Classification (CIE A Level Biology)

A team of biologists discovered a new organism inside a previously unexplored cave. They conducted several investigations in order to classify it.

They observed the following:

• The organism is multicellular and possesses a well-defined nucleus within its cells
• It has a cell wall composed of chitin
• It reproduces asexually by budding
• It exhibits heterotrophic nutrition

Identify the kingdom to which this organism most likely belong. Justify your answer.

Describe three other characteristics that would further support your classification of this organism within the identified kingdom.

[3]

Discuss the limitations of using only a few characteristics to classify organisms. Include examples to illustrate your points.

The organism found does not have either a gas exchange system or a vascular system.

Suggest how the organism can survive without these structures.

Part of the genome of many viruses is a gene for the enzyme reverse transcriptase. As its name suggests, this enzyme can synthesise DNA from an RNA template, which is the reverse reaction to transcription within most other organisms' cells. 

Suggest one advantage to the virus of producing the enzyme reverse transcriptase. 

Explain why viruses are not classified within the traditional three-domain system of Archaea, Prokaryotes and Eukaryotes. 

The number of fully assembled virus particles (virions) released from an individual host cell after an infection can vary from tens of virions to tens of thousands. For example, the HIV virus can burst out around 5 000 virions from a single infected host T-cell.

Calculate the maximum theoretical number of T-cells that can be destroyed by a single host cell infection followed by one further cycle of infection.

(1)

(ii)

Suggest the implications to a human of your answer to part (i) in the light of the fact that a healthy adult human possesses around 50 million T-cells.

(2)

Fig. 1 shows Ailuropoda melanoleuca, the giant panda (left), and Ailurus fulgens, the red panda (right), both mammals native to China.

Public domain, via pxhere	Mathias Appel, public domain, via Flickr

Fig. 1

Giant pandas and red pandas have the following characteristics:

• They live in similar environments
• They digest bamboo as a primary source of food
• They have a pseudo-thumb (a sixth digit) used to grip and shred food

Table 1 shows the differences between a comparable sequence of mitochondrial DNA from the giant panda and the red panda.

Table 1

A group of students concluded that the giant panda and the red panda are closely related species.

Use the information provided to discuss the students' conclusion.

The DNA analysed in Table 1 was extracted from mitochondria and codes for mitochondrial proteins.

Identify two examples of mitochondrial proteins. Precise names of proteins are not required.

[2]

(ii)

Suggest why scientists researching evolutionary relationships may choose to analyse mitochondrial DNA rather than DNA that codes for other characteristics, e.g. fur colour.

[2]

Table 2 contains the taxonomic classification of giant pandas, red pandas, and two other species considered to be relatives of the red panda.

Table 2

*Now extinct

Identify the order to which all four species belong.

[1]

(ii)

Identify the closest relative of the red panda shown within Table 2. Justify your answer.

[2]

(iii)

Suggest how scientists may have gone about classifying the now extinct Parailurus anglicus.

[2]

Current evidence shows that the closest living relative of the red panda shown in Table 2 is the common raccoon, though the common ancestor of the two species is thought to have lived between 30-40 million years ago, so the two groups of animals are now highly diverged from each other.

Explain how a common ancestor of red pandas and common raccoons may have diverged to give rise to new species.

The Fig. 1 below shows an image of Halobacterium salinarum, a member of the domain Archaea.

CC BY-SA 4.0, via Wikimedia Commons

Fig. 1

The scale bar in Fig. 1 measures 3 cm, and line X measures 12.2 cm.

Use this information to calculate the actual length of the archaeon, marked X in in Fig. 1.

[2]

Compare Halobacterium salinarum with a bacterial cell.

[4]

Halobacterium salinarum have a protein in their cell surface membranes known as bacteriorhodopsin. The role of bacteriorhodopsin is shown in Fig. 2 below.

Fig. 2

H. salinarum is a photoheterotroph, meaning that it can consume organic molecules, but it can also use light energy to produce ATP.

Suggest how bacteriorhodopsin allows H. salinarum to produce ATP.

[3]

H. salinarum is found in environments with extremely high salt concentration.

Suggest how bacteriorhodopsin might work together with other membrane proteins, such as K⁺ and Cl^- pumps, to aid survival in such extreme environments.

[3]

One study found that H. salinarum can exist in extreme environments alongside structures known as viroids. Fig. 3 below shows an example of a viroid structure. Note that viroid RNA does not code for proteins.

Fig. 3

Viroids can infect plant cells.

Explain how the infectious action of a viroid might differ from that of a virus.

[2]

(ii)

Suggest two possible mechanisms for the classification of viroids.

[2]

Viruses are not included in the three-domain classification system.

Explain why.

Discuss whether viruses should or should not be classed as living organisms.

State the two main features that are used to classify viruses.

Archaea and Eukarya are two domains of life that exhibit distinct characteristics and evolutionary relationships.

Contrast the cellular structures of Archaea and Eukarya.

Scientists compared four species from the kingdom Achaea. They compared the base sequence of their DNA to determine the evolutionary relationships between the species.

Their results are shown in Table 1. 

Table 1

Use the information in Table 1 to explain which two species are the closest evolutionary relatives.

Archaea were previously classified as bacteria due to similarities in their features. 

Describe three features of archaea that led to their classification as a separate domain. 

Outline the characteristic features of organisms in the domain Bacteria.

Table 1 below contains the taxonomic classification of orca, minke whales, and bottlenose dolphins.

Table 1

Identify the phylum to which all three species belong.

[1]

(ii)

Give the binomial Latin name for the common minke whale.

[1]

(iii)

Orca are also known as killer whales.

Suggest why many would consider the name 'killer whale' to be inappropriate for this species.

[2]

Identify the taxonomic rank that is missing from Table 1.

[1]

Orca show wide variation in appearance and behaviour, and have been divided by marine scientists into groups known as 'ecotypes'. Fig. 1 below shows the distribution of some of the known orca ecotypes.

Fig. 1

There is some debate among scientists over whether orca ecotypes should be classified as separate species.

Use Fig. 1 to suggest why it might be difficult to determine whether orca ecotypes are the same species or several separate species.

To avoid the classification difficulties discussed in part (b), scientists often use DNA data.

Explain how the scientists could use DNA data to determine whether or not orca ecotypes are different species.

There have been rare instances of hybridisation events between closely related species that result in fertile offspring, for example Fig. 2 below shows the offspring of a cross between a female bottlenose dolphin and a male false killer whale. The individual in the photograph is known to have bred with another bottlenose dolphin and to have given birth to healthy offspring.

Mark Interrante, CC BY-SA 2.0, via Flickr

Fig. 2

Scientists do not believe that the hybrid shown in Fig. 2 is an example of a new species.

Suggest why the individual in Fig. 2 is not considered to be a new species.