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AP Intermediate Zoology (BIEAP) study materials
AP Intermediate Zoology (BIEAP) study resources

AP Intermediate Zoology (BIEAP) practice questions

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Question 1Animal Diversity and Classification

The presence of a notochord, a dorsal hollow nerve cord and pharyngeal gill slits at some stage of life is characteristic of which phylum?

  1. Annelida
  2. Echinodermata
  3. Chordata Correct answer.
  4. Mollusca

Explanation

These three features together with a post-anal tail form the defining suite of chordate characters, present at least in the embryo. Echinoderms, annelids and molluscs are non-chordate invertebrates lacking a notochord at any stage.

Question 2Animal Diversity and Classification

In the phylum Porifera, the flagellated cells that generate water currents through the canal system are called:

  1. Pinacocytes
  2. Choanocytes Correct answer.
  3. Amoebocytes
  4. Sclerocytes

Explanation

Choanocytes, or collar cells, line the spongocoel and flagellated chambers and their beating drives the water current used in filter feeding. The other cell types serve covering, transport and skeletal secretion rather than current generation.

Question 3Animal Diversity and Classification

Which class of Cnidaria exists only in the polyp form and has no medusa stage in its life cycle?

  1. Cubozoa
  2. Scyphozoa
  3. Hydrozoa
  4. Anthozoa Correct answer.

Explanation

Anthozoans such as sea anemones and corals have a dominant polyp with a gastrovascular cavity divided by mesenteries, and the medusa is completely absent. Scyphozoa, Cubozoa and Hydrozoa all include a medusa in the life cycle, though it may be reduced.

Question 4Animal Diversity and Classification

Flame cells (protonephridia) used in excretion and osmoregulation are characteristic of which phylum?

  1. Arthropoda
  2. Nematoda
  3. Platyhelminthes Correct answer.
  4. Annelida

Explanation

Flame cells are the terminal excretory units of the protonephridial system of flatworms, where a tuft of cilia drives fluid into a duct network. Nematodes, annelids and arthropods use different excretory organs.

Question 5Animal Diversity and Classification

The cuticle of nematodes is periodically shed during growth in a process known as:

  1. Regeneration
  2. Ecdysis Correct answer.
  3. Metamorphosis
  4. Budding

Explanation

Nematodes are ecdysozoans: they moult the tough collagenous cuticle four times, passing through larval stages. Regeneration, metamorphosis and budding describe other developmental processes and are not the shedding of the cuticle.

Question 6Animal Diversity and Classification

Metameric segmentation with a true coelom divided by septa and closed circulatory system is best exhibited by which animal?

  1. Pheretima Correct answer.
  2. Aurelia
  3. Ascaris
  4. Octopus

Explanation

Pheretima, the earthworm, is a classic annelid showing repeated segments, a schizocoelous coelom partitioned by septa and a closed vascular system. The other animals lack this combination of annelid features.

Question 7Animal Diversity and Classification

Which feature of arthropods is regarded as a major factor in their success on land?

  1. A muscular hydrostatic skeleton
  2. A water vascular system
  3. A chitinous exoskeleton with jointed appendages Correct answer.
  4. A ciliated larval epidermis

Explanation

The chitinous exoskeleton provides protection against desiccation and, with jointed appendages and striated muscle, permits efficient locomotion on land. The other listed features belong to echinoderms, larval forms and soft-bodied invertebrates.

Question 8Animal Diversity and Classification

The mantle cavity of a mollusc primarily houses which structures?

  1. The gastric shield and the crystalline style
  2. The nerve ring and statocysts
  3. The radula and the oesophagus
  4. The ctenidia and the openings of the excretory and reproductive ducts Correct answer.

Explanation

The mantle cavity is the space between the mantle and the visceral mass; it contains the gills (ctenidia) and receives the nephridial and genital openings. The radula lies in the buccal cavity and the nerve ring surrounds the oesophagus.

Question 9Animal Diversity and Classification

Water enters the water vascular system of a starfish through which structure?

  1. Peristomial membrane
  2. Tube feet
  3. Ambulacral groove
  4. Madreporite Correct answer.

Explanation

The madreporite is the sieve-like plate on the aboral surface through which seawater enters the ring canal via the stone canal. Tube feet are effectors of the system and the ambulacral groove houses them, but neither is the entry point.

Question 10Animal Diversity and Classification

Which of the following is a distinguishing feature of the subphylum Cephalochordata?

  1. Mammary glands used in nursing the young
  2. A cartilaginous cranium enclosing a large brain
  3. A vertebral column replacing the notochord in the adult
  4. A notochord that persists throughout life and extends into the rostrum Correct answer.

Explanation

In cephalochordates such as Amphioxus the notochord is retained in the adult and extends to the anterior tip, giving the group its name. Vertebral columns, a cranium and mammary glands are vertebrate specialisations.

Question 11Animal Diversity and Classification

In the classification of animals, the presence of three germ layers with a body cavity lined by mesoderm on both sides is described as:

  1. Diploblastic and acoelomate
  2. Triploblastic and coelomate Correct answer.
  3. Diploblastic and pseudocoelomate
  4. Triploblastic and pseudocoelomate

Explanation

Three germ layers make an animal triploblastic, and a cavity fully lined by mesoderm is a true coelom. Diploblastic animals have only ectoderm and endoderm, and a pseudocoelom is lined by mesoderm on one side only.

Question 12Animal Diversity and Classification

Which pair of animals shows radial symmetry in the adult stage?

  1. Ascaris and Taenia
  2. Asterias and Aurelia Correct answer.
  3. Pheretima and Hirudinaria
  4. Periplaneta and Palaemon

Explanation

Adult echinoderms such as Asterias and adult cnidarians such as Aurelia are radially symmetrical, though echinoderm larvae are bilaterally symmetrical. All the other pairs are bilaterians.

Question 13Animal Diversity and Classification

The excretory organs of Pheretima, which open to the exterior through nephridiopores, are:

  1. Malpighian tubules
  2. Flame cells
  3. Green glands
  4. Nephridia Correct answer.

Explanation

Earthworms possess metanephridia, coiled tubular organs with ciliated funnels that open internally and drain to nephridiopores. Flame cells, Malpighian tubules and green glands are excretory organs of flatworms, insects and crustaceans respectively.

Question 14Animal Diversity and Classification

Which statement best explains why echinoderms are placed closer to chordates than to arthropods in animal classification?

  1. Both possess jointed appendages and a chitinous exoskeleton
  2. Both show spiral cleavage and determinate development
  3. Both are deuterostomes with enterocoelic coelom formation and radial cleavage Correct answer.
  4. Both have a ventral nerve cord and schizocoelous coelom

Explanation

Echinoderms and chordates share deuterostome features: the blastopore becomes the anus, the coelom arises by enterocoely and cleavage is radial and indeterminate. The other options describe protostome or arthropod-specific characters.

Question 15Animal Diversity and Classification

Which larval form is correctly matched with its group?

  1. Cercaria larva in Porifera
  2. Bipinnaria larva in Crustacea
  3. Glochidium larva in Echinodermata
  4. Trochophore larva in Mollusca Correct answer.

Explanation

Trochophore larvae occur in molluscs and several annelids, and are a classic link between the two lophotrochozoan phyla. The other matches pair each larva with the wrong phylum.

Question 16Animal Diversity and Classification

Which structure in cephalopods is modified into a siphon used in jet propulsion?

  1. The visceral mass
  2. The ctenidium
  3. The radula
  4. The muscular foot Correct answer.

Explanation

In cephalopods the muscular foot is modified into arms and tentacles and also into the funnel or siphon through which water is expelled from the mantle cavity. The radula is a feeding organ and the ctenidium is a gill.

Question 17Animal Diversity and Classification

In which group does the coelom arise as pouches pinched off from the archenteron during embryonic development?

  1. Annelida
  2. Arthropoda
  3. Echinodermata Correct answer.
  4. Platyhelminthes

Explanation

Enterocoely, in which mesodermal pouches bud from the archenteron, occurs in deuterostomes such as echinoderms and chordates. Annelids and arthropods form their coelom by schizocoely, while flatworms have no coelom.

Question 18Animal Diversity and Classification

The insect Malpighian tubules are best described as:

  1. Open funnels that filter blood and discharge through nephridiopores
  2. Blind-ended tubules opening into the gut that remove nitrogenous waste and regulate ions Correct answer.
  3. Glands that secrete digestive enzymes into the crop
  4. Flame cells that drive fluid by ciliary action

Explanation

Malpighian tubules are blind-ended at the haemocoel end and open into the gut at the midgut-hindgut junction; they secrete uric acid and ions into the intestine. Their arrangement is the reverse of metanephridia and their function is excretory.

Question 19Animal Diversity and Classification

Which of the following animals is a pseudocoelomate?

  1. Asterias rubens
  2. Wuchereria bancrofti Correct answer.
  3. Pheretima posthuma
  4. Taenia solium

Explanation

Wuchereria, a filarial nematode, has a body cavity that is not fully lined by mesoderm and is therefore a pseudocoelom. Taenia is acoelomate, while Pheretima and Asterias possess a true coelom.

Question 20Animal Diversity and Classification

Which feature distinguishes the class Aves from other vertebrates in relation to flight?

  1. A muscular diaphragm separating thorax and abdomen
  2. A three-chambered heart with a single systemic arch
  3. Pneumatic bones and a sternum bearing a keel Correct answer.
  4. A scaly skin with mucous glands

Explanation

Birds possess hollow air-filled (pneumatic) bones that reduce body mass and a keeled sternum for the attachment of flight muscles. The other options describe amphibian, mammalian or fish features respectively.

Question 21Animal Diversity and Classification

A researcher finds a marine animal with a calcareous endoskeleton of ossicles, pentaradial adult symmetry and bilateral larvae. To which phylum should it be assigned?

  1. Hemichordata
  2. Echinodermata Correct answer.
  3. Cnidaria
  4. Mollusca

Explanation

Ossicles of calcareous plates, secondary radial symmetry in the adult and bilateral larvae are diagnostic of echinoderms. Molluscs have a shell and a foot, cnidarians are diploblastic and hemichordates have a stomochord rather than ossicles.

Question 22Animal Diversity and Classification

Which statement about the notochord in vertebrates is correct?

  1. It is replaced by the vertebral column during development, persisting only as remnants such as the nucleus pulposus Correct answer.
  2. It is present only in the larval stage and disappears completely without trace
  3. It is found only in the tail region of the adult
  4. It persists as the main axial support throughout life in all vertebrates

Explanation

In vertebrates the notochord is embryonic and is largely replaced by the vertebral column, remaining as the nuclei pulposi of the intervertebral discs. Complete lifelong persistence is a cephalochordate or urochordate larval feature, not the vertebrate condition.

Question 23Animal Diversity and Classification

In the phylum Hemichordata, the structure that is often compared with the notochord but is not homologous to it is the:

  1. Pygochord
  2. Stomochord Correct answer.
  3. Collarette
  4. Proboscis gland

Explanation

The stomochord is a short buccal diverticulum in the proboscis of hemichordates that superficially resembles a notochord but differs in structure and origin. The other terms refer to different parts of the hemichordate body.

Question 24Animal Diversity and Classification

Which of the following correctly pairs an animal with its respiratory organ?

  1. Palaemon - book lungs
  2. Pila - ctenidia Correct answer.
  3. Hirudinaria - tracheae
  4. Periplaneta - gills

Explanation

Pila, the apple snail, breathes through ctenidia in water and a pulmonary sac on land. Book lungs occur in arachnids, cockroaches use tracheae and leeches respire through their moist skin.

Question 25Structural Organisation and Function in Animals

Which tissue type is characterised by cells embedded in an abundant extracellular matrix with collagen fibres, and is derived mainly from mesoderm?

  1. Muscular tissue
  2. Connective tissue Correct answer.
  3. Epithelial tissue
  4. Nervous tissue

Explanation

Connective tissue is defined by widely separated cells scattered in a large extracellular matrix containing collagen and other fibres. It arises predominantly from mesoderm and includes bone, cartilage, blood and adipose tissue. The other three tissue types are dominated by closely packed cells rather than matrix.

Question 26Structural Organisation and Function in Animals

In a typical mammalian neuron, which part contains the nucleus and most of the protein-synthesising machinery?

  1. The cell body (soma) Correct answer.
  2. The axon hillock
  3. The synaptic knob
  4. The node of Ranvier

Explanation

The soma, or cell body, houses the nucleus and the bulk of the rough endoplasmic reticulum and ribosomes needed for protein synthesis. Dendrites and axons are processes that extend from it and largely lack this machinery. The axon hillock and synaptic knob are specialised regions for impulse initiation and transmission.

Question 27Structural Organisation and Function in Animals

Which protein filaments slide past one another to shorten a skeletal muscle sarcomere during contraction?

  1. Collagen and elastin
  2. Tubulin and dynein
  3. Actin and myosin Correct answer.
  4. Keratin and vimentin

Explanation

Skeletal muscle contraction follows the sliding filament mechanism, in which thin actin filaments are pulled over thick myosin filaments. This shortens the sarcomere without changing filament length. The other listed proteins serve structural, cytoskeletal or transport roles, not sarcomeric contraction.

Question 28Structural Organisation and Function in Animals

Which type of epithelium lines the alveoli of the lung, where rapid gas diffusion is required?

  1. Simple squamous epithelium Correct answer.
  2. Stratified squamous epithelium
  3. Simple cuboidal epithelium
  4. Transitional epithelium

Explanation

Simple squamous epithelium is a single layer of flattened cells, giving the thinnest possible barrier for diffusion of oxygen and carbon dioxide. Cuboidal and stratified epithelia are thicker and suited to secretion or protection. Transitional epithelium is specialised for stretch in the urinary tract.

Question 29Structural Organisation and Function in Animals

Which component of blood is primarily responsible for oxygen transport from the lungs to tissues?

  1. Haemoglobin within erythrocytes Correct answer.
  2. Platelets
  3. Fibrinogen in plasma
  4. Albumins in plasma

Explanation

Oxygen is carried mainly bound to haemoglobin inside red blood cells, which greatly increases the blood's oxygen-carrying capacity. Plasma proteins and platelets have other functions. Only a small fraction of oxygen dissolves directly in plasma.

Question 30Structural Organisation and Function in Animals

Which feature distinguishes cardiac muscle from skeletal muscle in a histological section?

  1. Absence of any myofibrils
  2. Intercalated discs joining adjacent cells Correct answer.
  3. Presence of striations
  4. Multiple peripheral nuclei per fibre

Explanation

Cardiac muscle cells are joined end to end by intercalated discs, which contain desmosomes and gap junctions that mechanically and electrically couple the cells. Skeletal muscle fibres lack these junctions and instead are long syncytial cells with many peripheral nuclei. Both cardiac and skeletal muscle are striated.

Question 31Structural Organisation and Function in Animals

The basic functional unit of the kidney that filters blood and forms urine is the:

  1. Alveolus
  2. Sarcomere
  3. Villus
  4. Nephron Correct answer.

Explanation

The nephron is the structural and functional unit of the kidney, comprising the renal corpuscle and a tubule that processes the filtrate. Alveoli belong to the lung, villi to the small intestine, and sarcomeres to muscle. Each kidney contains many thousands of nephrons working in parallel.

Question 32Structural Organisation and Function in Animals

Which type of joint permits movement in only one plane, as seen between the humerus and the ulna?

  1. Pivot joint
  2. Hinge joint Correct answer.
  3. Ball-and-socket joint
  4. Saddle joint

Explanation

A hinge joint allows flexion and extension in a single plane, which is exactly the movement at the elbow between humerus and ulna. Ball-and-socket joints permit movement in several planes, pivot joints allow rotation, and saddle joints allow two planes of movement.

Question 33Structural Organisation and Function in Animals

During the sliding filament mechanism, what is the immediate trigger for myosin heads to bind actin?

  1. Binding of calcium ions to troponin, exposing actin-binding sites Correct answer.
  2. Hydrolysis of ATP by the sodium-potassium pump
  3. Stretching of the tendon by an external load
  4. Release of acetylcholine at the neuromuscular junction

Explanation

Calcium released from the sarcoplasmic reticulum binds troponin, which shifts tropomyosin and uncovers the myosin-binding sites on actin. Only then can the myosin head attach and execute the power stroke. Acetylcholine initiates the process upstream, but the direct molecular trigger is calcium binding to troponin.

Question 34Structural Organisation and Function in Animals

A patient's blood report shows reduced plasma albumin. Which function is most directly impaired?

  1. Clotting of blood at injury sites
  2. Maintenance of colloid osmotic pressure of blood Correct answer.
  3. Transport of oxygen to tissues
  4. Immune defence against pathogens

Explanation

Albumin is the most abundant plasma protein and is the chief contributor to the colloid osmotic (oncotic) pressure that holds water within capillaries. A fall in albumin therefore favours fluid loss into tissues and oedema. Oxygen carriage, clotting and immunity depend on haemoglobin, fibrinogen and globulins respectively.

Question 35Structural Organisation and Function in Animals

In a myelinated axon, action potentials appear to jump from one node of Ranvier to the next. This saltatory conduction mainly serves to:

  1. Increase the speed of impulse conduction Correct answer.
  2. Increase the amplitude of each action potential
  3. Allow conduction in both directions along the axon
  4. Remove the need for ion channels in the axon membrane

Explanation

Saltatory conduction restricts ion exchange to the nodes, so the local circuit currents skip along the internodes. This markedly increases conduction velocity without requiring a larger axon diameter. The action potential amplitude and the all-or-none nature of the impulse are unchanged.

Question 36Structural Organisation and Function in Animals

Which structural feature of the small intestine most directly increases the surface area available for absorption?

  1. Abundant serous glands in the submucosa
  2. Numerous villi and microvilli Correct answer.
  3. A thick muscularis externa
  4. A lining of stratified squamous epithelium

Explanation

Villi are finger-like projections of the mucosa, and microvilli are further folds of the apical epithelial membrane. Together they multiply the absorptive surface enormously. Thick muscle layers aid peristalsis, and stratified squamous epithelium would reduce absorption, not enhance it.

Question 37Structural Organisation and Function in Animals

Compared with a typical skeletal muscle fibre, a cardiac muscle cell is best described as:

  1. Short, branched and usually uninucleate Correct answer.
  2. Long, multinucleate and unbranched
  3. Non-striated and spindle-shaped
  4. Devoid of gap junctions and desmosomes

Explanation

Cardiac muscle cells are relatively short, branched and typically have one or two centrally placed nuclei. They are striated and are coupled by intercalated discs containing gap junctions and desmosomes. The long multinucleate unbranched description fits skeletal muscle.

Question 38Structural Organisation and Function in Animals

Which sequence correctly traces filtrate through the nephron?

  1. Glomerulus → distal tubule → loop of Henle → proximal tubule → collecting duct
  2. Proximal tubule → glomerulus → loop of Henle → collecting duct → distal tubule
  3. Glomerulus → proximal tubule → loop of Henle → distal tubule → collecting duct Correct answer.
  4. Glomerulus → loop of Henle → proximal tubule → distal tubule → collecting duct

Explanation

Filtration occurs at the glomerulus, after which filtrate enters Bowman's capsule and passes into the proximal convoluted tubule. It then descends the loop of Henle, enters the distal convoluted tubule and finally the collecting duct. This order reflects the anatomical arrangement of the nephron.

Question 39Structural Organisation and Function in Animals

The resting membrane potential of a neuron is closest to the equilibrium potential of which ion?

  1. Potassium Correct answer.
  2. Calcium
  3. Chloride
  4. Sodium

Explanation

At rest the membrane is far more permeable to potassium than to other ions because of open potassium leak channels. The resting potential therefore lies near the potassium equilibrium potential, around minus seventy millivolts. Sodium and calcium permeabilities are low at rest.

Question 40Structural Organisation and Function in Animals

Which cell junction allows small ions and molecules to pass directly between adjacent epithelial cells, enabling electrical coupling?

  1. Hemidesmosome
  2. Tight junction
  3. Desmosome
  4. Gap junction Correct answer.

Explanation

Gap junctions consist of aligned connexon channels that form cytoplasmic pores between neighbouring cells. They permit direct passage of ions and small molecules and so provide electrical coupling. Tight junctions seal the intercellular space, desmosomes anchor cells mechanically, and hemidesmosomes attach cells to the basal lamina.

Question 41Structural Organisation and Function in Animals

A person living at high altitude shows an increased red blood cell count. Which property of blood does this primarily help to maintain?

  1. Its colloid osmotic pressure
  2. Its capacity to buffer lactic acid
  3. Its oxygen-carrying capacity Correct answer.
  4. Its clotting ability

Explanation

At altitude the partial pressure of oxygen in the air is lower, so each unit of blood carries less oxygen. Raising the number of erythrocytes increases the total haemoglobin available and thus restores oxygen delivery to tissues. Clotting, osmotic pressure and buffering depend on other blood components.

Question 42Structural Organisation and Function in Animals

Which bone cell is responsible for resorbing bone matrix during remodelling?

  1. Osteocyte
  2. Osteoblast
  3. Chondrocyte
  4. Osteoclast Correct answer.

Explanation

Osteoclasts are large multinucleate cells that secrete acid and enzymes to dissolve bone mineral and matrix. Osteoblasts deposit new bone, osteocytes are mature cells within lacunae, and chondrocytes form cartilage. Resorption by osteoclasts is balanced against deposition by osteoblasts.

Question 43Structural Organisation and Function in Animals

In the mammalian heart, why is the wall of the left ventricle thicker than that of the right ventricle?

  1. It receives blood directly from the venae cavae
  2. It must pump blood against greater resistance in the systemic circuit Correct answer.
  3. It has a larger internal cavity than the right ventricle
  4. It contains more conducting tissue than the right ventricle

Explanation

The left ventricle drives blood into the aorta and the systemic circulation, which has much higher resistance than the pulmonary circuit. Its thicker muscular wall generates the higher pressure needed. The right ventricle pumps to the lungs at lower pressure, so its wall is thinner.

Question 44Structural Organisation and Function in Animals

Which structural adaptation of the mammalian lung is most directly responsible for keeping the alveolar surface moist and reducing surface tension?

  1. Ciliated columnar cells lining the bronchioles
  2. Cartilage rings in the trachea
  3. Surfactant secreted by type II alveolar cells Correct answer.
  4. Smooth muscle in the bronchial walls

Explanation

Type II alveolar cells secrete pulmonary surfactant, a lipoprotein mixture that lowers surface tension and helps keep alveoli patent and moist. Cilia, cartilage and smooth muscle serve clearance, support and airway calibre control. Surfactant is therefore the key adaptation for the alveolar surface itself.

Question 45Structural Organisation and Function in Animals

A competitive inhibitor of acetylcholinesterase at the neuromuscular junction would most immediately produce:

  1. Immediate relaxation of the muscle fibre
  2. Failure of acetylcholine to be released from the axon terminal
  3. Prolonged depolarisation of the muscle fibre membrane Correct answer.
  4. Loss of calcium stores from the sarcoplasmic reticulum

Explanation

Acetylcholinesterase normally breaks down acetylcholine so the postsynaptic membrane can repolarise. If it is inhibited, acetylcholine persists in the synaptic cleft and keeps binding its receptors, prolonging depolarisation and causing sustained contraction. Neurotransmitter release and calcium storage are unaffected by this enzyme.

Question 46Structural Organisation and Function in Animals

In the loop of Henle, the descending limb is permeable to water but relatively impermeable to solutes, while the ascending limb actively transports sodium and chloride out. What is the consequence of this arrangement?

  1. Glucose is reabsorbed mainly in the ascending limb
  2. The filtrate becomes progressively more dilute in the descending limb
  3. Potassium is secreted into the descending limb
  4. An osmotic gradient is established in the medulla that permits concentration of urine Correct answer.

Explanation

Water leaves the descending limb while solutes remain, and the ascending limb pumps out sodium and chloride while remaining impermeable to water. This countercurrent arrangement builds a hyperosmotic medullary interstitium that allows the collecting duct to concentrate urine under ADH. The descending limb concentrates the filtrate rather than diluting it.

Question 47Structural Organisation and Function in Animals

A toxin opens potassium channels in the axon membrane and keeps them open. Which effect on the neuron is most likely?

  1. The membrane becomes depolarised and spontaneously fires
  2. The threshold potential moves closer to the resting potential
  3. Sodium influx during the action potential increases
  4. The membrane becomes hyperpolarised and harder to excite Correct answer.

Explanation

Opening more potassium channels increases potassium efflux, driving the membrane potential closer to the potassium equilibrium potential. This hyperpolarises the cell, moving it further from threshold and making excitation harder. Sodium influx and threshold are not directly increased by this change.

Question 48Structural Organisation and Function in Animals

In a sarcomere, the A band remains constant in length during contraction while the I band and H zone shorten. Which interpretation follows directly?

  1. Actin filaments are broken down and resynthesised during each contraction
  2. Thin filaments slide toward the centre, increasing overlap with thick filaments Correct answer.
  3. The thick filaments themselves contract and become shorter
  4. The Z lines move apart as the sarcomere lengthens

Explanation

Constant A band length shows that thick filaments do not shorten; the shortening of the I band and H zone reflects greater overlap as thin filaments slide inward. This is the central evidence for the sliding filament model. The Z lines move closer together, shortening the sarcomere overall.

Question 49Structural Organisation and Function in Animals

A mutation prevents the formation of desmosomes in an epithelium. Which outcome is most likely?

  1. Adjacent cells lose mechanical cohesion and separate under stress Correct answer.
  2. Cells detach from the basement membrane
  3. The epithelium becomes unable to secrete mucus
  4. Small ions can no longer pass between cells

Explanation

Desmosomes anchor adjacent cells by linking their intermediate filament networks across the intercellular space. Without them, the epithelium loses mechanical strength and cells pull apart when stretched. Attachment to the basement membrane depends on hemidesmosomes, and ion coupling depends on gap junctions.

Question 50Structural Organisation and Function in Animals

Which type of epithelium lines the inner surface of the urinary bladder, and how is it suited to that organ's function?

  1. Transitional epithelium, whose umbrella cells flatten and stretch as the bladder fills Correct answer.
  2. Simple columnar epithelium with a dense brush border, which absorbs glucose from urine
  3. Simple squamous epithelium, which allows rapid diffusion of gases across the bladder wall
  4. Ciliated pseudostratified columnar epithelium, which sweeps urine toward the urethra

Explanation

The bladder must distend without rupturing, and transitional epithelium is specialised for this: its rounded surface (umbrella) cells flatten when stretched and the multiple cell layers slide over one another. It also forms a tight barrier that prevents urine from diffusing back into the blood. No other epithelial type combines this distensibility with an impermeable lining.