Topic 1: Cells – 1.1 Cell theory, cell specialization, and cell replacement

Topic 1: Cells

1.1 Cell theory, cell specialization, and cell replacement

Cell Theory

  • Cells are the basic unit of structure in all living things (smallest unit of life)
    Specialized structures within cells (organelles) carry out different functions. Organelles cannot survive alone.
  • All living organisms are composed of cells.
  • New cells are formed from pre-existing cells.
    Cells multiply through division
    All life evolved from simpler ancestors
    Mitosis results in genetically identical diploid daughter cells
    Meiosis generates haploid gametes (sex cells)

Cell Theory

  • Original Cell Theory
    • All living organisms are composed of cells. They may be unicellular or multicellular
    • The cell is the basic unit of life
    • Cells arise from pre-existing cells
  • Modernized ideas developed from cell theory
    • Energy flow occurs within cells
    • Hereditary information is passed from one cell to another
    • All cells have same basic composition
  • Characteristics of Life: Nutrition, Respiration, Growth, Excretion, Homeostasis, Reproduction, Movement
  • Young Cell → Cell Division → Cell Growth → Cell Specialization → Mature Cell
    • Programmed Cell Death

Functions of life (Mrs.H.Gren)

Metabolism: All the chemical reactions that occur within an organism
Reproduction: Hereditary molecules that can be passed to offspring
Sensitivity:
Homeostasis: the maintenance and regulation of internal cell condition e.g. temperature
Growth: limited but always evident in one way or another
Response: imperative for the survival of an organism
Excretion: Enable those chemical compounds that an organism cannot use or that may be toxic or harmful to it to be released from the organism’s system
Nutrition: Providing a source of compounds with many chemical bonds that can then be broken down to provide an organism with the energy necessary to maintain life

Cells and sizes

⇒ Why are cells small?

  • Diffusion pathway is smaller when cell is smaller, takes less time and energy to move.
  • Surface area to volume ration is large
  • Larger SA: VOL Ratio, more efficient diffusion
  • (However, loses heat and water more quickly)
  •  Cells need to exchange substances with their surroundings, such as food, waste, heat, and gases.
  • In the cytoplasm, chemical reactions take place which are known as metabolic reactions. These reactions produce heat, wastes, and also consume resources. The rate of these reactions is proportional to the volume of the cell, while the exchange of these materials and heat energy is a function of the cell’s surface area.
  • A cell increases in size, its surface area to volume ratio (SA/V) will decrease.
  • As the SA to volume ratio decreases, the rate or the cell’s ability to exchange materials through diffusion or radiation decreases.
  • If metabolism is to continue at an optimum rate, substances such as oxygen must be absorbed and waste products such as carbon-dioxide need to be removed.
  • Also if too much heat is produced during metabolism in comparison to the amount the cell is able to remove, the cell might overheat.
  • Therefore, the greater the SA/volume ratio is, the faster the cell can remove waste and heat, and absorb oxygen and nutrients essential for the
    cell to function properly.
  • Exception: small, warm-blooded mammals lose heat very quickly due to their large SA:Vol ratio e.g shrew; desert plants would lose water quickly with fat leaves, so they minimizetheir SA:Vol ratio in order to conserve more water e.g. cactus

Calculate magnification

Stem cells

Stem cells are characterized by the ability to divide through mitotic cell division and differentiate along different pathways to become a diverse range of specialized cell types.
At early embryonic stages, the stem cells can still divide have ability to become any type of cell, until they express certain genes and differentiate into a specific type of cell.
Two main types of stem cells are adult stem cells which are found in adult tissues such as the bone marrow and embryonic stem cells that are found in the inner cell mass of blastocysts.
Another source of stem cells is from the umbilical cord of newly born fetuses (cord blood stem cells)
Stem cells can be categorized into totipotent, pluripotent, multipotent and unipotent according to their ability to differentiate

The use of stem cells in some disease

Stargardt’s Macular Dystrophy Is a genetic disease that develops in children that can cause blindness
The disease affects a membrane protein in the retina causing the photoreceptor cells in the retina to become degenerative
The treatment involves injecting embryonic stem cells that can develop into retina cells in to the back of the eyeball
The cells attach to the retina and begin to grow, improving an individual’s vision, with limited side effects
Leukemia (same from above) – is caused by a mutation in the genes that control cell division, which will create an abnormal amount of white blood cells. These white blood cells are produced in the bone marrow
One of the greatest therapeutic successes for the use of stem cells has been for the treatment of leukemia or lymphomas through bone marrow transplants.
This involves using hematopoietic stem (HS) cells (blood stem cells) derived from bone marrow tissue.
These cells will divide continually to form new red and white blood cells.
Using a large needle, stem cells are removed from the bone marrow of the patient or from a donor person, such as a brother or a sister.
The patient undergoes chemotherapy and radiation therapy to kill the cancer cells in the bone marrow. However, normal dividing cells in the blood will also be killed.
After chemotherapy and radiation therapy the HS cells will be transplanted directly in to the bloodstream through a tube called a central venous catheter.
The stem cells find their way into the bone marrow, where they will begin reproducing and making healthy new blood cells.

Ethical concern of using stem cells

The therapeutic use of stem cells involves the creation and the death of an embryo that that has not yet differentiated in order to supply embryonic stem cell lines for stem cell research and stem cell therapies.
The biggest ethical concern involves the creation of a new human embryo. Is it ethically acceptable to create a human embryo for biomedical research even if the research and therapies developed from the research could save human lives? Different people have a views of when human life begins.

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