Cell biology

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Principles of the Cell Theory

  • All living things are composed of cells (or cell products)
  • The cell is the smallest unit of life
  • Cells only arise from pre-existing cells

Certain types of cells / tissues do not conform to a standard notion of what constitutes a cell

  • Striated muscle fibres
    • Muscle cells fuse to form fibres that may be very long (>300mm)
    • Consequently, they have multiple nuclei despite being surrounded by a single, continuous plasma membrane.
    • Challenges the idea that cells always function as autonomous units.
  • Aseptate fungal hyphae
    • Fungi may have filamentous structures called hyphae, which are separated into cells by internal walls called septa.
    • Some fungi are not partitioned by septa and hence have a continuous cytoplasm along the length of the hyphae.
    • Challenges the idea that living structures are composed of discrete cells.
  • Giant Algae
    • Certain species of unicellular algae may grow to very large sizes (e.g. Acetabularia may exceed 7 cm in length).
    • Challenges the idea that larger organisms are always made of many microscopic cells

Unicellular organisms are the smallest organisms capable of independent life. 

All living things carry out 7 basic functions integral to survival:

  • Metabolism – Living things undertake essential chemical reactions
  • Reproduction – Living things produce offspring, either sexually or asexually
  • Sensitivity – Living things are responsive to internal and external stimuli
  • Homeostasis – Living things maintain a stable internal environment
  • Excretion – Living things exhibit the removal of waste products
  • Nutrition – Living things exchange materials and gases with the environment
  • Growth – Living things can move and change shape or size

SA:Vol Ratio

  • The rate of metabolism of a cell is a function of its mass / volume  (larger cells need more energy to sustain essential functions)
  • The rate of material exchange is a function of its surface area  (large membrane surface equates to more material movement).

As a cell grows, volume (units3) increases faster than surface area (units2), leading to a decreased SA:Vol ratio

  • If metabolic rate exceeds the rate of exchange of vital materials and wastes (low SA:Vol ratio), the cell will eventually die.
  • Hence growing cells tend to divide and remain small in order to

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