Download AP Bio Ch 4

Chapter 7
A Tour of the Cell
cell
- basic unit of structure and function in all organisms
The cell demonstrates 4 themes:
emergent properties
correlation of structure and function
interactions of organisms within their environment
evolution
Robert Hooke - first described cells using a light microscope (1665)
- coined the term “cell”
light microscope
- condenser lens focuses light on a specimen
- light passing through the specimen is refracted (bent) with an
objective lens and an ocular lens
- image is thus magnified and inverted
magnification - how much larger an object is made to appear compared to its real size
- highest magnification of a light microscope with maximum resolution is
about 1000 times
resolution
- minimum distance between 2 points that can still be distinguished as 2
separate points (clarity)
- maximum possible resolution of a light microscope is 0.2 μm
electron microscope - far surpassed the resolving power of the light microscope
- uses a beam of electrons
- practical resolution of about 2 nm
- can only view dead specimens
2 types of electron microscopes:
transmission electron microscope (TEM)
- aims a beam of electrons at a thin section of specimen
electrons transmitted through the specimen are focused and the image magnified
by using electromagnetic lenses to bend the paths of the electrons
- image is focused onto a viewing screen or film
- used to study internal cell structures
scanning electron microscope (SEM)
- useful for studying the surface of a specimen
- electron beam scans the surface of a specimen
- this excites secondary electrons on the sample’s surface which are
collected and focused on a viewing screen
cytology
- study of cell structure
cell fractionation
- centrifuging disrupted cells to isolate components of various sizes,
densities, and shapes
- uses an ultracentrifuge (spins up to 80,000 rpm)
steps of cell fractionation:
homogenization of tissue using pistons, blenders, or ultrasound
slow centrifugation of homogenate  nuclei and other larger particles settle to
bottom of tube forming a pellet
unpelleted fluid (supernatant) decanted into another tube
supernatant centrifuged at a faster speed separating out smaller organelles
supernatant decanted again and centrifuged at higher speed (and so on)
2 major kinds of cells:
prokaryotic and eukaryotic
prokaryotic
eukaryotic
bacteria, archaebacteria
no true nucleus; lacks nuclear envelope
genetic material in nucleoid region
no membrane-bound organelles
protists, fungi, plants, animals
true nucleus; bounded by nuclear envelope
nuclear material in nucleus
cytoplasm (cytosol and membrane-bound
organelles)
10 – 100 μm
0.1 – 10 μm
cytoplasm
- region between cell membrane and nucleus
cytosol
- semi-fluid part of cytoplasm
Range of cell size limited by metabolic requirements.
lower limits: probably determined by the smallest size with enough DNA to program
metabolism and enough ribosomes, enzymes, etc. to sustain life
upper limits: determined by the surface area to volume ratio (as a cell increases in
size, its volume grows proportionately more than its surface area)
- surface area must be large enough for cell volume (to provide
adequate exchange surface for oxygen, nutrients, and wastes)
plasma membrane
- boundary of every cell
- selective barrier
- allows passage of materials into and out of cell
internal membranes in eukaryotic cells:
separate cell into compartments
have unique lipid and protein compositions
may take part in metabolic reactions (many enzymes incorporated into membranes)
provide localized environmental conditions necessary for specific metabolic processes
isolate reactions
Nucleus
nucleus
- membrane-bound organelle in a eukaryotic cell
- contains most of the genes that control the entire cell
- about 5 μm in diameter
- enclosed by nuclear envelope
nuclear envelope
- double membrane enclosing the nucleus
- 2 lipid bilayer membranes separated by a space
- contains pores (regulate passage of molecules into and out of nucleus)
nuclear lamina - network of protein filaments on envelope’s nuclear side
- stabilizes nuclear shape
chromatin
- genetic material (complex of DNA and histone proteins)
- make up chromosomes in eukaryotic cells
chromosomes - long, threadlike group of genes
- composed of chromatin
- each species has a characteristic number of chromosomes
- human cells have 46 chromosomes (except sperm and egg cells)
nucleolus
- spherical region in nucleus of nondividing cells
- may be 2 or more
- make ribosomal subunits from ribosomal RNA and proteins
- subunits pass through pores to cytoplasm where they are assembled
protein synthesis
- controlled by nucleus
- messenger RNA (mRNA) transcribed in nucleus from DNA
- mRNA passes through pores into cytoplasm
- mRNA attaches to ribosomes where “message” is translated into a
protein
Ribosomes
ribosome
- cytoplasmic organelle
- site of protein synthesis
- consist of RNA and protein
- made in nucleolus (in eukaryotic cells)
- many are found in cells with high rates of protein synthesis
- found free in cytoplasm or bound to endoplasmic reticulum
free ribosomes
- in cytosol
- most proteins made here function in cytosol
bound ribosomes
- attached to outside of ER
- usually make proteins for membrane inclusion or export
- many found in cells specializing in protein excretion (pancreas)
endomembrane system
vesicles
- membranes vary in structure and function
- may change in composition, thickness, and behavior
- includes:
nuclear envelope
ER
Golgi apparatus
lysosomes
vacuoles
plasma membrane
- membrane-enclosed sacs that are pinched off portions of membranes moving
from 1 membrane to another
Endoplasmic Reticulum
endoplasmic reticulum
cisternal space
2 regions of ER
smooth ER
rough ER
- membranous network of tubules and sacs (cisternae)
- most extensive portion of endomembranous system
- continuous with external membrane of nuclear envelope
- internal lumen of ER
- continuous with space between membranes of nuclear envelope
- cytoplasmic surface lacks ribosomes
- cytoplasmic surface studded with ribosomes
- continuous with outer membrane of nuclear envelope
functions of smooth ER:
participates in synthesis of lipids, phospholipids, & steroids
- example:
mammalian sex hormones
- cells that secrete these products are rich in smooth ER (testes, ovaries, skin oil glands)
participates in carbohydrate metabolism
- smooth ER in liver contains an enzyme that helps convert glycogen to glucose
detoxifies drugs and poisons
- smooth ER in liver contains enzymes which detoxify drugs and poisons
- enzymes catalyze addition of a hydroxyl group to drugs and poisons  they
become soluble in cytosol  can be excreted
- smooth ER in liver cells increases in response to barbiturates, alcohol and other
drugs (may increase drug tolerance)
store calcium ions necessary for muscle contraction
- in muscle cells, ER pumps Ca2+ from cytosol into cisternal space
- in response to nerve impulse, Ca2+ leaks back into cytosol which triggers cell
contraction
functions of rough ER
manufactures secretory proteins
membrane production
rough ER and protein synthesis
- ER ribosomes synthesize secretory proteins
- growing polypeptide is threaded through ER membrane into cisternal space
- protein folds into its native configuration
- if destined to be a glycoprotein, enzymes catalyze the bonding of an oligosaccharide
to the protein
- transport vesicle containing the protein pinches off from a specialized region of ER
(transitional ER)
glycoproteins
oligosaccharide
transport vesicle
- protein covalently bonded to carbohydrate
- small polymer of sugar units
- membrane vesicle in transport from 1 part of the cell to another
rough ER and membrane production
- membrane proteins produced by ER ribosomes (polypeptide is inserted into rough ER
membrane)
- enzymes within ER membrane synthesize phospholipids from materials in cytosol
- proteins and phospholipids are assembled and expand ER membrane
- expanded membrane transported as a vesicle to other parts of the cell
Golgi Apparatus
Golgi apparatus
- organelle made of flattened membranous sacs (cisternae)
- many transport vesicles leave ER and travel to Golgi
- finishes, stores, and ships ER products
- has 2 poles: cis face and trans face
cis face
- “forming face” of Golgi
- accepts transport vesicles from nearby transitional ER
- vesicle fuses its membrane with the membrane of the cis face and empties its
contents into Golgi cisternal space
trans face
- “maturing face” of Golgi
- vesicles pinch off from Golgi here to transport molecules to other sites
Golgi - enzymes in cisternae modify ER products as they move from cis to trans face
- each cisterna has unique enzymes
- products move from cisterna to cisterna in transport vesicles
some examples of what the Golgi does to ER products:
alters some membrane phospholipids
alters oligosaccharide portion of glycoproteins
tags some products to be sent to various cell parts (phosphate groups or
oligosaccharides may be added to act as molecular identification tags)
sorts products for secretion (these products leave trans face in transport vesicles which
fuse with plasma membrane)
Lysosomes
lysosomes
- membrane-enclosed bag of hydrolytic enzymes
- enzymes digest macromolecules
- probably pinch off from trans face of Golgi
lysosomal enzymes
- lipases, carbohydrases, proteases, nucleases
- optimal pH 5
- made in RER; processed in Golgi
lysosomal membrane - keeps potentially destructive enzymes separate from cytosol
- pumps H+’s inward to keep optimal pH for enzymes
- made in RER; processed in Golgi
functions of lysosomes
intracellular digestion - lysosomes fuse with food-filled vacuoles
- hydrolytic enzymes digest food
- example: amoeba eating by phagocytosis
- example: human cells called macrophages “eat” bacteria by
phagoctosis
phagocytosis - plasma membrane engulfs substances
and pinches off to form a particlecontaining vacuole
recycling of cell parts - lysosomes engulf other organelles or part of cytosol and digest them
- monomers released into cytosol where they can be recycled
programmed cell destruction - important during metamorphosis and development
lysosomal storage diseases
- inherited diseases resulting from impaired lysosomal function
- lack of specific lysosomal enzymes cause some substances to
accumulate  interferes with cell function
- examples:
Pompe’s disease
- a carbohydrase that breaks down
glycogen is missing
- glycogen accumulates and damages liver
Tay-Sach’s Disease - a lipase is missing
- lipids accumulate in brain
Vacuoles
vacuole
- membrane-enclosed organelle
- larger than a vesicle
food vacuole - formed by phagocytosis
- site of intracellular digestion in some protists and macrophages
contractile vacuole
- pumps excess water from cell
- found in some freshwater protozoa
central vacuole
- large vacuole found in most mature plant cells
- forms when smaller vacuoles from ER and Golgi coalesce
- enclosed by tonoplast (part of endomembrane system)
- functions: stores organic compounds (protein storage in seeds, etc.)
stores inorganic ions (K+, Cl-)
sequesters dangerous metabolic byproducts from cytoplasm
contains pigments in some cells (pigments in flowers)
may contain poisonous or unpalatable compounds for
protection against predators
absorbs water and elongates cell (promotes plant growth)
contributes to large ratio of membrane surface area to
cytoplasmic volume (only thin layer of cytoplasm
between tonoplast and plasma membrane)
relationships among endomembranes:
Nucl ear
IS A N
E X T E N S IO N O F
envel ope
IS C O N F L U E N T
W IT H
Rough ER
s moot h er
MEM BRANE AND
SECR ETO RY
P R O T E IN S P R O D U C E D
IN E R A R E
T R A N S P O R T E D IN
ves i cl es
F U S E W IT H T H E
F O R M IN G F A C E O F
gol gi
appar at us
P IN C H E S O F F
M A T U R IN G F A C E
G IV E R IS E T O
ves i cl es
Peroxisomes
F U S E W IT H A N D A D D T O
PLASMA M EM BR A NE AN D
M A Y RE LE AS E C E LLU LAR
P R O D U C T S T O O U T S ID E
Lys os omes
PLASMA
& VACUOLES
MEMBRANE
peroxisomes - membrane-bound organelles that contain specialized enzymes
- found in nearly all eukaryotic cells
- probably form by pinching off from existing peroxisomes
- contain peroxide-producing oxidases
 transfer hydrogen to oxygen producing hydrogen peroxide
oxidase
RH
+ O -------- R + H O
2
2
>
2
2
- contain catalase  converts hydrogen peroxide to water
catalase
2H2O2 ------->2H2O +O2
functions of peroxisomal reactions
break down of fatty acids into smaller molecules (acetyl coA)
detoxification of alcohol and poisons
glyoxysomes - specialized peroxisomes found in germinating seeds
- contain enzymes that convert lipids to carbohydrates
- make energy stored in seed oils available for the seedling
mitochondria and chloroplasts
mitochondria and chloroplasts
- organelles that change energy from surrounding into
useable forms of energy
- enclosed by double membranes
- not part of endomembrane system (membrane proteins
made by free ribosomes and by ribosomes in these
organelles themselves)
- contain ribosomes and some DNA that programs some of
their protein synthesis (most programmed by nuclear DNA
and synthesized in cytosol)
- semiautonomous  grow & reproduce within cell
endosymbiotic theory - says that mitochondria and chloroplasts originated when a eukaryotic cell
engulfed a prokaryotic cell
- mitochondria came about when a eukaryotic cell engulfed an oxygenusing non-photosynthetic prokaryote
- chloroplasts came about when a eukaryotic cell engulfed a
photosynthetic prokaryote
evidence:
both mitochondria and chloroplasts have 2 membranes surrounding them
both contain ribosomes and their own DNA
both are autonomous (grow & reproduce within the cell)
mitochondria
respiration
- catabolic, oxygen-requiring process that uses energy extracted from organic
molecules to produce ATP
mitochondria - sites of cellular respiration
- found in nearly all eukaryotic cells
- number per cell correlates with the cell’s metabolic activity
- dynamic structures (move, change shape, divide)
mitochondrion structure
- enclosed by 2 phospholipid bilayers with unique proteins embedded in them
- outer membrane
– smooth
- highly permeable to small solutes
- blocks passage of proteins and other macromolecules
- inner membrane
- contains cristae (membrane infoldings)
- contains embedded enzymes involved in respiration
- cristae increase surface area for respiration reactions
- 2 internal compartments (separated by internal membrane)
intermembrane space - narrow region between inner and outer membranes
- reflects solute composition of cytosol
mitochondrial matrix - enclosed by inner mitochondrial membrane
- contains enzymes that catalyze many steps of respiration
chloroplasts
plastids
- membrane-bound organelles in plants and algae
- contain amyloplasts, chromoplasts, and chloroplasts
amyloplasts
- colorless plastids
- store starch
- found in roots and tubers
chromoplasts - pigment-containing plastids (other than chlorophyll)
- responsible for colors of fruits, flowers, and autumn leaves
chloroplasts
- chlorophyll-containing plastids
- bound by a double membrane
- sites of photosynthesis
- found in eukaryotic algae, leaves, and other green plant parts
- dynamic structures (move, change shape, divide)
chloroplast structure:
- contain thylakoids (flattened membranous sacs)
- membranes contain chlorophyll
- take part in initial steps of photosynthesis
- some are stacked into grana
- 3 compartments formed by membranes
intermembrane space - narrow space between the 2 membranes surrounding the
chloroplast
thylakoid space
- space inside a thylakoid
stroma
- space between thylakoids and inner membrane
- contains viscous fluid
- site of photosynthetic reactions that convert CO2 to sugar
Cytoskeleton
cytoskeleton - network of fibers throughout the cytoplasm
- forms a dynamic framework for support, movement, and regulation
- supports cell; helps maintain cell shape
- enables a cell to change shape
- takes part in motility  interacts with proteins called motor molecules
(e. g. organelle movement, muscle contraction, locomotor organelles)
- takes part in regulation  transmits signals from cell’s surface to its interior
- consists of at least 3 types of fibers: microtubules (thickest)
microfilaments (thinnest)
intermediate filaments
microtubules
- found in cytoplasm of all eukaryotic cells
- structure:
straight hollow fibers
constructed from a globular protein called tubulin
- each tubulin molecule consists of 2 polypeptide subunits
(α-tubulin and β-tubulin)
- may be disassembled and recycled elsewhere in the cell
- functions: cell support; reinforce cell shape
tracks for organelle movement (protein motor molecules interact
with microtubules to translocate organelles (e.g. vesicles
from Golgi to plasma membrane)
separation of chromosomes during cell division
centrioles
- pair of cylindrical structures located in the centrosome in animal cells
- 9 sets of triplet microtubules arranged in a ring
- at right angles to each other
- replicate during cell division
- may organize microtubule assembly during cell division
cilia & flagella- locomotor organelles found in some eukaryotic cells
- made of microtubules
- propel many eukaryotic unicellular organisms through water
- flagella propel motile sperm cells (animals, algae, some plants)
- cilia draw fluid across the surface of stationary cells (ciliated cells of
trachea)
structure of cilia and flagella
- extensions of plasma membrane with a core of microtubules
- “9 + 2 pattern”
- core made of 9 microtubule doublets in a ring around 2
single microtubules in center
(doublet = pair of attached microtubules)
- radial spokes
- connect each doublet to the center
- side arms
- attach each doublet to neighboring doublet
- made of protein dynein
- basal bodies
- anchor the microtubular assembly of cilia & flagella
- structurally identical to centrioles (can convert into a
centriole or vice versa)
dynein - motor molecule that changes its shape using ATP as energy
- shape change and the resistance of the radial spokes causes cilia or
flagella to bend
microfilaments
- actin filaments
- structure:
consist of 2 long actin chains wound into a helix
each chain is made of globular protein monomers called
G-actin
- functions: provide cellular support
- form a network with other proteins just inside
plasma membrane
muscle contraction
- interact with thicker filaments made of the protein
myosin in muscle cells
localized contraction of cells
- small actin-myosin aggregates in some parts of
cell cause localized contractions (e.g. pinching an
animal cell in 2 during cell division; elongation and
contraction of pseudopodia during amoeboid
movement)
cytoplasmic streaming in plant cells
- circular flow of cytoplasm (to distribute
materials may involve actin-myosin interactions)
intermediate filaments
- different in diameter and composition depending on cell type
- structure:
made of keratin subunits
more permanent than microtubules and microfilaments
- functions: bear tension (may serve as framework for cytoskeleton)
reinforce cell shape
may fix organelles in place (e.g. nucleus)
make up nuclear lamina
Cell Surfaces and Junctions
cell wall
- external to plasma membrane in plant cells
- thicker than plasma membrane
- structure:
chemical composition differs from cell to cell and species to species
consists of strong cellulose fibers in a matrix of other
polysaccharides and proteins
- functions: protects plant cell
maintains cell shape
prevents excess water uptake
plasmodesmata
- channels through cell walls connecting cytoplasm of neighboring cells
development of plant cell walls:
young cell secretes thin primary cell wall
middle lamella (made of sticky polysaccharides called pectin) between adjacent primary
cell walls  cements cells together
cell stops growing
cells secretes hardening substances into primary cells wall
secondary cell wall added between plasma membrane and primary cell wall
secondary cell wall
- durable matrix
- supports and protects cell
extracellular matrix (ECM) of animal cells
ECM - meshwork of macromolecules outside plasma membrane
- secreted by cells
- composed mostly of glycoproteins (mostly collagen)
collagen
- about ½ total protein in vertebrates
- forms strong extracellular fibers embedded in a meshwork of
glycoproteins called proteoglycans
- some cells are directly attached to the ECM; others are attached to ECM by
glycoproteins called fibronectins
- functions: supports and anchors cells
helps control gene activity (chemical signals in ECM may trigger specific
gene transcription)
intercellular junctions
plants:
plasmodesmata
- channels that perforate plant cell walls connecting cytoplasm of adjacent
cells
- lined by plasma membrane
- allows passage of water and small solutes (enhanced by cytoplasmic
streaming)
animals:
tight junctions - hold cells together tightly enough to block transport of substances through the
intercellular space
- specialized membrane proteins in adjacent cells bond to each other
- usually occur in belts around each cell (may separate 2 kinds of solutions)
desmosomes - rivet cells together into strong sheets
- permit substances to pass freely through intracellular spaces
- made of:
glycoprotein filaments that penetrate and attach plasma
membranes of both cells
a dense disk inside plasma membrane reinforced by intermediate
filaments (keratin)
gap junctions - specialized for moving materials between the cytoplasm of adjacent cells
- formed by 2 connecting protein rings each embedded in plasma
membranes of adjacent cells
- form pores big enough to allow cells to share smaller molecules (ions, sugars,
amino acids, vitamins) but not macromolecules (like proteins)
- common in animal embryos and cardiac muscle (chemical communication
between cells is essential)