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O Levelbiology · Topic 13

Biology Paper 2 Topic 13: Excretion

Master kidney structure, nephron ultrafiltration, selective reabsorption, and dialysis with Cambridge topical questions.

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About Excretion

Excretion focuses on the vital physiological removal of metabolic waste products, toxic substances, and substances in excess of requirements from the human body. In Cambridge O Level Biology Paper 2 (5090), this chapter examines deamination in the liver, urinary system anatomy, nephron ultrafiltration, selective reabsorption of glucose, hormonal osmoregulation via ADH, and medical interventions including kidney dialysis and renal transplantation.

Why Is Excretion Important?

Excretion is essential for maintaining internal homeostasis and preventing the accumulation of toxic cellular by-products such as urea and carbon dioxide. In Cambridge O Level examinations, excretion questions evaluate students on complex physiological mechanisms, testing their ability to link cellular biochemistry (like amino acid deamination) with microscopic renal filtration.

Skills Tested In This Topic

Students are assessed on their ability to define excretion, distinguish between excretion and egestion, trace the urinary tract (kidneys, ureters, bladder, urethra), label nephron structures (glomerulus, Bowman's capsule, tubules, collecting duct), explain ultrafiltration and selective reabsorption, analyze table data comparing plasma, filtrate, and urine compositions, and evaluate dialysis machine mechanics and kidney transplant implications.

How This Topical Paper Helps

This topical past paper compiles authentic Cambridge Paper 2 questions dedicated to Excretion. Practising these curated questions helps students master nephron flowcharts, interpret relative solute concentration tables, and refine their explanations of active transport in proximal tubules and counter-current dialysis mechanics.

Exam Preparation Tips

Never confuse excretion (removal of metabolic wastes produced by cells) with egestion (elimination of undigested dietary fibre as faeces). Memorise the specific fate of each substance during urine formation: 100% of glucose is selectively reabsorbed in the proximal convoluted tubule via active transport, while urea remains concentrated in urine.

Why Practice Past Paper Questions?

Paper 2 frequently features questions where students must explain how dialysis fluid composition prevents glucose loss while removing urea, or interpret bar charts showing kidney function under varying hydration states. Working through past paper prompts ensures candidates apply exact biological keywords for maximum marks.

Quick Answer

Excretion in Cambridge O Level Biology (5090) is the removal of toxic metabolic waste products, particularly urea, carbon dioxide, and excess salts. Students should revise by diagramming the urinary tract and nephron, contrasting ultrafiltration with selective reabsorption, comparing fluid solute concentrations, and practising structured Paper 2 questions from topical past paper PDFs.

How To Revise Using This Paper

  • Review definitions of excretion, deamination, ultrafiltration, and selective reabsorption.
  • Practice drawing and labelling the urinary tract and the nephron with capillary networks.
  • Memorise the concentration changes of glucose, proteins, urea, and salts from plasma to urine.
  • Understand the operational principles of kidney dialysis machines, including dialysate composition.
  • Solve structured past paper questions in this PDF without looking at answer keys.
  • Verify answers against official Cambridge mark schemes to eliminate vague phrasing.

Summary

Excretion is a fundamental unit in O Level Biology Paper 2 covering metabolic waste removal, renal anatomy, and homeostatic osmoregulation. Success in this topic requires a clear understanding of deamination, glomerulus ultrafiltration, tubular selective reabsorption, fluid composition differences, and renal dialysis. Practising authentic Cambridge topical past papers ensures students build confidence, master technical terminology, and achieve top grades in Cambridge 5090 exams.

Frequently Asked Questions

Excretion covers the removal of toxic materials, metabolic waste products (especially urea and carbon dioxide), and substances in excess of requirements from the human body, including the anatomy of the urinary system, nephron function, and kidney dialysis in Cambridge O Level Biology 5090.

Excretion is a core human physiology unit in Paper 2. Cambridge examinations frequently assess deamination in the liver, ultrafiltration under pressure in the glomerulus, selective reabsorption of glucose, and the mechanism of dialysis machines.

Excretion is the removal of metabolic waste products produced by chemical reactions inside body cells (such as urea and carbon dioxide). Egestion is the passing out of undigested, unabsorbed food material as faeces through the anus, having never taken part in cellular metabolism.

Urea is produced in the liver by deamination of excess amino acids. The nitrogen-containing amino group (-NH2) is removed and converted into toxic ammonia, which is immediately converted into less toxic urea and transported via blood to the kidneys for excretion.

Ultrafiltration occurs in the glomerulus and Bowman's capsule, where high blood pressure forces small molecules (water, glucose, ions, urea) into the tubule. Selective reabsorption occurs mainly in the proximal convoluted tubule, where useful substances (100% of glucose and some salts/water) are actively pumped back into the blood.

Under normal physiological conditions, 100% of the glucose filtered into the Bowman's capsule is selectively reabsorbed back into the blood capillaries in the proximal convoluted tubule via active transport against a concentration gradient.

A dialysis machine passes patient blood through partially permeable dialysis tubing surrounded by fresh dialysate. Dialysate contains ideal glucose and ion concentrations (so no net loss occurs) but zero urea, allowing urea to diffuse down its concentration gradient out of the blood.

Excretion appears regularly in Paper 2 theory exams, often featuring kidney cross-section diagrams, nephron flowcharts, solute concentration data tables comparing plasma and urine, or dialysis evaluation scenarios.

Common student mistakes include confusing the ureter (carrying urine from kidney to bladder) with the urethra (carrying urine from bladder out of the body), stating that faeces are excretory waste, and failing to state that glucose reabsorption requires active transport.

Yes, this topical past paper PDF compiles official Cambridge O Level Biology Paper 2 (5090) questions from 2011 to 2024, providing structured theory practice and topic-focused revision for independent self-study.