• Alice F.
• Views(2433)

Hello, I'm Alice, and today we're going to go through a question that would be suitable for a Biology interview. It's not the one I've done an interview on but is one I just made up. But I think it is suitable because it's quite an open-ended question and you'd need to sort of bring a lot of your knowledge together - but it's quite broad, so you can touch on lots of different things depending on what you know. The question I've come up with is: “Discuss the competing demands experienced by plants and how they might be balanced.”

So, the first thing you need to think about really is what demands are there on a plant. So, a plant is an autotroph, and that means that it uses simple molecules and energy - in this case, sunlight - to produce complex organic molecules. So the first thing you've got there is that plant needs sunlight for photosynthesis, and other things you could think about are: plant obviously needs water (when they don't have water, they will die, so you need water to keep Turgor pressure to support them); you also need carbon dioxide. So, carbon dioxide - unlike us, we respire use oxygen, but plants use carbon dioxide and kick out oxygen instead. They use that carbon dioxide, which is fixed in sugars in the Calvin cycle. 

Plants also need minerals so you think of in fields they put on fertilizer, and if you think of the name of that fertilizer, it's often referred to as NPK fertilizer and that gives you a clue as to what minerals a plant might need. So, N - nitrogen. So if you think of what those minerals might be used for, nitrogen is found in amino acids. So the NCC back backbone of amino acids is then going to be found in proteins (which are really important for the plant) and say then you've got P, which is phosphorus.  If you think about phosphorus, it's found in the backbone of DNA and RNA, so it's really important for the genetic material and gene expression; it's also found in the head group of lipids as well, so it’s important for membranes. And then K, you've got potassium - so that's a solute and is really important for keeping turgor pressure inside cells, and it's also important for enzyme activation. So you can see there - I talked through as I've been thinking. It's really important to let the interviewers know what you're thinking and not just come up with an answer. They need to know how you're thinking because then even if you're wrong, if you have the correct logic, they will award you for that. 

One last thing we could also think about is protection from herbivory. You see things like spines on some plants like cacti that try to deter animals eating them. Now what we need to think about is how those demands on the plant are in competition. The first thing we're going to think about is photosynthesis and water. Obviously, a plant needs enough water so that it won't die. The problem is: water doesn't just stay in a plant. Water can exit through the stomata on the bottom of the leaves. The stomata is also there to let carbon dioxide in for the Calvin cycle - so there's a clear toss-up between evaporation of water out of the stomata and enter of CO2 in. Then you want to think about how those competing demands are managed. The stomata are bordered by guard cells, and those guard cells can open and close depending on the turgor pressure of the cells. When the turgor pressure is low and there's not very much water in the plant - they shrivel up and the stomata will close shut. But when there's plenty of water, the guard cells will expand and then that opens up the stomata, and CO2 you can get in, and then you'll get photosynthesis. However, even in times of drought you still need to photosynthesize so there are also CO2 sensors inside the cells, but they're not that well-known. 

All of that you might not know - obviously things like the CO2 sensor, you might not know - but you could logically think “hmmm, how am I going to know how a plant does something?” You could just make a suggestion of how a plant could do it even if you're not certain that it is the way it would be able to do it. Another thing that is important is water with minerals as well. Minerals are transported through the plant via the transpiration stream in a xylem, so therefore, if the stomata are closed, that transpiration stream is not going to be moving up the plant. So the stomata do need to open other
points as well in order to allow the minerals to move up as well. But minerals are also important for getting water into the roots as well, so water balance is also reliant upon minerals. Plants will import minerals into the roots to increase the solute potential and then draw water in by osmosis. 

Another thing you can think about is minerals themselves - because they have a curve of ideal concentration. So you need enough to be replete with minerals, but past a certain point, they get toxic. So, you need to think about how plants balance it (depending on) whether they’re replete or whether they’re toxic. Again, you might not know much about this but you could suggest ideas - so plants obviously have transporters to get the minerals into the roots. You could think where there are plants used to shortages (in not very nutrient rich soil), they may have high affinity transporters, using ATP to actively transport minerals into the roots. But where their levels might be toxic, you might think that they need sort of an external component to get rid of them as well, and those are all good suggestions. You could also think that maybe they have some way of storing those minerals in the cells to prevent them from being toxic, and another thing you can think about is (the) need for investing in a big photosynthetic area versus the need of it if they're in a shaded area (where they won't have that much photosynthesis and won't have that much energy). So there's a key play off that you want to have a bigger photosynthetic area as possible, in order to get as much light as possible if you’re in a shaded area, but that comes at a cost: you've got to make that, and if you're not getting much energy, you then can't obviously make that much. So there's two different types of plants, as we call them, the Sun plants which have really thick leaves and shade plants which tend to have thinner leaves and therefore that reduces the metabolic cost of producing them because they're not as thick.

A really good example of this is a plant called the Swiss cheese plant, which you might have seen. It's got big holes in it so that means you can get a really big area, but again, there's less cost of making that area. One of the tactics that shade plants might use is having pigments that absorb at different wavelengths, to take advantage of the light that's not been absorbed by plants higher up in the canopy. Another thing you have to think about is the sun plants: they may experience too much
light - so there's a thing called light stress, and that's when the light absorbed is exceeding the light that can be used. The idea of photosynthesis is that in the light dependent reaction, the light will excite an electron in the photosystem and then that (electron) will go down an electron transport chain. The energy released will then be able to be used to transport protons across the membrane, and when they flow back through the ATP Synthase, you produce ATP. The end acceptor of the electron will be NADP+ and allows you to produce NAPDH so that NADPH and ATP will be used in the Calvin cycle. So the Calvin cycle is the driver of the light dependent reactions, and the light dependent reactions won't happen unless the Calvin cycle is happening. 

So if you’re say in a drought condition and the stomata closed, the Calvin cycle is going to be a lot slower, but it could be a drought in very hot condition, so you're still getting a lot of light. That excess light - when the chlorophylls absorb the light, if they can't pass it on to further chlorophylls until they get to the reaction center, they can then pass it on to oxygen, and that creates something called reactive oxygen species. They can be damaging to things like proteins and DNA because they're very reactive. In high sunlight, photo damage (damage to proteins and DNA) is often experienced, so you want to prevent that. So in high sunlight, you actually want to prevent and reduce the absorption of sunlight. 

Again, you probably wouldn't know much about this and would have to theorize things. Say you want to think about how you can reduce the efficient transfer of energy from absorption to the reaction center. The first (thing) you can think about is what you know about the layout of all the chlorophylls and the photosystems in the leaf. You might know that there's chlorophylls that surround the reaction center in antenna complexes, so one strategy is to increase the spacing between the chlorophyll so that the transfer of energy from chlorophyll to chlorophyll to the reaction center (the efficiency of that) is reduced. Another way of doing it is by increasing another kind of pigment called carotenoids which you might have heard of: they're the ones that make them sort of orangey, yellow in colors. Carotenoids can't transfer the energy between them. Instead, what they can do is dissipate the sunlight (absorbed as heat) so you can directly absorb a photon of light with the carotenoids instead of in chlorophylls, and that will dissipate it as heat. Or, you could alternatively transfer it from a chlorophyll to a carotenoid and again remove it as heat, but there are other tactics that a plant could take. The plants could try to avoid it. There's a plant called Oxalis which lives in woodlands and is a shade plant. Shade plants aren't very good at coping with strong sunlight, so what this plant in particular does is: it has flat leaves that fold in in high sunlight so that less sunlight hits the leaves, and that reduces absorbance. Leaves of other plants change the angle so that they reduce the absorbance.

One nice thing you might want to think about is herbivory. So obviously you should first think about what mechanisms plants use to try and protect themselves. You've got things like cacti which have spines on them, some plants have hairs, and then there's just some that have toxins as well - things like nicotine from tobacco and morphine from poppies; they're designed to deter animals from eating them. But even just parts of a plant like the lignin that reinforces the xylem and cellulose (makes up the cell wall) are all very hard to digest and also deter animals from eating them. But specialized defenses come at a metabolic cost - you have to produce them; there's a playoff there between protection and the metabolic cost. 

So you might want to think about why some plants invest in defenses and others don't. One thing to think about is accessibility. Those that are low to the ground will be accessible to herbivores, so it's often found in rainforests - the vines there are all quite spiny to stop animals eating them. But then you also want to think about scarcity as well. In a desert, there's not many plants around. Only the toughest can survive, so things like cacti (they're actually quite scarce) therefore will be noticed by herbivores and therefore will invest quite a lot in defenses as well. But then also about seasonality of plants: things like Holly that are in forests (they're one of the relatively few evergreen plants in forests and therefore they are there all year round) are also quite accessible even in winter.

Those are the things I thought about. There is probably a whole heap of other things you could think about, and by no means would you be expected to talk about all those things in an interview. You'd be prompted probably along the way, and they may also take some of what you say and go off in another direction and pursue that as well. But I hope it was helpful!