Plastids III: The Endosymbiotic Theory

To all who may strongly dislike plastids: this will be the last post about them. Promise!

Endosymbiosis

When two organisms interact in a symbiotic relationship, one can live inside the other. This is an extraordinary biological principle. Obviously the word ''organisms'' would be limited to micro-organsisms (including and especially bacteria) as it would be quite un-endosymbiotic (not a real word!) if a human engulfed another... On the other hand, endosymbiotic relationships without the need to engulf do occur in humans and mammals, but I'm gonna leave that to you to figure out. Email me an answer and the one who gets it right gets to decide a blog topic for me to write within the next few months. Exciting, ain't it? :)

What happens during endosymbiosis?

Let's take a unicellular eukaryotic cell (a single-celled animal cell). Now, if it decides to engulf (but not digest) a friend called cyanobacterium, then the structure inside the animal cell has now got two membranes around it. Obviously these would be the cell-surface membranes of the cyanobacterium and the animal cell.

Cyanobacteria. Scientists theorise that it was these little things that caused the near-extinction of oxygen-intolerant organisms.

Chloroplasts evolved from these structures ages ago. That explains their double membranes (and mitochondria's double membranes too!). However this is just conjecture. According to the endosymbiotic theory, certain organelles started off as free-living bacteria that were engulfed by another cell as endosymbionts. Chloroplasts with two-membrane envelopes are primary plastids and are found in algae that have been here for millions of years. Land plants, have evolved from green chloroplast-containing algae.

A beautiful SEM micrograph of a mitochondrion.
You can see the matrix very clearly here, a rare sight indeed.

Membrane-o-philia

Some plastids like to have more than two membranes surrounding them. Scientists have concluded that this is likely due to multiple endosymbiotic events. If we take a green alga (the consequence of the example I showed you earlier) and make a eukaryotic cell engulf it, we achieve a complex plastid with four membranes. The nucleus of the eukaryote that JUST engulfed is the one that takes charge of the entire structure. What happens to the green agla's nucleus? Well, it becomes a nucleomorph!

A nucleomorph is a small reduced eukaryotic nucleus found in many (but not all) plastids.

Right, I'm going to go into some topics even more closely related to biochemistry (plastids MAY be slightly out of place here, but it's interesting nonetheless!). I'm going to delve into the world of biomolecules next week so stay tuned. As always, do email me your feedback at praveenprathapan28@gmail.com. See ya!

Exocytosis

Plastids II and their evolution

Hello again! Well, I DID say we will continue with the plastids. Let's get cracking shall we? :)

Chromoplasts

Look at a flower. Go on. Do it. See the petals? Pretty colours, eh? Well, they're caused by chromoplasts. ''Chromo'' comes from the Greek word khrōma which translates to colour! Simples!

Chromoplasts are found in colourful things (unsurprisingly!). They are found in flower petals, fruts and structures that have a yellow, orange or red pigmentation.

Lycopene: red (tomatoes)
Xanthophylls: yellow (egg yolk)
Carotene: orange (carrots)

They can form directly from proplastids but can be formed from some chloroplasts. In the same way, chromoplasts can develop into chloroplasts! Pretty neat, eh?

Take a carrot root. Expose it to some light. See if it turns green. It should turn green as a result of chromoplasts (found at the tops of carrot roots) being altered to form chloroplasts which are green in colour. The exposure to light causes the formation of chloroplasts. Personally, I don't really find this surprisng because it is most likely an adaptation that has been evolved into these plants over a long period of time over many generations through the process of natural selection. The plants whose root tips were able to photosynthesise when the plant was upturned (e.g. by an animal) survived as they were able to store and release more energy.

Beautiful, isn't it?

Leucoplasts

To be honest, I have no idea how to say that word. But I DO know that the ''leuco'' comes from the Greek word ''leucos'' which means white. These plastids store a lot of products. A type of leucoplast (as shown on the diagram above) is the amyloplast.

Amyloplasts store starch and are actually pretty huge. They are found in the roots, tubers and seeds of plants. Amyloplasts can convert to chloroplasts when exposed to light. This is quite odd because amyloplasts are actually highly specialised and will need to unspecialise very inconveniently in order to do so. Sometimes, you may find green patches on some raw potatoes. These potatoes have leucoplasts on the outside of them. Whilst the potato was growing, some light must have made contact with these storing leucoplasts.

Amyloplast organelles from a potato cell

The evolution of pastids

These specialised organelles don't come cheap. Chloroplasts, for example, have been accepted as once being bacteria (cyanobacteria, actually) in their own right.

Cyanobacteria have chlorophyll on their outer membranes, giving them a blue/green colour. If a non-green unicellular animal engulfed a cyanobacterium but didn't digest it (as you do), the bacterium would be nurtured like an embryo in a mother. Nurturing would involve the animal cell providing the bacterium with minerals, carbon dioxide and a place to live. This is a dual-beneficial system whereby the animal cell benefits from a supply of sugars (from photosynthsis) and oxygen. They call it love, baby.

Actually, they call it endosymbiosis. :I

Many people also believe that mitochondria formed as a result of an animal cell engulfing (but not digesting) a non-green bacterium. A symbiotic relationship was formed. Awww.


That's all for now! Next time, I'll provide a more thorough explanation of endosymbiosis just because I love you so much. :)

Exocytosis