Cambridge Lower Secondary CheckpointStage 6

Biology: Structure and function

Science Stage 6 Chapter Notes

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Biology: Structure and function
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In this chapter

  1. 1Levels of Organisation
  2. 2The Heart: A Powerful Double Pump
  3. 3Highways of the Body: Blood and Vessels
  4. 4The Lungs and Gas Exchange
  5. 5The Digestive System and Enzymes
  6. 6The Brain: The Body's Control Centre

1. Levels of Organisation

In complex multicellular organisms like humans, life is structured in a hierarchy. The simplest level is the specialised cell, which is adapted for a specific job (e.g., a muscle cell for contraction). Groups of similar specialised cells work together to form a tissue (e.g., muscle tissue). Different tissues then combine to form an organ, which performs a specific, complex function (e.g., the heart, made of muscle, nervous, and connective tissues). Several organs work together in an organ system to carry out a major function in the body (e.g., the heart, blood, and blood vessels form the circulatory system). Finally, all organ systems work together to make up a living organism.

Key term

Tissue: A group of similar cells that are specialised to work together to carry out a particular function.

Examiner insight

Marks are often awarded for using specific examples to illustrate each level of organisation, such as 'cardiac muscle tissue' instead of just 'muscle'.

Common pitfall

Stating that an organ is made of 'one type of cell' rather than multiple tissues working together.

Worked example 14 marks

The following are five terms relating to the organisation of the body.

Organism, Tissue, Cell, Organ System, Organ

(a) Arrange these terms in the correct order, from simplest to most complex. [2](b) Define the term 'Organ'. [2]

  1. 1

    (a) The correct order starts with the basic building block and builds up to the whole organism.

  2. 2

    Step 1: Identify the simplest level - the Cell.

  3. 3

    Step 2: Identify the next level - a group of cells is a Tissue.

  4. 4

    Step 3: Identify the next level - a group of tissues is an Organ.

  5. 5

    Step 4: Identify the next level - a group of organs is an Organ System.

  6. 6

    Step 5: Identify the final level - the whole Organism.

  7. 7

    Final Answer (a): Cell, Tissue, Organ, Organ System, Organism.

  8. 8

    (b) An organ is a structure made of different tissues working together to perform a specific function.

  9. 9

    Final Answer (b): A collection of different tissues (at least two) working together to perform a specific function(s).

Recap

  • Specialised cells are the basic building blocks of complex organisms.
  • Tissues are groups of similar cells performing a shared function.
  • Organs are structures made of different tissues working together.
  • Organ systems consist of several organs collaborating on a major body function.
  • The hierarchy is: Cell -> Tissue -> Organ -> Organ System -> Organism.

Quick check

  1. Give an example of an organ and name one tissue found within it.2 marks

2. The Heart: A Powerful Double Pump

The heart is a muscular organ responsible for pumping blood around the body. It has four chambers: two atria (singular: atrium) at the top which receive blood, and two ventricles at the bottom which pump blood out. The left and right sides are separated by a muscular wall called the septum, which prevents oxygenated and deoxygenated blood from mixing. Humans have a double circulatory system. The right side of the heart pumps deoxygenated blood to the lungs (the pulmonary circuit), while the left side pumps oxygenated blood to the rest of the body (the systemic circuit). Valves are present to ensure blood flows in one direction and prevent backflow.

Cardiac Output (CO) = Heart Rate (HR) × Stroke Volume (SV)

Key term

Double Circulatory System: A circulatory system in which the blood passes through the heart twice for each complete circuit of the body.

Examiner insight

Students who can accurately trace the path of a red blood cell through the right side of the heart, to the lungs, and back through the left side gain full marks.

Common pitfall

Confusing the pulmonary artery and pulmonary vein. The pulmonary artery carries deoxygenated blood away from the heart, while the pulmonary vein carries oxygenated blood towards the heart.

Fun fact

The sound of a heartbeat is caused by the heart valves closing. The 'lub' is the atrioventricular valves closing, and the 'dub' is the semilunar valves closing.

Worked example 16 marks

The diagram shows a human heart. Describe the path of blood from when it enters the right atrium until it leaves the aorta. In your answer, name the chambers, valves, and blood vessels involved. [6]

  1. 1

    Step 1: Blood enters the right atrium from the body via the vena cava.

  2. 2

    Step 2: The right atrium contracts, pushing blood through the tricuspid valve into the right ventricle.

  3. 3

    Step 3: The right ventricle contracts, pumping the deoxygenated blood through the pulmonary valve into the pulmonary artery, which leads to the lungs.

  4. 4

    Step 4: In the lungs, the blood becomes oxygenated and returns to the heart via the pulmonary vein, entering the left atrium.

  5. 5

    Step 5: The left atrium contracts, pushing blood through the mitral (bicuspid) valve into the left ventricle.

  6. 6

    Step 6: The muscular left ventricle contracts powerfully, pumping the oxygenated blood through the aortic valve into the aorta to be distributed to the rest of the body.

Worked example 23 marks

Explain why the wall of the left ventricle is much thicker than the wall of the right ventricle. [3]

  1. 1

    Step 1: State the function of each ventricle. The right ventricle pumps blood to the lungs.

  2. 2

    Step 2: The left ventricle pumps blood to the entire rest of the body.

  3. 3

    Step 3: Link the function to the structure. Pumping blood to the whole body requires much greater force and pressure than pumping it only to the nearby lungs.

  4. 4

    Step 4: Conclude. Therefore, the left ventricle has a thicker, more muscular wall to generate the necessary high pressure.

Recap

  • The heart has four chambers: two atria and two ventricles.
  • The right side handles deoxygenated blood; the left side handles oxygenated blood.
  • Valves prevent the backflow of blood.
  • The pulmonary circuit goes to the lungs; the systemic circuit goes to the body.
  • The muscular wall of the left ventricle is thicker than the right.

Quick check

  1. Which blood vessel carries oxygenated blood away from the heart to the body?1 mark
  2. What is the function of the septum in the heart?1 mark

3. Highways of the Body: Blood and Vessels

Blood is transported around the body in three types of blood vessels: arteries, veins, and capillaries. Arteries carry high-pressure blood away from the heart; they have thick, muscular, and elastic walls to withstand this pressure and a narrow lumen. Veins carry lower-pressure blood towards the heart; they have thinner walls, a wider lumen, and contain valves to prevent backflow. Capillaries are tiny vessels that form vast networks in tissues. Their walls are only one cell thick, providing a very short diffusion path for efficient exchange of substances like oxygen, carbon dioxide, and glucose between the blood and body cells. Blood itself is composed of four main components: plasma (the liquid matrix), red blood cells (for oxygen transport via haemoglobin), white blood cells (for immunity), and platelets (for clotting).

Key term

Capillary: A microscopic blood vessel with walls one-cell thick that allows for efficient exchange of substances between the blood and body tissues.

Examiner insight

Examiners look for comparative language when asked to contrast arteries and veins, using words like 'thicker' or 'wider' rather than just listing features for each in isolation.

Common pitfall

Stating that 'all arteries carry oxygenated blood'. The pulmonary artery is a key exception, carrying deoxygenated blood to the lungs.

Worked example 14 marks

Compare the structure of an artery with the structure of a vein. For each structural difference, explain how it relates to its function. [4]

  1. 1

    Point 1 (Wall Thickness): Arteries have thick, muscular and elastic walls whereas veins have thinner walls. This is because arteries must withstand high-pressure blood pumped from the heart, while veins carry low-pressure blood.

  2. 2

    Point 2 (Lumen Size): Arteries have a narrow lumen relative to their diameter, while veins have a wide lumen. The narrow lumen helps maintain high pressure in arteries, while the wide lumen reduces resistance to blood flow in veins.

  3. 3

    Point 3 (Valves): Veins contain valves, whereas arteries do not (except semilunar valves leaving the heart). Valves in veins are needed to prevent the backflow of blood under low pressure, especially in the limbs.

Recap

  • Arteries carry blood away from the heart; they have thick walls and no valves.
  • Veins carry blood towards the heart; they have thinner walls and contain valves.
  • Capillaries are one-cell thick for efficient exchange of substances with tissues.
  • Red blood cells transport oxygen using haemoglobin.
  • White blood cells are part of the immune system.
  • Platelets are involved in blood clotting.

Quick check

  1. State the function of platelets.1 mark
  2. Why do capillaries have walls that are only one cell thick?1 mark

4. The Lungs and Gas Exchange

The respiratory system's primary function is gas exchange: taking in oxygen and removing carbon dioxide. Air enters via the trachea, which branches into two bronchi, one for each lung. The bronchi divide into smaller and smaller tubes called bronchioles, ending in millions of tiny air sacs called alveoli. It is in the alveoli that gas exchange occurs. Alveoli are perfectly adapted for this job. They have an enormous total surface area, their walls are only one cell thick (as are the capillary walls surrounding them) creating a very short diffusion distance, they have a rich blood supply to maintain a steep concentration gradient, and a moist lining for gases to dissolve in. Oxygen diffuses from the high concentration in the alveoli into the blood, while carbon dioxide diffuses from the high concentration in the blood into the alveoli to be exhaled.

Key term

Alveoli: Tiny air sacs in the lungs where the exchange of oxygen and carbon dioxide between the air and the blood takes place by diffusion.

Examiner insight

High-scoring answers on gas exchange always mention the four key adaptations of the alveoli and link each one to how it speeds up the rate of diffusion.

Common pitfall

Confusing breathing (the mechanical process of moving air in and out of the lungs) with respiration (the chemical process in cells that releases energy). Gas exchange supports cellular respiration.

Fun fact

If you could spread out all the alveoli in your lungs, they would cover a surface area roughly the size of a tennis court!

Worked example 15 marks

Explain how the alveoli in the lungs are adapted for efficient gas exchange. [5]

  1. 1

    Step 1: Large surface area. There are millions of alveoli, which collectively provide a huge surface area (like a tennis court) for diffusion to occur over.

  2. 2

    Step 2: Thin walls. The walls of the alveoli and the surrounding capillaries are each only one cell thick. This creates a very short diffusion pathway for gases, speeding up exchange.

  3. 3

    Step 3: Rich blood supply. A dense network of capillaries surrounds each alveolus. This constantly transports blood to and from the lungs, maintaining a steep concentration gradient for oxygen and carbon dioxide.

  4. 4

    Step 4: Moist surface. The inner surface of the alveoli is covered in a thin layer of moisture. This allows oxygen to dissolve before it diffuses across the membrane into the blood.

  5. 5

    Step 5: Ventilation. Breathing maintains a high concentration of oxygen and a low concentration of carbon dioxide in the alveoli, which also maintains the concentration gradient for diffusion.

Recap

  • Air travels from the trachea to the bronchi, bronchioles, and finally the alveoli.
  • Gas exchange occurs between the alveoli and the blood in the capillaries.
  • Alveoli have a large surface area, thin walls, a good blood supply, and a moist lining.
  • Oxygen diffuses from alveoli to blood; carbon dioxide diffuses from blood to alveoli.
  • This process relies on maintaining a steep concentration gradient.

Quick check

  1. Name the process by which oxygen moves from the alveoli into the blood.1 mark
  2. State two features of an alveolus that helps with gas exchange.2 marks

5. The Digestive System and Enzymes

The digestive system breaks down large, insoluble food molecules into small, soluble molecules that can be absorbed into the bloodstream. This involves both mechanical digestion (chewing, churning) and chemical digestion (enzymes). Key organs include the stomach, where acid and protease enzymes (like pepsin) begin protein digestion. The food then moves to the small intestine, where digestion is completed and absorption occurs. The pancreas releases a cocktail of enzymes into the small intestine: amylase to break down starch into sugars, proteases to break down proteins into amino acids, and lipases to break down fats (lipids) into fatty acids and glycerol. The liver produces bile, which is stored in the gall bladder and released into the small intestine to emulsify fats, increasing the surface area for lipase to act on. The small intestine is lined with villi to maximise the surface area for absorption of the digested nutrients. Finally, the large intestine absorbs water from the remaining undigested food, and the faeces are egested.

Key term

Enzyme: A biological catalyst that speeds up the rate of a specific chemical reaction without being used up.

Examiner insight

Examiners expect students to name specific enzymes, their substrates, and their products (e.g., 'Amylase breaks down starch into maltose'). Vague answers like 'enzymes break down food' will not score high marks.

Common pitfall

Confusing the roles of the small and large intestine. Most digestion and absorption of nutrients happens in the small intestine, while the main role of the large intestine is water absorption.

Worked example 15 marks

Describe the role of the liver, pancreas, and small intestine in the digestion of fat. [5]

  1. 1

    Step 1: The liver produces bile, which is stored in the gall bladder.

  2. 2

    Step 2: When fat enters the small intestine, bile is released into it.

  3. 3

    Step 3: Bile emulsifies the fat. This means it breaks down large fat globules into smaller droplets.

  4. 4

    Step 4: This emulsification increases the surface area of the fat for enzymes to work on.

  5. 5

    Step 5: The pancreas produces the enzyme lipase, which is also released into the small intestine. Lipase then chemically digests the fat droplets into fatty acids and glycerol, which can be absorbed.

Recap

  • Amylase digests starch into simple sugars.
  • Protease digests protein into amino acids.
  • Lipase digests fats (lipids) into fatty acids and glycerol.
  • Bile, produced by the liver, emulsifies fats.
  • Most chemical digestion and all absorption of nutrients occurs in the small intestine.
  • The large intestine primarily absorbs water.

Quick check

  1. Which enzyme digests protein and where does this digestion begin?2 marks
  2. What is the function of villi in the small intestine?1 mark

6. The Brain: The Body's Control Centre

The brain, part of the central nervous system, is the body's primary control centre. It is a highly complex organ with different regions specialised for different functions. The largest part is the cerebrum, which is divided into two hemispheres. The cerebrum is the site of all conscious thought, memory, learning, language, and personality. It also processes information from our senses. The cerebellum, located at the back of the brain, is responsible for coordinating movement, posture, and balance. It ensures our movements are smooth and precise. The medulla oblongata (or brain stem) controls vital, unconscious activities such as heart rate, breathing rate, and digestion. Finally, regions like the hypothalamus are crucial for homeostasis, regulating factors like body temperature and water balance.

Key term

Cerebrum: The largest part of the human brain, responsible for conscious functions such as thought, language, memory, and processing sensory information.

Examiner insight

Clear, concise descriptions of the function of each brain part are required. For example, linking the cerebellum specifically to 'coordination of movement and balance' is better than a vague answer like 'controls movement'.

Common pitfall

Attributing all 'thinking' to the brain as a whole, without differentiating the specific roles of the cerebrum, cerebellum, and medulla.

Fun fact

Although the brain is only about 2% of your body weight, it uses around 20% of the oxygen and calories your body consumes.

Worked example 12 marks

A person suffers an injury to their cerebellum. Describe two effects this might have on their daily life. [2]

  1. 1

    Step 1: Recall the function of the cerebellum. It controls coordination of voluntary movement and balance.

  2. 2

    Step 2: Based on this function, identify a potential difficulty. A person might have difficulty with coordinated movements, leading to jerky or uncontrolled actions.

  3. 3

    Step 3: Identify a second difficulty. They might have poor balance and find it difficult to walk or stand without falling.

  4. 4

    Final Answer: They may have difficulty with fine motor skills like writing or buttoning a shirt. They may also have poor balance and be prone to falling.

Worked example 24 marks

A diagram of the brain is shown. Identify region X (the cerebrum) and region Y (the medulla) and state one function of each. [4]

  1. 1

    Step 1: Identify region X as the cerebrum, the large, folded outer part of the brain.

  2. 2

    Step 2: State one function of the cerebrum, for example, memory, conscious thought, or processing sensory information.

  3. 3

    Step 3: Identify region Y as the medulla oblongata, located at the top of the spinal cord.

  4. 4

    Step 4: State one function of the medulla, for example, controlling heart rate or breathing rate.

Recap

  • The cerebrum is responsible for conscious thought, memory, and intelligence.
  • The cerebellum coordinates movement and balance.
  • The medulla oblongata controls involuntary actions like breathing and heart rate.
  • Different regions of the brain have highly specialised functions.

Quick check

  1. Which part of the brain is responsible for keeping your balance when riding a bicycle?1 mark
  2. If you are learning a new language, which part of your brain is most active?1 mark

End-of-chapter exercise

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

  1. Define the terms 'tissue' and 'organ'.2 marks
  2. Name the four main components of blood and state the function of each.4 marks
  3. Describe the path taken by a red blood cell starting from the vena cava, passing through the heart and lungs, and ending in the aorta. You do not need to name the valves.5 marks
  4. Explain how the structure of the small intestine is adapted for the efficient absorption of nutrients.4 marks
  5. Compare the structure of an artery with the structure of a capillary, relating each feature to its function.4 marks
  6. A patient has a damaged cerebellum. Describe two difficulties they might experience in their daily life.2 marks
  7. Explain in detail how the structure of the lungs enables efficient gas exchange to occur.6 marks
  8. Describe the chemical digestion of a protein molecule as it passes through the human alimentary canal. Name the organs, enzymes, and products involved at each stage.6 marks
  9. An athlete's heart rate is 180 beats per minute during exercise. Their stroke volume is 110 ml per beat. Calculate their cardiac output in litres per minute (L/min).3 marks
  10. State the function of the medulla oblongata.1 mark

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