Cell biology

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…