Support and Locomotion

Lessons

Senior 4 · Biology

Support and Locomotion

5 lessons

Lesson 1 of 5

Locomotion and movement without muscles

25 min

Three swimmers in a drop of pond water

Put a drop of water from a pond, or from a vase where flowers have stood for a week, under a microscope. An amoeba creeps across the slide, its whole body flowing into finger-like bulges. A slipper-shaped Paramecium darts past, spinning as it goes, then suddenly backs away from an obstacle. A tiny green Chlamydomonas zigzags towards the light. None of them has a single muscle, yet all three travel. This unit is about locomotion: what it requires, and how it is done in water, on land and in the air. We begin with the smallest movers.

Definition

Movement: a change in position of the whole organism or of part of it (a leaf turning, an arm bending). Locomotion: movement of the whole organism from one place to another.

Support: holding the body in shape and up against gravity, and giving muscles or other motors something firm to pull or push against.

Drag: the force that resists motion through water or air. Streamlined: shaped so that drag is small.

Pseudopodium (plural pseudopodia, "false foot"): a temporary bulge of cytoplasm used for amoeboid movement and feeding.

Cilia and flagella: hair-like extensions of eukaryotic cells, with the same internal "9 + 2" arrangement of microtubules. Cilia are short and many; flagella are long and few.

Why move, and what locomotion requires

Animals and many protists move to find food, water and mates, to escape predators and bad conditions (heat, drought, poisons), and to spread to new places. Plants and fungi cannot travel; they disperse seeds and spores instead.

Requirement	Why it is needed
A propulsive force	By Newton's third law, an organism moves forward only by pushing backwards on something: the water, the air or the ground. The force comes from muscles or, in single cells, from protein motors.
Support	A skeleton, a stiff cell surface or fluid under pressure keeps the body in shape and transmits the force.
Coordination	Muscles, legs, fins or cilia must act in the right order; in animals the nervous system controls them.
Energy	Every contraction or beat uses ATP from respiration.
Overcoming resistance	Drag in water and air (reduced by streamlining); friction and gravity on land.

Medium	Density	What it means for locomotion
Water	About $1000\,\mathrm{kg\,m^{-3}}$	Buoyancy supports most of the body's weight; but water is dense and viscous, so drag is large: streamlining matters
Air	About $1.2\,\mathrm{kg\,m^{-3}}$	Almost no support: a flier must make lift; little drag
Land	Solid	The full weight must be held up by a skeleton and limbs; the ground gives grip (friction) to push against

Worked example 1: how much does water hold you up?

A fish's body has a density of $1050\,\mathrm{kg\,m^{-3}}$ without a swim bladder. (a) What fraction of its weight must it still support in fresh water (density $1000\,\mathrm{kg\,m^{-3}}$ )? (b) How many times denser is water than air?

(a) The upthrust equals the weight of water displaced, so the unsupported fraction is $\frac{1050 - 1000}{1050} = \frac{50}{1050} = 0.048$ , only 4.8 % of its weight.
(b) $\frac{1000}{1.2} \approx 830$ times. Air gives almost no support, which is why flying animals need lift and very light bodies.

Movement without muscles

Amoeboid movement is used by amoebae and by human white blood cells moving through tissues. The cytoplasm has a stiff outer layer, the ectoplasm (a gel), and a runny inner part, the endoplasm (a sol). At the front, the cell pushes out a pseudopodium; endoplasm streams into it and turns to gel at its tip, while at the rear gel turns back to sol and is squeezed forward (Figure 1). The pseudopodium sticks to the surface, and the cell flows after it. The forces come from protein filaments of actin that assemble at the front and from myosin that contracts the rear, using ATP.

Figure 1. Amoeboid movement (simplified).

Ciliary movement: Paramecium is covered by thousands of cilia. Each one beats in two phases (Figure 2): a fast, stiff power stroke that pushes water backwards, and a slow, bent recovery stroke close to the cell that pushes little water back. Neighbouring cilia beat slightly out of step, so waves pass over the surface, like wind over a field of grass (a metachronal rhythm). If Paramecium bumps into an obstacle, its cilia reverse their stroke and it swims backwards briefly, then sets off in a new direction. In humans, cilia do not move the body but move fluids over it: mucus up the trachea, and the egg along the oviduct.

Figure 2. The beat of a cilium, and the 9 + 2 structure that bends it.

Flagellar movement: a eukaryotic flagellum has the same 9 + 2 structure as a cilium but is much longer. Waves of bending pass from its base to its tip and push water backwards, driving a sperm cell or Euglena forwards. Chlamydomonas pulls itself through the water with two front flagella in a breaststroke-like beat. In both, dynein arms use ATP to slide neighbouring microtubule pairs past each other, which bends the whole structure. Bacterial flagella are different: a rigid, helical protein filament with no microtubules, spun like a propeller by a rotary motor in the cell wall.

A living Paramecium caudatum under a light microscope: a slipper-shaped cell whose whole surface is fringed with a fine haze of short cilia, visible along its edges. Photo: Deuterostome, CC BY-SA 3.0, via Wikimedia Commons

Worked example 2: comparing speeds

Illustrative values: a Paramecium $200\,\mu\mathrm{m}$ long swims at $1000\,\mu\mathrm{m\,s^{-1}}$ ; an amoeba $500\,\mu\mathrm{m}$ long creeps at $2\,\mu\mathrm{m\,s^{-1}}$ . (a) Express each speed in body lengths per second. (b) How long would each take to travel 1 cm?

(a) Paramecium: $\frac{1000}{200} = 5$ body lengths per second. Amoeba: $\frac{2}{500} = 0.004$ body lengths per second.
(b) $1\,\mathrm{cm} = 10\,000\,\mu\mathrm{m}$ . Paramecium: $\frac{10\,000}{1000} = 10\,\mathrm{s}$ . Amoeba: $\frac{10\,000}{2} = 5000\,\mathrm{s} \approx 1.4\,\mathrm{h}$ .
Thousands of beating cilia push water far faster than cytoplasm can flow into pseudopodia. Speed in body lengths per second is a fair way to compare organisms of different sizes.

Worked example 3: explaining the backward escape

When a Paramecium hits an obstacle, it swims backwards for a moment, turns, and swims forward again. Explain this in terms of its cilia.

Touching the obstacle changes the cell's membrane, and the cilia reverse the direction of their power stroke.
Now the power stroke pushes water forwards, so by Newton's third law the cell moves backwards.
When the cilia return to their normal beat, the cell, now pointing in a slightly different direction, swims forwards again. Repeating this lets it find a way round obstacles with no brain at all.

Journey to the Microcosmos: "Paramecium: The White Rat of Ciliates". Watch the living cilia beating and Paramecium backing away and turning when it meets an obstacle.

Warning

Common mistakes

Using "movement" and "locomotion" as if they mean the same. A plant moves its leaves, but only organisms that travel show locomotion.
Saying cilia "row" both ways equally. The power stroke and the recovery stroke differ; that difference is what gives a net push.
Describing a bacterial flagellum as a 9 + 2 structure. Only eukaryotic cilia and flagella have microtubules; bacterial flagella rotate.
Forgetting energy: amoeboid, ciliary and flagellar movement all use ATP.

Tip

Exam technique. For any kind of locomotion, answer the same four questions: what pushes (cilia, flagellum, fins, legs, wings), against what (water, air, ground), what supports (cell surface, skeleton, fluid pressure) and what supplies energy (ATP). Then add how drag or gravity is overcome.

✦

Practice: locomotion and movement without muscles

Distinguish between movement and locomotion, with an example of each.
State four requirements for locomotion and explain one of them.
Describe how an amoeba moves, using the terms ectoplasm, endoplasm and pseudopodium.
Explain why a cilium produces a net push even though it moves back and forth.
A Euglena $50\,\mu\mathrm{m}$ long swims at $150\,\mu\mathrm{m\,s^{-1}}$ (illustrative). Calculate its speed in body lengths per second and the time it takes to swim 3 mm.
Compare a eukaryotic flagellum with a bacterial flagellum.
Suggest why white blood cells use amoeboid movement rather than flagella to reach an infection in the tissues.

Show answersHide answers

Movement is a change in position of part or all of an organism (a sunflower head turning to the light); locomotion is movement of the whole organism from place to place (a fish swimming).
Any four: a propulsive force, support, coordination, energy (ATP), ways to overcome drag, friction or gravity. For example: by Newton's third law, the organism must push backwards on water, air or ground to be pushed forwards.
The fluid endoplasm streams forward into a bulge, the pseudopodium, where it turns into gel-like ectoplasm; at the rear, ectoplasm turns back into endoplasm and flows forward. The pseudopodium attaches to the surface and the cell flows after it; actin and myosin, powered by ATP, drive the flow.
In the power stroke the cilium is stiff and sweeps fast through the water, pushing a lot of water backwards; in the recovery stroke it bends and returns slowly close to the cell, pushing little water. The backward push is greater than the forward push, so the cell moves forwards.
$\frac{150}{50} = 3$ body lengths per second; $3\,\mathrm{mm} = 3000\,\mu\mathrm{m}$ , $\frac{3000}{150} = 20\,\mathrm{s}$ .
Eukaryotic: 9 + 2 microtubules inside the cell membrane, bends in waves driven by dynein and ATP. Bacterial: a rigid hollow protein helix outside the cell, with no microtubules, rotated by a motor at its base.
Tissues are crowded, solid spaces, not open water; amoeboid movement lets a cell change shape, squeeze between other cells and out of blood vessels, and crawl along surfaces, where a flagellum would have nothing to swim in.

Quick check

Match each organism to its main way of moving.

Tap one on the left, then its partner on the right.

Key points

Locomotion needs a propulsive force pushing against water, air or ground, support, coordination and energy, and must overcome drag, friction or gravity.
Water supports the body but resists motion; air gives little support; on land, the skeleton must carry the full weight.
Amoeboid movement: endoplasm flows into pseudopodia (actin and myosin). Ciliary and flagellar movement: 9 + 2 microtubules bent by dynein using ATP; a power stroke and a recovery stroke give net thrust. Bacterial flagella rotate.

Sources and credits

5.3.3: Characteristics of Protists and 2.1.6: The Cytoskeleton, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for protist motility, pseudopodia, cilia and flagella. Adapted by Ganzaa.
"Paramecium" and "Amoeboid movement", Wikipedia contributors, licensed under CC BY-SA 4.0. Used for the avoidance reaction and the mechanism of amoeboid movement. Adapted by Ganzaa.
Figures 1 and 2, the tables, worked examples (the speeds and the fish density are illustrative), practice and quick check: written by Ganzaa.
Images: SmallRex, CC BY-SA 4.0; Deuterostome, CC BY-SA 3.0; Dartmouth Electron Microscope Facility, public domain; all via Wikimedia Commons.
Video: "Paramecium: The White Rat of Ciliates", Journey to the Microcosmos, embedded from YouTube.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Three swimmers in a drop of pond water

2Why move, and what locomotion requires

3Movement without muscles

Three swimmers in a drop of pond water

Why move, and what locomotion requires

Movement without muscles