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Light & Plants

Seedling with sunlight

Seedling with sunlight (amenic181, iStockphoto)

Seedling with sunlight

Seedling with sunlight (amenic181, iStockphoto)

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Learn about the process of photosynthesis and how it is affected by light levels.

Light Factors Affecting Photosynthesis

Organisms that can photosynthesize are a very important part of all . Photosynthesis is a chemical process in green plants. In it light energy is converted into chemical energy. Plants use energy from light to combine water (H2O) and carbon dioxide (CO2). The products of this reaction are simple  and oxygen (O2). An example of a simple carbohydrate is glucose.

Did you know?

People often call simple carbohydrates "sugars".

 In plants, the opposite of photosynthesis is cellular respiration. Cellular respiration is a series of chemical reactions. They convert glucose back into water, carbon dioxide and energy. This energy is used for basic  processes as well as growth. Different cell organelles are responsible for photosynthesis and respiration. Photosynthesis takes place inside . Respiration takes place inside .

Organelles and processes involved in cellular respiration and photosynthesis
Organelles and processes involved in cellular respiration and photosynthesis (Let’s Talk Science using an image by ttsz via iStockphoto).
Image - Text Version

Shown is a colour diagram illustrating the stages in cellular respiration and photosynthesis.

Four illustrations are laid out in a loop, joined by curved black arrows pointing clockwise, in a continuous cycle.

At the top, centre is a pale blue rectangle with the chemical symbols of carbon dioxide and water inside. This is labelled Carbon dioxide + Water. 

Next, below and to the right, is an illustration of a pale green oval with a flat bottom. The outer layer of this structure is cut away to reveal a stack of dark green discs. This is labelled Chloroplast (plant cells only). Above is an illustration of the sun with a yellow arrow pointing down to the surface of the chloroplast. This is labelled Energy (Light). 

Next, below and to the left is a pale green rectangle with the chemical symbols for oxygen and glucose inside. This is labelled Oxygen + Glucose. Between the chloroplast and this rectangle is the word Photosynthesis, in large green font. 

Finally, above and to the left ,is an illustration of a kidney-bean shaped structure with a reddish-brown surface. It appears to be cut in half, revealing a long, thick purple squiggle zig-zagging back and forth through the space inside. This is labelled Mitochondrion (plant and animal cells). Pointing up from this is a thick reddish-brown arrow labelled Energy (ATP). 

An arrow leads from this illustration back to the first stage: carbon dioxide and water. Between the two stages are the words Cellular Respiration in large, reddish-brown font.

A Day in the Life of a Plant

Both photosynthesis and cellular respiration occur within plant cells. During the day, photosynthesis is the dominant process. At night, or in the absence of light, photosynthesis in plants stops. This is when cellular respiration becomes the dominant process.

Comparison of photosynthesis and cellular respiration
Comparison of photosynthesis and cellular respiration (Let’s Talk Science using an image by KajaNi via iStockphoto).
Image - Text Version

Shown is a colour illustration with two panels comparing photosynthesis and cellular respiration.

The left panel is labelled Day, in the top left corner. In the lower half of the panel is a green plant with pointed oval shaped leaves, growing in a pile of brown soil. The sky behind the plant is pale blue with three small dark blue clouds. In the top right corner, a pale yellow sun shines down on the plant. 

To the right of the plant are four labels. At the top, the word Oxygen is over an arrow pointing away from a leaf. Next, the phrase Carbon dioxide is beside an arrow pointing towards a leaf. Third, the phrase Glucose produced is beside the plant, with no arrows. Finally, the word Water is above an arrow pointing towards a leaf. 

The right panel is labelled Night, in the top left corner. It shows the same plant, with a dark blue sky sprinkled with white stars. A full, pale grey moon is in the top right corner.

The labels here are white with pale blue arrows. The word Oxygen has an arrow painting towards a leaf. The phrase Carbon dioxide has an arrow pointing away from a leaf. The phrase Glucose consumed is beside the plant, with no arrows. Finally, the word Water has an arrow pointing away from the plant.

Misconception Alert

Not all plants release oxygen during the day. Cacti and similar plants keep their stomata closed during the day to reduce water loss. These plants only release oxygen at night when their stomata open.

For a green plant to survive, grow, and reproduce, the rate of photosynthesis must be greater than the rate of cellular respiration. In other words, the plant must produce more glucose than it consumes.

There are two ways to increase the amount of glucose a plant can produce.

  1. Increasing the intensity or brightness of light.
    This can increase glucose production, but only up to a certain point. Beyond that point, the extra light energy can damage plant cells. Increasing intensity of light can also increase . This can cause leaves to wilt.
  2. Increasing the duration of the light.
    It is generally not possible to increase the number of sunlight hours in a day. To increase the length of time beyond this requires artificial light. 

Light Compensation Point 

There is a specific intensity of light at which the rate of CO2 uptake is the same as the rate of CO2 production. We call this intensity the light compensation point.

Photosynthesis, respiration and the light compensation point
Photosynthesis, respiration and the light compensation point (©2022 Let’s Talk Science)
Image - Text Version

Shown is a graph identifying the levels of photosynthesis and respiration through the day and night.
The background is divided into three sections. On the far left is a dark blue area identified as night, in the centre is a top to bottom gradient of yellow to blue identified as day, and on the right is another dark blue area identified as night. Overlaid on the background is a a line in the shape of a parabola. The peak of the parabola is in the middle of the day segment. The line segment that overlays the night segments is coloured red and the line segment that overlays that day segment is coloured green. The green part of the line represents the time during which plants take up carbon dioxide and which photosynthesis is greater than respiration. The red part of the line represents the time during which plants produce carbon dioxide and which photosynthesis is less than respiration. This information is presented in text on both sides of the background image. A horizontal white dashed line extends horizontally across the background image where the parabola crosses the day-night divide. This line is labelled as the light compensation point.

Light Quality and Photosynthesis

Photosynthesis happens within the chloroplasts in a plant’s leaves. The   chlorophyll is central to this process. Chlorophyll molecules capture light energy from the Sun. They mainly absorb energy in the blue and red regions of the visible light spectrum. They also reflect energy in the green part of the spectrum. This is why leaves look green to us. To learn more about chlorophyll and other plant pigments, see Role of Pigments in Plants.

When growing plants indoors, the type of light bulb used is important. Plants grow best under light bulbs that give off blue and red light. Light bulbs that do not give off red or blue light can have a negative impact on plant growth. When growing plants under artificial light, look for full-spectrum, daylight bulbs or bulbs with red and blue LED lights.

Tomato seedlings under a plant grow light
Tomato seedlings under a plant grow light (Source: Olga Evtushkova via iStockphoto).
Image - Text Version

Shown is a colour photograph of tomato seedlings growing in a plastic tray under a large lamp.

A clear plastic tray filled with soil takes up most of the frame. 17 pale green seedlings with long cotyledons grow straight up from the soil. Above and to the left of the tray, the shade of a large, round metal lamp is visible. Its lower surface is covered with many rows of light emitting diodes. Most of them appear orange, but there is an inner circle of blue diodes close to the edge of the shade.  

The area around the tray appears dark, and the lamp shines purple light on the seedlings.

Plant Photoperiodism

The length of exposure to light is also important when growing plants indoors. For example, some plants will only flower in response to a certain number of hours of light. The term used to describe this is .

Some plants flower in response to the length of the dark and light periods in a day. ‘Long-day’ plants wait until the night is short enough to flower. This includes plants that bloom in the summer, such as poppies, sunflowers and spinach. ‘Short-day’ plants wait until the night is long enough to flower. This includes plants such as chrysanthemums, poinsettias and Christmas cactus. Other plants, like tomatoes, are ‘day-neutral’. They only flower once they reach a certain age.

 

Learn More

Photosynthesis
This page by Biointeractive offers an interactive video on photosynthesis.

Photosynthesis Science Games
This page by Legends of learning offers 4 games to learn more about photosynthesis.

The Best Plant Light Spectrum for Growing Flowering Plants
This page by SFGate explains the types of light that are ideal for plant growth, followed by an article on light and seedlings.

Photosynthesis
This page by the BBC explains the processes and plant cell structures involved in photosynthesis.

Respiration
This page by the BBC explains the processes and plant cell structures involved in respiration.

References

Crop King Incorporated. (n. d.) Light in the Greenhouse: How Much is Enough?.

Khan Academy. (n. d.) Phototropism & photoperiodism.

Lanoue, J., Leonardos, E. D., and Grodzinski, B. (2018). Effects of Light Quality and Intensity on Diurnal Patterns and Rates of Photo-Assimilate Translocation and Transpiration in Tomato Leaves. Frontiers in Plant Science, 04 June 2018. DOI: https://doi.org/10.3389/fpls.2018.00756.

May, P. (n. d.) Photosynthetic Cells. University of Bristol School of Chemistry.