16 terms in 3.3
Digestion and absorption
Your body breaks large food molecules into small pieces using digestion. Only these small pieces are tiny enough to pass through cell membranes and enter the bloodstream.
Organisms exchange substances with their environment
Digestion and absorption
Enzymes are biological catalysts that speed up hydrolysis — the breaking of chemical bonds using water. Each food type requires a specific set of enzymes. **Carbohydrates:** 1. Amylase (produced in the salivary glands and pancreas) breaks starch into maltose, a disaccharide. 2. Membrane-bound disaccharidases — enzymes fixed to the surface of intestinal epithelial cells — then split disaccharides
Organisms exchange substances with their environment
Digestion and absorption
The ileum (the final section of the small intestine) absorbs digested nutrients through two distinct mechanisms depending on the molecule type. **Co-transport — for glucose and amino acids:** 1. Sodium-potassium ATPase pumps on the basolateral (blood-side) membrane actively pump sodium ions out of the epithelial cell, keeping the sodium concentration inside the cell low. 2. This low intracellular
Organisms exchange substances with their environment
Digestion and absorption
Food molecules such as starch, proteins and lipids are too large to cross cell membranes, so the body uses hydrolysis — enzyme-driven reactions that break chemical bonds using water — to split them into smaller, absorbable units. Specific enzymes including amylases, lipases, endopeptidases and exopeptidases each target different molecule types, breaking them down in a precise sequence along the digestive tract. Once digestion is complete, the small intestine absorbs the products through specialised mechanisms, including co-transport (where glucose or amino acids hitch a ride alongside ions moving into cells) and micelles (tiny fat-soluble droplets that ferry lipid products to the intestinal lining).
Organisms exchange substances with their environment
Gas exchange
All gas exchange surfaces share the same core features: they are thin, moist, and large in surface area relative to volume. How each organism achieves this depends on its body plan and habitat. **Single-celled organisms** (e.g. Amoeba) are so small that their surface area to volume ratio (SA:V ratio — the proportion of outer surface relative to body size) is naturally high. Gases diffuse directly
Organisms exchange substances with their environment
Gas exchange
Insects and drought-resistant plants both need to let gases in and out for respiration and photosynthesis. Every adaptation that helps gas exchange also risks letting water escape, so each organism uses structures that balance both needs.
Organisms exchange substances with their environment
Gas exchange
The human gas exchange system moves air from your mouth and nose down into your lungs. It branches from one large tube into millions of tiny air sacs where oxygen enters your blood.
Organisms exchange substances with their environment
Gas exchange
Alveoli are tiny air sacs in the lungs. Their walls have special features that allow oxygen and carbon dioxide to diffuse rapidly between air and blood.
Organisms exchange substances with their environment
Gas exchange
Ventilation moves fresh air into the lungs and stale air out. This keeps oxygen levels high and carbon dioxide levels low at the alveolar surface, so diffusion into the blood continues rapidly.
Organisms exchange substances with their environment
Gas exchange
Breathing moves air by changing pressure inside the thoracic cavity (the airtight space in your chest that contains the lungs). Boyle's Law explains the link: when volume increases, pressure falls; when volume decreases, pressure rises. During inspiration (breathing in): 1. The diaphragm — a dome-shaped sheet of muscle below the lungs — contracts and flattens downward. 2. The external intercostal
Organisms exchange substances with their environment
Gas exchange
Every living organism needs to move oxygen in and carbon dioxide out, and the structures that make this possible are shaped by the same core problem: maximising the rate of diffusion across an exchange surface while minimising harmful water loss. This subtopic compares how organisms as different as insects, fish, flowering plants, and humans have each evolved specialised gas exchange systems — from the tracheal system (a network of air-filled tubes) in insects to the alveoli (tiny air sacs) deep in the human lung — revealing the structural trade-offs each solution involves. Understanding these adaptations also underpins how ventilation (the active movement of air or water over an exchange surface) maintains the concentration gradients that drive diffusion, which connects directly to mass transport and whole-body physiology.
Organisms exchange substances with their environment
Surface area to volume ratio
As an organism grows larger, its volume increases faster than its surface area. This means larger organisms have less outer surface available, relative to the amount of living tissue inside.
Organisms exchange substances with their environment
Surface area to volume ratio
As organisms get larger, their surface area to volume ratio falls. Larger organisms compensate by changing body shape or evolving specialised exchange systems, such as lungs or gills.
Organisms exchange substances with their environment
Surface area to volume ratio
Smaller organisms have a higher surface area to volume ratio. This means they lose heat faster and must run their chemical reactions — their metabolic rate — more quickly to compensate.
Organisms exchange substances with their environment
Surface area to volume ratio
As an organism gets larger, its volume — the amount of living tissue that needs supplying — grows much faster than its surface area, meaning there is proportionally less outer surface available to exchange substances like oxygen and carbon dioxide with the environment. This reduction in surface area to volume ratio creates a problem for larger organisms, because simple diffusion across a body surface is no longer fast enough to meet the demands of a high metabolic rate (the speed at which chemical reactions occur in cells). Understanding this constraint explains why larger organisms have evolved specialised exchange surfaces and transport systems — such as lungs and circulatory systems — which are explored in the subtopics that follow.
Organisms exchange substances with their environment