CHAPTER 4
A TOUR OF THE
CELL
Sect. 4.2
Cell Size
-Most cells are between 1 & 100um in
diameter (Fig. 4.2A p. 54)
-Plasma membrane (PM): selective barrier
that consists of proteins & lipids
(phospholipid bilayer)
-Cell size is limited due to:
-a single nucleus controlling the
entire cell
-enough surface area in relation to
volume to obtain nutrients and dispose
of wastes
Sect. 4.3
•
•
•
•
Two Types of Cells
Prokaryotic
No membrane
bound organelles,
only nucleoid region
Has ribosomes &
DNA
Cell wall, capsule &
pili help attach,
flagella
Found in the
Domains Bacteria &
Archaea
Eukaryotic
• Have cytoplasm –
region between the
nucleus and the cell
membrane (consists
of a semifluid medium
called cytosol)
• Contain organelles –
structures w/special
functions
• Found in protists,
plants, fungi, and
animals
Sect. 4.5
The Nucleus
(genetic control center of a
eukaryotic cell)
Nuclear Envelope
-boundary around nucleus, separating
it from the contents of the cytoplasm
-Double membrane – each with a
phospholipid bilayer
Contents of the Nucleus
- nearly all of the cell’s DNA is here
(organized along w/proteins into
chromosomes)
- unless the cells are dividing,
chromosomes are seen as a tangled up
mess called chromatin
- nucleolus functions in the synthesis of
ribosomes
Sect. 4.4
Ribosomes
Function
• assemble enzymes and the entire
cell’s other proteins
• there are genetic instructions for this
to occur
•not membrane bound
Location
• exist in 2 places w/in cytoplasm
1) free ribosomes - suspended in
cytosol (make proteins for cell use)
2) bound ribosomes – attached to
endoplasmic reticulum (make proteins
for export outside cell)
• ribosomes usually occur in
clusters called polysomes
Sect. 4.6
Endomembrane System
Endoplasmic Reticulum (ER)
• a network of interconnected
compartments, continuous w/the outer
membrane on the nuclear envelope
p. 58
W/in the ER is a
space known as
the lumen
Sect. 4.8
• 2 distinct regions of the
ER (interconnected
flattened sacs)
Rough ER - has
ribosomes on surface
- makes proteins
destined to be secreted
- makes membranes
Sect. 4.7
Smooth ER - lack
ribosomes (p. 59),
membrane is continuous
w/rough ER
- serves as a
transition area for ER
products
- makes steroids and
sex hormones
- tolerance to drugs
- calcium for muscles
Sect. 4.9 Golgi Complex (p. 60)
• after leaving the ER,
most transport vesicles
travel to this organelle
•manufactures,
warehouses, and ships
(products of the ER are
modified, stored, and
shipped to other
locations in transport
vesicles)
- consists of
flattened sacs
stacked up like
pancakes (not
connected)
- molecules move
from sac to sac
Sect. 4.10
Lysosomes
• membrane-bound bag of hydrolytic
(digestive) enzymes
- cell uses these enzymes to digest
macromolecules
- these enzymes work best at pH 5
• made by the rough ER,
processed, and released by the
Golgi
• function by using
phagocytosis (engulfs
particles) then digests them
• also function in recycling cell’s
own organic material (recycling
centers for damaged organelles)
• ex: webbing between fingers in
embryonic development
Sect. 4.11
• lysosomal storage diseases (missing one
of the hydrolytic enzymes):
Pompe’s disease (liver can’t break down
polysaccharide glycogen)
Tay-Sachs (overload of lipids on nervous
system)
Microbodies
• bounded by a single membrane
• compartments specialized for
specific metabolic pathways (each
has a particular kind of enzymes)
2 kinds of microbodies:
Peroxisomes - have enzymes which
transfer H from various substrates
to O (produce H2O2 as a byproduct)
Glyoxysomes - contain enzymes to
convert fats to sugar (in plants)
Sect. 4.12
Vacuole
• a large membrane sac
• various functions:
food vacuole - formed by phagocytosis
contractile vacuole found in freshwater
protists (pumps out
excess water from the
cell)
Central vacuole - found in mature
plant cells - bounded by a
tonoplast (membrane)
- stores organic compounds
Sect. 4.14
Energy Transducers
(have their own DNA)
Chloroplasts
• formed only in plants
• functions in photosynthesis
• chloroplast – contains a green pigment
(chlorophyll)
p. 63
• chloroplast’s elaborative structure:
-consists of flattened green sacs
which are stacked up
-sacs called thylakoids
-stack of thylakoids called a granum (if
more than one granum, it’s called grana)
-stroma: thick fluid surrounding
thylakoids
Sect. 4.15
Mitochondria (p. 63)
• site for cellular respiration
• found in plants and animals
• these foldings divide inner region of
mitochondria
- intermembrane space – region
between inner and outer membranes
- has inner membrane that’s
convoluted within foldings called cristae
(this greatly increases surface area to
enhance ATP production)
- mitochondrial matrix – is enclosed
by the inner membrane (most of the
respiration takes place here)
Sect. 4.16
Cytoskeleton – supportive meshwork
of fine fibers for structural support
Microfilaments
(thinnest type of fiber)
• solid rods of globular proteins called
actin – linked in chains
• best known for role in muscle
contraction
• protein called myosin is imbedded in
the actin molecule
• in plants, microfilaments are involved
in cytoplasmic streaming (cytoplasm
flows in a particular direction ex:
moving chloroplasts)
Intermediate Filaments
- ropelike
- reinforces cell
shape
- anchors certain
organelles (nucleus is
caged by these to
keep in place)
Microtubules
p. 64
• hollow rods of globular proteins
called tubulins
• found in cytoplasm of all eukaryotic
cells
• radiate out from cell centers
• give cell shape and reinforce it
• help w/animal cell division
• also help other organelles navigate
through cytoplasm
• w/in cell center are 2 centrioles –
consist of 9 sets of triplet microtubules
and must be 2 centrioles (a pair) in the
center
9 sets
p. 65 Fig. 4.17C
Sect. 4.17
Cilia and Flagella
Cilia – short fingerlike projections used
for locomotion
Flagellum – a whip-like tail
• both have a core of
microtubules and protrude
from the cell
• wrapped in plasma membrane
• anchored to the cell by a basal body –
similar to a centriole
Ex: cilia on cells in wind pipe
flagellum on sperm
Sect. 4.18
Cell Surface
Cell Wall
• plants only - thicker than PM
• there is a thin, flexible primary cell
wall (between the 2 cell walls is the
middle lamella - like glue) (pectin)
• cell wall strengthens as the cell matures
• some plants add a secondary cell wall
between the PM and the primary cell wall
• plants use the plasmodesmata
- channels in the cell wall
- strands of cytoplasm connect one cell
to another
Extracellular Matrix
• no cell wall in animals
• glycocalyx - fuzzy coat
• made of carbohydrates and helps cells
stick together - strong surfaces
Intercellular Junctions
• provides a means by which many
cells can be integrated into one
functional organism
• cell to cell contact in animals
provided by 3 junctions:
1. Tight junctions - binds cells
together ex: digestive tract
2. Anchoring junctions - attach
adjacent cells to each other to stretch
ex: skin
3. Gap junctions - allow water and
other small molecules to flow between
neighboring cells (similar to
plasmodesmata in plants)