Most people know that plants get their energy from the sun using photosynthesis. But what does that process actually look like? How does light become a nutrient molecule that the plant can actually use for energy? And are all plants equal when it comes to their ability to create energy?
Unless you’ve taken some plant physiology classes, the answers to those questions can be difficult to find! We’ve broken down the major steps of the process in this post to make it a bit less mysterious, and used some of the invasive species in our area as examples. This blog is part of a series of posts called How Do Plants Work?
Photosynthesis
To put it simply, life as we know it on Earth wouldn’t be possible without photosynthesis. Plants’ ability to harvest usable energy from the sun is essential, as all other species higher up on the food chain rely on this lowest rung to produce energy in the first place.
On top of this, all of us air breathers need the oxygen that they produce in the process to survive. While we breathe oxygen in and breathe out carbon dioxide (CO2), plants take in CO2 and emit oxygen. Without them constantly replenishing the supply, we’d run out pretty fast!
So what exactly is photosynthesis? The word itself gives a couple of clues. ‘Photo’ in Latin means light, and ‘synthesis’ means to put together. Essentially, the word describes building things using light!
The overall chemical reaction is quite straightforward. Plants take in water, CO2, and light and turn it into oxygen and carbohydrates. The oxygen is released for us to breathe, and the plant keeps the carbohydrate to use like food! However, photosynthesis has a few more steps that happen in between before those final products can be made.
There are two major parts of photosynthesis – the light reactions and the carbon reactions. Both of these happen inside of specialized structures in the cell called chloroplasts. Light reactions happen in the membranes of structures within the chloroplast called thylakoids, while the carbon reactions occur in the ’empty’ space within the chloroplast, called the stroma.
Light reactions
The light reactions, as the name suggests, are the part of photosynthesis where light is taken in and turned into a form of energy that the plant can actually use.
Light waves hit the plant’s leaves and are absorbed by molecules in the chloroplast called chlorophyll. The sudden increase in energy from the light waves makes the chlorophyll molecules much more energetic than normal and very unstable. The plant has a window of about seven nanoseconds to harvest this energy before it gets to be too much for the chlorophyll, and it releases the energy. This reaction is one of the fastest known chemical reactions to occur naturally. The process uses up water from the plant and releases oxygen into the atmosphere.
Chlorophyll isn’t the only molecule that can be used in these light reactions; it’s just the most common! If any part of a plant is green, that means that it has chlorophyll. It’s green because it absorbs all colours in the light spectrum except for green, which reflects back to our eyes. Other molecules cause plants to be different colours. Carotenoids tend to make plant parts red, purple, orange, and yellow, and help to protect the delicate photosynthetic reaction centres from sun damage. There are several other pigments in the plant world, but too many to easily mention here.
Suppose you’ve ever been in a high school biology class. In that case, you’ll probably have heard that the mitochondria are the powerhouse of the cell – that’s because it creates ATP, a molecule that all living things use for energy during their chemical reactions. In plants, the mitochondria aren’t the only structure that creates ATP! Once the energy has been transformed into a type that the plant can use, it moves on to the next phase of the light reaction, where it’s transformed into ATP still inside the chloroplast. This is the final step of the light reaction before this ATP is passed on to the carbon reaction in the stroma.
Carbon reactions
The second major part of photosynthesis is the carbon reactions, in which all of the ATP produced by the light reactions is used to turn CO2 from the atmosphere into sugars and starches for the plant to consume as energy.
CO2 from the atmosphere is unusable to the plant, as the molecule is too big to easily cross cell membranes and be used in reactions to produce sugars. Instead, the plant has to take the carbon out of the CO2, and turn it into a molecule that’s easier to use. Different plants do this differently, depending on their environment and overall anatomy.
The specific cycle in which CO2 is transformed into sugars and carbohydrates is called the Calvin cycle. The cycle is helped along by rubisco, a plant enzyme that’s actually very bad at its job. So bad, in fact, that some plants in hotter climates have had to make adaptations to their photosynthesis cycle because of it! On top of this, it’s surprisingly slow at its job. It’s only able to fix three carbon molecules per second. Compared to the speed of other enzymes (thousands of molecules per second), that’s painfully slow.
The main problem with this enzyme is that it can take either carbon or oxygen as its intake molecule – if it binds to oxygen, it begins a process that uses up all of the carbon the plant needs instead of creating it! Plants have several creative ways to get around this and ensure that rubisco binds to carbon when the plant wants it to. These are the three kinds of carbon reaction adaptations:
C3 Plants
C3 plants are the most common. These plants are ‘normal’, and haven’t had to adapt to living in difficult climates. They can perform the whole carbon reaction during the daytime and in one cell.
These plants have a typical carbon fixation cycle, with rubisco turning CO2 into a C3 compound and then into sugars and carbohydrates.
C4 Plants
C4 plants have adapted to hotter climates by separating the carbon reaction between two cells. In the first cell, CO2 is turned into a C4 compound, so it can be transported across cell membranes. (Remember, CO2 is too big of a molecule to be able to do this.)
Once in cells further away from the leaf surface, the C4 compound is turned back into CO2. The high amount of CO2 in the cell stops oxygen from getting close to rubisco and binding! Rubisco then takes the CO2, transforming it into sugars and carbohydrates like C3 plants do.
CAM Plants
CAM plants are well adapted to dry, arid climates, like deserts. These plants use time to separate the process rather than space as C4 plants do. During the cool nighttime, they open the small pores on their leaves called stomata to allow CO2 to enter. Like C4 plants, the CO2 is turned into a different carbon compound for storage.
During the day, when it’s too hot and dry for the plants to keep their stomata open, the carbon compounds are turned back into CO2, and then rubisco turns them into sugars and carbohydrates like C3 and C4 plants. Like C4 plants, the steady release of CO2 around rubisco keeps it from binding to oxygen.
Nutrient storage and use
At this point in the process, the plants have made their carbohydrates and sugars, but how do they use them for energy?
During the day, when most plants produce carbohydrates, they’re converted into starch and stored in the cells. When nighttime comes, and the plant doesn’t have any light to use for photosynthesis, it redirects its attention to the stored starch.
At night, the starch is broken down into sucrose, which are sugar molecules that are easily transported around the plant. The sucrose molecules are sent all around the plant, which gives the plant energy to grow and function.
Invasive plants’ photosynthetic adaptations
Many invasive plants have adaptations to allow for more efficient photosynthesis. This could look like larger leaves to increase the surface area that can collect light, non-waxy leaves for more light absorbtion, specialized colouring for more efficient light use, and many other traits. Here are just a few examples that can be seen in invasive plants in the Sea to Sky:
Brazilian Elodea
One of the only known C4 aquatic plants, Brazilian Elodea is well adapted to hot climates and intense light. It easily outcompetes other plants in the same climate because of how much more efficient its photosynthesis is. While this species is not present in the Sea to Sky (yet), it is on our Prevent watchlist and is a very common aquarium plant.
Scotch Broom
If you look closely, you’ll notice that Scotch Broom has very few leaves to use for photosynthesis. Instead, it has green, ridged stems. Scotch Broom’s photosynthesis is split about 50/50 between the leaves and the stems, which allows for year-round growth although the small leaves fall off early in the year. This gives Scotch Broom a competitive advantage over other plants that must go dormant when their leaves fall off!
Yellow Lamium
This plant has interesting leaves for a couple of reasons. Firstly, Yellow Lamium leaves are covered in small hairs on the upper side. This adaptation protects the delicate photosynthetic complexes from sun damage by reducing leaf temperature. Secondly, the white variegation on the leaves is also helpful for sun protection. These areas lack green chlorophyll, and reflect almost all of the light that hits those spots. This also works to protect the plant from the sun, and makes it very efficient at photosynthesizing in the shade, where it thrives.
Giant Hogweed
Giant Hogweed has a couple different special adaptations. It has extremely large leaves, which allows the plant to collect much more light than plants with regularly-sized leaves. Giant Hogweed is part of the Carrot family of plants (this family also includes Wild Parsnip, Wild Chervil, and Poison Hemlock, among many others). These plants emerge from the ground earlier in the year than most others. The headstart on growing gives them access to more light and makes it easy to outcompete other species by blocking the sun as they grow larger.
References
- Taiz, L., Zeiger, E., Møller, I. M., & Murphy, A. S. (2015). Plant Physiology and Development (6th ed.). Sinauer Associates Inc.
- Khan Academy, C3, C4, and CAM Plants, https://www.khanacademy.org/science/biology/photosynthesis-in-plants/photorespiration–c3-c4-cam-plants/a/c3-c4-and-cam-plants-agriculture
Species Mentioned
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