Chapter 6
A Tour of the Cell
How We Study Cells
Microscopes opened up the world of cells
Characteristics of Microscopes
• magnification: ability to make an image larger than
actual size
• resolution: power to show details clearly while enlarged
(if poor, objects seem fuzzy)
• constrast: accentuates different parts of the sample
Types of Microscopes
I.
compound light
- light passes through one or more lenses
- object must be sliced thinly enough to be
transparent
- upper limitation is 2000X or 0.5 microns
(um) in diameter
- resolution limitation: 0.2 microns
II. Electron Microscopes (1950’s)
- limited by physical characteristics of light
- can magnify an image up to 200,000 X
- beams of electrons produces enlarged
image
- resolution limitation: 0.002 nm across object
Types of Electron Microscopes
1. transmission electron microscope
(TEM)
- used to study internal structures
- transmits a beam of electrons
through very thinly sliced specimen stained with heavy metals
- change density of cellular structures and electron transmission
- dead specimens
- 200,000 X magnification
- black and white only
Plant Cell
2. scanning electron microscope (SEM)
-
used to study surface structures
surface covered with thin film of gold
beam excites electrons on surface
produces three dimensional images
100,000 X mag.
- dead specimens only
Isolating organelles
1. Cell fractionation - take apart
cells, separate major organelles
2. Ultracentrifuge - applies force 1
million times the force of gravity
to separate further the cell
organelles with the most dense
at the bottom
CELL THEORY
1. all living things are composed of cells
2. cells are basic units of structure and
function
3. all cells come from pre- existing cells
Activities of Life
• Most everything you think of a whole
organism needing to do, must be done at
the cellular level…
– reproduction
– growth & development
– energy utilization
– response to the
environment
– homeostasis
Common features of all cells
- plasma membrane (cell membrane)
- cytosol: semi-fluid substance, holds cellular structures
- cytoplasm: cystsol + organelles
- cytoskeleton: microscopic protein fibers that keep cells shape
- ribosomes: make proteins
- DNA: controls all cell activities
Prokaryote vs Eukaryote
DNA in nucleoid region, without
a membrane separating it from
rest of cell
Cell Wall: Made of
Peptidoglycans
Ribosome: Smaller, Free in
cytoplasm, Simultaneous
Transcription/Translation
chromosomes in nucleus,
membrane-enclosed organelle
Cell Wall: made of Cellulose or
Chitin
Ribosomes: Larger, Free and
Attached, Non-simultaneous
Cell Size
- most cells are 5-50 microns
surface area ratio (limits size of cells)
inside of cell grows faster: cubed
(V = L x W x H)
outside of cell grows slower: squared
(A = L x W)
Relationship of Surface Area to Volume
LENGTH OF
SIDE
(CM)
TOTAL
SURFACE
AREA (CM2)
TOTAL
VOLUME
(CM3)
SURFACE
AREA TO
VOLUME
RATIO
1
(1X1X6)= 6
(1X1X1)=1
6:1
(2X2X2)=8
24:8
3:1
(3X3X3)=27
54:27
2:1
2
3
(2X2X6)=24
(3X3X6)= 54
Surface Area Example
Cells must be small to maintain a large surface area to
volume ratio
Large S.A. allows  rates of chemical exchange between
cell and environment
Small Intestine: highly folded surface to increase
absorption of nutrients
Villi: finger-like projections on small intestine wall
Microvilli: projections on each cell
Surface Area Example
Root hairs increase surface area for water and mineral
absorption
Limits to Cell Size
• Metabolic requirements set uper limit of size
– In large cell, cannot move material in and out
fast enough to support life
How to get Bigger
• Become multi-cellular
Cell Shape
- most spherical or cuboidal
nerve cell
- different shapes reflect
function
dermal epidermal cells
white blood cells
goblet cell
red blood cells
Eukaryotic Cells
Animal
Plant
Cytoplasmic channelsConnnect adjacent
Cell’s cytoplasm
Cell Structure
Main components of eukaryotic cells
1. cell membrane (outer boundary)
2. nucleus (control center)
3. cytoplasm (material between nucleus
and membrane)
Nucleus
- control center of cell: directs all cell activities
- contains DNA
- continuous with rough ER
- site of DNA and RNA synthesis
- located in center of most cells
Structure:
•
- nuclear matrix - protein skeleton helps
maintain nucleus shape
- nuclear envelope (double membrane)
- contains chromatin: combination of
strands of DNA and protein
- nuclear pores: control substance
movement
- nucleoplasm: dense, protein rich
- nucleolus: region that forms ribosomal
subunits
Cytosol
(between membrane and nucleus)
- Cytoplasm = cytosol + organelles
– gel like material between
– contains water, salts, organic molecules
– in constant motion (cytoplasmic streaming)
– holds organelles
animation
amoeba animation
Organelles
Organelle: tiny structure that performs
special functions in the cell
to maintain life
Mitochondria
•
•
•
•
•
•
powerhouse of cell (cell respiration)
provides energy for cell in form of ATP
membrane bound
double membrane:
most numerous in cells which use a lot of energy (muscle)
self replicating, contain their own DNA
- cristae: folds of inner membrane
greatly enlarge surface area of inner membrane
(more area for chemical reactions of respiration)
- matrix: fluid filled inner compartment
Ribosomes
•
•
•
•
spherical structures which make proteins
not surrounded by membrane
composed of protein and rRNA
site of protein synthesis
• free ribosomes: float in cytosol- make proteins used
within cell
• bound ribosomes: attached to rough ER- make proteins
for export from cell (secretion)
Endoplasmic reticulum: (ER)
intercellular highway
complex membrane system of folded sacs and tunnels
regulates protein traffic and performs metabolic functions
Rough ER
Smooth ER
- ribosomes stuck to membrane
- no ribosomes
- also stores and acts as an
intercellular highway for proteins and
enzymes
- involved in:
- synthesis of steroids in gland
- cell regulation of Ca levels
in muscle
- cells break down toxic
substances in liver cells
surface
- package proteins for secretion and
inserted into ER
- can be stored or exported to
smooth ER
- prominent in cells that make a lot
of protein
Golgi Apparatus
• flattened system of membranes and sacs piles on each other
(like pancakes)
• very close to ER
• processes, packages, and secretes proteins for transport (o other
parts of cell (vesicles) and produce lysosomes
– Cis face: receives vesicles
– Trans face: ships vesicles
animation
Steps of Protein Production and
Transport
1.
ribosomes make proteins on the rough ER- packaged into vesicles
2.
vesicles transport the newly made proteins from the rough to the Golgi
apparatus
3.
in Golgi, proteins are processed and then packaged into NEW vesicles
4.
vesicles move thru Golgi to cell membrane and release contents outside cell
animation 2
Lysosomes
• small round vesicles that contain digestive hydrolytic enzymes
• formed from Golgi Apparatus
• digest and remove waste from cell (old organelles, byproducts,
bact., viruses)
animation
Vacuoles
• Storage of materials (food, water, minerals,
pigments, poisons)
• Membrane bound
• Ex: food vacuoles, contractile vacuoles
Peroxisomes
• contain different oxidative enzymes than lysosomes
• break down toxic substances into H2O2
(remove H from substances and transfer them to O2)
then converts H2O2 to H2O
- detox alcohol and drugs
- break down fatty acids
• formed from proteins in cytosol, not Golgi
– Glyoxysomes (fat tissues of plant seeds) FA  sugar
Cytoskeleton (cell framework)
•
maintains shape and size of cell
•
composed of network of long protein strands
located in cytosol
• provides movement for organelles within
cytosol
• regulate biochemical activities
Cytoskeleton Structure
A. Intermediate Filaments (medium size fibers)
- protein fibers coiled into cables
- maintain shape of cell
- permanent fixtures
- anchor nucleus and organelles
Cytoskeleton Structure
B. Microtubules (largest fibers)
- long hollow coiled protein tubes (tubulin)
- maintain shape and support cells
- internal cell highways – move organelles thru cell
- form centrioles, spindles (cell division)
- motility (cilia and flagella)
• flagella: long whip-like structures used for movement (motility)
• cilia: short numerous hair like projections
- movement
- transport of substances across cell
- signal receiving antenna for
ex: ear drum: transmits sound waves
respiratory tract: moves mucus etc.
Motility requires interaction
of cytoskeleton fibers with
motor proteins.
Basal body- anchors
cilia/flagella to cell
Internal Organization
9 + 2 Arrangement
dynein animation
respiratory system animation
• Centrosomes: region near nucleus
–
–
–
–
–
Microtubule organizing center
Contains centrioles
Used in cell division
Not found in plants or fungi
9 + 0 arrangement
Cytoskeleton Structure
C. Microfilaments (smallest fibers)
- two strands fine protein (actin) intertwined
- used in cytoplasmic streaming, muscle
contraction, ameboid movement
- smallest strands of cytoskeleton
cytoplasmic streaming
Extracellular Matrix (ECM)
•
•
•
•
Outside plasma membrane
Composed of glycoproteins (collagen/ proteoglycans)
Strengthens tissues and transmits external signals to cell
Fibronectins/integrins: attach cells to ECM
Intercellular Junctions (Animal Cells)
• Tight junctions: 2 cells
are fused to form
watertight seal
• Desmosomes: “rivets”
that fasten cells into
strong sheets
• Gap junctions: channels
through which ions,
sugar, small molecules
can pass
Intercellular Junctions (Plant Cells)
Plasmodesmata
• Channels in plant cell
walls that attach plasma
nmembranes of bordering
cells connect
• Water, solutes, some
proteins, RNA move
through channels
Plant Cells
Contain the same
organelles as
animal cells plus the
following:
1. cell walls
2. vacuoles
3. plastids
Plant Cell wall
• rigid covering of plant cells, algae, and some bacteria
• composed of long chains of cellulose embedded in
hardened lignin and pectin
• very porous (O, H2O, CO2 easily
pass through)
• function: support & protection
Structure
- middle lamella
• laid first, formed from the cell plate
during cytokenesis
• pectin: gluey substance holds cells
together
- primary cell wall
• forms next, expanded inside the
middle lamella
• cellulose: structure and support
- secondary wall
• constructed between the plant cell
and primary wall after a maximum
size has been reached and stops
growing
• lignin: very stiff and hard, in woody
plants in bark structure and support
Plastids
Convert solar energy into chemical energy to be stored.
3 types (arise from proplastids)
1. chloroplasts- chlorophyll (green pigment)
used in photosynthesis
2. chromoplasts- synthesize and store red, orange, and yellow
pigments (give plants unusual colors)
3. leucoplasts- store starches, proteins, and lipids
colorless