Climate change

Do crops have different metabolisms – like people?

Are you friends with a person who can eat anything they want and still not gain weight? Part of that could be in how their body metabolizes the food they eat. Of course, there are many other factors to that issue.

Foggy field with rows of green cut hay
Different crops have different efficiencies when it comes to capturing energy from the sun (photosynthesis). Shown, a grassy hay crop, which is one of the more efficient processors of sunlight. Credit: Stephen DeGloria

Many people don’t realize that one plant metabolizes its food in different ways than other plants. Plants rely on the same resources – water, sunlight, air and soil. So, why do they metabolize these resources differently?

Metabolism can be divided into three main functions:

  1. Conversion of food or energy to maintain cellular processes
  2. Conversion of food or energy to building blocks of the organism
  3. Elimination of nitrogenous waste

A few important discoveries beginning in the 19th century inform plant scientists today.

  • Pasteur’s studies on the fermentation of sugar to alcohol by yeast, and Wohler’s discovery of the chemical synthesis of urea by Friedrich Wohler, showed that the chemical reactions that happen in a cell were the same (in principle) as any other part of chemistry.
  • Buchner’s discovery of enzymes in the 20th century. Enzymes are proteins that act as biological catalysts to speed up chemical reactions.
  • Krebs’ studies that became the foundation of the Krebs’s cycle – the way all oxygen-using organisms release energy from carbohydrates, proteins, and fats. You might recall fun in biology reviewing ATP and how our own bodies make and release energy in chemical forms. (Fun fact, the citric acid cycle occurs in unicellular organisms as well as elephants! I’m always excited when I learn of ways life on earth is more similar than different.)

Photosynthesis can be broken up into 2 stages: the light dependent and light independent stages. The light dependent stage takes place in photosynthetic reaction centers. These are complicated proteins, pigments, and co-factors such as chlorophyll. The light-dependent stage produces energy for the plant in the form of ATP and NADPH. Both of these molecules can easily transfer different chemical groups in energy-using processes. Think of them as “molecular currency” for transferring energy within cells. (Fun fact – the human body uses close to its own weight in ATP per day!)

The light independent stage uses this energy currency to convert carbon dioxide into carbohydrates. Scientists refer to this process as carbon fixation. This process is carried out with the help of enzymes in the plant. Specifically, an enzyme called RuBisCO is important in this process. The result is carbohydrates that the plant can use to build, grow, and reproduce.

Converting resources like carbon dioxide into carbohydrates is part of a plant’s metabolic cycle. So is using these resources to grow and reproduce. And it’s in this light-independent stage where most plants differ, metabolically speaking.

Beginning in the 1960s, plant scientists began to discover the differences in plant metabolism. They are now categorized into three types: the C3 and C4 metabolic pathways, and CAM photosynthesis. C4 and CAM plants have adaptations that help them survive in hot, dry areas. Their adaptations have to do with the way they are able to use carbon dioxide. The discovery of these different metabolic pathways in plants is important to our world, and could lead to discoveries to help us adapt more crops to climate change. Let’s look a little further into each to learn why.

Plants that are in the C3 category can fix carbon directly to glucose. That means the plant takes carbon dioxide from the atmosphere, and makes their own food sources. Ninety-five percent of plant biomass on earth is categorized C3. These plants are not well adapted to hot, dry areas. They can lose up to 97% of the water they absorb through their roots to the air during the transpiration process (they’re some pretty sweaty plants).

Bright fronds of giant miscanthus up against a dark background of a forest
Giant Miscanthus grass is a C4 plant – very efficient at capturing the sun’s energy and turning it into sugars. This makes it a good source for biofuels. Credit: Brian Richards

Even though C4 plants make up close to five percent of our Earth’s plant biomass, they account for thirty percent of carbon fixation on land! This means that this hard-working type of plant is doing some heavy lifting – capturing carbon from the atmosphere better than their C3 counterparts, and turning that carbon into food for itself (and in turn for us).

The largest group of species that are categorized as C4 are grasses. This is an important factor for both grazing animals and biofuels. Food crops like maize, sugar cane and sorghum also are C4 plants. They are all able to capture energy from the sun and turn it into valuable food better than C3 plants in hot and dry conditions. They are more drought and heat tolerant than their C3 counterparts.

What’s the secret that C4 plants use to be more efficient in carbon fixation? C4 plants have adapted to use the RuBisCO enzyme differently. They are able to concentrate carbon dioxide spatially within certain parts of the plant. C4 plants have adapted to make sure RuBisCO is operating in a part of the leaf where there is a lot of carbon dioxide and very little oxygen. This seemingly small difference is actually really huge! C4 plants do not need to keep their stomata open as long as C3 plants to fix the same amount of carbon dioxide. And that means that C4 plants have an advantage under drought conditions.

purple cactus stems with large thorns
Cacti and other succulents are classified as CAM plants – and able to tolerate heat and drought better than many of their non-CAM counterparts. Credit: SV Fisk

CAM plants are usually species we consider succulents. It is no surprise that a desert cactus is able to tolerate heat and drought better than, say, an oak tree. The adaptations made by CAM plants help them thrive where other plants can’t. And, this is a metabolic adaptation.

CAM plants also use the RuBisco enzyme differently. They concentrate carbon dioxide temporally – during the night. At night, stomata open to collect carbon dioxide. The plants store this carbon dioxide and use it in photosynthesis during the day. Being able to keep their stomata closed during the daytime gives them an advantage to prevent loss of precious water. CAM plants express the most flexible photosynthesis known and can live and photosynthesize even in the virtual absence of water and carbon dioxide.

Metabolism is a fascinating topic. Further study and understanding of this relatively new area of science can help provide us with more creative solutions for increasing our food security while adapting to climate change. Who knows, maybe succulents will be on the dinner menu in the future!

Answered by Amanda Ramcharan, Pennsylvania State University

About us: This blog is sponsored and written by members of the American Society of Agronomy and Crop Science Society of America. Our members are researchers and trained, certified, professionals in the areas of growing our world’s food supply while protecting our environment. We work at universities, government research facilities, and private businesses across the United States and the world.

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