Respiration In Plants Explained

What Is Plant Respiration?

The process of respiration in plants (called cellular respiration) involves using the sugars produced during photosynthesis plus oxygen, to produce energy for metabolic cellular activities and plant growth. The process exerts a significant influence on Earth’s climate system, because it involves the release of carbon dioxide (CO2) into the atmosphere, where it adds to the greenhouse effect on Earth’s temperature.

Plant Respiration vs Photosynthesis

Basically, respiration is the reverse of photosynthesis.

Photosynthesis uses carbon dioxide to convert light energy into chemical energy – stored as glucose – with oxygen as a by-product.

The basic formula is: carbon dioxide + water + light energy -> oxygen + glucose.

Respiration, by contrast, uses oxygen to convert the chemical energy (glucose) into usable energy for its immediate needs, with carbon dioxide as a by-product.

The basic formula for respiration is: oxygen + glucose -> carbon dioxide + water + heat energy.

The process of photosynthesis requires energy, which makes it an endothermic reaction, whereas respiration releases energy, making it an exothermic reaction. Photosynthesis only occurs in sunlight, while respiration occurs 24/7.

Despite these basic differences, photosynthesis and respiration together make up the complete plant energy system. First, photosynthesis cleverly converts sunlight into energy, and stores it as glucose for future use (say) at night, or whenever there is no sunlight. Then respiration is used by plants to turn this stored energy into usable heat.

The products of photosynthesis are the reactants of respiration and vice versa. Photosynthesis generates the glucose that is used in cellular respiration to make ATP (adenosine triphosphate). The glucose is then converted back into CO2, which is used in photosynthesis. While water is broken down to make oxygen in photosynthesis, during respiration oxygen is joined with hydrogen to form water (H2O). While photosynthesis needs CO2 and releases oxygen, respiration requires oxygen and emits CO2.

Plant Respiration Process: Diagram
Like all living things, plants need energy to live. To obtain this energy, they absorb oxygen from the air and release carbon dioxide. This process is called respiration. Photo: © JagranJosh

Respiration & Biogeochemical Cycles

Plant respiration and photosynthesis play complementary roles in the fast carbon cycle and the oxygen cycle – the biogeochemical pathways through which these important chemicals are recycled around the biosphere.

While respiration releases carbon dioxide into the atmosphere, photosynthesis removes it. While respiration takes oxygen out of the atmosphere, photosynthesis puts it back. And although plants take out more CO2 than they put back (which is good for the planet) and use up a little less oxygen than they give out (also good for the planet), the two interlocking processes of respiration and photosynthesis help to maintain atmospheric CO2 and oxygen at relatively stable levels.

Plant Respiration & Climate Change

In the context of global warming, plants are coming under increasing scrutiny because of their relationship with carbon dioxide (CO2), one of the planet’s most abundant greenhouse gases. Climatologists and botanists have not yet managed to develop climate models that are capable of unlocking the contradictory outcomes of photosynthesis and respiration. However, the issue remains a live one, not least because the relative contribution of these two processes to atmospheric CO2 levels informs the debate on land use and deforestation, both of which impact on climate change across the world.

Professor Chris Huntingford, of the UK Centre for Ecology & Hydrology, and lead author of a recent study into plant respiration and climate change (see below) says: “Plants both capture carbon dioxide and then release it by respiration. Changes to either of these processes in response to climate change have profound implications for how much ecosystems soak up carbon dioxide emissions from burning fossil fuels.”

Note: Scientists estimate that plants respire about 60 billion tons of CO2 each year: roughly six times more than humans produce through burning of fossil fuels, like coal and oil. 1 For more about climate, see: 7 Effects of Climate Change on Plants.

How Do Plants Respire?

Respiration occurs in all parts of the plant, including the roots, the leaves, the stem, even the flowers – unlike photosynthesis, which takes place only in the leaves and stems. When respiring, the parts of the plant above ground obtain their oxygen directly from the air through the stomata – tiny pores located in the epidermis of stems, leaves, and other organs, that enable the entry and exit of CO2 and oxygen. Each stoma is flanked by a pair of parenchyma cells, called guard cells, whose function is to regulate the size of the stomatal opening. Trees respire in a similar way but instead of stomata, they take in oxygen through pores in their branches, known as lenticels.

The chemical reactions that drive respiration in plants actually occur in the mitochondria of each cell. Mitochondria are specialized compartments within cells where photosynthesized energy is stored, usually as sugar or glucose. When this glucose is broken down into usable energy during respiration, it, too, is stored in the mitochondria, this time as molecules of adenosine triphosphate (ATP).

The chemical equation for respiration goes like this: 2

Plant Respiration Equation

The glucose breaks down in the presence of oxygen, forming carbon dioxide, water and ATP molecules (energy molecules). The amount of “energy units” generated by respiration is significantly less than the amount of solar energy needed to create the glucose during photosynthesis. Cell respiration is only about 40-60 percent efficient: a good example of the Second Law of Thermodynamics at work. (See also: What is Transpiration?)

Why Do Plants Respire?

Answer: In order to create energy. Every cell in a plant needs energy to live. The only way to obtain this energy is to convert the chemical energy (created from sunlight by photosynthesis and stored as glucose) into usable ATP energy through the process of cellular respiration. Energy is needed by plant cells to drive the numerous chemical reactions needed for growth, such as the transport of substances in the phloem. It also fuels the reactions needed to generate complex carbohydrates, proteins and lipids from the products of photosynthesis. It is also needed in large quantities for transpiration, in order to cool down and to drive the plant’s circulatory system: for example, it takes almost 600 calories to transpire 1 gram of water.

Aerobic And Anaerobic Respiration In Plants

What’s the difference between aerobic and anaerobic respiration? Answer: Respiration that occurs in the presence of oxygen is aerobic. Respiration in the absence of oxygen is anaerobic. Most plant respiration is aerobic, because it is much more efficient. For example, regular aerobic respiration in plants produces 38 ATP molecules of energy. By contrast anaerobic respiration generates only two ATP molecules.

However, in certain situations – such as, when the cells in the roots of a plant become completely waterlogged (and unable to access oxygen) – it is very useful to be able to respire anaerobically. 

During anaerobic respiration, glucose molecules are broken down into ethanol, carbon dioxide and a small amount of energy.

How Does Plant Respiration Affect The CO2/Oxygen Balance?

  • During the hours of darkness or in the absence of sunlight, photosynthesis isn’t possible so only respiration can take place. Thus, only oxygen is absorbed from the atmosphere and only carbon dioxide is released.
  • In dim sunlight, plants photosynthesize at the same rate as they respire. Therefore, the amount of carbon dioxide absorbed during photosynthesis is about the same as the amount that the plant emits as it respires. Likewise, it emits no more oxygen during photosynthesis than it absorbs during respiration.
  • In bright sunlight, plants photosynthesize faster than they respire. The extra oxygen produced is released into the atmosphere.

Plants Respire More CO2 Than We Thought

A recent study, which represents the most informed picture to date of current and future carbon emissions from trees and plant vegetation, suggests that plant respiration is a larger source of CO2 emissions than previously thought. It warns that as global temperatures rise, this will reduce the capacity of Earth’s biomass to absorb emissions from fossil fuels.

Using plant respiration data from over 100 remote sites around the world – covering all different biomes, from burning deserts in Australia, to the deciduous forests and taiga of North America and Europe, the Alaskan tundra, and the tropical rainforests of South America, Asia, Africa and northern Australia – the study shows that carbon release as a result of plant respiration may be around 30 percent higher than previous estimates. 3

According to lead author Professor Chris Huntingford: “For too long, plant respiration losses of carbon dioxide to the atmosphere have been the Cinderella of ecosystem computer modelling, with carbon dioxide gains via photosynthesis stealing the attention. Here we address that, using extensive measurements of respiration to guide computer-based calculations of how carbon cycles through trees and plants.”

“The study shows that with rising temperatures, the amount of carbon dioxide released through plant respiration will increase significantly,” says co-author Professor Atkin, from the ARC Centre of Excellence in Plant Energy Biology at the Australian National University. “Currently, around 25 per cent of carbon emissions from the use of fossil fuels is being taken up and stored by plants, which is good, as it helps reduce the concentration of greenhouse gases in the atmosphere. Our work suggests that this positive contribution of plants may decline in the future as they begin to respire more as the world warms.” For more on this, see: Is more CO2 good for plants?

Plants Respire Less CO2 Than Predicted

Interestingly, another report has come to a completely different set of conclusions, despite sharing several of the same co-authors. Up to now, digital climate models have tended to assume that as temperature doubles, so does plant respiration, no matter what the timeframe. Now, the most comprehensive global study of its kind suggests that this assumption may no longer be true, and that increases in plant respiration may not be as large as previously thought.

The study subjected leaves to rising temperatures over 30-minute periods. Plant responses were measured in 231 species, from herbs and grasses to shrubs and trees. Study sites, located in regions with widely differing growing-season temperatures, included boreal forests in Minnesota and Sweden; Alaskan tundra; temperate forest in New York’s Hudson Valley; tropical areas in French Guiana, Peru and Costa Rica; and grasslands in Australia and Texas.

Findings show that rates of respiration increase slow in an easily predictable way as temperatures mount, in every region of earth, from tropics to the Arctic Circle. The newly defined curve leads to sharply reduced estimates of respiration, particularly in the coldest regions. For example, on the North Slope of Alaska plant respiration was 28 percent lower than expected; in Black Rock Forest, north of New York City, about 15 percent lower.

According to co-author Kevin Griffin, a plant physiologist at Columbia University’s Lamont-Doherty Earth Observatory: “with this new (climate) model, we predict that some ecosystems are releasing a lot less CO2 through leaf respiration than we previously thought.” 4 See also: Which is the Largest Carbon Reservoir?

The Debate Continues

With global energy consumption up, emissions of greenhouse gases up, global temperatures up and fossil fuels still accounting for more than 80 percent of our energy needs, it seems highly doubtful that the goals of the Paris Climate Agreement will be met, and that global warming is almost certain to exceed 2 degrees Celsius, perhaps even 3 degrees Celsius by the end of the century.

A key element in the climate change mitigation strategies so far proposed, by the IPCC and others, is the idea of “net-carbon emissions”, or the offsetting of fossil fuel emissions by carbon capture and storage. This is where trees and plants come in, and why the role of respiration and photosynthesis is so important. The debate continues. 

References

  1. Lamont-Doherty Earth Observatory, Columbia University. “Scientists say many plants don’t respond to warming as thought: From tundra to New York exurbs and tropics, new data lowers estimates of carbon release.” ScienceDaily. ScienceDaily, 21 March 2016. []
  2. “Environmental Science.” Richard T. Wright, Dorothy F. Boorse. Eleventh Edition. p.66. Pearson Benjamin Cummings. San Francisco. []
  3. Implications of improved representations of plant respiration in a changing climate.” Chris Huntingford, Owen K. Atkin, Alberto Martinez-de la Torre, Lina M. Mercado, Mary A. Heskel, Anna B. Harper, Keith J. Bloomfield, Odhran S. O’Sullivan, Peter B. Reich, Kirk R. Wythers, Ethan E. Butler, Ming Chen, Kevin L. Griffin, Patrick Meir, Mark G. Tjoelker, Matthew H. Turnbull, Stephen Sitch, Andy Wiltshire & Yadvinder Malhi. (2017) Nature Communications volume 8, Article number: 1602. []
  4. Convergence in the temperature response of leaf respiration across biomes and plant functional types.” Mary A. Heskel, Odhran S. O’Sullivan, Peter B. Reich, Mark G. Tjoelker, Lasantha K. Weerasinghe, Aurore Penillard, John J. G. Egerton, Danielle Creek, Keith J. Bloomfield, Jen Xiang, Felipe Sinca, Zsofia R. Stangl, Alberto Martinez-de la Torre, Kevin L. Griffin, Chris Huntingford, Vaughan Hurry, Patrick Meir, Matthew H. Turnbull, and Owen K. Atkin. PNAS April 5, 2016 113 (14) 3832-3837. []
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