1. Chapter 6. The Cell: Basic Unit of Life
Adapted from: Kim Foglia

2. Why do we study cells?

3. Cell Theory All organisms are made up of cells
The cell is the basic living unit of organization for all organisms
All cells come from pre-existing cells
Today, we take many things in science for granted. Many experiments have been performed and much knowledge has been accumulated that people didn’t always know. For centuries, people based their beliefs on their interpretations of what they saw going on in the world around them without testing their ideas to determine the validity of these theories — in other words, they didn’t use the scientific method to arrive at answers to their questions. Rather, their conclusions were based on untested observations.
Among these ideas, for centuries, since at least the time of Aristotle (4th Century BC), people (including scientists) believed that simple living organisms could come into being by spontaneous generation. This was the idea that non-living objects can give rise to living organisms. It was common “knowledge” that simple organisms like worms, beetles, frogs, and salamanders could come from dust, mud, etc., and food left out, quickly “swarmed” with life. For example:
Observation: Every year in the spring, the Nile River flooded areas of Egypt along the river, leaving behind nutrient-rich mud that enabled the people to grow that year’s crop of food. However, along with the muddy soil, large numbers of frogs appeared that weren’t around in drier times.
Conclusion: It was perfectly obvious to people back then that muddy soil gave rise to the frogs.
Observation: In many parts of Europe, medieval farmers stored grain in barns with thatched roofs. As a roof aged, it was not uncommon for it to start leaking. This could lead to spoiled or moldy grain, and of course there were lots of mice around.
Conclusion: It was obvious to them that the mice came from the moldy grain.
(continued)

4. Biological diversity & unity
Underlying the diversity of life is a striking unity
DNA is universal genetic language
Cells are the basic units of structure & function
lowest level of structure capable of performing all activities of life
Observation: Since there were no refrigerators, the mandatory, daily trip to the butcher shop, especially in summer, meant battling the flies around the carcasses. Typically, carcasses were “hung by their heels,” and customers selected which chunk the butcher would carve off for them.
Conclusion: Obviously, the rotting meat that had been hanging in the sun all day was the source of the flies.
In 1668, Francesco Redi, an Italian physician, did an experiment with flies and wide-mouth jars containing meat. This was a true scientific experiment — many people say this was the first real experiment — containing the following elements:
Observation: There are flies around meat carcasses at the butcher shop.
Question: Where do the flies come from? Does rotting meat turn into or produce the flies?
Hypothesis: Rotten meat does not turn into flies. Only flies can make more flies.
Prediction: If meat cannot turn into flies, rotting meat in a sealed (fly-proof) container should not produce flies or maggots.
Testing: Wide-mouth jars each containing a piece of meat were subjected to several variations of “openness” while all other variables were kept the same.
Control Group:These jars of meat were set out without lids so the meat would be exposed to whatever it might be in the butcher shop.
Experimental Group(s): One group of jars were sealed with lids, and another group of jars had gauze placed over them.
Replication: Several jars were included in each group.
(continued)

5. 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
Data: Presence or absence of flies and maggots observed in each jar was recorded. In the control group of jars, flies were seen entering the jars. Later, maggots, then more flies were seen on the meat. In the gauze-covered jars, no flies were seen in the jars, but were observed around and on the gauze, and later a few maggots were seen on the meat. In the sealed jars, no maggots or flies were ever seen on the meat.
Conclusion(s): Only flies can make more flies. In the uncovered jars, flies entered and laid eggs on the meat. Maggots hatched from these eggs and grew into more adult flies. Adult flies laid eggs on the gauze on the gauze-covered jars. These eggs or the maggots from them dropped through the gauze onto the meat. In the sealed jars, no flies, maggots, nor eggs could enter, thus none were seen in those jars. Maggots arose only where flies were able to lay eggs. This experiment disproved the idea of spontaneous generation for larger organisms.
The cell senses and responds to its environment and exchanges materials & energy with its surroundings. All cells are related through common descent, but evolution has shaped diverse adaptations.

6. How do we study cells? Microscopes opened up the world of cells
Robert Hooke (1665)
the 1st cytologist
Drawings by Hooke
cork
flea
Examples of Hooke's detailed drawings can be seen in the illustration of cork & flea above. It was in his description of cork that he first used the term "cell" even though he did not kno how important his discovery would become. The cell wasn't really understood until 1839 when scientists began to discover its importance.
As it turned out, the flea, Ceratophyllus faciatus in this illustration was the carrier of the Bubonic Plague that was sweeping through Europe at time. However, this was not known by Hooke.
Bubonic plague was a bacterial disease carried by fleas of infected Old English rats. At its worst, it killed two million people a year. Those that caught the disease had a 90% chance of dieing from it.

7. Technology advancing science
How do we study cells?
Microscopes
light microscopes
electron microscope
transmission electron microscopes (TEM)
scanning electron microscopes (SEM)
Microscopes provide windows to the world of the cell
Technology advancing science

8. Light microscopes 0.2µm resolution ~size of a bacterium
visible light passes through specimen
can be used to study live cells
While a light microscope can resolve individual cells, it cannot resolve much of the internal anatomy, especially the organelles.
To resolve smaller structures we use an electron microscope (EM), which focuses a beam of electrons through the specimen or onto its surface.

9. Electron microscope 1950s 2.0nm resolution
100 times > light microscope
reveals organelles
but can only be used on dead cells

10. Transmission electron microscopes
TEM
used mainly to study internal structure of cells
aims an electron beam through thin section of specimen
rabbit trachea
cucumber seed leaf
to enhance contrast, the thin sections are stained with heavy metals

11. Scanning electron microscopes
SEM
studying surface structures
sample surface covered with thin film of gold
beam excites electrons on surface
great depth of field = an image that seems 3-D
rabbit trachea

12. SEM images
grasshopper

13. SEM images
spider head

14. Isolating organelles Cell fractionation separate organelles from cell
variable density of organelles
ultracentrifuge
What organelle would be heaviest?
What organelle would be lightest?
Make sure you know the difference between supernatant & pellet
What organelle would be heaviest? nucleus
What organelle would be lightest? ribosome

15. Ultracentrifuge spins up to 130,000 rpm
forces > 1 million X gravity (1,000,000g)
Why is it in a BIG thick lead-lined housing?
Ultracentrifuge: really fast, but really big.
1 foot thick lead walls
Why? And why don’t we have one?
Because every now and then one of these things disintegrates while its spinning.
When they blow, they really blow!!
A real lot of energy in something spinning 100,000 rpm & the rotor is made of titanium…
Titanium shrapnel!

16. Microcentrifuge Biotechnology research
study cells at protein & genetic level

17. Adapted from: Kim Foglia
Tour of the Cell
Adapted from: Kim Foglia

18. Cell characteristics All cells: surrounded by a plasma membrane
have cytosol
semi-fluid substance within the membrane
cytoplasm = cytosol + organelles
contain chromosomes which have genes in the form of DNA
have ribosomes
tiny “organelles” that make proteins using instructions contained in genes

19. Types of cells Prokaryotic vs. eukaryotic cells
Location of chromosomes
Prokaryotic cell
DNA in nucleoid region, without a membrane separating it from rest of cell
Eukaryotic cell
chromosomes in nucleus, membrane-enclosed organelle

20. Cell types
Prokaryote
Eukaryote
internal membranes

21. The prokaryotic cell is much simpler in structure, lacking a nucleus and the other membrane-enclosed organelles of the eukaryotic cell.

22. Eukaryotic cells
Eukaryotic cells are more complex than prokaryotic cells
within cytoplasm is a variety of membrane-bounded organelles
specialized structures in form & function
Eukaryotic cells are generally bigger than prokaryotic cells

25. Limits to cell size Lower limit Upper limit
smallest bacteria, mycoplasmas
0.1 to 1.0 micron (µm = micrometer)
most bacteria
1-10 microns
Upper limit
eukaryotic cells
microns
micron = micrometer = 1/1,000,000 meter
diameter of human hair = ~20 microns

26. What limits cell size? Surface to volume ratio
as cell gets bigger its volume increases faster than its surface area
smaller objects have greater ratio of surface area to volume
What cell organelle governs this?
square - cube law
As cell gets larger, volume increases cubically, but surface area only increases by the square.
The volume of the cell is demanding… it needs exchange. The surface area is the exchange system… as cell gets larger, the surface area cannot keep up with demand.
Instead of getting bigger, cell divides -- mitosis.
Why is a huge single-cell creature not possible?
s:v
6:1
~1:1
6:1

27. Limits to cell size Metabolic requirements set upper limit
in large cell, cannot move material in & out of cell fast enough to support life aa aa
What process is this? CH
NH3 aa
CHO O2 CH
CHO
CO2
CHO
CO2
CO2
What process is this?
diffusion
What’s the solution?
cell divides
make a multi-celled creature = lots of little cell, rather than one BIG cell aa
NH3
NH3 O2 O2
CHO
NH3 aa
CO2 CH aa CH O2 aa O2
What’s the solution?

28. How to get bigger? Become multi-cellular (cell divides) aa aa aa O2 aa
But what challenges do you have to solve now?
CO2
CO2 O2
NH3 aa
NH3 aa
CO2
NH3 O2
CO2
CO2 CH
CHO
CO2
NH3
Larger organisms do not generally have larger cells than smaller organisms — simply more cells
What’s challenges do you have to solve now?
how to bathe all cells in fluid that brings nutrients to each & removes wastes from each aa O2
NH3
NH3
CO2
CO2
CO2
CHO aa
NH3
NH3
NH3 CH
CHO
CO2
CO2 O2 aa aa CH

29. Cell membrane Exchange organelle
plasma membrane functions as selective barrier
allows passage of O2, nutrients & wastes

30. Organelles & Internal membranes
Eukaryotic cell
internal membranes
partition cell into compartments
create different local environments
compartmentalize functions
membranes for different compartments are specialized for their function
different structures for specific functions
unique combination of lipids & proteins
To do their job effectively, each organelle has specific resource requirements and need to maintain internal environment distinct from the general cytosol, so each organelle is bound by a membrane… a selectively permeable membrane that allows it to control the local environment