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Cell Theory Summary

Retrospectively we regard the observations of Hooke and Leeuwenhoek as important contributions to the development of cell biology. However, at the time, the microscope was regarded by many as a toy and not as a serious scientific instrument. Furthermore, the philosophy of science was dominated by the physical sciences and ‘reductionism’ which held the view that all natural phenomena could ultimately all be best explained in terms of their constituent parts, namely molecules and atoms. Consequently, biology at this time was not established as a serious autonomous science. Rather, physics and chemistry were regarded as ‘science’.

The Dutch were regarded as the best lens makers of the time, and it is there that the best telescopes and microscopes were being made. However, Newton himself was not appointed to the Royal Society until (December 21, 1671 ) his reflecting telescope. Newtonian’s work became more famous this time and began with his discussion on how light is composed of a spectrum. The outstanding BBC has covered the context and historical development of optics broadcast In Our Time.

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Discuss the evidence of cell theory

a. All living things are made of cells:

When living things are observed under the microscope, they consistently appear to be composed of cells. However, several examples of cells do not conform to the standard notion of what a cell looks like at the microscopic level. This is a typical example of how variation in biological systems means that ‘biological theories’ are based on concepts or regularities.

Muscle cells:

Muscle cells have more than one nucleus per cell

Muscle Cells called fibres can be very long (300mm).

A single plasma membrane surrounds them, but they are multi-nucleated.(many nuclei).

This does not conform to the standard view of a small single nucleus within a cell.

Fungal Cells

Fungal Hyphae: again very large with many nuclei and a continuous cytoplasm

The tubular system of hyphae forms dense networks called mycelium.

Like muscle cells, they are multi-nucleated

They have cell walls composed of chitin.

The cytoplasm is continuous along the hyphae with no end cell wall or membrane.


A cell is capable of all necessary functions Amoeba Single-celled organisms have one region of cytoplasm surrounded by a cell membrane.

The Protoctista cell is unusual in that it performs all functions. In addition, such cells are usually much larger than other cells, such that some biologists consider them ‘acellular’, that is, non-cellular.

This is an image of an amoeba. A single cell Protoctista capable of all essential functions. What cell organelles can you see?

b. Cells are the smallest unit of life.

  • The cell is the smallest unit of organization that can show all the characteristics of living processes.
  • Organelles often require the cooperation of other organelles for their successful function.
  • An interesting example is the mitochondria which can replicate and carry out metabolism. However, the mitochondria need a metabolic product from the cytoplasm to begin their respiratory processes.

c. Cells come only from other cells.

  • Cells carry out a form of cell division to form new cells. This process of cell replication in eukaryotes is called mitosis, and in prokaryotes is called binary fission. This is because the parental cell divides to produce identical daughter cells.
  • This aspect of cell theory suggests that all cells, therefore, have a common ancestor, the original ancestral cell from which all other cells have arisen by descent. (origin of cellular life).
  • This relationship of a common ancestor, therefore, suggests that all organisms are related.
  • Cell theory replaces the former ideas of spontaneous generation or abiogenesis in which inanimate matter assembles itself into living forms. This was particularly believed to be the case in outbreaks of diseases. These ideas are then replaced by the work of Francesco Redi, Agostino Bassi, John Snow and Louis Pasteur. However, there remains the necessary idea that cellular life developed from a non-cellular form at some point. No doubt, this pre-cellular form was a replicating macromolecule, perhaps like RNA. Much of this discussion is based on the notion that life takes cellular form and that it is possible to define the exact boundary between living and non-living. However, some scientists suggest that too much importance is attached to resolving the question and that the definition of ‘life’ is of little value.

2.1.3 Unicellular organisms carry out all the functions of life.

Unicellular organisms (Protoctista) evolved some 3-4 Billion (109 ) years ago. These unicellular life forms remained the dominant ones until the Precambrian period 600million years ago, after which multi-cellular life forms proliferated. Other unicellular groups include the prokaryotes (Bacteria), which lack a true nucleus.

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These organisms can carry out all the processes which are characteristic of living things, such as:

a. metabolism, which includes respiration the synthesis of ATP.

b. response to a change in the environment

c. homeostasis the maintenance and regulation of internal cell conditions.

d. growth which for a unicellular organism means an increase in cell size and volume.

e. reproduction which for the unicellular organism is largely asexual through cell division to form a clone.

f. nutrition which means either the synthesis of organic molecules or the absorption of organic matter.

2.1.4 Relative sizes of structure and unicellular organism

We depend on the microscope for our observation of cellular structures. Observations of this time are, for the most part, dependable, but we must consider the introduction of ‘artifacts’ by those processes that prepare the material for microscopy. These artifacts are a consequence of specimen dehydration, contrast enhancement (staining), radiation and microscope function. These artifacts can lead to image or data distortions and misinterpretation.

Relative sizes:

1. molecules (1nm).

2. cell membrane thickness (10nm).

3. virus (100nm).

4. bacteria (1um).

5. organelles (less than 10um).

6. cells (<100 um).

7. generally, plant cells are larger than animal cells.

nm= nanometer (10-9m) um= micrometer (10-6m)

2.1.5 Magnification

On an image of a specimen, it is useful to show how much larger/smaller the image is than the real specimen. This is called magnification.

To calculate magnification:

using a ruler, measure the size of a large clear feature on the image

Measure the same length on the specimen

convert to the same units of measurement

* Magnification = measured length of the image /measured length of the specimen

* Length of the actual specimen = length on the image/ magnification ( e.g. rose-leaf = image length 4.2cm/ magnification 0.82 = 5cm real length

Scale Bars: images often carry a scale bar which is a horizontal line drawn on the image. The scale bar shows how long the line is in the real specimen.

This example shows a plant cell.

The scale bar indicates the length of 10 microns = 10um

Notice that 10 um is about the vertical length of the diameter of the nucleus.

All other measurements from the image are made relative to this scale bar.

2.16 Surface area: Volume ratios as a factor limiting cell size:

* As the size of a structure increases the surface area to volume ratio decreases.

* This can be seen by performing some simple calculations concerning different-sized organisms.

if you would like to make an Excel spreadsheet that calculates and shows the ratio of surface area to volume the you should use the formula




A1 would be the surface area cell, and B1 would be the volume cell.

* Therefore, the rate of exchange (diffusion/radiation) decreases

* This is true for organelles, cells, tissues, organs and organisms. It is also true for those structures that organisms build, e.g. termite mound

* All organisms need to exchange substances such as food, waste, gases and heat with their surroundings.

* These substances must be exchanged between the organism and its surroundings.

* The rate of exchange of substances depends on the organism’s surface area that is in contact with the surroundings.

* As organisms get bigger, their volume and surface area both get bigger, but not by the same amount.

* Conclusions:

* As the organism gets bigger, it is surface area: volume ratio decreases

Biological Consequences:

* Large organisms, the rate of exchange with their surroundings occurs more slowly.

Loxodonta africana (African elephant).

This is the largest land-living mammal. Adult African elephants can reach a length of 18-24 feet and a height of 10-13 feet. They weigh in at 8,800-15,500 pounds . The maximum size is reached at around 25 years of age.

According to the calculations above, the elephant should have a small surface area to volume ratio compared to smaller animals.

Heat exchange should be quite slow, which would be a problem for an animal living in a warm region.

The elephant has evolved large ears to increase the surface area for heat exchange allowing the elephant to remain cool.

* Small organisms have the opposite problem of a rapid rate of exchange with the environment.

Sorex minutus (pygmy shrew)

Size: Approximately 60mms from the tip of the nose to the base of the tail. The tail is around 40mms long.

Mass: 4 grams.

This shrew looses body heat so fast that it consumes food at a furious rate to produce heat from respiration. I understand that this particular species needs to eat every two hours to stay alive.

This is a brief introduction to scaling in biological systems. There are a variety of other fascinating examples.

2.1.7 Multicellular organisms show emergent properties

syllabus: ‘Emergent prrise from the interaction of the component parts; the whole is greater than the sum of the parts.

‘I define life as….a whole that this pre-supposed by all its parts’

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S. Coleridge

Systems biologists attempt to put together the parts that make up a system and then observe the properties that ’emerge’ from the system but could not have been predicted from the parts themselves.

As a model, consider the electric light bulb. The bulb is composed of a filament made of tungsten, a metal cup, and a glass container. We can study the parts individually, how they function and the properties they posses. These would be the properties of tungsten, the properties of the metal cup, and the glass container’s properties. When studied individually, they do not allow the prediction of the properties of the light bulb. Only when we combine them to form the bulb can these properties be determined. There is nothing supernatural about the emergent properties rather. It is simply the combination of the parts that result in new properties being shown.

Emergence and reductionism 2

The physical sciences approach is to reduce an inanimate phenomenon to its constituent parts, and that knowledge of these will explain the phenomena as a whole. The parts do not vary (otherwise, there would be more parts), and these are predictable within the laws and principles that describe them. Since the smallest parts are predictable, then the system as a whole is predictable. No new properties will arise from the sum of the parts, this is explanatory reductionism.

Biological systems need a different approach, population thinking, which acknowledges the role of variation in a population. Consequently, the deterministic laws and theories of the physical sciences do not apply to all aspects of biological systems. The ‘parts of the living system vary on both a phenotypic level and at the level of the genetic program. This is an important feature of the biological system (compared to the non-living) that is not just affected by the physicochemical laws but also by a genetic program.

Theory reduction is the concept that theories and laws in one science field are special cases of theories that are to be found in the physical sciences.

Emergence is the occurrence of unexpected characteristics or properties in a complex system. These properties emerge from the interaction of the ‘parts of the system. Remember that biology insists on population thinking so that we know the interacting ‘parts’ vary in themselves, and therefore their ’emerging’ properties can only be generalised. One of the classic examples cited is the emergent properties of water (fluidity) that cannot be predicted from knowledge of the constituent gases hydrogen and oxygen. On a biological scale, consider the current debate about the nature of human consciousness or the origin of life itself.

1Concise Oxford English Dictionary 10th edition revised: (2002), Oxford University Press: New York

2 Mayr, E (2004) What Makes Biology Unique? Cambridge University Press: Cambridge

2.1.8 Multicellular organisms and specialised cell function

* Multi-cellular organisms are large and have to specialise parts of their structure to complete the various functions characteristic of life.

* What is the benefit of differentiation and specialisation of tissues rather than all tissues carrying out all functions?

* An interesting parallel with economic theories is that the larger the collective economic group, the greater the number of specialisms, a rough guide that holds in living systems.

* Differentiation: Cells within a multi-cellular organism specialise in their function.

* Specialised cells have switched on particular genes (expressed) that correlate to these specialist functions.

* These specific gene expressions produce particular shapes, functions and adaptations within a cell.

* Therefore, a muscle cell will express muscle genes but not those genes for nerve cells.

* Specialisation is more efficient than the generalised plan when competing for a specific resource in a multi-cellular organism. Consider the role of water transport through the plant. In higher plants, we have specialisation for a tubular system called the xylem. This is a more efficient water transport than been passed by the mass movement of water from cell to cell. In the xylem, water can be moved very efficiently from underground to the canopy of the highest trees at minimal cost to the plant. If there is no specialised tissue for carrying water, the plant will rely on the movement of water by mass flow of diffusion, which is very slow. The plant is therefore limited in size and therefore cannot compete with larger species.

* The study of how cells become specialised is called embryology. This study area in biology has been developing very fast in recent times. Some of the discoveries about why some embryonic cells become nerves, muscles or blood cells have led to new ideas about the evolution of life. The new discipline is called evolutionary developmental biology or ‘Evo-devo’. The following text is a great introduction to what will become one of the most important aspects of biology for this century

Carrol, S.(2005). Endless forms are most beautiful.New York: Norton

2.1.9 Stem cells


A stem cell retains the capacity to divide and has the ability to differentiate along different pathways.

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* A stem cell can divide but has not yet expressed genes to specialise in a particular function. However, under the right conditions, stem cells can be induced to express particular genes and differentiate into a particular type of cell.

* Stem cells can be obtained from a variety of different places, including the blastocyte. Adults still possess stem cells in some organs but much less so than a child. Even the placenta can be a useful source of stem cells.

2.1.10 Therapeutic use of stem cells

1.Non-Hodgkins Lymphoma is a cancerous disease of the lymphatic system. Outline of the disease.

1. patient requires a heavy dose of radiation and or chemotherapy. This will destroy healthy blood tissue as well as the diseased tissue.

2. Blood is filtered for the presence of peripheral stem cells. Cells in the general circulation that can still differentiate into different types of a blood cell.

3. Bone marrow can be removed before treatment.

4. Chemotherapy supplies toxic drugs to kill the cancerous cells.

5. Radiation can be used to kill the cancerous cells, but in time, they adapt to this treatment so that radiation and chemotherapy are often used together.

6. Post radiation/ chemotherapy, the patient’s health blood tissues are also destroyed.

7. Health stem cells or marrow cells can be transplanted back to produce blood cells again

You may wish to think about more elaborate forms of stem cell therapy. The following information provides an introduction to these technologies.

2. Embryonic Stem cell therapy this animation is an excellent introduction to the use of embryonic stem cells for therapies.

3. Therapeutic cloning. This is a method of obtaining ES cells from someone who has already been born. These stem cells can be used to treat the individual without generating an immune response. Unfortunately, the human body recognizes and attacks foreign cells, including stem cells. This is a serious barrier to stem cell therapy.

The process of therapeutic cloning is shown in this diagram. It begins by taking a somatic (body) cell from the individual. Next, the somatic cell is fused with an egg that has had its nucleus removed. The resulting cell is genetically identical to the individual because it contains the DNA from its somatic cell. The new cell behaves like a fertilized egg and develops into a blastocyst. ES cells can be harvested from the blastocyst and grown in culture. These ES cells could be used to treat the individual without encountering resistance from their immune system.

Notice that we do not refer to this type of blastocyst as an embryo. This is because, technically speaking, an embryo is the result of the union of an egg and a sperm, which has not happened in this case. �

1. The patient requires the replacement of some diseased tissue. But, first, we obtain a health cell from the same patient.

2. At the same time, we require a human egg cell. This is mainly as the cell retains the tendency to divide, unlike the sample tissue from the patient.

3. The nucleus is removed from the egg and discarded. However, the cell body itself is retained.

4. The nucleus of the patient’s cell is removed and retained. The cell body of the patient’s cell is discarded.

5. The nucleus from the patient’s cell is transferred to the enucleated cell body.

6. The cells are then stimulated to divide, forming a clone.

7. The cell mass forms a blastocyst.

8. The inner cell mass becomes a source of totipotent stem cells. Totipotent means they are capable of being stimulated to become one of any cell.

9. Cells are stimulated using differentiation factors to become the type of cell required for therapy.

10. Therapy would require the transfer of the new healthy cell to the patient. In therapeutic cloning, these cells have the same immune system identified as the patient therefore, there is not immune rejection problem.

This technique mustn’t be confused with embryonic stem cell cultures or with reproductive cloning.



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