Heterotrophic protists are not capable of making their own food. In order to live, they have to obtain carbon containing nutrients from the environment or by ingesting other living organisms. This lesson will describe a few of the mechanisms they use for feeding.
What are Protists?
Because they are such a diverse kingdom, it's easier to define protists by describing what they are not rather than describing what they have in common. The biggest shared feature of protists is that they don't belong in any of the other eukaryotic kingdoms. Protists are not plants, animals, or fungi. For example, kelp or seaweed appear to have leaves and roots like plants, but do not have the same specialized tissues.
All organisms need to obtain organic carbon and energy to stay alive. Protists have a variety of ways getting both carbon and energy. Some protists are autotrophic and are able to make organic carbon-containing nutrients like glucose. Other protists are heterotrophic, and can't make their own carbon containing nutrients. Heterotrophic protists have to obtain carbon-containing nutrients by ingesting them -- by 'eating' other organisms or decaying organic matter in the environment.
How Do Heterotrophic Protists 'Eat'?
Heterotrophic protists have many ways to obtain nutrients from the environment. Some have specialized structures designed for obtaining food while others are passive and have to wait on the nutrients to come into contact with their plasma membrane.
Many protists are predatory and have specialized structures designed to capture and aid in bringing nutrients inside their cells. Some have structures that are kind of like mouths, and others fold themselves around and slowly dissolve their prey in order to obtain the carbon necessary to live. Moreover, some protists have 'weapons' that allow them to trap, paralyze, or kill their prey.
Filter Feeding
Protists such as paramecium use cilia to filter feed. Some filter feeders such as the paramecium have trichocyst, which are specialized cilia that the organism projects in order to paralyze or kill prey. Filter feeders use their cilia to create a current that brings prey close their oral groove that connects to the cytopharynx, which is a structure directs the ingested food into the cytostome. The cytostome is similar to a mouth in that it functions as an opening to allow food to enter the cell. Once inside the cytostome, the food will enter a food vacuole that contains enzymes that break down food into smaller components the protists can then use as nutrients. Once the food is digested, the waste is expelled from the cell through the cytopyge, which serves as the anus.
Figure 1: Paramecium uses the cilia surrounding their oral groove to filter food from the environment.
Filter Feeding
Phagocytosis
Phagocytosis is a process where a cell extends its plasma membrane around a large particle to be internalized. Extensions of the plasma membrane are called pseudopodia. Some amoebas use phagocytosis to ingest prey. To accomplish this, they extend and wrap pseudopodia around the organism to be eaten. Think of the process as a fatal hug. Once inside the amoeba, the prey is digested in a food vacuole.
Figure 2: Amoebas embrace their prey until they are taken inside of a food vacuole and digested.
Phagocytosis
The foraminifera are amoebas that have shells. The presence of a shell makes phagocytosis a bit more complicated, so these amoebas extend thin pseudopodia called filopodia, which capture prey. Once the prey is trapped, the amoeba is able to steal nutrients.
Figure 3: Amoebas with shells entangle their prey in filopodia.
Filopodia
Peduncle
Some species of dinoflagellate protists form a peduncle ,which is a tube-shaped structure used to suck nutrients out of prey. Some dinoflagellates even steal mitochondria and chloroplasts from their prey. When dinoflagellates steal chloroplast, they temporarily gain the ability to do photosynthesis in addition to carbon containing nutrients.
Figure 4: Protists like the dinoflagellate dinophysis can steal nutrients, chloroplasts, and mitochondria using a peduncle.
Peduncle
Pallium
A pallium is similar to a peduncle except that instead of being tubular, it is shaped like a veil or net. It is as if the protists wrap their prey in a blanket from which there is no escape. The pallium encompasses the prey and removes the cytoplasm.
Figure 5: Protoperidinium uses a pallium to capture and remove the cytoplasm from its prey.
Pallium
How Can They Eat if They Don't Have Mouths?
A few species of protists do not have specialized structures to catch or ingest nutrients. These protists must absorb nutrients from the environment through their plasma membranes. Many of these organisms are saprophytic and feed on dead organisms. Since most carbon-containing nutrients are too large to fit through the plasma membrane, specialized proteins embedded in the membrane bring the nutrients into the cell.
Explanation:
