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ILEARN Biology ECA practice questions

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Question 1Molecules to Organisms: Structures and Processes

Which statement describes the main function of a ribosome?

  1. It packages proteins for shipment out of the cell.
  2. It produces ATP through cellular respiration.
  3. It stores the genetic instructions for the cell.
  4. It builds proteins from amino acids. Correct answer.

Explanation

Ribosomes are the sites of translation, where mRNA codons are matched to tRNA molecules carrying amino acids, forming polypeptide chains. The Golgi apparatus modifies and packages proteins after they are made, but it does not assemble them.

Question 2Molecules to Organisms: Structures and Processes

Which description best characterizes the plasma membrane?

  1. A double strand of nucleotides that stores energy
  2. A phospholipid bilayer that controls movement of substances Correct answer.
  3. A rigid layer of cellulose that surrounds the cell
  4. A solid layer of protein that blocks all molecules from entering

Explanation

The plasma membrane consists of a phospholipid bilayer with embedded proteins that regulate which substances cross. It is described as selectively permeable because some molecules pass freely while others require channels or energy-driven pumps.

Question 3Molecules to Organisms: Structures and Processes

Starch and glycogen are polymers built from which monomer?

  1. Nucleotides
  2. Glucose Correct answer.
  3. Amino acids
  4. Fatty acids

Explanation

Starch, glycogen, and cellulose are polysaccharides assembled from glucose monomers joined by glycosidic bonds. Amino acids build proteins, fatty acids build lipids, and nucleotides build nucleic acids.

Question 4Molecules to Organisms: Structures and Processes

Which structure is found in prokaryotic cells?

  1. Mitochondria
  2. Ribosomes Correct answer.
  3. A Golgi apparatus
  4. A membrane-bound nucleus

Explanation

Prokaryotic cells lack membrane-bound organelles and a true nucleus, so their genetic material sits directly in the cytoplasm. Ribosomes are the one organelle they share with eukaryotes, and they are required for protein synthesis.

Question 5Molecules to Organisms: Structures and Processes

Which structure is present in plant cells but absent in animal cells?

  1. Nucleus
  2. Mitochondrion
  3. Ribosome
  4. Cell wall Correct answer.

Explanation

Plant cells have a rigid cellulose cell wall outside the plasma membrane, plus chloroplasts and a large central vacuole. Animal cells lack these three structures but share the nucleus, mitochondria, and ribosomes with plant cells.

Question 6Molecules to Organisms: Structures and Processes

Which molecule serves as the main energy currency that cells use to power work?

  1. ATP Correct answer.
  2. Glucose
  3. DNA
  4. NADPH

Explanation

ATP transfers energy from exergonic reactions to energy-requiring processes. Breaking its terminal phosphate bond releases energy that powers cellular work such as ion pumps and synthesis reactions.

Question 7Molecules to Organisms: Structures and Processes

Which sequence correctly shows the levels of organization from simplest to most complex?

  1. Cell, organ, tissue, organ system
  2. Organ, tissue, cell, organ system
  3. Cell, tissue, organ, organ system Correct answer.
  4. Tissue, cell, organ system, organ

Explanation

Multicellular organisms are organized from simple to complex: cells form tissues, tissues form organs, and organs work together as organ systems. The order reflects increasing complexity and cooperation among parts.

Question 8Molecules to Organisms: Structures and Processes

In which organelle does photosynthesis take place?

  1. Nucleus
  2. Ribosome
  3. Chloroplast Correct answer.
  4. Mitochondrion

Explanation

Chloroplasts contain thylakoid membranes and the enzyme rubisco needed to capture light energy and fix carbon dioxide into sugars. Mitochondria carry out cellular respiration instead of photosynthesis.

Question 9Molecules to Organisms: Structures and Processes

A cell whose interior is 1% solute is placed in a beaker of 5% solute solution. What will most likely happen?

  1. Water enters the cell and the cell swells.
  2. Water moves in and out at equal rates, so nothing changes.
  3. Water leaves the cell and the cell shrinks. Correct answer.
  4. Solutes move into the cell until the concentrations are equal.

Explanation

Water moves by osmosis from a region of lower solute concentration to a region of higher solute concentration. Because the surrounding solution is hypertonic, water leaves the cell and the cell shrinks.

Question 10Molecules to Organisms: Structures and Processes

An enzyme that works best at 37 degrees Celsius is heated to 70 degrees Celsius. What most likely happens to the reaction rate?

  1. The reaction rate keeps rising as temperature increases.
  2. The reaction rate stays the same because enzymes are unaffected by heat.
  3. The reaction rate drops sharply because the enzyme loses its shape. Correct answer.
  4. The reaction rate rises slowly because more substrate becomes available.

Explanation

Enzymes are proteins whose shape determines the shape of their active site. High temperatures break the bonds holding that shape, denaturing the enzyme so substrate no longer fits and the reaction rate falls sharply.

Question 11Molecules to Organisms: Structures and Processes

A cell moves potassium ions into a region that already has a high potassium concentration. Which process does this describe?

  1. Active transport Correct answer.
  2. Facilitated diffusion
  3. Osmosis
  4. Simple diffusion

Explanation

Moving a substance against its concentration gradient requires energy, which is the defining feature of active transport. Osmosis and diffusion are passive and move material only down a gradient.

Question 12Molecules to Organisms: Structures and Processes

During which phase of mitosis do sister chromatids separate and move toward opposite poles?

  1. Anaphase Correct answer.
  2. Metaphase
  3. Telophase
  4. Prophase

Explanation

In anaphase, motor proteins pull sister chromatids apart toward opposite poles of the cell. Prophase is when chromosomes condense, metaphase is when they line up at the equator, and telophase is when nuclei re-form.

Question 13Molecules to Organisms: Structures and Processes

Which substances are the direct inputs of the light-independent reactions of photosynthesis?

  1. Water and light energy
  2. Carbon dioxide, ATP, and NADPH Correct answer.
  3. Oxygen, ATP, and NADPH
  4. Glucose and oxygen

Explanation

The Calvin cycle uses ATP and NADPH made by the light reactions to reduce carbon dioxide into G3P, which can be assembled into glucose. Oxygen, water, and light are involved in the light-dependent reactions rather than in carbon fixation.

Question 14Molecules to Organisms: Structures and Processes

Where does glycolysis take place, and what does it produce?

  1. In the cytoplasm; it produces glucose and oxygen
  2. In the mitochondrion; it produces carbon dioxide and most of the cell's ATP
  3. In the mitochondrion; it produces NADPH and water
  4. In the cytoplasm; it produces pyruvate and a small amount of ATP Correct answer.

Explanation

Glycolysis occurs in the cytoplasm and splits one glucose into two pyruvate molecules, with a net gain of two ATP and two NADH. The mitochondrion hosts the later stages that generate most of the ATP.

Question 15Molecules to Organisms: Structures and Processes

After exercise, a person's body temperature rises, and sweating cools the body back toward normal. Which term describes this control mechanism?

  1. Negative feedback Correct answer.
  2. Passive transport
  3. Positive feedback
  4. Cellular respiration

Explanation

Negative feedback counteracts a change to bring a variable back toward its set point, as sweating does when body temperature rises. Positive feedback instead amplifies a change away from the set point.

Question 16Molecules to Organisms: Structures and Processes

A large spherical cell divides into many smaller cells of the same total volume. What happens to the surface-area-to-volume ratio?

  1. The ratio increases, but exchange across the membrane slows down.
  2. The ratio increases, so materials diffuse in and out faster. Correct answer.
  3. The ratio decreases, so the cell exchanges materials more slowly.
  4. The ratio stays the same because the total volume does not change.

Explanation

As a cell gets smaller, its surface area decreases more slowly than its volume, so the surface-area-to-volume ratio rises. A higher ratio lets a cell exchange gases and remove wastes fast enough to meet the needs of its volume.

Question 17Molecules to Organisms: Structures and Processes

A cell secretes large amounts of digestive enzymes. Which organelles would be most abundant in this cell?

  1. Cilia and flagella
  2. Smooth endoplasmic reticulum and lysosomes
  3. Rough endoplasmic reticulum and Golgi apparatus Correct answer.
  4. Chloroplasts and mitochondria

Explanation

Proteins destined for export are built on ribosomes attached to the rough ER, then travel through the Golgi for modification and packaging into vesicles. Cells specialized for secretion therefore contain abundant rough ER and Golgi membranes.

Question 18Molecules to Organisms: Structures and Processes

DNA replication occurs during which part of the cell cycle?

  1. G1 phase
  2. G2 phase
  3. S phase Correct answer.
  4. M phase

Explanation

DNA is copied during the S phase of interphase, so each chromosome consists of two identical sister chromatids when mitosis begins. G1 and G2 are growth phases that flank the S phase.

Question 19Molecules to Organisms: Structures and Processes

A molecule is made of carbon, hydrogen, oxygen, and nitrogen, and it speeds up a reaction without being consumed. Which class of macromolecule is it?

  1. Nucleic acid
  2. Carbohydrate
  3. Protein Correct answer.
  4. Lipid

Explanation

Proteins are polymers of amino acids and always contain nitrogen, sometimes sulfur; many proteins act as enzymes that catalyze reactions. Carbohydrates and lipids are built mainly from carbon, hydrogen, and oxygen.

Question 20Molecules to Organisms: Structures and Processes

Which pair of organelles both contain their own DNA and are surrounded by double membranes?

  1. Nucleus and smooth endoplasmic reticulum
  2. Chloroplasts and mitochondria Correct answer.
  3. Ribosomes and Golgi apparatus
  4. Lysosomes and central vacuole

Explanation

Mitochondria and chloroplasts both have double membranes, circular DNA, and their own ribosomes, evidence supporting the endosymbiotic theory. The other organelles listed lack DNA of their own.

Question 21Molecules to Organisms: Structures and Processes

An inhibitor binds directly to the active site of an enzyme. Which statement best describes the effect of this inhibitor?

  1. It competes with the substrate, so adding more substrate can restore activity. Correct answer.
  2. It binds to the substrate instead of the enzyme and changes the substrate's shape.
  3. It permanently destroys the active site so that no substrate can ever bind.
  4. It lowers the activation energy more than an uninhibited enzyme does.

Explanation

A competitive inhibitor occupies the active site, so substrate and inhibitor compete for the same binding site; raising the substrate concentration can outcompete the inhibitor. Inhibitors slow reactions, and only noncompetitive inhibitors act by altering enzyme shape elsewhere.

Question 22Molecules to Organisms: Structures and Processes

A plant is sealed inside a clear, lighted chamber. The oxygen level rises during the day and falls at night. Which statement best explains this pattern?

  1. Photosynthesis happens only during the daylight hours, and respiration never occurs at all.
  2. Respiration releases oxygen at night, so the oxygen level in the chamber should rise.
  3. The plant stops all gas exchange with the chamber as soon as the light is turned off.
  4. In the light, photosynthesis exceeds respiration; in darkness, only respiration occurs. Correct answer.

Explanation

Plants photosynthesize and respire at the same time; in bright light, photosynthesis fixes more carbon dioxide and releases more oxygen than respiration consumes. In darkness only respiration continues, so oxygen is consumed and the level falls.

Question 23Molecules to Organisms: Structures and Processes

A leaf is placed in a concentrated salt solution, and its cells show chloroplasts bunched together, pulled away from the cell wall. Which statement best explains this observation?

  1. Salt from the solution entered the cell and pushed the chloroplasts toward the center.
  2. The cell wall dissolved in the saltwater, so the cell collapsed and its contents clumped together.
  3. Water moved into the cell, causing the cell contents to press tightly against the wall.
  4. Water left the cell by osmosis, shrinking the vacuole and pulling the membrane inward. Correct answer.

Explanation

In a hypertonic solution water leaves the cell by osmosis, and the flexible plasma membrane pulls away from the rigid wall in a process called plasmolysis. Solute does not need to enter the cell for this shrinkage to occur.

Question 24Molecules to Organisms: Structures and Processes

Muscle cells contain far more mitochondria than most other body cells. Which statement best explains this observation?

  1. Mitochondria store glucose that the muscle cell uses during contraction.
  2. Mitochondria produce ATP without oxygen, which muscles need during exercise.
  3. They need large amounts of ATP, which mitochondria produce aerobically. Correct answer.
  4. Mitochondria build the contractile proteins that muscle cells use to shorten.

Explanation

Muscle contraction depends on ATP for myosin cross-bridge cycling and ion pumps, so active muscle cells contain many mitochondria. Aerobic respiration in the mitochondria yields far more ATP per glucose than glycolysis alone.

Question 25Molecules to Organisms: Structures and Processes

A cell begins dividing repeatedly and without restraint, forming a mass of cells. Which explanation best fits this behavior?

  1. The cells no longer need ATP because they produce energy in a different way.
  2. Checkpoints that normally control the cell cycle no longer work. Correct answer.
  3. The cells skip interphase, so they can divide much more quickly than normal cells.
  4. The plasma membranes of the cells disappear, allowing them to divide freely.

Explanation

Cell cycle checkpoints verify that DNA is replicated correctly and that chromosomes are properly attached before division proceeds. When these controls fail, cells divide without restraint and may form tumors.

Question 26Molecules to Organisms: Structures and Processes

Which feature of the plasma membrane best explains why small nonpolar molecules can cross it while most large polar molecules cannot?

  1. A single layer of proteins covering the surface
  2. A phospholipid bilayer with a hydrophobic core Correct answer.
  3. Channels made only of cholesterol
  4. A rigid layer of cellulose fibers

Explanation

The plasma membrane is a phospholipid bilayer with hydrophobic fatty acid tails facing inward. Small nonpolar molecules can dissolve through this hydrophobic core, while large polar molecules generally require transport proteins.

Question 27Molecules to Organisms: Structures and Processes

Which structure is found in a human liver cell but not in a typical bacterial cell?

  1. A membrane-bound nucleus Correct answer.
  2. Cytoplasm that fills the cell
  3. Ribosomes that assemble proteins
  4. A plasma membrane that controls entry

Explanation

Bacterial cells are prokaryotes and lack a membrane-bound nucleus; their DNA is in a nucleoid region. Human liver cells are eukaryotes and have a true nucleus enclosed by a nuclear envelope.

Question 28Molecules to Organisms: Structures and Processes

Which cell structure is the site of protein synthesis?

  1. Golgi apparatus
  2. Smooth endoplasmic reticulum
  3. Lysosome
  4. Ribosome Correct answer.

Explanation

Ribosomes translate messenger RNA into polypeptide chains during protein synthesis. The Golgi apparatus modifies and packages proteins, while smooth ER mainly makes lipids and lysosomes digest materials.

Question 29Molecules to Organisms: Structures and Processes

Which organelle contains enzymes that break down worn-out cell parts and ingested material?

  1. Lysosome Correct answer.
  2. Vacuole
  3. Nucleolus
  4. Chloroplast

Explanation

Lysosomes are membrane-bound organelles containing hydrolytic enzymes. They digest worn-out organelles, food particles, and material brought into the cell.

Question 30Molecules to Organisms: Structures and Processes

When ATP is used for cellular work, which bond is most directly responsible for releasing usable energy?

  1. The bond between adenine and ribose
  2. The bond between two carbon atoms in ribose
  3. The bond holding the terminal phosphate Correct answer.
  4. The bond between the first and second phosphate groups only

Explanation

ATP releases usable energy when the bond to its terminal phosphate is hydrolyzed, producing ADP and inorganic phosphate. The adenine and ribose portions are structural components and are not the main source of energy for cellular work.

Question 31Molecules to Organisms: Structures and Processes

Which statement about the cell wall of a plant cell is correct?

  1. It is made mainly of phospholipids.
  2. It provides support and is made mainly of cellulose. Correct answer.
  3. It controls which molecules enter and leave the cell.
  4. It is found in animal cells as well as plant cells.

Explanation

The plant cell wall is a rigid layer made mainly of cellulose that provides support and protection. The plasma membrane, not the wall, regulates which substances enter and leave the cell.

Question 32Molecules to Organisms: Structures and Processes

Which plant tissue transports water and dissolved minerals from roots to leaves?

  1. Epidermis
  2. Xylem Correct answer.
  3. Phloem
  4. Cambium

Explanation

Xylem tissue contains dead, hollow cells that transport water and dissolved minerals upward from roots to shoots. Phloem transports sugars and other organic compounds.

Question 33Molecules to Organisms: Structures and Processes

Alveoli in the lungs are tiny sacs with thin walls and a large total surface area. What does this structure most directly help with?

  1. Exchanging oxygen and carbon dioxide Correct answer.
  2. Filtering waste from the blood
  3. Moving food through the digestive tract
  4. Producing red blood cells

Explanation

Alveoli have thin walls and a very large total surface area, which increases the rate of diffusion. This structure supports exchange of oxygen and carbon dioxide between air and blood.

Question 34Molecules to Organisms: Structures and Processes

Glucose moves from a high concentration outside a cell to a low concentration inside the cell through a membrane protein, without using ATP. Which process is this?

  1. Phagocytosis of a large particle
  2. Active transport using ATP
  3. Facilitated diffusion Correct answer.
  4. Osmosis of water across a membrane

Explanation

Facilitated diffusion uses membrane proteins to move substances down their concentration gradient without ATP. Active transport moves substances against a gradient and requires energy.

Question 35Molecules to Organisms: Structures and Processes

A white blood cell surrounds and takes in a bacterium, forming a vesicle inside the cell. Which process is shown?

  1. Simple diffusion
  2. Exocytosis of a vesicle
  3. Endocytosis Correct answer.
  4. Osmosis of water

Explanation

Endocytosis is the process by which the plasma membrane surrounds material and forms a vesicle inside the cell. Phagocytosis is a type of endocytosis used by white blood cells to engulf bacteria.

Question 36Molecules to Organisms: Structures and Processes

Chlorophyll appears green. Which statement best explains why?

  1. It reflects ultraviolet light and absorbs green light.
  2. It absorbs green light strongly and reflects red and blue light.
  3. It absorbs all wavelengths of visible light equally.
  4. It reflects green light and absorbs red and blue light. Correct answer.

Explanation

Chlorophyll absorbs red and blue wavelengths efficiently and reflects or transmits green light. The reflected green light is why chlorophyll and many leaves appear green.

Question 37Molecules to Organisms: Structures and Processes

Which set of products is formed during the Krebs cycle in the mitochondrial matrix?

  1. Glucose and oxygen gas
  2. NADH, FADH2, ATP, and CO2 Correct answer.
  3. Pyruvate and NADP+ produced in glycolysis
  4. ATP and carbon dioxide only

Explanation

The Krebs cycle oxidizes acetyl-CoA and produces NADH, FADH2, ATP, and carbon dioxide. These electron carriers then deliver electrons to the electron transport chain.

Question 38Molecules to Organisms: Structures and Processes

During intense exercise, human muscle cells may produce lactic acid. What is a main purpose of this fermentation pathway?

  1. To convert lactic acid into glucose immediately
  2. To produce large amounts of ATP directly
  3. To increase oxygen delivery to mitochondria
  4. To regenerate NAD+ so glycolysis can continue Correct answer.

Explanation

Lactic acid fermentation regenerates NAD+ from NADH so that glycolysis can continue when oxygen is limited. It does not produce a large amount of ATP directly; most ATP from glucose still comes from glycolysis in this situation.

Question 39Molecules to Organisms: Structures and Processes

An enzyme normally functions at pH 7. When placed in a strongly acidic solution, its activity drops sharply. What is the best explanation?

  1. The enzyme's shape changed, altering its active site. Correct answer.
  2. The reaction no longer required an enzyme to occur.
  3. The substrate concentration increased.
  4. The enzyme was converted into a carbohydrate molecule.

Explanation

A strong pH change can alter the hydrogen bonds and ionic interactions that maintain an enzyme's shape. This can distort the active site so the substrate no longer binds efficiently.

Question 40Molecules to Organisms: Structures and Processes

Which event occurs during the G1 phase of the cell cycle?

  1. The cell grows and performs its usual functions. Correct answer.
  2. The nuclear envelope breaks down completely.
  3. Sister chromatids separate and move to opposite poles.
  4. DNA is replicated before division.

Explanation

G1 is the first gap phase of interphase, when a cell grows and performs its normal functions. DNA replication occurs later, in the S phase.

Question 41Molecules to Organisms: Structures and Processes

How does cytokinesis differ in plant cells and animal cells?

  1. Animal cells divide by binary fission, while plant cells do not.
  2. Plant cells do not undergo cytokinesis.
  3. Plant cells form a cell plate, while animal cells form a cleavage furrow. Correct answer.
  4. Plant cells form a cleavage furrow, while animal cells usually form a cell plate.

Explanation

Plant cells build a cell plate in the middle of the cell, which becomes a new cell wall. Animal cells pinch inward to form a cleavage furrow.

Question 42Molecules to Organisms: Structures and Processes

During meiosis, homologous chromosomes pair and exchange segments. What is this exchange called?

  1. Cytokinesis
  2. Binary fission
  3. Crossing over Correct answer.
  4. DNA replication

Explanation

Crossing over is the exchange of genetic segments between homologous chromosomes during prophase I of meiosis. It increases genetic variation in the resulting gametes.

Question 43Molecules to Organisms: Structures and Processes

A cell from an early embryo can develop into many different cell types. Which term best describes this ability?

  1. Potency Correct answer.
  2. Osmosis
  3. Homeostasis
  4. Fermentation

Explanation

Potency is the ability of a cell to develop into different cell types. Embryonic stem cells have high potency because they can differentiate into many specialized cell types.

Question 44Molecules to Organisms: Structures and Processes

After a meal, blood glucose rises, and the pancreas releases insulin, which helps cells take up glucose. Blood glucose then falls. Which statement best describes this regulation?

  1. A response that is unrelated to homeostasis
  2. A process that occurs only in plant cells
  3. Negative feedback toward a set point Correct answer.
  4. Positive feedback that increases blood glucose

Explanation

Insulin lowers blood glucose, and the falling glucose level reduces the stimulus for more insulin release. This is negative feedback because the response opposes the initial change and returns the variable toward a set point.

Question 45Molecules to Organisms: Structures and Processes

Water moves rapidly through some cell membranes with the help of aquaporins. What are aquaporins?

  1. Enzymes that break down water
  2. Channel proteins for water movement Correct answer.
  3. Pumps that use ATP to move water against its gradient
  4. Lipids that repel water

Explanation

Aquaporins are channel proteins that allow water to cross membranes rapidly. They increase water permeability without requiring ATP for each water molecule.

Question 46Molecules to Organisms: Structures and Processes

Where do the light-dependent reactions of photosynthesis take place, and what do they produce?

  1. In the stroma; they produce glucose and release carbon dioxide gas
  2. In the mitochondrial matrix; they produce ATP and water molecules
  3. In the cytoplasm; they produce lactic acid and NAD+ molecules
  4. In thylakoid membranes; they produce ATP, NADPH, and oxygen Correct answer.

Explanation

The light-dependent reactions occur in the thylakoid membranes of chloroplasts. They split water, release oxygen, and produce ATP and NADPH for the Calvin cycle.

Question 47Molecules to Organisms: Structures and Processes

What is the role of oxygen in the electron transport chain of aerobic cellular respiration?

  1. It donates electrons to NADH.
  2. It is broken down into carbon dioxide.
  3. It accepts electrons and forms water. Correct answer.
  4. It directly phosphorylates ADP to make ATP.

Explanation

Oxygen is the final electron acceptor in the electron transport chain. It accepts electrons and protons to form water, which allows the chain to keep passing electrons.

Question 48Molecules to Organisms: Structures and Processes

In the Calvin cycle, what is the main role of carbon dioxide?

  1. It is converted directly into ATP energy.
  2. It is fixed into organic compounds. Correct answer.
  3. It accepts electrons from NADPH during reduction.
  4. It is split to release oxygen gas.

Explanation

In the Calvin cycle, carbon dioxide is fixed onto an organic molecule and eventually used to build sugars. This carbon fixation step incorporates inorganic carbon into organic compounds.

Question 49Molecules to Organisms: Structures and Processes

A hormone binds to a receptor on the outside of a target cell, and this leads to a series of reactions inside the cell. Which term best describes this process?

  1. Fermentation in muscle cells
  2. Photosynthesis in chloroplasts
  3. Signal transduction Correct answer.
  4. Osmosis of water across a membrane

Explanation

Signal transduction is the process by which a cell converts an extracellular signal, such as a hormone, into an intracellular response. Receptor binding typically triggers a series of molecular reactions inside the cell.

Question 50Molecules to Organisms: Structures and Processes

Which function is most closely associated with microtubules in a eukaryotic cell?

  1. Digesting macromolecules inside lysosomes
  2. Producing ATP in the mitochondrial matrix
  3. Synthesizing lipids in the smooth endoplasmic reticulum
  4. Forming spindle fibers that move chromosomes Correct answer.

Explanation

Microtubules are cytoskeletal filaments that form spindle fibers during cell division. The spindle fibers attach to chromosomes and help move them to opposite poles.