(9) The structure and features of some unicellular organisms

Protoctists (protists) are a varied collection of microscopic single-celled organisms. Some protoctists, like amoeba, that live in pond water and have features like an animal cell, but others, like chlorella, have chloroplasts and are more like plants because they can perform photosynthesis. Plasmodium is a pathogen responsible for causing malaria.

Although eukaryotic, they cannot be classified as plants, animals or fungi.

Protists

Protists are single celled organisms e.g. protozoan pathogens like those causing malaria, but this diverse group of organisms includes simple single celled algae and amoebas.

An amoeba is a classification of protist (single-celled eukaryotic organism that is neither plant, animal, bacteria, nor fungus) that are amorphous in shape.

They move via forming 'feet-like' pseudopodia, which are also used for feeding.

The simplified diagram below shows the structure of three singled cell organisms found in fresh water.

The cell membrane encloses the cytoplasm and the usual cell features like the nucleus.

The diagram also illustrates some structural adaptations via evolution to help the organism to survive e.g.

flagellum to move towards food or away from predators,

chloroplasts for photosynthesis, so the organism can make its own food.

Dinoflagellate

Dinoflagellates are protists, and in this case, it contains chloroplasts for photosynthesis and can manufacture its own food.

It also has flagellum that enables it to move at will in a forward direction with a 'swimming' action.

Other dinoflagellates do not have chloroplasts and feed on other microscopic creatures. feed on

Amoeba

The amoeba is a single celled primitive organism that does not contain chloroplasts, so cannot photosynthesize its own food and must scavenge for smaller organisms or waste decayed organic material.

The nucleus, mitochondria and vacuoles are shown.

The amoeba has a very flexible shape and can surround a food source and 'digest' it.

The amoeba can use a contractile vacuole to collect excess water and expel it out through the cell membrane.

Amoeba feed on algae and bacteria and tiny fragments of dead plant or animal material.

Euglena

Euglena are microscopic protists containing chloroplasts to photosynthesize sugars for food, but they can also ingest bacteria and small microbes for food too.

They has have flagellum to enable independent movement.

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Key points - Summary of ideas

Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

Eukaryotic Unicellular Organisms – Protists/Protoctists

Protists (or protoctists) are a diverse group of eukaryotic organisms that are mostly unicellular.

They do not fit into the categories of plants, animals, or fungi, and exhibit characteristics from each. Some are autotrophic (photosynthetic), while others are heterotrophic (consume organic matter.

Below are three examples: Amoeba, Dinoflagellate, and Euglena.

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Amoeba (Genus: Amoeba)

Amoebas are simple, unicellular organisms that move using pseudopodia (temporary extensions of the cytoplasm).

They are heterotrophic and engulf food particles through phagocytosis.

Key Features:

• Structure: No definite shape; changes as it moves.

• Locomotion: Moves by pseudopodia (extensions of cytoplasm).

• Nutrition: Heterotrophic; consumes bacteria and other small organisms.

• Osmoregulation: Uses a contractile vacuole to expel excess water.

• Reproduction: Binary fission (asexual).

Example of Adaptation:

Amoebas live in freshwater environments, where osmosis constantly brings water into the cell. The contractile vacuole expels excess water to prevent the cell from bursting.

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Dinoflagellate (Genus: Gonyaulax, Ceratium, etc.)

Dinoflagellates are unicellular planktonic organisms found in marine and freshwater ecosystems. Some species can be bioluminescent, while others contribute to harmful algal blooms known as red tides.

Key Features:

• Structure: Tough cellulose plates and two flagella for movement.

• Locomotion: Moves using two flagella (one encircling the body, one trailing).

• Nutrition: Some are autotrophic (photosynthetic); others heterotrophic.

• Bioluminescence: Some species glow due to luciferase enzymes.

• Reproduction: Mostly binary fission.

Example of Adaptation:

Certain species of dinoflagellates exhibit bioluminescence, which acts as a defense mechanism against predators.

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Euglena (Genus: Euglena)

Euglena is a mixotrophic unicellular organism that exhibits both plant-like and animal-like characteristics.

It contains chloroplasts for photosynthesis but can also consume organic material.

Key Features:

• Structure: Has both chloroplasts and an eyespot to detect light.

• Locomotion: Moves using flagellum (a whip-like tail).

• Nutrition: Can photosynthesize or absorb nutrients heterotrophically.

• Reproduction: Asexual (binary fission).

• Eyespot: Detects light to help Euglena move towards sunlight for photosynthesis.

Example of Adaptation:

In the absence of sunlight, Euglena switches to a heterotrophic mode by absorbing nutrients from the environment.

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Summary Table

Organism	Locomotion	Nutrition	Key Feature
Amoeba	Pseudopodia	Heterotrophic	Engulfs food via phagocytosis
Dinoflagellate	Flagella (2)	Mix of autotrophic and heterotrophic	Some species glow (bioluminescence)
Euglena	Flagellum	Mixotrophic	Photosynthesizes and detects light

These organisms demonstrate diverse modes of survival, from engulfing food to photosynthesis.

They play essential roles in ecosystems, acting as primary producers or decomposers

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Summary of learning objectives and key words or phrases

Be familiar with the diagram showing the structure of unicellular organisms - protists, amoeba, dinoflagellate euglena, and some of their adaptations e.g. flagellum for movement and chloroplasts for photosynthesis, and note whether they have a cell wall or cell membrane or both.

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