Organization of the Cell

Robert Hooke -- 1665
1. Coined the term cell
2. Studied cork and described the cell walls

Cytology
1. Study of cell structure and function, including the tools for study, i.e.., microscopes, centrifuges, etc.

Cell Theory - A Review
1. 1838: Matthias Schleiden (zoologist) and Theodor Schwann (botanists) proposed the theory.
2. 1855: Rudolph Virchow (physician) modified the theory.
3. Three Tenets of Cell Theory:
        - Cells are the basic units of life
        - All living organisms are composed of cells
        - Cells arise from previously existing cells (not by spontaneous generation)

August Weismann (1880) added a corollary to cell theory:
1. The ancestry of all cells today can be traced back to ancient times. Thus, all cells have a common origin (evolution). This helps to explains the basic similarities in cell structure and molecular make-up of all organisms.

Cell
1. Basic structural and functional unit of life.
2. Defined as a mass of protoplasm surrounded by a semipermeable membrane.
3. Protoplasm = cytoplasm + nucleus
4. Cytoplasm = protoplasm - nucleus
5. The cytoplasm is the cell contents with the exception of the nucleus (including the membrane).

Metric Measurements
Meter = m
Millimeter = mm
Micrometer = um
Nanometer = nm

1 m = 1000 mm
1 mm = 10-3 m

1 mm = 1000 um
1 um = 10-3 mm = 10-6 m

1 um = 1000 nm
1 nm = 10-3 um = 10-6 mm = 10-9 m

Cell Size
1. Review (but do not memorize) Figure 4-1, page 75 of the text.

What limits cell size?
1. Ability to move raw materials into the cell.
2. Ability to move wastes out of the cell.
3. Surface area / volume ratio limits cell size.
4. Several models of surface area/volume were examined. See text Figure 4-2, page 76.
5. FOR UNDERSTANDING: Computer Lab CL127. CD-ROM "Explorations in Cell Biology and Genetics," do Topic 2: Cell Size. (This is a CD-interactive exercise.)

Types of Cells
1. Prokaryotic cell: lacks a membrane-bound nucleus.
2. Eukaryotic cell: has a membrane-bound nucleus.
3. Electron micrographs of each cell type was examined.

Eukaryotic Cell Structure

Typical Plant Cell and Typical Animal Cell
1. Both electron micrographs and models of both types of eukaryotic cells were examined.

Plasma Membrane
1. Also called the cell membrane and the plasmalemma in plants.
2. Delineates the cell boundary.
3. Controls the movement of materials into and out of the cell.
4. It is selectively permeable or semipermeable.
5. All cells have one.

Organelles
1. Means "small organ."
2. Subcellular structure in cell that perform specialized functions.
3. They are typically membrane-bound (but not always).
4. Examples include mitochondria, nucleus, ribosomes, lysosomes, Golgi complex, etc.

Cytosol
1. Is the fluid component of the cytoplasm in which the organelles are suspended.

Double Membrane-bound Organelles
1. Includes the nucleus, mitochondria, and plastids.

Nucleus
1. Genetic center of the cell.
2. Flow of genetic information: DNA ---> RNA ---> Protein.

Nuclear Structure
1. Nuclear envelope or membrane, nuclear pores, nucleolus, nucleoplasm or nuclear sap, and chromatin (= DNA + nuclear proteins).
2. TEM micrographs of the nucleus was examined.

Nuclear Pores
1. Several TEM micrographs were examined. A model of nuclear pores was also examined. See text Figure 4-12 for details.
2. Nuclear pores regulate the passage of materials between nucleoplasm and cytoplasm.

Nucleolus
1. Darkly stained
2. Contains the nucleolar organizer or the genes for the synthesis of RNA for ribosomes (rRNA).
3. TEM micrograph of nucleolus was examined.

Chromatin -- Chromosomes
1. Chromatin = DNA + nuclear proteins (such as histones).
2. In dividing cells, chromatin condenses to form the chromosomes. A picture of chromosomes was examined. See text Figure 9-14, page 212 for details.
3. An Interesting Fact: If the DNA in the 46 chromosomes of one human cell could be stretched end to end, it would extend for two meters.

Mitochondria (sing., mitochondrion)
1. Self-replicating organelles (by fission).
2. Contain DNA (about 1% of total DNA in the cell).
3. "Powerhouse of the cell," or the site of cellular respiration or aerobic respiration.
4. Hydrolysis of sugar to make ATP
        - C6H12O6 + 6O2 ----> 6CO2 + 6H2O + ATP
5. Mitochondrial Structure: Outer and inner membranes, intermembrane space, cristae (sing., crista), and matrix.
6. A TEM micrograph and model of a typical mitochondrion were examined. See text Figure 4-19, page 95 for details.

Plastid Types
1. Chloroplast
2. Proplastids
3. Chromoplasts
4. Leucoplasts (Amyloplast)

Chloroplasts
1. Self-replicating organelles (by fission).
2. Contain DNA.
3. They are the site of photosynthesis (= sugar anabolism [synthesis]):
        - 6CO2 + 6H2O + light energy ---> C6H12O6 + 6O2
4. Essentially the reverse of cellular respiration.

Aerobic Respiration and Photosynthesis
1. See text Figure 4-18, page 94.

Chloroplast Structure
1. Outer and inner membranes, thylakoids, grana (sing., granum), thylakoid (interior) space, and stroma.
2. A TEM micrograph and model of the chloroplast was examined. See text Figure 4-20, page 96 for details.

Chromoplast
1. A TEM micrograph was examined.
2. Plastids containing pigments other than chlorophylls, usually yellow and orange carotenoid pigments.

Leucoplasts
1. A TEM micrograph was examined.
2. Colorless plastid.
3. Also called amyloplast, which serve as centers for starch storage.

Organelles of the Endomembrane System
Endoplasmic Reticulum (ER)
1. Is a maze of parallel internal membranes.
2. Lumen -- internal space enclosed by the membranes.
3. ER lumen forms a continues "internal canal" system that runs from the cell membrane to the nuclear envelope.
4. Two Types of ER:
        - Rough ER = RER
        - Smooth ER = SER
5. The 2 types of ER are distinguished on appearance and function.
6. Although they have different functions, their membranes are continuous.
7. TEM micrographs and models of RER and SER were examined. See text Figure 4-13, page 90 for details.

RER
1. Has ribosomes attached on the cytosolic side giving it a serrate appearance.
2. Lumen side is bare (no ribosomes).
3. RER Function: Major site of synthesis of proteins to be:
        - Secreted from the cell, or
        - Membrane-bound, or
        - Intracellular digestive enzymes.

SER
1. More tubular in appearance
2. No ribosomes on surface, therefore it appears smooth.
3. SER Function: Primary site of lipid synthesis, including:
        - Phospholipids
        - Steroids
        - Fatty acids
4. Cells which synthesize and secrete sterols, have extensive amounts of SER. Example: Human liver cells, which synthesize and process cholesterol, have an extensive SER.

Golgi Complex
1. Consists of a stack of flattened membranous sacs surrounded by many vesicles, much like a "stack of plates surrounded by ping pong balls."
2. Each sacs has a lumen, but the lumen of one sac is not continuous with that of other sacs.
3. A TEM micrograph and model of the Golgi complex was examined. See text Figure 4-14, page 91 for details.
4. Each Golgi stack has three distinct areas:
        - Cis or forming face
        - Trans or mature face
        - Medial region
5. Cis (Forming) Face
        - Face nearest the nucleus and ER.
        - Receives (and is formed by the fusion of) transport vesicles from the ER.
6. Trans (Mature) Face
        - Face nearest the cell membrane
        - Pinches off vesicles to transport material out of the Golgi.
7. Golgi Function: to process, concentrate and package molecules (especially proteins) into vesicles for transport.
8. The transport is either in or out of the cell.

Types of Vesicles Formed by the Golgi
1. Secretory vesicles (for export of materials out of cell)
2. Lysosomes (for intracellular digestion)

Flow of Material Through the Endomembrane System
As an example, we will examine the path of a proteins to be secreted from the cell (such as digestive enzymes from the pancreas). See text Figure 4-14, page 91 for details. (Models of this pathway from other textbooks were also examined.)

Direction:
1. ER
2. Transport vesicles (from ER to Golgi)
3. Golgi complex
4. Secretory vesicles
5. Exocytosis (via cell membrane).

Exocytosis
1. The active transport of materials out of the cell by fusion of cytoplasmic vesicles with the plasma membrane.
2. A model and TEM micrograph of exocytosis were examined.

Intracellular Digestion via the Lysosome
1. Digestion of material in a food vacuole (such as a white blood cell destroying a bacterium).
2. Involves the lysosome.

Lysosome
1. Formed from the Golgi complex.
2. Can be called "suicide bags," since they are membrane bound sac of hydrolytic enzymes.
3. Occur in all eukaryotes except plants.
4. Lysosome Functions: Intracellular digestion of:
        - Material taken into the cell via endocytosis;
        - Non-functional organelles (autophagy).
5. Types of Lysosomes:
        - Primary lysosome
        - Secondary lysosome
        - Residual body
6. Several TEM micrographs of lysosomes were examined. See text Figure 4-15, page 92 for examples of secondary lysosomes.

Endocytosis
1. Is the opposite of exocytosis.
2. Is the uptake of material through the formation of a vesicle by the cell membrane.
3. A model and TEM micrograph of endocytosis were examined.
4. Autophagy is the digestion of damaged or non-functional organelles.
5. Two models of intracellular digestion via the lysosome were examined. There are no models in your textbook.

Microbodies
1. Single membrane-bound organelles which have specialized functions.
2. Two types of microbodies:
        - Peroxisomes
        - Glyoxysomes

Peroxisomes
1. Occur in all eukaryotic cells.
2. Contain enzymes (peroxidases) that detoxify hydrogen peroxide.
3. Example: Catalase which splits H2O2 (= hydrogen peroxide)
        - H2O2 --catalase---> H2O + ½ O2
4. A TEM micrograph of a peroxisome was examined. See text Figure 4-16, page 93 for details.

Glyoxysomes
1. Occur only in plants and fungi.
2. During spore and seed germination, they assist the conversion of stored lipids to sugars.

Structure Found Only in Plants and Fungi
Cell Wall
1. Plants: cellulose (is a polymer of glucose)
2. Fungi: chitin (is a polymer of N-acetyl-glucosamine)
3. Is rigid and non-living.
4. CW Function:
        - Rigidity: determines cell shape.
        - Structural strength (helps keep plants erect).
        - Protection (against insect damage and osmotic rupture)
5. Several TEM micrographs were examined. See text Figure 4-28, page 101.

Central Vacuole
1. May occupy 90% of cell volume.
2. Is membrane-bound. The vacuole membrane is called the tonoplast.
3. A TEM micrograph of the vacuole was examined.
4. Vacuole Functions:
        - Site of waste collection
        - Antiherbivory (toxins)
        - Food and pigment storage
        - Serves a lysosomal function
        - Aid cell elongation
        - Structural support of plant

Other Organelles Found in All Eukaryotic Cells: Non-Membrane Bound
Ribosomes
1. Site of protein synthesis
2. Composition: 50% rRNA and 50% protein.
3. Location:
a. Attached to ER (called bound ribosomes), where they are synthesizing proteins for secretion or to be bound in membranes, or
b. Free in cytosol (called free ribosomes), where they are synthesizing proteins for the cell.
4. Ribosome structure: composed of two units, termed the small subunit and the large subunit.
5. A model of ribosome structure was examined.

Microtubules
1. They are hollow cylinders (i.e., think "McDonald's straw").
2. Composed of tubulin
3. Have polarity, that is, they have a plus end and a minus end.
4. The minus end is anchored to the MTOC (= microtubule-organizing center)
5. Microtubules (mt) are assembled at the plus end and disassembled at the minus end.
6. See Figure 4-22, page 98 for details.

Microtubules usually associate together to form more complex structures
1. These complex structures include centrioles and basal bodies, flagella (flagellum), cilia (cilium), the spindle apparatus, and the cytoskeleton.

Centrioles
1. In animals, the centrioles are associated with the MTOC.
2. Absent in plants and most fungi.
3. Minus end of a mt is anchored to a centriole.
4. The centriole plays a role in mt assembly and disassembly (in animals).
5. They have a 9 x 3 structures of mt, that is, a "9 triplets in a ring" structure.
6. See text Figure 4-23, page 98.

Basal Bodies
1. Appear to be centrioles.
2. Have the 9 x 3 structures of mt.
3. Function: direct the synthesis of and anchor flagella and cilia in the cell.
4. Only cells with flagella and cilia have basal bodies.
5. Several models and TEM micrographs were examined.

Flagella and Cilia
1. Composed of a "9 + 2" arrangement of mt (which is called the axoneme).
2. The axoneme is surrounded by cell membrane (which is called the sheath).
3. The flagellum or cilium extends out from the surface of the cell.
4. Several models and TEM micrographs were observed. See text Figure 4-25, page 99 for details.

Flagella vs. Cilia
1. Flagella:
        - One to few per cell
        - Long -- typically 10-20 times the length of the cell
        - But can be up to 1 mm
2. Cilia:
        - Many per cell (100's)
        - Short -- typically just 2-10 um long
        - Typically much shorter than the length of the cell
3. Flagella/Cilia -- Function:
a. To move cells through an aqueous environment, or
b. To pass (move) liquids and particles across the cell surface.

Spindle Apparatus
1. Also called the mitotic spindle.
2. It functions to separate chromosomes during cell division.
3. See text Figure 9-6, page 204.

Cytoskeleton
1. Composed of microtubules and filaments (both microfilaments [such as actin filaments] and intermediate filaments).
2. A light micrograph (see Fig. 4-26, p. 100 of the text), an electron micrograph, and a model of the cytoskeleton were examined.
3. The cytoskeleton is a dynamic system, that is, it is constantly being formed and disassembled.
4. The filaments of the skeleton are anchored to membrane proteins. This provides mechanical support of the cell.
5. Cytoskeleton - Functions:
a. Provide mechanical support for the cell (shape).
b. Aid in cell movement (and change in shape).
c. Anchor organelles in fixed positions or move organelles within the cell (vesicles).
d. Anchor cytoplasmic enzymes in place (organize cell activities).
 
 

Prokaryotic Cell Structure

Prokaryotic Cell
1. A TEM micrograph and a model of the prokaryotic cell were examined. See text Figure 4-7, page 81.

Prokaryotic Cell vs. Euckaryotic Cell
1. A sketch of a typical prokaryotic cell and a typical eukaryotic cell were examined. These sketches were passed out to the class.
2. A table listing some major differences between prokaryotes and eukaryotes were examined. This table was passed out to the class.
3. A comparison between the prokaryotic flagellum and eukaryotic flagellum was made. A sketch comparing the two types was passed out to the class.

Eukaryotic Flagellum
1. "9 + 2" arrangement of microtubules (axoneme) surrounded by the sheath (= cell membrane).
2. Large and located within the cytosol.

Prokaryotic Flagellum
1. No microtubules. Just a simple filament.
2. Located outside the cell.
3. Anchored into the cell wall.
 
 

Symbiosis and the Origin of Eukaryotes

Endosymbiont Theory
1. Primary proponent is Lynn Margulis. Introduced in the 1970's.
2. "Organelles, such as mitochondria and chloroplasts, may have originated from mutually advantageous symbiotic relationships between two prokaryotic organisms."
3. Remember, mitochondria and chloroplasts are double membrane bound organelles. How could the double membrane system have evolved? To answer this question, a model of double membrane evolution was observed in class.
4. Two different models showing the endosymbiosis of mitochondria and chloroplasts were shown in class. See text Figure 20-6, page 432 for details.

Evolution of Mitochondria
1. Occurred first (before the chloroplast).
2. Occurred only one time. This helps to explains the similarity of mitochondrial structure in all eukaryotic cells (i.e., evolution from a common ancestor).
3. Origin of the mitochondria:
        - From an aerobic respiring bacterium
        - Possibly a purple sulfur bacterium

Evolution of Chloroplast
1. Occurred after mitochondrial evolution.
2. Occurred at least three separate times.
3. This helps to explain the great diversity in chloroplast structure and pigmentation (i.e., multiple ancestors)
4. Origin of chloroplasts:
        - Different types of cyanobacteria

Diversity Based on the Type of Chloroplast
1. Green Line: green algae and land plants.
2. Brown Line: brown algae, diatoms, golden algae, dinoflagellates, chrysophytes
3. Red Line: red algae

Evidence Supporting the Endosymbiotic Theory
Mitochondria
1. Ribosomes in matrix are prokaryotic type (70S).
2. DNA in matrix is prokaryotic type.
3. Same size as bacteria.
4. Reproduce by fission, just like bacteria.

Chloroplasts
1. Contain chlorophyll a and evolve O2.
2. Ribosomes in stroma are prokaryotic type (70S).
3. DNA in stroma is prokaryotic type.
4. Same size as cyanobacteria.
5. Reproduce by fission, just like cyanobacteria.

General Evidence Supporting evolution of Both
1. 1000's of examples of endosymbiotic relationships exist today in which the two partners are still recognizable
2. Example: Cyanophora and Cyanobacteria (TEM micrograph seen)
3. Example: Zooxanthellae (Dinoflagellates + Coral); (light micrograph seen)
 
 

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