Apicomplexa

Some tiny things live inside animals.

They are too small to see. They use a special tip to get inside. This tip helps them enter cells.

Once inside, they find food. They can make people and animals sick. Some of these tiny things move by gliding. They do not have legs or wings. They use small motors to move.

They can have very complex life cycles. They often need more than one host to live.

Apicomplexa are a large group of tiny, single-celled organisms.

Most of them are parasites. This means they live inside other animals to survive.

These tiny life forms have a special part called an apical complex. This is a set of parts at the top of the cell. It helps them break into and enter host cells. The name comes from Latin words meaning "top" and "infolds."

They also have a part called an apicoplast. This is a small part that helps the cell stay alive.

Many Apicomplexa move by gliding. They do not have legs. Instead, they use tiny motors and sticky parts to slide along.

Their life cycles are very complex. They often use two different hosts to grow. For example, some move from an insect to a vertebrate. Some can cause diseases like malaria. This happens when they live in blood cells.

Because they are so much like their hosts, it is hard to make medicine to stop them.

Apicomplexa are a large group of tiny, single-celled living things.

Most of these organisms are parasites. This means they live inside other animals to survive. Some live inside marine animals as partners, like the Chromerida that live with corals. However, many others are obligate parasites, which means they must live inside a host to complete their life cycle. This group is very diverse and includes many different types of organisms. They include coccidia, gregarines, and plasmodia.

These tiny cells have a special way of working. They use a unique structure called an apical complex at the top of the cell. This complex helps the parasite break into and enter a host cell. It contains parts like the rhoptry and micronemes, which are small bodies that release enzymes. These enzymes help the parasite invade the host. The cell also has an apicoplast. This is a small part that helps the cell make things like lipids and heme to stay alive.

The name Apicomplexa comes from two Latin words. The first word is apex, which means top. The second word is complexus, which means infolds. This describes the set of parts found in the sporozoite stage. Scientists used to call this whole group the Sporozoa. This name is now considered old or sometimes used as a synonym. Some members of this group move by gliding. They do this using small motors and sticky parts to slide through tissues.

The life cycles of these organisms are very complex. They often use both sexual and asexual ways to make more of themselves. Many species need two different hosts to finish their cycle. For example, some move from an insect to a vertebrate. In the blood, they can cause serious diseases. Malaria is caused by Plasmodium, which lives in red blood cells.

Other diseases include Babesiosis, Toxoplasmosis, and Cryptosporidiosis.

Understanding these parasites is a hard job for scientists. Because these organisms are eukaryotic, they are very similar to the animals they infect. They share many of the same ways of working. This makes it very difficult to create medicine to stop them. A drug that harms the parasite might also harm the human host. Scientists also find it hard to grow these parasites in a lab. This makes it difficult to study them and find new ways to fight the diseases they cause.

The Apicomplexa are a massive phylum of single-celled organisms known as alveolates.

Most members of this group are obligate endoparasites. This means they must live inside a host animal to survive and complete their life cycles. While most are parasites, some exceptions exist. For example, the Chromerida act as photosynthetic partners to corals. Another member, Nephromyces, lives as a symbiont in marine animals. Despite these differences, the group is defined by its unique cellular structures. They are primarily spore-forming and lack structures like cilia or most flagella.

What makes an apicomplexan truly unique is the apical complex. This name comes from the Latin words *apex*, meaning top, and *complexus*, meaning infolds. This structure is located at the anterior end of the cell. It is essential for the parasite to invade host cells. The complex includes a conoid, which is a spiral of microtubules. It also contains secretory organelles called rhoptries and micronemes. These bodies release enzymes that allow the parasite to penetrate host membranes.

Additionally, the cell contains dense granules with a mean diameter of 0.7 μm. These granules secrete contents after the parasite has entered the host's protective vacuole.

The internal machinery of these cells is highly specialized. Most apicomplexans contain an apicoplast. This is a non-photosynthetic plastid that helps with lipid and heme biosynthesis. It possesses its own 35 kilobase circular genome. This organelle is necessary for the parasite's survival. The cell is also surrounded by a pellicle made of three membrane layers. This structure is supported by vesicles called alveoli. This specific arrangement of membranes is why they are classified as alveolates.

Apicomplexans move in a way that is quite different from many other cells. They use a method called gliding motility. This process relies on adhesions and small, static myosin motors. This ability allows them to slide through tissues and enter host cells efficiently. While they lack most moving parts, certain gamete stages do possess flagella. These flagella are usually posteriorly directed and come in numbers of one to three. This movement is vital for the various stages of their complex life cycles.

The phylum is divided into several distinct groups. The coccidia are parasites of vertebrates, often living in the gut. They undergo stages called merogony, gametogony, and sporogony. The gregarines are different because they typically parasitize annelids, arthropods, and molluscs. Unlike coccidians, gregarine gamonts are large and live outside of host cells.

The haemosporidians, such as the genus *Plasmodium*, have even more complex cycles. They often alternate between an insect vector and a vertebrate host. Finally, the Marosporida is a newly recognized lineage. These organisms infect marine invertebrates and have very reduced apicoplast genomes.

Many apicomplexans are significant pathogens that cause serious human diseases. *Plasmodium* species cause malaria, which targets red blood cells.

Other notable examples include *Toxoplasma gondii*, which causes toxoplasmosis.

Other diseases include babesiosis, cryptosporidiosis, cyclosporiasis, and cystoisosporiasis. These parasites are difficult to study because they are eukaryotes. This means they share many metabolic pathways with their animal hosts. Because of this similarity, it is hard to develop drugs that kill the parasite without harming the human host.

Historically, these organisms were grouped together as the Sporozoa. This older classification was based on the fact that they generally lacked flagella, cilia, or pseudopods. However, modern science has refined this. The Sporozoa name is now often considered a synonym for Apicomplexa. Other groups once thought to be related, like the Myxozoa or Microsporidia, are now known to be different. Today, we understand the Apicomplexa through their complex evolutionary relationships and their highly specialized tools for survival within other living things.