Characteristics of Living Organisms (Edexcel IGCSE Biology: Double Science)

Human populations are at risk from infectious disease.

Describe the different types of pathogen.

Refer to a disease that each type of pathogen causes in your answer.

Compare the structure of a plant cell with the structure of a bacterial cell.

Some bacteria are able to carry out photosynthesis, despite not containing chloroplasts. 

Using your knowledge of photosynthesis, suggest how this could be possible. 

The diagram below shows a bacterium cell. 

Use the following equation to calculate the image size of this bacterium cell. 

Give your answer in mm. (1mm = 1000μm)

Name the group of organisms that this species belongs to.

Compare this process to saprotrophic nutrition in fungi. 

(3)

As well as nutrition, which other life process is being demonstrated during this process? 

(1)

The Amoeba is found in fresh water and must control its internal conditions by balancing the amount of water that moves in and out of the cell across the cell membrane. 

The Amobea contains a structure called a contractile vacuole, which expels excess water from the cell. As well as excess water, they also remove waste materials from the cell. 

Which of the life processes does the contractile vacuole carry out? 

A mosquito is an example of an animal. The image below shows an Anopheles mosquito.

Photo Credit: James Gathany Content Providers(s): CDC, Public domain, via Wikimedia Commons

Mosquitoes can spread diseases with their bite. Why is the mosquito not classed as a pathogen?

(2)

The diagram shows a simplified version of a cell of Plasmodium falciparum. This cell can be carried in the mouthparts of female Anopheles mosquitoes and can cause malaria in humans.

What kind of organism is the Plasmodium?

(1)

A student reaches the conclusion: 

"Plasmodium is not alive because it cannot respire. The evidence from this diagram shows that it does not have lungs and cannot breathe". 

Using evidence from the diagram, explain why the student is incorrect in their conclusion. 

(2)

When inside the human body Plasmodium can live inside red blood cells and get their nutrition by digesting haemoglobin and absorbing the broken down nutrients. 

Haemoglobin carries oxygen around the body in the blood.

(3)

(2)

The diagram below shows three different types of virus. 

• Virus A is a bacteriophage, which injects its genetic material into bacterial cells
• Virus B is TMV, which infects the cells of certain species of plants, such as tobacco, tomatoes and peppers. 
• Virus C is a coronavirus, such as the one that causes the disease Covid-19, and it can infect human cells. 

The diagrams are not drawn to scale. 

Name two structural features that are universal for all types of virus.

(2)

Suggest why it is necessary for the three types of virus to have differences in their structure. 

(3)

Another example of a virus pathogen that can infect humans is HIV. 

HIV can be spread through exchange of bodily fluids.

List three other ways pathogens can spread between people. 

People with AIDS are susceptible to infections by pathogens, and can die from infections that most people without AIDS could recover from easily. 

HIV infects the white blood cells of the infected person and stops them from working. Why might that cause people to be more susceptible to infections?

(1)

One way of preventing this from occurring is for people with HIV to take medication to prevent the viruses from replicating. Explain why this can prevent the infected person from developing AIDS. 

(2)

The graph below shows an estimation of the number of people worldwide that died from AIDS-related illnesses between the years 1990 and 2020.

Describe the trend of data in the graph.

(2)

Suggest two reasons why the number of AIDS-related deaths has declined in recent years. 

(2)

Some students wanted to investigate sensitivity in plants. 

They set up their equipment as shown in the diagram below: 

The experimental method was as follows: 

1. Grow two pots of plant seedlings of the same species in the same conditions.
2. Place pot A in a box to prevent light reaching the seedlings.
3. Place pot B with a unidirectional light source hitting the seedlings.
4. Leave the seedlings for three days, keeping all other conditions the same.
5. After three days measure the angle of the curvature of the seedling stems using a protractor, as shown in the diagram below.
6. Repeat the experiment three times with three different pots of seedlings

Identify the independent variable in this experiment. 

(1)

Other than sensitivity, name a life process that is being demonstrated in this experiment.

(1)

Complete the table by calculating the mean angle of stem growth for Pot B. 

(1)

Plot a graph of the results of the experiment in the space provided. 

(4)

The results of the experiment show that the plants exposed to unidirectional light grow towards the light. 

Explain how this response benefits the plant. 

The students wanted to adjust their experiment to find out if the plant response varies with different levels of light intensity. 

Suggest how the students could alter their experiment method in order to carry out this investigation. 

Different types of organisms store their carbohydrates in different forms.

The images below show the structure of two of these types of carbohydrates. 

Using the information provided in the diagram, compare the structure of starch and glycogen. 

(2)

For the three examples of organisms below, state whether they store their glucose as starch or glycogen.

• Plants 
• Animals
• Fungi

(3)

The structure of the two types of carbohydrate shown in (a) are adapted for the needs of the organism. 

The more exposed ends of the glucose strands in the carbohydrate, the faster it can be broken down and the faster the glucose can be made available for use in the cell. 

Animals store carbohydrate in their muscle cells.

State why this is beneficial to the animal.

(1)

Suggest why some types of organisms may need to have adaptations to release their glucose faster than other types of organisms.

(3)

The table below shows the approximate maximum length of one molecule of the carbohydrate types shown in part (a). The size is given in terms of the number of glucose molecules present.

Using the data in the table, calculate which carbohydrate molecule (starch or glycogen) will take the least amount of time to be fully hydrolysed. 

Another type of carbohydrate that can be found in many types of cells is cellulose. 

What is the function of cellulose in the cell?

(1)

Name two types of organisms that can contain cellulose in their cells. 

(2)

Gut flora, or the gut microbiome, is the name given to the huge number of microorganisms that live inside the digestive system of animals, such as humans. 

Most of these microorganisms are bacteria.  

State how the presence of the gut flora demonstrates that not all bacteria are pathogens.

(2)

Explain how human gut bacteria get their nutrition. 

(2)

If a patient is unwell with a bacterial infection they may take a type of medication called an antibiotic. 

Different types of antibiotics affect bacteria in different ways, for example some damage the cell walls of the bacteria, and some prevent bacteria from reproducing. 

Use the information provided to explain how taking an antibiotic would affect the bacteria that live in the gut.

To increase the number of bacteria living in the gut, people are encouraged to consume probiotics, such as health yogurts. 

These contain cultures of gut bacteria that can start to grow in number inside the intestines. This helps to keep the gut flora healthy. 

Name one example of a species of bacteria that is used in the production of yogurt from milk. 

When manufacturers are producing probiotics they must grow cultures of bacteria in a factory. They can put a small number of bacteria in a large vat called a fermenter and provide them with nutrients such as amino acids and glucose.

The graph below shows how the number of bacteria in the fermenter changes over time when no additional nutrients are added during the growth process. 

Explain the shape of the curve between point A and point B.

(2)

Use your knowledge of the characteristics of living organisms to explain the shape of the curve after point C in the graph above.

(2)