The Cell
 
The Microscope: Two factors play an important role in microspy: magnification and resolving power. Magnification is the comparison of the real size of a specimen with that of the one viewed under the microscope. Resolution refers to the clarity of the specimen viewed under the scope.
Light microscopes have a maximum resolving power of 1500X . The wave length of light is 0.2um. This property of light makes it impossible to go beyond this magnification. Electron microscopes have shorter wave lengths 0.2nm and can magnify an object several 100,000 times.
 
 
 

 Compound Light Microscope
 
Electron Microscopes: The 2 types of electron microscopes are the Transmission electron microscope or TEM and the Scanning electron microscope or SEM.
 
1. TEM: Thin sections of a specimen are coated with a heavy metal. A beam of electrons are shot at the specimen. This beam is guided by electromagnets. A negative picture is produced. This microscope is used to detail the inner ultra structures of a cell.
 
2. SEM: Surface of a specimen is coated with gold. This produces a 3-D picture of the surface of the specimen being observed.
  
 
Cell Fractionation: This a process of taking a cell apart, separating the major organelles so that their functions can be studied. Cells are broken up to help release their contents. The contents are spun in a machine called a centrifuge separate the parts. The heavier parts sink to the bottom and form a pellet. The liquid left over is called the supernatant. This is then transferred to another test tube and respun at a higher rate. This will continue until several pellets have been formed of decreasing size.
 
Prokaryotic and Eukaryotic Cells:
 
Prokaryotic
Eukaryotic

1. No nuclear membrane ( nucleoid region)

1. Definite membrane bound nucleus

2. No membrane bound organelles

2. Contains membrane bound organelles

3. Found only in The kingdoms Eubacteria and Archaebacteria

3. Found in Kingdoms: Protista, Animal, Fungi, and Plantae

4. Size 0.1 um - 10 um.

4. 10 um-100 um.

 
Cell Size: is governed by several factors: The size of the cell is controlled by metabolic requirements. DNA must be available to produce the enzymes and proteins needed for proper functioning. A too small cell will not have enough DNA to support life and a cell too large will need an enormous amount of DNA to carry on its functions. A second restriction involves surface area to volume ratio. As the cell increases in size, the volume increases geometrically while the surface area increases arithmetically. Eukaryotic cells cope with these problems in that they contain membrane bound organelles. These organelles break up the volume of the cell performing distinct functions which cuts down on the raw materials needed. Each part of the cell does not need the same material to function.
 
Cellular Organelles:
The Nucleus: contains the genes of the cell. Its size is about 5um. in diameter. It is surrounded by a lipid bilayer perforated with pores. There is a 20-40 nm. space between the layers of the membrane. Chromosomes are rod shaped structures containing DNA and protein. Chromosomes are usually broken down into chromatin. The Nucleolus is also found in the nucleus. It produces ribosomes at a rate of 10,000 / min. The nucleus controls the cell's functioning through the production of m-RNA.
 
 
Ribosomes: functions in protein synthesis. They are not considered a membrane bound organelle. Ribosomes are built from 2 subunits. Its only when these subunits are properly placed together that the ribosome becomes functional. The large subunit is composed of 45 proteins and 3 molecules of RNA while the small subunit consists of 33 proteins molecules and 1 RNA. Fully assembled it measures 20-30 nm. Sometime ribosomes work in units called polysomes.
There are 2 types of ribosomes: bound and free. The bound are found attached to the ER, while the free ones float around in the cytosol. The proteins made by the free will usually stay within the cell, while the bound are destined to become part of the membrane system or exported from the cell.
 
Endoplasmic Reticulum: consists of a network of membranous tubules and sacs called cisternae. The ER is continuous with the nuclear membrane. There are 2 types of ER: smooth and rough. Smooth does not contain ribosomes where the rough form does.
 
Smooth ER: Functions: synthesis of lipids, carbohydrate metabolism, and detoxification of drugs and poisons. It also helps in muscle contraction by regulating the calcium ions in the muscle.
 
 

 

Rough ER: Antibody production occurs in the rough ER. Many secretory proteins are also produced here. These proteins are glycoproteins, proteins and carbohydrates combined. The carbohydrate is called an oligosaccharide. These secretory proteins are bound in special structures called transport vesicles. The Rough ER is involved with membrane production.
How does a ribosome know if it will be free or bound? Signal sequence( 20 amino acids found at the start of a protein being coded by the ribosome alerts the ribosome to attach itself to the ER. If the sequence is missing it will remain free.
 
Golgi Apparatus: a series of flattened membranous sacs, stacked like pita bread. Each stack is called a dictyosome. The functions are : synthesis, storing, sorting and shipping of materials. The dictysomes have 2 distinct faces: the cis face receives the material and the trans face ships materials. The cis face is close to the ER. The trans face gives rise to vesicles full of material and pinch off.
 
Lysosomes: a bag of hydrolytic enzymes. pH of 5 is optimum and is maintained by the continuous pumping of H ions into the vesicle. They are produced by the trans face of the Golgi Apparatus. Their functions include, intracellular digestion, autophagy, and programmed destruction. In humans Tay-Sachs disease is caused by a lack of lipid digesting enzymes and an accumulation of lipids in the brain. Pompe's disease deals with carbohydrate enzymes and ultimate liver damage.
 
Microbodies: are single membrane bound enzyme filled structures able to function in a particular metabolic pathway. There are 2 types: Peroxisomes and glyoxysomes. Peroxisomes detoxifies alcohol and other poisons by producing hydrogen peroxide by transferring a Hydrogen from the poison to water. Glyoxysomes are usually found in fat storing tissue of seeds and plants.
 
Vacuoles: Sacs that are larger than vesicles having functions in digestion, pumping excess material, storage, cell stabilizer, and are found in many protists and plants.
 
Mitochondria: converts energy to forms the cell can use. Site of cellular respiration. 1-10um. in length.
It is a double membrane structure. The outer membrane is smooth while the inner is folded, these folds are called cristae. The area between the 2 membranes is called the inner membrane space. The inner most area of the mitochondrion is called the mitochondrial matrix.
 
Plastids: a specialized member of a family of closely related plant organelles called plastids.
Amyloplasts store starch, Chromoplasts contain pigments and Chloroplasts contain chlorophyll.
 
Chloroplast measure 2um- 5um in diameter. It is a double membrane structure. The outer membrane is smooth wile the inner membrane takes the form of stacks of coins called the thylakoid membrane. The thylakoid membrane is divided into coin like structures called grana. The area between the thylakoid and outer membrane is called the stroma.
 


 
Cytoskeleton: contains microtubules and microfilaments. Microtubules: straight hollow rods measuring about 25 nm. thick . They are constructed from a and b tubulin. They shape, support and help move organelles around the cell. They form centrioles, which are not found in plants. They have a "9 + 2 " pattern. Microfilaments: are constructed of 2 interwoven pieces of actin about 7nm. in diameter. They function in muscle contraction, cytoplasmic streaming, ameboid movement, and changes in cell shape.
 
Cell Wall: From the cell membrane out the cell wall is as follows: Primary wall, secondary wall, and a sticky area between adjacent cells called the middle lamella. In animal cells a fuzzy coat called the glycocalyx is found. This is made of sticky oligosaccharides that act as glue to keep the cells together.
 
Intercellular Junctions:
 
1. Tight junctions. bind cells together in such a way that no material can pass through the intercellular spaces. Epithelial cells are held together by tight junctions.
 
2. Desmosomes: These bind the cells together like rivets. They let material pass through the intracellular spaces.
 
3. Gap Junctions: They connect cells but allow material to pass from one cell to another through the opening in the center of the joint. They are analogous to the plasmodesmata in plants.