Photosynthesis in plants – Introduction Part I

• Photosynthesis is a process where green plants and some other organisms (algae and some bacteria – such as Proteobacteria , Chloroflexi, Chlorobi and cyanobacteria ) uses trapped sunlight to convert carbon di oxide and water to produce chemical energy in the form of carbohydrates (most commonly glucose) and oxygen.
• In other words, photosynthesis is a process where in green plants and some other organisms prepare their own food from inorganic molecules such as carbon dioxide (CO₂) and hydrogen sulfide (H₂S) using sunlight. Photosynthesis ensures that these organisms don’t have to eat or depend on other living organisms for food.
• Photosynthesis in green plants is important for two reasons – Important source for all food on Earth and also releases oxygen into atmosphere for us to breath.
• So to eat and breathe, humans are completely dependent on the organisms that carry out photosynthesis – plants.
• Autotroph is an organism that can produce its own food by means of photosynthesis using CO₂ as principal carbon source.
• Heterotrophs are incapable of making their own food and depends directly or indirectly on photosynthesis process of autotrophs.
• Photosynthesis is a complex process which can be simplified by following equation : CO₂ + H₂O in presence of sunlight gives rise to Glucose and O₂
• 6CO₂ + 12H₂O + Light energy → C₆H₁₂O₆ (sugar) + 6H₂O + 6O₂. It is important to note that this is not a single reaction but a summary of multiple steps involved in photosynthesis process.
• In green plants, water molecule is the Hydrogen donor, which is oxidized to Oxygen, so oxygen released in the process comes from water but not from CO₂. This process is termed as “Oxygenic Photosynthesis“.
• In some bacteria (Green Sulphur bacteria). Hydrogen sulfide (H₂S) acts as Hydrogen donor and Sulphur is the oxidation product and not Oxygen as in green plants. This process is called as “Anoxygenic Photosynthesis“.
• oxidation in a biological process – involves the loss of electrons as compared to the reduction process in which electrons are gained.
• Oxidation and reduction, always occur together and termed as a ‘redox‘ reaction, which is an energy-producing reaction within the cell. 
• C.B. Van Niel proposed that photosynthesis was essentially a process of oxidation–reduction. 6CO₂ + 12H₂A + Light energy → C₆H₁₂O₆ (sugar) + 6H₂O + 6A
• In the above equation , H2A is an electron donor (reducing agent) – Either H2O or H2S depending on the organism. CO2 acts as oxidizing agent , where it is reduced to form glucose.

• Photosynthesis occurs in a membrane bound cell organelle called as Chloroplast found in green plants and algae.
• In photosynthetic bacteria (eg: Cyanobacteria), chloroplasts are absent and photosynthesis occurs in thylakoid membranes found in cytoplasm.

In this article we will understand mostly about photosynthesis in evolved plants.

Chloroplast:

• Photosynthesis occurs in the leaves of plants and that too in the middle layer calls as mesophyll.
• The exchange of CO₂ and O₂ takes place through small regulated openings termed as stomata.
• Mesophyll harbor many cells which contains a double membrane bound cell organelle called as Chloroplast. The space between the membranes is called as Stroma (not stomata , the opening for gas exchange in leaves).
• Photosynthesis takes place in the Chloroplasts. German biologist T. Engelmann identified Chloroplasts at the site of photosynthesis in green algae Spirogyra.
• Chloroplast are found in cytoplasm of the cell and similar to mitochondria posses their own genetic material as DNA.
• Genetic material is double stranded, circular DNA along with ribosomes are found in stroma.
• Based on the theory of endosymbiosis, ancestors of Cyanobacteria may have evolved into present-day chloroplasts that exist in plants of today.
• Another structure of interest in chloroplast is Thylakoid membrane, which is vital for photosynthesis.
• Thylakoid forms the third membrane (apart from double membrane surrounding chloroplast) that forms stacked, disc-shaped structures.  A stack of thylakoids is called granum, and the space surrounding the granum is called stroma . Stroma contains the enzymes required of synthesis of Glucose. 
• Inside these thylakoid membrane, we find a green pigment (light absorbing molecule) termed as Chlorophyll. It absorbs light of red and blue wavelength and emits green light. That’s why we see plants with chlorophyll as green (The color of any objects depend of the wavelength of light they emit).
• In addition to Chlorophyll, a complex chain of electron carriers, and an ATP-synthesizing machinery is also present in thylakoids.
• Chlorophyll imparts green color to plants and is starting point for photosynthesis.

The process of Photosynthesis can be divided into two parts :

1) The light dependent reactions – “Photo” part where reactions triggered by light occurs which leads to synthesis of chemical energy in the form of ATP , NADPH and release of oxygen.

2) Calvin Cycle / Dark Reactions or “Synthesis” part of photosynthesis , where Glucose is synthesized from capture of carbon in carbon dioxide molecules using chemical energy formed during light-dependent reactions.

Light Reactions Overview :

• Both light and dark reactions occur in chloroplast. There is a clear division of labor within the chloroplast when it comes to different events of photosynthesis. Light reactions occur in Thylakoid membranes and dark reactions takes place in Stroma.
• The thylakoid membrane system helps in trapping the light energy from sun and convert it in the form of chemical energy by synthesizing two key biological molecules – Adenosine Tri Phosphate (ATP) and Nicotinamide adenine dinucleotide phosphate (NADPH). Both ATP and NADPH allows a cell to store energy.
• ATP is the cell’s main source of chemical energy and often referred to as “energy currency of a cell“, and NADPH is its primary source of reducing power.
• The ATP and NADPH formed during light reactions phase are used in subsequent phase of Dark reactions for synthesis of carbohydrates from CO₂.
• Important to note that in light reaction, sugars are not made but it only produces the energy (ATP and NADPH) required synthesis of sugars.
• Apart from ATP , NADPH, light reactions also releases something really very important for survival of all animals – Oxygen.

Light Energy :

• The sun emits energy in the form of electromagnetic radiation. Solar electromagnetic spectrum includes ultraviolet light, infrared, radio waves, X-rays, and gamma rays. Most of the high energy harmful radiations are prevented from reaching Earth’s atmosphere by varying means.
• Visible light are the only radiation waves that are in the frequency range (700 nm to 400 nm) easily seen by the naked eye.
• Light travels in packet of energy called as Photons. These Photons have no mass but have energy and the energy depends on wavelength of light.
• The longer the wavelength, the less energy is carried. Short, tight waves carry the most energy.
• Plants pigment molecules also absorb light only in the visible range of 700 nm to 400 nm.

Steps in Light Reaction:

• Light reactions can be subdivided into following important events : Light absorption, splitting of water, Oxygen release and formation of high energy chemical intermediates in the form of ATP and NADPH.
• Photosynthesis is initiated when the light is absorbed by a leaf, its energy excites the Chlorophyll pigment present in thylakoid membrane. This trapping of sunlight is key event of photosynthesis.
• Chlorophylls are excellent photoreceptors due to alternating single and double bonds in their structure and they exhibit very strong absorption bands in the visible region of the spectrum.
• Even though Chlorophyll a is a major pigment for trapping light, but other pigments present in thylakoid, such as chlorophyll b, xanthophyll’s and carotenoids also contribute in light absorption.
• These accessory pigments (chlorophyll b, xanthophyll’s and carotenoids) absorb light and transfer the energy to chlorophyll a. Accessory pigments also protect chlorophyll a from photo-oxidation.
• When a photon is absorbed by a molecule like Chlorophyll, a electron gets sufficient energy to be excited from its ground energy level to an excited energy level.
• In absence of an efficient electron acceptor, the excited electron will return to the ground state by losing heat.
• If a acceptor is present, then the excited electron will be transferred to acceptor molecule. This will give positive charge (due to loss of electron) to the initial molecule and the acceptor molecule becomes negatively charged. This process is called “photoinduced charge separation”.
• The place where this process occurs is called as “reaction centers”.

In the next blog post we talk more about Light reactions such as Reaction centers, splitting of water, ATP synthesis and more.

Part II – Photosynthesis – Light Reactions

PART III – Calvin Cycle – Photosynthesis in plants

Part VI – Differences between Light and Dark reactions or Calvin Cycle

Part V – Photorespiration, C4 Pathway and CAM metabolism pathway

Part VI – C3, C4 and CAM types of photosynthesis comparison

Part VII – factors affecting the rate of photosynthesis

Part VIII – Importance of Photosynthesis

Photolysis of Water

Anoxygenic Photosynthesis

Image credit : Photosynthesis process – Freepix.

Site of Photosynthesis : Openstax Biology