Cells:- the building blocks
Animal cell
Plant cell
The secret of life lies within the
cell!
Outline of the cell theory
 all living things are made of one or more cells
 the cell is the smallest functional unit of life
 all cells come from pre-existing cells through cell
division
 Cell Theory is attributed to Schleiden and Schwann
(1838).
 the term 'cell' was first coined by Robert Hooke after
observing the structure of cork in 1655.
What is a theory?
 a theory is a general system of ideas used to explain or
interpret observations
 theories provide predictive power by generating
hypotheses
 a hypothesis is a specific prediction than can be tested
through observation or experiment
 each aspect of cell theory is based on evidence obtained
from observations and experiments
Evidences for the cell theory
1. All living things are made of one or more cells
☺microscopes allow us to observe that all living things
are either unicellular (made up of one cell) or
multicellular (made up of several cells)
☻exceptions:
a)skeletal muscle, some fungal hyphae, and some algae
have multinucleate cytoplasm, lacking normal cell
separations
b)connective tissues, bone, blood and cartilage, are
composed of both cellular and extracellular structures,
especially extracellular proteins and fluids; however,
these extracellular structures are products of cellular
activities
2. The cell is the smallest unit of life
☺nothing smaller than a cell can survive independently
☺ sub-cellular structures (i.e. organelles) such as
nuclei, ER, Golgi apparatus, chloroplasts,
mitochondria e.t.c. cannot survive independently
☺ the lower limit on cell size is about 200nm, large
enough for DNA, ribosomes, and membranes
3. All cells come from pre-existing cells
☺ this seems to imply that life has always existed, which
is incompatible with geological evidence about the age
of Earth
☺ therefore, an exception is made for the origin of life,
when cells must have arisen from non-living
substances
☺ since the conditions of early Earth were anaerobic,
they allowed for cells to form from non-living
substances
☺ the conditions of present Earth are aerobic,
precluding the formation of cells from non-living
substances
Unicellular organisms
Amoeba
Rhizobium bacteria
Functions of life carried out by
unicellular organisms
metabolism: chemical reactions inside the cell,
including cell respiration to release energy
sensitivity: perceiving and responding to changes in the
environment
homeostasis: keeping conditions inside the organisms
within tolerable limits
growth: an irreversible increase in size
reproduction: producing offspring either sexually or
asexually
nutrition: obtaining food, to provide energy and the
materials needed for growth
Relative sizes of structures in SI
units
1 Centimeter(cm) = 10 m
-2
1 Millimeter(mm) = 10-3 m
1 Micrometer(μm) = 10-6 m
1 Nanometer(nm) = 10-9 m
Structure
Relative size
SI unit
molecules
1
nm
cell membrane
10
nm
Viruses
100
nm
bacteria
1
μm
organelles
10
μm
cells
100
μm
Linear magnification of drawings
☺Magnification shows how much larger or smaller the
image is compared to the real specimen.
☺ Magnification = measured length of the image
/measured length of the specimen
☺ Magnification could be stated (for example, ×250)or
indicated by means of a scale bar, for example:
1 μm
☺Is the actual leaf bigger or smaller than the image
shown above? Explain your answer.
☺ If the image length = 4.2cm, calculate the actual size
of the leaf.
☺Magnification of the image:
X 1:- image same size as the specimen
X<1:- image is smaller than the specimen
X > 1:- image is bigger than the specimen
Importance of the surface area to volume
ratio as a factor limiting cell size
☺surface area (SA)
•SA = f(x2) i.e. it increases two fold as the size of the cell
increases
 the rate of exchange of materials and energy (heat) is a
function of its surface area.
☺ Volume (Vol)
 Vol., = f(x3) i.e. it increases three folds as the size of
the cell increases
 rate of heat production/waste production/resource
consumption of a cell is a function of its volume
Cuboids Length SA (cm2) Vol(cm3) SA:Vol
(cm)
ratio
A
1
6
1
6:1
B
2
24
8
3:1
C
3
54
27
2:1
D
4
96
64
3:2
E
5
150
125
6:5
Which of the two animals has a
bigger SA: Vol ratio?
African Elephant
(Loxodonta africana)
Pygmy Shrew
(Sorex minutus)
Biological Consequences:
 As the size of an organism increases the surface area to
volume ratio decreases.
 For elephant heat exchange is quite slow which is a
problem for elephants living in a warm region.
 Elephant has evolved large ears to increase the surface
area for heat exchange allowing them to remain cool
 Small organisms have the opposite problem of a rapid
rate of exchange with the environment.
 Shrew looses body heat so fast that it consumes food
at a furious rate simply to produce heat from
respiration.
Emergent properties in multicellular
organisms
 Multicellular organisms show emergent properties i.e.
occurrence of unexpected characteristics or properties
in a complex system.
 Emergent properties arise from the interaction of the
component parts - the whole organism is greater than
the sum of the parts.
 Example: organisms that can regulate their body
temperature using their hairs, sweating, changing
blood flow’ altering rate of metabolism etc can exploit
more habitats than ones that cannot.
Differentiation of cells in multicellular
organisms
 multicellular organisms can differentiate into a variety




of interdependent cell types each specialized to carry
out a subset of functions thereby achieving a greater
efficiency through division of labor among a
multicellular cooperative
cellular differentiation is achieved through differential
gene expression
all cells in an organism have identical DNA = genome
different cell types make different proteins usually as a
result of transcriptional regulation
each cell type expresses a closely regulated subset of its
genome through “turning on” some genes and
“turning off” others
Therapeutic use of stem cells
 stem cells are undifferentiated cells that retain the
capacity to divide and have the ability to differentiate
along different pathways i.e. they are multipotent.
 Stem cells can be obtained from: bone marrow,
umbilical cord blood, inner cell mass of embryo
(embryonic stem cells)
 Stem cells can be used to treat: leukaemia, heart
disease, diabetes, graft new skin for people with severe
burns, stroke due to loss of neurons
 Patients’ differentiated cells i.e. cells causing the
condition are killed by chemotherapy and then
replaced with stem cells through transplant
Revision Questions
Q1.
(a) State two characteristics
of stem cells that can be used
to distinguish them from
other body cells.
[2]
(b) Outline one therapeutic
use of stem cells.
[3]
(c) Describe the importance
of stem cells in
differentiation.
[3]
Q2.
(a) Outline the cell theory . [3]
(b) Discuss the evidence for the
cell theory .
[5]
Q3.
(a) Explain the importance of
the surface area to volume ratio
as a factor limiting cell size . [4]
(b)The scanning electron micrograph below shows the surface of
the nuclear envelope with numerous nuclear pores.
(i) Calculate the power of magnification of the
image.
[1]
(ii) State the diameter of the pore labelled X.
[1]
(c) Compare the relative sizes of viruses and bacteria
to this cell.
[2]
Q4. Explain how multicellular organisms develop
specialized tissues.
[4]