AH Biology - Unit 2 - KA5(b) - Parasitic Life Cycles

While some parasites can complete their life cycle using only one host, what do many parasites require to complete their life cycle?

While some parasites can complete their life cycle using only one host, many parasites require more than one host to complete their life cycle. In these cases hosts are split into two groups:

• The definitive host

• Intermediate host

What is a definitive host?

A definitive host is an organism in or on which a parasite reaches sexual maturity.

What is an intermediate host?

An intermediate host is an organism that is required to complete the parasites life cycle but the parasite does not reach sexual maturity in or on them.

In addition to hosts, what else is crucial for parasite transmission?

In addition to hosts, vectors are crucial for parasite transmission.

What are vectors?

Vectors are organisms that play an active role in parasite transmission. They may also be hosts themselves, but not always.

What are the two key examples of parasites?

The two key examples of parasites are:

• Plasmodium and malaria

• Schistosomes and schistosomiasis 

What is the plasmodium life cycle?

The plasmodium life cycle is:

1. An infected mosquito is a vector. It bites a human and transmits the plasmodium, the plasmodium enters the human blood stream.

2. The plasmodium reproduces asexually in the human liver, then red blood cells. They produce gametocytes (cells that divide by meiosis to form gametes).

3. The red blood cells burst, releasing gametocytes into the human blood stream.

4. Another mosquito bites an infected human and the gametocytes enter the mosquito.

5. The gametocytes mature into male and female gametes and sexual reproduction takes place. The mosquito can now infect another human host.

What is stage one of the plasmodium lifecycle?

Stage one of the plasmodium lifecycle is “An infected mosquito is a vector. It bites a human and transmits the plasmodium, the plasmodium enters the human blood stream.”

What is stage two of the plasmodium lifecycle?

Stage two of the plasmodium lifecycle is “The plasmodium reproduces asexually in the human liver, then red blood cells. They produce gametocytes (cells that divide by meiosis to form gametes).”

What is stage three of the plasmodium lifecycle?

Stage three of the plasmodium lifecycle is “The red blood cells burst, releasing gametocytes into the human blood stream.”

What is stage four of the plasmodium lifecycle?

Stage four of the plasmodium lifecycle is “Another mosquito bites an infected human and the gametocytes enter the mosquito.”

What is stage five of the plasmodium lifecycle?

Stage five of the plasmodium lifecycle is “The gametocytes mature into male and female gametes and sexual reproduction takes place. The mosquito can now infect another human host.”

What is plasmodium?

Plasmodium is a degenerate endoparasite.

In the plasmodium lifecycle what is the human?

In the plasmodium lifecycle, the human is the intermediate host.

In the plasmodium lifecycle, what is the mosquito?

In the plasmodium lifecycle, the mosquito is the definitive host and the vector.

What is the schistosome life cycle?

The schistosome life cycle is:

1. Schistosomes reproduce sexually in the human intestine.

2. The fertilised eggs pass out via the faeces into the water.

3. The fertilised eggs develop into larvae.

4. The larvae infect water snails. Asexual reproduction occurs in the water snails, producing another type of motile larvae.

5. The motile larvae escape the snail and penetrate the skin of the human, entering the bloodstream.

Is there a vector in the schistosome lifecycle?

There is no vector in the schistosome lifecycle.

What are schistosomes?

Schistosomes are degenerate endoparasites.

In the schistosome lifecycle, what is the human?

In the schistosome lifecycle, the human is the definitive host.

In the schistosome lifecycle, what is the water snail?

In the schistosome life cycle, the water snail is the intermediate host.

What does the water borne dispersal stage in the schistosome life cycle allow for?

The water borne dispersal stage in the schistosome life cycle allows for the parasite to complete it’s life cycle even if the host is incapacitated. 

What is stage one of the shistosome life cycle?

Stage one of the schistosome life cycle is “Schistosomes reproduce sexually in the human intestine."

What is stage two of the schistosome life cycle?

Stage two of the schistosome life cycle is “The fertilised eggs pass out via the faeces into the water.”

What is stage three of the schistosome life cycle?

Stage three of the schistosome life cycle is “The fertilised eggs develop into larvae”

What is stage four of the schistosome life cycle?

Stage four of the schistosome life cycle is “The larvae infect water snails. Asexual reproduction occurs in the water snails, producing another type of motile larvae.”

What is stage five of the schistosome life cycle?

Stage five of the schistosome life cycle is “The motile larvae escape the snail and penetrate the skin of the human, entering the bloodstream.”

What are viruses?

Viruses are parasites that only replicate inside a host cell.

What is the structure of a virus?

The structure of a virus is:

• Viruses contain genetic material in the form of DNA or RNA, which is packaged inside a protective protein coat.

• Some viruses are then surrounded by a phospholipid membrane which is made from host cell materials.

• The outer surface of a virus contains antigens. The host cell may, or may not, be able to detect these as foreign.

What is the viral life cycle?

The viral life cycle is:

1. The host cell is infected with genetic material.

2. Host cell enzymes replicate the viral genome.

3. Transcription of viral genes occurs.

4. Translation of viral proteins, such as the new viral coats, occurs

5. Assembly of new viral particles (genetic material is inserted into the protein coats).

6. Release of new viral particles occurs.

What is stage one of the viral life cycle?

Stage one of the viral life cycle is “The host cell is infected with genetic material.”

What is stage two of the viral life cycle?

Stage two of the viral life cycle is “Host cell enzymes replicate the viral genome.”

What is stage three of the viral life cycle?

Stage three of the viral life cycle is “Transcription of viral genes occurs.”

What is stage four of the viral life cycle?

Stage four of the viral life cycle is “Translation of viral proteins, such as the new viral coats, occurs”

What is stage five of the viral life cycle?

Stage five of the viral life cycle is “Assembly of new viral particles (genetic material is inserted into the protein coats).”

What is stage six of the viral life cycle?

Stage six of the viral life cycle is “Release of new viral particles occurs”

What are retroviruses and how do they form new viral particles?

Retroviruses are viruses that contain RNA as their nucleic acid. They use their host’s enzyme reverse transcriptase to form DNA. The DNA is then inserted into the host’s cell’s genome. Once this has happened, the viral genes can be expressed to form new viral particles.