Unit 5, Part 1: Cytology

Objectives:• Describe the accomplishments of Hooke, Leeuwenhoek, Schleiden, Schwann,

and Virchow.• Discuss the cell theory.• Distinguish between prokaryotic and eukaryotic cells.• Describe the purpose and function of each organelle.• Compare and contrast plant and animal cells.• Explain the importance of proteins & glycoproteins on the cell membrane• Explain the huge significance of mitochondria.

Vocabulary:Ribosomes * Golgi Body/Apparatus * Nucleus * Mitochondria * flagella *Endoplasmic Reticulum (smooth and rough) * Prokaryote * Eukaryote *Cell Membrane * Cell Wall * Spontaneous Generation * Cell Theory *Bacteria * Nucleolus * Lysosomes * Microtubules/filaments * vacuoles *Cytoplasmic streaming * nuclear pore * nucleoplasm * nuclear envelope tRNA * mRNA * rRNA * nucleolus * enzymes * cytoplasm * cilia *peroxisomes * cell membrane * hydrophobic * hydrophilic * respiration *spindle fibers * microtubules/fibers * vacuoles * chloroplasts * plastids *chromoplasts * leukoplasts * ATP * endosymbiosis * cellulose * pectin

Robert Hooke was the first to see the dead, therefore empty, cells of cork through his microscope. He referred to them as cells because they reminded him of the rather empty “cells” monks retreated to. Antoine van Leeuwenhoek built microscopes and was the first to notice “animalcules”, or bacteria in the tartar scraped from his teeth, though he had no idea what they were.

Matthias Schleiden viewed many plants and declared that the basic unit of plant life was the cell. Shortly thereafter, Theodore Schwann declared that all animals are composed of cells. The cell theory combined these statements with Rudolph Virchow’s declaration that all cells come from pre-existing cells. This was in direct opposition to the earlier belief of spontaneous generation, life being able to generate from non-living things.

The Cell Theory:

• All living things are composed of 1 or more cells.

• Cells are the basic structural and functional units of life.

• All cells come from pre-existing cells.

There are 2 to 3 basic types of cells. Prokaryotes (bacteria) and eukaryotes have long been accepted as the 2 distinct cell types. But recently, a third category of cell (Archaean cells) has gained recognition. These have some similarities to both prokaryotes and eukaryotes.

Characteristics of Prokaryotes:• lack a true nucleus and membrane bound organelles• size is between 1 and 10 micrometers

ONLY BACTERIA ARE PROKARYOTIC

Characteristics of Eukaryotes:• have a true membrane bound nucleus and organelles• size is between 10 and 100 micrometers

The third cell type, the Archaean cells are similar to prokaryotic cells in size but have some characteristics similar to eukaryotic cells and some characteristics found in neither prokaryotes or eukaryotes.

Animals, plants, fungi, everything EXCEPT bacteria and archaean cells, are eukaryotes. Organelles within eukaryotic cells are like separate compartments due to membranes surrounding the organelles.

Organelles include:

1) The nucleus is the “control center” of the cell. The chromatin within the nucleus contains the gene sequences of DNA (deoxyribonucleic acid) which direct the cell as to which proteins to make. In order to get the directions from the nucleus to the ribosomes, where proteins are made, mRNA (messenger ribonucleic acid) must be transcribed from DNA and then travel to the ribosomes to be translated to make proteins. Within the nucleus is the nucleolus. This is where ribosome parts are made and partially assembled before export to the cytoplasm. The nucleus is filled with a gel-like nucleoplasm and is surrounded by a nuclear envelope. This membrane is a lipid bi-layer that has nuclear pores which act as channels for proteins to enter the nuclear area and RNA to leave.

The area outside the nucleus is bathed by the gel-like cytoplasm which is constantly moving in an effort to nourish the organelles and carry away their wastes. This movement is called cytoplasmic streaming.

2) The Endoplasmic Reticulum (“endo” = inside, “reticulum” = network, so it’s the network inside the “plasm” of the cell) folds back and forth extending from the nuclear envelope to the cell membrane. The “ER” appears rough near the nucleus because it has numerous ribosomes attached to it. These ribosomes make proteins that will be exported out of the cell or become part of the cell’s membrane. Further away from the nucleus, the ER becomes smooth. Smooth ER synthesizes fatty acids and many membrane components. It contain enzymes used in detoxification. It acts as a pathway for the proteins synthesized by the rough ER. At the end of the smooth ER pathway, proteins and fatty acid lipids are put in vesicles to be carried to the Golgi apparatus.3) The Golgi apparatus, or Golgi bodies, process and finish folding the proteins made by the rough ER. It also makes simple carbohydrates and attaches them to proteins or lipids to make glycoproteins or glycolipids. Those substances for cell export will be put in vesicles that will travel to the cell membrane. Some proteins will be sent to the lysosomes.

4) Lysosomes (“lyse” means to cut apart) contain enzymes (special

proteins) important for killing bacteria, worn-out cell organelles, and other

debris. Enzymes from lysosomes are also responsible for dissolving the

tissues at the finger and toe buds so that fingers and toes can grow. In

addition, these enzymes can break down fats, carbohydrates, and proteins into

forms usable at the cellular level. The enzymes within the lysosomes are made

by the ribosomes on the rough ER and then sent to the lysosomes by the Golgi

bodies.

Enzymes from lysosomes can only function under acidic (low pH)

conditions. Certain cells, like white blood cells, have more lysosomes because

their enzymes are needed to break apart bacteria and debris. Liver cells also

have numerous lysosomes to break down cholesterol and toxins.

There are more than 40 enzymes in lysosomes. If one is missing, it can

lead to a “storage disease” where a body product is not broken down or

removed properly. With Pompe’s disease, glycogen in the liver can not be

broken down so the liver becomes damaged. In Tay-Sachs disease, the brain

is damaged as fats begin to accumulate due to a missing lysosomal enzyme.

5) The ribosomes are the most numerous of all of a cell’s organelles. This is where proteins are made. Ribosomes attached to the rough ER make proteins that will be exported by the cell for use elsewhere. Ribosomes loose in the cytoplasm make proteins that will stay in the cell. Ribosomes are made of protein and ribosomal RNA (rRNA). When proteins are being made, transfer RNA (tRNA) carries the amino acids to the ribosome to add to the chain.6) Microfilaments and microtubules are long, slender protein structures that help maintain the shape of a cell by creating the cytoskeleton. Spindle fibers are specialized microtubules that help chromosomes move during cell division. A weak or broken spindle fiber may cause a birth defect, such as trisomy 21, because sister chromatids may not be separated to opposite areas in the egg or, possibly, the sperm cell during meiosis. 7) Peroxisomes help protect cells from toxins. They use catalase to convert H2O2 to water. Liver and kidney cells contain a lot of these organelles to help disarm toxins in the blood. In adrenoleukodystrophy (ALD) - the disease in “Lorenzo’s Oil” - there is a defect in one of the proteins in the peroxisome’s membrane. This protein normally helps transport an enzyme that breaks down a very long chain fatty acid. Without this protein, the fatty acid builds up in the brain and spinal cord. This causes behavioral problems, loss of muscle strength/control, and death if not treated correctly.

8) The cell membrane is a lipid bi-layer. The carboxyl and phosphate, “head” end of the 2 lipid layers point away from each other, forming both the inner and outer portion of the membrane. Therefore, the innermost and outermost portion of the cell membrane is hydrophilic (water loving) because it is polar. The hydrocarbon chains, or “tails” of the lipids are turned inward, sandwiched between the carboxyl heads. The tails are hydrophobic (fear water) because they are nonpolar. The cell membrane also has numerous proteins imbedded along its length. Some proteins act as channels for transport of substances into and out of the cell. Some glycoproteins help mark the cell as “self” so that it can be recognized from foreign substances and will not be attacked by the immune system. These glycoproteins are also important for distinguishing between blood types: A, B, AB, and O and for letting the body know that this is a “bone” cell or a “skin” cell, etc..9) Cilia are short, numerous, hair-like structures that extend out from the cell membrane of some cells. For example, cells of our respiratory tract. The cilia help mucus and irritants get coughed out of the body. Unicellular organisms, such as paramecium use cilia for movement.

10) Flagella are longer whip-like structures found on some cells, usually in small numbers, used for movement by sperm cells and some unicellular organisms like Euglena,Trypanosomes, Giardia, Trichomonas, etc.

11) The mitochondria are the cell’s energy providers. Here, energy from the foods we eat is converted into a form of energy our cells can use, that is ATP (adenosine triphosphate). This process is called respiration. This is probably the single most important process going on at the cellular level! Cells requiring more than usual amounts of energy, for example, the liver for detoxification purposes and the muscles for contraction/strength, have far more mitochondria than other cells. The mitochondria also have their own DNA! ***This DNA is inherited 100% from the mother!*** Very few structures besides the nucleus ever contain their own DNA. In plants, the best example of an organelle containing DNA, other than the nucleus, would be the chloroplasts.Some scientists believe more advanced eukaryotic cells were created when one prokaryote ingested another, but instead of digesting it, a symbiotic relationship occurred where the ingested prokaryote started producing energy for its host. This is how eukaryotic organelles like mitochondria and chloroplasts arose. This evolution from prokaryote to eukaryote is called endosymbiosis.

Plant cells have several organelles not usually found in animals. They include:

12) A cell wall. The cell wall is made of a long chain polysaccharide called

cellulose. This is strengthened by pectin and lignin. Since these are not

Digestible by humans, we consider them “fiber” or roughage which helps clean

out the digestive tract. Pectin, usually from apples or other fruit, is used to set

jelly, for example. The cell wall surrounds the cell membrane, adding shape

and support to the cell structure. Primary cell walls form in young growth.

Wood is formed when primary cells stop growing and lignin fills in, thus forming

secondary cells which can no longer grow. So wood is made of secondary cell

walls.

13) Vacuoles in plants store enzymes, toxins, and wastes. Usually, the older

the cell, the larger the vacuole.

14) Plastids are primarily storage and photosynthesis areas. Chloroplasts

are green from chlorophyll, the primary plant pigment that traps sunlight for

photosynthesis. Chromoplasts make and store secondary plant pigments

(orange = carotenes, yellow = xanthophylls, etc.) used to catch different light

wavelengths and leukoplasts store starch as well as proteins and lipids. (Think

potatoes)