Life Process

Water transport in plants occurs mainly through xylem tissue. The most important force is transpiration pull. When water evaporates from the leaves through stomata (transpiration), it creates a negative pressure (suction) at the top of the plant. This pull draws water upward from the roots through the xylem vessels, like drinking through a straw. Root pressure also helps but is a minor force compared to transpiration pull.

Xylem is a complex permanent tissue consisting of vessels, tracheids, xylem fibers, and xylem parenchyma. The vessels and tracheids are dead cells with thick lignified walls, forming continuous hollow tubes from roots to leaves. Water and dissolved minerals absorbed by root hairs move upward through these tubes. Xylem transports only in one direction: upward from roots to shoots.

Phloem is a complex living tissue composed of sieve tubes, companion cells, phloem fibers, and phloem parenchyma. Sieve tubes are living cells with perforated ends (sieve plates) that allow sap to flow. Phloem transports the products of photosynthesis (mainly sucrose, amino acids, hormones) from source (leaves) to sink (roots, fruits, growing tips, storage organs). This transport is bidirectional and occurs through a process called translocation.

Xylem transports water and minerals unidirectionally from roots to leaves because it is driven by transpiration pull. Phloem transports organic food bidirectionally. For example, during summer, food moves from leaves (source) downward to roots for storage. In spring, stored food moves upward from roots (source) to growing buds (sink). This two-way movement is possible because phloem uses an active process called pressure flow hypothesis.

Transpiration is the evaporation of water from the leaves, stems, and flowers of plants. About 90-99% of water absorbed by roots is lost as water vapor through stomata (small pores on leaves). While this seems wasteful, transpiration serves important functions: it creates transpiration pull for water transport, cools the plant, and allows mineral uptake. Transpiration rate is affected by light, temperature, humidity, and wind.

Root hairs are extensions of epidermal cells that increase surface area for absorption. The cell sap inside root hairs contains dissolved sugars, minerals, and other solutes, making it hypertonic (lower water concentration) compared to the soil water (higher water concentration). Therefore, water moves by osmosis from the soil into the root hairs. This creates root pressure, which pushes water upward slightly, but the main upward movement is due to transpiration pull.

The endodermis is a single layer of cells surrounding the vascular tissue (xylem and phloem) in roots. Its cells have a waxy band called the Casparian strip. This strip blocks the apoplast pathway (through cell walls), forcing water and dissolved minerals to enter the symplast pathway (through the cytoplasm). This allows the plant to selectively control which minerals reach the xylem, preventing harmful substances from entering the transpiration stream.

Translocation specifically refers to the movement of organic food (mainly sucrose) and other substances like amino acids and hormones through the phloem tissue. The source is where food is produced (leaves) or released from storage (e.g., germinating seeds). The sink is where food is used or stored (roots, fruits, developing buds, storage organs like tubers). Translocation is an energy-requiring process that can move food up or down.

According to the pressure flow hypothesis, at the source (leaf), sucrose is actively loaded into phloem sieve tubes. This makes the sap hypertonic, so water enters from xylem by osmosis, increasing hydrostatic pressure. At the sink (root), sucrose is unloaded (used or stored), making the sap hypotonic, so water leaves, decreasing pressure. The pressure difference pushes the sap from source to sink. This is why phloem transport is bidirectional depending on source-sink locations.

Transpiration pull depends on the difference in water vapor concentration between the leaf interior (saturated) and the outside air. When humidity is very high, the outside air is nearly saturated, so the gradient is small. Therefore, transpiration slows down, reducing the transpiration pull, and water transport through xylem decreases. This is why plants in humid greenhouses need less watering. Wind and high temperature increase transpiration.

Excretion specifically refers to getting rid of wastes that are produced inside the body as a result of metabolic reactions (e.g., breakdown of proteins, respiration). This is different from egestion (elimination of undigested food from the digestive tract, which never entered body cells). Major excretory products in humans are urea (from protein breakdown), carbon dioxide (from respiration), and excess water and salts.

Humans are ureotelic animals, meaning they excrete urea as their main nitrogenous waste. Urea is produced in the liver through the ornithine cycle (urea cycle) from ammonia, which is highly toxic. Ammonia is converted into less toxic urea, which can be transported in blood and excreted in urine. Uric acid is excreted by birds and reptiles (less water loss). Ammonia is excreted by fish (requires lots of water). Urea is a good balance: moderately toxic and moderately water-soluble.

Urine formation occurs in the nephrons (functional units of the kidney). First, blood pressure forces water, urea, glucose, amino acids, and ions from the glomerulus into Bowman’s capsule (filtration). Second, as the filtrate passes through the tubules, useful substances like glucose, most water, and amino acids are reabsorbed into blood (reabsorption). Third, additional wastes like hydrogen ions and drugs are added to the filtrate (secretion). The remaining fluid is urine.

The glomerulus is a tuft of capillaries inside Bowman’s capsule. Blood pressure in these capillaries is high because the afferent arteriole is wider than the efferent arteriole. This pressure forces water, glucose, amino acids, urea, uric acid, and ions through the filtration slits into Bowman’s capsule. Red blood cells, white blood cells, platelets, and large plasma proteins are too big to pass and remain in the blood. This filtrate is called glomerular filtrate or primary urine.

After filtration, the filtrate contains many valuable substances that the body needs. As the filtrate flows through the proximal convoluted tubule, loop of Henle, and distal convoluted tubule, the tubule cells actively reabsorb glucose, amino acids, and ions, while water follows passively by osmosis. About 99% of the filtrate is reabsorbed. If reabsorption fails (e.g., in diabetes, glucose appears in urine), it indicates a problem. Reabsorption is selective and regulated.

Tubular secretion is the opposite of reabsorption. It occurs mainly in the distal convoluted tubule and collecting duct. The body uses secretion to get rid of excess ions (like K+), maintain blood pH (by secreting H+ or ammonia), and remove toxins or drugs that were not filtered. Secretion allows the kidneys to rapidly eliminate substances that might have been bound to plasma proteins and thus not filtered. This is the third step in urine formation.

The human excretory system (urinary system) consists of a pair of kidneys (filter blood to produce urine), a pair of ureters (muscular tubes that carry urine from kidneys to bladder), a urinary bladder (muscular sac that stores urine), and a urethra (tube that carries urine out of the body). While lungs excrete CO2, skin excretes sweat, and liver produces urea, the kidneys are the primary excretory organs for nitrogenous wastes.

Each kidney contains about 1 to 1.5 million nephrons. A nephron is a microscopic tube consisting of a renal corpuscle (glomerulus + Bowman’s capsule) and a renal tubule (proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct). Each nephron performs the three processes of urine formation: filtration, reabsorption, and secretion. The collecting ducts from many nephrons merge to carry urine to the renal pelvis and then to the ureter.

The urethra is a muscular tube that connects the urinary bladder to the external urethral orifice. In males, it is longer and also carries semen; in females, it is shorter and only carries urine. The expulsion of urine through the urethra is called micturition (urination). The urethra has two sphincters (muscular valves) that control the release of urine; the internal sphincter is involuntary, and the external sphincter is under voluntary control.

Hemodialysis is a life-sustaining treatment for patients with kidney failure (end-stage renal disease). The patient’s blood is pumped from an artery through a dialysis machine, which contains a semipermeable membrane (dialyzer or artificial kidney). On the other side of the membrane flows a dialysis fluid (dialysate) with a composition similar to normal blood without waste. Wastes (urea, creatinine, excess ions) diffuse from blood into the dialysate, and cleaned blood is returned to a vein.

The dialyzer contains thousands of hollow fibers made of semipermeable membrane. Blood flows inside the fibers, and dialysate flows outside. Urea, creatinine, uric acid, and excess ions are small enough to diffuse through the membrane pores from high concentration (blood) to low concentration (dialysate). The dialysate is constantly refreshed to maintain the concentration gradient. Large molecules like proteins and blood cells cannot cross. This mimics glomerular filtration without reabsorption.

When kidneys fail (less than 10-15% function), dangerous levels of urea, creatinine, potassium, and excess fluid accumulate in the blood (uremia). This can lead to coma and death. Hemodialysis temporarily performs the excretory and regulatory functions of the kidneys: removing waste, balancing electrolytes (sodium, potassium, calcium), and removing excess water. However, it is not a cure; patients usually need dialysis several times per week until a kidney transplant is possible.

Plants do not have specialized excretory organs like kidneys. Their metabolic wastes include: oxygen (from photosynthesis, released through stomata), carbon dioxide (from respiration, released through stomata), water vapor (transpiration), and excess salts. Additionally, plants produce secondary metabolites like tannins (in bark), resins (in conifers), gums, latex (rubber), and alkaloids (caffeine, nicotine) which are stored in vacuoles, old leaves, bark, or shed with fallen leaves, thus removing them from active metabolism.

Plants respire continuously, day and night, breaking down glucose to produce energy, carbon dioxide, and water. The carbon dioxide produced diffuses out of the plant cells, into intercellular spaces, and exits mainly through the stomata (open pores on leaves) and also through lenticels (small pores on stems and bark). During the day, some of this CO2 may be reused for photosynthesis, but at night, it is released as a waste product.

Many plant wastes are toxic or not easily excreted. Instead, plants sequester (store) them in non-essential parts. For example, tannins are stored in bark and old wood; resins are stored in specialized ducts; calcium oxalate crystals are stored in vacuoles or as druses. When the plant sheds old leaves, flowers, fruits, or bark, these wastes are removed from the plant’s body. This is sometimes called “excretion by shedding.” Rubber (latex) is also a waste product collected from some plants.

Each kidney has a ureter, a narrow muscular tube about 25-30 cm long. Urine formed in the kidney collects in the renal pelvis and then enters the ureter. The smooth muscle in the ureter wall undergoes rhythmic peristaltic contractions (waves of squeezing) that push urine downward toward the bladder, regardless of gravity. The ureter enters the bladder obliquely, creating a one-way valve that prevents urine from flowing back into the kidney (reflux).

The kidneys are homeostatic organs. For example, on a hot day with little water intake, the pituitary gland releases antidiuretic hormone (ADH), which makes the collecting ducts more permeable to water, so more water is reabsorbed, producing concentrated urine (dark yellow, small volume). If you drink a lot of water, ADH decreases, less water is reabsorbed, producing dilute urine (pale, large volume). Similarly, aldosterone hormone regulates sodium and potassium reabsorption/secretion.

The urinary bladder is a hollow, muscular, elastic sac located in the pelvic cavity. Its wall contains smooth muscle called the detrusor muscle and specialized transitional epithelium that can stretch. As urine enters from the ureters, the bladder expands to accommodate increasing volume (up to about 400-600 mL in adults). Stretch receptors signal the brain when it is about half full, creating the urge to urinate. Urination (micturition) is a reflex that can be voluntarily controlled until an appropriate time.

Plants absorb minerals from soil, but excess salts (like sodium chloride, calcium oxalate) can be toxic. Many plants compartmentalize excess salts into the vacuoles of cells, where they are harmless. Some plants deposit calcium oxalate crystals (raphides) in leaves or bark. Halophytes (salt-tolerant plants like mangroves) have specialized salt glands on their leaves that actively secrete salt crystals onto the leaf surface, which are then washed away by rain or wind. Shedding old leaves also removes accumulated salts.

Plants have a low metabolic rate, produce fewer toxic wastes, and can store many wastes in non-living tissues (bark, dead leaves) or vacuoles without harming themselves. They do not have a circulatory system dedicated to waste transport to specific excretory organs. Humans have high metabolic rates, produce toxic nitrogenous wastes (urea), and have evolved complex excretory systems (kidneys, ureters, bladder, urethra, plus lungs for CO2 and skin for sweat) to rapidly and efficiently remove wastes to maintain homeostasis.