The Cell: Basic Unit of Life
AP Biology
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
AP Biology
2005-2006
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
AP Biology
2005-2006
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

AP Biology
2005-2006
How do we study cells?
 Microscopes opened up
the world of cells
 Robert Hooke (1665)
 the 1st cytologist
Drawings by Hooke
cork
flea
AP Biology
2005-2006
How do we study cells?
 Microscopes
light microscopes
 electron microscope
 transmission electron
microscopes (TEM)
 scanning electron
microscopes (SEM)

AP Biology
2005-2006
Light microscopes
0.2µm resolution
 ~size of a bacterium
 visible light passes
through specimen
 can be used to study
live cells

AP Biology
2005-2006
Electron microscope
1950s
 2.0nm resolution
 100 times > light
microscope
 reveals organelles
 but can only be used
on dead cells

AP Biology
2005-2006
Transmission electron microscopes
 TEM

used mainly to study internal structure
of cells
 aims an electron beam through thin section
of specimen
rabbit trachea
AP Biology
cucumber seed leaf
2005-2006
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
AP Biology
2005-2006
What are these?
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
Why is a huge
single-cell
creature not
possible?
AP Biology
s:v
6:1
~1:1
6:1
2005-2006
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

O2
CH
aa
CHO
CO2
CHO
CH
AP Biology
aa
aa
O2
CO2
NH3
O2
NH3
CHO
O2
CO2
CO2
CHO
O2
NH3
CH
aa
2005-2006
What’s the solution?
How to get bigger?
 Become multi-cellular (cell divides)
But what challenges do you have to solve now?
CO2
CO2
aa
O2
CH
aa
CO2
CHO
NH3
CH
AP Biology
NH3
CO2
CO2
CHO
O2
aa
aa
O2
aa
CH
NH3
NH3
CO2
NH3
CO2
CO2
NH3
O2
NH3
NH3
CO2
CO2
NH3
CO2
CHO
aa
2005-2006
Lab 2A: Relationship between
Surface Area and Cell Size
 Cell size and shape are
important factors in
determining the rate of
diffusion.
16
Pre-lab assignment (HW)
Workbook p. 31-33
 Read Lab sheet and think about how to
design your experiment
17
 Think about cells with specialized
functions, such as the epithelial cells
that line the small intestine or plant root
hairs.
18
Questions to ponder…
 What is the shape of these cells?
 What size are the cells?
 How do small intestinal epithelial and
root hair cells function in nutrient
procurement?
19
Starting material
20
Materials
2% agar containing NaOH and the pHindicator dye
phenolphthalein
•
1% phenolphthalein solution
•
0.1M HCl
•
0.1 M NaOH
•
Petri dishes and test tubes
•
2% agar with phenolphthalein preparation
Squares of hard, thin plastic (from disposable plates);
Metric rulers
•
21
Procedure
 Step 1 Place some phenolphthalein in



two test tubes.
Add 0.1 M HCl to one test tube, swirl to
mix the solutions, and observe the
color.
Using the same procedure, add 0.1 M
NaOH to the other test tube.
Remember to record your observations
as you were instructed.
22
Be sure you’re clear on these
questions…
 Which solution is an acid?
 Which solution is a base?
 What color is the dye in the base? In

the acid?
What color is the dye when mixed with
the base?
23
What is your hypothesis?
 If you put each of the blocks into a


solution, into which block would that
solution diffuse throughout the entire
block fastest? Slowest?
How do you explain the difference?
These are your test questions: Come
up with a hypothesis!
24
 2 Using a dull knife or a thin strip of



hard plastic, cut three blocks of agar of
different sizes.
These three blocks will be your models
for cells.
What is the surface area of each of your
three cells?
What is the total volume of each of your
cells?
25
Results Section
 Show all calculations in your results
section
 Pictures of your three cells are helpful –
before and after you put them in acid..
26
Designing and Conducting Your
Investigation
 Using the materials listed earlier, design
an experiment to test the predictions you
just made regarding the relationship of
surface area and volume in the artificial
cells to the diffusion rate using the
phenolphthalein–NaOH agar and the HCl
solution.
27
 Once you have finished planning your


experiment, have your teacher check
your design.
When you have an approved design,
run your experiment and record your
results.
Do your experimental results support
your predictions?
28
Tips for lab report
 Make sure you have a hypothesis in


your introduction; be sure to refer to
the hypothesis in your conclusions.
You can use your workbook (p. 32-33)
as one of your references; you may find
the graph you plotted there helpful in
writing your conclusions.
Note any sources of error, confounding
variables that may be operating in this
experiment.
29
Tips for lab report
Consider the following questions in the
final part of your conclusions:
 What is average kinetic energy?
 How does the average kinetic energy of
a solution influence diffusion rates?
 Design an experiment to test the
following hypothesis: Diffusion rates
increase when average kinetic energy
increases.
30