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
- 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…