Introduction of cells

The Cell Theory

• living organisms are composed of cells

Discrepancies (exceptions to the cell theory)

• organisms or parts of organisms that do not consist of typical cells.

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Organisms consisting of only one cell carry out all functions of life in that cell.

• Metabolism: chemical reactions inside the cell, including the cell respiration to release energy
• Reproduction: producing offspring either sexually or asexually
• Growth: an irreversible increase in size
• Response: the ability to react to changes in the environment
• Excretion: getting rid of the waste products of metabolism
• Nutrition: obtaining food, to provide energy and the materials needed for growth
• Homeostasis: keeping conditions inside the organism within tolerable limits

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Limitations on cell size

The surface area to volume ratio is important in the limitation of cell size.

• Substances move into and out of cells through the plasma membrane at the surface of the cell rate of substances across the membrane depends on SA
• if the ratio is small (elephant):
  • substances will not enter the cell as quickly as they are required
  • waste products will accumulate because they are produced more rapidly than excreted
  • cells may overheat because the metabolism produces heat faster than it is lost over the cell surface

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Multicellular Organisms

Multicellular organisms have properties that emerge from the interaction of their cellular components.

• organisms consisting of a single mass of cells, fused together, are multicellular
• example of a multicellular organism: Caenorhabditis elegans
  • the adult body is 1mm long and is made up of exactly 959 cells.
  • it feeds on the bacteria that cause decomposition
  • has a mouth, pharynx, intestine, and anus
  • hermaphrodite (contains both male and female reproductive organs
  • has neurons

Emergent properties

• individual cells in a group can organize themselves and interact with each other to form a living organism with distinctive overall properties. (1+1>2)
• properties that arise from the interaction of component parts

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Cell differentiation in multicellular organisms

Specialized tissues can develop by cell differentiation in multicellular organisms.

• the development of cells in different ways performs different functions
• a group of cells specialize in the same way to perform the same function is called tissue
• becoming specialized makes the cell more efficient than they had many different roles
• thus multicellular organisms are able to carry out multiple and complex functions because different cells responsible for different jobs

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Gene expression and cell differentiation

Differentiation involves the expression of some genes and not others in a cell's genome

• • differentiated cells get exactly the same set of genes
  • 25000 genes in the human genome are all present in every body cell
  • gene is being expressed means gene is being used, which can switch specific gene to make a protein or other gene product
  • cell differentiation happens because a different sequence of genes is expressed in different cell types

Stem cells

The capacity of stem cells to divide and differentiate along different pathways is necessary for embryonic development. It also makes stem cells suitable for therapeutic uses.

• stem cells can divide again and again to produce copious quantities of new cells, therefore they are useful for the growth of tissues or the replacement of cells that have been lost or damaged
• Stem cells are not fully differentiated. They can differentiate into different cell types. Scientists now add cytokines to ESCs to make them differentiate into the specific cell type
• 2 characteristics
  • self-renewal: the ability tounmergo numerous cycles of cell division while maintaining an unspecialized state
  • potency: the ability to differentiate into specialized cells

Totipotent (zygote) = can develop into any type of cell;

Pluripotent (embryonic stem cell) = can develop into most types of cells;

Multipotent (adult stem cell...) = can develop into a restricted range of cells;

Unipotent = can develop into one type of cell

• Stargardt's macular dystrophy
  • genetic disease develops in children between 6-12
  • Caused by the death of photoreceptor cells in the central portion of the retina called the macula.
  • The macula is responsible for sharp central vision — for tasks like reading, watching television, and looking at faces. It is also important for colour vision.
  • Symptoms are reduced central vision and reduced color vision, side vision is usually kept

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Magnification

image size = actual size ✖️ magnification

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About Microscope

SI unit

• Examples of sizes

  

  

Resolution-the ability to distinguish between two separate objects, depends on the wavelength of EM radiation used

Light Microscopes

Transmission Electron Microscope

• The specimen must be kept in a vacuum – air molecules would collide with electrons.

• Dense parts of the specimen absorb electrons – heavy metal stains increase the density gradient.

• The image is viewed on a screen or focussed onto photographic film to produce an electron micrograph.

• Advantages

  Very high resolution (0.5 nm)

• Disadvantages

  Specimen must be dead. Specimen deteriorates. Preparation may damage specimen. Preparation is difficult & time consuming. Very expensive.

Scanning Electron Microscope

Electron beam is scanned across the surface of the specimen, electrons are reflected and collected.

Forms a 3D image on a cathode ray tube.

The image is viewed on a screen or focussed onto photographic film to produce an electron micrograph.

• Advantages

  Excellent 3D images.

  Surface structures shown.

  Larger samples can be examined.

• Disadvantages

  Same as TEM. Resolution only 5 – 20 nm.

Light vs. Electron

• EM’s have a much greater resolution so are used to investigate the ultrastructure of a cell.
• LM’s are used to get an overall picture. They are also much quicker, easier and cheaper to use and can be used to view living specimens.

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IB question BANK

• Describe the characteristics of stem cells that make them potentially useful in medicine

  (stem cells) have/retain the capacity to divide; can be used to produce cell cultures/large number of identical cells; can be used to repair/replace damaged/lost cells/tissue; (stem cells) are undifferentiated / have not yet differentiated/specialized; can differentiate/specialize in different ways / are pluripotent/totipotent; can be used to form a variety of different tissues / form organs; used in medical research; used in treatment of (named) disease;

• Outline cell theory

  a. living things are composed of cells;

  b. cells are the basic/smallest unit of life;

  c. cells come from pre-existing cells;

• Explain the importance of surface area to volume ratio as a factor limiting cell size.

  as volume of a cell increases, the ratio of its surface area to volume decreases;

  food/oxygen enters through the surface of cells;

  wastes leave through the surface of cells;

  the rate of substance crossing the membrane depends on surface area;

  more metabolic activity in a larger cell means more food and oxygen required;

  large volume means longer diffusion time;

  (large volume) means more wastes produced;

  excess heat generated will not be lost efficiently (with low surface area to volume ratio);

  eventually surface area can no longer serve the requirements of the cell;

  this critical ratio stimulates mitosis;

  (thus) the size of the cell is reduced and kept within size limits;