The Photosynthesis
The Photosynthesis
The Photosynthesis
-is a process used by plants and other organisms to convert light energy that, through cellular
respiration, can later be released to fuel the organism’s activities.
Photosynthesis is the process by which plants, algae and some mocro organisms converts solar energy
and CO2 into chemical energy
Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes
These organisms feed not only themselves but also most of the living world
Autotrophs sustain themselves without eating anything derived from other organisms
Autotrophs are the producers of the biosphere, producing organic molecules from CO 2 and other
inorganic molecules Almost all plants are photoautotrophs, using the energy of sunlight to make organic
molecules
Heterotrophs obtain their organic material from other organisms
Heterotrophs are the consumers of the biosphere
Almost all heterotrophs, including humans, depend on photoautotrophs for food and O 2
some heterotrophs consume the remains of dead organisms by decomposing and feeding on organic
litter such as carcasses, feces and fallen leaves- decomposers.
Pigments -Are chemical compounds which reflect only certain wavelengths of visible lights. This makes
them appear “colorful”. Flowers, corals, and even animal skin that contain pigments which give them
colors. More important than their reflection of light is the ability of pigments to absorb certain
wavelengths.
Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria
The structural organization of these cells allows for the chemical reactions of photosynthesis
Leaves are the major locations of photosynthesis
Their green color is from chlorophyll, the green pigment within chloroplasts
Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf
Each mesophyll cell contains 30–40 chloroplasts
Microscopic pores called stomata
The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast);
thylakoids may be stacked in columns called grana
Chloroplasts also contain stroma
Photosynthetic Pigments
CHLOROPHYLL A
CHLOROPHYLL B
XANTHOPHYLLS
CAROTENOIDS
CHLOROPHYLL A (blue-green)
IS A ESSENTIAL FOR MOST PHOTOSYNTHETIC ORGANISMS TO RELEASE CHEMICAL ENERGY BUT IS NOT
THE ONLY PIGMENT THAT CAN BE USED FOR PHOTOSYNTHESIS.
XANTHOPHYLLS (yellow)
CAN FUNCTION AS ACCESSORY LIGHT-HARVESTING PIGMENTS, AS STRUCTURAL ENTITIES WITHIN THE
LHC, AND AS MOLECULES REQUIRED FOR THE PROTECTION OF PHOTOSYNTHETIC ORGANISMS FROM
THE POTENTIALLY TOXIC EFFECTS OF LIGHT
PROCESS OF PHOTOSYNTHESIS
At the cellular level, the photosynthesis process takes place in cell organelles called chloroplasts. These
organelles contain a green-coloured pigment called chlorophyll, which is responsible for the
characteristic green colouration of the leaves.
PHOTOSYNTHESIS STEPS
-During the process of photosynthesis, carbon dioxide enters through the stomata, water is absorbed by
the root hairs from the soil and is carried to the leaves through the xylem vessels
-The hydrogen from water molecules and carbon dioxide absorbed from the air are used in the
production of glucose
-Glucose is a source of food for plants that provide energy for growth and development, while the rest is
stored in the roots, leaves and fruits, for their later use.
-Pigments are other fundamental cellular components of photosynthesis. They are the molecules that
impart Colour and they absorb light at some specific Wavelength and reflect back the unabsorbed
light.
The Calvin cycle (in the stroma) forms sugar from CO 2, using ATP and NADPH
The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules
The light reactions convert solar energy to the chemical energy of ATP and NADPH
Chloroplasts are solar-powered chemical factories
Their thylakoids transform light energy into the chemical energy of ATP and NADPH
Photosynthetic Pigments: The Light Receptors
Pigments are substances that absorb visible light
Different pigments absorb different wavelengths
Wavelengths that are not absorbed are reflected or transmitted
Leaves appear green because chlorophyll reflects and transmits green light
A spectrophotometer measures a pigment’s ability to absorb various wavelengths
This machine sends light through pigments and measures the fraction of light transmitted at
each wavelength
The action spectrum of photosynthesis was first demonstrated in 1883 by Theodor W. Engelmann In
his experiment, he exposed different segments of a filamentous alga to different wavelengths
Areas receiving wavelengths favorable to photosynthesis produced excess O 2
He used the growth of aerobic bacteria clustered along the alga as a measure of O 2 production
Chlorophyll a is the main photosynthetic pigment
Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis
Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll
Excitation of Chlorophyll by Light
When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable
When excited electrons fall back to the ground state, photons are given off, an afterglow called
fluorescence
If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat
The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO 2 to sugar
The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter
and leave the cycle
The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons
carried by NADPH
Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phospate (G3P)
For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO 2
The Calvin cycle has three phases
◦ Carbon fixation (catalyzed by ruBisco)
◦ Reduction
◦ Regeneration of the CO2 acceptor (RuBP)
Alternative mechanisms of carbon fixation have evolved in hot, arid climates
Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic
processes, especially photosynthesis
On hot, dry days, plants close stomata, which conserves H 2O but also limits photosynthesis
The closing of stomata reduces access to CO 2 and causes O2 to build up
These conditions favor an apparently wasteful process called photorespiration
The Importance of Photosynthesis:
The energy entering chloroplasts as sunlight gets stored as chemical energy in organic
compounds
Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the
organic molecules of cells
Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits
In addition to food production, photosynthesis produces the O 2 in our atmosphere