Download 1 Cytology (Cells) Cells are the lowest level of organization that can

Cytology (Cells)
Cells are the lowest level of organization that can perform all activities required for life;
they’re life’s fundamental unit of structure and function
A. Historical & Cell Theory
‘Cells’ were named by Robert Hooke in 1665 after looking at cork that was made
up of chambers that looked like monks rooms
Leewenhoek’s microscope allowed them to be viewed
1. Schleiden & Schwann
Schleiden was a German Botanist; he came up with the idea that all plants were
made of cells in 1838
Schwann was a German Zoologist; he came up with the idea that all animals were
made of cells in 1839
Virchow: idea that new cells could only be produced from the division of existing
cells
2. Cell Theory
a. all things are composed of one or more cells
b. cells are the basic unit of structure & function (organizational unit)
c. new cells all come from existing cells
B. Sizes & Limiting Factors (Figure 6.2)
1. Microscopic (measured in micrometers) most cells are between 1-100 micrometers
except for egg yolk (largest of all cells), & nerve cells (can be 1m long)
2. Factors-Volume & Surface Area (Figure 6.7)
the greater the volume, the more difficult it is to receive oxygen & nutrients;
the greater the volume, the greater the surface area the cell needs; more cellular
structures are necessary to adequately exchange materials and energy
examples: plant root hairs, alveoli cells, villi, microvilli
C. Types of Cells
1. prokaryotic (most primitive)
Nuclear or nucleoid region (no nuclear membrane)
Only one chromosome found here; no membrane bound organelles or internal
membranes but usually have a cell wall
example: bacteria, archaea
2. eukaryotic
nuclear membrane or envelope
membrane bound organelles
3. common features—all bound by plasma membrane, cytosol, chromosomes, & ribosomes
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D. Plants vs. Animal cells
1. In animal cells but not plants:
a. lysosomes
b. centrioles
c. flagella (but in some plant sperm)
2. In plant cells but not animal cells
a. chloroplasts
b. central vacuole & tonoplast
c. cell wall
d. plasmodesmata
E. Cellular plasma membrane
1. living
2. functions: outer boundary, regulates what enters & leaves the cell
has a selective barrier which allows sufficient oxygen, nutrients, & wastes to
come & go to service the entire volume of the cell
3. Features: porous, flexible, thickness 100-200 Angstroms
4. Structure (Figure 6.8, 6.9)
a. bilaminar (phospholipid bylayer)—hydrophobic fatty acid tails + hydrophilic
phosphate groups
*water insoluble, but does allow oxygen & carbon dioxide to pass
b. “collage” of proteins embedded/attached to phospholipid bilayer; have
pores or channels. 2 main types:
1) peripheral proteins—not embedded
2) integral proteins—penetrate through hydrophobic core—if all the way
through, called transmembrane proteins
c. six main functions of plasma membrane proteins:
1) transport of specific solutes
2) enzymatic activity
3) signal transduction (relay hormonal messages to cell)
4) cell to cell recognition (allows other proteins to attach cells together)
5) intercellular joining of adjacent cells with gap or tight junctions
6) attaching to cytoskeleton & extracellular matrix (maintaining cell shape
& stabilization)
d. carbohydrate side chains: glycolipids & glycoprotein receptors on surface;
they’re SPECIFIC to cell & involved in cell to cell recognition
example: blood type depends on sugars in the glycoproteins on the surface
of the blood cell
e. cholesterol wedged between phospholipids in animal cells
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F. Nucleus-genetic control center
- often spherical
-contains most of the genes in a eukaryotic cell (some in mitochondria &
chloroplasts)
1. Nuclear envelope or membrane—double membrane, each a lipid bilayer with proteins
a. pores perforate envelope: regulate entry & exit of macromolecules
b. double & continuous with the ER
c. nuclear side of envelope lined by nuclear lamina (except at pores);
mechanically supports the envelope
2. nucleoplasm-semifluid (contains proteins, granules & nuclear matrix). Nuclear matrix
is framework of fibers; maintains shape & allows for attachment sites for enzymes
3. chromosomes: contain the genetic info passed from one generation to the next
a. DNA bound to proteins
b. each chromosome is made up of chromatin—elongated threads
(granular material usually spread throughout the nucleus except during
cell division)
c. chromosomes-shorter condensed fibers
d. each eukaryotic species has characteristic number of chromosomes
ex/ humans: 46 (except sex cells); fruitfly: 8 (4 in sex cells)
4. nucleolus-small, dense region (RNA) darkly stained
a. produces ribosomes: particles consisting of RNA & protein. Ribosomes serve
as sites of protein synthesis in the cytoplasm
G. Membranous Organelles
1. Vacuoles & vesicles: membrane bound, fluid containing storage sacs
a. size: vesicles are smaller
b. storage: water, salts, proteins, carbs. In plants, the vacuole pressure makes it
possible to support heavy leaves & flowers. Sometimes, they’re used as pumps
as well
2. Endoplasmic Reticulum (ER) “within cytoplasm”+” little net”
a. structure: system of membranous passageways
b. two types: RER & SER
c. separates cytoplasm into areas & for transportation (membranes make
passageways)
i. RER: has ribosomes
makes proteins for secretion
phospholipids in membrane
ii. SER: no ribosomes
various functions (synthesis of lipids, steroids, metabolism of
carbs, detox of drugs, etc)
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3. Golgi apparatus-stack of flattened membranous sacs. (look like pita bread stacks)
called cisternae
a. where products of the ER are refined and packaged & then secreted
b. secretory pathway
1. RER produces protein that enters the ER channel
2. vesicles to Golgi complex on the cis face
3. refined & vesicles discharged through membrane by exocytosis on
trans face
4. Lysosomes-single membrane sacs
a. contain strong hydrolytic enzymes (lysosomal enzymes; work best at acid pH)
b. carry out intracellular digestion of food & organelles, recycle organic materials/
self destruction (autophagy)
examples: tadpoles get rid of extra tissue to become frogs, caterpillars to
butterflies, embryos “paddle” to fingers
c. Tay Sachs: example of lysosomal storage disease; leads to lipid accumulation
in brain
5. Mitochondria-organelles that convert chemical energy stored in food into compounds
more convenient for the cell to use (Figure 6.17)
a. enclosed by outer & inner membrane (each a phospholipid bilayer)
b. matrix & inner membrane has infoldings called cristae
1) purpose of cristae: to increase SA
2) cristae divide mitchondria into 2 internal compartments; the
intermembranous space & the mitochondrial matrix
*c. burn/oxidize molecules to release energy in the form of ATP (sites of cellular
respiration)
d. in humans, inherited from the cytoplasm of the egg cell
6. Chloroplasts-only found in plants & algae. Sites of photosynthesis
a. double membrane covering
b. system of flattened internal discs called thylakoids (contain chlorophyll)
stacked like poker chips. each stack is called a granum
c. fluid matrix outisde the thylakoids: stroma
7. Peroxisomes—transfer hydrogen from different substrates to oxygen, producing
hydrogen peroxide as a byproduct
a. some use oxygen to break down fatty acids
b. some detox alcohol (those in liver)
c. peroxisomes also contain an enzyme that converts peroxide to water
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H. Other Cellular Components
1. Ribosomes—organelles that carry out protein synthesis—made up of RNA & protein
a. production: millions produced by the nucleolus. 2 subunits squeezed through
nuclear pores to get to the cytoplasm
b. distribution: throughout cytosol
free ribosomes: suspended in cytosol; bound ribosomes are attached to
outside of ER or nuclear envelope (both bound & free are structurally
identical
c. ribosomes (and nucleoli) are not enclosed in membrane
d. function: produce proteins by following the coded instructions that come from
the nucleus.
2. Cytoskeleton- network of fibers extending through the cytoplasm; for internal support,
movement & regulation
a. microtubules: hollow rods made of protein tubulin (thickest); often, but not
always, grow out from a centrosome (MTOC)
b. microfilaments: solid rods made of protein actin (thinnest)
c. intermediate filaments-diameter in between
3. Cilia & Flagella—enable movement—locomotor appendages (Figure 6.24, p. 115)
a. microtubules in a “9 + 2” array (means 9 doublets in a ring around 2 single)
b. differences
i. flagella are longer
ii. beating patterns: flagella undulate (snakelike motion) in the same
direction as their axis when the move; cilia move like oars
4. Cell Walls-extracellular structure of plants, algae, fungi & prokaryotes (fig 6.28 p 119)
function: protects plant cell, maintains its shape, prevents excess water uptake
a. nonliving; lie outside the cell membrane
b. middle lamella; common layer; rich in pectins (thickening agents used
in jams). Forms between primary cell walls of adjacent cells
c. primary layer; layer secreted by young plant. Made of cellulose
d. secondary layer: deposited by some cells (in several layers) between plasma
membrane and primary wall. Made of lignin. Strong; for rigidity,
support, protection (example: wood)
e. plasmodesmata: channels in cell walls of adjacent cells
5. Intercellular Junctions
a. plasmodesmata: channels in cell walls of adjacent plant cells
cytoplasm of one plant cell is continuous with adjacent cell via plasmodesmata
b. animal cells: tight junctions
desmosomes
gap junctions
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I. Movement Across Cell Membranes
1. Fluid Mosaic Model (Singer & Nicholson 1972)
 amphipathic membrane proteins are embedded in the phospholipid bilayer with
their hydrophilic regions (phosphate heads) in maximum contact with water and
their hydrophobic tails (hydrocarbons) within the membrane.
 membrane molecules are held in place by weak, hydrophobic interactions
 Factors influencing fluidity:
a. Temperature- as temp cools, membranes switch from fluid to solid state
(phospholipids pack together more closely) like bacon grease forming lard
b. Components- unsaturated fatty acids have greater fluidity because they can’t
pack tightly “kinky”
c. Cholesterol- wedged between phospholipid molecules in animal cells
i. warm (37 degrees C) restrains movement & fluidity (decrease
phospholipid movement, makes membrane less fluid)
ii. cool-prevents solidification; maintains fluidity by disrupting tight
packing
therefore, cholesterol = temp buffer
 to work properly with active enzymes, membranes need to be about as fluid as
salad oil.
 cells can change the lipid component of their membranes to compensate for
changes in fluidity caused by changes in temperature
example: winter wheat is adapted to the cold; cells increase the percentage of
unsaturated phospholipids in their membranes in the autumn so the membranes
won’t solidify in the winter.
2. Permeability
a. amphipathic phospholipid bilayer & embedded proteins-selectively permeable
b. passage by two basic means
i. passive transport
ii. active transport
3. Diffusion-passive transport-DOES NOT REQUIRE THE CELL TO USE ENERGY!
a. definition: process by which molecules of a substance move from areas of
higher concentration to areas of lower concentration; tendency of molecules of
a substance to spread out in the available space
b. factors: most rapid in gas; least rapid in solids.
c. Arises from random movements (Brownian motion), & occurs because of
the kinetic energy (energy of motion) of the particles. Continues down the
concentration gradient until dynamic equilibrium is reached.
d. examples: i. uptake of oxygen by cell performing cellular respiration
ii. carbon dioxide movement in & out, also small, uncharged polar
molecules and small nonpolar molecules like N2
e. dynamic equilibrium: occurs when the concentration of the solute is the same
throughout a system. Movement still continues afterwards
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4. Osmosis
definition: (passive) diffusion of WATER across a selectively permeable
membrane
a. solutions can be
1) hypertonic: solution has higher solute concentration than the cell. Water
rushes out of the cell & the cell shrinks. (why an increase in salinity of a
lake can kill animals)
2) hypotonic: solution has lower solute concentration than the cell. Water
rushes into the cell, the cell swells (hydrates) and then bursts
3) isotonic: same on both sides—equal solute concentrations
ex/ most land animals have cells bathed in extracellular fluid isotonic to
the cells
4) kinds of solutes don’t matter
examples: tap water is hypertonic compared to distilled water, but hypotonic
compared to seawater
specializations for life: example: paramecium
hypertonic to pond water in which it lives, so water
continuously enters its cells. To solve this, it has a
specialized cell, the contractile vacuole, that pumps
water out.
b. turgor pressure: internal osmotic pressure in plant cells; gives plant firmness or
rigidity. Survival depends on balancing water uptake and loss
example: rain is hypotonic. Plant in rain swells to a point, then “pushes
back” due to its elastic cell wall (doesn’t allow further uptake); cell called
turgid
c. plasmolysis: loss of water in a cell due to osmosis—if plant is put into a
hypertonic solution, it loses its water to its surroundings, shrivels &
shrinks. The plasma membrane pulls away from the wall. Cell wall gives
no advantage here; usually lethal
d. if plant cell is isotonic to its surroundings, no movement of water into cell, so
cell becomes ‘flaccid’ (limp) & plant may wilt
5. Facilitated Diffusion
* still diffusion, but fast & specific*
allows hydrophilic substances such as large polar molecules and ions to move across
the membrane
a. Transport Proteins: 2 kinds
1) Channel Proteins: provide ‘hydrophilic hallways’ for specific molecules or
ions to pass;
examples: aquaporins (water channel proteins);
gated ion channels (open & close depending on presence or absence
of a chemical or physical stimulus)
2)Carrier Proteins: physically bind & transport a specific molecule.
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a) carrier (transport) proteins; bind to molecules (substances are carried across)
b) happens with specific molecules—examples: glucose, amino acids;
glucose transport protein in liver will carry a glucose into cell, but won’t
transport fructose, its structural isomer
c) goes with the concentration gradient & DOES NOT REQUIRE ENERGY
In some inherited diseases, a carrier protein can be missing & cause problems
example: cystinuria (cysteine & other amino acids can’t be carried across kidney
cell membranes, leading to kidney stones as amino acids accumulate and
crystallize in the kidneys.
6. Active Transport
movement against the concentration gradient by carrier proteins (from side where
solutes are less concentrated to side where they’re more concentrated)
a. requires considerable energy. ATP supplies the energy for most active
transport; can transfer a phosphate group from ATP (forming ADP) to a specific
carrier protein embedded in membrane
b. examples:
--sodium potassium pump (see other sheet) ions of Na+ and K+ moved
across the membrane
--ENDOCYTOSIS: intake of larger molecules; ex/polysaccharides &
proteins. 3 types:
1) phagocytosis: cell eating. solids are engulfed, packaged into a
vacuole, fuse with a lysosome, then lysosome digests them
2) pinocytosis: specific receptors on membrane form cytoplasmic
vesicle around a droplet of extracellular fluid--any solutes inside it
are taken into cell
3) receptor mediated endocytosis: allows greater specificity—
transporting only certain substances. Steps:
a) extracellular substances (ligands) bind to specific receptors on
membrane surface; receptor proteins are clustered in coated
pits of membrane
b) triggers formation of vesicle by coated pit, bringing the specific
material in
c example: in humans, how cholesterol is brought in to synthesize
membranes & steroids
c. exocytosis: elimination of large particles
1) transport vesicle budded from the Golgi apparatus is moved by the
cytoskeleton to the plasma membrane.
2) when they come in contact, the bilayers fuse & spill the contents to the
outside of the cell.
3) used by many secretory cells to export products
d. co-transport: coupled transport by a membrane protein
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