IB Guides

Unicellular Organisms

Understanding: Organisms consisting of only one cell carry out all functions of life in that cell.

Unicellular organisms consist of one cell. This single cell carries out all the functions of life which are:

- Nutrition: the ability to get food in order to provide energy as well as the substances needed for growth

- Growth: an increase in size which is irreversible 

- Response: the ability to react to changes in the environment 

- Excretion: the ability to remove waste products that occur as a consequence of metabolism 

- Metabolism: chemical reactions that occur inside the cell for the purpose of releasing energy (e.g. cell respiration) 

- Homeostasis: keeping the internal conditions of the organism stable and relatively constant 

- Reproduction: producing offspring (sexually or asexually) 


Application: Investigation of functions of life in Paramecium and one named photosynthetic unicellular organism.













Function		Paramecium

Figure 1.1.6 - Paramecium		Chlamydomonas 

 

 Figure 1.1.7 - Chlamydomonas 

Nutrition
Ingest small organisms and digest these            
through endocytosis (vesicles)


Contains a chloroplast and
thereby produces its own food
via photosynthesis
Growth
Nutrients from digestion are used to provide
energy and materials required for growth 


Is able to grow through the absorption
of minerals and photosynthesis
Excretion
Waste products from metabolism (e.g. CO2)
are expelled from the cell by diffusing out
the membrane  


Waste products such as oxygen deriving
from photosynthesis are expelled
outside the cell through diffusion
Response
The cilia help the cell to move around,
the paramecium moves toward or away
from external stimuli and therefore
responds to environmental changes

Has an eyespot that is able to detect the
brightest light, the chlamydomonas
moves towards the light stimuli and is
therefore capable of responding to
environmental changes  
Metabolism
Contains enzymes in the cytoplasm
which catalyse the metabolic reactions


Contains enzymes in the cytoplasm 
which catalyse the metabolic reactions
Reproduction
  Asexual (mitosis) as well as sexual
  reproduction (meiosis & gametes)

 
Asexual (mitosis) as well as sexual 
  reproduction (meiosis & gametes)
Homeostasis 
Contractile vacuoles in the cell fill up
with water and remove this water from
the cell  by expelling it through the
plasma membrane, this maintains the
water levels inside the cell relatively constant
Contractile vacuoles in the cell fill up
with water and remove this water from
the cell  by expelling it through the 
plasma membrane, this maintains the 
water levels inside the cell relatively constant

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