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Biology: Life processes

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Biology: Life processes
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In this chapter

  1. 1Defining Life: The Seven Processes
  2. 2Movement and Sensitivity
  3. 3Respiration: Releasing Energy
  4. 4Nutrition: Obtaining Food
  5. 5Excretion and Reproduction
  6. 6Growth: A Permanent Increase

1. Defining Life: The Seven Processes

Biology is the study of life, but what does it mean to be 'alive'? All living things, from the smallest bacterium to the largest blue whale, share seven fundamental characteristics, also known as life processes. To be classified as a living organism, something must exhibit all seven of these traits. A useful mnemonic to remember them is MRS GREN. Non-living objects, like a rock or a car, might show one or two of these characteristics, but never all seven.

Key term

Organism: A living thing, which carries out all seven of the life processes.

Examiner insight

Examiners look for the ability to list all seven characteristics and apply them to distinguish between living and non-living objects in unfamiliar contexts.

Common pitfall

Simply listing the MRS GREN letters without stating the full name of the characteristic, for example, writing 'R' instead of 'Respiration' or 'Reproduction'.

Fun fact

Viruses are on the borderline of living and non-living. They can't carry out life processes on their own and need to infect a living cell to reproduce, essentially hijacking the cell's machinery.

Worked example 13 marks

A car takes in fuel (nutrition), 'breathes' in air for its engine, releases exhaust fumes (excretion), and can move. Explain why a car is not considered a living organism.

  1. 1

    Step 1: Recall the seven characteristics of life. A living thing must show all seven: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, and Nutrition.

  2. 2

    Step 2: Evaluate the car against the missing characteristics. While a car shows some similarities, it cannot grow in a biological sense (a permanent increase in size and complexity).

  3. 3

    Step 3: Identify another missing characteristic. A car cannot reproduce to make more cars; they are built in factories. It also does not show sensitivity to its environment in a biological way.

  4. 4

    Step 4: Conclude based on the evidence. Since a car does not show all seven life processes, particularly growth, reproduction, and sensitivity, it is not a living organism.

Recap

  • All living things are called organisms.
  • Organisms share seven characteristics: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, and Nutrition.
  • The mnemonic MRS GREN helps you remember the seven life processes.
  • Non-living things may show some, but not all, of these characteristics.

Quick check

  1. Use the mnemonic MRS GREN to list the seven characteristics of living organisms.7 marks

2. Movement and Sensitivity

Movement is the action of an organism or part of an organism causing a change of position or place. Animals move their whole bodies to find food or escape danger, which is called locomotion. Plants also move, but usually much more slowly, such as a shoot growing towards sunlight (phototropism). Sensitivity is the ability to detect changes in the environment, called stimuli, and to make responses. For example, you pull your hand away from a hot object (a response to the stimulus of heat), and a plant's roots grow towards water (hydrotropism).

Key term

Stimulus: A detectable change in the internal or external environment of an organism that leads to a response.

Examiner insight

Marks are often awarded for explaining that plants also move, even though they are rooted in one place. Giving a specific example, like phototropism, demonstrates a strong understanding.

Common pitfall

Confusing movement with locomotion. Locomotion (moving the whole body) is a type of movement mainly seen in animals, but the definition of movement also includes plants turning their leaves to the sun.

Fun fact

The mimosa pudica plant, also known as the 'touch-me-not' plant, rapidly folds its leaves inwards when touched. This is a dramatic example of sensitivity and movement in plants.

Worked example 12 marks

Describe one way a plant shows movement and one way it shows sensitivity.

  1. 1

    Movement: A plant shows movement by its shoots and leaves growing towards a light source. This is a slow change in the position of a part of the organism.

  2. 2

    Sensitivity: A plant shows sensitivity by detecting the direction of light. The plant then responds to this stimulus by growing towards it to maximise light absorption for photosynthesis.

Worked example 22 marks

A person accidentally touches a hot stove and immediately pulls their hand away. Identify the stimulus and the response in this situation.

  1. 1

    Step 1: Identify the change in the environment. The stimulus is the high temperature of the hot stove detected by receptors in the skin.

  2. 2

    Step 2: Identify the action taken by the person. The response is the rapid withdrawal of the hand from the stove, caused by muscle contraction.

Recap

  • Movement is a change in position or place of an organism or its parts.
  • Animals show locomotion, while plants show growth movements called tropisms.
  • Sensitivity is the ability to detect and respond to stimuli.
  • Stimuli can be external (like light, sound, temperature) or internal (like blood sugar level).

Quick check

  1. Give one example of a stimulus that a plant might respond to.1 mark
  2. What is the scientific term for an animal moving its whole body from place to place?1 mark

3. Respiration: Releasing Energy

Every living cell needs a constant supply of energy to carry out its functions, from moving to growing. Respiration is the vital chemical process that releases this energy from food. It happens inside the mitochondria within the cells of all living organisms, both plants and animals, 24 hours a day. In aerobic respiration, glucose (a simple sugar from food) reacts with oxygen to produce carbon dioxide, water, and a large amount of energy. It is crucial not to confuse respiration with breathing. Breathing (or gaseous exchange) is the physical act of getting oxygen into the body and carbon dioxide out, while respiration is the chemical reaction that uses that oxygen inside cells.

Glucose + Oxygen → Carbon Dioxide + Water (+ Energy released)

Key term

Respiration: The chemical reactions in cells that break down nutrient molecules and release energy for metabolism.

Examiner insight

Examiners frequently test the difference between respiration and breathing. A clear, precise answer that defines respiration as a chemical process in cells releasing energy will always score highly.

Common pitfall

Stating that respiration is 'breathing'. Breathing is a mechanical process of moving air; respiration is a chemical process that happens in every cell to release energy.

Fun fact

Hummingbirds have one of the highest metabolic rates of any animal. To power their rapid wing beats (up to 80 times per second), their cells respire at an incredible rate, requiring them to consume more than their own body weight in nectar each day.

Worked example 13 marks

A student says, 'Plants photosynthesise, animals respire'. Explain why this statement is incorrect.

  1. 1

    Step 1: Analyse the first part of the statement. 'Plants photosynthesise' is correct; this is how they make their food.

  2. 2

    Step 2: Analyse the second part. 'Animals respire' is also correct; they respire to release energy from their food.

  3. 3

    Step 3: Identify the error. The error is the implication that plants do not respire. All living organisms, including plants, must respire to release energy from the food they have made or consumed.

  4. 4

    Step 4: Provide a corrected statement. Plants photosynthesise to make food (glucose) and also respire continuously (day and night) to release energy from that glucose for their life processes.

Recap

  • Respiration is a chemical reaction in all living cells that releases energy from food.
  • The energy released is used for all other life processes, like movement and growth.
  • Aerobic respiration requires oxygen and produces carbon dioxide and water as waste products.
  • Respiration is not the same as breathing.

Quick check

  1. What is the main purpose of respiration for an organism?1 mark
  2. Name the gas required for aerobic respiration and the gas produced as a waste product.2 marks

4. Nutrition: Obtaining Food

Nutrition is the process of taking in materials (food) that are needed for energy, growth, and to keep the body healthy. Organisms obtain these nutrients in different ways. Plants and some bacteria are autotrophs (literally 'self-feeders'). They make their own food using simple inorganic molecules and an energy source. Green plants use light energy to make glucose from carbon dioxide and water, a process called photosynthesis. Animals, fungi, and some bacteria are heterotrophs ('other-feeders'). They cannot make their own food, so they must obtain their nutrients by consuming other organisms.

Carbon Dioxide + Water --(light energy/chlorophyll)--> Glucose + Oxygen

Key term

Nutrition: The taking in of materials for energy, growth and development; plants require light, carbon dioxide, water and ions; animals need organic compounds and ions and usually water.

Examiner insight

Students who can clearly differentiate between how plants and animals obtain nutrition, using the correct scientific terms 'autotrophic' for plants and 'heterotrophic' for animals, score highly.

Common pitfall

Thinking that the minerals a plant absorbs from the soil are its 'food'. The plant's food is the glucose it makes during photosynthesis; the minerals are essential nutrients but not its primary energy source.

Fun fact

Carnivorous plants, like the Venus flytrap, have adapted to grow in nutrient-poor soil. They get essential minerals, especially nitrogen, by trapping and digesting insects, supplementing their photosynthetic nutrition.

Worked example 13 marks

Compare the method of nutrition in a lion and a blade of grass.

  1. 1

    Step 1: Identify the type of nutrition for the grass. The grass is a plant, so it is an autotroph. It makes its own food via photosynthesis, using light energy, water, and carbon dioxide.

  2. 2

    Step 2: Identify the type of nutrition for the lion. The lion is an animal, so it is a heterotroph. It obtains nutrients by eating other animals (it is a carnivore).

  3. 3

    Step 3: State the key difference. The key difference is that the grass produces its own organic food molecules from simple inorganic substances, while the lion must consume complex organic molecules ready-made from other organisms.

Recap

  • Nutrition provides the raw materials for energy, growth, and repair.
  • Plants are autotrophs; they make their own food by photosynthesis.
  • Animals are heterotrophs; they get food by eating other organisms.
  • The food obtained through nutrition is used in respiration to release energy.

Quick check

  1. What is the term for an organism that must eat other organisms to obtain nutrients?1 mark
  2. What is the source of energy for photosynthesis in plants?1 mark

5. Excretion and Reproduction

Excretion is the removal of waste products that have been made by the body's chemical reactions (metabolism). These substances, such as carbon dioxide and urea, can be toxic if they build up. A key example is carbon dioxide, a waste product of respiration, which we breathe out. Another is urea, a waste product from the breakdown of excess proteins, which is removed from the blood by the kidneys and released in urine. It's vital not to mix up excretion with egestion, which is the removal of undigested food (faeces). Reproduction is the process of making more of the same kind of organism. While not essential for an individual's survival, it is crucial for the continuation of a species. If a species did not reproduce, it would eventually become extinct.

Key term

Excretion: The removal from organisms of the waste products of metabolism (chemical reactions in cells), toxic materials, and substances in excess of requirements.

Examiner insight

A common exam question asks for the definition of excretion. Be precise: it's the removal of *metabolic* waste. Mentioning this key term demonstrates a clear understanding and separates your answer from lower-level responses.

Common pitfall

Confusing excretion with egestion. Egestion is getting rid of undigested food (faeces), which has never been absorbed into the body's cells. Excretion is the removal of waste products made *by* the body's cells.

Worked example 13 marks

When you breathe out on a cold day, you can see your breath. This contains water vapour and carbon dioxide. Explain why breathing out is a form of excretion.

  1. 1

    Step 1: Define excretion. Excretion is the removal of metabolic waste products from the body.

  2. 2

    Step 2: Identify the source of the waste products. Both carbon dioxide and water are produced during aerobic respiration, which is a metabolic process occurring in all body cells.

  3. 3

    Step 3: Link the products to excretion. Because carbon dioxide and water are waste products of metabolism, their removal from the body via the lungs is an act of excretion.

Recap

  • Excretion is the removal of waste products from metabolic reactions.
  • Examples of excretory products include carbon dioxide and urea.
  • Excretion is not the same as egestion (removal of faeces).
  • Reproduction is the process of making new individuals.
  • Reproduction is essential for the survival of a species, but not an individual.

Quick check

  1. Is passing faeces an example of excretion? Explain your answer.2 marks

6. Growth: A Permanent Increase

Growth is defined as a permanent increase in size and dry mass of an organism. This is achieved by an increase in the number of cells (through cell division), an increase in the size of cells, or both. 'Dry mass' is the mass of the organism after all water has been removed. Using dry mass is a more accurate measure of growth than just measuring total mass, as water content can fluctuate significantly. For multicellular organisms like humans, growth involves not just getting bigger but also development, which is the process of becoming more complex as cells differentiate to form specialised tissues and organs.

Key term

Growth: A permanent increase in size and dry mass by an increase in cell number or cell size or both.

Examiner insight

High-scoring answers on growth often include the concept of a 'permanent increase in dry mass'. This shows you understand the difference between true biological growth and temporary changes in size, for example, due to drinking water.

Common pitfall

Simply defining growth as 'getting bigger'. A complete definition must mention that it is a permanent increase and ideally refer to dry mass or an increase in cell number.

Fun fact

The largest organism on Earth is a fungus in Oregon, USA, nicknamed the 'Humongous Fungus'. It covers nearly 4 square miles and has been growing for thousands of years, with most of its mass being underground.

Worked example 13 marks

A scientist is measuring the growth of a bean plant. Why is it better to measure the change in dry mass over time rather than the change in fresh mass?

  1. 1

    Step 1: Define fresh mass and dry mass. Fresh mass is the total mass of the plant, including its water content. Dry mass is the mass of the plant after all water has been removed.

  2. 2

    Step 2: Explain why fresh mass can be misleading. The water content of a plant can vary greatly depending on the time of day, how recently it was watered, and the humidity. This means fresh mass can go up and down without any real growth occurring.

  3. 3

    Step 3: Explain why dry mass is a better measure. Dry mass represents the actual organic material the plant has produced through photosynthesis. A permanent increase in dry mass is therefore a true and accurate measure of growth.

Recap

  • Growth is a permanent increase in the size and dry mass of an organism.
  • It is caused by an increase in cell number (cell division) or cell size.
  • Dry mass is a more accurate measure of growth than fresh mass.
  • Development is the increase in complexity of an organism.

Quick check

  1. How do multicellular organisms, like humans, grow from a single fertilised egg?1 mark
  2. What is meant by the 'dry mass' of an organism?1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Use the mnemonic MRS GREN to list the seven characteristics of living organisms.7 marks
  2. Define 'respiration' and state the word equation for aerobic respiration.3 marks
  3. A motor car can move, it releases waste gases, and it requires a source of energy (fuel). Explain why a car is not considered a living organism.3 marks
  4. Compare and contrast nutrition in a green plant and a human.4 marks
  5. Explain the difference between movement in a plant and movement in an animal, giving an example for each.4 marks
  6. Distinguish between excretion and egestion, giving a clear example of a substance that is excreted by humans.3 marks
  7. An astronaut discovers a new object on Mars. It appears to grow by crystallising from a liquid, and it moves when the wind blows it. Describe two further tests the astronaut could perform to determine if the object is truly alive, explaining what a positive result for each test would indicate.4 marks
  8. Explain why reproduction is essential for the survival of a species, but not for the survival of an individual organism.3 marks
  9. A student places a plant in a dark cupboard for a week. They observe that the leaves turn yellow and the plant looks unhealthy. Relate these observations to two of the life processes.4 marks
  10. A student states, 'Breathing and respiration are the same thing'. Write a paragraph to correct the student's misconception, clearly explaining the difference between the two processes and how they are linked.5 marks

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