Proteins Transmit Force

woman in red athletic outfit throwing wooden javelin midair during the daytime

Biological Strategy

Vertebrates (Mammals, Fish, Birds, Reptiles)

Eleanor Banwell

Image: Tableatny / Flickr / Some rights reserved

Physically Assemble Structure

Living systems use physical materials to create structures to serve as protection, insulation, and other purposes. These structures can be internal (within or attached to the system itself), such as cell membranes, shells, and fur. They can also be external (detached), such as nests, burrows, cocoons, or webs. Because physical materials are limited and the energy required to gather and create new structures is costly, living systems must use both conservatively. Therefore, they optimize the structures’ size, weight, and density. For example, weaver birds use two types of vegetation to create their nests: strong, a few stiff fibers and numerous thin fibers. Combined, they make a strong, yet flexible, nest. An example of an internal structure is a bird’s bone. The bone is comprised of a mineral matrix assembled to create strong cross-supports and a tubular outer surface filled with air to minimize weight.

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Manage Shear

The effect of shear stress on a living system is parallel internal surfaces sliding past each other. Slippage occurs in parallel with the force. Think about holding two wooden boards on top of each other and sliding one to the right and the other to the left. This may be easy until you add glue, which increases their shear strength and makes them harder or impossible to slide. Shear can occur in solids, liquids, and gases. Living systems must increase their shear strength to overcome these types of forces. For example, darkling beetles lock their wings together in flight to prevent lateral movement by using many small hairs on each wing. These hairs interlock to provide shear strength, just as two hair brushes put together would be difficult to slide past each other.

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Manage Tension

When a living system is under tension, it means there is a force pulling on it, like a person pulling on a rope tied to a horse. When applied to a living system, unless the system is completely rigid, the result is that it gets stretched. If stretching exceeds the strength of the living system’s material, it can damage it. Living systems manage tension using materials that are flexible and stretchable enough to survive most tension that occurs in their environment. The ocean’s intertidal zone offers a good example. The waves and incoming and outgoing tides put tension on soft-bodied organisms. Mussels resist tension with flexible threads that hold them onto rocks; in contrast, large algae have stretchy fronds.

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Attach Permanently

A living system can conserve energy by attaching permanently to a particular site because it can take advantage of resources that come its way, rather than expending energy to move to resources. A permanent attachment, intended to last the lifetime of the living system, creates special challenges. For example, physical mechanisms, such as the anchor that holds a marine algae to the ocean’s bottom, must be able to withstand forces that can pull it off its substrate. Chemical mechanisms, such as a barnacle’s glue, must avoid both physical and chemical breakdown, such as being dissolved by water.

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Move in/Through Gases

Living systems must move through gases (which are less dense than liquids and solids) such as those in the earth’s atmosphere. The greatest challenge of moving in gases is that because the living system is heavier than the gas, it must overcome the force of gravity. Moving efficiently in this light medium presents unique challenges and opportunities for living systems. As a result, they have evolved countless solutions to optimize drag and increase lift so that they can stay aloft and take advantage of variable currents. Additionally, they must overcome gravity when moving from a liquid or solid into the air. The fairyfly, the smallest known insect, is a tiny wasp that must move through the air. To the wasp, air feels like a heavy liquid and to move through it, it uses special feathery oars rather than wings.

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Vertebrates (Mammals, Fish, Birds, Reptiles)

Subphylum Vertebrata (“jointed”): Mammals, fish, birds, reptiles

Vertebrates are a subgroup of chordates, which all have a flexible, rod-shaped structure that supports the body called a notochord. Non-vertebrate chordates include tunicates, hagfish, and lancelets. In vertebrates the notochord ultimately becomes part of the spine, usually encased in bony joints. All chordates also have a dorsal hollow nerve cord that forms the nervous system and pharyngeal slits that open outside the body during development (and persist to form gills in aquatic animals). Lastly, chordates have a tail at the back of the body––it’s just that sometimes you need an x-ray to see it.

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Muscle proteins in vertebrates ensure efficient force transmission because they occur along the length of muscle fibers

Muscles are how we, and almost all animals, move. As animals cannot get energy from the sun, movement is crucial for finding food, as well as for reproducing: without muscles, we cannot survive.

Muscles are complex tissues. Muscle cells are packed full of tiny cylinders of specialized contractile proteins. These cylinders, called sarcomeres, are arranged end to end into myofibrils. Muscle cells, or myofibers, contain many myofibrils. Inside a single muscle, there are multiple hierarchical levels of bundling: myofibrils are bundled into myofibers, then multiple myofibers are bundled into fascicles and then several fascicles are bundled together to form whole muscles. At each hierarchical level, each bundle is covered with a layer of connective tissue that supports the fibers inside it by supplying energy and oxygen via blood vessels and control via nerves.

Muscle contraction occurs at the level of the individual sarcomeres. Although each tiny cylinder only contracts by a minuscule amount, once that contraction is repeated down the entire length of the myofibril and across every fibril, myofiber and fascicle all at once, muscles are able to contract by large amounts and generate relatively huge movements.

In order to transmit large movements to the skeleton, muscles must be attached to bones. In vertebrates, this attachment is via tendons. Muscles are capable of generating very large forces. For example, in humans, the masseter, or jaw muscle, is capable of generating a force of over 170 lbs at the molars. Remarkably, this force is transmitted entirely via vast numbers of weak non-covalent bonds between proteins that connect the sarcomeres inside muscle cells to the connective tissue outside.

There are a large number of proteins that are crucial for force transmission in muscles. Two of the most important are laminins, which are found in the connective tissue that coats each hierarchical bundle, and integrins, which are membrane-spanning proteins that connect indirectly to the sarcomeres themselves and transmit the force through the cell membrane to the laminins outside. These proteins are found along the length of all the muscle fibers as well as at the junction between the muscle and the tendon (the myotendinous junction). The spread of these force-transmitting proteins along the length of the myofibers ensures that, when sarcomeres contract, the force is spread evenly. In this way, the force at the myotendinous junction is transferred extremely efficiently.