Photosynthesis

Energy from sunlight, water absorbed by the roots and carbon dioxide from the atmosphere produce glucose and oxygen by photosynthesis

Photosynthesis is the process by which plants and other things make food. It is an endothermic (takes in heat) chemical process that uses sunlight to turn carbon dioxide into sugars that the cell can use as energy. As well as plants, many kinds of algae, protists and bacteria use it to get food. Photosynthesis is very important for life on Earth. Most plants either directly or indirectly depend on it. The exception are certain 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 CO_2(g) + 6 H₂O + photons → C₆H₁₂O_6(aq) + 6 O_2(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 Chloroplast (found in leaf cells) and contain Chlorophyll, the green pigment that absorbs light energy.

Palisade cell has numerous chloroplasts to maximise the absorption of light.

Oxygen is a waste product: it is used for respiration or diffuses back out of the leaf through the stomata.

Glucose is used for 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 eg. cellulose, nectar, fructose, amino acids and fats.

Reactions

Diagram of a chloroplast

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 reaction

Light energy from the sun is used to split the water molecules (photolysis). The sunlight hits chloroplasts in the plant, causing an enzyme to break apart the water. Water, when broken, makes 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 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.

Light-independent 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

Template:Graphical timeline There are three main factors affecting photosynthesis:

• Light intensity
• Carbon dioxide concentration
• Temperature

Light intensity

If there is little light shining on a plant, the light-dependent reactions will not work efficiently. This means that photolysis 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.

Increasing the light intensity will increase the rate of photosynthesis, so long as there is enough carbon dioxide and the temperature isn't too cold.

The light intensity required is easily investigated in an aquatic plant such as pondweed. The bubbles of gas (oxygen) given off can easily be counted or the volume measured. By changing the distance between light and plant, the light intensity will vary.

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.

You can enrich the carbon dioxide concentration in enclosed environments by using gas burners. This has a dual function: will increase the temperature as well as producing carbon dioxide.

Temperature

There are many enzymes working in photosynthetic reactions – such as the enzyme in photolysis. These enzymes will not work as well, or stop working at all at high or low temperatures and therefore, so will the light-dependent and light-independent reactions. 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 maintain the optimum temperature.

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

• Cellular respiration