Download BioH_Tour of the Cell

I. Cell Size
Figure 1: Expressing Cell Size
1/10 mm
• Some cells, such as bacteria, can be as small as 1 x 10-6 (one-millionth) meters in diameter! In order to more
conveniently express the size of such objects, “smaller” units must be used. The unit often used to express cell size is
called the Micrometer (µm). One µm is equal to 1000 millimeters.
• Sample Problem: Converting millimeters to microns
Consider a cell that is 1 x 10-6 m (.000001m) in diameter. Convert this measurement into micrometers:
STEP 1: multiply the initial measurement by the following “conversion factor”
INITIAL MEASUREMENT
X
NEW UNIT
OLD UNIT
STEP 2: convert meters into centimeters, then centimeters into millimeters.
.000001m
X
100cm
=
cm
1m
STEP 3: convert centimeters into millimeters.
________cm
X
10mm
=
mm
1cm
STEP 4: finally, convert millimeters into micrometers.
________mm
X
1000µm
=
µm
1mm
*It can now be seen that expressing the cell size as ____________µm’s is more convenient than .000001m’s!
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• Practice Problem 1: consider a skin cell that is .0001 mm’s in diameter. Express this cell’s size in terms of µm’s.
• Practice Problem 2: suppose this cell grows to 2x its initial size. Express the cell’s new diameter in terms of mm’s.
• General Rule: when converting mm to µm,
When converting µm to mm,
the initial measurement by
the initial measurement by
.
.
Figure 2: Surface Area Ratio to Volume Ratios
• Cells are small to maximize transport rates of substances can pass across its Plasma Membrane. As a cell’s size
increases, the cell’s volume grows faster than the surface area of its plasma membrane & it becomes increasingly
difficult for to transport vital substances to all parts of the cell (see below).
• In the smaller cell (50:1), substances can diffuse to reach more of its internal volume in the same amount of time
compared to a larger cell (25:1).
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• The Cell:
II. Cell Theory
• Robert Hooke is credited with the discovery & description of cells in 1665 when examining cork sections.
• Anton van Leeuwenhoek is credited as the first person to observe & describe living cells.
Table 1: Modern Cell Theory
Claims
Apparent “Contradictions”
Cell is basic unit of structure & function
Viruses are self-replicating, non-cellular entities
Cells arise from preexisting cells
1st cell self- assembled from inorganic materials
All cells carry out metabolism (chemical reactions)
Origin of mitochondria & chloroplasts (now known
to be due to endosymbiosis (see fig. 2.1)
All cells contain hereditary information (DNA)
III. Observing Cells
Figure 3: Compound Light Microscope
Figure 3.1: Resolution of the Compound Light Microscope
*Resolution = clarity of magnified image
Resolution at 1 Micron
Resolution at Less Than 1 Micron
Both points distinguishable
Points merge & are not distinguishable
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Figure 3.2: Image Appearance Under the Compound Light Microscope
• Due to the use of multiple lenses to view specimens, images viewed under the compound light microscope appear
upside-down & backwards compared to how they appear when viewed with the naked eye.
Determining Magnifying Power
Total Magnification = Ocular Lens x Objective Lens
Example: consider a microscope having a 10x ocular lens with a 10x low power objective & a 40x low power objective.
The maximum magnification that that can be achieved is: 10x X 40x = 400x.
Figure 3.3: Microscope Field Diameter
*Field Diameter is the physical measure of the distance across the viewing field (can be expressed in mm or µm).
Δ Field in Field Diameter = Ratio between Objectives x Initial Diameter
*When switching from low power to high: low power objective/high power objective
** When switching from high power to low: high power objective/low power objective
Example: consider a microscope having a 10x ocular lens with a 10x low power objective & a 40x low power objective.
If the field diameter using the 10x objective is 100 microns (μm), what will the field diameter be under 40x?
Δ Field in Field Diameter = 10x/40x = ¼ x 100 μm = 25 μm
• Sample Problem 1: If 20 paramecia can fit across the field of view under low power, how many can be seen under
high power?
• Sample Problem 2: If the high power field diameter is 500 μm, what would it be under low power in μm? In mm?
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• Light Microscope:
a) Maximum magnification = 1000x. Maximum resolving power = 1micron.
b) Can be used to observe both living & nonliving specimens.
Figure 4: Electron Microscope
• Electron Microscope:
a) Transmission Electron Microscope: used to observe internal cellular structures.
b) Scanning Electron Microscope: used to observe cell surface features.
Figure 5: Isolating Cell Components -Cell Fractionation
•Homogenization: cells broken into a soupy mixture to release their contents.
•Centrifugation: mixture is spun to separate it into 2 fractions: Pellet = collection of heavier organelles at the
bottom of the tube. Supernatent = unsettled mixture consisting of lighter particles, dissolved molecules, & ions. The
supernantent is then poured off into another test tube & centrifuged again at a faster speed. The process is
repeated, increasing the speed with each step, collecting smaller & lighter components of the mixture.
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III. Eukaryotic Cell Characteristics
Table 2: Comparing Major Cell Types
Eukaryotic Cells
Large Size (10-100 µm)
Nucleus (DNA + Proteins)
Internal Membranes
Large Ribosomes
Membrane bound flagella (whip-like motion)
Peroxisomes
Ex. Animals, Plants, Fungi, Protists
Prokaryotic Cells
Small Size (1-10
µm)
Nucleoid Region (circular chromosome –no protein)
Internal Membranes Absent
Small Ribosome & Lack Mitochondria
“Naked” flagella (spinning motion)
Peroxisomes Absent
Ex. Bacteria & Archaebacteria
Figure 6: Endomembrane System
Importance of Internal Cell Membranes
• Advantage 1: creates “compartments” that limit reactants to a small area, increasing the likelihood they will react
& increasing reaction rates. These compartments allow many reactions to occur simultaneously w/o interfering with
each another:
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• Advantage 2: often associated with embedded enzymes; allows enzymes that catalyze successive steps of a reaction
to be in close proximity (increases likelihood that a reaction will progress to completion).
•Advantage 3: Allow for energy storage –the difference in concentration of a substance on both sides of a membrane is
a form of potential energy (PE). As it moves across the membrane, the cell can convert the PE to perform various
activities.
• Endomembrane System:
IV. Eukaryotic Cell Structural Components
Figure 7: Eukaryotic Animal vs Plant Cells
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Figure 8: Plasma Membrane
• Plasma Membrane:
Figure 9: Cell Wall
• Cell Wall:
a) Perforated (plasmodesma) to allow nutrients & wastes to pass into or out of the cell as well as to allow for the
exchange of nutrients & chemical signals between adjacent cells.
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Figure 10: Cytoskeleton
• Cytoskeleton:
a) Microtubules are protein fibers composed of Tubulin that have a large diameter & help maintain the cell’s shape
by resisting compressional forces. As a component of cilia & flagella, microtubules play a role in cell movement
(see below).
*Motor proteins associated with certain organelles can move along microtubules to transport materials throughout the
cell. The protein Kinesin moves material toward the plasma membrane while Dynein moves material toward the cell
interior.
b) Microfilaments are protein fibers composed of Actin that have a smaller diameter & help maintain the cell’s
shape by resisting pulling forces.
c) Intermediate Filaments are protein fibers that are between microtubules & microfilaments in diameter. Like
microfilaments they help maintain cell shape by resisting pulling forces. They also help to fix the position of many
organelles within the cell (i.e. nucleus).
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IV. Eukaryotic Organelles
Figure 11: Cytoplasm & Cytosol
• Cytoplasm / Cytosol:
Figure 12: Centrosome & Centrioles
• Centrosome:
a) Contains a pair of Centrioles oriented at right angles composed of microtubule bundles arranged in a ring (9x3).
Centrioles inserted into the plasma membrane can give rise to cilia & flagella. They function in, but are not
necessary for, cell division. Are NOT found in plant cells.
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Figure 13: Vacuoles
• Vacuole:
Figure 13.1: Central Vacuole (Plant Cells)
• Central Vacuole:
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Figure 13.2: Contractile Vacuole (Some Animal Cells)
• Contractile Vacuole:
Figure 13.3: Lysosomes (Digestive Vacuole)
• Lysosome:
a) Lysosomes are vesicles that originate from the Golgi that contain enzymes to break down complex molecules that
originate inside or outside the cell.
b) During Intracellular Digestion, a lysosome merges with a food vacuole where enzymes mix with the ingested
molecules, reducing them their simplest subunits. The “digested” products then pass from the vacuole to the
cytoplasm to be utilized for various metabolic processes.
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Figure 14: Nucleus
• Nucleus:
a) Surrounded by a Nuclear Envelope consisting of two continuous membranes. At intervals the membranes come
together to form Nuclear Pores, which allow specific molecules associated with RNA synthesis & gene control to
enter/exit.
b) DNA is associated with proteins around which it is wrapped (enables DNA to fit in the nucleus). Normally, they exist
as a loose mass called Chromatin. During cell division, DNA condenses (becomes more tightly wrapped) into
Chromosomes.
Figure 14.1: Nucleus & Nucleolus
• Nucleolus:
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Figure 15: Ribosomes
• Ribosome:
Figure 16: Endoplasmic Reticulum
• ER:
a) Smooth ER: tubular in shape. Functions in phospholipid & steroid hormone synthesis, carbohydrate & fatty acid
metabolism, & detoxification of poisons.
b) Rough ER: covered with ribosomes, which synthesize proteins that are included into membranes, lysosome
enzymes, or proteins that are exported from the cell. The proteins are packaged in transport vesicles that bud off
of the ER membrane & make their way to the Golgi apparatus.
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Figure 17: Golgi Body
• Golgi Body:
a) Vesicles from the ER are received by the Golgi –proteins are modified through the addition of chemical groups that
act as sorting signals to direct the protein to their final destination within (or out of) the cell. The modified
products are packaged in transport vesicles arising from the Golgi.
Figure 18: Peroxisome
• Peroxisomes:
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V. Eukaryotic Organelles: Mitochondria & Chloroplasts
Figure 18: Mitochondria
• Mitochondria:
a) Consists of an inner & outer membrane separated by an Intermembrane Space. The folds of the inner membrane,
Cristae, give rise to the mitochondrial Matrix –site of many of the reactions associated w/ATP production.
Figure 19: Chloroplasts
• Chloroplast:
a) Consists of an inner & outer membrane. The inner membrane encloses a fluid-filled space called the Stroma, which
contains enzymes for photosynthesis.
b) Possess flattened disks called Thylakoids, organized into stacks called Grana. The thylakoid membranes house
photosynthetic pigments (chlorophyll).
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Figure 19.1: Endosymbiotic Origin of Mitochondria & Chloroplasts
Evidence for Endosymbiosis:
1) ______________________________________________________________________________________________
2) ______________________________________________________________________________________________
3) ______________________________________________________________________________________________
4) ______________________________________________________________________________________________
5) ______________________________________________________________________________________________
Figure 20: Surface Extensions: Cilia & Flagella
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