7.2 Cell Structure
2. Describe basic cell structure and explain their functions.
3. Distinguish among bacteria, viruses and eukaryotic cells.
Readings: 2: 42-46, 3: 66-69, 18: 467-476
Recall the observations of Anton van Leeuwenhoek and Robert Hooke who first observed microscopic
organisms. They gave us the cell theory: all livings things are composed of cells.
"In the morning I used to rub my teeth with salt and rinse my mouth
with water and after eating to clean my molars with a toothpick.... I
then most always saw, with great wonder, that in the said matter
there were many very little living animalcules, very prettily a-
moving. The biggest sort had a very strong and swift motion, and shot
through the water like a pike does through the water; mostly these
were of small numbers."
-Anton van Leeuwenhoek
Every cell consists of a plasma membrane surrounding internal fluids or cytoplasm and a form of DNA. All
cells also have a mechanism for making proteins involving tiny ribosomes.
Prokaryotic and eukaryotic cells share these characteristics in common but have many more differences.
Archaea Bacteria Eukarya
Cell structure Prokaryotic cells Eukaryotic cells
Domains Bacteria and Domain Eukarya
size 1- 10 µm 10 – 100 µm
Location and in cytoplasm contained inside nucleus
arrangement of arranged in a circular DNA strands are coiled around histone
genetic information chromosome and small proteins and highly condensed into
circular plasmids chromosomes
ribosomes √ √
Microtubules some √ for cytoskeleton
Endoplasmic reticulum No √ for processing proteins
Golgi apparatus No √ for further processing proteins
Mitochondria No, but may have same Most have except few anaerobic protists
enzymes in cytoplasm Perform energetic rxns in cytoplasm involving
Chloroplasts No, if photosynthetic, Plantae, some protists
perform in cytoplasm
1. Flagella some some
2. Plasma membrane
All cells have a plasma membrane made of
phospholipids. In membranes, lipids are arranged in a
double layer and only allow some small uncharged
molecules to pass through (eg. CO2) (Fig 3.11a).
Protein carriers/channels in membranes
Water enters cells through “aquaporins” (small pores in the plasma membrane made of proteins).
Other small molecules (eg. glucose) can also be transported by protein carriers through membranes.
Charged molecules (eg. Na+, sodium ions) must pass
through ion channels which are also composed of
proteins sitting in the membrane and have specificity to
only allow specific ions through.
Facilitated diffusion (Fig 3.11b) means no energy is
required to move some molecules as movement goes
along the molecules’ concentration gradient (from an
area of high concentration where there are many of these
molecules to an area of low conc.) Active transport (Fig
3.12) means energy must be expended to move a
molecule against its concentration gradient.
Large molecules enter or exit cells through vesicles by
endo or exocytosis. (endo = in exo=out)
Cell structure Prokaryotic cells Eukaryotic cells
3. Cell wall -differing compositions of Found in Plantae, Fungi, some
carbohydrate in wall protists, with differing major
components (eg. cellulose, chitin)
Not in animal cells
4. External Outer membrane Found in Gram - bacteria (in
Domain Bacteria) none
Bacterial membranes and cell walls:
Note: Peptidoglycan is a carbohydrate. Lipopolysaccharides are, as the name suggests, lipids with sugars
(carbohydrates) linked to them. In pathogenic bacteria these lipolysaccharides often contain toxins.
Prokaryotes divide by binary fission
Circular chromosome is copied into two.
Two copies separate
Cell grows larger and divides in two with one
identical chromosome in each new cell.
Also shown in Fig 18.2
Bacteria can have a form of sex by sharing plasmids
Note: This form of genetic transfer
allows bacteria (often of different
species) to share genes. E coli 0157
(pathogenic strain of E. coli that killed
the people in Walkerton Ontario) make
virtually the same toxin as found in
Shigella dysenteriae, the organism
which causes dysentery. It is thought
that S. dysenteriae transferred the
gene to make the toxin to E. coli
through a plasmid.
Viruses Fig 18.4
-consist of genetic material (DNA or RNA) and a protein coat (capsid) They may or may not have an
additional covering called an envelope made of more protein.
-require a living cell to make more of themselves
1. inject DNA
2. take over cell’s machinery for
making DNA and proteins
3. make many copies of viral DNA and
viral coat proteins assemble into
4. cell lyses (breaks open) viruses are
released and spread to nearby cells
Viral infection ends when cells of the immune system are able to recognize and degrade viruses.
Viruses can sometimes incorporate into a cell’s DNA and stay for many cell divisions/years (lysogenic cycle).
Such viruses are latent. A certain environmental signal can cause virus production to suddenly begin Eg.:
chickenpox virus can become active in nerve cells of adults (who once had chicken pox) causing “shingles”
-Viruses usually only recognize very specific cell types egs:
- bacteriophage specific sp. of bacteria. -adenoviruses cells of lungs and respiratory system
Prions (Fig 18.6) A prion is a misfolded protein which in brain cells
causes spongiform encephalopathy (holes in the brain
tissue) The word comes from proteinaceous infectious
particle. It seems to be one specific protein which,
when misfolded, can cause spongiform
encephalopathy. The normal form of the protein is PrPC
and the misfolded disease-causing form is PrPSC. It is
not known what the normal protein does, although it is
found in many nerve cells.
One misfolded protein can
stimulate normal proteins to
become misfolded themselves
(Fig 18.6). This is the only
protein known to have this
infectious capability. Misfolded
proteins build up in nerve cells
causing the cells to stop
functioning and die.
Normally when proteins in a cell are misfolded they are either refolded by proteins called chaperones or
destroyed (in lysosomes).
What makes prions different from other pathogens (viruses, bacteria, eukaryotic pathogens) is that they do
not carry any genetic material (DNA or RNA) with them. Previously it was thought that all pathogens needed
to have their genetic material with them in order to multiply. The normal form of prions (PrPc) is found in
many nerve cells and like other proteins it is coded for by a gene. QUESTION: Where is the PrP gene found?
Questions to Consider:
1. Although mutations are rare they happen frequently enough for species to evolve. Because bacterial species
reproduce so quickly, a mutation which provides some advantage (i.e. allows a bacterium to resist an
antibiotic, become more infectious or pathogenic etc) will increase quickly in a population. Besides the quick
reproduction rate of bacteria what other feature assists their quick evolution (gaining new advantageous
2. Why could it be advantageous for a virus to remain dormant for some time rather than reproducing and
causing cells to lyse rapidly?
3. Antibodies in your immune system recognize specific shapes of foreign macromolecules (proteins,
carbohydrates, lipids and combinations of all three) that are different from any shape of self macromolecules
(normally found in the host’s body). Based on what you know now about bacterial cells, what components of
bacterial cells may be recognized by antibodies. Hint: what structures or molecules do bacteria have that are
different from eukaryotic cells? (FYI: we will talk about how antibodies work in greater detail next week)
4. In order to evade the immune system some pathogens get inside the cells of the host organism. They
usually do not initially kill the cells they enter. Considering what you know about how molecules enter cells,
how might pathogens get inside?
5. When you get a cold, you have been infected with a virus that infects cells of your airways (bronchial
passages and lungs). A virus which infects a lung cell will have the cell make many copies of the virus and
then the cell bursts and the viruses spread to nearby cells. Aside from mucus, what is in a tissue when you
blow your nose?