Photosynthesis facts for kids
Photosynthesis is the process by which plants and some microorganisms make substances like carbohydrates. The word "photosynthesis" comes from Greek words: "photo," meaning light, and "synthesis," meaning putting together.
Photosynthesis is an endothermic (takes in heat) chemical process that uses sunlight to turn carbon dioxide into sugars. The sugars are used by the cell as energy, and to build other kinds of molecules.
Photosynthesis is very important for life on Earth. Green plants build themselves using photosynthesis. Algae, protists and some bacteria also use it. Some exceptions are organisms that directly get their energy from chemical reactions; these organisms are called chemoautotrophs.
Photosynthesis can happen in different ways, but there are some parts that are common.
- 6 CO2(g) + 6 H2O + photons → C6H12O6(aq) + 6 O2(g)
- carbon dioxide + water + light energy → glucose + oxygen
- The carbon dioxide enters the leaf through the stomata by Diffusion from the atmosphere.
- The water is absorbed from the soil by root hair cells, which have an increased surface area for an increased uptake of water.
Photosynthesis occurs in the chloroplasts in leaves (or other green tissues). They contain chlorophyll, the green pigment that absorbs light energy. In leaves, palisade cells have chloroplasts to capture light.
Oxygen is a waste product of photosynthesis: it comes out of the plant in respiration. All the oxygen in the atmosphere has its origin in plants (including those microorganisms which do photosynthesis)
Glucose is used in respiration (to release energy in cells). It is stored in the form of starch (which is converted back to glucose for respiration in the dark). Glucose can also be converted into other compounds for growth and reproduction e.g. cellulose, nectar, fructose, amino acids and fats.
Contents
Reactions of the process
Photosynthesis has two main sets of reactions. Light-dependent reactions need light to do work; and light-independent reactions, which do not need light to do work.
Light-dependent reactions
Light energy from the sun is used to split water molecules (photolysis). The sunlight hits chloroplasts in the plant. This causes an enzyme to split the water. Water, when split, gives oxygen, hydrogen, and electrons.
Hydrogen, along with electrons energized by light, converts NADP into NADPH which is then used in the light-independent reactions. Oxygen gas diffuses out of the plant as a waste product of photosynthesis, and ATP is synthesized from ADP and inorganic phosphate. This all happens in the grana of chloroplasts.
Dark reaction
During this reaction, sugars are built up using carbon dioxide and the products of the light-dependent reactions (ATP and NADPH) and various other chemicals found in the plant in the Calvin Cycle. Therefore, the light-independent reaction cannot happen without the light-dependent reaction. Carbon dioxide diffuses into the plant and along with chemicals in the chloroplast, ATP, and NADPH, glucose is made and finally, transported around the plant by translocation.
Factors affecting photosynthesis
Orange labels: known ice ages.
Also see: Human timeline and Nature timeline
There are three main factors affecting photosynthesis:
Light intensity
If there is little light shining on a plant, the light-dependent reactions will not work efficiently. This means that photolysis (breakdown of water by light) will not happen quickly, and therefore little NADPH and ATP will be made. This shortage of NADPH and ATP will lead to the light-independent reactions not working as NADPH and ATP are needed for the light-independent reactions to work.
The light intensity required is easily investigated in an aquatic plant such as pondweed. The oxygen bubbles given off can be counted or the volume measured. By changing the distance between light and plant, the light intensity is made to vary. Change in light intensity will affect the change in rate of photosynthesis . Artificial lighting can be used in the dark to maximise the photosynthetic rate.
Carbon dioxide levels
Carbon dioxide is used in the light-independent reactions. It combines with NADPH and ATP and various other chemicals (such as Ribulose Bisphosphate) to form glucose. Therefore, if there is not enough carbon dioxide, then there will be a buildup of NADPH and ATP and not enough glucose will be formed.
Temperature
There are many enzymes working in photosynthetic reactions – such as the enzyme in photolysis. All enzymes work best at their optimum temperature. All light-dependent and light-independent reactions will occur normally at average or optimum temparatures. Tropical plants have a higher temperature optimum than the plants adapted to temperate climates.
When the temperatures are too low, there is little kinetic energy, so the reaction rate decreases. If the temperatures are too high, the enzymes become denatured and the catalysis of photosynthesis reaction stops.
Greenhouses must keep an optimum temperature for normally functioning of plants.
Early evolution
The first photosynthetic organisms probably evolved early in the history of life. They may have used reducing agents such as hydrogen or hydrogen sulfide as sources of electrons, rather than water. Cyanobacteria appeared later, and the excess oxygen they produced contributed to the oxygen catastrophe. This made the evolution of complex life possible.
Effectiveness
Today, the average rate of energy capture by photosynthesis globally is approximately 130 terawatts, which is about six times larger than the current power used by human civilization. Photosynthetic organisms also convert around 100–115 thousand million metric tonnes of carbon into biomass per year.
Related pages
Images for kids
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Composite image showing the global distribution of photosynthesis, including both oceanic phytoplankton and terrestrial vegetation. Dark red and blue-green indicate regions of high photosynthetic activity in the ocean and on land, respectively.
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The "Z scheme"
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Plant cells with visible chloroplasts (from a moss, Plagiomnium affine)
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Portrait of Jan Baptist van Helmont by Mary Beale, c.1674
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Melvin Calvin works in his photosynthesis laboratory.
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The leaf is the primary site of photosynthesis in plants.
See also
In Spanish: Fotosíntesis para niños
