Chloroplasts

Before we can dig deeper into the details of photosynthesis, you first need some spatial context of where this happens in plants-the chloroplast.

Chloroplasts are specialized organelles within plant cells where photosynthesis takes place, among other interesting biochemical reactions. They are what make plants green. The cells of the green parts of plants each contain 10 – 100 chloroplasts. Check out this link for a great overview of plant cell structure to put chloroplasts in context.

Chloroplasts contain their own DNA, a genome separate from that in the plant cell nucleus. This means that they can make their own proteins and contain all of the biological machinery to do so. But they don’t make all of the proteins they need. Some chloroplast-localized proteins are encoded on nuclear DNA and must be synthesized in the cytoplasm then transported into the chloroplast.

While the proteins are essential for photosynthesis (and the other biochemical reactions that take place in the chloroplast), the membrane structure of the chloroplast is equally important for this process. The chloroplast is surrounded by two separate membranes (the outer and inner chloroplast envelope membranes). Inside the inner envelope membrane, the enclosed aqueous space is called the chloroplast stroma. The chloroplast stroma is site of the photosynthetic ‘dark reactions’ (really, just light-independent reactions), which convert carbon dioxide into sugar. Starch granules comprised of the storage form of the sugars generated from photosynthesis accumulate in the chloroplast stroma. The thylakoid membrane system is a separate membrane system internal to the envelope membranes. The distinct aqueous space enclosed by the thylakoids is called the thylakoid lumen. The thylakoid membranes are the site of the photosynthetic ‘light reactions,’ where solar energy is converted to biochemical energy. The chlorophyll-containing membrane protein complexes that perform these reactions give the thylakoids (and thus the chloroplast) their green color.

Chloroplast structure. By Kelvinsong (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)

Thylakoid structure. From By Kelvinsong (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons  with text additions be J. Roose

Thylakoid structure. From By Kelvinsong (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)%5D, via Wikimedia Commons with text additions by J. Roose

The thylakoid membranes have an elaborate architecture that is important for photosynthetic function. The membrane system folds back on itself to form stacked regions called grana. The unstacked regions are called stroma lamellae; these stroma lamellar thylakoids extend from the grana stack regions. Current ultrastructural research suggests that at of the thylakoids (grana and lamellae) are connected in a single system thereby creating a lumenal space that is continuous for the entire system. Plastoglobuli are protrusions of the thylakoid stroma lamellae that contain enzymes involved in lipid synthesis and other specialized metabolism. The plastoglobule structures are unique because they are lipid monolayer protrusions. All other membranes of the chloroplast are the lipid bilayers typical of biological membranes.

Don’t think there’s nothing more to know about chloroplasts and their membrane structure- far from it. Below are some areas of active research:

Chloroplasts must communicate with the nucleus to coordinate the appropriate synthesis of proteins for optimal photosynthetic function, and we don’t understand exactly how that works. What signals are traveling between these two organelles, and what factors are interpreting these signals?

Another process we don’t know a lot about is the chloroplast protein targeting system. Nucleus-encoded proteins are made in the cytoplasm, but end up in a handful of possible locations within the chloroplast (inner membrane, outer membrane, thylakoid membrane, stroma, and thylakoid lumen). How the chloroplasts sort these individual proteins so they end up in the proper place is an active area of research.

Another outstanding question is how the chloroplasts shape their thylakoid membranes into the elaborate grana stacks and stroma lamellae structures discussed above. Plants also have the ability to change their thylakoid membrane structure to have more or fewer grana stacks in response to environmental conditions. Little is known about the mechanism of this rearrangement.

Plant cells can also independently position their chloroplasts within the cell for optimal light harvesting depending on the light conditions of the plant. In this way, chloroplasts do not block on another when light is limiting.

Understanding chloroplast division is another mystery. Chloroplasts reproduce themselves independently of plant cells. In this way, as plant cells grow and divide, roughly the same number of chloroplasts per cell is maintained. Otherwise, plant cells would have fewer chloroplasts after several rounds of division. Scientists are not sure exactly what controls chloroplast division with respect to proper timing and size.

If these questions haven’t blown your mind yet, think about this. Plant seeds don’t have fully formed chloroplasts with the membrane structures pictured above. How chloroplasts develop their thylakoid membrane system ‘from scratch’ in these proplastids is also largely unknown.

If you still already knew all of this, stay tuned for graduate level cell biology next time. There will be more to come on these topics here because chloroplast structure and its relationship to the rest of the plant cell are intimately linked to photosynthetic function.

Johnna

Further reading on chloroplasts:

Nevo, R., Charuvi, D., Tsabari, O. and Reich, Z. (2012), Composition, architecture and dynamics of the photosynthetic apparatus in higher plants. The Plant Journal, 70: 157–176. doi: 10.1111/j.1365-313X.2011.04876.x

Bréhélin, C. and Kessler, F. (2008), The Plastoglobule: A Bag Full of Lipid Biochemistry Tricks. Photochemistry and Photobiology, 84: 1388–1394. doi: 10.1111/j.1751-1097.2008.00459.x

Daum, B. and Kuhlbrandt, W., Electron tomography of plant membranes. J. Exp. Bot. (2011) 62 (7): 2393-2402. doi: 10.1093/jxb/err034

http://en.wikipedia.org/wiki/Chloroplast

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4 thoughts on “Chloroplasts

  1. Pingback: Stealing Photosynthesis | New Under The Sun Blog

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