IB Biology Topic B4.2.5: Mixotrophic Nutrition in Some Protists Notes

Mixotrophy: Some Protists Combine Two Ways of Feeding

1. Mixotrophs use both autotrophy and heterotrophy to meet their nutritional needs.
   1. Autotrophy makes organic compounds from inorganic sources, usually by photosynthesis.
   2. Heterotrophy takes in organic compounds made by other organisms.
2. This is common among protists, a diverse group of mostly single-celled eukaryotes.

Facultative vs Obligate: Whether the Second Mode Is Optional

1. Facultative mixotrophs can switch between the two modes as conditions change.
2. Obligate mixotrophs need both modes and cannot survive on one alone.

Euglena: A Facultative Mixotroph That Switches

Example

• Euglena gracilis is a classic facultative mixotroph.

• In bright, nutrient-poor water it uses its chloroplasts to photosynthesise.
• In dim or nutrient-rich water it switches to heterotrophy, taking in food by endocytosis.

1. This flexibility lets facultative mixotrophs cope where conditions keep changing.

Obligate Mixotrophs: Both Modes Are Essential

1. Obligate mixotrophs eat food to obtain carbon compounds they cannot make themselves.
2. Some lack their own chloroplasts and instead take chloroplasts from algae they consume, a process called kleptoplasty.

Why Mixotrophy Is an Advantage

1. Flexibility lets the organism survive in both nutrient-rich and nutrient-poor water.
2. Switching modes lets it keep taking in energy as conditions change.