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8
Units
60
MCQs on exam
6
FRQs on exam
3 hours
Exam length
Most universities grant credit for AP Biology scores of 4 or 5.
Unit 1 explores the molecular foundation of all living things, starting with water. Water is a polar molecule, meaning one end carries a slight negative charge and the other a slight positive charge. This polarity allows water molecules to form hydrogen bonds with each other and with other polar substances, giving water its remarkable properties: cohesion, adhesion, high specific heat, and the ability to act as a universal solvent. These properties make water essential for transporting nutrients, regulating temperature, and supporting nearly every chemical reaction in cells.
Living organisms are built from four classes of large molecules called macromolecules: carbohydrates, lipids, proteins, and nucleic acids. Each is assembled from smaller subunits through dehydration synthesis reactions, which release water, and broken apart through hydrolysis reactions, which use water. Carbohydrates provide quick energy and structural support, lipids store long-term energy and form membranes, proteins carry out an enormous range of structural and functional roles, and nucleic acids store and transmit genetic information. Understanding the relationship between the structure of each macromolecule and its function is a central theme of this unit.
Enzymes are biological catalysts, almost always proteins, that speed up chemical reactions by lowering activation energy. Each enzyme has a specific active site that binds a particular substrate, making enzymes highly specific for the reactions they catalyze. Enzyme activity can be influenced by temperature, pH, and the presence of inhibitors or cofactors. Competitive inhibitors block the active site directly, while noncompetitive inhibitors bind elsewhere on the enzyme and change its shape, reducing its effectiveness. Understanding how enzymes are regulated helps explain how cells control their metabolism in response to changing conditions.
Cohesion
The attraction of water molecules to other water molecules via hydrogen bonds. This property allows water to form droplets and move through plant vessels as a continuous column.
Adhesion
The attraction of water molecules to other polar or charged surfaces. Adhesion allows water to climb the walls of narrow tubes, such as xylem vessels in plants.
Specific Heat
The amount of energy required to raise the temperature of one gram of a substance by one degree Celsius. Water has a high specific heat, allowing it to resist rapid temperature changes and stabilize environments.
Dehydration Synthesis
A chemical reaction in which two monomers are joined together by removing a water molecule. This process builds all four types of macromolecules from their smaller subunits.
Hydrolysis
A chemical reaction in which a water molecule is used to break a covalent bond between two monomers. Hydrolysis is how organisms digest macromolecules during digestion and cellular recycling.
Enzyme
A biological catalyst, typically a protein, that lowers the activation energy of a chemical reaction without being consumed in the process. Enzymes are highly specific to their substrates due to the shape of their active site.
Active Site
The specific region of an enzyme where the substrate binds to form an enzyme-substrate complex. The shape and chemical properties of the active site determine which substrates the enzyme will accept.
Denaturation
The process by which a protein loses its three-dimensional shape due to extreme changes in temperature, pH, or other environmental conditions. Denaturation disrupts the interactions that maintain protein structure, rendering the enzyme nonfunctional.
Competitive Inhibition
A type of enzyme regulation in which a molecule resembling the substrate blocks the active site, preventing substrate binding. The effect of a competitive inhibitor can be overcome by increasing the concentration of the substrate.
Noncompetitive Inhibition
A type of enzyme regulation in which an inhibitor binds to an allosteric site rather than the active site, changing the enzyme's shape and reducing its activity. Increasing substrate concentration cannot overcome noncompetitive inhibition.
1. A researcher places an enzyme in a solution and gradually increases the temperature from 20°C to 80°C while measuring the rate of reaction. Which of the following best explains why the reaction rate first increases and then sharply decreases?
2. A student adds a molecule that is structurally similar to an enzyme's natural substrate. Even when the concentration of the natural substrate is greatly increased, the reaction rate does not return to its uninhibited level. What type of inhibition is most likely occurring?
3. Which of the following best explains why the phospholipid bilayer is an effective barrier for the cell membrane?
All living cells fall into one of two broad categories: prokaryotic or eukaryotic. Prokaryotic cells, like bacteria, lack a membrane-bound nucleus and other internal organelles, keeping their DNA floating freely in the cytoplasm. Eukaryotic cells, found in plants, animals, fungi, and protists, are far more complex, housing their genetic material inside a nucleus and containing specialized organelles that divide labor within the cell.
Each organelle in a eukaryotic cell performs a specific job that contributes to the cell's overall survival. The mitochondria generate ATP through cellular respiration, while chloroplasts (in plant cells) capture light energy to produce sugars through photosynthesis. The endomembrane system, which includes the endoplasmic reticulum, Golgi apparatus, and lysosomes, works as an assembly line to build, modify, package, and deliver proteins and lipids to their correct destinations inside or outside the cell.
The cell membrane is the gatekeeper of everything that enters and exits the cell. Described by the fluid mosaic model, it is a flexible phospholipid bilayer embedded with proteins that regulate transport, cell communication, and recognition. Cells move substances across this membrane through passive transport, which requires no energy, or active transport, which uses ATP to move substances against their concentration gradient, ensuring the cell maintains the precise internal environment it needs to function.
Fluid Mosaic Model
The scientific model describing the cell membrane as a dynamic, flexible phospholipid bilayer with proteins, cholesterol, and carbohydrates embedded throughout. The term mosaic refers to the diverse mix of molecules, while fluid reflects the membrane's ability to shift and move.
Selective Permeability
The property of the cell membrane that allows certain substances to pass through while restricting others. Small, nonpolar molecules cross freely, while ions and large polar molecules require protein channels or carriers.
Osmosis
The passive diffusion of water molecules across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration. No energy is required because water moves down its own concentration gradient.
Tonicity
The relative solute concentration of a solution compared to the inside of a cell, which determines the direction of water movement via osmosis. Solutions are described as isotonic, hypotonic, or hypertonic relative to the cell.
Facilitated Diffusion
A form of passive transport in which specific membrane proteins, either channel proteins or carrier proteins, assist molecules that cannot cross the phospholipid bilayer on their own. Movement still occurs down the concentration gradient and requires no ATP.
Active Transport
The movement of molecules across a membrane against their concentration gradient, from low to high concentration, requiring the cell to expend ATP. The sodium-potassium pump is a classic example of a primary active transport protein.
Endomembrane System
A network of membrane-bound organelles including the endoplasmic reticulum, Golgi apparatus, lysosomes, and vesicles that work together to synthesize, modify, and transport proteins and lipids within the eukaryotic cell.
Endocytosis
A process by which a cell engulfs external material by wrapping the plasma membrane around it to form an internal vesicle. Phagocytosis and pinocytosis are two major types, differing in whether the cell takes in large particles or fluids.
Exocytosis
The process by which vesicles inside the cell fuse with the plasma membrane and release their contents to the outside of the cell. This is the primary mechanism by which cells secrete proteins, hormones, and other large molecules.
Concentration Gradient
The difference in the concentration of a substance across a space, such as on either side of a cell membrane. Substances naturally tend to move from areas of higher concentration to lower concentration, a process that drives passive transport.
1. A researcher places red blood cells into three different solutions and observes the results. In solution X, the cells shrink and become crenated. In solution Y, the cells maintain their normal shape. In solution Z, the cells swell and burst. Which of the following correctly identifies the tonicity of solution X?
2. A protein synthesized by a ribosome will ultimately be secreted from the cell. Which of the following correctly describes the sequence of organelles involved in processing and exporting this protein?
3. Which of the following best explains why the sodium-potassium pump is classified as active transport rather than facilitated diffusion?
Cellular energetics is the study of how cells capture, transform, and use energy to power life. At the heart of this unit is a simple but profound idea: energy from sunlight or food must be converted into a usable form, ATP, before cells can do any work. Two major processes drive this conversion — photosynthesis in plant cells and cellular respiration in nearly all living organisms — and understanding how they connect is essential for mastering this unit.
Enzymes are the molecular machines that make all of this chemistry possible. They lower the activation energy of reactions, speeding up processes that would otherwise take far too long to sustain life. Factors like temperature, pH, and substrate concentration all influence how well enzymes function, and the AP exam frequently asks students to interpret graphs showing these relationships and predict what happens when conditions change.
Cellular respiration and photosynthesis are both organized into stages that occur in specific locations within the cell. Respiration moves through glycolysis in the cytoplasm, then into the mitochondria for pyruvate oxidation, the Krebs cycle, and finally oxidative phosphorylation along the inner mitochondrial membrane. Photosynthesis runs in reverse fashion energetically, using light energy captured in the thylakoids to ultimately build sugar in the stroma through the Calvin cycle. Redox reactions and electron transport chains are central to both processes.
ATP (Adenosine Triphosphate)
The primary energy currency of the cell, releasing usable energy when the bond between its second and third phosphate groups is hydrolyzed. Nearly all cellular work, from muscle contraction to active transport, is powered by ATP.
Enzyme
A biological catalyst, almost always a protein, that speeds up a chemical reaction by lowering its activation energy without being consumed in the process. Each enzyme has an active site that is complementary in shape to its specific substrate.
Substrate
The specific reactant molecule that binds to an enzyme's active site to undergo a chemical reaction. The enzyme-substrate interaction is highly specific due to the complementary shapes of the active site and substrate.
Glycolysis
The first stage of cellular respiration, occurring in the cytoplasm, in which one glucose molecule is broken down into two pyruvate molecules with a net gain of 2 ATP and 2 NADH. Glycolysis does not require oxygen and is common to nearly all living organisms.
NAD+ / NADH
NAD+ is an electron carrier molecule that accepts high-energy electrons during cellular respiration, becoming NADH, which then donates those electrons to the electron transport chain to drive ATP synthesis. This redox pair is essential for transferring energy through the stages of respiration.
Chemiosmosis
The process by which ATP is synthesized using the potential energy of a proton gradient across a membrane, as H+ ions flow through ATP synthase from high to low concentration. This mechanism operates in both mitochondria during respiration and in chloroplasts during photosynthesis.
Electron Transport Chain (ETC)
A series of protein complexes embedded in the inner mitochondrial membrane (or thylakoid membrane in chloroplasts) that pass electrons from one carrier to the next, releasing energy used to pump protons and ultimately produce a large amount of ATP. Oxygen serves as the final electron acceptor in cellular respiration, forming water.
Calvin Cycle
The light-independent reactions of photosynthesis occurring in the stroma of the chloroplast, in which carbon dioxide is fixed and reduced using ATP and NADPH to produce G3P, a precursor to glucose. The cycle must turn three times to produce one net molecule of G3P.
Fermentation
An anaerobic process that regenerates NAD+ from NADH so that glycolysis can continue producing ATP in the absence of oxygen. Common types include lactic acid fermentation in animal muscle cells and alcoholic fermentation in yeast.
Competitive Inhibitor
A molecule that reduces enzyme activity by binding to the active site and blocking the substrate from entering, competing directly with the substrate for the same binding location. Its effect can be overcome by increasing substrate concentration.
1. A researcher adds a molecule that is structurally similar to glucose and binds to the active site of hexokinase, the enzyme that phosphorylates glucose in glycolysis. When glucose concentration is significantly increased, enzyme activity returns to near-normal levels. Which of the following best describes this inhibitor?
2. During the light reactions of photosynthesis, water molecules are split in a process called photolysis. Which of the following correctly identifies where this occurs and what it directly provides to the light reactions?
3. A cell is treated with a chemical that makes the inner mitochondrial membrane freely permeable to H+ ions, allowing protons to flow across the membrane without passing through ATP synthase. Which of the following would be the most likely immediate consequence?
Cell communication allows cells to receive and respond to signals from their environment and from other cells. This process, called signal transduction, follows a three-step sequence: reception, where a receptor protein binds a signaling molecule called a ligand; transduction, where the signal is converted and often amplified through a cascade of molecular events; and response, where the cell carries out a specific action such as activating a gene or triggering cell division. Understanding this pathway is fundamental to understanding how multicellular organisms coordinate the behavior of trillions of individual cells.
Cells can communicate over varying distances using different signaling types. Endocrine signaling involves hormones traveling through the bloodstream to reach distant target cells, while paracrine signaling acts locally on neighboring cells. Autocrine signaling occurs when a cell responds to signals it produces itself, and synaptic signaling is the rapid, targeted communication between neurons across a synapse. Second messengers like cyclic AMP and IP3 are small intracellular molecules that amplify and relay signals from the cell surface to the interior, allowing one extracellular signal to trigger a large and coordinated cellular response.
The cell cycle is the ordered series of events by which a cell grows and divides. It consists of interphase, which includes the G1, S, and G2 phases where the cell grows and replicates its DNA, followed by the mitotic phase where the cell physically divides. The cycle is tightly regulated by checkpoints that monitor cell size, DNA integrity, and chromosome attachment, and these checkpoints are controlled by proteins called cyclins and cyclin-dependent kinases. When this regulation breaks down, cells can divide uncontrollably, which is the underlying basis of cancer.
Signal Transduction
The process by which a cell converts an extracellular signal into a specific intracellular response through a cascade of molecular events involving reception, transduction, and response.
Ligand
A signaling molecule that binds specifically to a receptor protein, triggering a conformational change that initiates a cellular response.
Second Messenger
A small intracellular signaling molecule such as cAMP or IP3 that relays and amplifies a signal from a surface receptor to target molecules inside the cell.
G-protein Coupled Receptor (GPCR)
A transmembrane receptor that, upon ligand binding, activates an associated G-protein, which then triggers downstream signaling cascades including the production of second messengers.
Cyclin
A regulatory protein whose concentration fluctuates cyclically throughout the cell cycle and whose binding to a cyclin-dependent kinase activates that kinase to drive the cell forward through a checkpoint.
Cyclin-Dependent Kinase (CDK)
A protein kinase that is only active when bound to its corresponding cyclin; active CDK complexes phosphorylate target proteins to advance the cell through the cell cycle.
Cell Cycle Checkpoint
A critical control point in the cell cycle where internal monitoring mechanisms verify that conditions are suitable before allowing the cell to proceed to the next phase.
Apoptosis
A highly regulated program of controlled cell death in which a cell systematically dismantles itself, playing essential roles in development, tissue homeostasis, and elimination of damaged cells.
Proto-oncogene
A normal gene that promotes cell growth and division in a controlled manner; mutations can convert it into an oncogene that drives unregulated cell proliferation.
Tumor Suppressor Gene
A gene that normally inhibits cell division or promotes apoptosis; loss-of-function mutations in these genes remove critical brakes on the cell cycle and contribute to cancer development.
1. A researcher treats cells with a drug that permanently activates adenylyl cyclase, the enzyme that produces cAMP. Which of the following best predicts the effect of this drug on signal transduction?
2. A cell has a mutation that prevents the degradation of M-phase cyclin after chromosomes have aligned at the metaphase plate. Which of the following is the most likely consequence of this mutation?
3. Cancer cells often show reduced expression of proteins that promote apoptosis. Which of the following best explains why this contributes to tumor growth?
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Continue with Study Them →Heredity is the process by which traits are passed from parents to offspring, and understanding it requires knowing how genetic information is packaged, shuffled, and transmitted. Meiosis is the central mechanism here: it is a specialized form of cell division that produces haploid gametes from diploid parent cells, reducing chromosome number by half so that fertilization restores the correct diploid number. Without meiosis, chromosome counts would double with every generation.
What makes meiosis especially powerful for generating diversity is what happens before and during the division process. During prophase I, homologous chromosomes pair up and exchange segments of DNA in a process called crossing over, creating new combinations of alleles on a single chromosome. Then, during metaphase I, homologous pairs line up independently of one another, meaning each gamete receives a random assortment of maternal and paternal chromosomes. These two mechanisms together are the physical basis for the inheritance patterns Gregor Mendel observed in his pea plant experiments.
Mendelian genetics describes how discrete heritable units called alleles interact to produce phenotypes. In the simplest cases, one allele is dominant over another, and straightforward ratios emerge from crosses. But many traits do not follow this simple pattern. Incomplete dominance, codominance, multiple alleles, sex linkage, and non-Mendelian phenomena like maternal inheritance and genomic imprinting all reveal that the relationship between genotype and phenotype is far more nuanced than Mendel's original model suggested. Mastering both the foundational rules and these extensions is essential for AP Biology success.
Meiosis
A two-stage cell division process in sexually reproducing organisms that reduces chromosome number by half, producing four genetically unique haploid gametes from one diploid parent cell.
Crossing Over
The exchange of corresponding segments of DNA between non-sister chromatids of homologous chromosome pairs during prophase I, creating new combinations of alleles called recombinant chromosomes.
Independent Assortment
Mendel's second law stating that alleles of different genes on non-homologous chromosomes are distributed to gametes independently of one another during meiosis I.
Codominance
A pattern of inheritance in which both alleles in a heterozygous organism are fully and simultaneously expressed in the phenotype, as seen in human ABO blood type with genotype I^A I^B.
Incomplete Dominance
A pattern of inheritance in which neither allele is fully dominant, producing a heterozygous phenotype that is an intermediate blend of the two homozygous phenotypes, such as pink flowers from red and white parents.
Sex-Linked Trait
A trait controlled by a gene located on a sex chromosome, most often the X chromosome, causing the trait to appear at different frequencies in males and females because males are hemizygous for X-linked genes.
Linked Genes
Genes located on the same chromosome that tend to be inherited together and do not assort independently, though crossing over can separate them and produce recombinant offspring.
Recombination Frequency
The proportion of offspring that show new combinations of parental alleles, used to estimate the map distance between two linked genes in centimorgans (cM), where 1% recombination equals 1 cM.
Genomic Imprinting
An epigenetic phenomenon in which the expression of a gene depends on which parent it was inherited from, achieved through differential methylation that silences either the maternal or paternal copy.
Dihybrid Cross
A genetic cross between two organisms that are each heterozygous for two different traits, typically producing a 9:3:3:1 phenotypic ratio among offspring when the two genes assort independently.
1. In a species with a diploid number of 2n = 8, how many unique gamete combinations are theoretically possible from independent assortment alone, before accounting for crossing over?
2. A woman who is a carrier of an X-linked recessive condition and a man who is unaffected have children. Which of the following correctly describes the expected outcomes for their sons?
3. Two genes are located on the same chromosome. In a testcross, a researcher observes 18% recombinant offspring. Which of the following conclusions is best supported by this data?
Gene expression is the process by which the information encoded in DNA is used to build functional proteins, and it follows a two-step pathway known as the central dogma: DNA is transcribed into messenger RNA, which is then translated into a protein. This flow of information is tightly controlled at every step, allowing cells to produce only the proteins they need at any given moment. Understanding this process is foundational to nearly every other topic in modern biology.
DNA replication ensures that every time a cell divides, each daughter cell receives an exact copy of the genetic information. The process is called semiconservative because each new double helix retains one original strand and one newly synthesized strand. Key enzymes like helicase unwind the double helix, DNA polymerase builds the new strand by adding nucleotides in the 5 to 3 direction, and ligase seals the gaps between Okazaki fragments on the lagging strand.
Gene regulation allows cells with identical DNA to behave differently depending on their environment or developmental stage. Prokaryotes use operons like the lac operon to switch groups of genes on or off in response to environmental signals such as the presence of lactose. Eukaryotes use more complex mechanisms including enhancers, repressors, and chromatin remodeling to fine-tune gene expression. Mutations and modern biotechnology tools like CRISPR and PCR allow scientists to study, edit, and exploit these regulatory systems in powerful ways.
Semiconservative replication
The mode of DNA replication in which each new double helix consists of one original parental strand and one newly synthesized strand, confirmed by the Meselson-Stahl experiment.
DNA polymerase
An enzyme that synthesizes new DNA strands by adding complementary nucleotides in the 5 to 3 direction, and also proofreads newly added bases to reduce errors.
Okazaki fragments
Short segments of DNA synthesized discontinuously on the lagging strand during replication, later joined together by DNA ligase into a continuous strand.
Promoter
A specific DNA sequence upstream of a gene where RNA polymerase binds to initiate transcription, acting as a regulatory on-switch for gene expression.
Codon
A sequence of three consecutive nucleotides on mRNA that specifies a particular amino acid or signals the start or stop of translation.
Transfer RNA (tRNA)
An RNA molecule with an anticodon loop that carries a specific amino acid to the ribosome and matches it to the correct codon on the mRNA during translation.
Lac operon
A set of prokaryotic genes in E. coli that are controlled together and code for enzymes needed to metabolize lactose, serving as a classic model of gene regulation.
Frameshift mutation
A mutation caused by an insertion or deletion of a number of nucleotides not divisible by three, which shifts the reading frame and typically alters every amino acid downstream of the mutation.
CRISPR-Cas9
A molecular tool adapted from a bacterial immune system that uses a guide RNA to direct the Cas9 enzyme to a precise location in the genome for targeted gene editing.
Enhancer
A regulatory DNA sequence that can be located far from the gene it controls and increases transcription by binding activator proteins that interact with the transcription complex.
1. During DNA replication in a eukaryotic cell, which of the following best explains why the lagging strand is synthesized discontinuously?
2. A researcher introduces a single adenine nucleotide into the third position of a gene coding sequence. Which of the following most accurately describes the likely effect on the resulting protein?
3. In E. coli, glucose is present but lactose is absent. Which of the following correctly describes the state of the lac operon under these conditions?
Natural selection is the driving mechanism of evolution, and it works through four key principles that Charles Darwin identified: there must be variation among individuals in a population, that variation must be heritable, individuals must produce more offspring than can survive, and individuals with favorable traits must survive and reproduce at higher rates. Over generations, beneficial traits become more common in the population while harmful ones are filtered out. It is important to understand that natural selection acts on phenotypes, but it is the underlying genotypes that are actually passed to the next generation.
Populations can change over time through mechanisms other than natural selection. Genetic drift refers to random changes in allele frequencies due to chance events, and it has the strongest effect in small populations. The bottleneck effect occurs when a population is drastically reduced in size by a random event, while the founder effect occurs when a small group splits off to start a new population. Gene flow, the movement of alleles between populations through migration, and mutation, which introduces entirely new alleles, are also critical forces that shape genetic diversity. Together, these forces are known as the mechanisms of microevolution.
When populations become reproductively isolated, they can diverge enough to become separate species, a process called speciation. Allopatric speciation occurs when a physical barrier geographically separates a population, while sympatric speciation happens within the same geographic area, often through polyploidy or disruptive selection. Scientists use phylogenetic trees and cladograms to map evolutionary relationships, and molecular evidence such as DNA sequence comparisons and the presence of homologous genes provide powerful support for common ancestry. Hardy-Weinberg equilibrium serves as a useful null model, describing a hypothetical non-evolving population against which real populations can be compared.
Natural Selection
The process by which individuals with heritable traits better suited to their environment survive and reproduce more successfully, causing those traits to increase in frequency over generations.
Hardy-Weinberg Equilibrium
A mathematical model stating that allele and genotype frequencies in a population remain constant across generations in the absence of evolution, requiring no mutation, random mating, no gene flow, infinite population size, and no natural selection.
Genetic Drift
A mechanism of evolution in which allele frequencies change due to random chance events rather than natural selection, with the strongest effects seen in small populations.
Gene Flow
The transfer of alleles from one population to another through the movement and interbreeding of individuals, which can reduce genetic differences between populations.
Allopatric Speciation
The formation of new species that occurs when a population is geographically isolated from the parent population, preventing gene flow and allowing independent divergence over time.
Reproductive Isolation
Biological barriers that prevent members of two populations from interbreeding and producing fertile offspring, categorized as either prezygotic or postzygotic depending on when the barrier acts.
Founder Effect
A type of genetic drift that occurs when a small number of individuals establish a new population, resulting in reduced genetic diversity and potentially different allele frequencies than the original population.
Stabilizing Selection
A type of natural selection that favors intermediate phenotypes and acts against extreme variants, reducing variation and maintaining the most common form of a trait in a population.
Cladogram
A branching diagram that represents the evolutionary relationships among organisms based on shared derived characters, with each branch point representing a common ancestor.
Disruptive Selection
A type of natural selection that favors both extreme phenotypes over intermediate ones, increasing variation in a population and potentially leading to the formation of two distinct groups.
1. A population of beetles lives on a rocky hillside. Beetles with medium-sized body mass are most likely to survive harsh winters, while very large and very small beetles die at higher rates. Which of the following best describes the type of selection occurring and its expected long-term effect on the population?
2. A researcher calculates that the frequency of a recessive allele in a large, randomly mating population is 0.3. Assuming the population is in Hardy-Weinberg equilibrium, what is the expected frequency of heterozygous individuals?
3. Two frog populations live in the same geographic region but breed in different seasons, preventing interbreeding. This is best described as which type of reproductive isolation, and at which stage does it act?
Ecology is the study of how organisms interact with each other and with their physical environment. In AP Biology Unit 8, you will zoom out from the cell and organism level to examine populations, communities, and entire ecosystems. These interactions are governed by the same evolutionary pressures you have studied throughout the course, meaning natural selection shapes not just individual traits but entire ecological relationships.
Population ecology focuses on how and why the number of individuals in a group changes over time. When resources are unlimited, populations grow exponentially, producing a J-shaped curve. In the real world, however, resources like food, water, and space impose a ceiling called the carrying capacity, which causes growth to slow and level off into an S-shaped logistic curve. Factors that regulate populations can be density-dependent, meaning their effect intensifies as population size increases, or density-independent, meaning they affect populations regardless of size.
At larger scales, communities of interacting species form complex webs of competition, predation, and symbiosis that determine which species survive and in what numbers. Energy flows through these communities in one direction, from sunlight captured by producers through successive trophic levels, with roughly 90 percent lost as heat at each step. Nutrients, unlike energy, are recycled through biogeochemical cycles such as the carbon and nitrogen cycles, linking living organisms to the abiotic environment in continuous loops that sustain all life on Earth.
Carrying Capacity (K)
The maximum population size that a given environment can sustainably support, determined by the availability of limiting resources such as food, water, and space.
Logistic Growth
A pattern of population growth that starts exponentially but slows as the population approaches carrying capacity, producing a characteristic S-shaped curve.
Density-Dependent Factor
A population-regulating factor whose effect on survival and reproduction intensifies as population density increases, such as competition for food or disease transmission.
Keystone Species
A species that has a disproportionately large effect on community structure relative to its abundance, and whose removal causes dramatic shifts in biodiversity.
Trophic Level
A feeding position in a food web, with producers occupying the first level and successive levels occupied by primary, secondary, and tertiary consumers.
10 Percent Rule
The ecological principle that only approximately 10 percent of the energy stored at one trophic level is transferred to the next, with the remaining 90 percent lost primarily as heat.
Primary Productivity
The rate at which producers convert solar energy into organic compounds through photosynthesis; gross primary productivity minus plant respiration yields net primary productivity available to consumers.
Nitrogen Fixation
The biological conversion of atmospheric nitrogen gas into ammonia by specialized prokaryotes, making nitrogen available in a form that plants and other organisms can use.
Symbiosis
A close, long-term ecological relationship between two species, categorized as mutualism when both benefit, commensalism when one benefits and the other is unaffected, or parasitism when one benefits at the other's expense.
Invasive Species
A non-native species introduced to a new ecosystem where it thrives without natural predators or competitors, often reducing native biodiversity and disrupting ecological balance.
1. A population of deer in a forest grows rapidly after hunting is banned but eventually stabilizes at approximately 500 individuals despite continued food availability. Which of the following best explains why growth slowed and stabilized?
2. In a grassland ecosystem, 10,000 kilocalories of energy are fixed by grasses. Assuming 10 percent energy transfer efficiency between trophic levels, approximately how many kilocalories are available to a tertiary consumer such as a hawk that feeds on snakes that feed on mice?
3. Researchers studying a coastal marine ecosystem remove all sea otters from the area. Over the following years, sea urchin populations explode and kelp forests are nearly eliminated, causing dozens of other species to disappear. What does this experiment most directly demonstrate?
6 questions · 90 minutes · 2 long-form + 4 short
Describe the graph before you explain it
On any FRQ with a graph or data set, start by stating the trend in plain language (e.g., "As substrate concentration increases, reaction rate increases until it plateaus"). Only then give the biological explanation.
Be specific, then be specific again
Vague answers like "because the cell needs energy" earn zero points. Name the specific molecule, organelle, or mechanism. AP graders look for scientific vocabulary used correctly.
Follow the causal chain to the organism
Many FRQs ask how a molecular change affects the organism. Work from molecular → cellular → tissue/organ → organism level. One sentence per level.
Answer every task verb, separately
If a question says "Describe AND explain," write two distinct responses. "Predict" asks for a direction AND a mechanism. "Design an experiment" requires hypothesis, variables, controls, and expected results.
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An enzyme found in human cells catalyzes the conversion of compound X to compound Y. A researcher measures enzyme activity under the following conditions: - Condition 1: Normal pH (7.4), 37°C - Condition 2: pH 5.0, 37°C - Condition 3: pH 7.4, 50°C - Condition 4: pH 7.4, 37°C, with 5x excess substrate (a) Predict the relative enzyme activity in Conditions 2 and 3 compared to Condition 1. Justify your predictions. (4 points) (b) The cell produces a regulatory molecule that binds to a site on the enzyme that is NOT the active site. When bound, the enzyme's active site changes shape. Identify the type of inhibition and explain how it affects enzyme activity. (2 points) (c) Explain how the data from Condition 4 would differ if the concentration of enzyme rather than substrate were increased. (2 points)
Time management
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