Transport System in Living Things

Singapore Lower Secondary Science — Complete Study Notes

1. Overview — What Is the Circulatory System?

Purpose

The circulatory system transports substances around the body:

Oxygen and glucose to respiring cells
Carbon dioxide and other waste products away from cells
Hormones, antibodies, and heat

Components

The circulatory system consists of:

Heart — muscular pump that keeps blood moving
Blood vessels — arteries, veins, and capillaries (the "pipes")
Blood — the fluid that carries dissolved substances

Key point: Humans have a double circulatory system — blood passes through the heart twice for each complete circuit of the body.

2. Structure of the Heart

Four Chambers

Chamber	Side	Receives blood from	Sends blood to
Right atrium	Right	Vena cava (superior & inferior)	Right ventricle
Right ventricle	Right	Right atrium	Pulmonary artery → lungs
Left atrium	Left	Pulmonary vein (from lungs)	Left ventricle
Left ventricle	Left	Left atrium	Aorta → rest of body

Valves

Valve	Location	Function
Tricuspid valve	Between right atrium & right ventricle	Prevents backflow of blood into right atrium when right ventricle contracts
Bicuspid (mitral) valve	Between left atrium & left ventricle	Prevents backflow of blood into left atrium when left ventricle contracts
Aortic semilunar valve	Between the left ventricle & aorta	Prevents backflow of blood into the left ventricle
Pulmonary semilunar valve	Between the right ventricle & pulmonary artery	Prevents backflow of blood into the right ventricle

The bicuspid valve is also called the mitral valve. Both names are acceptable in exams.

Septum

The septum is a muscular wall that separates the left and right sides of the heart. It prevents oxygenated blood and deoxygenated blood from mixing.

Wall Thickness Adaptation

The left ventricle has a much thicker muscular wall than the right ventricle because it must pump blood to the entire body (systemic circulation), whereas the right ventricle only pumps blood to the nearby lungs (pulmonary circulation).

Exam favourite: "Explain why the left ventricle has a thicker wall than the right ventricle." — It must generate much higher pressure to push blood all the way around the body.

Heart Diagram

Note: Blue = deoxygenated blood Red = oxygenated blood Green lines = valves

3. Pathway of Blood Through the Heart

Follow the path step by step:

Vena cava (superior & inferior) carries deoxygenated blood into the right atrium
Right atrium contracts → blood passes through the tricuspid valve into the right ventricle
Right ventricle contracts → blood is pumped through the pulmonary semilunar valve into the pulmonary artery
Pulmonary artery carries blood to the lungs, where gas exchange occurs (CO₂ removed, O₂ absorbed)
Oxygenated blood returns via the pulmonary vein to the left atrium
Left atrium contracts → blood passes through the bicuspid (mitral) valve into the left ventricle
Left ventricle contracts → blood is pumped through the aortic semilunar valve into the aorta
The aorta carries oxygenated blood to the rest of the body

Exception alert! The pulmonary artery carries deoxygenated blood (the only artery that does). The pulmonary vein carries oxygenated blood (the only vein that does). Arteries are defined by direction of flow (away from heart), NOT by oxygen content.

4. Double Circulation

Humans have a double circulatory system — blood passes through the heart twice for each full circuit:

Pulmonary Circulation

Right side of heart → lungs → back to left side

Right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium
Purpose: Gas exchange — oxygen enters blood, carbon dioxide leaves blood

Systemic Circulation

Left side of heart → body organs → back to right side

Left ventricle → aorta → body organs → vena cava → right atrium
Purpose: Deliver oxygen and nutrients to cells; remove CO₂ and waste

Advantage of double circulation: Blood can be pumped at higher pressure to the body after receiving oxygen in the lungs. This ensures oxygen reaches respiring tissues faster and more efficiently.

5. Blood Vessels

Comparison Table

Feature	Artery	Vein	Capillary
Function	Carries blood away from the heart	Carries blood towards the heart	Exchange of materials between blood and cells
Wall thickness	Thick (muscle + elastic fibres)	Thin (less muscle + elastic fibres)	One cell thick (endothelium only)
Lumen size	Small lumen	Large lumen	Tiny lumen (just fits RBCs)
Valves	None	Present (prevent backflow)	None
Pressure	High	Low	Falling (high → low)
Pulse	Felt (pulsatile)	Not felt (steady flow)	Not felt

Why do arteries have thick, elastic walls? To withstand and smooth out the high pressure of blood pumped by the ventricles. The elastic fibres stretch and recoil to maintain steady blood flow.

Why do veins have valves? Blood in veins is at low pressure. Valves prevent backflow and ensure blood returns to the heart, especially in the limbs where blood must travel against gravity.

Why are capillary walls one cell thick? To provide a short diffusion distance for efficient exchange of oxygen, carbon dioxide, glucose, and waste products between blood and cells.

6. Blood — Components & Functions

Component	Description	Function
Red blood cells (erythrocytes)	Biconcave disc shape; no nucleus; contain haemoglobin	Transport oxygen (haemoglobin binds O₂ to form oxyhaemoglobin). Biconcave shape increases surface area for gas exchange. No nucleus → more room for haemoglobin.
White blood cells (leucocytes)	Have a nucleus; larger than RBCs; fewer in number	Defend against disease. Phagocytes engulf pathogens (phagocytosis). Lymphocytes produce antibodies.
Platelets	Small cell fragments; no nucleus	Involved in blood clotting — seal wounds to prevent blood loss and entry of pathogens
Plasma	Straw-coloured liquid (mostly water)	Transports dissolved substances: glucose, amino acids, urea, hormones, antibodies, CO₂ (as hydrogen carbonate)

Key adaptations of red blood cells:

Biconcave disc — increases surface-area-to-volume ratio for faster oxygen diffusion
No nucleus — more space for haemoglobin molecules
Flexible membrane — can squeeze through narrow capillaries

Blood composition by volume: Plasma ~55%, Red blood cells ~45%, White blood cells and platelets <1%

7. Coronary Heart Disease (CHD)

What Is CHD?

Coronary heart disease occurs when the coronary arteries (which supply the heart muscle with oxygen and glucose) become narrowed or blocked by fatty deposits (atheroma). This reduces blood flow to the heart muscle, causing chest pain (angina) or a heart attack (myocardial infarction).

Causes & Risk Factors

Factor	How It Contributes
High-fat diet (saturated fats)	Increases blood cholesterol → fatty deposits (atheroma) build up on artery walls
Smoking	Nicotine constricts blood vessels; carbon monoxide reduces oxygen in blood; damages artery lining
Lack of exercise	Reduces cardiovascular fitness; contributes to obesity and high blood pressure
Obesity	Increases workload on the heart; linked to high blood pressure and diabetes
Excessive stress / alcohol	Raises blood pressure; damages blood vessel walls
Genetic predisposition	Family history increases risk

Prevention

Balanced diet — reduce saturated fats and cholesterol; increase fibre
Regular aerobic exercise
Avoid smoking
Maintain healthy body weight
Manage stress

The coronary arteries are the heart's own blood supply. If they are blocked, the heart muscle is starved of oxygen and can die — this is a heart attack.

8. Why Multicellular Organisms Need Transport Systems

Unicellular vs Multicellular Organisms

Unicellular organisms (e.g. amoeba) are very small. They have a large surface area to volume ratio. This means that diffusion alone is sufficient to transport oxygen, nutrients and waste products in and out of the cell — the distances are very short.

Multicellular organisms (e.g. humans) are made up of many cells. They have a small surface area to volume ratio. The distance from the outside environment to the cells deep inside the body is too great for diffusion alone to be efficient. Substances would take far too long to reach inner cells by diffusion.

Multicellular organisms need a transport system because their small surface area to volume ratio and large internal distances make diffusion too slow to meet the needs of all cells.

What the Transport System Carries

Oxygen — from lungs to all cells for respiration
Glucose and nutrients — from digestive system to all cells for energy and growth
Carbon dioxide — from cells to lungs for removal
Urea — from cells to kidneys for excretion
Hormones — from glands to target organs
Heat — from the liver and muscles to the rest of the body

9. Diffusion

Definition

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient.

Diffusion is a passive process — it does not require energy from respiration. Particles move due to their random kinetic energy.

Factors Affecting the Rate of Diffusion

Factor	Effect on Rate	Why
Steeper concentration gradient	Increases rate	Larger difference in concentration means more particles move from high to low
Higher temperature	Increases rate	Particles have more kinetic energy and move faster
Larger surface area	Increases rate	More space for particles to diffuse across
Shorter diffusion distance	Increases rate	Particles travel a shorter distance, so they cross faster

Examples of Diffusion in Living Organisms

Oxygen diffuses from the alveoli (high concentration) into the blood capillaries (low concentration) in the lungs
Carbon dioxide diffuses from the blood (high concentration) into the alveoli (low concentration) in the lungs
Digested food molecules (e.g. glucose, amino acids) diffuse from the small intestine (high concentration) into the blood capillaries (low concentration)
Carbon dioxide diffuses into the leaf through the stomata for photosynthesis

10. Osmosis

Definition

Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane.

Osmosis is a special case of diffusion — it involves only water molecules moving through a partially permeable membrane. Like diffusion, osmosis is passive and does not require energy.

Osmosis in Plant Cells

Water enters root hair cells by osmosis because the soil water has a higher water potential than the cell sap inside the root hair cell.

When water enters a plant cell, it becomes turgid (firm and swollen). This supports the plant structure.
If water leaves a plant cell, it becomes flaccid (soft and wilted). The plant wilts.

Osmosis in Animal Cells

In a dilute solution (higher water potential outside): water enters the cell by osmosis, the cell swells and may burst (lyse) — animal cells have no cell wall to prevent this.
In a concentrated solution (lower water potential outside): water leaves the cell by osmosis, the cell shrinks and becomes crenated (shrivelled).
In an isotonic solution (equal water potential): water enters and leaves at the same rate, the cell stays normal.

Diffusion vs Osmosis

	Diffusion	Osmosis
Substance moving	Any particles (gases, dissolved substances)	Water molecules only
Partially permeable membrane required?	No	Yes
Energy required?	No (passive)	No (passive)
Direction	High concentration → low concentration	High water potential → low water potential
Gradient	Concentration gradient	Water potential gradient

11. Plant Transport System

Overview

Plants do not have a heart or blood, but they still have a transport system made up of two types of vascular tissue arranged in vascular bundles:

Xylem — transports water and dissolved mineral salts
Phloem — transports manufactured food (sucrose and amino acids)

Xylem Tissue

Feature	Detail
Transports	Water and dissolved mineral salts
Direction	One direction only — upward (roots → stem → leaves)
Cell type	Dead, hollow cells with no cytoplasm or nuclei
Structure	No cross-walls — forms a continuous hollow tube
Strengthening	Walls strengthened with lignin (makes them waterproof and provides structural support)
Energy required?	No — water moves mainly by transpiration pull (a passive process)

Phloem Tissue

Feature	Detail
Transports	Manufactured food (sucrose and amino acids)
Direction	Both directions — upward and downward (called translocation)
Cell type	Living cells — sieve tube elements and companion cells
Structure	Sieve plates with pores between cells allow sap to flow; companion cells provide energy for translocation
Energy required?	Yes — translocation is an active process that requires energy from respiration

Xylem vs Phloem

	Xylem	Phloem
Transports	Water and mineral salts	Sucrose and amino acids
Direction	Upward only	Both directions (translocation)
Cells	Dead	Living
Cross-walls	None (continuous tube)	Sieve plates with pores
Energy required	No	Yes
Strengthening	Lignin	None (companion cells provide energy)

Root Hair Cells

Root hair cells are specialised cells found on the surface of plant roots. They have tiny hair-like projections called root hairs that grow outward into the soil.

Adaptation	How It Helps
Root hairs (long, thin projections)	Greatly increase surface area for absorption of water and mineral salts from the soil
Thin cell wall	Provides a short diffusion distance for water and minerals to enter the cell
Located close to xylem	Water absorbed by root hair cells can be quickly transferred to the xylem for transport upward
High concentration of solutes in cell sap	Maintains a lower water potential inside the cell than the soil water, so water enters by osmosis

Water enters root hair cells by osmosis (from higher water potential in soil to lower water potential in cell sap). Mineral salts are absorbed by active transport (against the concentration gradient, requiring energy).

Transpiration

Transpiration is the loss of water vapour from the leaves of a plant through the stomata.

When stomata are open (to allow CO₂ in for photosynthesis), water vapour also escapes. This creates a transpiration pull — a suction force that draws water up through the xylem from the roots, like drinking through a straw.

Factors Affecting Transpiration Rate

Factor	Effect on Rate	Why
Higher temperature	Increases rate	Water molecules have more kinetic energy and evaporate faster
Lower humidity (drier air)	Increases rate	Steeper water potential gradient between leaf air spaces and outside air
More wind	Increases rate	Wind carries away water vapour from leaf surface, maintaining a steep gradient
Higher light intensity	Increases rate	Stomata open wider in light to let in CO₂ for photosynthesis, allowing more water vapour to escape

Transpiration pull is the main force that moves water up the xylem. It is a passive process driven by evaporation — the plant does not use energy to pump water upward.

12. Coronary Arteries — The Heart's Own Blood Supply

The heart is a muscle that works constantly. But the walls of the heart are too thick for oxygen and glucose to reach all the muscle cells by diffusion from the blood inside the chambers.

Instead, the heart has its own blood supply through the coronary arteries.

The coronary arteries branch off the aorta just above the aortic semilunar valve
They carry oxygenated blood to the heart muscle (myocardium)
The coronary veins return deoxygenated blood to the right atrium

If a coronary artery becomes narrowed or blocked by fatty deposits (atheroma), the heart muscle does not receive enough oxygen — this is coronary heart disease (CHD). If the blockage is complete, the heart muscle dies — this is a heart attack.

13. Effects of Exercise on the Heart

During Exercise

When you exercise, your muscles need to work harder. This means they need more:

Oxygen — for aerobic respiration to release energy
Glucose — the fuel for respiration

At the same time, more waste products are produced:

Carbon dioxide — must be removed from the muscles and carried to the lungs

To meet these increased demands:

The heart beats faster (heart rate increases) — more blood is pumped per minute
Blood vessels to the muscles dilate (widen) — more blood reaches the muscles
Breathing rate also increases — more oxygen enters the blood and more CO₂ is removed

After Exercise (Recovery)

After exercise stops, the heart rate does not return to normal immediately. It remains elevated for a while because:

Extra oxygen is needed to repay the oxygen debt — the oxygen that the muscles needed but could not get fast enough during intense exercise
Lactic acid (produced during anaerobic respiration) must be broken down, which requires oxygen

The heart rate then gradually returns to the resting heart rate as the body's oxygen demand returns to normal.

Physical Fitness

Fitter people tend to have a lower resting heart rate — their heart is stronger and pumps a larger volume of blood per beat, so it needs to beat fewer times at rest
Fitter people also have a faster recovery rate — their heart rate returns to normal more quickly after exercise

14. Drug Abuse and the Transport System

How Drugs Affect the Heart and Blood Vessels

Drug Type	Examples	Effects on the Circulatory System
Stimulants	Amphetamines, cocaine	Increase heart rate and blood pressure; can cause irregular heartbeat (arrhythmia); increased risk of heart attack or stroke
Nicotine	Found in tobacco smoke	Constricts blood vessels (narrows them), raising blood pressure; increases risk of blood clots; damages the lining of arteries (contributes to atheroma build-up)
Alcohol (excessive)	Spirits, beer, wine	Raises blood pressure; over time weakens the heart muscle (cardiomyopathy); contributes to weight gain which strains the heart
Injected drugs	Heroin and other intravenous drugs	Risk of infection from unsterile needles; collapsed or damaged veins; risk of blood-borne diseases (HIV, hepatitis B and C) from shared needles

Drugs place severe strain on the circulatory system. They can damage the heart muscle, raise blood pressure, narrow blood vessels, and increase the risk of heart attacks and strokes.

15. Ethical Issues in Heart Transplant

Discussion Points

Heart transplants save lives, but they also raise important ethical questions. Consider the following:

1. Shortage of Donor Organs

There are many more patients who need a heart transplant than there are donor hearts available. Who should receive the limited number of organs?

Should a younger patient be prioritised over an older one because they may live longer with the transplant?
Should someone who has damaged their own heart through smoking or drug abuse receive the same priority as someone whose heart disease is genetic?

2. Risk of Rejection

The immune system may recognise the donated heart as foreign and attack it (rejection). To prevent this, patients must take immunosuppressant drugs for the rest of their lives. These drugs weaken the immune system, making the patient more susceptible to infections.

3. Artificial Hearts

Mechanical hearts and ventricular assist devices can be used as a bridge to transplant — keeping a patient alive until a donor heart becomes available. However, they are expensive, can develop mechanical faults, and increase the risk of blood clots.

4. Lifestyle and Responsibility

Some people argue that patients whose heart disease was caused by their lifestyle choices (smoking, poor diet, drug abuse) should have lower priority. Others argue that everyone deserves equal access to medical treatment regardless of how their illness developed.

These are ethical questions with no single correct answer. In Singapore, organ allocation is managed by the National Organ Transplant Unit, which considers factors such as medical urgency, tissue match, and waiting time.

16. Single vs Double Circulation

Comparison Table

	Single Circulation (e.g. Fish)	Double Circulation (e.g. Mammals)
Passes through heart per circuit	Once	Twice
Pathway	Heart → gills → body → heart	Heart → lungs → heart → body → heart
Pressure to body	Low (pressure drops after gills)	High (heart pumps again before going to body)
Oxygen delivery efficiency	Lower	Higher
Metabolic rate supported	Lower (cold-blooded)	Higher (warm-blooded)

The advantage of double circulation is that blood is pumped twice by the heart in one complete circuit — once to the lungs and once to the body. This means blood reaching the body is at high pressure, allowing faster delivery of oxygen and glucose to tissues. This higher efficiency supports the higher metabolic rate of warm-blooded mammals.

17. Test Yourself!

Section A — Multiple Choice (25 questions)

Q1. Which blood vessel carries blood away from the heart?

A. Vena cava B. Pulmonary vein C. Aorta D. Capillary

Answer: C. Aorta
Arteries carry blood away from the heart. The aorta is the main artery. The vena cava and pulmonary vein are veins (carry blood towards the heart).

Q2. Which chamber of the heart has the thickest muscular wall?

A. Right atrium B. Right ventricle C. Left atrium D. Left ventricle

Answer: D. Left ventricle
The left ventricle pumps blood to the entire body (systemic circulation) at high pressure, so it needs a thick muscular wall. The right ventricle only pumps to the nearby lungs.

Q3. The pulmonary artery carries:

A. Oxygenated blood to the body B. Deoxygenated blood to the lungs C. Oxygenated blood to the heart D. Deoxygenated blood to the body

Answer: B. Deoxygenated blood to the lungs
The pulmonary artery is the only artery that carries deoxygenated blood. It goes from the right ventricle to the lungs. "Artery" means away from the heart, NOT oxygenated.

Q4. What is the function of valves in veins?

A. Speed up blood flow B. Prevent backflow of blood C. Filter waste products D. Produce red blood cells

Answer: B. Prevent backflow of blood
Blood in veins is at low pressure. Valves ensure one-way flow back to the heart, particularly important in the limbs where blood must travel against gravity.

Q5. Which component of blood is responsible for clotting?

A. Red blood cells B. Plasma C. Platelets D. White blood cells

Answer: C. Platelets
Platelets are cell fragments that initiate blood clotting at wound sites, sealing the wound to prevent blood loss and entry of pathogens.

Q6. Why do red blood cells have a biconcave disc shape?

A. To look larger B. To increase surface area for oxygen diffusion C. To store more water D. To move faster

Answer: B. To increase surface area for oxygen diffusion
The biconcave shape gives a large surface-area-to-volume ratio, allowing oxygen to diffuse in and out rapidly.

Q7. In double circulation, blood passes through the heart:

A. Once B. Twice C. Three times D. Four times

Answer: B. Twice
In one complete circuit, blood passes through the heart twice — once during pulmonary circulation (right side → lungs → left side) and once during systemic circulation (left side → body → right side).

Q8. The bicuspid valve is located between:

A. Right atrium and right ventricle B. Left atrium and left ventricle C. Right ventricle and pulmonary artery D. Left ventricle and aorta

Answer: B. Left atrium and left ventricle
The bicuspid (mitral) valve is on the left side of the heart. The equivalent valve on the right side is the tricuspid valve.

Q9. Which of the following is a risk factor for coronary heart disease?

A. Low blood pressure B. Regular exercise C. A diet high in saturated fats D. High fibre intake

Answer: C. A diet high in saturated fats
High saturated fat intake raises blood cholesterol, leading to fatty deposits (atheroma) narrowing the coronary arteries. Regular exercise and high fibre are protective factors.

Q10. Which statement about capillaries is correct?

A. They have thick walls to withstand high pressure B. They have valves to prevent backflow C. Their walls are one cell thick for efficient exchange D. They carry blood away from the heart

Answer: C. Their walls are one cell thick for efficient exchange
Capillaries are the site of material exchange. Their thin walls (one cell thick) provide a short diffusion distance for oxygen, CO₂, glucose, and waste products.

Q11. Diffusion is defined as the net movement of particles:

A. Against the concentration gradient using energy B. From a region of lower concentration to higher concentration C. From a region of higher concentration to lower concentration down a concentration gradient D. Only through a partially permeable membrane

Answer: C
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is passive (no energy required) and does not need a membrane.

Q12. Which factor would increase the rate of diffusion?

A. Lowering the temperature B. Decreasing the surface area C. Increasing the diffusion distance D. Steepening the concentration gradient

Answer: D. Steepening the concentration gradient
A steeper concentration gradient means a larger difference in concentration between two regions, so more particles move from high to low concentration per unit time.

Q13. Osmosis is the net movement of:

A. Any particles through a membrane from high to low concentration B. Water molecules through a partially permeable membrane from higher to lower water potential C. Water molecules against the water potential gradient using energy D. Dissolved substances through a partially permeable membrane

Answer: B
Osmosis specifically involves water molecules moving through a partially permeable membrane from a region of higher water potential to a region of lower water potential. It is passive (no energy required).

Q14. Which process requires a partially permeable membrane?

A. Diffusion only B. Osmosis only C. Both diffusion and osmosis D. Neither

Answer: B. Osmosis only
Osmosis requires a partially permeable membrane to allow only water molecules to pass through. Diffusion can occur without a membrane (e.g. gas exchange in the lungs).

Q15. Xylem tissue transports:

A. Sucrose from leaves to roots B. Water and mineral salts from roots to leaves C. Water and mineral salts from leaves to roots D. Amino acids from roots to leaves

Answer: B
Xylem transports water and dissolved mineral salts from the roots to the stems and leaves, in one direction only (upward). Sucrose and amino acids are transported by phloem.

Q16. The transport of manufactured food in plants is called:

A. Transpiration B. Osmosis C. Translocation D. Diffusion

Answer: C. Translocation
Translocation is the transport of manufactured food (sucrose and amino acids) through the phloem. It occurs in both directions (up and down the plant). Transpiration is the loss of water vapour from leaves.

Q17. Root hair cells increase their surface area for absorption by having:

A. Thick cell walls B. Many mitochondria C. Long, thin projections (root hairs) D. Large vacuoles

Answer: C
The root hairs are long, thin projections that greatly increase the surface area of the root hair cell, allowing more water and mineral salts to be absorbed from the soil.

Q18. Transpiration is the loss of:

A. Liquid water from the roots B. Water vapour through the stomata on leaves C. Carbon dioxide from the leaves D. Oxygen from the roots

Answer: B
Transpiration is specifically the loss of water vapour (not liquid water) through the stomata on the underside of leaves. This process creates the transpiration pull that draws water up the xylem.

Q19. Why does heart rate increase during exercise?

A. To lower blood pressure B. Because the heart muscle is warming up C. To deliver more oxygen and glucose to muscles and remove more CO₂ D. To slow down breathing

Answer: C
During exercise, muscles need more oxygen and glucose for aerobic respiration to release energy. More CO₂ is also produced and must be removed. The heart beats faster to increase blood flow and meet these demands.

Q20. Which drug constricts blood vessels and raises blood pressure?

A. Paracetamol B. Nicotine C. Antibiotics D. Insulin

Answer: B. Nicotine
Nicotine constricts (narrows) blood vessels, which raises blood pressure. It also increases the risk of blood clots and damages the lining of arteries, contributing to atheroma formation.

Q21. Why do multicellular organisms need a transport system?

A. They have too many cells to count B. Their small surface area to volume ratio means diffusion alone is too slow C. They do not have a transport system D. Their cells are too small

Answer: B
Multicellular organisms have a small surface area to volume ratio. The distance from the outside to inner cells is too great for diffusion to be fast enough. A transport system is needed to carry substances to all cells efficiently.

Q22. In osmosis, water moves from a region of:

A. Lower concentration to higher concentration B. Lower water potential to higher water potential C. Higher water potential to lower water potential D. Higher concentration to lower concentration without a membrane

Answer: C
In osmosis, water molecules move from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane. Note: we use "water potential" not "concentration" for osmosis.

Q23. What creates the transpiration pull in plants?

A. Root pressure pushing water up B. Evaporation and loss of water vapour from stomata creating a suction force C. Capillary action in the xylem D. Active transport of water

Answer: B
The transpiration pull is created when water evaporates from the spongy mesophyll cells inside the leaf and water vapour is lost through the stomata. This creates a suction force that pulls water upward through the xylem from the roots.

Q24. A person with a very low resting heart rate is most likely:

A. Unhealthy B. Very physically fit C. A smoker D. Suffering from heart disease

Answer: B. Very physically fit
Fitter people tend to have a lower resting heart rate because their heart is stronger and pumps a larger volume of blood per beat (larger stroke volume). Therefore, fewer beats are needed at rest to maintain adequate blood flow.

Q25. Coronary arteries supply blood to:

A. The lungs B. The brain C. The heart muscle itself D. The liver

Answer: C. The heart muscle itself
The coronary arteries branch off the aorta and supply oxygenated blood to the heart muscle (myocardium). The heart's walls are too thick for diffusion from the blood inside the chambers to supply all the muscle cells.

Section B — Structured Questions (15 questions)

Q1. Describe the pathway of blood from the vena cava to the aorta. Name all chambers and valves the blood passes through.

Answer:
Vena cava → right atrium → tricuspid valve → right ventricle → pulmonary semilunar valve → pulmonary artery → lungs → pulmonary vein → left atrium → bicuspid (mitral) valve → left ventricle → aortic semilunar valve → aorta.

Key points: Blood is deoxygenated from vena cava to lungs. It becomes oxygenated in the lungs. From pulmonary vein onwards, blood is oxygenated.

Q2. Explain why the left ventricle has a thicker muscular wall than the right ventricle.

Answer:
The left ventricle pumps blood through the aorta to the entire body (systemic circulation), which requires high pressure to overcome the resistance of the long network of blood vessels throughout the body. The right ventricle only pumps blood through the pulmonary artery to the lungs, which are nearby, so a lower pressure is sufficient. The thicker muscular wall allows the left ventricle to contract more forcefully and generate this higher pressure.

Q3. Compare the structure of an artery and a vein. Give three differences and explain the reason for each.

Answer:
1. Arteries have thicker walls (more muscle and elastic fibres) — to withstand and smooth out the high pressure of blood pumped by the heart. Veins have thinner walls as blood is at lower pressure.
2. Arteries have a smaller lumen — helps maintain high blood pressure. Veins have a larger lumen to reduce resistance to low-pressure blood flow.
3. Veins have valves; arteries do not — valves in veins prevent backflow because blood pressure is low. Arteries do not need valves because the high pressure ensures blood only flows in one direction (away from the heart).

Q4. State two adaptations of red blood cells and explain how each adaptation helps their function.

Answer:
1. Biconcave disc shape — increases the surface-area-to-volume ratio, allowing faster diffusion of oxygen into and out of the cell.
2. No nucleus — creates more space inside the cell for haemoglobin molecules, increasing the oxygen-carrying capacity of each red blood cell.
(Also acceptable: Flexible membrane — allows red blood cells to squeeze through capillaries that are narrower than the cell's diameter.)

Q5. Explain how coronary heart disease develops and state two ways it can be prevented.

Answer:
Development: Fatty deposits (atheroma) build up on the inner walls of the coronary arteries, narrowing them. This reduces blood flow to the heart muscle, meaning less oxygen and glucose reach the cardiac muscle. This can cause chest pain (angina). If an artery becomes completely blocked, the heart muscle is starved of oxygen and may die — this is a heart attack (myocardial infarction).

Prevention (any two):
- Eat a balanced diet low in saturated fats and cholesterol to reduce fatty deposit build-up
- Exercise regularly to strengthen the heart and improve circulation
- Avoid smoking — smoking damages blood vessel lining and raises blood pressure
- Maintain a healthy body weight to reduce strain on the heart

Q6. Explain what diffusion is and name two factors that affect its rate.

Answer:
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is a passive process that does not require energy.

Factors affecting rate (any two):
1. Concentration gradient — a steeper gradient increases the rate because there is a greater difference in concentration.
2. Temperature — higher temperature increases the rate because particles have more kinetic energy and move faster.
(Also acceptable: surface area, diffusion distance)

Q7. Describe osmosis. How is it different from diffusion?

Answer:
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.

Differences from diffusion:
1. Osmosis involves only water molecules, while diffusion involves any type of particle.
2. Osmosis requires a partially permeable membrane, while diffusion does not need a membrane.
3. The direction in osmosis is described using water potential gradient, while diffusion uses concentration gradient.
Both are passive processes and do not require energy.

Q8. Compare xylem and phloem in terms of: substance transported, direction of transport, and whether the cells are living or dead.

Answer:

	Xylem	Phloem
Substance transported	Water and dissolved mineral salts	Manufactured food (sucrose and amino acids)
Direction	Upward only (roots to leaves)	Both directions — translocation (leaves to rest of plant)
Cells	Dead (no cytoplasm or nuclei; hollow tubes)	Living (sieve tube elements with companion cells)

Q9. Describe the path of water from the soil to the leaf, naming the structures involved.

Answer:
1. Water in the soil is absorbed by root hair cells by osmosis (water moves from higher water potential in soil to lower water potential in cell sap).
2. Mineral salts are absorbed by active transport (against the concentration gradient, requiring energy).
3. Water passes across the root cortex cells by osmosis to the xylem in the centre of the root.
4. Water moves up the xylem in the stem, pulled by transpiration pull.
5. Water reaches the leaf via the xylem in the leaf veins.
6. Water evaporates from the spongy mesophyll cells into the air spaces inside the leaf, and water vapour exits through the stomata by transpiration.

Q10. Explain why heart rate increases during exercise and why it returns to normal afterwards.

Answer:
During exercise: The muscles are working harder and need more energy. They require more oxygen and glucose for aerobic respiration to release this energy. More carbon dioxide is also produced as a waste product and must be removed. The heart rate increases to pump blood faster, delivering more oxygen and glucose to the muscles and removing more CO₂.

After exercise: The muscles no longer need the extra oxygen and glucose, and CO₂ production decreases, so the demand on the circulatory system reduces. However, the heart rate does not drop instantly because the body needs to repay the oxygen debt — extra oxygen is needed to break down lactic acid that built up during exercise. Once the oxygen debt is repaid, the heart rate returns to its resting rate.

Q11. Describe the adaptations of a root hair cell and explain how each helps its function.

Answer:
1. Long, thin projections (root hairs) — greatly increase the surface area for absorption of water and mineral salts from the soil.
2. Thin cell wall — provides a short diffusion distance, allowing water and minerals to enter the cell quickly.
3. Close to xylem — water absorbed by the root hair cell is quickly transferred to the xylem for transport to the rest of the plant.
4. High solute concentration in cell sap — maintains a lower water potential inside the cell than the soil water, so water enters by osmosis.

Q12. What is transpiration? Name two environmental factors that increase its rate and explain why.

Answer:
Transpiration is the loss of water vapour from the leaves of a plant through the stomata. It creates a transpiration pull that draws water up the xylem.

Factors that increase the rate (any two):
1. Higher temperature — water molecules have more kinetic energy and evaporate faster from the mesophyll cells, increasing the rate of water vapour loss.
2. Lower humidity (drier air) — creates a steeper water potential gradient between the moist air spaces in the leaf and the drier air outside, so water vapour diffuses out faster.
3. More wind — carries away water vapour from the leaf surface, maintaining a steep water potential gradient.
4. Higher light intensity — stomata open wider in light for photosynthesis, allowing more water vapour to escape.

Q13. Discuss two ethical issues related to heart transplants.

Answer (any two):
1. Shortage of donor organs — there are far more patients needing a heart transplant than there are donor hearts available. This raises the question of who should receive the limited organs. Criteria such as age, likelihood of success, and waiting time are considered, but there is no perfect system.
2. Rejection and immunosuppressants — the patient's immune system may reject the donated heart. To prevent rejection, the patient must take immunosuppressant drugs for life, which weaken the immune system and make them more vulnerable to infections.
3. Lifestyle and priority — some argue that patients whose heart disease was caused by their own lifestyle choices (e.g. smoking, poor diet) should have lower priority. Others argue that everyone deserves equal access to treatment regardless of how their illness developed.

Q14. Explain how drug abuse can damage the circulatory system. Give two specific examples.

Answer:
Drug abuse places severe strain on the circulatory system, damaging the heart and blood vessels in multiple ways.

Example 1 — Nicotine: Constricts (narrows) blood vessels, which raises blood pressure. It also increases the risk of blood clots and damages the lining of arteries, contributing to atheroma build-up and coronary heart disease.

Example 2 — Stimulants (e.g. amphetamines, cocaine): Increase heart rate and blood pressure. They can cause irregular heartbeat (arrhythmia) and increase the risk of a heart attack or stroke from the excessive strain on the cardiovascular system.
(Also acceptable: Excessive alcohol — raises blood pressure and weakens the heart muscle over time. Injected drugs — risk of damaged/collapsed veins and blood-borne diseases from shared needles.)

Q15. Compare single and double circulation. Why is double circulation more efficient?

Answer:
Single circulation (e.g. fish): Blood passes through the heart once per complete circuit. Blood flows: heart → gills → body → heart. After passing through the gills, blood pressure drops, so blood reaches the body at low pressure. This limits the rate of oxygen delivery to tissues.

Double circulation (e.g. mammals): Blood passes through the heart twice per complete circuit. The right side pumps blood to the lungs (pulmonary circulation), and the left side pumps blood to the body (systemic circulation). Blood is pumped again before going to the body, so it reaches body tissues at high pressure.

Why double circulation is more efficient: Because blood is pumped twice, it reaches the body at higher pressure. This means oxygen and glucose are delivered faster to tissues, supporting a higher metabolic rate. This is why mammals (warm-blooded) can maintain a constant body temperature and be more active than fish (cold-blooded).

— End of Study Notes —

Aligned with Singapore MOE Lower Secondary Science Syllabus