Mobility of Life Forms

in early work

currently only a section about virions

Mobility of Virions

Mobility of Virions

in work and to be extended, April 2021

Once virions are released from a cell the virions are moved by whatever movements are in their surroundings.

Swim with the Tide

All forms of life on earth are water based. Both protein and RNA/DNA synthesis occur in water. So virions are pro therefore start out their journey in watery fluids, often the cytoplasm of a cell. Upon release they are exposed to whatever substances there are around. For most viruses their virions will always be dissolved in watery fluids and will not survive being dissolved in other fluids such as alcohol or air.

Fluid dissolved in:
- Salty water in oceans: Water in the oceans is also likely where life and RNA viruses as an early form of life developed and still contains a huge number of virions.
- Water in lakes and rivers: These waters serve as an intermediate place for many virions.
- Fluid in an organism: Virions of viruses infecting multicellular life first start out in the fluids of the hosts and in many cases never leave the host fluids.
- Water droplets can be dissolved in air.
- Air is challenging to use for viruses due to dilution and activation factors such as sunlight, washed out by rain …
Material attached to: These can be any solid material e.g. a part of living organism or any kind of non living matter. The movement of virions follows the movement of the materials attached to. These particles are in turn dissolved in fluids, mostly water, or attached to other particles.

Movement within a Host

Often virions get moved within a host. Some even evolved not to leave the host organism at all. Within a host the movement of virions depends on the location they are released and the kind of material they are attached to:

movements with fluid flows:
- the blood flow if the virion is in the bloodstream
- the mucus flow
- respiratory tract lining fluid
- the air breathed in and out and the particle dynamics in the respiratory tract associated.
solid movements within the host i.e. the virion is attached to a moveable part:
- inside cells by the centrosome. Especially important for viruses which need to enter the cell nucleus e.g. influenza viruses.
- attached to particles get moved around within a host.
- the movement of body parts e.g. an extremity (hands and feet) or the lungs.
Brownian Motion: The Brownian motion of viruses is slow and by Brownian motion alone a virion usually just moves a few micro meters to reach close by cells. However if given enough time virions can diffuse to many locations both within tissues or within cells.

Movement between hosts

[in work] Many forms of life cannot move well on their own: Plants, bacteria and fungi. In the oceans this is less a problem thanks to the currents. On land however, all forms of life with no ability to move have the challenge to spread and genetically interact. The main options are the same for them all: Being carried by wind, water or moving organisms. Virions have to rely on this options to spread through a host populations:

Moveable Organism: This is often the host where the virus was built but it can be an animal primarily transporting the virion called a transporter.

Transporter

A transporter for a virus is an organism which is important for its transport function in the life cycle of a virus.

When the transporter moves the virions move with it. The farther, faster and the more possible hosts at new location the better for the virus to distribute. Optimal transportation are migrating and/or fare flying animals: flying insects, birds and bats. The usage of horses and the inventions of trains, cars and airplanes made us humans new optimal hosts. Jetting around the world from city to city is optimal for viruses since: fast travel to a new set of hosts.

Wind: Many plants rely on wind for spread. Some viruses too, but not many since wind driven movement has challenges:
- The movements induced by wind are random in the sense that evolutionary adaption to directional movement is hardly possible. => Organisms relying on wind usually compensate this randomness often with huge numbers. As can be seen each season by the pollen powdering the landscape.
- The air contains reactive oxygen and sunlight exposure is frequent. Therefore genetic material needs to be well protected.
- Particles need to have large surface in comparison to their mass, else gravity will ground them (explained in Objects Moving in the Air). Except for very small particles only dry particles have a surface large enough to fly.
Different life forms solved these challenges:
- Many plants rely on wind both for genetic recombination and spread.
- Viruses spread with the winds is quite rare, since
  - The large numbers needed to compensate the undirected movement with the wind, are hard to achieve for viruses.
  - viruses since they don’t encode their entire metabolism have fewer options to optimize for stability and survive the harsh life condition in the air.
  - Viruses and especially RNA viruses aren’t stable in sunlight.
  Exception: smallpox. Coronaviruses may do so in theory.
Water: Water movement is directional to the oceans and particles need to be “fished out” not to end up there. One of the few escape options are drinking animals.
- Land-plants rarely use water since ending up in the oceans is not the goal of land plants.
- Viruses infecting animals rely on water. Animals drinking brings them directly to possible hosts.

Travel Probabilities

Getting matched to an infectable cell involves the following steps (steps in […] are facultative):

[reach a host]
[reach a tissue]
reach an infectable cell

Each step has usually a low success rate. So the combined probability of reaching a target tends to be very small.

The probability to reach a next exploitable host, is especially low, due to many traps and hostile environment: