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Plant Functions

Leaf and the Sun

Leaf and the Sun (pictureimpressions, iStockphotos)

Leaf and the Sun

Leaf and the Sun (pictureimpressions, iStockphotos)

Let's Talk Science

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Learn about the important plant functions of photosynthesis, respiration and transpiration.

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).


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.


is the term for the  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  (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).

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.