Transportation and Excretion in Plants and Humans

5.4 Transportation — Moving Materials Around

Once food is absorbed and oxygen picked up at the lungs, they must reach every corner of the body; at the same time, wastes produced by cells must be carried away. In a single-celled organism, simple diffusion handles all of this — distances are tiny. But in a large multicellular body like ours, diffusion is far too slow. Specialised transport systems take over the job.

5.4.1 Transportation in Human Beings

Our transport system has three parts: a muscular pump (the heart), a network of tubes (blood vessels) and the fluid that flows through them (blood). A separate drainage system, lymph, collects the fluid that leaks out of capillaries.

The Human Heart — A Four-Chambered Pump

The heart is a fist-sized muscular organ with four chambers — two upper thin-walled atria and two lower thick-walled ventricles. A muscular wall (septum) divides the left side from the right side so that oxygenated and deoxygenated blood never mix.

• Right atrium receives deoxygenated blood from the body (via the venae cavae).
• Right ventricle pumps this blood to the lungs through the pulmonary artery.
• Left atrium receives oxygenated blood from the lungs (via pulmonary veins).
• Left ventricle pumps this oxygen-rich blood to the whole body through the aorta. Its wall is the thickest because it must push blood the longest distance at the highest pressure.

Valves between atria and ventricles, and at the base of the great arteries, prevent back-flow.

Blood Vessels — Arteries, Veins and Capillaries

Exceptions: The pulmonary artery carries deoxygenated blood (heart to lungs) and pulmonary veins carry oxygenated blood (lungs to heart).

Components of Blood

• Plasma — pale-yellow liquid (~55%); carries digested food, hormones, CO₂, wastes.
• Red blood cells (RBCs) — contain haemoglobin, the red iron-containing protein that binds oxygen.
• White blood cells (WBCs) — fight infections.
• Platelets — help blood clot at a wound.

Double Circulation — Two Loops, One Heart

Blood passes through the human heart twice for each complete round of the body. This arrangement is called double circulation. It has two loops:

1. Pulmonary circulation: Right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium. CO₂ leaves and O₂ is picked up.
2. Systemic circulation: Left ventricle → aorta → body tissues → venae cavae → right atrium. O₂ and nutrients delivered; CO₂ and wastes collected.

Why is it necessary? Warm-blooded animals like us need a high rate of respiration to keep body temperature constant. Separating oxygenated from deoxygenated blood means tissues always get fully-loaded oxygen-rich blood — maximum efficiency.

Lymph

As blood flows through capillaries, some plasma leaks out into the spaces between cells. This fluid (now called tissue fluid) is collected by blind-ended lymph capillaries. Lymph is colourless, has less protein and fewer RBCs. It drains back into the veins near the heart. Lymph also carries digested fats absorbed by the villi to the blood.

5.4.2 Transportation in Plants

Plants have two entirely separate transport tissues: xylem (water + minerals, upward only) and phloem (food, any direction).

Xylem — Upward Transport of Water

Xylem is made of dead, hollow, tube-like cells (tracheids and vessels) joined end-to-end. Two forces push water up:

1. Root pressure: Root cells actively take up minerals from the soil, lowering the water potential inside. Water follows by osmosis and gets pushed up — but only for short distances.
2. Transpiration pull: Water evaporates from the leaves through stomata (transpiration). This evaporation creates a suction that pulls the column of water up through the xylem all the way from the roots — like sipping through a straw, except the straw is hundreds of metres long in a tall tree.

Phloem — Distributing Food

Phloem is made of living cells (sieve tubes + companion cells). It carries the sugars made in leaves to all growing or storage parts — roots, fruits, seeds, flowers. Transport in phloem is called translocation and uses energy (ATP). Unlike xylem, phloem can move material in either direction — downward in summer, upward from storage organs in spring when new shoots grow.

5.5 Excretion — Throwing Out Waste

Cellular respiration and other reactions constantly produce wastes — especially nitrogen wastes like urea and uric acid from protein breakdown. Left inside, these would poison the body. Excretion is the biological process of removing them.

5.5.1 Excretion in Humans

The excretory system consists of a pair of bean-shaped kidneys, two ureters, the urinary bladder and the urethra. Kidneys filter the blood continuously. The useful substances (glucose, amino acids, most water and salts) are reabsorbed; the waste plus excess water leaves as urine.

The functional unit of the kidney is the nephron. Blood at high pressure enters a tuft of capillaries called the glomerulus sitting inside a cup — Bowman's capsule. Small molecules (water, urea, salts, glucose) are pushed out of the blood into the capsule — this is the filtrate. As the filtrate trickles along the tubule, useful items are reabsorbed into the surrounding capillaries. Whatever remains is urine.

Urine flows: kidney → ureter → bladder (stored) → urethra → out of body.

5.5.2 Excretion in Plants

Plants handle their wastes differently because they grow slowly and produce less waste than animals. Their methods include:

• Gaseous wastes (O₂ from photosynthesis, CO₂ from respiration) are released through stomata and lenticels.
• Excess water is lost by transpiration.
• Solid wastes are often stored safely — in old leaves that later fall off, in the bark, or inside vacuoles of certain cells.
• Resins and gums accumulate in the older (inner) wood.
• Some wastes are secreted into the soil through the roots.