Cytology

Discussion Unit 2: Cytology 

I. Cells

Three unifying theories of biology - Cell theory genetic theory, theory of evolution.

Form the foundation of biology. Do not exist or function independently, rather interact with + influence one another, allowing life to exist

Cell Theory - a three component theory (developed between 1839 and 1858)

1) All living organisms are composed of one or more cells

2) Cells are the fundamental unit of structure, function, and organization in living organisms

3) Cells arise only from pre-existing cells



Current (modern) interpretations of cell theory include three additional components:

4) cells contain genetic material DNA and RNA that is passed to new generations of cells through cell division

5) all cells essentially same in chemical composition

6) metabolism (transfer of energy from chemical bonds of molecules into form that cells can use, ATP) and biochemistry (chemical reactions that sustain life) utilized by living organisms and occur only within cells



B. Cell Function: Cells are basic unit of life and smallest unit capable of sustaining life. Life depends upon chemical reactions which occur in fluid portion of the cell called cytosol. The cytosol is contained within the cell, providing environment necessary for these reactions to occur

  1. Cell Size: Cell microscopic, typically ranging from 1-100 micrometers in diameter. Human cells generally ranging from 10-100 micrometers. Cell size limited by amount of material that can enter + leave the cell. The rate at which substances diffuse is affected by several variables, one of which is surface area. As a cell increases in size, its volume increases much more rapidly than its surface area. If a cell doubles in diameter:

  • Its surface area increases by a factor of 4 ( it is squared).

  • Its volume increases by a factor of 8 (it is cubed).


As cells become larger, more chemical reactions are required to maintain cellular function. These reactions require nutrients and oxygen to enter the cell whuile metabolic waste must leave the cell. Eventually, a cell reaches a size at which its surface area is not longer sufficient to transport enough materials into the cell to sustain these chemical reactions. If sufficient nutrients cannot enter, the cell dies. Likewise, if metabolic wastes cannot leave the cell rapidly emoguh, they accumulate and poison the cell. Therefore, cells must remain relatively small to function efficiently and divide (replicate) to maintain an optimal surface area-to-volume ratio.



C. Types of Cells 

1. Prokaryotes : Means “before nucleus”. These cells lack a nucleus and membrane-bound organelles. They possess ribosomes (for protein synthesis) that are smaller and structurally different from those of eukaryotes. Their DNA consists of double helix that floats in cytoplasm (a region called nucleoid) as single circular chromosome. These cells very small (1-15 micrometers) + found only in Domains Bacteria and Archaea. All prokaryotes are unicellular, although many are colonial, giving the appearance of multicellularity.



  1. Domain Archaea - one of the three domains of life. This domain contains only prokaryotic cells + differs from Domain Bacteria in four ways: 

  • Lacks a peptidoglycan cell wall (its cell wall is composed primarily of lipids)

  • Contains introns (doesn’t code for protein, new chapter, segmentation of a gene)

  •  Utilizes methionine as the initiator amino acid for protein synthesis

  • Utilizes histones for DNA organization and possesses  singular circular chromosome

  1. Domain Bacteria - one of the three domains of life. This prokaryotic domain differs from domain archaea in four major ways:

  • Possesses a peptidoglycan (interwoven peptides and carbohydrates) cell wall

  • Uses formyl-methionine as the initiating amino acid for protein synthesis

  • Lacks introns in its genes

  • Does not utilize histones for DNA organization and possesses a single circular chromosome



2. Eukaryotes: means “true nucleus” (or good nucleus). These cells possess a nucleus (where the DNA is enclosed) and membrane-bound organelles that perform specific functions. These organelles allow eukaryotic cells to function in much more complex manner than prokaryotes. Eukaryotic cells are found in the Domain Eukarya. It is hypothesized that Domain Eukarya results from a symbiotic relationship between two prokaryotic organisms (Endosymbiotic Theory).

 
3.  Endosymbiotic Theory: explains how eukaryotes evolved from prokaryotes. Theory proposes that eukaryotes evolved from prokaryotes that developed symbiotic relationships. It is thought that larger prokaryotes engulfed a smaller prokaryote. The engulfed prokaryote benefited larger cells (e.g. producing ATP or converting sunlight into carbohydrates), while the larger cells benefited the smaller one by providing protection. Eventually, over hundreds of millions of years, the two organisms began to behave, reproduce, and function as single organism, ultimately evolving into eukaryotic cells. This process gave rise to certain organelles found in modern eukaryotic cells.



II. Eukaryotic Cell Structure

Eukaryotic cells are composed of numerous structure, some of which are membrane-bound while others are not 

  1. Plasma (cell) membrane: Separates cell from its environment, contains cytoplasm, and regulates what enters and exits the cell. Composed of a phospholipid bilayer. Both the extracellular environment and the cytoplasm (intraclelular fluid) contain a high concentration of water. Each phospholipid contains of a polar (hydrophilic) head attached to two nonpolar (hydrophobic) fatty acid tails. The phospholipids arrange themselves into two layers (a bilayer), with the hydrophobic tails facing one another and the hydrophilic heads facing the aqueous environment inside and outside the cell. Because the fatty acid taisls are nonpolar (hydrophobic), large polar molecules and ions cannot pass directly through the membrane. The phospholipid bilayer is permeable primarily to small uncharged molecules (ex co2 and o2) and larger hydrophobic molecules.

1. Phospholipids: Composed of a glycerol backbone attached to a phosphate group (containing phosphorus), collectively forming the polar head, with two fatty acid tails attached. Because the head is polar, it attracts other polar molecules (such as water) and is therefore hydrophilic. The fatty acid tail are nonpolar, hydrophobic, and interact primarily with other lipids. Molecules possessing both polar and nonpolar regions are called amphiphatic. Phospholipids form the bilayers that make up all cellular membranes.

2. Membrane Proteins: within the phospholipid bilayer are proteins, membrane proteins perform specific functions

- integral proteins are embedded within the membrane + function as channels, carriers, pumps, receptors, or enzymes

-peripheral proteins are attached to inner or outer surface of the membrane + function as enzymes or glycoproteins (proteins attached to carbohydrates) involved in cellular recognition

3. Solutions: A solution is a mixture (usually aqueous) of two or more substances

  1. Solvent: the dissolving agent, usually a liquid, most commonly water. 

ex) In seawater, the solvent is water

  1. Solute: the substance being dissolved. 

ex) in seawater, the solute is salt.

  1. Hypotonic: a solution possessing a greater concentration of solute than another solution. 

ex) seawater is more hypertonic than pure water

  1. Hypotonic: a solution possessing a lower concentration of solute than another solution. 

ex) pure water is more hypotonic than sea water

4. Transport Across the Plasma Membrane: there are several mechanisms by which substances substances cross the plasma membrane

  1. Diffusion: the movement of molecules from regions of higher concentration to regions of lower concentration. Diffusion can occur directly across the plasma membrane if the molecules are capabel of passing through it.


  1. Osmosis: The diffusion of water (h2o) across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration. 

  2. Facilitated Diffusion: molecules that are too large, polar, or electrically charged to pass directly through the membrane move from areas ofo higher concentration to lower concentration with the assistant of specific transport proteins. This process does not require energy (ATP). (carbohydrates, water, ions)

  3. Active Transport: The movement (pumping) of substances from areas of lower concentration to area of higher concentration through specific membrane proteins (pumps). Requires cellular energy (ATP).

B. Cell Wall: found in plant cells, fungal cells, some protists. Surrounds plasma membrane. Composed primarily of cellulose in plants and chitin in fungi. Provides rigidity, protection, and structural support. Contains pores that allow water, gases (co2 and o2) and certain dissolved substances to pass through


C.Nucleus: surrounded by a double phospholipid bilayer called the nuclear envelope (nuclear membrane).. Contains the cells genetic material in the form of chromatin (DNA wrapped around histone proteins). The nucleolus is a dense region composed primarily of rRNA and proteins where ribosomes are assembled. Controls protein production by regulating gene expression. Nuclear pores allow mRNA to leave the nucleus while DNA remains inside. 


D. Cytoplasm: the region of cell located between plasma membrane and the nucleus. Consists of cytosol and all cellular organelles. 


  1. Cytosol: a fluid matrix composed primarily of water, dissolved ions, and proteins (especially enzymes) the site of many cell’s metabolic reactions. (biological catalyst = enzyme) 

E. Chloroplasts: found only in plants and algae. Believed to have originated through endosymbiosis. Surrounded by double membrane and contain chlorophyll. Possess their own DNA (cpDNA). Which resembles bacterial dna. Site of photosynthesis, where light energy is coveted into chemical energy stores in carbohydrates.


F. Mitochondria: found in nearly all eukaryotic cells. Believed to have originated through endosymbiosis, posses their own dna (mtDNA), which resembles bacterial DNA. Site of aerobic cellular respiration, where energy stores in food molecules is converted into ATP.


G. Ribosomes: found in all cells (prokaryotic and eukaryotic). The smallest cellular structures and lack membrane. Composed of ribsomal RNA (rRNA) and ribosomal proteins. Site of protein synthesis, where amino acids are assembled acccording to the genetic instructions carried by the mRNA. May be free within the cytosol or attached to rough endoplasmic reticulum.




H. Endoplasmic Reticulum: a network of flattened, interconnected membranous channels extending from the nucleus throughout the cytoplasm. Function in intracellular transport and provides surfaces for many chemical reactions. Two types: 


  1. Smooth Endoplasmic Reticulum - lacks ribosomes. Site of lipid, phospholipid, cholesterol, fatty acid synthesis

  2. Rough Endoplasmic Reticulum - studded with ribosomes. Site where proteins destined for secretion (extracellular purposes) or membrane insertion are synthesized before transported to the golgi apparatus

I. Golgi apparatus: Stacks of flattened membranous sacs resembling a stack of pancakes. Modifies, sorts, packages, and distributes cellular products in vesicles.


J. Lysosomes: membrane bound vesicles produced by the golgi apparatus. Remain within cell. Contain hydrolytic (digestvie) eynzymes that break down worn-out organelles, damaged cellular components, and foreign material.

K. Peroxisomes: small membrane-bound vesicles produced by the nedoplasmis reticulum. Contain enxymes that detoxify harmful substances by oxidation and reduction erections. Partically important in detoxifying alcohl and other toxic compounds.

L. Vacuoles: membrane-bound vesicles that generally remain within cell. Especially prominent implant cells. Functions in the storage of water, carbohydrates, proteins, pigments, and other substances.

M. Cytoskeleton: a network of protein filaments visible within an electron microscope. Provides structural support, maintains cell shape, and enables cellular movement. Composed primarily of microtubules and microfilaments.

  1. Microtubules: Thick protein filaments composed of tubulin. Maintain cell shape and provide structural suppprort.

    1. Centrosome: region of the cell that organizes microtubules and assembles centioles. Centrioles participate in chromosome separation during cell division.

  2. Microfilaments: thin protein filaments composed of actin. Function primarily in cell movement and changes in cell shape (centriole is what pulls chromosomes to pole of each cell)

N. Structure of Cell Locomotion: Some human cells possess structures (cilia and flagella) that either move the cell itself or move substances across the cell surface.

  1. Cilia: Short hair-like projections extending from the plasma membrane. Contain a 9+2 arrangement of microtubules (nine pairs surrounding one central pair). Move substances across the surfaces of cell.

  2. Flagella: Similar in structure to cilia but much longer. Fcunction primarily in cell movement. In humans, the only flagellated cell is the sperm cell.

  3. Differences between Prokaryoiic and Eukaryotic Flagella: Prokaryotic flagella lack microtubules. They are composed of protein flagellin and move by rotary motion rather  than by bending