Secret life of plants 2: photosynthesis

Secret life of plants 2: photosynthesis

This is my second science related blog post which is intended to give plant hobbyists a deeper appreciation of the precise nature of how their plants grow as well as a base of knowledge from which to better comprehend phenomena such as variegation and cresting. After my last blog post -  Secret life of plants 1: the Meristem - I expected the this next one would focus on variegation. But I realized that to really be able to grasp variegation and, in particular, the origin and inheritance of variegation, one needs of have a fundamental understanding of the role chlorophyll plays in plant life. So please join me for a look at chlorophyll and the fascinating process of photosynthesis. Your life depends on it. Literally. 

Feed me

What do you do when you are hungry? If you are home you might go to the kitchen and find something to munch on. Or you might indulge in a bowl of Demon Ramen in Tokyo. 

But Imagine if your body could make its own food. Imagine being hungry. Instead of heading to the fridge or a restaurant, you head outside. You lift your arms up to sky at an angle that maximizes their sun exposure.  Your feet absorb minerals from the earth. Your pores inhale CO(carbon dioxide). You look at your arms and see they are green due to a pigment - chlorophyll. As your body bathes in sunlight, you start to feel satiated as your cells fill with sugars made from COusing the energy of the sun. That sounds bizarre. but it's how plants get their food. Plants can't go to a restaurant and eat a burger.

Photosynthesis: Your life depends on it.

Most people associate photosynthesis with plant life. But it's also an essential process for human life. Think of all the things you eat. Everything is a direct or indirect product of photosynthesis: vegetables, fruits, meat (which is essentially converted plant mater).  When you are are enjoying a tasty meal at your favorite restaurant, you should stop for a second to contemplate the process of photosynthesis which has been the ultimate source of all of the food you are eating.     

Photosynthesis is a multilayered processes. There are entire books written on this topic and research labs investigating every aspect. Here are the basics you need to know.

Chloroplasts: sugar making factories

Chloroplasts are little compartments inside plant cells where photosynthesis occurs.  The story of how they originated reads like science fiction. Chloroplasts originated as photosynthetic bacteria (cyanobacteria) that were gobbled up by the primordial ancestor of plants. These bacteria became trapped inside these promordial plant cells and made these cells their home. Over time, a kind of beneficial symbiotic relationship evolved that ultimately resulted in the evolution of plant life. Chloroplasts have their own genome which is related to bacterial genomes. (Aside: plants aren't the only creatures on earth to evolve this kind of weird symbiotic relationship with bacteria.  Your cells have mitochondria, the result of bacteria being gobbled up and trapped inside the primordial eukaryotic cell. Yes, you are part bacteria).

For those interested in variegation, the fact that chloroplasts have their own genome (usually inherited from the mother) plays a role in how certain forms of variegation are inherited, as will be discussed in the next blog post.

This image shows chloroplasts inside plant cells. It's from a cool youtube video showing chloroplats moving around plant cells.   

Chlorophyll - why plants are green

Chloroplasts contain a high concentration of the green pigment chlorophyll.  Here is the chemical structure of chlorophyll taken from wikipedia:

Chlorophyll molecules are arranged in intricate assemblies called photosystems embedded in the membranes of chloroplasts. 

 Photosynthesis - sugar from sunlight + CO2

Photosynthesis is a series of two chemical reaction chains that occur in chloroplasts, the light and dark reactions.

Light dependent reactions

More science-y explanation: the unique structure of chlorophyll allows it to capture photons (particle) of light energy, which then allow the chlorophyll to transfer energy in the form of an electron to another molecule (quinone). The electron is passed along to different molecules in this first reaction chain, and this process creates an energy gradient in the chloroplast membrane (outer shell) that drives the formation of energy rich ATP.

Non-sciency explanation: Chlorophyll captures the energy of the sun and transfers it to other compounds creating sort of an energy storage battery inside the plant's cells.  

The light reactions also result in the release of a byproduct you may have heard of: oxygen. You have plants to thank for the air you breath.

Dark reactions

More science-y explanation: Using the ATP and NADPH created by the light-dependent reactions, the enzyme rubisco (the most abundant protein on earth) catalyzes the conversion of CO2 into the sugar glucose. The reaction is shown here:

3 CO2 + 9 ATP + 6 NADPH + 6 H+ → C3H6O3-phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O

Non-science-y explanation: The storage battery produced by the light reactions provides energy that fuels a molecular factory that creates sugars from the carbon dioxide that plants inhale.

The simple sugars produced by photosynthesis are used by plants to fuel cellular processes. They are also converted to polysaccharides (medium sized sugar assembliess), starches (large sized sugar assemblies) and lipids (fats) used by the plants to store energy and build cellular structures. Animals, humans included, are dependent on these sugars for their sustenance and survival. 

For those who wish to dig deeper there is a good review at the Nature website here

Albino plants

I am uneasy with calling plants albino because this is a term typically used to describe animals lacking the pigment melanin, but there really isn't a better term. So I will refer to plants lacking chlorophyll as albino. Albino plants should not be able to survive because they lack the compound essential to the crucial first step of photosynthesis. They would starve. Yet albino plants do exist. Take a look at this picture, taken by my iPhone this morning.

It's a branch from a jade tree in my front yard. The branch is white. Yet it's alive. How can that be? It's because it is attached to a large tree which contains chlorophyll. And the lower leaves of the branch look like they have some chlorophyll. The albino branch is dependent on the green parts of the tree to provide it with sugars. People have taken advantage of this phenomenon to rescue albino plants by grafting them onto green stocks. An example with which you are probably familiar are colorful grafted albino cacti.

The bottom green part of the graft provides the albino plant grafted on top with sugars so it has food to survive. I have had this one for at-least 15 years. Truly albino plants can't survive on their own. Or can they? 

Rule Breakers

To the consternation of scientists, Mother Nature doesn't always follow rules. She likes to throw a monkey wrench in the mix to keep things interesting. Here are some exceptions to things I discussed above:

I said "Truly albino plants can't survive on their own".  But not all plants are photosynthetic. There are a small number of plants that have given up on the process entirely. As a child growing up on Long Island, I was fascinated by these weird white plants that would grow up from the leaf litter in our backyard. I was so excited and happy when they appeared. I didn't know what they were at first but managed to identify them. Back in the olden days we used something called an encyclopedia. Even then I loved weird plants. These plants are called Indian Pipe or ghost plants. The picture below is from Wikipedia which has a good description of them. How do these plants get food? These plants have essentially adopted a lifestyle similar to mushrooms and get their sustenance from decaying organic matter and associations with mycorrhizal fungi and certain tree roots. 

Another plant that does not photosynthesize (has lost it's chloroplast genome altogether) is Rafflesia from Southeast Asia. This is a parasitic plant with giant flowers. Read more here.

I implied that only plants can photosynthesize. But it was pointed out to me by Rohit that there is in fact a photosynthetic animal:

Imagine being able to genetically engineer yourself to become photosynthetic.  Sounds science-fictiony and implausible. But that's essentially what this creature has done. This sea slug has absorbed photosynthesis genes from algae and makes it's own chlorophyll. How cool is that? Read more here.

I said "Plants can't go to a restaurant and eat a burger". But carnivorous plants do eat meat. No, they can't go to a restaurant. They make the food come to them. So humans have to go out to a restaurant and pay for food. While pitcher plants make the food come to them. Which is the more advanced life form?  


What about variegated plants? What's going on there? What's with the stripes? How can plants be both albino and non-albino? What's going on in the meristem of variegated plants? That's the topic of my next blog post. Promise.

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  • Thank you for this discussion which even I could follow. I’ve got a much better grasp of photosynthesis as a result. Now I’m off to find the first article on the meristem.

    Tom Everman on
  • Thank you for this discussion which even I could follow. I’ve got a much better grasp of photosynthesis as a result. Now I’m off to find the first article on the meristem.

    Tom Everman on

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