CELL STRUCTURE
CH 6
• Cells are the smallest collection
of matter that is alive
• All living things are made of cells
• All cells descend from preexisting cells
• All cells share several basic
features
I. Why are cells so small?
A. The surface – to – volume theory
• Metabolic requirements limit cell size
• Surface area to volume ratio of a cell is
critical to its metabolic efficiency
• Surface area increases by n2 while volume
increases by n3
• Smaller cells have a larger surface area to
volume ratio
B. How cells can increase in size without
decreasing the SA/V ratio:
• increase their surface area or decrease their
volume
o Organelles
o Plant cells : large central vacuole
o Nerve cells: long skinny extensions
o Intestinal cells: highly folded cell membrane
II. Prokaryotic cells vs Eukaryotic cells
A. Prokaryotic cells
• No nucleus or
organelles
• Circular DNA in a
nucleoid region
• Cytoplasm and
ribosomes
surrounded by cell
membrane
B. Eukaryotic cells
• Have linear chromosomes in nucleus
• Organelles
• Bigger than prokaryotic cells
C. All cells have:
• Cell membrane to let things in and out
• Ribosomes for protein synthesis
• Cytoplasm where many enzymatic reactions
occur
• Chromosomes that contain genes
D. Cell wall: protection and support
•
•
•
•
in prokaryotic and some eukaryotic cells
cellulose in plants
chitin in fungi
peptidoglycan in bacteria (not archaea)
III. A Tour of Eukaryotic Cells
• Eukaryotic cells have internal membranes
forming organelles
• This localizes cellular activities to different
parts of cell
• All eukaryotic cells have nearly the same
organelles
prokaryote
Nucleus
ER
Golgi
Mitochondri
a
Chloroplast
lysosome
ribosomes
Cell
membrane
Cell wall
Central
vacuole
cytoskeleton
Plant
Animal
Fungus
Protist*
A. The endomembrane system is
composed of the nucleus, ER, Golgi,
lysosomes, vacuoles, and cell
membrane
1. Evolution
most widely accepted model is thru
invagination of the cell membrane
2. nucleus
• contains most of the cell’s DNA
• surrounded by nuclear envelope that is
double membrane with pores
• contains chromosomes and nucleolus
(synthesize ribosomal RNA)
• pores regulate what enters and leaves
• What enters and leaves?
• RNA that leaves nucleus is used by
ribosome to make protein
– Free ribosomes are in cytoplasm and
make proteins that stay in cytoplasm
– Bound ribosomes are on RER and make
membrane bound proteins or proteins
that leave the cell
3. endoplasmic reticulum (ER): the cell’s
factory
• highly folded membrane continuous with
nuclear membrane
• types
– rough ER (RER)
• membrane is studded with ribosomes
• it helps synthesize and modify proteins
that will be transported
– smooth ER (SER)
• no ribosomes on it
• it synthesizes carbs and lipids
• it detoxifies drugs
4. Golgi apparatus: shipping and
receiving
• Flatted stack of membranes found near
the RER
• It receives proteins from the RER and
packages them for transport out of the
cell
5. Lysosomes: digesting center
• membrane bound organelles with digestive
enzymes in acidic environment
– digest macromolecules (phagocytosis)
– digest and recycle old organelles
(autophagy)
– digest bacteria and viruses
– digest cells destined to be destroyed
(apoptosis)
6. Vacuoles: storage depots
• Derived from ER and Golgi
• Central vacuole in plants stores water and
gives plant cell its rigid shape
B. Mitochondria and Chloroplasts:
convert energy from one form to another
1. mitochondria
• double membraned organelle where the
inner membrane is folded into cristae
• site of ATP synthesis via aerobic respiration
• purpose of cristae?
2. chloroplast
• double membraned organelle which contains
stacks of membrane sacs (thylakoids) called
grana
• contain chlorophyll
• use the energy from the sun to make a
precursor to glucose by photosynthesis
• purpose of thylakoids?
3. Evolutionary Origins of Mitochondria
and Chloroplasts: Endosymbiont theory
• Ancestral eukaryote engulfed prokaryotic
aerobe and prokaryotic autotroph and they
coexisted
• Have similarities with bacteria
– Have ribosomes and DNA similar in size
and structure to bacteria
– Same size
– Divide independently from rest of cell
http://www.sumanasinc.com/webcontent/animations/conten
t/organelles.html