Cells: Why do they divide?
Doing Life’s Work
Types of cells
bacteria
cells
Prokaryote
-no organelles
-no nucleus
Eukaryotes
-Organelles
-nucleus
animal cells
plant cells
Cell size comparison
most bacteria
Animal cell
Bacterial cell
 1-10 microns
eukaryotic cells
 10-100 microns
 micron = micrometer = 1/1,000,000 meter
 diameter of human hair = ~20 microns
Why study cells?
 Cells  Tissues  Organs  Bodies
bodies are made up of cells
 cells do all the work of life!

The Work of Life
 What jobs do cells have to do for an organism to live…

“breathe”
 O2 in vs. CO2 out







ATP
Eat
make energy
build molecules
remove wastes
control internal conditions
respond to external environment
build more cells
 growth, repair, reproduction & development
The Jobs of Cells
 Cells have 3 main jobs

make energy
 need energy for all activities
ATP
 need to clean up waste produced
while making energy

Our organelles
do all these
jobs!
make proteins
 proteins do all the work in a cell,
so we need lots of them

make more cells
 for growth
 to replace damaged or diseased cells
Making more cells is OUR focus
 Think about it… why do we need to make
more?
Before we get too far...
 What limits cell size?
How do you think the size of the cell
affects the distribution of chemicals
throughout the cell?
 Write your hypothesis to this question:
If… then____

Modelling Limits to cell size
 We will use different sized agar cubes
as models of living cells.
You will submerge them in a NaOH
solution for 10 min. and observe
whether it diffuses to the centre
 We will investigate a relationship
between the surface area and the
volume of different sized cells, and
drawing conclusions about maximum
size

I need a volunteer!
 Measure the three cubes, and record the
values on the blackboard

Cube
Small
Medium
Large
Everyone needs to copy this table down,
and calculate the first 3 columns while we
wait
Surface
area (cm2)
Volume
(cm3)
SA/Volume
ratio
Diffusion
Depth
(mm)
Diffusion
Rate
(mm/min)
Analysis:
Answer in your notes:
1. What does the colour
change in the cells
indicate about the
diffusion of NaOH?
2. Did all the cells
change colour all the
way through?
3. Calculate the
diffusion rate per min
for each cell, and
record it in your table
4. Compare the results to
your hypothesis. Was
your prediction
correct?
5. If the coloured part of
the cell is thought of as
the part that gets a lot
of food and oxygen,
and can get rid of
wastes by diffusion,
which of the three cells
is best able to carry
out these functions?
Explain
Analysis
6. If all cell activity
takes place in the
cell’s interior, but
all materials enter
and exit through
the cells surface,
explain the
importance of the
surface area to
volume ratio.
7. So... Why do cells
divide!?
8. How might
temperature affect
the results of this
activity? How
would this relate to
your cells?
Back to our focus: Cells gotta work
to live!

They need to make more cells
 growth
 repair
 reproduction

To reproduce, repair & grow,
the cell must…
 copy their DNA
 make extra organelles
 divide the new DNA & new
organelles between 2 new
“daughter” cells
Cell Division for Growth:
 Main characteristic


of living things is to
grow
As you grow, the
number of cells
increase
Chemical signals
cannot get across a
cell efficiently if they
are too large, so
they must divide
Cell Division for Repair:
 Dead/ injured cells
 Your body sheds

millions of cells
daily! All of which
need to be
replaced
Broken bones,
blisters, cuts, you
need new cells to
heal the break
Cell lifespans
 Not all cells are created equal,
they all have a predetermined
self-destruct time!






red blood cells live 120 days
liver cells live 200 days
intestine lining cells live 3 days
skin cells live 20 days
stomach lining cells live 2 days
brain cells live 30-50 years
Cell Division for:
REPRODUCTION
 Important characteristic
for ‘living’ things
 Asexual reproduction:
involves only one
parent, and the
offspring all have
identical DNA to the
parent and eachother
 Occurs in many single
celled organisms, and
ALL your body cells
excluding sex cells
REPRODUCTION
 Sexual Reproduction:
sex cells/gametes
(which contain only
half of the necessary
DNA) from two parents
join to create offspring
with characteristics
from both parents
 Offspring are not
identical
Some Questions…
For you to answer:
1. A cleaning product claims to kill 99.9% of all
2.
3.
4.
5.
bacteria. Will a cleaned surface stay bacteria free
forever? Explain your reasoning.
What is the major difference between sexual and
asexual reproduction?
Why do cells divide instead of just getting bigger
as organisms grow?
Provide some evidence that suggest that not all
cells in your body divide at the same rate (speed).
At one time, Dr.’s transfused blood from younger
individuals into the elderly. They believe that the
younger blood would provide the elderly with more
energy. Do older people have older blood?
And some research for homework:
6. Why might scientists want to get mature nerve cells
to divide? [Read this, it might help! – Brain cells are
not just precious, they are irreplaceable. Although
we each have many billions of neurons, as they are
known, we cannot make any more to replace those
that die. During development, neurons lose the
capacity to divide to make new cells or even grow
the processes that form circuits in the brain.
Scientists are trying to discover how to restore
these capacities in order to treat degenerative
diseases of the brain.]
 Find a brain cell (a.k.a. neuron) on the internet and
print it out. Explain in 1 paragraph how brain cells
communicate with one another.