Photosynthesis

I seem to recall on my very first 'A' level chemistry lesson, being told that I had to forget a whole load of stuff I learnt at 'O' level, and the need to 're-learn' it at a higher level. The trouble was that I now seem to have forgotten both.

 

Funnily enough, that's exactly the situation that stopped me from studying physics after school, a subject that I really enjoyed and did ok at.

I remember a news article on telly that happened within days of me having finished my GCSEs. It was something to do with NASA having observed lightning at a considerably higher altitude than was previously thought possible, shattering my deep seated 'knowledge' that lightning happened when electrical charges in the clouds reached high enough levels to arc between cloud and ground.

I figured, if the top boffins don't know, and have been lying and/or making unfounded assumptions for all these years, then how can I trust them to teach me anything.

Since then, I've also discovered that many of the things I was taught as scientific fact in the subject of electronics, which I studied at college, were also not true. Once again, physics's fault. Quantum physics came along and said 'You know you believe an electron is a particle? Well it is, but also it isn't, it is both and neither all at the same time, but when we say the same time, what we really mean is not time as you know it, but another kind of time that exists and doesn't exist both at the same, erm, time'.

 

Thanks for all your answers.

Thank you Peter S, and please answer one more query on this same subject: in your first reply you state that a half-reaction is not possible, and if the plant receivesinsufficient molecules of H2o, photosynthesis cannot take place. You give a very clear reason as to why this is not usually an issue, but later in this thread you have stated that if plants receive too little water, they will grow "less well." What I'd like to ask, then, is whether the proccess of dying due to insufficient water is a gradual one whereby the plant actually photosynthesizes a little (a half-reaction, etc) but not enough to survive; ot whether the plant fails to photosynthesize at all if an insufficiency of H2o is absorbed, and thus still dies, slowly or quickly. What I really want to know is what happens to a plant when it receives an insufficient amount of co2, and whether a half-hearted version of photosynthesis can take place?
Thanks.

 

Thanks Simon for your appreciation. I think plants are like people. People also need air, food and water. However if you reduce any of these, the person will still continue to live but will get weaker and weaker, because the body is not able to continue to function properly. Think of people coming out of Japanese prisoner of war camps. Chemically speaking, I wouldn't really call this a half reaction. Because the individual molecules are fully reacting - but there just aren't enough of them to sustain the body as a whole.

If you deprive a person of water, they will continue to use the water stored in their body, but the body will fairly quickly dehydrate (ie lose water). A person will also lose energy and be less active in a bid to save water. And so with a plant, it will utilise the water and CO2 stored, but the plant's metabolism will slow down. A plant senses when it is short of water, the stems go limp and the leaves will hang down. This way the plant does not lose as much water by exhalation. That may be enough to get the plant through a dry period - such as a very hot day. But if the drought persists some of the leaves will die - further saving water loss. Ultimately all the leaves will die, and I suspect that the cause of death that will go on the death certificate is loss of cell regeneration, caused by lack of food (the sugars and starches I mentioned above). This in turn would have been caused by lack of photosynthesis, which in turn would have been cause by lack of water or CO2 - or indeed sunlight.

There is a further strange aspect that I never realised for a long time. If you overwater a plant, you can kill it. The plant in fact dies, not from too much water, but from too little water! When you overwater a plant, you prevent oxygen getting to the roots. The oxygen is needed, because photosynthesis creates sugars and starches, which are transported to the roots and all the other parts of the plant. They are burnt within the roots using oxygen, which they must get from the soil, and CO2 is given off as a byproduct. The act of consuming these sugars gives the fuel needed to build cell tissue. Thats how cells are created in the roots, where photosynthesis never takes place. But if there is no oxygen round the roots the process cannot take place and the roots will ultimately die. If the roots are dead they cannot draw up water for the rest of the plant. So the plant dies of dehydration.

 

Tuesday 13th June 9:00 pm BBC 4 - Botany: A Blooming History, Photosynthesis (2nd programme in this series).


If you missed the first episode (last week) it's here on iPlayer

 

Thank you for both of these excellent responses! I barely get time to go online these days so its good that when I do, there is usually something great to read on Gardener's Corner!

Plant toxicity is for some reason an interest of mine, so I appreciate your further comments on over-watering, Peter. One of my concerns is that many of my plants are kept in containers and pots within saucers or trays, so often after watering a pool of water will fill up and if I am out during the day I don't get chance to let this drain or pour it back into other plant pots.

Scrungee - thank you for the link. A friend actually sent me that theother day and I'm looking forward to watching it soon.

 

Simon, I think we all have difficulty in getting watering right. I certainly do. But I think there are some basic guide lines in watering:-

1) To have compost that drains well. Pure compost can sometimes hold too much water, so I like to add 50% extra sharp sand to help the drainage. Its not a magic formulae. You could use Perlite, grit or a number of other things that could help drainage and hence allow pockets of air to accumulate.

2) Use a deep pot. Gravity makes water sink to the bottom of a pot, leaving it drier at the top. But as long as there is moisture at the bottom, the capilliary effect will still allow water to be pulled upwards so the plant won't be short of water. If the pot is short, the plant roots will be at the bottom where all the water is.

3) Water according to the amount of foliage a plant has. Water is lost through the leaves, so it stand to reason that the more leaves a plant has the more water it needs. If a plant is very young or has very little foliage, you are at greatest risk of overwatering.

4) Know your plant. This is a learning curve that we are all on. Some plants need more water than others. In general, plants with large smooth leaves, such as Cannas and Brugmansias need a lot of water. And in summer can quite happily sit permanetly in saucers of water. But plants with small silvery leaves covered with hairs have adapted to hot climates and have developed leaves that loose very little water. Consequently, in our climate, they need much less water.

5) you could always put your pot on a little block of wood, so that the bottom of the pot is lifted above the bottom of the saucer. Using gravel in a tray will do the same thing.

 

Interestingly enough, you can also overfeed a plant (if you use soluble plant feed). It took me a while to realise this, thinking a plant would only take what it needed, and leave the rest in the soil for later.

It doesn't work that way I was interested to learn (partly the hard way, and partly from a good horticulture book I came across).

If the food is in the form of, say, rotted manure, then the nutrients in there have to be unlocked by microbial activity. The microbes partly depend on the activity of plant roots, so the whole thing is kind of self regulating.

However, if the feed is in the former of soluble chemical feed (basically mineral salts), then it is already available to the plants without bacteria having to unlock it. This sounds great, BUT (and here's the bit where the chemistry goes slightly over my head, I understand the basic principle but can't explain it), salts have an affinity to water, meaning that water molecules are attracted to the salts. In a health plant, there will be the same or slightly more of these salts in the plant than there is readily available salts in the surrounding soil, so it is easy for the plants to draw water (and dissolved salts) in. However if the salt availability in the ground is too much higher than in the plant cells, then water will be drawn from the plant and it will die.