Photosynthesis: Site, Type & Factors Affecting | Biology for JAMB PDF Download

What is Photosynthesis?

“Photosynthesis is the process used by green plants and a few organisms that use sunlight, carbon dioxide and water to prepare their food.”

The process of photosynthesis is used by plants, algae and certain bacteria that convert light energy into chemical energy. The glucose formed during the process of photosynthesis provides two important resources to organisms: energy and fixed carbon.

Site of Photosynthesis

Types of Photosynthesis

There are two different types of photosynthesis:

• Oxygenic Photosynthesis
• Anoxygenic Photosynthesis

1. Oxygenic Photosynthesis
   Oxygenic photosynthesis is more common in plants, algae and cyanobacteria. During this process, electrons are transferred from water to carbon dioxide by light energy, to produce energy. During this transfer of electrons, carbon dioxide is reduced while water is oxidized, and oxygen is produced along with carbohydrates.
   During this process, plants take in carbon dioxide and expel oxygen into the atmosphere.
   This process can be represented by the equation:
   6CO₂ + 12H₂O + LIGHT ENERGY → C₆H₁₂O₆ + 6O₂ + 6H₂O
2. Anoxygenic Photosynthesis 
   This type of photosynthesis is usually seen in certain bacteria, such as green sulphur bacteria and purple bacteria which dwell in various aquatic habitats. Oxygen is not produced during the process.
   The anoxygenic photosynthesis can be represented by the equation:
   CO₂ + 2H₂A + LIGHT ENERGY → [CH₂O] + 2A + H₂O

Photosynthesis Apparatus
The photosynthesis apparatus includes the following essential components:

1. Pigments
   Pigments not only provide colour to the photosynthetic organisms, but are also responsible for trapping sunlight. The important pigments associated with photosynthesis include:
   (i) Chlorophyll: It is a green-coloured pigment that traps blue and red light. Chlorophyll is subdivided into, “chlorophyll a”, “chlorophyll b”, and “chlorophyll c”. “Chlorophyll a” is widely present in all the photosynthetic cells. A bacterial variant of chlorophyll known as bacteriochlorophyll can absorb infrared rays.
   (ii) Carotenoids: These are yellow, orange or red-coloured pigments that absorb bluish-green light. Xanthophyll and carotenes are examples of carotenoids.
   (iii) Phycobilins: These are present in bacteria and red algae. These are red and blue pigments that absorb wavelength of light that are not properly absorbed by carotenoids and chlorophyll.
2. Plastids
   Plastids are organelles found in the cytoplasm of eukaryotic photosynthetic organisms. They contain pigments and can also store nutrients. Plastids are of three types:
   (i) Leucoplast: These are colourless, non-pigmented and can store fats and starch.
   (ii) Chromoplasts: They contain carotenoids.
   (iii) Chloroplasts: These contain chlorophyll and are the site of photosynthesis. 
3. Antennae
   Antennae is the collection of 100 to 5000 pigment molecules that capture light energy from the sun in the form of photons. The light energy is transferred to a pigment-protein complex that converts light energy to chemical energy.
4. Reaction Centers
   The pigment-protein complex responsible for the conversion of light energy to chemical energy forms the reaction centre.

Factors Affecting Photosynthesis

(1) Light

(a) Light Quality or Wavelength → Maxm photosynthesis takes place in red light than in Blue light. But rate of photosynthesis is highest in white light. Minimum in green light.

(b) Light Intensity – Rate of photosynthesis is greater in intense light than diffused light. But at higher light intensity photooxidation (solarization) occurs and photosynthetic apparatus may get destroyed.

• P/R (Photosynthesis : Respiration)  Ratio at mid day is 10 : 1; but can reach upto 20 : 1 ratio. At the time of evening & morning rate of photosynthesis equals to respiration, this situation called as light compensation point.

• Intensity of light, at which rate of photosynthesis, becomes equal (or compensate) with the rate of respiration in plants is known aslight compensation point. (Net photosynthesis or net primary productivity at this point is zero.)Plants which are adapted to grow in high intensity of light is called heliophytes & plants which are adapted to grow in shade is sciophytes. (c) Duration of Light – On the basis of effect of light on plants may be LDP & SDP. Product of photosynthesis is greater in intermittent light than continuous light – Warburg.

(2) Temperature

• Optimum temp. for photosynthesis is 20–35°C

• At high temp. rate of photosynthesis decreases due to denaturation of enzymes.

• Conifers & lichens can perform photosynthesis at –35°C, while thermal algae Oscillatoria at 70–80°C.

• Generally different habitat plants show, different response to photosynthesis on a given temperature.

(3) CO₂ (0.03%/314 ppm)

• An increase in CO₂ concentration upto 1% rate of photosynthesis is increased. Higher CO₂ concentration. is toxic to plant & also closes stomata.

• C₄–Plants can photosynthesize at low CO₂ concn (upto 10 ppm). “CO₂ concn at which CO₂ fixation in photosynthesis is equal to volume of CO₂ released in respiration is"CO2compensation point”, when plant saturated with full light.

• CO₂ compensation point for C₄ plants is 8-10 ppm, while for C3 plants it is 40-100 ppm.

4) O₂ 
High O₂ concn. reduces photosynthesis due to photorespiration.

(5) Water – Less availability of water reduces the rate of photosynthesis (stomata get closed)

(6) Chlorophyll– The amount of CO₂  in grams absorbed by 1 gm. of chlorophyll in 1 hour is called as photosynthetic number or assimilatory number (Willstatter & Stoll).

(7) Product – Rate of photosynthesis decreases, when sugar accumulates in mesophyll cells.

(8) Leaf – Various leaf factors likeleaf ageandleaf orientationeffect the rate of photosynthesis. In young & mature leaves photosynthesis is more than old (senescent) leaves.

(9) Inhibitors – DCMU (Diuron/Dichlorophenyl Dimethyl Urea)CMU (Monuron), PAN,Atrazine, Simazime, Bromocil, Isocil–inhibit the photosynthesis by blocking PS–II. They stop e– flow between P-680 & PQ. In cyclic ETS diquat, paraquat (Viologen dyes) inhibit e– flow between P-700 & Fd. All these chemicals are used as herbicides, which mostly block ETS.

(10) Minerals :- Mg and Nitrogen are essential for structure of chlorophyll and enzymes. Thus reduction in N₂ and Mg supply to plants effects adversely the rate of photosynthesis. Rubisco alone accounts for more than half of total leaf nitrogen.

Generally all essential element affect the rate of photosynthesis.

• Concept of three cardinal points (Von Sachs) :- The effect of the various external factors on the rate of biological processes were centred around the attempts to establish minimum, optimum and maximum values known as cardinal points.

• Law of minimum (Liebig) :-  According to it, when a process is governed by a number of separate factors, then the rate of process is controlled by that factor present in minimum amount.

• Law of limiting factors – (Blackman) :- It is the modification of Law of minimum by Liebig. "When a process is conditioned to its rapidity by a number of factors, then rate of process is limited by the pace of the slowest factor" (CO₂, light, chlorophyll, water, temp.)

• CO₂ becoming limiting in clear sky, but light limiting in cloudy days.

• Atmospheric CO₂ is not limiting factor for C₄ plants & submerged hydrophytes.

Old Syllabus NCERT
BACTERIAL PHOTOSYNTHESIS

• Certain bacteria are capable for photosynthesis Eg :- Chlorobium (Green Sulphur), Chromatium (Purple Sulphur), Rhodospirillum, Rhodopseudomonas (Purple non sulphur).

• Cyclic photophosphorylation is an important method in bacterial photosynthesis.

• Absorption of Infra red spectrum takes place during bacterial photosynthesis thus no red drop.

• Pigment system of bacteria denoted by – B–890 or 870

• Evolution of O₂ is not related to bacterial photosynthesis, because water is not e– donor and PS II is absent.

• Only one ATP is produced in each turn of cyclic photophosphorylation, in bacteria.

• Olson 1970 gave a non cyclic scheme in bacterial photosynthesis.

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• Bacteria has only one pigment system, PSI.