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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. 2Human Nutrition and Digestion
  3. 3Respiration and Gas Exchange
  4. 4Excretion in Humans

1. The Seven Life Processes

To be classified as a living organism, a thing must carry out seven essential activities, often remembered by the mnemonic MRS GREN. These are the fundamental processes that distinguish living things from non-living objects. Each process is vital for survival, growth, and the continuation of the species.

Key term

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

Examiner insight

Examiners expect students to define each life process precisely, especially distinguishing between respiration (a chemical reaction in cells) and breathing (the mechanical process of gas exchange).

Common pitfall

Confusing excretion with egestion. Excretion is the removal of metabolic waste (e.g., urea), while egestion is the removal of undigested food (faeces).

Worked example 13 marks

A student observes a wooden chair and a potted plant. Describe three life processes shown by the plant that are not shown by the chair.

  1. 1
    1. Growth: The plant will increase in size and complexity over time, which is a life process. The chair will not grow.
  2. 2
    1. Nutrition: The plant makes its own food via photosynthesis. The chair does not require nutrition.
  3. 3
    1. Sensitivity: The plant will respond to stimuli, for example, its shoots will grow towards a light source. The chair does not respond to stimuli.

Recap

  • All living organisms exhibit seven life processes: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, and Nutrition (MRS GREN).
  • Movement is a change in position or place, which can be the whole organism or parts of it.
  • Respiration is the release of energy from food, not to be confused with breathing.
  • Sensitivity is the ability to detect and respond to changes in the environment (stimuli).
  • Excretion is the removal of metabolic waste products like urea and carbon dioxide.
  • Nutrition involves obtaining or making food to provide energy and substances for growth.

Quick check

  1. State the life process that involves the removal of metabolic waste.1 mark
  2. Give the full term for each letter in the mnemonic MRS GREN.7 marks

2. Human Nutrition and Digestion

Nutrition is how organisms get food. Humans have a specialised digestive system, a long tube called the alimentary canal, to break down large, insoluble food molecules into small, soluble ones that can be absorbed into the blood. Digestion is achieved through both mechanical means (chewing, churning) and chemical means (enzymes). The main enzymes are amylases (for carbohydrates), proteases (for proteins), and lipases (for fats/lipids).

Starch --(Amylase)--> Maltose

Protein --(Protease)--> Amino Acids

Lipids --(Lipase)--> Fatty Acids + Glycerol

Key term

Absorption: The process where small, soluble food molecules move from the digestive system into the bloodstream.

Examiner insight

Marks are often awarded for correctly sequencing the organs of the alimentary canal and matching each organ with its specific digestive function and associated enzyme(s).

Common pitfall

Stating that all digestion happens in the stomach. Its main roles are killing pathogens and starting protein digestion; most digestion and almost all absorption occurs in the small intestine.

Fun fact

The surface area of the small intestine, with all its folds, villi, and microvilli, is roughly 250 square meters – about the size of a tennis court!

Worked example 16 marks

Describe the journey of a protein molecule from the stomach to its absorption in the small intestine. Name the key enzymes and organs involved.

  1. 1
    1. In the stomach, hydrochloric acid provides the optimal acidic pH for the protease enzyme, pepsin.
  2. 2
    1. Pepsin begins the chemical digestion of the large protein molecule into smaller polypeptide chains.
  3. 3
    1. The food (now chyme) moves into the small intestine (duodenum).
  4. 4
    1. The pancreas releases another protease, trypsin, which works in the alkaline conditions of the small intestine.
  5. 5
    1. Trypsin continues to break down the polypeptides into amino acids.
  6. 6
    1. These small, soluble amino acid molecules are then absorbed through the walls of the small intestine (ileum) into the bloodstream.

Worked example 23 marks

Explain the role of bile in digestion.

  1. 1
    1. Bile is produced by the liver and stored in the gall bladder. It is released into the small intestine.
  2. 2
    1. It neutralises the acidic chyme coming from the stomach, creating the optimal alkaline pH for enzymes in the small intestine.
  3. 3
    1. It also emulsifies large fat globules, breaking them into smaller droplets. This increases the surface area for the enzyme lipase to act upon, speeding up fat digestion.

Recap

  • The alimentary canal consists of the mouth, oesophagus, stomach, small intestine, large intestine, and anus.
  • Chemical digestion uses enzymes: amylase for starch, protease for protein, and lipase for lipids.
  • The stomach has an acidic environment for pepsin to digest protein.
  • Most chemical digestion and absorption occurs in the small intestine.
  • The small intestine is adapted for absorption with a large surface area, provided by villi and microvilli.
  • Bile, made in the liver, emulsifies fats and neutralises stomach acid.

Quick check

  1. Which enzyme digests starch, and what does it break it down into?2 marks
  2. State two functions of the stomach in digestion.2 marks

3. Respiration and Gas Exchange

Cellular respiration is the vital chemical process in all living cells that releases energy from glucose. Aerobic respiration uses oxygen and releases a large amount of energy. Anaerobic respiration occurs without oxygen and releases much less energy. In humans, anaerobic respiration produces lactic acid, causing muscle fatigue. Gas exchange is the physical process where oxygen enters the body and carbon dioxide leaves. In humans, this occurs in the lungs, specifically in millions of tiny air sacs called alveoli, which are adapted for efficient diffusion.

Aerobic Respiration: C6H12O6 + 6O2 → 6CO2 + 6H2O + large amount of energy

Anaerobic Respiration (in muscles): C6H12O6 → 2C3H6O3 + small amount of energy

Key term

Gas Exchange: The diffusion of gases, primarily oxygen and carbon dioxide, down a concentration gradient across a respiratory surface.

Examiner insight

Candidates who can clearly link the features of the alveoli (e.g., one-cell thick walls) to their function in efficient gas exchange (e.g., short diffusion distance) score highly.

Common pitfall

Forgetting that anaerobic respiration still produces some energy, just much less than aerobic respiration. Another common error is stating that the alveoli walls are 'thin' without specifying they are 'one-cell thick'.

Worked example 14 marks

Explain how the alveoli in the lungs are adapted for efficient gas exchange.

  1. 1
    1. Large surface area: There are millions of alveoli, which collectively provide a huge surface area for gases to diffuse across.
  2. 2
    1. Thin walls: The walls of the alveoli and the surrounding capillaries are each only one cell thick, creating a very short diffusion distance for gases.
  3. 3
    1. Good blood supply: A dense network of capillaries surrounds each alveolus, maintaining a steep concentration gradient by rapidly carrying oxygen away and bringing carbon dioxide.
  4. 4
    1. Moist surface: The inner surface is moist, allowing oxygen to dissolve before diffusing into the blood.

Recap

  • Aerobic respiration requires oxygen and releases a large amount of energy.
  • Anaerobic respiration does not require oxygen and releases a small amount of energy.
  • In humans, anaerobic respiration produces lactic acid, leading to an 'oxygen debt'.
  • Gas exchange occurs in the alveoli of the lungs.
  • Alveoli are adapted with a large surface area, thin walls, a moist lining, and a good blood supply.
  • Breathing (inhalation and exhalation) ventilates the lungs, maintaining concentration gradients for diffusion.

Quick check

  1. Write the balanced chemical equation for aerobic respiration.2 marks
  2. State two differences between aerobic and anaerobic respiration.2 marks

4. Excretion in Humans

Excretion is the removal of metabolic waste products and substances in excess from the body. It is crucial to prevent the build-up of toxic materials. The main excretory organs are the lungs (excreting CO2) and the kidneys. The kidneys are part of the urinary system and perform two key roles: filtering waste products like urea from the blood to form urine, and osmoregulation (regulating the water content of the blood). Urea is produced in the liver from excess amino acids and transported in the blood to the kidneys.

Key term

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

Examiner insight

Examiners reward precise definitions of excretion that clearly distinguish it from egestion. Questions often trace the path of urea, so knowing the organs in the correct sequence is vital.

Common pitfall

Confusing urea with urine. Urea is the specific chemical waste molecule (CO(NH2)2), whereas urine is the fluid mixture containing urea, water, and salts.

Fun fact

Your kidneys filter all the blood in your body approximately every 30 minutes, removing waste and ensuring your internal environment stays stable.

Worked example 16 marks

Describe the path taken by a urea molecule from its formation in the liver to its removal from the body.

  1. 1
    1. Urea is formed in the liver from the breakdown of excess amino acids.
  2. 2
    1. It enters the bloodstream and travels around the body.
  3. 3
    1. The blood containing urea enters the kidney via the renal artery.
  4. 4
    1. In the kidney, the blood is filtered and the urea, along with excess water and salts, is removed to form urine.
  5. 5
    1. The urine passes from the kidney down the ureter to the bladder, where it is stored.
  6. 6
    1. The urine is then expelled from the body via the urethra.

Worked example 24 marks

A person has a disease that prevents their kidneys from functioning. Explain why dialysis is necessary for their survival.

  1. 1
    1. If the kidneys fail, they cannot filter the blood.
  2. 2
    1. Toxic waste products, primarily urea, would build up in the blood to dangerous levels.
  3. 3
    1. The body's water and salt balance would also not be regulated, which can lead to cell damage and high blood pressure.
  4. 4
    1. Dialysis is a process where a machine artificially filters the blood, removing urea and balancing water/salt levels, performing the function of a healthy kidney.

Recap

  • Excretion removes metabolic waste and toxic substances from the body.
  • The main nitrogenous waste in humans is urea, produced in the liver.
  • The kidneys filter blood to remove urea, excess water, and excess salts.
  • The mixture of urea, water, and salts forms urine.
  • Urine travels from the kidneys, down the ureters, is stored in the bladder, and exits via the urethra.
  • Kidney failure is life-threatening because of the build-up of urea and loss of water balance.

Quick check

  1. Name the organ that produces urea and the organ that excretes it from the blood.2 marks
  2. List the three main components of urine.3 marks

End-of-chapter exercise

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

  1. List the seven characteristics of living organisms.3 marks
  2. Compare aerobic and anaerobic respiration in humans in terms of oxygen requirement, products, and energy released.5 marks
  3. Explain the importance of a balanced diet, referring to the main food groups and their functions.6 marks
  4. Describe the roles of the stomach and small intestine in the chemical digestion of a meal containing protein and starch.5 marks
  5. Explain how the structure of a villus in the small intestine is adapted for absorption.4 marks
  6. Distinguish between 'excretion' and 'egestion', giving an example of a substance removed by each process in humans.4 marks
  7. A sprinter runs a 100m race. Explain why they are likely to build up an 'oxygen debt' and how this is removed after the race.4 marks
  8. Describe the function of the kidneys in maintaining the composition of the blood.4 marks
  9. Explain the role of bile in the digestion of fats.3 marks
  10. Trace the pathway of an oxygen molecule from the air outside the body to a muscle cell where it will be used in respiration. Name the key structures it passes through.6 marks

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