- Investigation of functions of life in Paramecium and one named photosynthetic unicellular organism.
- Students are expected to be able to name and briefly explain these functions of life: nutrition, metabolism, growth, response, excretion, homeostasis and reproduction.
- Surface area to volume ratio is important in the limitation of cell size.
Unicellular Organisms
Unicellular organisms = organisms with one cell
The cell theory indicates that a cell exerts all functions of life so even though is a single cell, it is a life!
Functions of life:
- Metabolism: undergoes a chemical reaction to sustain life
- Growth: increase the size (multicellular) or cell number (unicellular)
- Response: response to the change of the external or internal environment
- Nutrition: obtain food from outside or self-synthesis process
- Reproduction: produce offspring sexually or asexually
- Excretion: Excrete the waste out from the body
- Homeostasis: control and maintain a stable internal condition
Unicellular Organisms Examples
Paramecium
- 200 um in size, can be observed under the light microscope
- Having cilia to move according to the environment alteration (Response)
- Having cilia to sweep food through the oral groove into the body. The food is engulfed and digested in the food vacuoles (Nutrition, Metabolism)
- To regulate water content inside the cell through contractile vacuole to prevent bursting (Homeostasis)
e.g. high salty water, the vacuole is filled with water.
distilled water, the vacuole excrete water. (Excretion) (you will learn the detail in osmosis chapter)
Chlorella
- Produce food through photosynthesis (Nutrition)
- Move toward the light (Response)
- The reproduction rate is fast, nitrate and phosphate boost up the growth (Reproduction/Growth)
Multicellular Organisms
multicellular organisms = body composed of >1 cells
Surface area to volume ratio
We human or your pet are multicellular organisms. But as an IB biology student, have you ever wondering why the multicellular organisms don't grow into a single big cell but require plenty of cells to constitute our body? It is due to the Surface area to Volume ratio
- Volume determines the cell metabolic activity, higher volume --> greater metabolic activity
Surface area determines the rate of material exchange, greater surface area --> better contact with the outside --> better material exchange with the outside
In the graph, the volume of a single cube to a large cube constituted by 27 cubes increases by 27 times, while the surface area just increases by 9 times. The increase is not proportional. (Imaging you are blowing a balloon)
The Larger the Volume, The Lower the ratio of Surface Area : Volume
If we grew as large single cell, sure the cell volume increases, but the ratio of Surface Area : Volume decreases which is not favouring material exchange. The cell can't efficiently excrete the waste to the outside and obtain sufficient food from the outside.
Increase Surface Area : Volume Ratio enhance the material exchange
To maintain a desirable ratio, the cells have two adaptations: folding, microvilli (you will learn in digestion chapter) to increase the surface area which boosts up the material exchange.
But once the cell reaches a limit, it divides --> multicellular, thus, a cell cannot unlimitedly grow.
In this IB Biology topic, you have to:
- state the Paramecium as the unicellular organisms example
- state the Chlorella as the Photosynthetic unicellular organism
- the concept of surface area : volume ratio, and how it affects the efficiency of material exchange
That is the end of today topics!~
Photo References:
1. Medical News. Retrieved URL from: news-medical.net
2. Diagram of Paramecium. Retrieved URL from: http://33.dv.zimmer-in-zwickau.de/diagram-of-paramecium.html
3. Chlorella: Therapeutic powers of the billion-year-old algae now within your reach. (24 June 2019). FactDr. Retrieved URL from: https://factdr.com/supplements/wellness/chlorella/
4. Paramecium. Wikipedia. Retrieved URL from: https://en.wikipedia.org/wiki/Paramecium