Cell origins have always been a mystery. How could cell membranes form in the sea when their fatty acids can’t survive in salt? Find out the answer.
I can still remember the first time I saw a living cell. It belonged to me.
Everyone in our biology class took a swab and scraped the inside of their cheek. We spread the residue onto a glass slide and added some methylene blue.
After placing a coverslip over the sample, we slid it into the microscope. Under the lens, as I zeroed in and gradually raised the magnification, I could see a few of my cheek epithelial cells.
I COULD MAKE OUT THE NUCLEUS, CYTOPLASM AND MEMBRANE
They were flat looking and many of them were folded over on themselves. Still, I could make out the nucleus of each cell, the cytoplasm and the cell membrane.
We learned that cells are a complex little structure. There’s the nucleus, the cytoplasm, various organelles and the cell membrane.
Anyone who’s ever seen human cells finds them awe-inspiring. Where could a formation like this come from and are we really made of 37 trillion of these tiny beings?
ARE WE REALLY MADE OF 37 TRILLION OF THESE TINY BEINGS?
Science has shown that evolution is a proven fact, but cell origins have been a tougher challenge. Cell biology has evolved over time, but that doesn’t explain how the simple, original cell types emerged.
To even be worth calling a cell, an organism needs to have three things. These are the complex molecules that can store our genetic code (RNA or DNA), proteins and a cell membrane.
A cell that wasn’t membrane-bound, would be indistinguishable from the fluid around it. What makes a cell an individual living entity is the thin sheet of tissue that encloses it.
EVEN PRIMITIVE PROTOCELLS HAVE THREE KEY ELEMENTS
Biologists believe that even the most primitive forerunners of cells called protocells would have had the three key elements we just described. It’s not all that difficult to imagine how the membranes could have formed.
Fatty acids consist of organic molecules called phospholipids. These molecules have a head that water attracts and a tail that water repels.
This makes them align with their heads pointing outward toward the surrounding water and their tails pointing inward, away from it. The molecules form a bubble-like sphere that can contain the protocell contents like a sack.
MOLECULES CAN CONTAIN THE PROTOCELL CONTENTS LIKE A SACK
Professor Sarah Keller of the University of Washington described it like this, “You can imagine different types of molecules moving within the primordial soup as fuzzy tennis balls and hard squash balls bouncing around in a big box that is being shaken. If you line one surface inside the box with Velcro, then only the tennis balls will stick to that surface, and they will end up close together.”
Fair enough, but there is another challenge to explaining cell origins and the origin of life. We know that salt breaks down fatty acids.
If life formed in the ocean, as we now think it did, this would mean that sea salt would dissolve cell membranes before life on earth could form. The membranes also couldn’t withstand magnesium ions, yet RNA needs these ions to encode our gene expression.
SEA SALT WOULD DISSOLVE CELL MEMBRANES BEFORE LIFE FORMED
Professor Keller is part of a research team whose lead author is doctoral student Kaitlin Cornell. The team published their results in the Proceedings of the National Academy of Sciences and shed light on the cell origins enigma.
The researchers used light-based microscopes, electron microscopes and spectroscopes. They tested the ways in which 10 amino acids combined with membranes.
They found that certain amino acids, the molecules behind protein synthesis, can bind to cell membranes. This combined structure functions to stabilize the membranes, protecting them from salts and the magnesium ions.
AMINO ACIDS BIND TO CELL MEMBRANES AND STABILIZE THEM
Some amino acids cause major changes in membrane structures. A few can cause the membranes to divide into layers like an onion.
As Kaitlin Cornell put it, “Amino acids were not just protecting vesicles from disruption by magnesium ions, but they also created multilayered vesicles — like nested membranes.”
Amino acids are often called the building blocks of proteins. This means that if amino acids collected together supporting membranes, proteins in cells could have formed in the cell origin process.
“ASSUMPTION WAS JUST SOMEHOW THEY DID COME TOGETHER”
Team member Professor Roy Black explained it this way, “Cells are made up of very different types of structures with totally different types of building blocks, and it has never been clear why they would come together in a functional way. The assumption was just that — somehow — they did come together.”
This discovery explains how cells formed into proteins inside a sack of fatty acids. The fatty acids started to form the membrane and the amino acids stabilized it.
The more stable the fatty acids became, the more amino acids they attracted and vice versa. The amino acids then made it easier for proteins and RNA to form inside the cell.
LIFE MAY HAVE STARTED IN SHALLOW POOLS
Another cell origins knot that this discovery untangles is that life may have started in shallow pools. If so, the concentrations of amino acids would vary as some water evaporated and new water flowed in.
The study showed that varying the concentration of amino acids stabilized the membranes even more. This supports the idea that cells first formed in shallow basins.
From here, the team plans to focus on the next stage in how cells formed. If cell membranes brought all of the ingredients for life together, then how did these ingredients bind together to become a functioning cell?
“That is the next step,” said Professor Black.
We always have more to learn if we dare to know.
University of Washington
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