Thursday, 11 August 2011

The Advantages Of Early Multicellularity

Yeast cells under a microscope (Saccharomyces cerevisiae)
We know that around 2.3 billion years ago, cells became more complex. We believe that they made the change from prokaryote to eukaryote through a perfect form of symbiosis - where an organism living inside a host cell for protection provides the host with energy until the two organisms eventually fuse into one. The second step in this story is multicellularity. The rather puzzling question is 'why.' Why bother becoming multicellular when you can survive as a single celled organism? The change would have been unnecessary.

To answer this question, a team of scientists at Harvard University, led by Dr Andrew Murray, set up a model that used yeast cells (Saccharomyctes cerevisiae) in a solution of sucrose, more commonly known as table sugar. Sucrose in its molecular form is too big to pass through the yeast cell's membrane. The yeast therefore excrete an enzyme called invertase to break down the sucrose into two simpler sugars called glucose and fructose. These are small enough to pass across the cell membrane.

The yeast cells capture them using chemicals known as transporter molecules which are an integral part of the cell membrane. The problem arises once the sucrose has been broken down. The glucose and fructose are carried from where they encountered the invertase to the cell by diffusion. Diffusion is when a substance moves from an area of high concentration to an even concentration in a liquid. The yeast take advantage of this movement to bring the sugar molecules to their cells.

Francevillian Group Fossils are the oldest
known remains of early multicellular organisms
However at this micro scale and low concentration of usable sugars, diffusion is slow and therefore very inefficient. The team calculated that for every 100 sugar molecules created, only one will be captured by the yeast cells. This does not provide the yeast with enough energy to reproduce through cell division. The scientists found that in such a situation, the individual yeast cells will clump together to increase the concentration of invertase in one area which in turn increases the concentration of usable sugars and therefore the rate of diffusion and absorption.

The team believe that early cells would have done a similar thing with one slight twist. Instead of existing as a large colony of individual cells, these early eukaryotes would have evolved bodies with multiple cells all of which would have excreted digestive enzymes, increasing the concentration of usable molecules in the area. It is likely that these multicelled creatures would have been very like sponges in shape and lifestyle. If such creatures were predators, they would have been able to digest their prey faster and absorb more energy.