Biology

photosynthesis

photosynthesis.

photosynthesis, the process by which green plants and certain other organisms transform light energy into chemical energy. During photosynthesis in green plants, light energy is captured and used to convert water, carbon dioxide, and minerals into oxygen and energy-rich organic compounds.

It would be impossible to overestimate the importance of photosynthesis in the maintenance of life on Earth. If photosynthesis ceased, there would soon be little food or other organic matter on Earth. Most organisms would disappear, and in time Earth’s atmosphere would become nearly devoid of gaseous oxygen. The only organisms able to exist under such conditions would be the chemosynthetic bacteria, which can utilize the chemical energy of certain inorganic compounds and thus are not dependent on the conversion of light energy.

Energy produced by photosynthesis carried out by plants millions of years ago is responsible for the fossil fuels (i.e., coal, oil, and gas) that power industrial society. In past ages, green plants and small organisms that fed on plants increased faster than they were consumed, and their remains were deposited in Earth’s crust by sedimentation and other geological processes. There, protected from oxidation, these organic remains were slowly converted to fossil fuels. These fuels not only provide much of the energy used in factories, homes, and transportation but also serve as the raw material for plastics and other synthetic products. Unfortunately, modern civilization is using up in a few centuries the excess of photosynthetic production accumulated over millions of years. Consequently, the carbon dioxide that has been removed from the air to make carbohydrates in photosynthesis over millions of years is being returned at an incredibly rapid rate. The carbon dioxide concentration in Earth’s atmosphere is rising the fastest it ever has in Earth’s history, and this phenomenon is expected to have major implications on Earth’s climate.

Chemical Reaction as a Formula.

The formula that describes photosynthesis is 6CO2 + 6H20 + light energy = C6H1206 + 602. What this chemical equation means is that photosynthesis combines light energy with six molecules of carbon dioxide and six molecules of water to produce six molecules of oxygen and one molecule of sugar.

How Photosynthesis Works.

A key component that drives photosynthesis is the molecule chlorophyll. Chlorophyll is a large molecule with a special structure that enables it to capture light energy and convert it to high energy electrons, which are used during the reactions of the two phases to ultimately produce the sugar or glucose.

In photosynthetic bacteria, the reaction takes place in the cell membrane and within the cell, but outside of the nucleus. In plants and photosynthetic protozoans — protozoans are single-celled organisms belonging to the eukaryote domain, the same domain of life which includes plants, animals and fungus — photosynthesis takes place within chloroplasts. Chloroplasts are a type of organelle or membrane-bound compartments, adapted for specific functions like creating the energy for plants.

Chloroplasts — An Evolutionary Tale.

While chloroplasts exist today within other cells, such as plant cells, they have their own DNA and genes. Analysis of the sequence of these genes has revealed that chloroplasts evolved from independently-living photosynthetic organisms related to a group of bacteria called cyanobacteria.

A similar process occurred when the ancestors of mitochondria, the organelles within cells where oxidative respiration, the chemical opposite of photosynthesis, takes place. According to the theory of endosymbiosis, a theory which was given a boost recently, because of a new study published in the journal Nature, both chloroplasts and mitochondria once lived as independent bacteria, but were engulfed within the ancestors of eukaryotes, leading ultimately to the emergence of plants and animals.

Two Stages of Photosynthesis.

Photosynthesis represents the biological process by which plants convert light energy into sugar to fuel plant cells. Comprised of two stages, one stage converts the light energy into sugar, and then cellular respiration converts the sugar to Adenosine triphosphate, known as ATP, the fuel for all cellular life. The conversion of unusable sunlight makes plants green.

While the mechanisms of photosynthesis are complex, the overall reaction occurs as follows: carbon dioxide + sunlight + water —> glucose (sugar) + molecular oxygen. Photosynthesis takes place through several steps which occur during two stages: the light phase and the dark phase.

Stage One: Light Reactions.

In the light-dependent process, which takes place in the grana, the stacked membrane structure within chloroplasts, the direct energy of light helps the plant to make molecules that carry energy for utilization in the dark phase of photosynthesis. The plant uses light energy to generate the co-enzyme Nicotinamide adenine dinucleotide phosphate, or NADPH and ATP, the molecules that carry energy. The chemical bonds in these compounds store the energy and are used during the dark phase.

Stage Two: Dark Reactions.

The dark phase, which takes place in the stroma and in the dark when the molecules that carry energy are present, is also known as the Calvin cycle or C3 cycle. The dark phase uses the ATP and NADPH generated in the light phase to make C-C covalent bonds of carbohydrates from carbon dioxide and water, with the chemical ribulose biphosphate or RuBP, a 5-C chemical capturing the carbon dioxide. Six molecules of carbon dioxide enter the cycle, which in turn produces one molecule of glucose or sugar.

Calvin Cycle.

The dark reaction employs ATP and NADPH created in the light reaction to transform carbon dioxide into sugar. This phase happens within the plant’s stoma in the dark. The main cycle in this stage is called the Calvin cycle, which consists of three stages. Stage one, also called carbon fixation phase, is when carbon dioxide combines with ribulose bisphosphate, a five-carbon sugar. In stage two, ATP helps convert the product of stage one into sugar. The third stage, or regeneration phase, again uses ATP to regenerate the reserve levels of RuBp in the cell, completing the cycle.

Light Absorption.

Plants use light energy to start the photosynthesis process and fuel the storage of energy in sugars. Light is divided into various colors with their characteristic wavelengths with each wavelength represented by an individual pigment. Chlorophyll, a specific plant pigment, takes in blue and red light while carotenoid, another type of plant pigment, utilizes blue-green light waves. Green wavelengths are not absorbed efficiently by plants and is reflected by the plant’s leaves and stems, which makes plants appear green.

Factors affecting photosynthesis.

There are several ways of measuring the rate of photosynthesis in the lab. These include:

▪️ the rate of oxygen output.

▪️ the rate of carbon dioxide uptake.

▪️ the rate of carbohydrate production.

These are not perfect methods as the plant will also be respiring, which will use up some oxygen and carbohydrate and increase carbon dioxide output.

Several factors can affect the rate of photosynthesis:

▪️ light intensity. Without enough light, a plant cannot photosynthesise very quickly – even if there is plenty of water and carbon dioxide and a suitable temperature.

Increasing the light intensity increases the rate of photosynthesis , until some other factor – a limiting factor – becomes in short supply.

At very high light intensities, photosynthesis is slowed and then inhibited, but these light intensities do not occur in nature.

▪️ carbon dioxide concentration. Carbon dioxide – with water – is one of the reactants in photosynthesis.If the concentration of carbon dioxide is increased, the rate of photosynthesis will therefore increase.

Again, at some point, a factor may become limiting.

▪️ temperature. The chemical reactions that combine carbon dioxide and water to produce glucose are controlled by enzymes. As with any other enzyme-controlled reaction, the rate of photosynthesis is affected by temperature.

At low temperatures, the rate of photosynthesis is limited by the number of molecular collisions between enzymes and substrates. At high temperatures, enzymes are denatured.

The amount of chlorophyll also affects the rate of photosynthesis:

▪️ plants in lighting conditions unfavourable for photosynthesis may synthesise more chlorophyll, to absorb the light required.

▪️ the effects of some plant diseases affect the amount of chlorophyll, and therefore the ability of a plant to photosynthesise .

Chlorophyll.

Chlorophyll absorbs the light energy required to convert carbon dioxide and water into glucose.

Chlorophyll is green – so absorbs the red and blue parts of the electromagnetic spectrum and reflects the green part of the spectrum.Leaves with more chlorophyll are better able to absorb the light required for photosynthesis.

Reference :

Hans Lambers,James Alan Bassham, “Photosynthesis”، www.britannica.com

Dr. David Warmflash (Updated 26-4-2018), “Two Stages of Photosynthesis”، sciencing.com

Andrew Latham (Updated 27-4-2018), “How Do Plants Store Energy During Photosynthesis?”، sciencing.com

Factors affecting photosynthesis”, www.bbc.co.uk

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