Delivered on: 16 OCT 2023
The antenna complex, or light-harvesting complex (LHC), plays a pivotal role in photosynthesis. This intricate structure consists of hundreds of chlorophyll molecules and dozens of carotenoids, organized within protein scaffolds. Its primary function is to capture light energy and efficiently transfer it to the reaction center, where photosynthesis occurs. However, it also serves as a safety mechanism, dissipating excess absorbed energy as heat to prevent damage.
In plants and most algae, chlorophyll a and b are the key pigments found in the antenna complex. The ratio of these pigments, chlorophyll a to chlorophyll b, is a reliable indicator of the antenna's size. Seasonal fluctuations in light intensity can influence this ratio, leading to changes in antenna size. For example, under low-light conditions, plants increase the synthesis of chlorophyll b, expanding the antenna complex to maximize light absorption. Conversely, during periods of high light intensity, the antenna size may be reduced to avoid excessive energy capture and potential damage.
In this session, we will explore how antenna complex funnels the light energy to the reaction center.
Video Transcript:
Okay so let's look at this the antenna funnels that's the keyword here. Funnels energy to the reaction center. What do we understand by the word funnel? From big to small. What about the direction of it? One way both way sideway multi-way? One way okay so two concepts of funnels here meaning that from big to small and unidirectional nobody that has reached one and go back backward no it's not happening.
Okay so for each antenna system, island, you will have in essence this setup of pigments molecule okay I'm still not referring to either photosystem 1 or photosystem 2 but both of them kind of like this. But for simplicity most book will say that this is the setup general setup for photosystem 2. Okay remember this photosystem 1 photosystem 2, biochemically photosystem 2 happen first then only photosystem 1. The reason that it's not named that way I don't know why scientist didn't correct this. Photosystem 1 happened to be found first then later out they found out oh by the way uh energy come from this guy first. Yeah so photosystem 2 just written it's number two but sequence it is number one right
Okay so you got your light, light the energy is high. Okay will be accepted by molecules of pigments this is called collectively as antenna complex. Meaning that in this complex not only one pigment or one colour there are various colour there are chlorophyll there are carotenoid, xanthophyll, violaxanthin and so on depending on plant species. But in essence all plant species have this thing carotenoids and chlorophyll AB. So these will keep on passing the energy maybe the carotenoids if you remember from your note here the first week note where's that note from the first week note I think figure 9.7 you can see that for the carotenoid or beta carotene the peak absorption happens around 470 nanometer. Okay meaning that 470 nanometer absorbed by the carotenoids however this is not the energy that can excite or activate the reaction center, because the reaction center requires 680 nanometer so 470 here will be passed to the second molecule now it becomes 500 and it it pass to the second molecule over and over and over again, until the energy becoming lower enough to hit 680 nanometer. Only this energy will activate the reaction center. So what exactly is reaction center? Reaction center is actually a special chorophyll a molecule with a loosely bound electron. Meaning that the electron can be ejected out if it gets excited it's kind of like.
Remember I give the analogy of uh excitation like uh you and your friend laughing together? Yeah you keep tickling on your friend she's laughing right? She's laughing so she's in excit state. Yeah your other friend is also laughing but none of them laughing and anything coming out from their body. So, they are all this antenna complex but this one friend this one friend that's about to go and snitch to the teacher that you guys are laughing nonstop. She's the one with the denture get excited and then the denture come out and that come out is kind of like electron get ejected out of this. Go to where? Go to the teacher and tell snitch on you. You have been creating lots of noises in the class.
Okay right so another thing that you need to understand in physiology about this asterisk or star. The moment you see a substance or molecule followed by a star like this meaning that it has been excited. Excited carotenoid, chlorophyll b excited, chlorophyll a excited, the one without the star we call it ground state carotenoid, ground state chlorophyll, ground state reaction center. Excited state reaction center. The moment it has star in it. Okay so keep on passing and then activate or excite the p680. Oh yeah it's mentioned here for the excitation to be transferred back to the chlorophyll b, it is just not possible yeah because this energy here is already low, low energy cannot go back up to go against the energy gradient. That's why it's called funnelling, meaning that p60 cannot transfer back to the carotenoid this carotenoid it requires higher energy to excite it otherwise it's not going to accept okay.
It's kind of like tomorrow you tell the same joke again your two friend not going to laugh and I already know this new joke please. Not easy to excite you you need the the right switch.
Reference book: Plant Physiology and Development 7th Edition
by Lincoln Taiz, Ian Max Møller, Angus Murphy, Eduardo Zeiger
Full video: https://www.youtube.com/watch?v=dYs_TczAsM0
Attribution 4.0 International — CC BY 4.0 - Creative Commons
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