Back Story

These days it’s ‘just known’ that we/animals take in oxygen to survive (exhaling carbon dioxide as a waste product), while plants take in carbon dioxide to survive (releasing oxygen as a waste product).

Yet right up into the early 1770s not much was known about ‘air’ and its components, except that it was regarded as a “a simple elementary substance, indestructible and unalterable”, and that it might have contained ‘phlogiston’.

The English scientist Joseph Priestley charted a new field of discovery into ‘the airs’ (gases) with his famous experiment with a bell jar, mint plant and mouse. This experiment, performed in August 1771, established that “…plants, instead of affecting the air in the same manner with animal respiration, reverse the effects of breathing, and tend to keep the atmosphere sweet and wholesome…” (link to a PDF).

Three years later to the month, in August 1774, Priestley discovered ‘dephlogisticated air’; we would call it oxygen gas (O₂).

The Dutch scientist Jan Ingenhousz became interested in the gaseous exchange of plants after meeting Priestley in 1771, and in 1779 made two very important discoveries.

One was that plants, when submerged, produced bubbles around their leaves when exposed to light, but ceased this bubble production when in the dark. He identified the bubble gas as oxygen and further observed that only a plant’s green parts ever produced this gas — Ingenhousz had discovered photosynthesis.

His second discovery was that plants give off carbon dioxide in the dark — in other words, they can respire just as animals do.

Even today, 241 years later, most people don’t realise this, that plants both photosynthesise and respire. And understandably so, as high school level botany is quite rudimentary at best! It’s fair to say that most people would assume that photosynthesis is to a plant what breathing is to us.

And yet both are very different, though complementary, processes. Let’s explore these now!

Photosynthesis

Photosynthesis is a two-stage process that converts the energy in light photons into the chemical energy of carbon molecule bonds which can be later used for food and other cellular functions. It occurs in the chloroplasts of plant cells and in the cell membranes of photosynthetic bacteria.

In the first stage, light energy is stored in adenosine triphosphate (ATP) (the ‘energy currency’ of cells) and another compound, nicotinamide adenine dinucleotide phosphate (NADPH). It goes without saying that this step is light-dependent!

These two molecules then feature in the second stage, whereby the energy stored within them is used to take the carbon out of air (specifically, out of inorganic carbon dioxide) and ‘fix’ it into organic three-carbon molecules. This stage is called the Calvin Cycle and is light-independent.

These three-carbon molecules are 3-phosphoglycerate and D-glyceraldehyde 3-phosphate (G3P). Two are needed to make the six-carbon glucose molecule (though the biochemical pathways involved are far more complex than this casual statement may imply!). Other combinations make more complex carbohydrates such as sucrose, starch and cellulose. Starch can be stored as an energy reserve (’food’), and cellulose is an important structural material that reinforces cell walls (we know it as ‘fibre’).

Photosynthesis is how a plant makes its own food, but to use that food, a different process altogether is required: cellular respiration.

Cellular Respiration

The word ‘respiration’ when commonly used means to inhale and exhale air, or to breathe. But in biochemistry there is much more to ‘respiration’ than simply breathing. Respiration — more accurately cellular respiration — is a series of reactions occurring at the cellular level in which organic molecules are converted into carbon dioxide and water, releasing energy along the way that can then be used by the cell.

Cellular respiration is undertaken by every single living cell on the planet - in every cell in multicellular organisms such as plants, and in every unicellular bacterium.

Respiration is the sole means by which all cells obtain energy, and, like photosynthesis, cellular respiration is a two-stage process.

The first stage occurs in a cell’s cytoplasm, and involves the splitting of glucose to ultimately produce two molecules of pyruvate and two molecules of adenosine triphosphate (ATP), the ‘energy currency’ of cells. These products then enter further reactions in the second stage to generate still more ATP again.

Aerobic respiration uses oxygen as an electron acceptor, and occurs in all eukaryotic organisms (all plants, animals and fungi) and the aerobic prokaryotes (bacteria and archae).

Anaerobic respiration does not require oxygen, but is less efficient than aerobic respiration, as oxygen is by far the strongest electron acceptor. It is for this reason that anaerobic respiration only occurs in anaerobic prokayotes — the energy obtained isn’t enough to sustain multicellular organisms.

Electron acceptors in anaerobic respiration include sulfate (SO₄^2-), nitrate (NO₃^-) and even elemental sulfur (S) — and this has significant consequences in soil microbiology generally, including the nitrogen cycle and the release of nutrients to plants.

In aerobic respiration, the second stage takes place in the mitochondria of eukaoryotic cells, and in either the cytoplasm or cell membrane of aerobic prokaryotes, and is called the Krebs cycle or the citric acid cycle.

In anaerobic respiration, the second stage takes place in the cytoplasm or cell membrane of anaerobic prokaryotes. There is no one common process like the Krebs cycle, owing to the different possible electron acceptors determining different chemical reactions. Anaerobic pathways are shorter and less complex than aerobic ones. And some result in delicious foodstuffs such as alcohol, vinegar, and Emmental cheese!

Some microbes are capable of both aerobic and anaerobic respiration, and these are called facultative anaerobes. Again this can be significant in soil microbiology if the presence of oxygen drives some reactions and its absence drives others. Denitrification — the return of nitrogen gas to the atmosphere during the nitrogen cycle — is only done anaerobically by facultative anerobes, for example.

Fun facts: mitochondria are, believe it or not, prokaryotes our eukaryotic ancestors (probably themselves prokaryotes) long ago engulfed and formed a beneficial relationship with. Thus cellular respiration is a prokaryotic creation no eukaryote ever independently developed for itself. All eukaryotes to this day literally cannot exist without prokaryotes, though prokaryotes certainly don’t need eukaryotes.

Chloroplasts are also prokaryotic in origin, meaning plant cells contain two prokaryotes: a chloroplast and a mitochondrion.

Putting it All Together

People are familiar with the concept of a simple food chain, with, say, grass at the bottom, a herbivore next, and a carnivore at the top.

But what isn’t as well-known is that for each level up the food chain, from grass to herbivore to carnivore, up to 90% of the energy is ‘lost’ as heat each time from biochemical processes!

As energy cannot be created from nothing, it must be continually input from some external source to keep that food chain going.

That external source is light photons, and the mechanism by which it enters a food chain as usable energy is via photosynthesis.

Photosynthesis only occurs during daylight, converting light energy into ATP and NADPH chemical energy, which then enter the Calvin cycle to ‘fix’ the carbon in carbon dioxide into carbohydrates.

These carbohydrates are ‘food’, both for the plant and for anything that eats it. Either way, the carbohydrates end up in cellular respiration to produce usable energy via the Krebs cycle. (Organisms higher up the food chain also extract energy from their food via cellular respiration and the Krebs cycle.)

A plant can simultaneously photosynthesise and undergo cellular respiration in the daytime — the two processes are not mutually exclusive and in fact complement each other. Photosynthesis makes the food the plant needs to grow and develop, and the energy for that growth and development can only come from breaking down that food via cellular respiration.

Growth and development are more likely to occur during daylight, partly because of the carbohydrate production going on at the same time that can fuel that, and partly because temperatures are warmer, which speeds up enzymatic reactions.

Cellular respiration still occurs during the night when photosynthesis stops, as discovered by Ingenhousz in 1779, but here the plant is more likely to undergo maintenance respiration in repairing/replacing the DNA, proteins and injured tissues needed to maintain a healthy living state rather than actively growing.

In the Context of Deciduous Trees and Dormancy

With jujube trees now beginning to bud and come back to life, it’s fitting to draw on all of the above and return to last week’s post and the recent topic dealing with winter dormancy to put everything in context.

It might be clearer now how a tree leaving dormancy is able to feed itself, via cellular respiration, with the stem and root reserves it laid down during dormancy induction. And despite its lifeless appearance throughout winter, low-level cellular respiration — maintenance respiration — was in play to keep that tree going throughout endodormancy and ecodormancy.

Cellular respiration ramps up after exiting dormancy to produce new leaves and the chloroplasts needed for photosynthesis. (Even now that little Ta-Jan continues to develop leaves with no signs of ill-health and with its roots yet to meet soil again — not that I suggest for a moment anyone else ever do this. This is my little experiment.)

Once photosynthesis is restored, a tree can again produce its carbohydrate food source instead of drawing on depleting reserves. It will continue to fuel its own growth and development this way, until shortening days once again signal the approach of winter.

When this signal comes, the tree will delegate more carbohydrates to storage than to growth, until photosynthesis stops completely and all leaves are dropped. Cellular respiration slows to a subsistence level as the tree settles into dormancy, and the cycle begins anew.