Photosynthesis is the process that plants and other organisms use to convert light energy into chemical energy that can be used to fuel the organism’s activities. The rate of photosynthesis, or how quickly it occurs, is affected by several factors, including temperature, carbon dioxide levels, and light levels. Light is an essential component in photosynthesis and is the driving force that catalyzes the reaction. The amount and quality of light available plays a major role in determining the rate at which photosynthesis occurs.
How Light Affects Photosynthesis
Light provides the energy that is required to split water molecules and release electrons, protons, and oxygen in the early steps of photosynthesis. When light strikes the chloroplasts in plant cells, it initiates a series of light-dependent reactions that convert solar energy into chemical bonds. The amount of light absorbed determines the rate at which this light phase of photosynthesis can occur.
The light-dependent reactions are so named because they require light to take place. Light excites electrons in the chloroplast and provides the energy to generate ATP and NADPH, which are used in the next phase of photosynthesis. The more light that is absorbed, the more ATP and NADPH will be produced, resulting in faster rates of photosynthesis.
Light Intensity and Photosynthetic Rates
The intensity or brightness of light has a direct impact on the rate of photosynthesis. Light intensity is measured in lux or micromoles per square meter per second. At low light intensities, the rate of photosynthesis is limited and increases linearly with increasing light. As light intensity increases, photosynthetic rates increase proportionally until an optimal point is reached.
Any further increase in brightness does not increase the rate substantially. This saturation point is reached because other factors, such as the amount of chlorophyll and carbon dioxide available, limit the reactions. Excess light beyond the saturation point will not enhance photosynthesis rates further.
Different plants have adapted to different optimal light intensities. Plants that thrive in full sunlight, such as cacti, have higher light saturation points than shade-loving plants like orchids.
| Light Intensity (μmol m-2 s-1) | Rate of Photosynthesis |
|---|---|
| 50 | 5 μmol CO2 m-2 s-1 |
| 100 | 10 μmol CO2 m-2 s-1 |
| 200 | 15 μmol CO2 m-2 s-1 |
| 400 | 20 μmol CO2 m-2 s-1 |
| 600 | 20 μmol CO2 m-2 s-1 |
Light Quality and Wavelength
Not all light is equally effective at driving photosynthesis. The specific wavelength (color) of light absorbed by a plant strongly influences the light-dependent reactions. Within the visible light spectrum, blue and red wavelengths are the most efficient while green light is the least efficient in terms of promoting photosynthesis.
Chlorophyll, the primary photosynthetic pigment in plants, strongly absorbs violet-blue and red light while reflecting green light wavelengths. Therefore, red and blue light generate greater rates of photosynthesis compared to green light.
Shorter wavelengths of light have higher energy than longer wavelengths. Blue has a wavelength of 450-520 nm and red light 600-700 nm. The high energy blue photons can effectively split water molecules during the light-dependent reactions. Red light, while lower in energy, provides the optimum wavelength for absorption by chlorophyll to drive photosynthesis most efficiently.
Green light is not absorbed as well because it has a wavelength of 500-600 nm. Therefore, it does not provide enough energy or an optimal chlorophyll absorption wavelength to maximize photosynthetic rates.
Duration of Light Exposure
The duration of light exposure also affects the rate of photosynthesis. Under low light, the longer a plant is exposed to light, the greater the photosynthetic rate. This cumulative effect occurs until saturation at optimal light levels is achieved.
If the period of illumination is shortened below the time required to reach saturation, then photosynthetic rates decrease proportionally. For example, 5 seconds of saturating light will produce less photosynthesis than 30 seconds of saturating exposure.
Once saturated, extending the duration does not increase photosynthesis further. This is because the reactions are operating at their maximal rate given other limiting factors.
Light Fluctuations
Natural sunlight conditions fluctuate throughout the day with changes in cloud cover, sun angle, and shadows. Plants have adapted to these light variations. When light levels drop below the saturation point, photosynthetic rates decline rapidly. However, when bright light returns, photosynthesis increases again.
Plants can resume maximal photosynthesis within seconds after low light exposure. Therefore, the total daily photosynthetic output depends on the total amount of light they receive over the course of a day, not just the peak intensity at noon.
Fluctuating light allows plants to achieve near maximal rates when the sunlight is brightest, while minimizing photodamage at the daily peaks. Compare this to steady high light exposure, where photosynthesis remains saturated but other processes that protect the leaf cannot match the constant high rates.
Effect of Light on Photosynthetic Products
The carbohydrates and other compounds produced during photosynthesis vary depending on light exposure. Under low light, plants produce more soluble sugars, starches, and amino acids. This promotes plant growth but not as quickly as under brighter illumination.
High light exposure increases production of insoluble carbohydrates and storage starches. It also causes plants to accumulate secondary pigments like carotenoids and anthocyanins, which help shield chloroplasts from excess light. Therefore, the optimal balance of products needed for plant growth and protection are achieved under fluctuating moderate light conditions.