Campbell Biology Chapter 50 Review: Sensory and Motor Mechanisms

50.1 How sensory receptors turn stimuli into signals

pp. 1085–1090

On the AP exam? Background

Sensory receptors and nerve signals aren't in the current course. What carries over: receptors that open or close ion channels and pathways that amplify a signal (Topic 4.2), neurons signaling to nearby cells (Topic 4.1) and animals detecting and responding to cues around them (Topic 8.1).

In the course: Topic 4.1 Cell Communication, Topic 4.2 Introduction to Signal Transduction, Topic 8.1 Responses to the Environment (notes, videos and more questions)

Key points

Every sense starts with some form of energy: light, heat, pressure or a molecule binding. A sensory receptor turns that energy into a change in the voltage across its membrane. That step is called sensory transduction.
The voltage change is a receptor potential, and it's graded: a stronger stimulus gives a bigger change. If the receptor is itself a neuron, a bigger change makes it fire action potentials more often. If it's a separate receptor cell, it releases more neurotransmitter onto a sensory neuron.
Your nervous system reads how strong a stimulus is from how often action potentials fire and how many receptors are firing. It reads what kind of stimulus it is from which neurons carry the signal and where in the brain they end. That's why pressing on a closed eyelid can make you 'see' light.
Perception happens in the brain. A ripe tomato reflects certain wavelengths of light, but the 'red' you experience is something your brain builds from the incoming signals.
Receptors can boost a faint stimulus, often through second-messenger pathways in which each activated enzyme turns out many messenger molecules. They also adapt: when a stimulus holds steady, their response fades, which is why a strong smell in a room seems to disappear after a few minutes while a new smell still stands out.
Receptors are grouped by the energy they detect: mechanoreceptors (touch, stretch, sound), chemoreceptors (particular molecules or total solute concentration), electromagnetic receptors (light, electric fields and, in some animals, magnetic fields), thermoreceptors (heat and cold) and nociceptors (harmful stimuli that cause pain).
Many receptors are ion-channel proteins. Piezo channels open when a membrane is stretched and handle much of your sense of touch, and TRP channels open at particular temperatures. Their discovery won the 2021 Nobel Prize in medicine.

Key terms (13)

sensory receptor: A cell, or a protein on a cell, that responds to a particular kind of stimulus, like light or pressure. Some are neurons; others are cells that pass the message to a neuron.
sensory transduction: Turning a stimulus's energy into a change in a receptor cell's membrane voltage. It's the first step in every sense.
receptor potential: The voltage change in a sensory receptor caused by a stimulus. It gets bigger as the stimulus gets stronger, unlike an all-or-none action potential.
perception: The brain's interpretation of incoming sensory signals, such as recognizing a color or a song. It happens in the brain, not in the receptor.
amplification: Strengthening of a sensory signal, often because each step in a signaling pathway switches on many molecules in the next step.
sensory adaptation: A drop in a receptor's response when a stimulus stays the same, so steady background stimuli fade from notice. It's a short-term change, unrelated to adaptation by natural selection.
mechanoreceptor: A receptor that responds to being bent, stretched or pressed, as in touch, hearing and the stretch sensors in muscles.
chemoreceptor: A receptor that responds to chemicals, either specific molecules (like a scent) or the overall concentration of solutes.
electromagnetic receptor: A receptor that detects electromagnetic energy, such as light, infrared radiation or electric fields.
thermoreceptor: A receptor that responds to heat or cold. Different types are tuned to different temperature ranges.
nociceptor: A pain receptor. It responds to stimuli strong enough to damage tissue, like extreme heat, crushing pressure or harsh chemicals.
muscle spindle: A stretch receptor wrapped around special fibers inside a skeletal muscle. It tells the spinal cord and brain how stretched the muscle is.
osmoreceptor: A chemoreceptor that monitors the overall solute concentration (osmolarity) of body fluids, helping the body decide when to take in or hold on to water.

Check yourself: 50.1 How sensory receptors turn stimuli into signals

4 questions on 50.1 How sensory receptors turn stimuli into signals. Pick an answer to see if you got it, and why.

Question 1 of 4

Sensory cells on the front legs of a tick respond when carbon dioxide from an animal's breath binds to proteins on their membranes. Which kind of receptor are these cells?

Question 2 of 4

A researcher recorded from a single touch-sensitive sensory neuron while pressing on the skin with different forces (invented data). Force applied (g) | Action potentials per second | Height of each action potential (mV) 0 | 2 | 100 5 | 10 | 100 10 | 24 | 100 20 | 45 | 100 Which conclusion is best supported by these data?

Question 3 of 4

With your eyes closed, you press gently on one eyelid and see a faint glowing pattern, even though no light reaches the eye. Which explanation is best?

Question 4 of 4

A swimmer jumps into a cool lake. At first, cold receptors in her skin fire about 40 action potentials per second, and the water feels cold. Five minutes later, with the water temperature unchanged, the same receptors fire about 8 per second, and the water feels comfortable. This change is an example of

0 of 4 answered

50.2 Hearing and balance

pp. 1090–1094

On the AP exam? Not tested

Ear anatomy, hearing and balance aren't in the current course. The links: animals sending and receiving sound signals (Topic 8.1), and hair cells working the same way in every vertebrate, a sign of common ancestry (Topic 7.6).

In the course: Topic 8.1 Responses to the Environment, Topic 7.6 Evidence of Evolution (notes, videos and more questions)

Key points

Hearing and balance both rely on hair cells. Each one has a bundle of stiff, actin-filled projections on top. Bending the bundle one way opens ion channels and depolarizes the cell; bending it the other way closes them and hyperpolarizes it. That changes how much neurotransmitter the cell releases onto a sensory neuron.
Many invertebrates have statocysts: hollow balance organs in which heavy particles called statoliths rest on whichever sensory cells are at the bottom, telling the animal which way is down. Insects pick up sound with hairs that vibrate or with thin, eardrum-like membranes.
In your ear, sound waves in the air vibrate the eardrum. Three tiny middle-ear bones carry it on to the cochlea's oval window, turning it into stronger pressure waves in the fluid of the inner ear.
Inside the coiled cochlea, those pressure waves shake the basilar membrane. Hair cells sitting on it bend against a membrane that hangs over them, and the auditory nerve carries the signal to the brain. Leftover wave energy drains out through a second flexible membrane, the round window, so one sound doesn't keep echoing inside the cochlea.
A louder sound (larger amplitude) shakes the basilar membrane harder, so hair cells change their output more and the auditory nerve fires faster. Pitch depends on frequency, and each stretch of the membrane is tuned to its own frequency: high notes shake the end nearest the oval window, where the membrane is slim and rigid, and low notes shake the floppier far tip. The brain knows the pitch from which hair cells are active.
For balance, the utricle and saccule hold hair cells under a gel weighed down with tiny calcium carbonate crystals, so they sense head tilt and speeding up or slowing down in a straight line. Three semicircular canals, set at right angles, sense turning: the fluid inside lags behind as your head rotates and bends a gel cap over the hair cells.
Fishes have no eardrum or cochlea. Vibrations reach their inner ears through the skull, and a lateral line of hair-cell clusters along each side senses water movement. Hair cells work the same way in all vertebrates.

Key terms (14)

hair cell: A mechanoreceptor with a bundle of stiff projections on top. Bending the bundle changes the cell's voltage. Hair cells handle hearing, balance and the fish lateral line.
statocyst: An organ in many invertebrates that senses gravity. Dense grains inside it settle onto the lowest sensory cells and show which way is down.
statolith: A heavy particle, often a mineral grain, inside a statocyst. Gravity pulls it down onto the sensory cells beneath it.
tympanic membrane: The eardrum: a thin membrane that vibrates when sound waves hit it. Many insects have eardrum-like membranes too.
middle-ear bones: Three tiny bones (malleus, incus and stapes) that pass the eardrum's vibrations to the inner ear and boost their pressure.
oval window: The membrane-covered opening where the last middle-ear bone pushes on the cochlea, starting pressure waves in its fluid.
cochlea: The coiled, fluid-filled part of the inner ear that turns sound vibrations into nerve signals.
basilar membrane: The membrane inside the cochlea that hair cells sit on. Different spots along it vibrate most to different pitches.
organ of Corti: The strip of hair cells and supporting cells on the basilar membrane where sound is actually detected.
amplitude: The height of a sound wave. Bigger amplitude means a louder sound and faster firing in the auditory nerve.
frequency: How many vibrations a sound wave makes per second, measured in hertz (Hz). Higher frequency means a higher pitch.
utricle and saccule: Two inner-ear chambers whose hair cells sense head tilt and straight-line acceleration, using crystals that shift with gravity.
semicircular canals: Three fluid-filled loops in the inner ear, set at right angles, that sense the head turning in any direction.
lateral line: A row of hair-cell sensors along the sides of fishes and some amphibians that detects water currents and vibrations.

Check yourself: 50.2 Hearing and balance

4 questions on 50.2 Hearing and balance. Pick an answer to see if you got it, and why.

Question 1 of 4

A worker spent years next to machinery that made a loud, high-pitched whine. Testing now shows he has lost much of his hearing for high pitches but hears low pitches normally. Where are the damaged hair cells most likely located?

Question 2 of 4

Even in silence, a hair cell releases a small amount of neurotransmitter, so its sensory neuron fires at a steady low rate. What is the main advantage of this resting activity?

Question 3 of 4

A passenger has a cold that blocks the tube connecting his middle ear to his throat. As the plane descends and cabin air pressure rises, his ear hurts and sounds become muffled. Which explanation is best?

Question 4 of 4

In orbit, astronauts float, so gravity no longer pulls the crystals in the utricle and saccule down against the hair cells below them. Which ability would be most disrupted at first?

0 of 4 answered

50.3 Light detection and vision

pp. 1095–1101

On the AP exam? Background

Eye anatomy isn't in the current course, but vision is full of tested ideas: rhodopsin is a G protein-coupled receptor with a second messenger (Topics 4.2 and 4.3), the X-linked inheritance of red-green color blindness (Topic 5.4), shared eye-building genes point to common ancestry (Topic 7.6) and night-active animals have rod-rich eyes (Topic 8.1).

In the course: Topic 4.2 Introduction to Signal Transduction, Topic 4.3 Signal Transduction Pathways, Topic 5.4 Non-Mendelian Genetics, Topic 7.6 Evidence of Evolution, Topic 8.1 Responses to the Environment (notes, videos and more questions)

Key points

Every animal eye, from a simple eyespot to a compound eye to a camera-type eye, uses photoreceptor cells holding a light-absorbing pigment. The genes that switch on eye development are shared by flatworms, insects and vertebrates, which suggests light sensing traces back to early animals.
Simple eyespots are partly shaded by dark pigment, so they only tell an animal which direction light comes from. Compound eyes, found in insects and crustaceans, are built from many units, each with its own lens. They're excellent at catching movement, and some insects can see ultraviolet light.
Camera-type eyes with a single lens evolved in vertebrates and, separately, in squids and octopuses. In your eye, the cornea and lens focus light on the retina, the iris adjusts the pupil, and the lens changes shape to focus on near or far objects.
Rods are very sensitive and work in dim light but don't see color. Cones need brighter light and give color and fine detail. The fovea, the spot of sharpest vision, is packed with cones and has no rods. Animals active at night tend to have rod-rich retinas.
Rhodopsin, the pigment in rods, is a small molecule called retinal (made from vitamin A) attached to a protein called opsin. Absorbing light flips retinal from its cis form to its trans form, which switches opsin on. Opsin is a G protein-coupled receptor, so this starts a signaling relay.
Light actually turns a rod's output down. In the dark, a second messenger called cyclic GMP holds Na⁺ channels open, so the rod is depolarized and keeps releasing the neurotransmitter glutamate. Light activates an enzyme that breaks down cGMP, the channels close, the rod hyperpolarizes, and it releases less glutamate.
The retina starts processing images before the brain does: bipolar, horizontal, amacrine and ganglion cells sharpen edges by having active cells inhibit their neighbors. Ganglion cell axons form the optic nerve. Humans have three kinds of cones, and since the red and green opsin genes are on the X chromosome, red-green color blindness is far more common in males.

Key terms (15)

photoreceptor: A cell that responds to light because it contains a light-absorbing pigment. Rods and cones are your photoreceptors.
ocellus: A simple eyespot found in some invertebrates. It senses light and its direction but can't form an image.
compound eye: An eye made of many separate light-detecting units, each with its own lens, as in insects and crustaceans. It's very good at spotting motion.
ommatidium: One of the many light-detecting units in a compound eye. Each one covers a small patch of the view.
retina: The layer of photoreceptors and neurons lining the back of the eye, where light is turned into nerve signals.
lens: A clear, flexible structure behind the pupil that focuses light on the retina. In mammals, it changes shape to focus near or far.
fovea: A small spot in the center of the retina packed with cones and no rods. It gives your sharpest, most detailed vision.
rod: A very light-sensitive photoreceptor that lets you see in dim light, but only in shades of gray.
cone: A photoreceptor that works in brighter light and allows color vision. Humans have three types, each most sensitive to different wavelengths.
rhodopsin: The light-absorbing pigment in rods, made of retinal bound to an opsin protein. Light switches it on.
retinal: A small molecule made from vitamin A that absorbs light and changes shape. It's the light-catching part of visual pigments.
opsin: The membrane protein that holds retinal in a visual pigment. It's a G protein-coupled receptor, and different opsins tune cones to different colors.
optic nerve: The bundle of ganglion cell axons that carries visual signals from each eye to the brain.
lateral inhibition: When a stimulated neuron suppresses its neighbors. In the retina, it sharpens edges and boosts contrast.
ganglion cell: A retinal neuron that collects signals from bipolar cells and sends action potentials to the brain. Its axons make up the optic nerve.

Check yourself: 50.3 Light detection and vision

4 questions on 50.3 Light detection and vision. Pick an answer to see if you got it, and why.

Question 1 of 4

In rods, light activates an enzyme that breaks down cyclic GMP (cGMP), and cGMP holds Na⁺ channels open. A drug blocks this enzyme. What would most likely happen when light strikes a treated rod?

Question 2 of 4

Opsin is a membrane protein that activates a G protein when the retinal attached to it absorbs light and changes shape. In a typical hormone-signaling pathway, which step plays the same role as retinal changing shape?

Question 3 of 4

A biologist compared the retinas of four mammal species (invented data). Species | Rods per cone | Number of cone types P | 2 | 3 Q | 60 | 1 R | 8 | 2 S | 4 | 3 Which species is most likely active mainly at night and has the poorest color vision?

Question 4 of 4Calculator allowed

Red-green color blindness is caused by a recessive allele on the X chromosome. A woman with normal color vision whose father was red-green color-blind has children with a man who has normal color vision. What fraction of their sons would be expected to be red-green color-blind?

0 of 4 answered

50.4 Taste and smell

pp. 1101–1103

On the AP exam? Background

Taste and smell themselves aren't in the current course, but they run on tested signaling: G protein-coupled receptors, cAMP as a second messenger and ion channels (Topics 4.2 and 4.3). Each smell neuron using just one receptor gene is differential gene expression (Topic 6.6), and pheromones are chemical signals between animals (Topic 8.1).

In the course: Topic 4.2 Introduction to Signal Transduction, Topic 4.3 Signal Transduction Pathways, Topic 6.6 Gene Expression and Cell Specialization, Topic 8.1 Responses to the Environment (notes, videos and more questions)

Key points

Taste (gustation) and smell (olfaction) both rely on chemoreceptors. On land the split is simple: taste handles molecules dissolved in saliva or another liquid, and smell handles molecules floating in air. Animals use these senses to find food, avoid poisons, mark territory and find mates.
Mammals sense five basic tastes: sweet, salty, sour, bitter and umami (a savory taste set off by the amino acid glutamate). Taste cells are modified epithelial cells grouped into taste buds, and any part of the tongue that has taste buds can sense all five. The old 'tongue map' is a myth.
Each taste cell carries one kind of taste receptor. The brain decides 'sweet' or 'bitter' from which cells, and so which nerve pathways, are active.
Sweet, bitter and umami are detected by G protein-coupled receptors. Humans have one sweet receptor and one umami receptor but about 25 bitter receptors, so we can notice many different possible poisons. Sour and salty use ion channels: sour cells have a channel that lets H⁺ ions in, and salt is sensed partly through sodium channels.
Unlike taste cells, smell receptor cells are neurons. One end reaches into the mucus on the roof of the nasal cavity, and the other end is an axon that connects to the brain's olfactory bulb with no relay cell in between.
An odor molecule binds a G protein-coupled odorant receptor, which leads to cyclic AMP being made. cAMP opens ion channels, positive ions rush in, and the neuron fires. Humans have about 400 working odorant receptor genes, and each smell neuron expresses only one of them.
Taste and smell have separate receptors and brain pathways but combine into flavor. Much of what you think of as taste is really smell.

Key terms (11)

gustation: The sense of taste: detecting chemicals dissolved in liquid, usually in the mouth.
olfaction: The sense of smell: detecting chemicals carried in the air (or, for aquatic animals, in water).
tastant: Any chemical that triggers a taste, like sugar, salt or an acid.
odorant: Any chemical that can be smelled. It has to be able to drift through air to reach your nose.
taste bud: A cluster of taste receptor cells, mostly on the tongue. Each bud has cells for all five tastes.
umami: The savory taste, triggered mainly by the amino acid glutamate in foods like tomatoes, mushrooms, soy sauce and broth.
G protein-coupled receptor (GPCR): A membrane receptor that, when a signal binds, switches on a G protein inside the cell to start a relay. Sweet, bitter, umami, smell and vision all use GPCRs.
odorant receptor: A G protein-coupled receptor on smell neurons that binds particular odor molecules. Each smell neuron makes just one kind.
olfactory bulb: The part of the brain where smell neurons send their axons and smell signals are first sorted.
cyclic AMP (cAMP): A small second messenger made inside a cell after a receptor is activated. In smell neurons, it opens ion channels.
pheromone: A chemical one animal releases to send a message to others of its species, such as a signal to attract a mate.

Check yourself: 50.4 Taste and smell

4 questions on 50.4 Taste and smell. Pick an answer to see if you got it, and why.

Question 1 of 4

Domestic cats carry a broken copy of one of the two genes needed to build the sweet-taste receptor. Researchers offered cats and dogs a choice between a bowl of plain water and a bowl of a test solution (invented data). Test solution | Dogs: % drunk from test bowl | Cats: % drunk from test bowl Sugar water | 82 | 51 Bitter solution | 12 | 9 Plain water | 50 | 49 Which conclusion is best supported?

Question 2 of 4

Which sequence correctly describes how an odor molecule causes a smell neuron to fire?

Question 3 of 4

People with two copies of a particular variant of one odorant receptor gene report that a certain compound smells very faint or has no smell, yet they smell other odors normally. Which explanation is best supported?

Question 4 of 4

Humans have about 25 different bitter-taste receptor genes but only one sweet receptor. Which is the most likely evolutionary explanation for the large number of bitter receptors?

0 of 4 answered

50.5 How muscles contract

pp. 1103–1110

On the AP exam? Background

Muscle structure and the sliding-filament model aren't in the current course. What carries over: acetylcholine as a neurotransmitter and ligand-gated channels (Topics 4.1 and 4.2), Ca²⁺ pumped by active transport (Topic 2.8), ATP powering work and coming from respiration or fermentation (Topics 3.3 and 3.5) and a protein's shape setting its job (Topic 1.7).

In the course: Topic 4.1 Cell Communication, Topic 4.2 Introduction to Signal Transduction, Topic 2.8 Mechanisms of Transport, Topic 3.3 Cellular Energy, Topic 3.5 Cellular Respiration, Topic 1.7 Proteins (notes, videos and more questions)

Key points

Muscle contraction depends on two proteins: actin, which forms thin filaments, and myosin, which forms thick filaments. A muscle can only pull. It gets longer again only when something else stretches it.
A skeletal muscle is a bundle of long cells called fibers. Each fiber is one giant cell with many nuclei, the result of many precursor cells merging during development. Fibers are packed with myofibrils made of repeating units called sarcomeres, and their lined-up bands give skeletal muscle its striped (striated) look.
In the sliding-filament model, neither kind of filament gets shorter. Myosin heads grab actin, pull the thin filaments toward the middle of the sarcomere and let go, over and over. The filaments overlap more and the sarcomere shortens.
ATP powers each cycle. Splitting ATP cocks the myosin head into its high-energy position; the head binds actin and pulls; then a new ATP must bind before the head can let go. Muscles restock ATP from creatine phosphate (good for seconds), glycolysis with fermentation (about a minute) and aerobic respiration (long-lasting work).
Calcium is the on switch. In a resting fiber, tropomyosin lies over the spots on actin where myosin would bind. A motor neuron releases acetylcholine, which opens ligand-gated channels on the fiber and starts an action potential. The signal runs down T tubules and makes the sarcoplasmic reticulum release Ca²⁺, which binds troponin and moves tropomyosin aside. Pumps then use ATP to move Ca²⁺ back into storage, and the muscle relaxes.
A single fiber's twitch is all-or-none, but a whole muscle's force is graded. The nervous system adds force by recruiting more motor units (one motor neuron plus every fiber it controls) and by firing faster. When signals come too quickly for the fiber to relax in between, the twitches stack up and blend into a steady, maximal pull called tetanus.
Fibers differ: slow oxidative fibers have many mitochondria and lots of myoglobin and resist fatigue, while fast glycolytic fibers are powerful but tire quickly. Cardiac muscle is striated, sets its own rhythm and is linked by gap junctions. Smooth muscle has no stripes, contracts slowly and uses calmodulin instead of troponin to respond to Ca²⁺.

Key terms (15)

actin: The protein that makes up the thin filaments in muscle. Myosin heads grab it and pull.
myosin: The motor protein that makes up thick filaments. Its heads use energy from ATP to pull actin filaments along.
muscle fiber: A single skeletal muscle cell: long, with many nuclei, and packed with myofibrils.
myofibril: A long strand inside a muscle fiber made of sarcomeres lined up end to end.
sarcomere: The basic repeating unit of a myofibril and the smallest part of a muscle that contracts.
sliding-filament model: The explanation of contraction in which the two kinds of filaments glide along each other without changing length, shortening the sarcomere.
cross-bridge: The link formed when a myosin head attaches to actin. Repeated cross-bridge cycles drive contraction.
tropomyosin: A thin protein strand lying along actin that blocks myosin's binding sites while the muscle is relaxed.
troponin complex: A set of proteins on the thin filament that binds Ca²⁺ and then shifts tropomyosin to expose actin's binding sites.
sarcoplasmic reticulum (SR): The muscle cell's specialized endoplasmic reticulum. It stores Ca²⁺, releases it to start contraction and pumps it back to end it.
T tubules: Inward folds of a muscle fiber's membrane that carry action potentials deep into the cell, close to the SR.
acetylcholine: The neurotransmitter motor neurons release onto skeletal muscle. It opens channels that start the fiber's action potential.
motor unit: One motor neuron together with every fiber it connects to. They always contract together.
tetanus: A steady, maximal contraction that happens when action potentials arrive so fast the fiber never relaxes between them. (Tetanus is also an illness, in which a toxin from soil bacteria causes muscle spasms.)
myoglobin: A red, oxygen-storing protein inside muscle cells. Its affinity for O₂ is higher than hemoglobin's, so O₂ that the blood releases ends up held in the muscle.

Check yourself: 50.5 How muscles contract

4 questions on 50.5 How muscles contract. Pick an answer to see if you got it, and why.

Question 1 of 4

An experimental drug locks tropomyosin in place over actin's myosin-binding sites, even when Ca²⁺ binds troponin. A motor neuron then fires repeatedly onto a treated muscle fiber. What is the most likely result?

Question 2 of 4

Muscle fibers were treated so their membranes let small molecules in freely, then placed in different solutions (invented data). Solution | What happened to the fibers Ca²⁺ and ATP | Contracted; relaxed once Ca²⁺ was removed ATP, no Ca²⁺ | Stayed relaxed and could be stretched easily Ca²⁺, no ATP | Became stiff and could not be stretched Which conclusion best explains all three results?

Question 3 of 4

Question 4 of 4

Muscles that move the eye have motor units of about 10 fibers each. A large thigh muscle has motor units of about 1,000 fibers each. Which statement best explains how this difference suits each muscle's job?

0 of 4 answered

50.6 Skeletons and moving around

pp. 1110–1115

On the AP exam? Not tested

Skeleton types and locomotion aren't in the current course. Useful links: volume growing faster than area as size goes up (Topic 2.2), smaller animals spending more energy per gram (Topic 8.2), chitin as a structural carbohydrate (Topic 1.4) and body shapes as adaptations (Topic 7.2).

In the course: Topic 2.2 Cell Size, Topic 8.2 Energy Flow Through Ecosystems, Topic 1.4 Carbohydrates, Topic 7.2 Natural Selection (notes, videos and more questions)

Key points

Muscles can only pull, so moving a joint back and forth takes an antagonistic pair: one muscle bends the joint and another straightens it. The same push-pull setup works whether the muscles attach to bones or to the inside of hard tubes, as in a crab's claw.
Skeletons also hold the body up and protect soft organs, the way a turtle's shell shields everything inside it.
A hydrostatic skeleton has no hard parts at all: muscles press on a trapped body fluid, and because water barely compresses, squeezing the body in one direction makes it stretch in another. Worms and sea anemones use them. They suit soft-bodied animals that swim, crawl or dig, but a fluid-filled body can't stand up on legs.
An exoskeleton is a hard outer covering. A snail enlarges its calcium carbonate shell by adding new material along the rim. An insect's or crab's jointed cuticle, made of chitin fibers in a protein matrix, can't grow, so the animal must molt and build a bigger one.
An endoskeleton is a hard frame buried in soft tissue, such as the cartilage and bone of vertebrates or the mineral plates under a sea urchin's skin. Ligaments connect bones at joints, and tendons attach muscles to bones.
Size matters. Double an animal's length and its mass goes up about eightfold, but a leg bone's cross-section, and so its strength, goes up only fourfold. Large mammals cope partly with thicker bones but mostly with posture: upright, pillar-like legs let muscles and tendons carry much of the weight.
Moving means working against gravity and friction. Water holds swimmers up but resists them, so streamlining pays off; walkers must hold themselves up against gravity; fliers need wings shaped to make lift. Moving a kilogram of body one meter is cheapest in water and most expensive on legs, and a large animal pays less per kilogram than a small one using the same mode.

Key terms (13)

antagonistic muscles: A pair of muscles that move a joint in opposite directions, like one that bends your elbow and one that straightens it.
skeleton: A supporting structure that muscles pull against. It can be fluid-filled, external or internal.
hydrostatic skeleton: Support from a body fluid that muscles squeeze against, used by soft-bodied animals such as worms and sea anemones.
peristalsis: Movement driven by a ripple of muscle squeezes that travels along a tube-shaped body. Worms crawl this way, and your gut uses it to move food.
exoskeleton: A hard covering on the outside of the body, such as a snail's shell or a beetle's cuticle.
cuticle: The tough outer covering of arthropods: chitin fibers set in protein, stiff on the plates and flexible at the joints.
chitin: A tough, nitrogen-containing polysaccharide that forms arthropod exoskeletons and fungal cell walls.
molting: Shedding an old exoskeleton and making a larger one. Arthropods can only grow this way.
endoskeleton: A hard supporting frame inside the body, such as the bones and cartilage of vertebrates.
ligament: Tough connective tissue that holds bones together at a joint.
tendon: Tough connective tissue that attaches a muscle to a bone, passing the muscle's pull to the skeleton.
locomotion: An animal moving its whole body to a new place, whether by swimming, walking, running, crawling or flying.
drag: The friction-like resistance an animal meets moving through water or air. Streamlined bodies cut it down.

Check yourself: 50.6 Skeletons and moving around

4 questions on 50.6 Skeletons and moving around. Pick an answer to see if you got it, and why.

Question 1 of 4

In an isolated frog leg, an electrode stimulates only the muscle on the back of the thigh, and the knee bends. When stimulation stops, the knee stays bent. What must happen for the knee to straighten again?

Question 2 of 4

A peanut worm is a soft marine worm with one large, fluid-filled body cavity. It changes shape and burrows by squeezing that fluid with circular and lengthwise muscles. If a small wound let much of the fluid leak out, which effect would be most likely?

Question 3 of 4Calculator allowed

Imagine a lizard species whose body shape stays the same while its length doubles. Its weight depends on its volume, and the strength of its leg bones depends on their cross-sectional area. By what factor would the weight pressing on each square centimeter of leg bone increase?

Question 4 of 4

A biologist measured the shell width of one crab over several months (invented data). Day | Shell width (mm) 0 | 30 20 | 30 40 | 30 42 | 37 70 | 37 90 | 37 92 | 44 Which explanation best accounts for this pattern of growth?

0 of 4 answered