Cells and the Cell Theory
Cells and the Cell Theory
January 13, 2000
Readings: Starr text: Ch 4 front page, 4.1, 4.3-4.9, 4.11-4.12
"Little eels, or worms, lying all huddled up together and wriggily, and the whole water seemed to be alive with these animalcules and cells fill'd with juices."
-- Anton van Leeuwenhoek's observations of a drop of pond water, as he stared through the eyepiece of his newly built microscope in 1673
Outline
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I . The cell is the basic unit of life:
The discovery of the cell was made possible by the invention of the microscope (1600s) As microscopes improved, scientists observed that all living organisms were made up of cells ("little rooms")
What defines a cell? A cell
1. Is the lowest level of structure capable of performing all 7 activities of life
2. Is bounded by a plasma membrane "envelope"
3. Contains cytoplasm, DNA, enzymes, organelles
II. The Cell Theory (Matthias Schleiden and Theodor Schwann 1839) states that
(1) The cell is the smallest unit that retains a capacity for independent life
(1) All organisms are composed of one or more cells
(3) All cells arise from preexisting cells.There are, at present, no exceptions to this Theory. (We will discuss next week more specific about Theories)
III. How big are cells?
Our eyes can see things as small as ~0.1 mm. There are a few cells that can be seen without the aid of a microscope, but, a general rule is that most eukaryotic cells are ~ 0.1 mm (100 microns) to 0.005 mm (5 microns) long, bacterial cells are much smaller (~ 3 microns or less). Viruses are even smaller (nano-meters!)
Refer to Figure 4.5, or check out How Big are They?
Why aren't cells bigger? Surface -to- volume ratio.
IV. What do cells do?
Cells perform all of the 7 activities of life! They also eat, swim, beat, reproduce, protect your body, kill other cells, infect your body, etc...(note these are all movies and may not work well with a home computer)
V. There are 2 major types of cells: Eukaryotic and Prokaryotic.
Eukaryotic cells: (Kingdoms: protists, plants, fungi and animals) Figures 4.7 - 4.9 in text
1. Subdivided by internal membranes into different compartments
2. DNA is enclosed by a membrane-bound nucleus
3. DNA organized into chromosomes
4. Cytoplasm surrounds the nucleus and organelles
5. Plant cells have a tough cell wall, animal cells do not.
Prokaryotic cells: bacteria and blue-green algae (Kingdom: Monera) Figure 20.2 in text
1. Smaller than eukaryotic cells
2. Lack organelles
3. Lack a nucleus - DNA is not separated from the cytoplasm
4. One circular chromosome
5. Tough external walls
VI. Cells - and whats inside... - Figures 4.3 and 4.4 are particularly helpful
Plasma Membrane: a phospholipid bilayer studded with proteins
. Lipids have a hydrophilic head and a hydrophobic tails that, in water, spontaneously form into a bilayer.
Function of the lipids: Keeps inside in and outside out!
Fig. 4.3 (Left)
Fig. 4.4 (Below) A cutaway view of the plasma membrane illustrating the Fluid-mosaic model of plasma membrane structure
Cytoplasm semifluid "soup" of proteins, enzymes, dissolved salts, sugars
Nucleus contains chromatin, a combination of DNA (genes) and associated proteins, floating in a liquid nucleoplasm, surrounded by nuclear envelope (another lipid bilayer). The nucleolus is an area where synthesis of ribosomal genes takes place
Ribosomes small structures in the cytoplasm made of RNA and protein that assemble protein chains. Can be "free" in the cytoplasm or bound to the ER
Rough Endoplasmic Reticulum (RER) sorts and modifies protein chains delivered by bound ribosomesSmooth Endoplasmic Reticulum (SER) lacks ribosomes. Site of lipid (membrane) synthesis
Golgi Body connects with the SER, completes lipid synthesis and sorts proteins to their correct destination in small vesicles
Vesicles transport proteins and lipids to the cell surface; bring proteins and lipids into cell from the cell surface, digest compounds (in lysosomes)Lysosomes: intracellular digestion - contain a potent brew of digestive enzymes
Peroxisomes: break down fatty acids, amino acids, and alcohol
Mitochondria: double membrane-bound organelles that make ATP for cellular energy (not just one lipid bilayer, but TWO)
Cytoskeleton (microtubles and microfilaments) provide cell shape, internal "skeleton" and cell movement
Centioles made of microtubules, assist in cell division
For a look at some more "way cool" cells, check
out the Cells Alive webpage.
Objectives for Chapter 3: 1. Explain the basis of the Cell Theory 2. Identify the essential structures within a eukaryotic cell and describe a function for each. 3. Compare and contrast a prokaryotic and a eukaryotic cell |
