An Online Introduction

to Advanced Biology


Terms and Concepts




Cells with Other Cells -  Communication, Coordination, and Multicellularity


Cells Respond to "Messages from Beyond" -

Taxis, Tropism, and Transduction


A cell is a wonderfully isolated system for carrying on the wide array of chemical processes that make up living metabolism, but there is a limit to how isolated it can truly be.  Weve talked about the movement of materials in and out from the environment, but what about the movement of information?  A cell must be able to pick up signals, both simple and complex, from its surroundings, and change its chemistry as circumstances change.  In some cases, a movement response, a taxis, will be produced, induced by some environmental factor.  A chemotaxis is a movement response to a chemical, moving up or down a concentration gradient;  phototaxis is a response to a light source (important in photosynthesizers but also to potential prey when predator shadows pass); geotaxis is a response to the "up and down" of gravity.  A taxis can be positive, toward the source, or negative, away from it.  In multicelled systems, a similar response that occurs in growth direction rather than overall movement is a tropism;  in a sprouting seedling, the section that is going to be the root grows with a positive geotropism that sends it down toward water, the future stem follows a negative geotropism that sends it up into the light.

In other responses to environmental stimuli,  new proteins must be made, production must be started or amended, reactions must be adjusted, perhaps response signals sent out.  This is even more critical when a cell is just a member of a vast team inside a multicellular system.  Such responses require a system that takes molecules picked up by external receptors and pass signals through into the cell, activating response systems.  This process of cell signaling is called transductionWe are going to give a simplified version of how this works - as with most biological systems, the actuality will often be much more complex.

The external molecule plays the role of the first messenger, some kind of signal ligand, attaching to a receptor that is probably very stereospecific for that particular molecule.  The receptor passes this message along, either directly to the cell interior (when the receptor molecule penetrates the whole membrane) or indirectly when its floating on one surface and other molecules in the membrane and on the other side carry the message on.  

The next step involves one of many types of G Protein, which get their name from a GTP component that plays a critical role in generating a second messenger, usually cyclic AMP (cAMP).  The job of the second messenger is to either activate a kinase, a molecule that transfers a phosphate group to the specific enzyme required to start whatever response the cell needs to make to the original signal, or to inhibitors that will shut off an ongoing process. 

Steroid molecules  work somewhat differently as signalling molecules;  since they are lipid in nature, they can pass through cell membranes.  Once inside the cell, they might be picked up by internal receptors, or travel on their own to activator segments for particular genesInstead of activating enzymes, they can actually "turn on" production of necessary proteins by acting on the DNA itself.  


Cells and Other Cells - 

The Development of Multicellular Systems


Almost all multicellular organisms began existence as single starting cells.  The organisms whose beginnings were through sexual reproduction virtually always began as zygotes, the single-celled product of fertilization, the union of sperm and egg cell.  Most of that starting cell originated as the egg cell, including machinery such as mitochondria;  there also is strong evidence that much of the early protein activity during the embryo stage (once the zygote becomes 2 cells, its an embryo) is using maternal mRNAs (a sort of epigenetic inheritance, as the alleles to produce it might actually not be present) as working codes.

Several mechanisms exist to prevent polyspermy, entrance by more than one sperm into the egg cell.  First, the sperm are using particular molecules that attach to and open up an entrance for the sperm nucleus and a centriole for the first cell division (but not sperm mitochondria, which might not be compatible with egg cell mitochondria).  Once a single sperm cell is successful, the egg cell membrane instantly changes its chemistry to shut down any other sperms entry system.  If too many sperm get inside, the cell will have entire extra sets of chromosomes (polyploidy) and centrioles, and is likely to die.  In some organisms, there are so many egg cells that they do just die;  in some organisms, the extra sperm material is ejected from the cell;  in others, it is broken down inside the cell.  Polyploidy in plant cells may not kill the offspring, resulting in a brand new type of plant that may still be reproductively compatible with non-mutant relatives - several plant species seem to have started that way.

Except for the most ancient groups, multicellular organisms consist of tissues, classes of cells that do particular general jobs.  Most have epithelium, a tissue used as, for one thing, an outer and inner "skin."  Another common tissue contributes to reproduction, by being a site for mitosis and / or meiosis.  Some tissue is specialized, such as muscles in animals or photosynthetic tissue in plants.  Somehow, a cluster of cells in an early embryo must go from being all very similar to becoming various tissues, and then integrating into structures and organs.  This process is called differentiation, which allows cells to access and express different combinations of the genes that they all share.  The genes that are in use early to produce basic structure, tissues, and patterns are generally called homeotic genes (sometimes hox genes, homeogenes or homeobox genes) and seem to be some of the most conserved genes around - a very similar allele is used to determine eye placement in a human and a fruit fly, for example.  It would make sense that such critical-at-the-beginning codes, once they worked properly, would tend to remain largely the same through tens of millions of generations, when you think of the implications from changing them.

Plant differentiation arises from its reproduction tissue, called a meristem, which expresses different types of genes as it grows.  This makes sense, since the cell walls of plants means that cells cant migrate and must be differentiated in their places as the plant is built.  Along with meristem, plant tissues include dermal tissue, which forms coverings;  ground tissue, for support, storage, and photosynthesis;  vascular tissue, tubes for moving water, nutrients, and photosynthetic products up and down the plant. 

Fungus seem to follow comparable patterns, with more ability later to change the nature of cells that already exist.  Fungi are capable of generating new hyphae, the structures that make up any fungus, from virtually any cell.  A fungus has somewhat differentiated areas involved in reaching out to potential new areas, growth in a nutrient-rich zone, aging into a resources-consolidation form, or fruiting, producing spores for dispersal by either sexual or asexual means (often when nutrient levels drop).  Spores are said to germinate (same term used for plant seeds), and like seeds the timing can be tied to environmental conditions where they land.

Animals follow developmental patterns that go with cells that can distort and migrate.  There are two basic approaches to development in the newer animal groups, depending upon a very early difference.  The protostomes follow a mosaic pattern, where the differentiation fate of each cell is "set" from the very first division.  The cells also divide unequally, forming large and small cells which wrap around the initial ball of cells in a pattern called spiral cleavage.  The deuterostomes follow a regulative pattern in which cells tend to have great flexibility until very late in the developmental process - separated clumps can even form entire organisms (so identical twins are only possible in this group).  Early cell divisions are equal splits of the starting cells, so division lines radiate out from the center of the ball - radial cleavage.  There are other differences between the groups that will be discussed within the discussion of animal development.

Although the particular details vary, animal embryos go from a single zygote cell through a series of many divisions to a hollow ball of cells called a blastula.  A dent on the surface becomes deeper in a process called gastrulation, until the ball is a two-layered bowl.  The opening of the bowl will be an opening into the animal - in protostomes, the mouth, and in deuterostomes, the anus.  Each layer of the gastrula, called germ layers, will go one to be different parts of the animal.  The outer layer will be the ectoderm, which gives rise to the surface features (skin and related structures, including shells and external skeletons) and the nervous system, which develops from a thickened surface plate and which extends nerves into the other tissues during later development.  The inner layer, the endoderm, goes on to be digestive system and related structures (including our lungs).  Between the layers a mesoderm appears (as a mass in protostomes and an outgrowth of endoderm in deuterostomes) and will eventually contribute to the rest of the internal organs and structures, including muscles and internal skeletons.

As differentiation progresses, newly-different cells express new receptors and secretions, communicating with one another and controlling the roles and placement of the cells that follow.  Limbs grow out from the body in response to pockets of secreting cells, and cells grow, change, and even die off to give form to the animal.  The embryo stage involves the establishment of a basic layout - when everything important is at least in place, even if the structures are far from functional, generally the embryo stage is over and the fetus stage has begun.  In some organisms, a functional but sexually immature stage, the larva, may follow.  But once the form is in place, a growth period usually involves a combination of cells getting bigger and cells multiplying.   And cells keep sending each other messages, often controlling distant production through feedback,  in which production is turned on and off according to either the levels of the produced material itself, or monitored levels of the effects produced by the material.  Feedback is often negative, where a rise in levels causes a lowering of production rates.  Rarer is positive feedback, which tends to magnify a weak stimulus.  This appears sometimes in nervous systems, and also in problems.

Many organisms go through a stage during their lifetimes called metamorphosis.  This usually involves a significant change in form and often a shift in niche;  in fact, one of the useful aspects of many larval forms is that young and old of the same species do not compete for the same slot of an ecosystem.  Adult forms may also reflect difficulties in accomplishing sexual reproduction, as happens in corals and funguses, or the need to disperse young, as happens with many water-breeding insects.




A time lapse video of early embryo development in vertebrates.


Terms and Concepts

In the order they were covered.

First Messenger  
G Proteins  
Second Messenger  
AMP, Cyclic  
Homeotic Genes  
Differentiation - Different Groups  
Fungi Zones
Protostomes / Deuterostomes  
Mosaic Development  
Spiral Cleavage  
Regulative Development  
Twins, Identical  
Radial Cleavage  
Germ Layers  
Embryo Stage  
Larva Stage  
Fetus Stage  
Growth in Multicelled Systems  
Feedback Control  





Online Introduction to Biology (Advanced)

Copyright 2003 - 2011, Michael McDarby.

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