10.2 Life Processes: Transportation & Excretion

10.2 Life Processes: Transportation & Excretion

Why It Matters

Once food is digested and oxygen is absorbed, they must be carried to every cell, and waste products (CO2, urea) must be carried away. The circulatory system does this in humans and other animals, while xylem and phloem do it in plants. Excretion removes the nitrogenous wastes produced when cells break down proteins.

Transportation in Humans

The human circulatory system has three parts: blood, blood vessels and the heart.

Blood is a fluid connective tissue with:

• Red blood cells (RBCs) — contain haemoglobin, which binds oxygen (as oxyhaemoglobin). RBCs have no nucleus in mammals, which maximises space for haemoglobin.
• White blood cells (WBCs) — fight infection (the body's defence).
• Platelets — help in clotting of blood.
• Plasma — the fluid part, which carries dissolved nutrients, hormones, CO2 (mostly) and urea.

Blood vessels — there are three types:

Capillaries have walls only one cell thick so that substances (O2, CO2, nutrients, waste) can diffuse rapidly between blood and the surrounding cells.

The Human Heart & Double Circulation

The human heart is a muscular organ, about the size of a fist, located in the chest cavity, slightly tilted to the left. It has four chambers: two upper, thin-walled atria (right and left) and two lower, thick-walled, muscular ventricles (right and left). The left ventricle is the thickest because it has to pump blood to the whole body.

Double circulation means blood passes through the heart twice during one complete cycle:

1. Pulmonary circulation — deoxygenated blood from the right ventricle goes to the lungs via the pulmonary artery, picks up oxygen, returns to the left atrium via the pulmonary vein.
2. Systemic circulation — oxygenated blood from the left ventricle goes to the whole body via the aorta, gives up O2 and picks up CO2, returns to the right atrium via the vena cava.

Double circulation is important because it keeps the oxygenated and deoxygenated blood completely separate, which means body tissues receive fully oxygen-rich blood, maintaining a high metabolic rate (essential for warm-blooded animals like birds and mammals).

Heartbeat: the heart beats about 70–80 times per minute in an adult at rest. The two atria contract together, then the two ventricles contract together, and then both relax. The sequence is controlled by a patch of specialised heart muscle called the pacemaker (SA node).

Transportation in Plants

Plants do not have a heart or blood, but they need to move water, minerals and food. They have two vascular tissues:

• Xylem — moves water and dissolved minerals from the roots UP to the leaves and other parts. Xylem vessels are mostly dead at maturity, hollow tubes. The upward pull of water is driven by transpiration (evaporation of water from leaf surfaces through stomata). This transpiration pull also cools the plant.
• Phloem — moves food (sucrose and amino acids) made in the leaves to all other parts, including roots, fruits and seeds. This is called translocation. Unlike xylem, phloem tubes are living cells. Translocation needs energy (ATP) and uses a pressure-flow mechanism: high concentration of sugars at the source (leaf) pushes the sap along the tube to the sink (root/fruit) where sugars are used or stored. Phloem transport is bidirectional (food can move up or down depending on where it is needed), unlike xylem which is mainly upward.

A useful mnemonic: Xylem up, Phloem wherever.

Excretion in Humans

Excretion is the removal of metabolic waste (mainly nitrogenous wastes like urea) from the body. (Note: egestion = removal of undigested food as faeces; excretion = removal of metabolic waste like urea, CO2, sweat.)

The human excretory system consists of:

• A pair of kidneys (bean-shaped, reddish-brown, located in the abdomen, one on each side of the backbone).
• A pair of ureters — tubes carrying urine from each kidney to the urinary bladder.
• The urinary bladder — stores urine until it is convenient to release it.
• The urethra — the tube through which urine leaves the body.

Each kidney has about a million tiny filtering units called nephrons. A nephron has a cup-shaped Bowman's capsule at the top, containing a cluster of capillaries called the glomerulus. Blood enters the glomerulus under high pressure; the filtrate (water, urea, salts, glucose, amino acids) is pushed into the Bowman's capsule. As the filtrate passes along the long tubule, useful substances (glucose, amino acids, most water, some salts) are selectively reabsorbed back into the blood by capillaries around the tubule. The remaining fluid — urine — contains water, urea, some salts and other waste, and flows to the collecting duct, then to the ureter.

Urine is about 95% water and 2.5% urea, the rest being other dissolved salts and wastes. An adult produces about 1–1.8 litres of urine per day.

If the kidneys fail, waste builds up in the blood. This is treated by dialysis (an artificial kidney machine that filters blood), but dialysis does not cure kidney failure — it only maintains the patient until a transplant can be done.

Excretion in Plants

Plants do not have a dedicated excretory system. Their wastes are dealt with differently:

• Gaseous wastes (O2 during the day from photosynthesis, CO2 from respiration) escape through stomata in leaves and lenticels in stems.
• Some waste products are stored in old leaves, which are then shed, or in bark, or in cell vacuoles, isolating them from the rest of the plant.
• Some plants store waste as resins, gums, latex or tannins in specialised tissues.

Exam Traps

1. The pulmonary artery carries deoxygenated blood (the only artery that does); the pulmonary vein carries oxygenated blood (the only vein that does). Examiners love this exception.
2. Xylem is mostly dead tissue; phloem is living tissue. If a question asks which tissue uses ATP for transport, the answer is phloem.
3. Excretion vs egestion: undigested food (faeces) is egested, NOT excreted. Excretion is the removal of metabolic waste.
4. The left ventricle is the thickest chamber because it pumps blood to the whole body; the right ventricle pumps blood only to the lungs.