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Cell theory, differentiation, stem cells

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Cell Theory
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Cells are the smallest units of life
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Only after exposure to pre-existing cells was life able to
re-establish itself in the sterilized chicken broth
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Metabolism
■ All chemical reactions in cells
■ Energy being converted from one form to another
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Reproduction
■ Either asexual or sexual
■ Hereditary molecules being passed to offspring
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Homeostasis
■ Organisms maintain a constant internal environment or at least close
to optimal state
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Excretion
■ Unusable or toxic compounds need to be disposed of
■ Essential for the organism

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Cell theory, differentiation, stem cells

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However, not all genes of the cell’s genome will be
expressed
○ This means that each cell will become a specific type of a cell depending on
which DNA segment (gene) becomes active
➢ Some cells can keep dividing after differentiation
○ For example, skin cells divide in order to heal wounds
○ Die very often and replace themselves
➢ Some are unable to do that
○ For example, nerve and muscle cells
○ Don’t die, until you die
○ Difficult to be repaired

Limiting cell size
➢ Most cells can only be seen by using a microscope
○ Cells are so small due to a principle called “the surface area to volume ratio”
■ As cells increase in volume, their surface area also increases, but at a
much slower rate

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Cell theory, differentiation, stem cells

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Therefore, they
need a lot of nutrition and have a lot of waste
But not all cells follow this norm
➢ Some cells have evolved into different shapes
○ Cells that are large in size have modifications that allow them to function
efficiently
■ Changes in shape, in- or out- foldings => increase of surface area
○ Some atypical examples
■ Striated muscles => flat and long rather than spherical
■ Giant algae => interconnected with each other with small openings
■ Acetate fungal hyphae => the roots of a certain fungi, physically
connected with each other, however they have their own organelles

Stem cells
➢ Stem cells are the cells that retain their ability to divide and differentiate into various
types of cells
➢ Plants have also stem cells in regions of meristematic tissue (near roots and stem
tips)
➢ Stem cells cannot be distinguished by their appearance
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Biology SL

○ Stem cells could replace lost or damages cells in the brain and pancreas
➢ The cells must come from an embryo, killing it in the process
○ Moral problem
○ Therapeutic cloning could also be used
■ The patient is cloned and the specially created embryo is then used to
harvest stem cells
➢ The blood from the umbilical cord also contains pluripotent stem cells
○ Small amount
○ Legally used
○ Cure some diseases

Tissue-specific stem cells
➢ Found in the bone marrow and deep inside the skin
○ Those from the bone can turn into blood cells
○ Those from the skin can turn into some skin tissues
➢ Can be transplanted to replace damaged bone marrow in Leukemia patients

Stargardt’s disease
➢ Victims cannot process vitamin A
○ Vitamin A is essential for the proper function of light-sensitive cells in the
retina
○ Victims lose central vision by their 20s and eventually go blind
○ Genetic, caused by both parents, quite rare
➢ Treated with the use of pluripotent stem cells since March 2010

Cells and sizes
➢ Resolution means the clarity of a viewed object
➢ In light microscopes light passes through an alive or dead specimen to form an image
○ Stains may be used to make it easier to see the details
○ Inexpensive to purchase and operate
○ Simple and easy specimen preparation
○ Magnifies up to 2000x
➢ In electron microscope, electrons pass through a dead specimen to form an image
○ Provide huge magnification and great resolution
○ Expensive to purchase and operate
○ Complex and lengthy specimen preparation
○ Magnifies over 500 000x
○ Specimens must be dead and fixed in a plastic material

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Cell theory, differentiation, stem cells

Topic 1

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