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Biology: Life processes

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Biology: Life processes
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In this chapter

  1. 1The Seven Life Processes
  2. 2Nutrition and Growth
  3. 3Respiration: Releasing Energy
  4. 4Excretion and Sensitivity

1. The Seven Life Processes

To be classified as a living organism, a thing must carry out seven essential activities, known as life processes. These are the fundamental functions that distinguish living things from non-living objects. A simple way to remember them is using the mnemonic MRS GREN: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, and Nutrition. Every living organism, from a tiny bacterium to a giant blue whale, performs all seven of these processes to survive.

Key term

Organism: A living being that is capable of displaying all the characteristics of life.

Examiner insight

Examiners often ask students to list the seven characteristics of living organisms, so memorising the MRS GREN mnemonic is highly beneficial.

Fun fact

Some viruses can reproduce and have genetic material, but they are not considered truly alive because they cannot perform the other life processes without a host cell.

Worked example 14 marks

A student observes a plant on a windowsill. Over a week, the stem bends towards the window. Name and describe two life processes demonstrated by the plant.

  1. 1
    1. Identify the first process: The plant is bending towards light. This is a response to a stimulus. The life process is Sensitivity.
  2. 2
    1. Describe Sensitivity: The plant detects the direction of the light (stimulus) and responds by changing its direction of growth.
  3. 3
    1. Identify the second process: The bending is a result of the stem growing. The life process is Growth.
  4. 4
    1. Describe Growth: The plant shows a permanent increase in size and dry mass by producing new cells, causing the stem to elongate and bend.

Recap

  • All living things carry out seven life processes.
  • The mnemonic MRS GREN helps remember Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, and Nutrition.
  • Movement is a change in position or place.
  • Sensitivity is the ability to detect and respond to stimuli.
  • Growth is a permanent increase in size and dry mass.
  • Excretion is the removal of metabolic waste.

Quick check

  1. List the seven characteristics of living organisms.2 marks

2. Nutrition and Growth

Nutrition is the process of providing or obtaining the food necessary for health and growth. Organisms need food for energy, to build and repair tissues, and to control chemical reactions. Plants are autotrophs, meaning they make their own food via photosynthesis. Animals are heterotrophs, meaning they must consume other organisms to get their nutrients. Growth is the permanent increase in the size and dry mass of an organism, achieved by an increase in cell number, cell size, or both. For growth to occur, the rate of synthesis of new materials must be greater than the rate of breakdown.

Key term

Nutrition: The process by which organisms take in and use food, which provides energy and materials for growth and repair.

Common pitfall

Confusing growth with simple swelling. Growth involves a permanent increase in dry mass (the mass without water), whereas swelling (e.g., a seed soaking up water) is a temporary increase in size due to water intake.

Fun fact

The world's largest organism by mass is a grove of quaking aspen trees in Utah, called Pando. It is a single genetic individual connected by a massive root system and is estimated to weigh 6,000,000 kg.

Worked example 14 marks

Explain why a lion is described as a heterotroph while the grass it walks on is an autotroph.

  1. 1
    1. Define autotroph: An autotroph is an organism that produces its own food, usually through photosynthesis. Grass uses sunlight, water, and carbon dioxide to make its own glucose.
  2. 2
    1. Define heterotroph: A heterotroph is an organism that cannot make its own food and must obtain nutrients by consuming other organisms.
  3. 3
    1. Apply to the lion: The lion cannot make its own food. It hunts and eats other animals (like zebras, which eat grass) to get the energy and nutrients it needs for life processes.
  4. 4
    1. Conclude: Therefore, the grass is an autotroph (a producer) and the lion is a heterotroph (a consumer).

Recap

  • Nutrition provides organisms with energy and materials for growth and repair.
  • Autotrophs, like plants, make their own food.
  • Heterotrophs, like animals, consume other organisms for food.
  • Growth is a permanent increase in an organism's size and dry mass.

Quick check

  1. What is the difference between an autotroph and a heterotroph?2 marks
  2. State two reasons why organisms need nutrition.2 marks

3. Respiration: Releasing Energy

Respiration is NOT breathing. It is a chemical reaction that happens inside every living cell to release energy from nutrient molecules like glucose. This energy powers all other life processes. Aerobic respiration requires oxygen and releases a large amount of energy. It happens in the mitochondria of cells. Anaerobic respiration does not require oxygen and releases a much smaller amount of energy. In humans, it occurs during intense exercise and produces lactic acid. The energy released is in the form of ATP (adenosine triphosphate), the 'energy currency' of the cell.

Aerobic Respiration (Word): Glucose + Oxygen -> Carbon Dioxide + Water (+ large amount of energy)

Aerobic Respiration (Chemical): C6H12O6 + 6O2 -> 6CO2 + 6H2O

Key term

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

Examiner insight

Examiners will penalise students who confuse respiration with breathing (gas exchange). Be precise: respiration releases energy in cells, while breathing is the mechanism to get gases in and out of the body.

Fun fact

Yeast performs anaerobic respiration to produce ethanol and carbon dioxide. This process, called fermentation, is used to make bread rise (CO2) and to produce alcoholic drinks (ethanol).

Worked example 14 marks

A long-distance runner and a 100m sprinter both respire to power their muscles. Compare the type of respiration that would be most dominant in each athlete during their race.

  1. 1
    1. Long-distance runner: This is a sustained, moderate-intensity activity. The runner's breathing and heart rate increase to supply enough oxygen to the muscles for aerobic respiration.
  2. 2
    1. Aerobic respiration in the runner: Glucose is completely broken down using oxygen to release a large amount of energy efficiently, which is needed for a long race.
  3. 3
    1. 100m sprinter: This is a short, maximum-intensity burst. The body cannot supply oxygen to the muscles fast enough for aerobic respiration alone.
  4. 4
    1. Anaerobic respiration in the sprinter: The muscles will predominantly use anaerobic respiration, breaking down glucose without oxygen to release a small amount of energy quickly. This produces lactic acid, leading to muscle fatigue.

Recap

  • Respiration is a chemical reaction in cells to release energy from glucose.
  • Respiration is essential for all other life processes.
  • Aerobic respiration uses oxygen and releases lots of energy.
  • Anaerobic respiration does not use oxygen and releases much less energy.
  • Do not confuse respiration with breathing (gas exchange).

Quick check

  1. Write the word equation for aerobic respiration.2 marks
  2. Where in the cell does most aerobic respiration take place?1 mark

4. Excretion and Sensitivity

Excretion is the removal of waste products made by the body's chemical reactions (metabolism). These products can be toxic if they build up. In humans, the main excretory products are carbon dioxide (from respiration, excreted via the lungs) and urea (from the breakdown of excess amino acids, excreted in urine by the kidneys). It's vital not to confuse this with egestion, which is the removal of undigested food (faeces). Sensitivity is the ability to detect changes in the environment, called stimuli, and to make appropriate responses. For example, the pupil of your eye gets smaller in bright light (stimulus) to protect the retina (response). This allows organisms to avoid danger and find food or mates.

Key term

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

Examiner insight

For sensitivity questions, always clearly state both the stimulus and the response. For excretion, be specific about the product (e.g., urea) and how it is formed (e.g., from amino acid breakdown).

Common pitfall

Confusing excretion with egestion. Egestion is the removal of undigested food (faeces), which has never been part of the body's cells, whereas excretion is the removal of waste made by cells.

Worked example 13 marks

A person accidentally touches a hot stove and immediately pulls their hand away. Identify the stimulus, the response, and the life process involved.

  1. 1
    1. Identify the stimulus: The stimulus is the change in the external environment. In this case, it is the high temperature of the stove.
  2. 2
    1. Identify the response: The response is the action taken by the organism. Here, it is pulling the hand away quickly.
  3. 3
    1. Identify the life process: The ability to detect a stimulus and make a response is sensitivity.

Worked example 23 marks

Explain why carbon dioxide is considered an excretory product in humans.

  1. 1
    1. State the origin of CO2: Carbon dioxide is produced as a waste product during aerobic respiration, which occurs in all living cells.
  2. 2
    1. Define metabolic waste: Respiration is a metabolic process. Therefore, CO2 is a waste product of metabolism.
  3. 3
    1. Explain its removal: If CO2 were to build up in the blood, it would become toxic by lowering the pH. It is removed from the body via the lungs during breathing. This removal of a metabolic waste product is excretion.

Recap

  • Excretion is the removal of metabolic waste like CO2 and urea.
  • Excretion is different from egestion (removal of faeces).
  • Sensitivity is the ability to detect stimuli and make responses.
  • Stimuli are changes in the internal or external environment.
  • Responding to stimuli helps organisms survive.

Quick check

  1. Name one excretory organ in humans and the substance it excretes.2 marks
  2. Define 'stimulus'.1 mark

End-of-chapter exercise

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

  1. Define the term 'life process' and list the seven life processes common to all living organisms.3 marks
  2. Compare and contrast aerobic and anaerobic respiration in terms of oxygen requirement, products, and the amount of energy released.5 marks
  3. A woodlouse is a small crustacean that lives in damp, dark places. Describe an investigation you could carry out to show that a woodlouse demonstrates sensitivity to light.4 marks
  4. Explain the difference between excretion and egestion in a mammal. Give one example of an excretory product and the organ that removes it.4 marks
  5. Distinguish between nutrition in a plant and nutrition in an animal.4 marks
  6. A growing seedling is an example of an organism undergoing growth. Explain what 'growth' means in biological terms.2 marks
  7. Reproduction is a life process. Explain why reproduction is essential for the species but not for the individual organism.3 marks
  8. During a 400m race, an athlete's muscles produce lactic acid. Explain why this happens, referring to the specific type of respiration occurring.4 marks
  9. A student claims that a car is a living organism because it moves, uses fuel (nutrition), and releases waste gases (excretion). Explain why the car is not considered a living organism, referencing at least three other life processes.5 marks
  10. A patient's kidneys have failed. Explain why the build-up of urea in their blood is dangerous, linking your answer to the definition of excretion.3 marks

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