Protozoa

INFECTIOUS AGENTS AND PARASITES PARASITES

Introduction

Balantidium coli - trofozoite and cyst - Wikimedia Commons

Flagella of E.coli - Nicolle Rager Fuller, National Science Foundation

All protozoa are unicellular eukaryotic organisms which store their genetic information in chromosomes in a nuclear envelope. Protozoa are classified depending on their structure and life cycle. This reflects the similarities of the diseases which they cause.

Protozoa usually range from 10μm-50μm but can grow up to 1mm. Thus, they are usually observed and classified using a microscope.

Protozoa multiply sexually, asexually and can also use a combination of both, for example, the coccidia class. Replication can be by binary or multiple fission. Different protozoa use different forms of motility, including flagella, cilia, pseudopodia and gliding.

Structure and function

Motile

Organelles and otehr cellular structures enable the protozoa to lead an independent exsistence

Cilia
- Fine, short hairs covering the protozoal surface
- Hairs beat in unison to enable the protozoa to move
- Wafts food towards the cytostome (mouth opening)
- E.g. Balantidium

Flagellum
- Contractile fibre
- Contracts in a whip like motion to propel protozoa
- Attached to body of protozoa by an undulating membrane
- E.g. Trypanosoma

Pseudopodia
- Extensions of the cellular cytoplasm
- Cytoplasm flows into the pseudopodia allowing movement of the protozoa
- Also acts in a phagocytic manner surrounding food particles and enclosing it in a vacuole
- E.g. Entamoeba

Gliding
- No obvious means of locomotion
- E.g. Eimeria

Nutrition and digestion

Pinocytosis - Mariana Ruiz Villarreal

Pinocytosis
- Droplets of fluid taken into the cell
- Generates small vesicles
- Usually used for extracellular fluid ingestion
- Requires ATP

Phagocytosis
- Larger particles of matter taken into the cell
- Usually solid particles ingested

Cell membrane envelops the fluid or food taking it into the cell

Lysosomes fuse with the fluid/food initiating digestion

Diffusion through the cell membrane allows excretion of metabolic products

Life Cycle

Most protzoal reproduction is asexual using binary fission, schizogony and sporogony

Some protozoa also use sexual reproduction called gametogony

In some species, sexual and asexual reproduction occurs in the same host, whilst in others asexual reproduction occurs in the vertebrate host and sexual reproduction in the arthropod vector