Eleonora Raimondo Introduction to cells Biology Exam
1.1Introduction to cells
The Big Picture
About 550 MYBP (million years before present), multicellular life appeared.
Multicellular life evolved from unicellular organisms that may have started to
work together, forming the first simple multicellular organisms. However, as
organisms kept on increasing in cell number and size, the distance between
some of their body cells and the environment kept on increasing. Therefore,
they could not continue to rely on a direct exchange of materials with their
environment to cater for all their needs and to get rid of waste products at an
adequate rate. This droves the exciting evolutionary processes of cell
differentiation, which allowed cells to specialize in doing specific functions.
These specialized cells were grouped together to form tissues that performed
specific functions. As a result, multicellular organisms evolved a range of
specialized tissues within their bodies, which
collectively allowed all life functions to be
performed.
The different cell types and tissues found in a
multicellular organism (a human being).
Cell Theory
In 1665, Robert hook, after observing plant cells in cork tissue under the
microscope, used the word ‘cell’ to refer to the smallest unit of life. As the
quality of light microscopes improved, more and more information about the
structure of cells was collected. Yet, there were no clear rules for telling the
difference between cells and other ‘non-cell’ structures.
The cell theory developed in 1838, by three scientists, namely: Theodor
Schwann, Matthias Schleiden and Rudolph Virchow. The cell theory offers a
widely accepted explanation of the relationship between cells and living
things.
Important: Development of cell theory was made possible thanks to advances
in microscopy.
Rules of the cell theory
• Living organisms are composed of cells (one or more) – cells are the
building blocks of organisms.
Eleonora Raimondo Introduction to cells Biology Exam
• Cells are the smallest units of life – a cell is the basic unit capable of
carrying out all the functions of a living organism.
• Cells come from pre-existing cells (omni cellulae e cellula) – cells do
not show spontaneous generation.
As microscopes improved, the cell theory was modified to include
additional statements.
Questioning the cell theory - atypical cells
Examples of atypical cells
Most organisms, whatever their size and complexity, conform to the cell theory.
However, there are a few types of cells that differ from typical cells (as
specified by the cell theory) and hence offer an interesting challenge to the cell
theory.
Striated muscle cell
Striated muscle tissue is composed of repeated units
called sarcomeres. These show a characteristic striped
(striated) pattern when viewed under the microscope.
This challenges the idea that a cell has one nucleus, as
the muscle cell (fibre) has more than one nucleus per
cell. That is, each cell is multinucleated. Additionally,
the average muscle fibre cell is about 30 mm long,
which is much larger than a typical cell.
Giant algae: Acetabularia
Acetabularia is a genus of single-celled green algae of
gigantic size, ranging from 0.5 to 10 cm in
length. Acetabularia consists of three easily distinguishable
parts, namely: the rhizoid (which looks like small roots), the
stalk and a top umbrella made of branches that may fuse
into a cap.
As a single-celled organism, Acetabularia challenges two
widely accepted notions about cells: that they must be
simple in structure and small in size.
Eleonora Raimondo Introduction to cells Biology Exam
Aseptate fungal hyphae (Septate and aseptate fungal cells)
Aseptate fungal hyphae are long threads (hyphae) with
many nuclei. They have no dividing cell walls,
called septa (singular: septum). The result of this is
shared cytoplasm and multiple nuclei (singular:
nucleus). This challenges the idea that a cell is a
single unit as the fungal hyphae have many nuclei, are very large and possess a
continuous, shared cytoplasm.
Investigating cells and tissues with a microscope
Size of a animal cell ranges from 10 to 20 μm in diameter, which is about one-
fifth of the size of the smallest particle visible to the naked eye.
Through light microscope (available at the beginning of the nineteenth century)
all plant and animal tissues were discovered to be made up of individual cells
and be able to be seen.
Similarly, the introduction of the far more powerful electron microscope in the
early 1940s allowed the full complexities of an internal fine structure of cells to
emerge.
Calculating magnification
1000 nm (nanometres) = 1 μm (micrometre)
1000 μm (micrometres) = 1 mm
(millimetre)
The formula used to calculate magnification is as
follows:
Magnification = Image size / actual size
Actual size= Image size/ magnification
Unicellular organisms
All living things can be classified as unicellular (single-celled) or multicellular
(many-celled) based on the number of cells that they possess. The whole body
of unicellular organisms is made up of only one cell and hence they should be
able to carry out all the life processes within this cell. Examples of unicellular
organisms include bacteria, archaea, protozoa, unicellular algae and unicellular
fungi.
Functions of life
Eleonora Raimondo Introduction to cells Biology Exam
Metabolism – The regular set of life-supporting chemical reactions that takes
place within the cells of living organisms.
Growth – An increase in size or shape that occurs over a period of time.
Response – A reaction by the living organism to changes in the external
environment.
Homeostasis – The maintenance of a constant internal environment by
regulating internal cell conditions.
Nutrition – The intake of nutrients, which may take different forms in different
organisms. Nutrition in plants involves making organic molecules (during
photosynthesis), while nutrition in animals and fungi involves the absorption of
organic matter.
Reproduction – The production of offspring, either sexually or asexually, to
pass on genetic information to the next generation.
Excretion – The removal of waste products of metabolism and other
unimportant materials from an organism.
A single living cell is capable of carrying out all life functions. In contrast, a
virus is a non-living example because it cannot carry out all the processes of
life. A virus has a protein coat and, has genetic material (DNA or RNA).
However, viruses do not metabolise or reproduce – this function is carried out
by the infected host cell. Because they exhibit no properties of life outside the
host cell and do not have a cellular structure, viruses are not regarded as living
entities.
Examples of unicellular organisms
Paramecium is a genus (group) of
unicellular protozoa. Paramecia are usually less
than 0.25 mm in size and are widespread in
aquatic environments, particularly in
stagnant ponds. They are heterotrophs, feeding
on food particles they encounter in their
environment. They can move in all directions
using their cilia, small hair-like structures, that
cover the whole body and beat rhythmically to
propel the cell in a given direction.
Eleonora Raimondo Introduction to cells Biology Exam
Chlamydomonas is a genus of unicellular green algae (Chlorophyta) distributed
all over the world, in soil, fresh water,
oceans, and even in snow on mountaintops.
The algae in this genus range in size from 10 to 30
µm in diameter and have a cell wall, a
chloroplast, an 'eye' that detects light, as
well as two flagella (whip-like structures),
which they use to swim using a breaststroke-
type motion. Chlamydomonas are autotrophs;
they can manufacture their own food using their large
chloroplast to photosynthesise.
Paramecium and Chlamydomonas exemplify how unicellular organisms carry
out all the life processes within the one cell that makes up their whole body.
Life functions Paramecium Chlamydomonas
Metabolism Most metabolic reactions are catalysed by enzymes and take
place in the cytoplasm.
Growth As it consumes food, the Production of organic
Paramecium enlarges. Once it molecules during
reaches a certain size it will divide photosynthesis and
into two daughter cells. absorption of minerals
causes the organism to
increase in size. Once it
reaches a certain size it
will divide into two
daughter cells.
Response The wave action of the beating cilia Chlamydomonas senses
helps to propel Paramecium in light changes in its
response to changes in the environment using its
environment, e.g. towards warmer eye spot and then uses
water and away from cool its flagella to move
Eleonora Raimondo Introduction to cells Biology Exam
temperatures. towards a brighter
region to increase the
rate of photosynthesis.
Homeostasis A constant internal environment is maintained by collecting
excess water in the contractile vacuoles and then expelling it
through the plasma membrane. This process is called
osmoregulation and helps Paramecium and Chlamydomonas
to maintain their water balance.
Nutrition Paramecium is a heterotroph. It Chlamydomonas is an
engulfs food particles in vacuoles autotroph; it uses its
where digestion takes place. The large chloroplast to
soluble products are then absorbed carry out
into the cytoplasm of the cell. It photosynthesis to
feeds on microorganisms, such as produce its own food.
bacteria, algae and yeasts.
Reproduction It can carry out both sexual and It can carry out both
asexual reproduction, though the sexual and asexual
latter is more common. The cell reproduction. When
divides into two daughter cells in a Chlamydomonas
process called binary fission reaches a certain size,
(asexual reproduction). each cell reproduces,
either by binary fission
or sexual reproduction.
Excretion Digested nutrients from the food It uses the whole
vacuoles pass into the cytoplasm, surface of its plasma
and the vacuole shrinks. When the membrane to excrete its
vacuole, with its fully digested waste products.
contents, reaches the Paramecium's
anal pore, it ruptures, expelling its
waste contents to the environment.
Definition
Heterotroph: organism that feeds by taking in organic substances (usually
other living things).
Autotroph: organism that can produce its own food from inorganic sources.
Surface area to volume ratio
Cells are microscopic, ranging in size from 1 to 100 μm. They cannot continue
to grow indefinitely – at a certain size, they will divide.
To survive, a cell needs to import molecules and
expel waste products through its plasma
membrane. If a cell's surface area is too small
Eleonora Raimondo Introduction to cells Biology Exam
compared to its volume, not enough of the necessary molecules can get in and
not enough can get out. By dividing into two smaller cells, a larger surface area
to volume (SA:V) ratio is restored. Therefore, the surface area to volume ratio
limits the overall size of a cell.
In the digestive system, the small intestine has small folds that increase the
surface area exposed to the digested food. These folds are called villi: where
nutrients are absorbed in the body. The villi increase the surface area and allow
more absorption to take place. Additionally, the cells making up the villi
have microvilli, which are small folds on their cell membranes which increase
the surface area even more. Celiac disease is a disorder where the body’s
immune system destroys the villi in the small intestine, part of a complicated
response to gluten in the diet.
Surface area 6 sides × 12 = 6 6 sides × 22 = 24 6 sides × 42 = 96
mm2 mm2 mm2
Volume 13 = 1 mm3 23 = 8 mm3 43 = 64 mm3
Surface area to 6:1 3:1 1.5:1
volume ratio
Multicellular organisms: From one cell to many
• Organisms grew larger because they were no longer limited by the
size of one cell.
• Cells in such an organism were able to specialise through
differentiation. Differentiation is a process in which unspecialised
cells develop into cells with a more distinct structure and function.
• Multicellular organisms displayed emergent properties. This means
that the whole organism can do more things than individual cells are
capable of, because of the interaction between the different parts.
In multicellular organisms, all started out as stem cells, but during the course of
their embryonic development these cells differentiated and became more
specialised cells.
Specialized cells have different functions results in emergent properties
appearing, and more complex functions may be performed compared to
individual cells.
Eleonora Raimondo Introduction to cells Biology Exam
Cell differentiation
Definition:
• Genome refers to the complete set of genes, chromosomes or genetic
material present in a cell or organism.
• When an unspecialised stem cell changes and carries out a specific
function in the body, the process is called cellular differentiation.
Cells differentiate to form different cell types due to the expression of
different genes.
• Emergent properties, a complex system possesses properties that its
constituent parts do not have – the whole is more than the sum of its
parts.
The process by which a fertilised egg develops into a fully grown human being
is long and complex. At a very early stage, once the fertilised egg starts to
divide, some cells start to develop differently. Up to that stage, the cells in an
embryo are pluripotent embryonic stem cells. This means that they can develop
into any type of body cell.
Although each cell has the same genome, only certain genes are switched on in
certain cells and not in others. Groups of cells differentiate along different
pathways to form the different specialised tissues of the embryo. Once a cell
starts to differentiate, this process cannot be reversed under normal conditions.
STEM CELLS
Definition: A stem cell is an undifferentiated cell of a multicellular organism
that can form more cells of the same type indefinitely, and from which certain
other kinds of cells arise by differentiation.
Stem cells are unspecialised cells that can give rise to a wide range of body cells
by differentiating along different pathways. They retain the capacity to divide
indefinitely and have the potential to differentiate into specialised cell types
when given the right stimulus. Not all stem cells can give rise to all body cells.
Type of stem cell Differentiated cells produced
Totipotent stem cells, e.g. the eight Can differentiate into any type of cell
cells of the morula (the first cells including placental cells.
formed following fertilisation of an
egg cell) Can give rise to a complete organism.
Eleonora Raimondo Introduction to cells Biology Exam
Pluripotent stem cells (e.g. Can differentiate into all body cells,
embryonic stem cells of the but cannot give rise to a whole
blastocyst) organism.
Multipotent stem cells (e.g. Can differentiate into a few closely
umbilical cord stem cells) related types of body cell.
Unipotent stem cells Can only differentiate into their
associated cell type. For example, liver
stem cells can only make liver cells.
Embryos are important sources of stem cells. Once an egg has
been fertilised, it starts to divide and forms totipotent
cells during the early stages, up until the eight-cell
stage of the morula. Theoretically, each cell can still
develop into a full and normal organism. These cells continue
to divide and develop to form the pluripotent cells of the blastocyst
from which all the specialised tissues of the
developing embryo are generated.
Modern medicine has identified enormous therapeutic
potential for these cells. Treatment for Parkinson's disease,
the growth of new transplant
organs and treatment of
leukemia are a few potential
uses for stem cells.
The diagram below shows the possible
uses of human stem cells. Practically
any type of differentiated cell can be
developed from stem cells.
Use of stem cells to treat disease
Using stem cells to treat diseases is a relatively new technology. Most
treatments are still at the experimental stage. However, scientists believe that
in the near future we will be able to treat a whole range of diseases using stem-
cell therapies. Likely candidates for treatment using stem-cell therapies include
many common diseases, such as heart disease and diabetes.
Use of stem cells in treatment of Stargardt’s disease.
Eleonora Raimondo Introduction to cells Biology Exam
Stargardt's disease is a disease of
the eye. It is an inherited form of
juvenile macular degeneration
(affects a small area near the centre
of the retina) that causes
progressive loss of central vision.
It is estimated that Stargardt's
disease affects 1 in 8,000 to 10,000
individuals. Appears in late
childhood to early adulthood.
Caused by a recessive genetic
mutation in gene ABCA4, which
causes an active transport protein on photoreceptor cells to malfunction. This
ultimately causes the photoreceptor cells to degenerate.
Stem-cell therapy has been shown to be effective in treating Stargardt's disease.
Patients are given retinal cells derived from human embryonic stem cells,
which are injected into the retina. The results obtained have been quite positive
as the inserted cells attach to the retina and become functional, suggesting that it
may be possible to restore sight to affected individuals using stem cells.
Stem cells' use in treatment of leukemia
Leukemia, a type of cancer of the blood or bone marrow, is caused by high
levels of abnormal white blood cells. People with leukemia have a higher risk of
developing infections, anemia and bleeding. Leukemia is among the first
diseases to have been successfully treated by using stem cells.
Treatment in this case involves harvesting hematopoietic stem cells (HSCs),
which are multipotent stem cells. HSCs can be taken from bone marrow,
peripheral blood or umbilical cord blood. The HSCs may come from either
the patient or from a suitable donor. The patient then undergoes chemotherapy
and radiotherapy to get rid of the diseased white blood cells. The next step
involves transplanting HSCs back into the bone marrow, where they
differentiate to form new healthy white blood cells.
Ethical issues of using stem cells
Stem cells can be obtained from different sources including specially created
embryos, umbilical cord blood of a newborn and from an adult’s own tissues.
But, given that some of the methods of obtaining stem cells may involve
destruction of an embryo, ethical issues are raised.
Nature of science
Eleonora Raimondo Introduction to cells Biology Exam
As we have seen, stem cell therapies are becoming increasingly important and
offer the potential to treat a huge number of diseases, reducing the suffering of
many. However, current methods of obtaining stem cells, particularly those
involving specially created embryos, present ethical questions
Stem cells from embryos
Of the different sources of stem cells, harvesting from embryos is the most
controversial. Arguments supporting the use of embryos for the harvest of stem
cells include:
Cells may be used in cell therapy (replacing bad cells with good ones) to
eliminate serious diseases or disabilities in the human population.
Transplants can be easily obtained without requiring the death of
another human or inflicting any kind of pressure on normal body
functioning which happens when someone donates an organ.
The stem cells are harvested from the embryo at an early stage when the
embryo has not yet developed a nervous system and thus it is not likely to
feel any pain.
The figure below shows how embryos are created
to produce stem cells that correspond to those of the
patient.
1. The seven functions of life are metabolism, growth, response,
nutrition, excretion, reproduction, and homeostasis.
2. A student is preparing a model of a cell. The purpose of this model cell is
to exchange materials with the external environment so that the exchange
can be studied. What is the best model to facilitate exchange of materials
between the cell and its environment? a small cell has a large surface area
to volume ratio
3. A Chlorella cell viewed through a microscope is 0.00005 m wide when
the magnification is ×2000. What is the actual size of the cell?
The actual size is the magnified size divided by magnification:
Eleonora Raimondo Introduction to cells Biology Exam
0.00005 m ÷ 2 000 = 0.000000025m. Since the answers are given in
micrometers, we must convert. 1 micrometer = 1e-6 m (0.000001 m) so
0.000000025 m/0.000001 m = 0.025 micrometers
4. A specimen is measured to be approximately 1μm in width when
observed under a microscope. Deduce the type of cell/structure that it is
most likely to be: Bacterium
5. Identify the unicellular organism shown in the diagram:
Paramecium
6. Striated muscle cells (i.e., cells found in skeletal muscle) are
atypical to cell theory. Which of the following statements is
challenged by striated muscle cells? That cells can have only one
nucleus
7. Which of the following is false about a living cell’s surface area to
volume ratio? The surface area to volume ratio is greater in larger cells
than in smaller ones.
8. In a cell, what is the effect of a large surface area to volume ratio? Faster
uptake of oxygen into the cell.
9. Which of the following criteria is a characteristic of life?
I: Locomotion
II: Reproduction
III: Growth
10.What is proportional to the cell's surface area? The rate of exchange of
materials
11.What describes tissues? Groups of similar cells from the same origin with
the same function.
WHAT DO I HAVE TO KNOW
Outline and discuss the cell theory.
Describe atypical examples of cells, including striated muscle, giant algae
and aseptate fungal hyphae.
Calculate the magnification of cells by using a scale bar or measurement
of drawings and specimen.
Describe all the functions of life in a cell.
List the functions of life in Paramecium and one named photosynthetic
unicellular organism.
Explain how cell size is limited by surface area to volume ratio.
Describe the properties that emerge from the interaction of their cellular
components in multicellular organisms.
Eleonora Raimondo Introduction to cells Biology Exam
State that differentiation involves the expression of some genes and not
others in a cell’s genome.
Explain the capacity of stem cells to divide and differentiate along
different pathways.
Explain how this is necessary in embryonic development and also how it
makes stem cells suitable for therapeutic uses.
Discuss the use of stem cells to treat Stargardt’s disease and one other
named condition.
Discuss the ethics of the therapeutic use of stem cells from specially
created embryos, from the umbilical cord blood of a newborn baby and
from an adult’s own tissues.
