Organismal biology is roughly one-third of the CLEP exam. These "how the body works" systems are high-yield and tested with short, direct stems and simple structure-function reasoning. This is Part I (intake, transport, and waste); a later lesson covers coordination systems.
Complex animals are organized in a hierarchy, smallest to largest:
Cell → Tissue → Organ → Organ system → Organism
A tissue is a group of similar cells doing one job (e.g., muscle tissue). An organ combines tissues for a function (e.g., the stomach). An organ system is a set of organs cooperating on a large task (e.g., the digestive system).
Digestion breaks large food molecules into absorbable units.
| Region | Main job |
|---|---|
| Mouth | Chewing; amylase in saliva starts starch digestion |
| Stomach | Acid + pepsin begin protein digestion |
| Small intestine | Most chemical digestion and nutrient absorption (villi) |
| Large intestine | Water reabsorption; forms feces |
The small intestine is the primary site of nutrient absorption; its inner wall is folded into tiny villi that vastly increase surface area. Accessory organs help: the liver makes bile (emulsifies fats) and the pancreas secretes digestive enzymes.
Gas exchange supplies O₂ and removes CO₂. In mammalian lungs, air travels trachea → bronchi → bronchioles → alveoli, tiny air sacs wrapped in capillaries. All gas exchange occurs across the thin alveolar walls by diffusion:
Gases always move down their partial-pressure gradient — passively, no ATP required. The alveoli's huge combined surface area and paper-thin walls make this fast.
[GRAPH: Cluster of grape-like alveoli sheathed in a capillary net — arrows show O₂ moving from alveolar air into blood and CO₂ moving from blood into the alveolar air.]
The circulatory system transports gases, nutrients, wastes, and heat.
Open vs. closed circulation: - Open (insects, most mollusks): blood (hemolymph) leaves vessels and directly bathes the tissues in open body cavities. - Closed (vertebrates, earthworms): blood stays within vessels the whole time; exchange happens across capillary walls.
Blood vessels: | Vessel | Role | |---|---| | Arteries | Carry blood away from the heart (high pressure, thick walls) | | Capillaries | Thin-walled exchange vessels; gases/nutrients pass to tissues | | Veins | Return blood to the heart (low pressure, valves) |
The human heart has four chambers. Oxygen-poor blood returns from the body to the right atrium → right ventricle → lungs; oxygen-rich blood returns from the lungs to the left atrium → left ventricle → body. (Right side pumps to lungs, left side pumps to body.)
Blood components: - Red blood cells (erythrocytes) — carry O₂ using hemoglobin. - White blood cells (leukocytes) — immune defense. - Platelets — clotting. - Plasma — fluid carrying cells, proteins, nutrients, and wastes.
The excretory system removes nitrogenous waste and balances water and salts (osmoregulation). In vertebrates the kidney filters blood; its functional unit is the nephron, which filters blood at the glomerulus and then reabsorbs useful substances, producing urine.
Osmoregulation adjusts to the environment. A freshwater fish is hypertonic to its surroundings, so water floods in; it copes by excreting large volumes of dilute urine and actively taking up salts through its gills. (A saltwater fish faces the opposite problem and conserves water.)
Homeostasis is the maintenance of a stable internal environment. The main mechanism is negative feedback: a change away from a set point triggers a response that reverses the change.
Example: body temperature rises above the set point → the body sweats → evaporation cools the body back toward the set point. Blood glucose, water balance, and pH are all regulated this way. (In contrast, positive feedback amplifies a change, as in childbirth contractions — the exception, not the rule.)
1. C — Cell, tissue, organ, organ system. Fix: Organization builds upward from cells to tissues to organs to organ systems. - A: Starts with organ and puts cell third — out of order. - B: Places tissue before cell. - D: Swaps organ and tissue. - E: Runs largest to smallest, the reverse of what's asked. - Fix rule: Build up: cell → tissue → organ → system.
2. B — Small intestine. Fix: The small intestine's villi make it the primary site of nutrient absorption. - A: The stomach mainly begins protein digestion, not major absorption. - C: The large intestine reabsorbs water and forms feces, not most nutrients. - D: The esophagus only transports food to the stomach. - E: The liver aids digestion (bile) but doesn't absorb nutrients from the gut lumen. - Fix rule: Villi + huge surface area = small intestine = absorption.
3. B — Starches. Fix: Salivary amylase breaks starch (a carbohydrate) into smaller sugars, starting in the mouth. - A: Proteins are handled by pepsin/proteases, not amylase. - C: Fats are broken down by lipase (aided by bile). - D: Nucleic acids are digested by nucleases. - E: Minerals are absorbed, not enzymatically digested. - Fix rule: Amylase → amylose (starch); "amyl" = starch.
4. A — Alveoli. Fix: The thin-walled alveoli, wrapped in capillaries, are where O₂ and CO₂ diffuse between air and blood. - B: Bronchi are conducting airways, not exchange surfaces. - C: The trachea is a conducting tube (the "windpipe"). - D: The pleura is the membrane lining the lungs, not an exchange surface. - E: The diaphragm is a muscle that drives breathing, not gas exchange. - Fix rule: Gas exchange happens at the alveoli — the end of the airway tree.
5. A — Oxygen diffuses from higher partial pressure in the alveolus to lower partial pressure in the blood. Fix: Gases move passively down their partial-pressure gradient; alveolar air is O₂-rich, incoming blood is O₂-poor. - B: Gas exchange is passive diffusion, not active transport. - C: Deoxygenated blood arriving at the lungs has less O₂ than the air, not more. - D: CO₂ does not "push" O₂; each gas follows its own gradient. - E: The diaphragm moves air by changing lung volume; it doesn't ferry oxygen into capillaries. - Fix rule: Gases diffuse high → low partial pressure, no ATP.
6. E — In an open system blood leaves the vessels and bathes the tissues directly, whereas in a closed system blood stays within vessels. Fix: The defining contrast is whether blood ever leaves the vessels. - A: Closed systems do have hearts (vertebrates, earthworms). - B: Open-system blood fills body cavities, not just gills. - C: Reversed — it's the open system that bathes tissues in hemolymph. - D: Open systems don't rely exclusively on hemoglobin; many use other pigments or none. - Fix rule: Open = blood exits vessels into cavities; closed = blood stays in vessels.
7. E — Right atrium. Fix: Oxygen-poor blood returning from the body enters the right atrium, then the right ventricle pumps it to the lungs. - A: The left atrium receives oxygen-rich blood from the lungs. - B: The left ventricle pumps oxygen-rich blood to the body. - C: The aorta carries oxygen-rich blood away from the left ventricle. - D: The pulmonary vein delivers oxygen-rich blood from the lungs to the left atrium. - Fix rule: Body → right side → lungs; lungs → left side → body.
8. D — Red blood cells. Fix: Red blood cells carry oxygen bound to hemoglobin. - A: White blood cells handle immune defense, not O₂ transport. - B: Platelets are for clotting. - C: Plasma proteins do many jobs but are not the main O₂ carrier. - E: Lymphocytes are a type of white blood cell (immunity). - Fix rule: Red cells + hemoglobin = oxygen delivery.
9. B — They are thin-walled vessels where gases and nutrients are exchanged with tissues. Fix: Capillaries are one-cell-thick, perfect for diffusion between blood and tissue. - A: Carrying blood away under high pressure describes arteries. - C: Returning blood via valves describes veins. - D: Veins, not capillaries, hold most of the blood volume. - E: Thick muscular walls that regulate pressure describe arteries/arterioles. - Fix rule: Capillary = thin exchange vessel between arteries and veins.
10. D — Nephron. Fix: The nephron is the kidney's functional unit, filtering blood and forming urine. - A: The alveolus is the lung's gas-exchange unit. - B: The villus is an absorptive structure of the small intestine. - C: The neuron is the nervous system's cell. - E: The glomerulus is only the filtering capillary tuft — part of the nephron, not the whole functional unit. - Fix rule: Kidney's working unit = nephron (glomerulus is just its filter).
11. C — Excrete large amounts of dilute urine and actively take up salts. Fix: Because water floods into the hypertonic fish, it dumps excess water as dilute urine and pumps salts back in. - A: Drinking lots of water and holding urine would worsen the overload. - B: Pumping in more water is the opposite of what's needed. - D: Stopping urine would let excess water build up dangerously. - E: Concentrated urine to conserve water fits a saltwater fish, not a freshwater one. - Fix rule: Freshwater fish = water in → dilute urine out + take up salts.
12. C — A rise in body temperature triggering sweating, which lowers temperature back toward the set point. Fix: Negative feedback reverses a deviation, restoring the set point — exactly what sweating does when you overheat. - A: Clotting that accelerates until sealed is positive feedback (amplifying). - B: Childbirth contractions intensifying is the classic positive-feedback example. - D: A signal speeding up on repetition is amplification, not correction. - E: Exponential population growth amplifies, not stabilizes. - Fix rule: Negative feedback opposes/reverses the change; positive feedback amplifies it.
1. C — Cell, tissue, organ, organ system. Fix: Organization builds upward from cells to tissues to organs to organ systems. - A: Starts with organ and puts cell third — out of order. - B: Places tissue before cell. - D: Swaps organ and tissue. - E: Runs largest to smallest, the reverse of what's asked. - Fix rule: Build up: cell → tissue → organ → system.
2. B — Small intestine. Fix: The small intestine's villi make it the primary site of nutrient absorption. - A: The stomach mainly begins protein digestion, not major absorption. - C: The large intestine reabsorbs water and forms feces, not most nutrients. - D: The esophagus only transports food to the stomach. - E: The liver aids digestion (bile) but doesn't absorb nutrients from the gut lumen. - Fix rule: Villi + huge surface area = small intestine = absorption.
3. B — Starches. Fix: Salivary amylase breaks starch (a carbohydrate) into smaller sugars, starting in the mouth. - A: Proteins are handled by pepsin/proteases, not amylase. - C: Fats are broken down by lipase (aided by bile). - D: Nucleic acids are digested by nucleases. - E: Minerals are absorbed, not enzymatically digested. - Fix rule: Amylase → amylose (starch); "amyl" = starch.
4. A — Alveoli. Fix: The thin-walled alveoli, wrapped in capillaries, are where O₂ and CO₂ diffuse between air and blood. - B: Bronchi are conducting airways, not exchange surfaces. - C: The trachea is a conducting tube (the "windpipe"). - D: The pleura is the membrane lining the lungs, not an exchange surface. - E: The diaphragm is a muscle that drives breathing, not gas exchange. - Fix rule: Gas exchange happens at the alveoli — the end of the airway tree.
5. A — Oxygen diffuses from higher partial pressure in the alveolus to lower partial pressure in the blood. Fix: Gases move passively down their partial-pressure gradient; alveolar air is O₂-rich, incoming blood is O₂-poor. - B: Gas exchange is passive diffusion, not active transport. - C: Deoxygenated blood arriving at the lungs has less O₂ than the air, not more. - D: CO₂ does not "push" O₂; each gas follows its own gradient. - E: The diaphragm moves air by changing lung volume; it doesn't ferry oxygen into capillaries. - Fix rule: Gases diffuse high → low partial pressure, no ATP.
6. E — In an open system blood leaves the vessels and bathes the tissues directly, whereas in a closed system blood stays within vessels. Fix: The defining contrast is whether blood ever leaves the vessels. - A: Closed systems do have hearts (vertebrates, earthworms). - B: Open-system blood fills body cavities, not just gills. - C: Reversed — it's the open system that bathes tissues in hemolymph. - D: Open systems don't rely exclusively on hemoglobin; many use other pigments or none. - Fix rule: Open = blood exits vessels into cavities; closed = blood stays in vessels.
7. E — Right atrium. Fix: Oxygen-poor blood returning from the body enters the right atrium, then the right ventricle pumps it to the lungs. - A: The left atrium receives oxygen-rich blood from the lungs. - B: The left ventricle pumps oxygen-rich blood to the body. - C: The aorta carries oxygen-rich blood away from the left ventricle. - D: The pulmonary vein delivers oxygen-rich blood from the lungs to the left atrium. - Fix rule: Body → right side → lungs; lungs → left side → body.
8. D — Red blood cells. Fix: Red blood cells carry oxygen bound to hemoglobin. - A: White blood cells handle immune defense, not O₂ transport. - B: Platelets are for clotting. - C: Plasma proteins do many jobs but are not the main O₂ carrier. - E: Lymphocytes are a type of white blood cell (immunity). - Fix rule: Red cells + hemoglobin = oxygen delivery.
9. B — They are thin-walled vessels where gases and nutrients are exchanged with tissues. Fix: Capillaries are one-cell-thick, perfect for diffusion between blood and tissue. - A: Carrying blood away under high pressure describes arteries. - C: Returning blood via valves describes veins. - D: Veins, not capillaries, hold most of the blood volume. - E: Thick muscular walls that regulate pressure describe arteries/arterioles. - Fix rule: Capillary = thin exchange vessel between arteries and veins.
10. D — Nephron. Fix: The nephron is the kidney's functional unit, filtering blood and forming urine. - A: The alveolus is the lung's gas-exchange unit. - B: The villus is an absorptive structure of the small intestine. - C: The neuron is the nervous system's cell. - E: The glomerulus is only the filtering capillary tuft — part of the nephron, not the whole functional unit. - Fix rule: Kidney's working unit = nephron (glomerulus is just its filter).
11. C — Excrete large amounts of dilute urine and actively take up salts. Fix: Because water floods into the hypertonic fish, it dumps excess water as dilute urine and pumps salts back in. - A: Drinking lots of water and holding urine would worsen the overload. - B: Pumping in more water is the opposite of what's needed. - D: Stopping urine would let excess water build up dangerously. - E: Concentrated urine to conserve water fits a saltwater fish, not a freshwater one. - Fix rule: Freshwater fish = water in → dilute urine out + take up salts.
12. C — A rise in body temperature triggering sweating, which lowers temperature back toward the set point. Fix: Negative feedback reverses a deviation, restoring the set point — exactly what sweating does when you overheat. - A: Clotting that accelerates until sealed is positive feedback (amplifying). - B: Childbirth contractions intensifying is the classic positive-feedback example. - D: A signal speeding up on repetition is amplification, not correction. - E: Exponential population growth amplifies, not stabilizes. - Fix rule: Negative feedback opposes/reverses the change; positive feedback amplifies it.