ScienceWeek

BIOPHYSICS: ON ANIMAL LOCOMOTION

The following points are made by Michael Dickinson (Nature 2003 424:621):

1) A glance at the surface of a pond reveals one of the more delightful images of summer: the shimmering ripples made by the graceful strokes of water striders. Water striders are insects that are adapted for locomotion and foraging on top of still water. Long hairy legs keep these animals afloat, but how do they glide so effortlessly across the surface? Models of water-strider locomotion have proposed that the animals move forwards by creating surface waves that carry momentum backwards. But recent work shows that this view is, quite literally, superficial. Like the oars of a rowing-boat, a water strider's legs create swirling vortices that carry momentum beneath the surface of the water. It is the rearwards motion of these vortices, and not the surface waves, that propels the animal forwards. This insight solves a paradox related to the motion of juvenile water striders, and helps to form a more cohesive picture of animal locomotion.

2) Much of animal locomotion distills down to a simple application of Newton's third law: to move forwards, animals must push something backwards. Just what that something is depends on the form of locomotion. Large terrestrial animals push against the solid ground, creating reaction forces in the opposite direction. The situation is a bit more complicated for swimming and flying animals, which must push against a fluid (from a physical standpoint, both air and water are fluids). As a fin or wing flaps, the fluid yields to form a pattern of swirling vortices. In some cases, the energy transmitted by an animal to the fluid in one stroke takes the form of a discrete vortex: a doughnut-shaped structure like a smoke ring. Birds, bats, insects and fish have all been shown to create a series of vortices as they move, although the precise arrangement may be complex and notoriously difficult to quantify. By carefully measuring the size, strength and velocity of the vortices generated during each stroke, it is theoretically possible to reconstruct the average force with which an animal propels itself through the air or water.

3) But what about tiny creatures such as water striders, that live between air and water? What do they push backwards to move forwards, and how do they stay afloat in the first place? Resting statically on the water surface poses no great problem to a small organism. Surface-tension forces at an air–water interface result from the mutual attraction of water molecules through hydrogen bonds. As the wax-covered, hairy legs of an insect dimple the water downwards, these surface-tension forces push the animal upwards. Whereas the total upward force is proportional to the perimeter of contact, which increases linearly with body size, the mass of the animal that must be supported by the tension forces increases proportionally to the cube of its body length. Thus, whereas a small water strider can easily stand atop water, larger insects need proportionally longer legs to ensure that they can rest on the surface with suitable safety. This requirement for ever-longer legs limits the maximum size of a water strider to about 25 centimeters.

Nature http://www.nature.com/nature

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CELL BIOLOGY: SUPRAMOLECULES AND BIOLOGICAL MOVEMENTS

Directed motion is one of the more dramatic characteristics of many living systems, and the movements of simple organisms, particularly of single-celled organisms, have fascinated biologists ever since the invention of the microscope. Under a microscope, a motile protozoan may appear as large as a rabbit, but there are no muscles or nerves in the single cell that constitutes such an organism, and the riddle is clear: How is chemical energy transduced to directed mechanical and kinetic energy in primitive biological systems? Until the era of the electron microscope and molecular biology, little progress was made in answering this question. That has changed: in recent decades molecular biology has provoked a renaissance in studies of cell movements. But if much has been learned and the questions refined, our fascination with motion in primitive organisms has grown rather than diminished. For it has become apparent that directed motions in primitive biological systems are examples of molecular-scale engineering that is often astonishing.

Vorticella, discussed below, is a ciliated protozoan common in ponds, a single-celled organism that can be envisioned as follows: Imagine a bell-shaped body 50 microns at its widest part. The rim of the open end of the bell is covered with cilia that beat synchronously to sweep water and nutrients into the open end of the body. The closed dome end of the bell is attached to a long thin stalk that may be 500 or more microns in length, and the far base of the stalk is attached to a leaf or to pond debris. When the organism is feeding, the stalk is extended. When the organism is physically or chemically disturbed, the stalk contracts like a spiral-shaped spring, quickly drawing the bell-shaped body of the organism to the protection of the debris where it is attached.

First described by Anton van Leeuwenhoek (1632-1723), Vorticella is a legendary organism in biology. Many children receive inexpensive microscopes as gifts when they are ten or eleven years old, and these children often use their new microscopes to examine everything available, including local pond water. At the first sight of Vorticella -- the lovely bell-shaped body with its synchronously beating cilia, the body at intervals suddenly pulled back by the contracting spring of the stalk, the stalk and body then slowly extending again with the cilia resuming their synchronized beating -- such children are often spellbound by the dynamic world of the small. If the fascination endures, and if they are fortunate in life, they often become biologists.

The following points are made by L. Mahadevan and P. Matsudaira (Science 2000 288:95):

1) The retraction of the stalk of Vorticella (and of other ciliates of this type: peritrich ciliates) is caused not by the sliding action of a motor protein but by a spring that operates according to a simple mechanism: the entropic collapse of polymeric filaments. Although they are considered unusual engines for motility, springs and ratchets composed of filaments and tubules power many of the largest, fastest, and strongest cellular and molecular movements. Just as muscles magnify forces and movements by a geometrical hierarchy, these unusual mechanochemical engines use a similar principle: small changes in a protein subunit are amplified by the linear arrangement of proteins in filaments and bundles. The authors suggest that, considering the biochemical and physical characteristics of several known molecular springs and ratchets, they apparently represent ancient and biologically commonplace molecular engines.

2) In general, biological springs are active mechanochemical devices that store the energy of conformation of proteins in certain chemical bonds that act as latches. In the absence of an external force, the potential energy is released and converted into mechanical movement when the chemical bonds are broken.

3) The contractile avoidance reaction of Vorticella, first described by Leeuwenhoek in 1676, is a dramatic example of an active mechanochemical spring. The body of Vorticella is attached to a leaf or to debris by a long slender stalk. Within the stalk lies a rod-like helical cytoplasmic organelle, the "spasmoneme". In its extended state, the spasmoneme is 2 to 3 millimeters long, depending on the species of ciliate. When exposed to calcium ions, but to no external energy source, the spasmoneme contracts in a few milliseconds to 40 percent of its length at velocities approaching 8 centimeters per second. Based on the hydrodynamics, the force of contraction is of the order of a millidyne, whereas the power generated is a few milliergs per second. In terms of specific power per unit mass, the spasmoneme is among the most powerful biological engines.

4) The authors state: "The dynamics and energetics of biological springs and ratchets are dominated by factors that are inconsequential on the large length scales associated with our everyday world. In a [biological] cell, viscous forces, Brownian motion, short-range hydrophobic interactions, screened electrostatics, and steric effects influence the kinetics of filament and subunit diffusion and growth. In this soft, wet, and dynamic world, structural features are dominated by filamentous and membranous objects, a constant reminder that all events at this level are mediated by interfacial interactions."

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