This lesson sits in the organismal-biology domain, which is roughly one-third of the CLEP exam. Expect terse, direct questions that test whether you can match a structure or hormone to its function and reason about feedback and development — not memorize every pathway.
A neuron receives signals on its dendrites, integrates them at the cell body (soma), and sends an impulse down its axon to the axon terminals. Many axons are wrapped in myelin, which speeds conduction.
At rest, the inside of the neuron is negative relative to the outside (the resting potential), maintained by the sodium–potassium pump. An action potential is a brief reversal of this charge:
| Phase | Ion movement | Effect |
|---|---|---|
| Depolarization | Na⁺ rushes into the cell | Inside becomes positive |
| Repolarization | K⁺ moves out of the cell | Inside returns to negative |
The signal is all-or-none and travels one way, from dendrite toward axon terminal.
Where one neuron meets the next is the synapse. When the action potential reaches the axon terminal, Ca²⁺ flows in, causing vesicles to release neurotransmitter into the synaptic cleft. The neurotransmitter binds receptors on the next cell, passing the signal chemically across the gap.
Endocrine glands release hormones into the blood to regulate distant target organs. A few high-value examples:
| Hormone | Gland | Main action |
|---|---|---|
| Insulin | Pancreas | Lowers blood glucose |
| Glucagon | Pancreas | Raises blood glucose |
| Thyroxine | Thyroid | Raises metabolic rate |
| Epinephrine | Adrenal medulla | "Fight-or-flight" response |
Most hormone systems run on negative feedback: a rising hormone level suppresses the signals that produced it, holding the body near a set point. For example, high thyroid hormone tells the pituitary to release less TSH.
Skeletal muscle contracts to move bones at joints. Connective tissues link the system: tendons connect muscle to bone; ligaments connect bone to bone. Muscles work in antagonistic pairs because they can only pull, not push.
After fertilization the zygote develops in stages:
1. D — Na⁺ moves into the cell. Fix: Depolarization is driven by voltage-gated Na⁺ channels opening, letting sodium rush inward and flip the inside positive. - A: K⁺ leaving is repolarization, the phase that follows depolarization. - B: Cl⁻ influx hyperpolarizes; it is not the depolarizing event. - C: Ca²⁺ matters at the axon terminal for release, not for the depolarizing spike itself. - E: Na⁺ moving out is the pump's slow restoration, not the fast depolarization. - Fix rule: Depolarize = Na⁺ IN; repolarize = K⁺ OUT.
2. B — Ca²⁺ influx causes vesicles to release neurotransmitter. Fix: Depolarization of the terminal opens Ca²⁺ channels, and the calcium triggers vesicle fusion and neurotransmitter release into the cleft. - A: Reuptake ends signaling; it is not what the incoming impulse triggers. - C: K⁺ channels in the dendrite are downstream, on the receiving cell. - D: Myelin synthesis is a long-term support process, unrelated to a single impulse. - E: The presynaptic membrane depolarizes; it does not hyperpolarize to release. - Fix rule: Impulse arrives → Ca²⁺ in → neurotransmitter out.
3. C — The spinal cord. Fix: The CNS is exactly the brain plus the spinal cord. - A: Skin receptors are peripheral sensory structures (PNS). - B: The sciatic nerve is a peripheral nerve (PNS). - D: A neuromuscular junction is a peripheral synapse onto muscle. - E: Ganglia outside the spinal column are PNS structures. - Fix rule: CNS = brain + spinal cord; everything else is PNS.
4. E — Insulin, from the pancreas. Fix: Insulin, released by pancreatic beta cells, drives glucose into cells and lowers blood sugar after a meal. - A: Glucagon raises blood glucose — the opposite direction. - B: Cortisol tends to raise blood glucose over time. - C: Thyroxine controls metabolic rate, not the immediate glucose set point. - D: Epinephrine raises glucose as part of the stress response. - Fix rule: High sugar → insulin (down); low sugar → glucagon (up); both from the pancreas.
5. B — Decrease secretion of TSH from the pituitary. Fix: Negative feedback means a high hormone level suppresses its own trigger, so more thyroid hormone lowers pituitary TSH output. - A: Increasing TSH would be positive feedback, not the normal control. - C: A "no effect" answer ignores the feedback loop entirely. - D: More stimulating hormone would amplify, not dampen, the signal. - E: Feedback exists precisely to prevent runaway metabolic increase. - Fix rule: Negative feedback = the product turns off its own trigger.
6. B — A rapid, nonspecific response such as phagocytosis and inflammation. Fix: Innate immunity acts fast and treats all invaders alike, using barriers, phagocytes, and inflammation. - A: Antigen-specific antibodies are adaptive, not innate. - C: Memory cells are a hallmark of adaptive immunity. - D: Clonal selection of B cells is an adaptive process. - E: Vaccination-based long-term immunity is adaptive. - Fix rule: Innate = fast + nonspecific + no memory; adaptive = specific + memory.
7. C — Memory lymphocytes generated during the first exposure. Fix: The first infection produces memory cells; on re-exposure they mount the quicker, larger secondary response. - A: Innate phagocytosis is fast but does not strengthen with repeat exposure. - B: A fever alone does not explain antigen-specific speed and strength. - D: Passive maternal immunity is borrowed antibody, not a self-made memory response. - E: A viral mutation would tend to weaken, not enhance, the person's recognition. - Fix rule: Faster, stronger second response = adaptive memory cells at work.
8. E — Skeletal muscle contracts to move bones at joints. Fix: Skeletal muscle attaches to and pulls on bones, producing voluntary movement at joints. - A: Ligaments connect bone to bone, not muscle to bone. - B: Tendons connect muscle to bone, not bone to bone. - C: Smooth muscle is involuntary and lines organs; it does not move the skeleton. - D: Cartilage cushions joints; it does not contract to generate force. - Fix rule: Tendon = muscle-to-bone; ligament = bone-to-bone; skeletal muscle moves the skeleton.
9. A — Binary fission in bacteria. Fix: Binary fission is asexual, copying one parent's genome to produce genetically identical offspring. - B: Fertilization mixes two parents' genes, producing variation. - C: Gamete fusion after meiosis is sexual and shuffles alleles. - D: Cross-pollination is sexual reproduction between plants. - E: Independent assortment is a source of variation, the opposite of cloning. - Fix rule: Asexual (one parent) → clones; sexual (two parents) → variation.
10. D — A rapid series of cell divisions with little net growth of the embryo. Fix: Cleavage divides the zygote into many small cells without the embryo getting bigger overall. - A: Forming germ layers is gastrulation, a later step. - B: Cell migration to form the gastrula is gastrulation. - C: Specialization into tissues is differentiation. - E: The neural tube forms after gastrulation, during organ development. - Fix rule: Cleavage = divide fast, grow little; the sequence is cleavage → gastrulation → differentiation.
11. A — Ectoderm. Fix: The ectoderm gives rise to the skin's outer layer and the entire nervous system. - B: Mesoderm forms muscle, bone, and blood, not the nervous system. - C: Endoderm forms the gut lining and associated organs. - D: The blastocoel is a fluid cavity, not a germ layer. - E: The yolk is a nutrient store, not a germ layer. - Fix rule: Ecto = outer + nervous system; meso = middle (muscle/bone); endo = inner (gut).
12. A — Differential gene expression in the two cells. Fix: Cells with the same DNA specialize because they switch different genes on and off, producing different proteins. - B: Cells keep their full genome; genes are regulated, not discarded. - C: A single random mutation cannot reliably produce two whole cell types. - D: Mitosis distributes the genome equally, so unequal division is false. - E: Differentiated body cells retain the same chromosome number. - Fix rule: Same DNA, different cells = differential gene expression, not gene loss.
1. D — Na⁺ moves into the cell. Fix: Depolarization is driven by voltage-gated Na⁺ channels opening, letting sodium rush inward and flip the inside positive. - A: K⁺ leaving is repolarization, the phase that follows depolarization. - B: Cl⁻ influx hyperpolarizes; it is not the depolarizing event. - C: Ca²⁺ matters at the axon terminal for release, not for the depolarizing spike itself. - E: Na⁺ moving out is the pump's slow restoration, not the fast depolarization. - Fix rule: Depolarize = Na⁺ IN; repolarize = K⁺ OUT.
2. B — Ca²⁺ influx causes vesicles to release neurotransmitter. Fix: Depolarization of the terminal opens Ca²⁺ channels, and the calcium triggers vesicle fusion and neurotransmitter release into the cleft. - A: Reuptake ends signaling; it is not what the incoming impulse triggers. - C: K⁺ channels in the dendrite are downstream, on the receiving cell. - D: Myelin synthesis is a long-term support process, unrelated to a single impulse. - E: The presynaptic membrane depolarizes; it does not hyperpolarize to release. - Fix rule: Impulse arrives → Ca²⁺ in → neurotransmitter out.
3. C — The spinal cord. Fix: The CNS is exactly the brain plus the spinal cord. - A: Skin receptors are peripheral sensory structures (PNS). - B: The sciatic nerve is a peripheral nerve (PNS). - D: A neuromuscular junction is a peripheral synapse onto muscle. - E: Ganglia outside the spinal column are PNS structures. - Fix rule: CNS = brain + spinal cord; everything else is PNS.
4. E — Insulin, from the pancreas. Fix: Insulin, released by pancreatic beta cells, drives glucose into cells and lowers blood sugar after a meal. - A: Glucagon raises blood glucose — the opposite direction. - B: Cortisol tends to raise blood glucose over time. - C: Thyroxine controls metabolic rate, not the immediate glucose set point. - D: Epinephrine raises glucose as part of the stress response. - Fix rule: High sugar → insulin (down); low sugar → glucagon (up); both from the pancreas.
5. B — Decrease secretion of TSH from the pituitary. Fix: Negative feedback means a high hormone level suppresses its own trigger, so more thyroid hormone lowers pituitary TSH output. - A: Increasing TSH would be positive feedback, not the normal control. - C: A "no effect" answer ignores the feedback loop entirely. - D: More stimulating hormone would amplify, not dampen, the signal. - E: Feedback exists precisely to prevent runaway metabolic increase. - Fix rule: Negative feedback = the product turns off its own trigger.
6. B — A rapid, nonspecific response such as phagocytosis and inflammation. Fix: Innate immunity acts fast and treats all invaders alike, using barriers, phagocytes, and inflammation. - A: Antigen-specific antibodies are adaptive, not innate. - C: Memory cells are a hallmark of adaptive immunity. - D: Clonal selection of B cells is an adaptive process. - E: Vaccination-based long-term immunity is adaptive. - Fix rule: Innate = fast + nonspecific + no memory; adaptive = specific + memory.
7. C — Memory lymphocytes generated during the first exposure. Fix: The first infection produces memory cells; on re-exposure they mount the quicker, larger secondary response. - A: Innate phagocytosis is fast but does not strengthen with repeat exposure. - B: A fever alone does not explain antigen-specific speed and strength. - D: Passive maternal immunity is borrowed antibody, not a self-made memory response. - E: A viral mutation would tend to weaken, not enhance, the person's recognition. - Fix rule: Faster, stronger second response = adaptive memory cells at work.
8. E — Skeletal muscle contracts to move bones at joints. Fix: Skeletal muscle attaches to and pulls on bones, producing voluntary movement at joints. - A: Ligaments connect bone to bone, not muscle to bone. - B: Tendons connect muscle to bone, not bone to bone. - C: Smooth muscle is involuntary and lines organs; it does not move the skeleton. - D: Cartilage cushions joints; it does not contract to generate force. - Fix rule: Tendon = muscle-to-bone; ligament = bone-to-bone; skeletal muscle moves the skeleton.
9. A — Binary fission in bacteria. Fix: Binary fission is asexual, copying one parent's genome to produce genetically identical offspring. - B: Fertilization mixes two parents' genes, producing variation. - C: Gamete fusion after meiosis is sexual and shuffles alleles. - D: Cross-pollination is sexual reproduction between plants. - E: Independent assortment is a source of variation, the opposite of cloning. - Fix rule: Asexual (one parent) → clones; sexual (two parents) → variation.
10. D — A rapid series of cell divisions with little net growth of the embryo. Fix: Cleavage divides the zygote into many small cells without the embryo getting bigger overall. - A: Forming germ layers is gastrulation, a later step. - B: Cell migration to form the gastrula is gastrulation. - C: Specialization into tissues is differentiation. - E: The neural tube forms after gastrulation, during organ development. - Fix rule: Cleavage = divide fast, grow little; the sequence is cleavage → gastrulation → differentiation.
11. A — Ectoderm. Fix: The ectoderm gives rise to the skin's outer layer and the entire nervous system. - B: Mesoderm forms muscle, bone, and blood, not the nervous system. - C: Endoderm forms the gut lining and associated organs. - D: The blastocoel is a fluid cavity, not a germ layer. - E: The yolk is a nutrient store, not a germ layer. - Fix rule: Ecto = outer + nervous system; meso = middle (muscle/bone); endo = inner (gut).
12. A — Differential gene expression in the two cells. Fix: Cells with the same DNA specialize because they switch different genes on and off, producing different proteins. - B: Cells keep their full genome; genes are regulated, not discarded. - C: A single random mutation cannot reliably produce two whole cell types. - D: Mitosis distributes the genome equally, so unequal division is false. - E: Differentiated body cells retain the same chromosome number. - Fix rule: Same DNA, different cells = differential gene expression, not gene loss.