Plant Science

Photosynthesis: what they didn’t teach you in school

When you look at a plant, it looks so calm and peaceful. Sitting still and silent in one spot. On the inside, however: mania. To many, photosynthesis is the Henry VIII of biology class; you know it should be interesting, but after the fourth or fifth time you just can’t find the energy to care.

But consider this: plants are able to make chocolate and oxygen out of thin air. I don’t know about you but that’s 90% of my current diet. So let’s take another look, and talk about some unlikely creatures who have stolen this ability and that one time photosynthesis almost killed everything.


First, let’s go right back to basics. Let’s imagine a leaf is an industrial estate. Within that industrial estate, there are thousands of tiny factories. These factories are called chloroplasts. When it’s time to work, the gates to the industrial estate opens. These gates are called stomata and they are tiny pores on the outside of the leaf.

The factories have to take in two deliveries to start working: water and carbon dioxide. Water is driven along the motorway of the plant. It’s taken in by the roots, flows (against gravity) up the stem and is delivered at the factory doorstep. So how does water defy gravity?

When the stomata open, water already in the leaf is lost to the air around it. Because water is “sticky” (cohesive), water leaving the leaf pulls on the water around it. This action has a knock-on effect, all the way from the leaf right down to the roots, drawing water from the bottom to the top of the plant (transpiration). Imagine the water park inside Giant Sequoias.

Getting carbon dioxide to the factory is a little easier. Carbon dioxide moves into the leaves through the open pores. Now that the two ingredients have been delivered, we need a way to power the factory. But you can’t just shine the sun on your mobile phone and expect it to charge. Chlorophyll is a pigment in plants that traps sunlight in order to power the reaction.

Water and carbon dioxide react to form oxygen and glucose (sugar). At this point, I imagine the tiny factory chimneys pumping out oxygen into the world, and tiny parcels of sugar coming out on conveyor belts, ready to be shipped around the plant.

Photosynthesis evolution almost killed everything

I’ve often considered plants to be one of the most advanced groups of organisms. They are so advanced that they grow where they’re planted (mostly – more on that in a future post), sit and sunbathe. They’re so advanced, even, that their waste products (sugar and oxygen) are incredibly useful. Once upon a time, however, evolving photosynthesis almost killed… well, everything.

Cast your mind back 3 billion years ago, when the Earth was covered mostly by iron rich oceans. These oceans would have been green, by the way, because iron reacts with other chemicals to produce a green colour when there’s a lack of oxygen, and 3 billion years ago oxygen was really lacking. Single-celled bacteria filled the oceans, and they mostly respired anaerobically (without oxygen).

All of a sudden (relatively speaking in terms of evolution) photosynthetic organisms, such as Cyanobacteria, appeared and started releasing massive amounts of oxygen. About 200 million years of oxygen excretion later, the oxygen reacting with the ocean had turned it blood red (think, the plagues of Egypt but the whole Earth).

Anaerobic bacteria started dying off because oxygen in large quantities is toxic. Oxygen started building up in the atmosphere, carbon dioxide was being depleted by the photosynthesisers, and the global temperature dropped so drastically that the Earth essentially froze over (the Huronian glaciation), pushing most life on Earth to extinction.

When the glacial period ended a few hundred million years later, oxygen was in a healthy abundance in the oceans and atmosphere. Thanks to this atmospheric oxygen, the O-zone layer appeared, blocking UV radiation from the sun.

Photosynthetic sheep

Plants are what we call “autotrophs”, ‘auto-‘ meaning ‘independent’ and ‘-troph’ meaning ‘feeder’. Specifically, plants are “photoautotrophs”, meaning they use light (‘photo-‘) to independently feed themselves. But they are not the only ones who can do this. Algae are also able to photosynthesise, and some animals have been able to take advantage of this.

Meet Leaf Sheep (Costasiella kuroshimae, pictured above). Leaf Sheep is a species of sea slug that lives around Japan, Indonesia and the Philippines. Adorably named due to its beady, little, cartoon eyes and round physique, Leaf Sheep appear to be covered in tiny leaves. Although these aren’t leaves, Leaf Sheep is able to photosynthesise.

When it comes to eating, Leaf Sheep are little kleptomaniacs. They undergo a process called kleptoplasty, meaning they hoard what they eat in their bodies. They suck the chloroplasts (those little photo-factories) out of the algae they eat and incorporate it into their own body.

This is a little aside because algae isn’t actually a plant. Algae is a term used to describe a group of photosynthetic organisms that aren’t plants or bacteria. This group is wildly diverse, containing tiny, single-celled Chlorella to giant seaweeds. Which yes, means seaweed isn’t actually a plant.

From blood red seas to photosynthetic sea slugs, the story of photosynthesis is incredibly interesting and I couldn’t possibly fit it all into one post. I hope I’ve convinced you that there is so much more to photosynthesis than A + B = C + D, and inspired you to do your own research around its rich history.

Image credits:

Underside of leaf feature image: Jeon Sang-O from Pixabay

Underside of leaves hero image: Kumiko SHIMIZU on Unsplash

Factory: This Photo / CC BY

Costasiella kuroshimae: Lynn Wu / ND Awards

Sunset: pictures101 from Pixabay

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