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Most tissue is comprised of cells. Why? It would seem inefficient to have so many individual nucleus, membranes, etc.?

Specifically: Not all tissue is cellular. Much tissue is extracellular matrix. Since the body can use noncellular tissue, what is the role of cells in cellular tissue?

Another way to ask my question: Do the cellular organelles directly contribute to the tissue and organ's function? Or are they just there to maintain the cell itself?

My hypothesis is:

  1. In some cases, the cellular organelles directly provide the tissue's function. Eg endocrine system - synthesized at the cellular level.
  2. In other cases, individual cells function discretely in the tissue. Eg, nervous or muscular tissue.
  3. In other cases, the cell doesn't directly support the tissue's function. But it's needed to grow and preserve the tissue. Eg connective tissue.
  4. And finally, in cases where preserving and growing the tissue is a small enough task, a small number of cells simply extracellular tissue. Eg bone tissue.

The above is my hypothesis. Is it correct? Fix, revise, or replace it with something better!

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I am not sure I get your premise. Would you think that one gigantic cell would be more efficient then? –  nico Sep 16 '12 at 7:12
    
Why "one gigantic cell"? Why cell at all? Why have zillions of small parts (nucleous, membranes) which don't directly contribute to tissue function? As a note, very few artificial machines work that way (though perhaps integrated circuits could be argued to). –  S. Robert James Sep 16 '12 at 13:46
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membranes are extremely important for things like compartimentalization and signalling. For instance, neurons could not work without membranes, as they are required to generate electrochemical gradients. One very important signalling pathways is that of DAG/IP3, one of the components of the membrane. Membranes allow for storage of calcium ions in high concentrations. And I could go on listing other millions of functions. –  nico Sep 16 '12 at 13:53
    
Also, the comparison of a cell to a machine is not really a good one. A cell is more like a factory, where you have the CEO's office (the nucleus), an assembly line (the ER) with workers (ribosomes), a packaging department (Golgi), power plants (mitochondria) and so on (blatantly copied from this PDF) –  nico Sep 16 '12 at 13:58
    
Also, I wouldn't trust anything artificial longer than 100 years, from experience. Eventually something gives, and then where is the repairman if it cannot repair itself like a cell that simply divides to survive? –  rwst Sep 16 '12 at 14:05

2 Answers 2

Biological tissue by definition is tied to cellularity. To start this post, it might be useful to include important definitions:

A definition from wikipedia:

Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of similar cells and from the same origin, that together carry out a specific function

The ECM or extracellular matrix, as the name suggest is a matrix.

WP: In biology, matrix (plural: matrices) is the material (or tissue) between animal or plant cells, in which more specialized structures are embedded, and a specific part of the mitochondrion that is the site of oxidation of organic molecules. The internal structure of connective tissues is an extracellular matrix.

Lastly,in chemistry, matrix generally refers to any chemical species which is not of direct immediate regard to the context of the observer (e.g. a chemist), but may or may not have significant effect on the chemistry or chemical environment in question.


Do the cellular organelles directly contribute to the tissue and organ's function?

No, they do not directly contribute to the tissue and organ's function but organelles contribute indirectly:

There is a hierarchy of structure: Everything is composed of smaller things and is a part of something larger. The character of structures with different scale changes according to the interplay of various physical forces. Quantum phenomena control the small scales, while gravity dominates on large scales, and both come into play at the beginning of the universe.

(Source: The Little Book of the Big Bang: A Cosmic Primer, pg 6 By Craig J. Hogan).

Mathematically the matter of hierarchies in science is tractable through renormalization. That isn't directly relatable to (structural) biology, but theoretical scientists have applied it.

Or are they just there to maintain the cell itself?

Out of necessity, the cell's organelles take part in the function of the cell, yes. The reason is that the organelles are enclosed by the cell and as such all prerequisites for sourcing energy are "enclosed by one additional level" as well. That being said, the organelles must maintain themselves, structurally (referred to as turnover) and information-wise. Information may not always be as accessible as through a designated external information carrier molecule, but may be a molecules properties or even another's molecules properties, serving for instance as a "nucleation template".

Thus, the word just in your question is not applicable.

Lastly, I would like to give you a few pointers related to cellularity and the question of "why" in a context of evolution.

Cellularity may be a universal restriction that follows from the laws of thermodynamics. These laws are by consensus equally valid anywhere in the physically tractable universe.
To do any work you need to have a gradient of a physical property, which is relatable to an interconvertable form of energy. To do work in an open environment in a sustainable manner over a long-term period, it is good to have insulation.

On this planet, inhabited by carbon-based lifeforms with evolution being confined to immediate (geo)chemistry**, that meant the use of lipids. Conveniently, lipids form micelles through intermolecular self-organization, in order to allow the system to reach a mimimum free energy state.
Chemical systems with say ions too spontanously assemble given favorable energetic conditions,- all you have to do is let table-salt water evaporate, yielding you little salt-cubes which even shed a bit of insight into the underlying makeup of the crystal's unit cell. But the energies of bonds that make up the insulation layer must be in a reasonable vicinity of the local thermal energy (k*T), where k is the Boltzmann constant, and T the temperature in Kelvin.

Thus, chemical species where the weak London forces outweigh any other intermolecular forces, is ideal for self-assembly of an insulation layer in a biological context. It is not the only solution that evolved. But the insulation layer crucially determines your abilities and interactions as an unicellular or multicellular organism in the tempo-spatial regime. (How large can you get? How fast can you hunt? How fast can you bud something off or engulf? How flexible are you towards shifts in your environment? Those things...)

enter image description here
(Dileptus - a predatory protozoan - hunting, hiding, and playing around)

Self-organization can be observed across many hierarchies, serving the minimization (or optimization) of one or several properties.

A lipid membrane is ideal on a planet whose surface is covered by an extremely polar molecule. Insulation-wise lipids are impermeable to ions to allow the formation of an membrane potential. As electrical insulators lipids are excellent thermal insulators, locally confining electrons, and finally at providing mechanical insulation. Additionally lipids can serve as fuel, nutrients, signalling molecules, and many other tasks - which really just follows from or less local molecular evolution.

Evolution:

Speaking of evolution. Just like biology doesn't make sense without the context of evolution, evolution doesn't make sense without the context of energy. For science, evolution starts at the earliest physically tractable point and transitions across many hierarchies. Matter itself is just another form of energy, and as poorly defined as space: as "anything that has mass and takes up space".

Concluding:

No-one can conclusively tell you what any given organelle is for (yet).

But science has furnished a comprehensive picture, by looking at the molecules that make up the organelle and tracing it back to genes to let scientists marvel at an organelles function from a bioinformatical perspective. What greatly helps the exploration of organelle function are many high-resolving imaging techniques, with a plethora of labeling techniques. This array of labeling methods was amassed over the course of a century. Recently imaging equipment has seen a rapid increase of better resolutions in the time domain, which is very important as each level of hierarchy has its own sort of "time-signature" - which is of course strongly influence by the masses (or inertial forces) involved.

The entire picture of organelle function is however subject to a major revision-cycle with the ongoing efforts to map out entire cells at an atomic level, with large scale preparations already underway.

This post isn't thoroughly sourced, and much of it is only a layman's perspective. Consider this only as a starting point.

Disclaimer: universe-review.ca has been around for almost a decade, despite an unclear author backing this project- for obvious copyright reasons of the textbook contents. it is scientifically sound.

**Immediate [geo]chemistry would exclude anything from say Alpha Centauri, but not for instance Kiuper belt objects that may have had a significant role in the composition of earth's geochemistry. This subject is so far outside my field, I cannot give you any better information.

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Since cells are the origin of every life, any tissue must at least be based on cells. Some tissue matter is outside the cells (plant cell walls), some apparently non-cellular tissue is really a bloated organelle. Both happens. As to your theses,

  1. yes, an example are oil reservoirs in plants
  2. no, these are just big cells. A striking example is spinal cord nerves that can be very long; irreparable if injured, because you can't repair single cells
  3. connective tissue is cartilage, bone, adipose tissue, blood, hematopoietic tissue and lymphatic tissue. These together fit in both the organelle and extracellular substance slots, so you would have to differentiate
  4. bone belongs to connective tissue

So, yes, you can differentiate between functions belonging to whole cells, cell organelles, or extracellular material. And there is no reason to narrow this question to humans.

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Thanks - can you clarify: Does the cellular nature directly contribute to tissue function? Do organelles directly contribute to tissue function? Why are cells the origin of every life? Has this question been explored ever? –  S. Robert James Sep 16 '12 at 13:44
    
Yes and yes, by their physical and chemical properties and those of their content, respectively. Because evolution started with cells and it proved the best solution over the years. And there is even enough evidence to see why it all starts with cells: because lipids will form spheres by themselves in water, with or without content. It's just the physical optimum. –  rwst Sep 16 '12 at 13:53

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