2.1: Cell Theory
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State the three points of the cell theory.
- The cell theory states that:
- All living things are made of cells and the product of cells
- All cells come from pre-existing cells
- The cell is the basic unit of life in which the processes of living take place.
Summarise the historical development of cell theory.
- 1590 - Zacharias Jensen - Invented the compound light microscope with two lenses to give greater magnification.
- 1685 - Robert Hooke - Studied cork using a compound light microscope and he named 'cells'.
- 1675 - Anotni van Leeuwenhoek - Observed micro-organisms
- 1824 - Rene Henn Dutrochet - The cell is the basic unit in living bodies.
- 1838 - Matthias Schleiden - All plants are made of cells
- 1839 - Theodor Schwann - All animals are made of cells. Created the term 'cell theory'
- 1840 - Jan Evangelista Purkinje - Names cellular contents 'protoplasm'
- 1858 - Rudolf Virchow - Proposed that cells come from pre-existing cells.
- 1880 - Walther Flemming - Described mitosis and cell division using living and sustained cells.
Outline why the development of the compound light microscope was essential in the formation of the cell theory.
The development of the compound light microscope provided the magnification necessary to observe cellular structures of plants, animals and unicells.
Outline how the development of the specific dyes assisted in the discovery of the internal structures of cells and assisted in the cell theory.
Specific dyes are used to show specific chemicals in cells and outline different structures inside cells. For example in 1849, Hartung developed the carminic acidic procedure and in 1882 Robert Koch introduced a staining technique to show the tuberculosis bacterium. The stained cells provided evidence for the cell theory and enabled further research, e.g. observation of cell and nuclear division.
Identify Robert Hooke's contribution to the development of the cell theory.
Robert Hooke made a microscope and through the microscope he observed a cork. In his book, there are various images which are seen through his microscope including animal, vegetable and minerals. The structures in cork he named 'cells' and his evidence provided the basis for the cell theory.
Identify one problem with staining.
Staining requires the cell to be preserved and 'fixed'. this reduces the ability to observe living cells carrying out specific functions.
What are the functions carried out by cells?
- Metabolism: chemical processes withing a cell where compounds are being broken down, synthesised and converted from one form to another. Includes respiration to release energy and excretion to remove unwanted products from the cell.
- Growth: cells can increase in size
- Reproduction: cells can divide to form new cells from the parent cell. Reproduction can be sexual or asexual
- Nutrition: Cells take in inorganic materials to form protoplasm and carry out the functions of living things
- Homeostasis: Cells maintain a constant internal environment
- Response to stimuli: Cells respond to external stimuli
Complete the following measurement conversions.
1 cm = ___ m?
1 mm = ___m?
1mm = ____ mm = ____ m?
1μm = ___ mm = ___ m?
1 nm = __ μm = ___ m?
10-3 = 10-6
10-3 = 10-9
Compare the relative sizes of molecules, cell membrane thickness, viruses, bacteria, organelles and cells, using the appropriate SI unit.
- A molecule = 1 nm
- Cell membrane thickness = 7.5 nm
- Virus = 100 nm (range: 20 - 200 nm)
- Bacteria = 1 - 5 um
- Organelles = <10 um
- Eukaryotic cells = <100 um
If you are given a photograph or diagram with a scale bar or information about magnification, what equation can you use to calculate the real size of the object in the photograph or diagram.
If given a photograph or diagram with given magnification, you can use the equation:
Real size = magnified size (measured by ruler)/magnification
Explain why cell size is dependent upon the surface area to volume ratio
Surface area to volume ratio determines the ability of a cell to exchange substances through the membrane with its environment and for the substances to reach all parts of the cell. A cell with a low surface area to volume ratio will retain heat and substances/wastes while a cell with a larger surface area to volume ratio will lose heat more quickly and diffusion will be more efficient in exchanging materials with the environment.
Observe the following table which summarises the surface area and identify which cube has the largest surface area:volume ratio and explain how this relates to the size of most cells.
Large cell -- 54cm2 -- 27mL3 -- 54:27 -- 1:2
Med cell -- 24cm2 -- 16 mL3 -- 24:8 -- 2:4
Small Cell -- 6cm2 -- 1ml3 -- 6:1 -- 1:6
The calculations show that the smallest cube has the greatest surface area:volume ratio. This means smaller cells are more efficient in exchanging substances with their environment than larger cells. Larger cells have a larger surface area, however it will take longer for substances to move throughout the entire large cell. To remain efficient, most cells remain small, e.g. they will divide when they reach a certain size.
Define emergent properties.
Emergent properties are properties that are found in higher biological orders where the sum of all the parts gives rise to new abilities.
Give an example of an emergent property.
An emergent property in humans is the ability to solve abstract problems. Lower biological orders have neurons to transmit signals and an anterior area that forms a brain. The architecture of the human brain enables rational thought processes, memory and abstract problem solving at a level not found in other species.
Explain why multicellular organisms show emergent properties.
A unicellular organism carries out all the basic requirements for life, e.g. growth, reproduction and homeostasis. Multicellular organisms show emergent properties as individual cells cooperate to form tissues, tissues form organs, organs form system and the combined systems cooperate to maintain life. The interaction and cooperation between cells, tissues, organs and systems provides multicellular organisms with abilities beyond the limitations of a single cell.
Discuss how the development of multicellular plants shows emergent properties.
There is a hierarchical organisation to form molecules, e.g. DNA molecules, to the organisation in a multicellular plant. The development of multicellular plants begins with a molecule to cell to tissue to organ to plant. At each step there are interactions that give the next level new properties. For example, chlorophyll is a compound; in chloroplasts it is involved in photosynthesis in a cell. Photosyntheitc cells are a part of leaf tissue which is also involved in gas exchange. Leaves, stems and roots are other organs that make up plants.
Differentiation is a process when cells become more specialized as they mature. They are no longer similar to the parent cell in structure or function.
Outline why the differences in cells is due to different gene expression and not differences in genes present.
The nucleus of each cell (except gametes which are haploid) contain a full set of chromosomes. Specialized cells change shape and function when specific genes are 'switched on'. This is differential gene expression and in humans lead to around 200 different cell types.
Discuss why various experiments were important in identifying the properties of differentiated cells.
The experiments show that mature cells, which have already differentiated (e.g. into root cells in the carrot), contain the genetic information and ability to produce a complete organism. Differentiation does not involve irreversible changes to DNA.
Discuss how altered gene expression can affect cell differentiation and lead to health issues.
When particular genes are 'switched on' they cause the cell to differentiate and become specialised. Proto-oncogenes code for proteins that regulate differentiation and cell growth. Health can be affected if the proto-oncogene becomes defective as it becomes an oncogene. Oncogenes increase the malignancy of tumour cells.
Define stem cell.
A stem cell is a relatively unspecialised cell that can divide by cell division to produce an identical daughter cell and to differentiate to form different specialised cells.
What is the difference between pluripotent stem cells, totipotent stem cells and multipotent stem cells?
Totipotent stem cells can give rise to any type of differentiated cell while pluripotent stem cells are able to give rise to many, but no tall cell types. Young embryos contain totipotent stem cells while adult stem cells are pluripotent. Multipotent stem cells, e.g. from the umbilical cord can become a limited number of particular types of cells.
What is the difference between an embryonic stem cell and an embryo cell that has differentiated?
Once a cell has differentiated cell determination occurs so that even if the cell is moved to another location it will not change into the type of cell at that location. Embryonic stem cells are totipotent and can reproduce indefinitely. Depending on the conditions they can become many types of cells. If these cells are removed from the embryo, the embryo dies.
The adult body has a variety of stem cells. What is the function of stem cells? Give some examples.
Adult stem cells are needed to replace non-reproducing specialised cells. Stem cells in bone marrow produce the different kinds of blood cells, e.g. T lymphocytes and B lymphocytes are produced by lymphoid stem cells and erythrocytes, monocytes and granulocytes are produced by myeloid stem cells. Stem cells in the intestinal wall regenerate the different cells in the lining of the intestine.
Identify an ethical issue associated with the use of stem cells.
The use of stem cells for research and medical procedures have involved debate and controversy. Most people are minimally concerned about the use of adult stem cells. Ethical issue arise as the procedure to obtain embryonic stem cells requires the embryo to be destroyed.
Outline why there is a great interest in using stem cells in medical treatments.
Since stem cells have the ability to develop and form specilaised tissues of the body, there is great interest in using stem cell cloning to treat a wide variety of degenerative diseases, e.g. multip sclerosis, Parkinson's disease. Research into the development of tissues and organs from stem cells, e.g. heart, kidney, spinal cord and insulin-producing cells from the pancreas could assist people who have limited treatment options for diseases that currently have no cure.
Bone marrow transplants have assisted people with leukaemia for over 40 years. Discuss how this is an example of a therapeutic use of stem cells.
Bone marrow contains haematopoietic stem cells (HS cells) - lymphoid stem cells that produce B lymphocytes and T lymphocytes and myeloid stem cells which produce erythorocytes, monocytes, granluocytes, and megakaryocytes. Healthy HS cells are used to treat several blood disorders, e.g. acute leukaemia, multiple myeloma and lymphoma. A lymphoma patient is given high doses of chemotherapy drugs to kill the cancer cells adn the HS cells are inserted to produce blood cells in the bone marrow. Bone marrow transplants have extended the life span of patients.
Discuss the potential of stem cell cloning for cell replacement therapy and tissue engineering.
Stem cell cloning has high potential for cell replacement therapy and tissue engineering. The process involves placing the nucleus from a body cell of the patient who needs an organ or tissue and placing it in a stem cell. The cell is stimulated to divide and then isolated cells are cultured to form the required organ or tissue. The culture tissue or organ can then be transplanted into the patient with no rejection problems.
Explain how adult stem cells are pluripotent while embryonic cells are totipotent.
Adult stem cells, e.g. from bone marrow can be cultured and when grown with specific conditions and growth factors will produce red blood cells. Adult stem cells are thus pluripotent producing multiple but not all cell types. The embryonic stem cells from the blastocyst can be cultured and, if placed in different condition with different growth factors, are able to produce differentiated cells of any type and thus totipotent.
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