents of the cell as the primary substance of living organisms. In 1840, J. E. Purkinje (POOR kin yay) first used the term protoplasm to refer to the entire contents of cells.

9 In 1840, J. E. Purkinje (POOR kin yay) first used the term protoplasm to refer to the entire contents of cells. The year 1838 brought a startling and daring statement from a German botanist, Matthais Schledin (SHLY den). After studying much botanical material under a microscope, he stated that all plants are composed of cells. The next year Theodor Schwann (SHVAHN), a German zoologist, made a similar statement about animals. In 1855, Rudolf Virchow proposed that cells arise only from preexisting cells. The observations of these scientists have been combined to form what is called the cell theory.

10 Continued research over the years by many scientists using more and more technically advanced instruments confirmed the validity of this theory, which is composed of three basic principles: -The cell is the basic unit of all living things. -Cells perform all the functions of living things. -Cells come from the reproduction of existing cells.

11 How Cell Theory Came to be v=4opbylwh9du

12 The Cell as a Basic Unit: The size of most organisms does not indicate the size of its cells but rather the number of its cells. For example, although a mouse and an elephant have cells of about the same size, their quantity of cells is different. *** Not all of an organism is made of cells. Those parts of an organism that are not composed of cells are made of materials that cells manufacture. An insect s body covering is a secretion of some of the insect s cells. It is also interesting that many of the materials composing bones are cellular secretions.

13 The Cell as the Function Unit Cells are responsible for all the functions of any living thing. Because the functions are so various and overlapping, it is impossible to easily describe them. The cellular functions given in Table 3A - 1 are more a list of the processes of life than a list of what cells do. Not all cells perform all these functions, but in order for an organism to be alive, all of these functions must be carried out. In complex organisms like human beings, some cells perform a few of these functions almost to the exclusion of other functions.

14 Table 3A - 1 Table 3A - 1 Cellular Functions and Processes Nutrition Absorption Transport dissolved substances into cells to serve as building blocks or energy sources Digestion Enzymatic breakdown of substances to obtain building materials or energy Internal functions Synthesis Integration of organic compounds from smaller units obtained from digestion, absorption, or some other synthesis reaction in the cell. Synthesis results in a cellʼs growth, secretion, or replacing worn out cellular parts. Respiration Break down of food (usually glucose) with the release of energy Movement Movement of the cell itself (locomotion) or movement of substances and structures inside the cell (internal movement) Irritability Ability to respond to external factors that affect the operations of the cell; in other words, the response or reaction of the cell to its environment Materials release Excretion Egestion Removal of soluble waste from the cell Elimination of nonsoluble waste from the cell Secretion Synthesis and release of substances from the cell Continuation of existence Homeostasis Reproduction Ability to maintain a steady state in the cell Formation of new cells

15 The Cell as the Reproductive Unit For most cells, the biosynthesis of materials results in maintenance and growth of the cell. Put in plain english... The science cell that grew so large it swallowed New York, is not possible. (has anyone ever heard of that?) When most cells reach a particular size, they either slow down their synthesis of materials, secrete certain synthesized materials, or divide. The built-in control of a cell (the DNA) can control only so much; therefore, cell division after a limited amount of growth is essential for most cells. Cells reproduce by dividing. Cellular division results in a new organism (if the organism is unicellular), more cells in the same organism, or a sex cell, like an egg or sperm. Processes of cellular reproduction are discussed in Chapter 5.

16 Cell Size: How large can a single cell grow? The maximum size a cell can attain is limited be several factors. First, the message encoded in the DNA in the nucleus governs the size of the cell. Another factor is the ratio between the surface area of the cell and its volume.

17 The nutrients a cell requires must pass from the outside of the cell, through the membrane, and to the inside. As the cell grows, its volume increases faster than does the surface area; at a certain size the cell will need more nutrients than can pass through the membrane. The table below compares the volume and surface area for hypothetical cuboidal cells of different sizes.

18 Ratios of Surface Area to Volume Side length Surface area (l x w x 6 sides) Volume (l x w x h) Surface area/ volume ratio 1 mm 2 6mm 3 1mm 6 : 1 2mm 2 24mm 3 8mm 3 : 1 3mm 2 54mm 3 27mm 2 : 1

19 Comparing the Size

Discovery of the Cell

Cell Structure Discovery of the Cell Who discovered cells? 1665 Robert Hooke used a compound microscope to examine a piece of cork (20X magnification) He saw little boxes in the cork and called them cells