Hey everyone, hope you had a wonderful Valentine's day last week! Sorry I forgot to mention it on the day! Anyway, we've been looking at a range of topics situated around the subject DNA. So I'm now going to delve into another topic that has bugged me for very long time. Plastids. Let's get straight to it!
What are they?
They are organelles. That means that they are parts of a cell. They are a type of ''organ'' of a cell, performing a function that is necessary for the overall function of the cell.
They are found in plants. Chloroplasts are plastids. Crazy, but true. Chloroplasts are weird in the sense that they are green (caused by the pigment, chlorophyll) - the only plastids to be of that insane amazing colour. Most plastids do not have a coloured pigment. However, all plastids contain a double-stranded DNA molecule that is free and not encased in a nucleus. The DNA is circular, like that of prokaryotes.
All plastids are able to change from a single type to another one. There are factors that affect the type that it is changed to:
1. The environment
2. The stage at which the plastid and cell are in terms of development
3. The tissue that the plastid's cell resides in
Features of plastids
Plastids display a huge variety of size and structure. The most remarkable thing about them, I believe, is that they display a large array different functions.
Plastids have a common feature: they produce and store substances. Another common feature is that all plastids are derived from a proplastid. These are found in meristematic parts of the organism. Meristematicism refers to the region that contains undifferentiated stem cells - e.g. the active regions or growing points of plants (like in the roots and shoots).
Much like a mitochondrion, plastids all have an envelope that surrounds a matrix. The matrix holds membranes, storage material and droplets containing pigments. The pigment is dependent on the type of plastid. For example, the pigment will be nice and green in a chloroplast.
Remember when I said the DNA wasn't in a nucleus? (don't worry, I was saying the truth!) Basically, the circular DNA is held in nucloids. These are DNA-protein complexes. Nucloids are linked directly to the inner membrane of the matrix.
The number of DNA copies in a nucloid and the number of nucloids are both dependent on the plastid type and its stage of development.
The chloro of the plastids
The chloroplast. The famous king of all the plastids. It is my personal favourite. Why? Because it's green! :D Their primary function is to photosynthesise. This is why they contain the green pigment, chlorophyll.
Chloroplasts have a matrix that is commonly referred to as the stroma. This is surrounded by an envelope (as is a common feature of all plastids). Inside the stroma, there are flattened sacs called thylakoids. These are stacked up to form... stacks. The stacks are called grana.
It is within the thylakoid membrane that the protein-chlorophyll complexes that trap the photons in the light from the sun are embedded.
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| If this ain't a chloroplast, I give up. |
Protoplasts develop into chloroplasts in the presence of light. How much light? I have no idea. But I'll do an experiment, find an answer, and tell you in 5 years! :)
In low levels of light, protoplasts form etioplasts. There are some leaves you see around you that have a yellow colour. These are partly due to the presence of these organelles. The yellow pigment is protochlorophyll. Etioplasts can quickly be converted to chloroplasts through the exposure of light. They have a prolamellar body that is quickly changed into the stroma and thylakoids upon contact with photons.
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| An etioplast. See the big red thing? That's the ball of tubules that are quickly converted to flat thylakoid membranes as soon as there is an introduction of light. |
Right, so I think that's enough brain food for you! Next week, I think I shall continue with this plastid business - there's a lot more in the bigger picture. I'll talk about chromoplasts, leucoplasts and their transformations too. Remember to send in your feedback at praveenprathapan28@gmail.com! So, until then, see ya!
Exocytosis
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