Plant Functions

Let's Talk Science

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Learn about the important plant functions of photosynthesis, respiration and transpiration and meet Dr. Smith, a professor of Plant Science at McGill University in Montreal.

Plants have an important feature that makes them different from other organisms – they are autotrophs (self-feeding). Autotrophs are able to make their own food from inorganic materials through a process known as photosynthesis. In contrast, humans and other animals are heterotrophs, which means that we get our food from outside of ourselves.


Photosynthesis is the process in which plants convert the light energy captured by chloroplasts to chemical energy needed for daily survival. Chlorophyll pigments in the chloroplasts use water and carbon dioxide from the air to form carbohydrates which store energy in their chemical bonds. Carbohydrates are compounds including all simple sugars (e.g., sucrose or ‘table sugar’) as well as complex sugars such as starch and cellulose which store energy in plants. Carbohydrates can be stored in different parts of the plant, such as in leaves and stems (e.g., potatoes). They can also be stored in fruit as pectin, which is the reason that fruit can be so sweet. Roots, such as carrots, are also storage organs for carbohydrates.

Process of photosynthesis
Process of photosynthesis (Let’s Talk Science using an image by lv_design via iStockphoto).

Spotlight on Innovation

Advanced Biofuels at ExxonMobil

Global demand for energy is expected to grow by about 25 percent by 2040. ExxonMobil is a company that is finding ways to supply world energy demand while reduce emissions. One way they do this is by creating biofuels. Unlike most fuels which are made from petroleum, biofuels are fuels made from plants.

Biofuels used today are mainly made from agricultural crops. Sugar cane and corn are used to make the biofuel ethanol and vegetable oils like soy are used to make the biofuel biodiesel. Scientists at ExxonMobil are working to turn algae and plant waste, such as corn stalks and wheat straw, into biofuels.

There are many benefits of using algae for biofuels. Unlike corn, which is harvested only once a year, algae can be harvested repeatedly throughout the year. Algae can also be produced on land that is not suitable for other purposes. And algae can use water that can’t be used for food production. Algae can even purify polluted water! Algae biofuels also have the potential to be produced on a large scale.

How does it work? Algae produce three things: protein, sugar, and fat (lipid). Biofuel is made from the lipid part of the algae. Most types of algae do not produce a lot of lipids. So ExxonMobil and Synthetic Genomics, Inc. (SGI) are working together to identify and modify algae to get it to make more lipids.

Biofuels can significantly reduce greenhouse gas emissions compared to today’s transportation fuels. This is because plants remove carbon dioxide (CO2) from the atmosphere while they grow. Biodiesel can be used by existing diesel automobiles without making major changes.

ExxonMobil is working to identify the best way to make these ground breaking technologies available to consumers. They combine science and engineering to develop biofuels that are affordable for customers and sustainable for the environment. ExxonMobil aims to produce 10 thousand barrels of algae biofuels per day by 2025.

Algae biofuel research
Scientists completing tests in the algae greenhouse (Source: © ExxonMobil. Used with permission.)


Respiration is essentially the opposite of photosynthesis. In the first step of this process, oxygen reacts with sugar in a plant cell, releasing the sugar’s stored chemical energy. The energy released is transferred to a new molecule called ATP (adenosine triphosphate). The ATP molecule can then be transported throughout the cell where it can be used to complete various tasks. This process releases carbon dioxide and water. Unlike photosynthesis which can only happen when there is light, respiration can happen both in the day and at night.


Transpiration is the term for the evaporation of water from the surface of leaves and stems. Transpiration is a necessary part of photosynthesis and respiration. Water produced during respiration exits the plant through specialized structures called stomata (little pores in the leaf that can open and close, as needed) (see Figure 16). Through these same stomata, the carbon dioxide needed for photosynthesis can enter the plant. Water initially enters the plant through the roots by the process of osmosis. The water travels up the stem of the plant through specialized tissue called the xylem, and then can exit the plant leaf through stomata which are located on the leaves.

When water availability is limited, the plant has to conserve water. It does this by closing its stomata, which decreases water evaporation, but also decreases the amount of carbon dioxide which can enter the cell. This results in decreased rates of photosynthesis which slows growth. However, at the same time the plant conserves water needed for survival. During this time, the plant can use its stored energy.

Since plants cannot move around to find energy and water, they have adapted several unique ways to be able to supply their needs. This happens through a balance between water and nutrient uptake through the roots and energy uptake through the leaves. The anatomy of both the plant cell and plant body allows plants to carry out special reactions to survive in a variety of habitats.

Stomata on the surface of a leaf
Stomata on the surface of a leaf (Source: Photohound [Public domain] via Wikimedia Commons).

My Career

Denise Hockaday

Climate Business Director Canada, Climate Corporation

Denise grew up on a dairy farm in eastern Ontario where she was actively involved in her farming community. She graduated from the University of Guelph with a Bachelor’s Degree in Commerce with a focus in Agribusiness.

Much like the rest of the world, Canada is going through a period of change in agriculture. There is an increasing interest in buying local food. And people are more interested in how and where their food is grown.

Canadians are also concerned about climate change. They want to know that farmers are taking steps to grow food today that will keep our environment healthy for future generations. But did you know that farmers will need to feed a world population of nearly 10 billion people by 2050? This will require an increase in global food production of more than 60 per cent. So how can farmers grow more food while using land, water and energy sustainably?

Increasingly, farmers today are using digital tools and data science. These technologies are enabling them to make better decisions on what, where and when to plant. This is where the Climate Corporation comes in.

The Climate Corporation’s Climate FieldView™ software is one of the most widely used platforms in the industry. Through the platform farmers can visualize and evaluate field conditions and crop performance instantaneously with weather data, satellite imagery, and field data maps. This helps them make better decisions so they can get the most out of every acre.

As Denise looks to the future, she believes that there will be great demand in agriculture for people that come from a variety of backgrounds. From business to communications to software development and science, all backgrounds have a role to play in the development of software such Climate FieldView™. A person interested in agriculture today does not need to have grown up on a farm or completed an Agricultural Science degree. Agriculture is a great and rewarding industry for everyone, so check it out!

Denise Hockaday
Denise Hockaday (Source: © Denise Hockaday. Used with permission).


Learn More

Plant Cells: Crash Course Biology #6 (2012)

This video by Crash Course explores why plants are so important, their evolution, different types of plant cells, their parts and functions. 

Xylem and Phloem - Part 2 - Transpiration - Transport in Plants (2016) 

This video by Fuse School Global Education shows the process of transpiration in plants. 


Crash Course. (2012). Plant cells: Crash Course biology #6.

Rader's Biology 4 Kids. (n.d.). Photosynthesis.

Virginia Cooperative Extension. (n.d.). 4-H virtual forest. (n.d.). Photosynthesis. Science Alberta Foundation.