Photosynthesis

Photosynthesis (from the Greek φώτο- [photo-], "light," and σύνθεσις [synthesis], "putting together", "composition") is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight.[1] Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can create their own food. In plants, algae, and cyanobacteria, photosynthesis uses carbon dioxide and water, releasing oxygen as a waste product. Photosynthesis is vital for all aerobic life on Earth. As well as maintaining the normal level of oxygen in the atmosphere, nearly all life either depends on it directly as a source of energy, or indirectly as the ultimate source of the energy in their food[2] (the exceptions are chemoautotrophs that live in rocks or around deep sea hydrothermal vents). The rate of energy capture by photosynthesis is immense, approximately 100 terawatts,[3] which is about six times larger than the power consumption of human civilization.[4] As well as energy, photosynthesis is also the source of the carbon in all the organic compounds within organisms' bodies. In all, photosynthetic organisms convert around 100–115 teragrams of carbon into biomass per year.[5][6]

Although photosynthesis can happen in different ways in different species, some features are always the same. For example, the process always begins when energy from light is absorbed by proteins called photosynthetic reaction centers that contain chlorophylls. In plants, these proteins are held inside organelles called chloroplasts, while in bacteria they are embedded in the plasma membrane. Some of the light energy gathered by chlorophylls is stored in the form of adenosine triphosphate (ATP). The rest of the energy is used to remove electrons from a substance such as water. These electrons are then used in the reactions that turn carbon dioxide into organic compounds. In plants, algae and cyanobacteria, this is done by a sequence of reactions called the Calvin cycle, but different sets of reactions are found in some bacteria, such as the reverse Krebs cycle in Chlorobium. Many photosynthetic organisms have adaptations that concentrate or store carbon dioxide. This helps reduce a wasteful process called photorespiration that can consume part of the sugar produced during photosynthesis.

Overview of cycle between autotrophs and heterotrophs. Photosynthesis is the main means by which plants, algae and many bacteria produce organic compounds and oxygen from carbon dioxide and water (green arrow).

The first photosynthetic organisms probably evolved about 3,500 million years ago, early in the evolutionary history of life, when all forms of life on Earth were microorganisms and the atmosphere had much more carbon dioxide. They most likely used hydrogen or hydrogen sulfide as sources of electrons, rather than water.[7] Cyanobacteria appeared later, around 3,000 million years ago, and drastically changed the Earth when they began to oxygenate the atmosphere, beginning about 2,400 million years ago.[8] This new atmosphere allowed the evolution of complex life such as protists. Eventually, no later than a billion years ago, one of these protists formed a symbiotic relationship with a cyanobacterium, producing the ancestor of many plants and algae.[9] The chloroplasts in modern plants are the descendants of these ancient symbiotic cyanobacteria.[10]

Plasmolysis

Plasmolysis is the process in plant cells where the plasma membrane pulls away from the cell wall due to the loss of water through osmosis. The reverse process, cytolysis, can occur if the cell is in a hypotonic solution resulting in a higher external osmotic pressure and a net flow of water into the cell. Through observation of plasmolysis and deplasmolysis it is possible to determine the tonicity of the cell's environment as well as the rate solute molecules cross the cellular membrane.

If a plant cell is placed in a hypertonic solution, the plant cell loses water and hence turgor pressure, making the plant cell flaccid. Plants with cells in this condition wilt. Further water loss causes plasmolysis: pressure decreases to the point where the protoplasm of the cell peels away from the cell wall, leaving gaps between the cell wall and the membrane. Eventually cytorrhysis – the complete collapse of the cell wall – can occur. There are some mechanisms in plants to prevent excess water loss in the same way as excess water gain, but plasmolysis can be reversed if the cell is placed in a weaker solution (hypotonic solution). Stomata help keep water in the plant so it does not dry out. Wax also keeps water in the plant. The equivalent process in animal cells is called crenation.

The liquid content of the cell leaks out due to diffusion. The cell collapse and cell membrane pulls away from the cell wall(in plants). Most animal cells consist of only a phospholipid bilayer and not a cell wall, therefore shrinking up under such conditions.

Plasmolysis only occurs in extreme conditions and rarely happens in nature. It is induced in the laboratory by immersing cells in strong saline or sugar solutions to cause exosmosis, often using Elodea plants or onion epidermal cells, which have coloured cell sap so that the process is clearly visible.

Plasmolysis can be of two types. It can be either concave plasmolysis or convex plasmolysis. Convex plasmolysis is always irreversible while concave plasmolysis is usually reversible


here is the picture of cell, before and after plasmolysis
BEFORE
AFTER

After the cells are plasmolyzed, the chloroplast is gather in the cytoplasm, it color is green