What is the process of making food by using light?. .As with all other types of energy, light can travel, change forms, and be harnessed to do work.With photosynthesis, light energy is converted into chemical energy, which autotrophs use to construct carbohydrate molecules.


Concept in Action


Watch the animation below to learn how a leaf performs photosynthesis.

The sun emits a huge amount of electromagnetic radiation (solar energy).In fact, humans see only a small fraction of this energy, called "visible light." Solar energy travels in waves, which can be described and measured.Wavelength is a measure of distance between successive, similar points in a wave, such as crest to crest or trough to trough (Figure 5.9).


Light from the sun is only one type of electromagnetic radiation emitted by the sun.The electromagnetic spectrum contains all wavelengths of radiation (Figure 5.10).Every wavelength represents a different amount of energy.


Electromagnetic radiation has a range of wavelengths characteristic to each type.Energy is carried less effectively by longer wavelengths (or stretched wavelengths).In other words, short, tight waves carry the most energy.You may think this is illogical, but picture a piece of ribbon that's moving.Long, wide waves are easy for a person to move with little effort.In order to move a rope in short, tight waves, a person would have to exert significantly more effort.

There are many types of electromagnetic radiation emitted by the sun, including X-rays and ultraviolet (UV) rays.Waves of higher energy are dangerous for living things; for example, X-rays and UV rays are harmful to humans.

During photosynthesis, pigments absorb light energy.Photosynthetic pigments absorb visible light only in plants.It is actually possible to perceive white light in rainbow colors as well as visible light.White light can be dispersed by certain objects, such as prisms and drops of water, to reveal these hues.In the visible light portion of the electromagnetic spectrum, the human eye perceives a rainbow of colors, with violet and blue having shorter wavelengths due to their higher energy.The wavelengths are longer and have a lower energy at the red end of the spectrum.

Different pigments absorb only certain wavelengths of visible light (colors).As a result, pigments reflect the colors of the wavelengths they are unable to absorb.

A pigment called chlorophyll a is found in all photosynthetic organisms, and it is this pigment that humans perceive as being associated with the color green.As a result, chlorophyll a absorbs wavelengths from either end of the spectral range (blue and red), but not from green.Chlorophyll is green because green wavelengths are reflected.

Besides chlorophyll b (which absorbs blue and red-orange light), there are other pigment types such as carotenoids.The absorption spectrum of a pigment lets us recognize its type based on the specific wavelengths it absorbs from visible light.

In many photosynthetic organisms, pigments are mixed together. Through a combination of pigments, they can absorb energy from a wider range of visible wavelengths.Photosynthesis is not available to all organisms.Light intensity decreases with depth, and certain wavelengths are absorbed by the water when organisms grow underwater.These organisms compete for light, however.As tall trees block most of the sunlight (Figure 5.11), the rainforest floor plants must be able to absorb any remaining light.


Essentially, light-dependent reactions are converted into chemical energy.The Calvin cycle uses this chemical energy to put together sugar molecules.

A photosystem, or group of pigment molecules and proteins, initiates the light-dependent reactions.In thylakoid membranes, photosynthetic activity is present.One pigment molecule in a photosystem absorbs one photon - a quantity or "packet" of light energy.

When a photon of light reaches a molecule of chlorophyll, it becomes a photon of light energy.A photon induces an electron in the chlorophyll molecule to become "excited." This energy allows the electron to break free from an atom in the chlorophyll molecule.Chlorophyll is therefore said to give an electron (Figure 5.12).

During chlorophyll synthesis, a molecule of water is split in order to replace the electrons.By splitting the electron, oxygen (O2) and hydrogen ions (H+) are released in the thylakoid space.Each time a water molecule breaks, two electrons are released, replacing two donated electrons.


A new electron can be substituted in chlorophyll for a response to another photon.The oxygen molecules produced as byproducts end up in the surrounding environment.In the remaining light-dependent reactions, hydrogen ions play an important role.

During the light-dependent reactions, solar energy gets converted into chemical carriers that will later be used in the Calvin cycle.Eukaryotes and some prokaryotes have two different kinds of photosystems.There is also the photosystem II, which was named after the order of its discovery rather than according to its function.

.Electrons pass along these proteins, allowing energy from the electron to power pumps that actively move hydrogen ions from the stroma into the thylakoid space.It is quite analogous to the process in the mitochondria in which hydrogen ions are pumped from the mitochondrial stroma across the inner membrane into the intermembrane space via an electron transport chain.Having used the energy, the electron is accepted by a pigment molecule in the next photosystem, known as photosystem I (Figure 5.13).


ATP and NADPH are the energy carriers for light-dependent reactions.Molecules store their energy in bonds anchored by a single atom.It is a phosphate atom in ATP, and it is a hydrogen atom in NADPH.A similar molecule, NADH, carried energy from the citric acid cycle to the electron transport chain in the mitochondrion.They each lose atoms as they release energy into the Calvin cycle to become the lower-energy molecules ADP and NADP+.

A gradient exists between the charge across the membrane created by protons (H+) in the thylakoid space, and the difference in concentration of protons (H+) that they create.

By allowing hydrogen ions to pass through the thylakoid membrane, ATP synthase allows the ions to pass.In the mitochondrion, this protein makes ATP from ADP.As a result of the energy created by the hydrogen ion stream, ATP synthase attaches a third phosphate to ADP, forming ATP through a process called photophosphorylation.Through ATP synthase, hydrogen ions move from an area of high concentration to a region of low concentration through a semi-permeable structure, a process called chemiosmosis.

It is the remaining function of the light-dependent reaction to generate the energy-carrier molecule NADPH.At photosystem I, the electron from the electron transport chain receives another photon from chlorophyll, which energizes it.This electron's energy leads to the formation of NADPH from NADP+ and a hydrogen ion (H+).The solar energy has been stored in energy carriers and can be used to make sugar molecules.

Pigment molecules absorb light energy during the light-dependent reaction that is the first part of photosynthesis.chlorophyll a is the most common pigment used in photosynthesis.The photon strikes photosystem II to begin photosynthesis.The electron transport chain transports energy through hydrogen ions into the thylakoid space.In turn, this produces an electrochemical gradient. .During the Calvin cycle, a second photon is absorbed by photosystem I, which forms a molecule of NADPH, another carrier of energy.