The Cell inIts Environment-Physical and Biophysical Processes
Substances can move into and outof a cellthroughits semi-permeable cell membrane. There are three different processes throughwhich materials can move in and outof a cell. They are:
• Through the processof diffusion,
• Through the processof osmosis and
• Through the processof active transport.
Diffusion is the majorprocess through which small moleculesmove in and out across the cellmembrane region of higherconcentrationto a region of lower concentration.
The concentrationof a substance is the amountof thatparticular substance in a given volume of liquid. Diffusion results asa resultof movement and collision of molecules. The collisions of the moleculesare what caused the molecules to thrust awayfrom one another and spread out.
Molecules would normallydiffuse throughthe cellmembrane intoa cell when the contents ofthe cellare of a lower concentrationthan the surroundingsolution.The diffusion of water molecules througha semi permeable membrane is knownas osmosis.
Dueto the factthatcellfunction effectively without sufficient water, the majority of cellular processesdepend onosmosis.
In osmosis, the molecules of water travelthroughthe process of diffusion from a region of higherconcentration to a region oflower concentration.
The movement of dissolved substances througha cellmembrane withoutthe use of cellular energyis referred to as passive transport. Diffusion and osmosis are two types of passive transport.
When there isa need for the cellto assimilate materials thatare inhigher concentrationinside the cellthan outside the cell, the movementof the materials requires energy.
Onthe contrary, active transportis the movement of materials througha cell membrane with the useof cellular energy. The major variation between passive transport and active transport is thatactive transportnecessitates the use of cell’s own energywhile passive transportdoes not.
Cellshave many ways of transporting materials throughactive transport. In one method, especially in the transportation ofprotein molecules, the cellmembrane “lift up” molecules outside the celland transportthem inside the cell.
Anothermethod of active transportis throughengulfing.In this case, the cell membrane envelops orengulfsa particle and forms a vacuole inside the cell.
The majority of the cells are highly small. This is because the all particles have to move in and outof the cell throughthe cellmembrane. On entering inside the cell, it istransported to itstargetthrougha stream of moving cytoplasm.
In anextremely large cell, streams of cytoplasm ought to travelfartherto transportmaterials from the cell membrane to every part of the cell.
In a nutshell, Osmosis can be defined as:
• Diffusion of a solvent (frequently water molecules) througha semi permeable membrane from an area of low solute concentrationto anarea of highsolute concentration.
• Netmovement of water molecules througha semi-permeable membrane from an area of higherwater concentrationto anarea of lower water concentration.
• The tendencyof water to move from a hypotonic solution (lower concentrationof dissolved substances) to hypertonicsolution (higher concentrationof dissolved substances) througha semi-permeable membrane
In biological processes, osmosis is very significant since a lotof biological membranes are semi permeable in nature, and it leads to diverse physiological effects.
For instance, when ananimal cellis placed into a hypertonic surrounding- a surroundingwith lower water concentration-the waterwilldepart from the cellresultingto a shrinking of the cell.
When ananimal cellis placed in a hypotonic environment, oran environmentthatis of higherwater concentration, the watermolecules will move into the cellcausing the cellto swell. If osmosis continues and becomes extreme the cellwillfinally burst.
In a plantcell, extreme osmosis is barred as a resultof the osmotic pressure exerted by the cellwallwhich stabilizes the cell.In fact, osmotic pressure is the majorcause of support inplants.
Onthe otherhand, if a plantcellis placed in a hypertonic surrounding, the cellwallcannot preventthe cellfrom losing water. This normallyleads to a condition knownas flaccidity orthe shrinking ofthe cell.
Plasmolysis
This can be defined as the shrinking of protoplasm away from the cellwallof a plantorbacterium as a resultof water loss from osmosis and in so doing leading to gaps between the cell walland cellmembrane.
When a plantcellis immersed intoa highly concentrated solution, water diffuses outof the cell, and turgor pressure of the cellis lost.This makes the cellto become flaccid. Additional loss of water willlead to plasmolysis, and finally to cytorrhysis which means the totalcollapse of cellwall.
Plasmolysis only occurs insevere situations and seldom happens in nature.Itis induced inthe laboratory by immersing cells instrong saline or sugar solutionsto give rise to exosmosis, frequently with the use of Elodea plants oronion epidermal cells.
Wilting and Plasmolysis
Plasmolysis is the separation of plant cellcytoplasm from the cellwalldue to excessive loss of water. Itis unlikelyto happen naturallyexcept inrigorous situations. Plasmolysis is induced in the laboratory by immersing a plant cellina powerfully saline or sugary solution, sothatthe plantlosses water throughthe process of osmosis.
Diffusion and Osmosis
Diffusion
Diffusion is the process by which molecules move from areas of high concentration, to areas of low concentration. When the moleculesare eventhroughout a space– it isreferred to as equilibrium state.
Concentration gradient: This is the difference between concentrations ina given environment.
Molecules willalways traveldown the concentrationgradient, toward areas of smaller concentration.Example: food coloring thatdisperses out ina glass of water the dispersal of the fume from an air freshenersprayed ina room.
Semi Permeable membranes: These are membranes that allow the passage of some things while at the sametime disallowing the passage of otherthings throughit. Example of a semi-permeable membrane is the cellwall. Itallows water and oxygen to pass freely throughthe cell’smembrane, by diffusion
Osmosis means the diffusion of water across a membrane. Water will normallytravelinthe direction where there isa high concentrationof solute. In otherwords where there isa lower concentrationof water.
Salt isa solute and when it is concentrated inside oroutside the cell, it willpull the waterup to itsdirection. This is also the reason why we are thirstyafter eating salty food.
Type of Solutions:
1. Isotonic Solutions: Iso means the same
When the concentrationof solute (salt) is the same onboth sides, the water willtravelback and forthbut it won’t have any effect onthe overall amount of water onthe twosides.
2. Hypotonic Solutions
“HYPO” means less. Therefore a hypotonic solution isa solution that containsless solute (salt) molecules outside the cellwhich leads to intake of water from the outside intothe cell.
This leads to the cellgaining water and growing bigger. In plantcells, the central vacuoles willget filledup and the plantturns firmerand more rigid. Thus the plant’scellwallprevents the plantfrom bursting.
In animal cells howeverwhen this happens, the cellpossibly willbe in risk of bursting.Animal cellorganelles knownas the contractile vacuoles would normallypump water outside of the cellto avert this da
nge
r.
3. Hypertonic Solution:
HYPERmeans more. Thus Ahypertonic solution isa solution thatcontains more solute orsalt moleculesoutside the cellthan inside the cell.This would normallyresultto waterbeing taking outside of the
cell.
In plantcells, the central vacuoleloses water to the surroundingleading to the shrinkage of the cells and wilting ofthe plant.
In animal cells, this also results to the shrinkage of the cell.The situation couldlead to deathboth inplantand animal cell.
This is why it is unsafe to drinksea water . Dehydration would normallybe hastened up by intake of salty water. This is also why “salting farm lands is a common war practice with the aim of causing the deathof crops inthe farm to createfamine.
Diffusion and Osmosis are both types of passive transport. What this means is thatto transportmaterials inand out of the cellthrougheither osmosis or diffusion, there is noenergy requirement.
Occasionally, big molecules are unable to pass throughthe plasma membrane, and are assisted to pass across by carrier proteins. This process is referred to asa facilitated diffusion.
Diffusion is which is the net movement of a substance like an atom, ion or molecule froma region of high concentrationto a region oflow concentrationcan as well be defined as the movementof a substance down a concentrationgradient.
A gradient is an alteration inthe value of a quantity like concentration, pressure and temperature with the change inanother variable like distance.
For instance, analteration ina concentrationacross a distance is referred to asa concentrationgradient. An alteration inthe pressure across a distance is referred to asa pressure gradient while analteration in temperature througha distance is referred to asa temperature gradient.
The word diffusion is obtained from the Latin word, “diffundere”, which means “to disperse”. If a substance is being dispersed, it is being transported from a region of highconcentrationto a region of low concentration.
A distinctive characteristic of diffusion is thatit leads to mixing ormass transport, withoutneeding bulkmotion orbulkflow. Therefore, diffusion oughtnotto be mistaken for convection, or advections, which are othertransportphenomena thatmake use of bulkflow to transportparticles from one place to another.
Osmosis on the otherhand is the spontaneousnet movementof solvent molecules via partly permeable membrane intoa region of higher solute concentration, inthe direction thathave the tendencyof equalizing the solute concentrations onthe either sides.
The osmotic pressure is used to explain the pressure needed to sustain equilibrium without any net movement of solvent.Osmotic pressure is a colligative property. This means that the osmotic pressure depends onthe molar of the solute but notonits characteristics.
In biological systems generally, biological membranes are semipermeable and are notpermeable to bigger and polar molecules, like ions, proteins, and polysaccharides. They are only permeable to non-polarand/or hydrophobic molecules suchas lipids and small molecules suchas oxygen, carbon dioxide, nitrogen, and nitric oxide.
Permeability of materials across the cellmembrane depends onsolubility, charge, or chemistry, in addition to the size of the solute.Water molecules pass throughthe plasma membrane, tonoplast membrane (vacuole) or protoplastby diffusing across the phospholipid bilayer throough aquaporins.
This means small trans-membrane proteinscomparable to those incharge of facilitated diffusion and ion channels.Osmosis makes available the key way throughwhich water is transportedinand outof cells.
The turgor pressure ofa cellis principally maintained by osmosis crosswise the cellmembrane stuck between the cellcenter and its comparatively hypotonic environment.

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Plant Aquaporins: Membrane Channels with Multiple Integrated Functions

Functions of aquaporin - In a typical plant, aquaporin contributes to plant growth and development by transporting water into the tissues. Water flows along various paths: (i) the apoplastic path, that is, within the cell wall continuum, (ii) the symplastic route through cytoplasmic continuities and plasmodesmata, and (iii) the transcellular path across cell membranes (mainly plasma membranes). Aquaporin also facilitates the transpiration, tissue desiccation and imbibition, tissue expansion in plants. Study showed that Aquaporin could function in Nitrogen fixation, CO2 transport and carbon metabolism and especially nutrient uptake in plant such as NIP5; 1 functions as a boric acid channel for boron uptake and as well essential for plant growth under low boron stress. Two T- DNA insertions were developed and the nip 5; 1 mutant were sensitive to low boron condition.

Structure of aquaporin

Structurally, aquaporin proteins consists of six transmembrane alpha helices arranged in a right handed bundle, with the amino and the carboxyl termini located on the cytoplasmic surface of the membrane. There are also five interhelical loop regions (A-E) that form the extracellular and cytoplasmic vestibules.

This structure depicts a typical plant aquaporin consisting of both the amino and carboxylic terminal, the six transmembrane and the five interhelical loop and the NPA motif in B and E.(modified from Training and Mobility Research network project).

Studies suggested that the loop B and E contains the asparagine, proline and alanine (NPA) motif.
http://www.prestarticles.com/plant-aquaporins-membrane-channels-multiple-integrated-functions/

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