Chlorophyll is the green pigment found in plants, algae and cyanobacteria that allows them to absorb energy from sunlight. It plays a crucial role in photosynthesis – the process used by plants and other organisms to convert light energy into chemical energy that can be used to fuel the organism’s activities.
Properties of Chlorophyll
Chlorophyll absorbs light most strongly in the blue and red regions of the visible light spectrum. The absorption peaks for chlorophyll a are at around 430 nm (blue) and 662 nm (red). Chlorophyll reflects green light, which is why plants appear green.
The chemical structure of chlorophyll contains a porphyrin ring similar to the heme group found in hemoglobin. At the center of the ring is a magnesium ion. Attached to the ring is a long hydrophobic phytol tail. These structural features allow chlorophyll to be soluble in fats and oils, but not water.
How Chlorophyll Absorbs Light
When a photon of light is absorbed by chlorophyll, it excites an electron in the porphyrin ring to a higher energy level. The excited electron is then transferred to an acceptor molecule. This creates a separation of charge, with the electron moving through an electron transport chain and the “hole” it leaves behind moving through the plant to drive the synthesis of ATP and carbohydrates.
Specifically, the light energy absorbed by chlorophyll transforms into a excited state when the electrons absorb photons and become excited. The excited electron from photosystem II is then captured by a primary electron acceptor. From here the electron is transported along an electron transport chain, which pumps protons from the stroma into the thylakoid space. This pumping of protons creates a proton gradient and membrane potential used by ATP synthase to generate ATP. The electron finally reduces NADP+ in photosystem I to NADPH. The chemical energy stored in ATP and NADPH is then used to fix CO2 into carbohydrates in the Calvin cycle.
The Fate of Excited Chlorophyll
When chlorophyll absorbs light energy and gets excited, there are three potential outcomes:
- Fluorescence – The excited electron returns to the ground state by emitting a photon of light (fluorescence). This causes the re-emission of the absorbed light at a longer wavelength, giving chlorophyll its red fluorescence.
- Heat – The excited state’s energy gets dissipated as heat to surrounding molecules through collisions and vibrations.
- Chemical work – The high energy state results in a chemical reaction. In photosynthesis, this involves the excited electron being used to transport an electron down the electron transport chain.
In most cases, the chemical work and heat dissipation pathways are favored over fluorescence. So only a very small fraction of absorbed light is re-emitted as fluorescence.
The Effect of Chlorophyll on Absorbed Light
When light strikes chlorophyll, several things can happen. Here is a summary of the effects chlorophyll has on the light it absorbs:
- Absorbance – Chlorophyll absorbs certain wavelengths of light very well, particularly violet-blue and orange-red wavelengths.
- Reflection – Chlorophyll reflects green light, giving leaves and plants their green color.
- Transmission – Chlorophyll allows other wavelengths of light to pass through and reach cells below the surface.
- Fluorescence – A small fraction of absorbed light is re-emitted at longer wavelengths as fluorescence.
- Chemical energy – Most of the light energy absorbed is converted into chemical energy and stored in the bonds of glucose and other carbohydrates.
- Heat – Some absorbed light energy is given off as heat to surrounding tissues.
So in summary, chlorophyll absorbs strongly in the blue and red regions. This light energy is transferred into chemical energy via photosynthesis. A small amount of absorbed light energy is dissipated as heat or re-emitted as fluorescence. The green and yellow wavelengths are reflected or transmitted through the leaf.
The Importance of Chlorophyll
Chlorophyll is an extremely important biomolecule, critical for photosynthesis and life on Earth. Here are some key functions and benefits of chlorophyll:
- Absorbs light energy for photosynthesis
- Acts as an accessory pigment, passing energy to reaction centers
- Drives electron transport chain to produce ATP
- Produces oxygen as a byproduct of photosynthesis
- Pumps protons across thylakoid membrane to generate proton gradient
- Provides the green color to most plants and algae
- Allows plants to absorb the energy they need to grow and flourish
Without chlorophyll, plants would be unable to absorb light energy and convert it into a usable form of chemical energy. They would be unable to perform photosynthesis and produce oxygen. Life as we know it on Earth depends on chlorophyll!
Absorption Spectrum of Chlorophyll
Chlorophyll does not absorb light evenly across the visible light spectrum. It preferentially absorbs light in the violet-blue and orange-red wavelength ranges. The absorption peaks for chlorophyll a are at 430nm and 662nm.
Here is a table showing the absorption spectrum of chlorophyll a and b:
| Pigment | Peak Absorption Wavelengths |
|---|---|
| Chlorophyll a | 430 nm, 662 nm |
| Chlorophyll b | 453 nm, 642 nm |
Chlorophyll reflects green wavelengths well. This strong green reflectance gives chlorophyll and most plants their green color. The absorbed light provides the energy that drives photosynthesis and allows plants to grow.
Factors Affecting Chlorophyll Absorbance
Several factors can affect the ability of chlorophyll to absorb light.
- Chlorophyll concentration – Higher chlorophyll levels mean more light absorbance.
- Leaf anatomy – Chloroplast and cellular structure impact light capture.
- Leaf area index – Total leaf surface area affects total light absorption.
- Temperature – Heat can degrade chlorophyll molecules.
- Damage – Physical damage reduces light absorption.
- Water stress – Drought causes chlorophyll breakdown.
- Nutrient deficiency – Lack of nitrogen, iron, magnesium reduces chlorophyll.
Optimizing these factors allows plants to absorb the maximum amount of light energy possible for use in photosynthesis. Strategies like proper watering, fertilization, and pest management help maintain healthy chlorophyll levels.
Measuring Chlorophyll Concentration
Since chlorophyll is so important for photosynthesis, it is often useful for farmers and researchers to measure the chlorophyll level or concentration in plant leaves and tissues. Some methods for measuring chlorophyll concentration include:
- Chlorophyll meters – Handheld devices that measure chlorophyll absorbance in intact leaves.
- Spectrophotometry – Measuring extracted chlorophyll against standards using a spectrophotometer.
- Fluorescence – Using fluorescence techniques to estimate chlorophyll content.
- Acetone extraction – Grinding leaves in acetone to dissolve and extract chlorophyll for quantification.
Knowing the chlorophyll concentration helps optimize crop growth and yield. It also allows detection of stressed plants and nutrient deficiencies.
Chlorophyll in Photosynthesis
The main function of chlorophyll in photosynthesis is to absorb light energy and transfer it to the reaction center where charge separation occurs. This light energy is converted into chemical energy in the form of ATP and NADPH which fuel carbon fixation and other cellular processes. Specifically, chlorophyll:
- Acts as an accessory pigment to pass absorbed light energy to reaction centers
- Drives the electron transport chain by exciting electrons from photosystem II
- Generates the proton gradient used to produce ATP synthase
- Ultimately provides the energy to reduce CO2 into carbohydrates
In plants, there are two main types of chlorophyll – chlorophyll a and chlorophyll b. Chlorophyll a is the primary pigment that captures light energy and converts it into chemical energy. Chlorophyll b acts as an accessory pigment, passing absorbed light to chlorophyll a reaction centers.
Without chlorophyll to absorb light energy, photosynthesis could not occur and plants would be unable to grow and survive. Chlorophyll allows photosynthetic organisms to tap into the vast energy resource of sunlight to power biological processes.
Conclusion
In summary, chlorophyll is the green pigment that allows plants to absorb energy from sunlight. It preferentially absorbs red and blue wavelengths while reflecting green light. When chlorophyll absorbs light, the energy can be re-emitted as fluorescence, converted to heat, or used to transport electrons during photosynthesis. This chemical energy is ultimately used to fix carbon dioxide into carbohydrates that the plant needs for food and growth. Chlorophyll is thus one of the most important biological molecules on Earth, enabling plants to harness solar energy through the process of photosynthesis.