Characteristics of living organisms

1. Overview

All living things, from microscopic bacteria to giant redwood trees, share seven fundamental characteristics. These processes distinguish living organisms from non-living objects and ensure the survival of individuals and species. This topic provides the "MRS GREN" framework used throughout the IGCSE Biology syllabus to define what it means to be alive.

Key Definitions

• Movement: An action by an organism or part of an organism causing a change of position or place.
• Respiration: The chemical reactions in cells that break down nutrient molecules and release energy for metabolism.
• Sensitivity: The ability to detect and respond to changes in the internal or external environment.
• Growth: A permanent increase in size and dry mass.
• Reproduction: The processes that make more of the same kind of organism.
• Excretion: The removal of the waste products of metabolism and substances in excess of requirements.
• Nutrition: The taking in of materials for energy, growth, and development.

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Core Content

To remember the seven characteristics, use the mnemonic: MRS GREN.

(M) Movement

• Most animals move their whole bodies to find food or avoid predators.
• Plants move parts of themselves (e.g., leaves turning toward the sun or roots growing downward).

(R) Respiration

• This is a chemical process that occurs inside every living cell.
• Energy is released from nutrients (like glucose) to power metabolic processes like protein synthesis and cell division.

(S) Sensitivity

• Living things sense "stimuli" (changes in temperature, light, sound, or chemicals).
• They produce a "response" to these stimuli to increase their chances of survival.

(G) Growth

• Growth involves a permanent increase in size.
• Dry mass refers to the mass of the organism after all water has been removed. This is the most accurate way to measure growth as water content can fluctuate daily.

(R) Reproduction

• Organisms produce offspring to ensure the species does not go extinct.
• This can be sexual (two parents) or asexual (one parent).

(E) Excretion

• Metabolism refers to all chemical reactions in the body. These reactions produce waste (e.g., CO2 from respiration, urea in urine).
• If these toxic substances build up, they can kill the organism.

(N) Nutrition

• Plants are autotrophic: They make their own food using sunlight, water, and CO2 (photosynthesis).
• Animals are heterotrophic: They must consume other living organisms to get their nutrients.
• Nutrients include proteins, carbohydrates, fats, vitamins, and minerals.

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Extended Content (Extended Only)

There is no additional content for the Extended curriculum for this specific sub-topic (1.1). Both Core and Supplement students must master the definitions provided above.

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Key Equations

While this topic focuses on definitions, the process of Respiration is defined by its chemical reaction:

Word Equation for Aerobic Respiration: Glucose + Oxygen → Carbon Dioxide + Water (+ Energy released)

(Note: Energy is not a chemical substance, so it is usually written in brackets or as a note alongside the equation.)

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Common Mistakes to Avoid

• ❌ Wrong: Respiration is "breathing."
• ✓ Right: Respiration is a chemical reaction in cells. Breathing (ventilation) is the physical mechanism of moving air in and out of lungs.
• ❌ Wrong: Excretion and Egestion are the same thing.
• ✓ Right: Excretion is the removal of metabolic waste (made inside cells). Egestion is the removal of undigested food (faeces) that never actually entered the cells.
• ❌ Wrong: Growth is just getting taller.
• ✓ Right: Growth is a permanent increase in dry mass and size.

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Exam Tips

• The "Define" Command Word: This topic often appears as 1-mark or 2-mark questions asking you to define a specific characteristic. You must use the exact wording from the syllabus to get full marks (e.g., for Respiration, you must mention "chemical reactions" and "release energy").
• Dry Mass Context: If an exam question asks why a scientist might dry out a plant before weighing it, the answer is always to measure Growth accurately by removing the fluctuation caused by water.
• Real-world Context: You may be asked to apply MRS GREN to a new discovery (e.g., a "robot" or a "virus"). Be prepared to explain why a virus is often considered non-living (it cannot reproduce without a host cell and does not perform its own metabolism/respiration).
• Metabolism: Always link "Excretion" to the word "metabolism" or "metabolic waste" in your answers.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0610 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

A student places a small bean seedling on a dish containing moist cotton wool. They then cover the dish with a clear plastic container and place it in a dark cupboard. After several days, they observe the seedling.

(a) State two characteristics of living organisms that the student could observe in the bean seedling. [2]

(b) Describe one process that the seedling is carrying out that demonstrates respiration. [3]

Worked Solution:

(a)

1. Growth is a characteristic of living organisms. The seedling will increase in size. This shows an increase in dry mass, a key aspect of growth.

2. Sensitivity is also a characteristic of living organisms. The seedling may grow towards the small amount of light entering the cupboard or away from gravity. This demonstrates a response to stimuli.

How to earn full marks:

• State 'growth' or 'sensitivity' (1 mark)
• Provide a valid observation related to the stated characteristic (1 mark)

(b)

1. Respiration involves the breakdown of glucose. The seedling is breaking down glucose. Glucose is a nutrient molecule stored in the seed.

2. This process releases energy. This releases energy for metabolic processes. These metabolic processes include growth and the transport of substances.

3. Carbon dioxide and water are produced as waste products. Carbon dioxide and water are produced. These are waste products of the respiration process.

How to earn full marks:

• Mention the breakdown of glucose (1 mark)
• State that energy is released (1 mark)
• Identify waste products such as carbon dioxide or water (1 mark)

Common Pitfall: Make sure you're describing general characteristics of living organisms, not specific features of a bean seedling. For example, stating "it has roots" doesn't demonstrate a characteristic like growth or sensitivity. Also, remember that respiration is a chemical process, not just breathing.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

A gardener notices that their tomato plants are growing slowly and producing fewer tomatoes than usual. They suspect the plants may not be getting enough of something.

(a) Define the term nutrition as it relates to living organisms. [2]

(b) Suggest two reasons why the tomato plants might not be growing well, linking each reason to a specific aspect of nutrition. [4]

Worked Solution:

(a)

1. Nutrition is the process of taking in materials. Nutrition is the process by which living organisms take in materials. This includes both energy sources and raw materials.

2. These materials are used for energy, growth, and development. These materials are used for energy, growth, and development. This is the purpose of taking in the materials.

How to earn full marks:

• State that nutrition involves taking in materials (1 mark)
• Mention that these materials are used for energy, growth, or development (1 mark)

(b)

1. Lack of minerals affects growth. The plants may not be getting enough essential minerals, such as nitrates or phosphates, from the soil. These minerals are needed for protein synthesis and other metabolic processes.

2. Lack of light affects photosynthesis. The plants may not be receiving enough sunlight. Sunlight is essential for photosynthesis, which produces glucose for energy and growth.

How to earn full marks:

• State a specific nutrient lacking (e.g., nitrates, phosphates, sunlight) (1 mark)
• Link the lack of that nutrient to a specific aspect of growth or energy production (1 mark)

Common Pitfall: When discussing nutrition, be specific about what the plant is lacking. Saying "not enough food" is too vague. Also, remember that plants need both light and minerals for healthy growth.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

A group of students is investigating the movement of woodlice in response to different moisture levels. They place 20 woodlice in a choice chamber with two compartments: one dry and one moist. They count the number of woodlice in each compartment every minute for 10 minutes.

(a) State the characteristic of living organisms that the woodlice are demonstrating in this experiment. [1]

(b) Describe the likely distribution of woodlice between the dry and moist compartments after 10 minutes. Explain your answer. [3]

(c) Explain how this behavior increases the woodlice's chances of survival. [4]

Worked Solution:

(a)

1. The woodlice are demonstrating sensitivity. The woodlice are demonstrating sensitivity. This is their ability to respond to changes in their environment.

How to earn full marks:

• State 'sensitivity' (1 mark)

(b)

1. More woodlice will be in the moist compartment. The majority of woodlice will likely be found in the moist compartment. This is the expected result based on woodlice behavior.

2. Woodlice prefer moist environments. Woodlice prefer moist environments because they need moisture to survive. Their exoskeletons are not waterproof, so they lose water easily in dry environments.

3. They move away from the dry compartment. They will actively move away from the dry compartment to avoid desiccation. This movement is a response to the stimulus of dryness.

How to earn full marks:

• State that more woodlice will be in the moist compartment (1 mark)
• Explain that woodlice prefer moist environments (1 mark)
• Link this preference to their need to avoid desiccation (1 mark)

(c)

1. Maintaining moisture is crucial. By moving to a moist environment, woodlice maintain a suitable internal water balance. This is essential for their cells to function properly.

2. Desiccation can be fatal. Avoiding dry conditions prevents excessive water loss, which can lead to desiccation and death. Woodlice are very vulnerable to drying out.

3. Moist environments support respiration. Moist environments often provide better conditions for respiration. Woodlice respire through their cuticle, which needs to be moist.

4. Increasing survival and reproduction. This behavior increases their chances of survival and reproduction. Survival allows them to reproduce and pass on their genes.

How to earn full marks:

• Explain that moving to a moist environment helps maintain water balance (1 mark)
• Explain that avoiding dry conditions prevents desiccation (1 mark)
• State that moist environments support respiration (1 mark)
• Link survival to reproduction (1 mark)

Common Pitfall: Don't just say "woodlice like moisture." Explain why they need moisture, linking it to their physiology and survival. Also, remember that sensitivity is about responding to stimuli, not just having a preference.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

A scientist is studying the growth of a population of bacteria in a nutrient-rich broth. They measure the population size every hour for 8 hours. The initial population size is 1000 bacteria.

(a) State two conditions, other than nutrient availability, that would be important to control in this experiment to ensure reliable results. [2]

(b) Explain why bacteria reproduce rapidly under favorable conditions. Relate your answer to the characteristics of living organisms. [3]

(c) After 8 hours, the population size is 256,000 bacteria. Assuming the bacteria reproduce by binary fission (one cell divides into two), calculate the average number of divisions per hour that each bacterium undergoes. Show your working. [4]

Worked Solution:

(a)

1. Temperature affects enzyme activity. Temperature should be kept constant. Temperature affects the rate of enzyme activity and therefore the rate of metabolic processes.

2. pH affects enzyme structure. pH should be kept constant. pH affects the structure of enzymes and therefore their ability to function.

How to earn full marks:

• State 'temperature' or 'pH' (1 mark)
• Explain why it is important to control that condition (1 mark)

(b)

1. Bacteria exhibit reproduction. Bacteria reproduce rapidly via binary fission. This is a characteristic of living organisms.

2. Binary fission is a simple process. Binary fission is a relatively simple and quick process compared to other forms of reproduction. This allows for rapid population growth.

3. Favorable conditions allow for rapid growth. Under favorable conditions (abundant nutrients, optimal temperature and pH), bacteria can dedicate all their energy to growth and reproduction. This leads to exponential population increase.

How to earn full marks:

• State that bacteria reproduce via binary fission (1 mark)
• Explain that binary fission is a quick and simple process (1 mark)
• Link favorable conditions to rapid growth and reproduction (1 mark)

(c)

1. Calculate the total number of divisions. The population increased from 1000 to 256,000, meaning it multiplied by 256. Since each division doubles the population, we need to find the power of 2 that equals 256. $2^8 = 256$. Therefore, there were 8 divisions per original bacterium in 8 hours. This step determines the total number of divisions each bacterium underwent over the entire experiment.

2. Calculate the average divisions per hour. To find the average number of divisions per hour, divide the total number of divisions by the number of hours: $8 \div 8 = 1$. This calculates the average rate of division per hour.

3. State the final answer. Each bacterium undergoes an average of $\boxed{1 \text{ division/hour}}$.

How to earn full marks:

• Correctly calculate that the population multiplied by 256 (1 mark)
• Recognize that 256 is 2 to the power of 8 (1 mark)
• Divide the total number of divisions (8) by the number of hours (8) (1 mark)
• State the final answer with the correct unit: $\boxed{1 \text{ division/hour}}$ (1 mark)

Common Pitfall: Remember to include the units in your final answer for calculations. Also, make sure you understand the relationship between binary fission and the exponential growth of bacterial populations. A common mistake is to misinterpret how many divisions occurred based on the population increase.