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Above Image: A plant absorbs sunlight

Light Factors Affecting Photosynthesis

Photosynthetic organisms are the primary source for all of the biotic energy requirements of an ecosystem. Photosynthesis is the process by which plants use light energy to produce carbohydrates, such as glucose, and oxygen (see Figure 1) from carbon dioxide and water. Respiration, on the other hand, is a series of reactions by which plants use the glucose molecules produced by photosynthesis to drive metabolic processes and growth; this process also produces carbon dioxide and water (see Figure 1).

A Day in the Life of a Plant

Both photosynthesis and respiration occur within plant cells. During the day, photosynthesis is the dominant process in plants. This means that the plant produces more glucose than it uses during respiration. At night, or in the absence of light, photosynthesis in plants stops, and respiration is the dominant process. The plant uses energy from the glucose it produced for growth and other metabolic processes.

The light compensation point of plants is the intensity of light at which the rate of carbon dioxide uptake through photosynthesis is exactly balanced by the rate of carbon dioxide production through respiration (see Figure 2). This can also be described as the light intensity at which the rate of oxygen production is exactly balanced by the rate of oxygen consumption.

For a green plant to survive, grow, and produce mature fruit, the rate of photosynthesis (production of glucose) must be greater than the rate of respiration (consumption of glucose).

The total amount of glucose production due to photosynthesis can be increased in two ways:

  1. Increasing the intensity (brightness) of the light that illuminates the plant leaves.
    Increasing the brightness of light can increase glucose production up to a certain point. Beyond that point, the extra light energy can damage the delicate plant cells. Increasing the intensity of light can also increase the transpiration rate, causing the leaves to wilt.
  2. Increasing the duration of the light that illuminates the plant leaves.
    It is generally not possible to increase the time during which plants receive natural sunlight beyond the length of natural daylight hours. To increase the length of time during which photosynthesis occurs requires the use of artificial lights.

Light Quality and Photosynthesis

Leaves are the primary site of photosynthesis in plants and the pigment chlorophyll is central to this process. Chlorophyll molecules are found in the chloroplast and trap the light energy available in sunlight. Chlorophyll primarily absorbs in the blue and red regions of the visible light spectrum. Green wavelengths of light are reflected which is why leaves appear green to us. To learn more about chlorophyll and other pigments found in plants, see Role of Pigments in Plants.

When selecting light bulbs to grow plants indoors the type of bulb is important to take into consideration. Because plants primarily absorb light in the red and blue regions of the light spectrum, plants grow best under broad spectrum light bulbs that emit blue and red wavelengths. Light bulbs that are missing wavelengths in either the red or blue regions of the spectrum, such as many incandescent bulbs, may have a negative impact on plant growth. For this reason, broad spectrum fluorescent lights are preferred for indoor growth.

Plant Photoperiodism

The duration or length of time that a plant is exposed to light is another important thing to consider when growing plants indoors. The term used to describe this response is photoperiodism. Some plants flower in response to the timing and length of the dark and light periods within a day. "Long day" plants wait until the night is sufficiently short enough to flower. "Short day" plants will begin to flower when the night period is long enough. Other plants, such as tomatoes, are "day neutral" and do not flower based on the length of the day or night. Instead, tomato plants simply flower after they have reached a certain developmental age.


Carbon dioxide

A gas used by plants for photosynthesis. A gas produced as waste by cellular aerobic respiration. Plants use more carbon dioxide during photosynthesis than they produce during cellular respiration during the day.


One of the main kinds of nutrients in food and a source of food energy for plants and animals. Energy comes from the carbohydrates sugar and starch. Sugar is a simple carbohydrate and starch is a complex carbohydrate made from many sugar molecules linked together.


A class of pigments produced in plants that give plants their green colour. These include chlorophyll a and b.

Light compensation point

The intensity of light at which the rate of carbon dioxide uptake through photosynthesis is exactly balanced by the rate of carbon dioxide production through respiration. Equivalently, light compensation point is the light intensity at which the rate of oxygen production through photosynthesis is exactly balanced by the rate of oxygen consumption through respiration.


The sugar or carbohydrate molecule produced by plants through photosynthesis.

Metabolic process

All chemical reactions that are involved in maintaining a living cell or organism. This process includes the conversion of food into proteins and other essential molecules, energy production, and elimination of waste.


A gas used during respiration; a gas produced during photosynthesis.

Photoperiodism (plants)

A plant's developmental response, including flowering, to changes in the relative lengths of day and night, or photoperiod.


The process used by plants to convert light energy into biochemical energy (sugar). Light energy is used to reduce carbon dioxide and water into oxygen and sugar.


The process used by plants and animals to get energy from sugar molecules. Aerobic respiration converts oxygen and sugar chemically into carbon dioxide and water and heat.

Transpiration rate

The rate at which water moves through plants and eventually evaporates from small pores or holes, in leaves.


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