Study guide
This chapter is educational content only and does not guarantee any exam outcome. It follows Units 3 and 4 of the AP Biology framework, tracing how cells capture, convert, and use energy through photosynthesis and respiration, how cells communicate through signal transduction, and how the cell cycle is regulated, including what happens when that regulation fails. Expect the exam to test these ideas through respirometer data, dose-response curves, and cell-cycle timing datasets, so practice reading graphs alongside the underlying mechanisms.
Photosynthesis: Capturing and Converting Light Energy
Photosynthesis converts light energy into the chemical energy of glucose across two linked stages. The light-dependent reactions occur in the thylakoid membrane, where chlorophyll and accessory pigments absorb light, exciting electrons that pass through an electron transport chain, splitting water molecules (releasing oxygen as a byproduct) and ultimately generating ATP and NADPH while building a proton gradient used for chemiosmosis. The Calvin cycle, in the stroma, uses that ATP and NADPH to fix carbon dioxide into organic molecules through the enzyme rubisco, running through carbon fixation, reduction, and regeneration of the starting molecule to eventually yield glyceraldehyde-3-phosphate, a precursor to glucose. Environmental factors such as light intensity, carbon dioxide concentration, and temperature each limit the overall rate up to a point, after which some other factor becomes limiting, a pattern visible in a classic dataset: a student named Elena measures oxygen production from aquatic plant sprigs under light intensities of 500, 1000, 1500, 2000, and 2500 lux, recording 3, 6, 8, 8.5, and 8.5 bubbles per minute. Oxygen production rises steeply at first as light is the limiting factor, then plateaus after 1500 lux, indicating some other factor, likely carbon dioxide availability or the plant's enzyme capacity, now limits the rate regardless of added light. C4 and CAM plants have evolved adaptations to reduce photorespiration (a wasteful process where rubisco binds oxygen instead of carbon dioxide under hot, dry conditions): C4 plants spatially separate initial carbon fixation from the Calvin cycle using a four-carbon intermediate, while CAM plants temporally separate these steps, fixing carbon dioxide at night when stomata are open and conserving water.
Cellular Respiration and Chemiosmosis
Cellular respiration extracts usable energy from glucose across glycolysis (in the cytoplasm, yielding a small net ATP and pyruvate), the citric acid cycle (in the mitochondrial matrix, further oxidizing pyruvate-derived acetyl-CoA and releasing carbon dioxide while generating NADH and FADH2), and oxidative phosphorylation (across the inner mitochondrial membrane). In oxidative phosphorylation, NADH and FADH2 donate electrons to the electron transport chain; as electrons pass through, energy released pumps protons into the intermembrane space, creating an electrochemical gradient. Chemiosmosis then allows those protons to flow back through ATP synthase, driving the phosphorylation of ADP into ATP, with oxygen serving as the final electron acceptor, forming water. Without oxygen, cells shift to fermentation, regenerating the NAD+ needed for glycolysis to continue, either producing lactate (in muscle and some bacteria) or ethanol and carbon dioxide (in yeast), though fermentation yields far less ATP than aerobic respiration. Respirometer experiments are a common data-analysis format: an invented lab measures oxygen consumption by germinating pea seeds and dry (non-germinating) pea seeds at 10 and 25 degrees Celsius, with corrected respirometer readings (milliliters of oxygen consumed per 10 minutes) of 0.5, 1.8, 0.1, and 0.3 for germinating-cold, germinating-warm, dry-cold, and dry-warm respectively. This dataset shows two independent variables at once: germinating seeds respire far more than dry seeds because they are metabolically active, and warmer temperatures increase respiration rate in the metabolically active tissue much more than in the dormant dry seeds, since enzyme-driven reaction rates are temperature sensitive only when the relevant metabolic machinery is actively running.
Signal Transduction Pathways
Cells communicate by releasing signaling molecules that bind receptors on or in target cells, triggering a signal transduction pathway that converts an extracellular signal into an intracellular response. Reception occurs when a ligand binds a specific receptor, such as a G-protein-coupled receptor or a receptor tyrosine kinase embedded in the plasma membrane; binding causes a conformational change that activates the receptor. Transduction often involves a phosphorylation cascade, in which a series of protein kinases activate one another by adding phosphate groups, amplifying the original signal so that one receptor-binding event can ultimately affect thousands of downstream molecules; second messengers like cyclic AMP or calcium ions further relay and amplify the signal within the cell. Response is the pathway's endpoint, such as activation of a transcription factor that turns specific genes on or off, or opening of an ion channel. Dose-response curves are a frequent data-analysis format: an invented experiment exposes muscle cells to increasing concentrations of a hormone analog (0, 1, 10, 100, and 1000 nanomolar) and measures the percentage of cells showing an intracellular calcium spike, recording 2%, 15%, 48%, 82%, and 85%. The steep rise between 1 and 100 nanomolar, followed by a plateau near 85%, suggests the receptor becomes saturated at high hormone concentration, and the concentration producing a half-maximal response (roughly 10 nanomolar here) estimates the receptor's sensitivity to the hormone. Such curves let researchers compare potency between different signaling molecules or test how a blocking drug shifts the curve rightward, indicating reduced potency.
The Cell Cycle, Checkpoints, and Mitosis
The eukaryotic cell cycle consists of interphase (G1, S, and G2 phases, where the cell grows, replicates its DNA, and prepares for division) and the mitotic phase (mitosis, dividing the nucleus, followed by cytokinesis, dividing the cytoplasm). Checkpoints regulate progression: the G1 checkpoint assesses cell size, nutrient availability, and DNA integrity before committing to DNA replication; the G2 checkpoint verifies that DNA replication is complete and undamaged before entering mitosis; and the M checkpoint (spindle assembly checkpoint) ensures all chromosomes are properly attached to spindle fibers before anaphase proceeds. These checkpoints are enforced by regulatory proteins, including cyclins and cyclin-dependent kinases (Cdks), whose fluctuating concentrations and activities drive the cell through each transition. Mitosis itself proceeds through prophase (chromosomes condense, spindle forms), metaphase (chromosomes align at the metaphase plate), anaphase (sister chromatids separate and move to opposite poles), and telophase (nuclei re-form). Cell-cycle timing data is often presented as the percentage of cells observed in each phase within a sample, which (assuming a roughly constant total cycle time) approximates the relative duration of each phase: if a sample of 200 onion root tip cells shows 160 in interphase, 20 in prophase, 8 in metaphase, 6 in anaphase, and 6 in telophase, then interphase (80% of cells) represents about 80% of the total cell cycle's duration, since more cells will be observed in whichever phase takes the most time. Cancer arises when mutations disable checkpoint controls or inactivate tumor suppressor genes (such as p53, which normally halts the cycle or triggers programmed cell death, apoptosis, in response to DNA damage) or overactivate proto-oncogenes into oncogenes, allowing uncontrolled division.
Key terms
- Chemiosmosis
- — The process by which a proton gradient across a membrane drives ATP synthesis as protons flow through ATP synthase.
- Rubisco
- — The enzyme in the Calvin cycle that fixes atmospheric carbon dioxide onto an organic molecule, and which can also bind oxygen, causing photorespiration.
- Photorespiration
- — A wasteful process where rubisco binds oxygen instead of carbon dioxide, reducing photosynthetic efficiency, especially in hot, dry conditions.
- Fermentation
- — An anaerobic pathway that regenerates NAD+ from NADH to sustain glycolysis, producing lactate or ethanol and carbon dioxide as byproducts.
- Signal transduction pathway
- — The sequence of molecular events, often a phosphorylation cascade, that converts an extracellular signal received at a receptor into a cellular response.
- Second messenger
- — A small intracellular molecule, such as cyclic AMP or calcium ion, that relays and amplifies a signal after receptor activation.
- Cyclin-dependent kinase (Cdk)
- — An enzyme that, when bound to its regulatory cyclin partner, phosphorylates target proteins to drive the cell cycle through its checkpoints.
- Tumor suppressor gene
- — A gene, such as p53, whose protein product normally restrains cell division or triggers apoptosis in response to DNA damage; its loss of function can contribute to cancer.
- Apoptosis
- — Programmed cell death, a regulated process that eliminates damaged or unneeded cells without releasing harmful contents into surrounding tissue.
- Dose-response curve
- — A graph relating the concentration of a signaling molecule or drug to the magnitude of a cellular or physiological response, often showing a sigmoidal, saturating shape.
Exam tips
- When a photosynthesis or respiration graph plateaus, identify which variable became newly limiting rather than concluding the reaction simply 'stopped.'
- For respirometer data with two independent variables (for example, tissue type and temperature), compare within one variable at a time before drawing an overall conclusion.
- On a dose-response curve, locate the plateau (maximum response, suggesting receptor saturation) and the midpoint concentration (a measure of potency) as the two features most likely to be asked about.
- When estimating phase duration from a cell count sample, remember the logic: phases seen in more cells are inferred to last longer, assuming a constant total cycle time across the sample.
- Distinguish a proto-oncogene (normal gene promoting division) from an oncogene (mutated, overactive version) and from a tumor suppressor (normally restrains division) — cancer typically requires losses or gains affecting several such genes.