Transportation across the cell membrane

Wednesday June 25, 2008

Transportation across the cell membrane

The cell is required for transport across the membrane, because it is important to expel from its internal debris Metabolism and acquire nutrients extracellular fluid, thanks to the ability of the cell membrane that allows the passage or leaving on a selective basis of certain substances.
Did you happened to be that a person smoking near you, as you see these "particles" in spreading suspension will gradually start to breathe? Have you ever wondered why?. The substances in the air are in a constant movement. Thus, the smoke particles move from where they are most concentrated (the smoker) where there is less (you). This phenomenon is called diffusion.
The transport routes across the cell membrane and the basic mechanisms for small molecules are:

Transportation passive or diffusion

The composition of molecules that are inside the cells is different from making up the environment that surrounds them. This is due to the existence of transport mechanisms that capture the substances they are useful to the cell and removes those that are not.. This tells us that the membrane is a "border" selective. But what is dependent transport? Will move all particles by the same mechanisms? Will this affect the transport of particle size, nature or chemical concentration as they are?

The passive transport is the exchange of simple molecules of a substance through the cell membrane, during which no expenditure of energy that brings the cell, because it goes in favour of the gradient of concentration or in favour of gradient of electric charge, that is, a place where there is a large concentration to one where there are lower. The process is carried out by cellular passive dissemination. As such, is the change from one means of further concentration (middle Hypertonic) to another lower concentration (half hypotonic) .
The investigations and our own experience show that the solutes are moving from areas with the highest concentration of the lowest concentration. This allows the cell to incorporate substances that require no expenditure of energy. If the particle is small pass through the membrane without major problems, but instead of the membrane through which it will be different if this is a composite or polar Apollo. In the first case the solutes pass freely through the lipid component of the membrane (for the dual layer). Sustancias apolares como el O2 el CO2 y el N2 lo hacen. Substances apolares as the O2 CO2 and N2 do.This is called diffusion simple.

Dissemination Standard

Some substances are indoor or outdoor cells from across a semi-permeable membrane, and move them inside for Dissemination Standard, being a physical process based on the movement at random. The spread is the movement of atoms, molecules or ions from one region to a higher concentration of concentration without requiring less energy expenditure.
The spread implies not only the movement of particles at random to achieve uniform distribution of the same (and this occurs when the particles come azarosamente are equated with those azarosamente van) but also the potential homogeneous chemical fluid, because there is a semi-permeable membrane that partition of a fluid into two distinct potential chemical, osmotic pressure will be generated from the higher potential chemical (eg pure solvent) to the lowest (eg solute and solvent) until both partitions are equivalent or hydrostatic pressure balances the osmotic pressure.

-Diffusion-supplied

It is the largest movement of molecules that can not pass through the cell membrane and need help from a protein or other mechanisms (exocytosis) to spend on the other side. Also called diffusion-mediated carrier because the substance transported in this way is rarely able to cross the membrane protein without a specific carrier who can help. It differs from the mere dissemination through conduits that while the extent of spread of simple diffusion increases in proportion to the concentration of the substance that spreads in the spread provided the magnitude of diffusion is approaching a maximum (Vmax), by increasing the concentration of the substance.

The polar solutes (ions as Na +, K + or larger molecules such as glucose and amino acids enter through its component molecules interact with lipid apolares. These proteins can be channels of transport or transport proteins.

- Filtration

Leaching is the movement of water and dissolved molecules through the membrane due to the hydrostatic pressure generated by the cardiovascular system. Depending on the size of the pores of the membrane, only solutes with a certain size can pass through the membrane. For example, the pores of the membrane of the capsule Bowman clusters in the kidney, are very small, and only albumin, the smallest of proteins, have the ability to be filtered through it. Moreover, the pores in the membranes of the hepatocytes are extremely large, so a wide variety of solutes can cross.

- Osmosis

Osmosis is a special kind of passive transport in which only water molecules are transported across the membrane. The movement of water is made from a point where there is greater concentration of one to match lower concentrations. According to the medium he finds a cell, Osmosis varies. The role of osmosis is to keep hydrated in the cell membrane. This process does not require energy expenditure. In other words osmosis or osmosis is a phenomenon consisting of the passage of a dissolution of the solvent from an area of low concentration of solute to a high concentration of solute, separated by a semipermeable membrane. browniano. Linked to the Brownian motion.

Osmosis in an animal cell

In half isotonic, there is a dynamic equilibrium, namely the constant passage of water.

• In half hypotonic, the cell absorbs water and swollen to the point where it can explode giving rise to cytolysis.
• In half Hypertonic, the cell removes water and wrinkle arriving dehydrated and dying, it's called Crenation.

Osmosis plant in a cell

In half isotonic, there is a dynamic equilibrium.

• In half hypotonic, the cell absorbs water and fill their vacuoles increasing pressure from swelling.
• In half Hypertonic, the cell removes water and the volume of the vacuole decreases, causing the cell membrane that is takeoff of the cell wall, the happening Plasmolysis

Transport assets

It involves the transport of substances against a concentration gradient, which requires an expenditure of energy. In most cases this is done active transport at the expense of a gradient of H ⁺ (potential electrochemical proton) previously created on both sides of the membrane processes by photosynthesis and respiration, for hydrolysis of ATP by ATP hydrolases membrane (F1F0). The transportation assets varies intracellular concentration and this leads a new movement of osmotic rebalancing by hydration. Transport systems are active among the most abundant bacteria, and have been selected evolutionarily because in their natural environments the majority of prokaryotes are on a permanent or transitory with a low concentration of nutrients.

NOTE: Perm: permeases.
Protein or group of proteins that facilitates the transfer of a solute (biomolecules, or protein ions) on one side or another of a biological membrane through a mechanism of active transport (energy expenditure).
Observations: belong to the class 2 (transferases) or 3 (hydrolases) from the classification of the IUBMB. The oligopéptido-permeases, for example, catalyses the reaction following:
ATP + + H2O = oligopéptidoexterior ADP + + oligopéptidointerior phosphate.
The common name of the latter enzyme is Oligopeptide-transporting ATPase (oligopéptidos transporting ATPase) and belongs to the subcategory of the IUBMB EC 3.6.3.23.

Transport systems are based on active and specific permeases inducidles. The way in which energy is coupled with metabolic transport of solute has not yet been elucidated, but generally handles the assumption that permeases, once captured the substrate with high affinity, experiencing a conformational change dependent on energy that makes them lose this affinity, which is releasing the substance inside the cell.

The active transport of molecules across the cell membrane takes place in upward direction or against a concentration gradient (chemical gradient) or against an electric pressure gradient (electrochemical gradient), ie the passage of substances from half just to concentrate a very concentrated.To move these substances against the tide is necessary input of energy from the ATP. The proteins are active carriers transport ATPase activity, which means they can split the ATP (adenosine phosphate Tri) to form ADP (two Phosphates) or WAP (phosphate) with release of energy links high-energy phosphate. Commonly obserban are three types of carriers:

• Uniport: are proteins that carry a molecule in a single direction across the membrane.

• Antiportadores: proteins include transporting a substance in one direction while simultaneously carrying another in the opposite direction.

• Simportadores: are proteins that carry a substance along with other, often a proton (H ^+).

Bomb sodium and potassium

It is found in all cells of the body, which was responsible for carrying potassium ions that are able to enter cells into the interior of these, giving a negative internal and cargo at the same time pumps sodium ions from inside to outside of the cell (axoplasma ), However the number of Na ⁺ ions (positively charged), both against gradient does not exceed that of negatively charged ions resulting in a load internal negative. In case of neurons in sleep this difference in charges on both sides of the membrane is called membrane potential or sleep.

When transportation asset is directly attached to the energy expended, is said to be primary. An example is the pump Na + / K + ATPPasa that couples transport Na + outwards with the transport of K + into the interior (antiporte) both against its gradient, the process takes place with consumption of ATP. This activity keeps the membrane potential and makes it possible functioning transport processes active secondary.

The animal cells maintain concentrations of Na + and K + intracellular that differ greatly from the extracellular.

	East intracellular	East extracellular
Na +	20 mmol / L	150 mmol / L
K +	140 mmol / L	4 mmol / L

Concentrations intra and extracellular Na + and K +

Responsibility for maintaining such a difference in concentrations of the pump is Na +-K + ATPase, pumping K + + Na inwards and outwards of the cell. Basically works as shown below.

Outline of the pump sodium potassium ATPase.

1. The ATP phosphorylation pump which is linked to sodium (3iones).

2. The union group phosphorylation it reduces the affinity for sodium, which is discharged to the environment and increases the extracellular affinity for potassium.

3. Potassium ions (2 ions) environmental extracellular join the protein.

4. Hydrolysis of link-protein phosphorylation allows the protein to revert to its original configuration and acquire high affinity for sodium and lower affinity for potassium. The latter is poured the cytoplasm.

5. Three sodium ions bind to the protein and resumed the cycle.

Transportation active secondary or cotransporter

Some molecules are transported against the gradient, taking advantage of a situation created by a primary active transport.

Many molecules, glucose and the amino acids enter the cell through a transport coupled with the entry of Na +, as presented in the figure below.

It is the transport of substances that are normally not cross the cell membrane such as amino acids and glucose, whose energy required for transportation stems from the gradient of concentration of sodium ions from the cell membrane (Bomb Glucose / Sodium ATPase).

• Bomb calcium: It is a protein of the cell membrane of all eukaryotic cells. Its role is to transport calcium ion (Ca ^{2 +)} outside the cell, thanks to the energy provided by ATP hydrolysis, in order to maintain the low concentration of Ca ^{2 +} in the cytoplasm that is about ten thousand times lower than in the external environment, necessary for the normal functioning of cells. It is known that variations in the concentration of intracellular Ca ^{2 +} (second messenger) are produced in response to various stimuli and are involved in processes such as muscle contraction, gene expression, cell differentiation, secretion, and a number of functions neurons. Given the variety of metabolic processes regulated by the Ca ^{2 +,} an increased concentration of Ca ^{2 +} in the cytoplasm can cause an abnormal functioning of the same. If the increasing concentration of Ca ^{2 +} in the aqueous phase of the cytoplasm is approximately one tenth of the external environment, the metabolic disorder produced leads to cell death. Calcium is the most abundant mineral in the body, in addition
  

The transportation asset What happens with the molecules that the cell must capture the medium and who are at a level lower than in the interior of herself? When sales of the cinema hall after seeing a movie can haberte happened that the departure is a "passive" given that your move is in part facilitated by the rest of the others who push in the same direction: exit . But imagine that when you are about to leave you realize you've forgotten briefcase, in those circumstances you must go back and move in the opposite direction effort to recover your garment. Clearly, the effort required in the latter situation is greater than the output. In other words, lower concentration to higher concentration.

What kind of transport would each situation? Can you explain the difference in terms of spending power in both processes?

How substances in the cell mobilizes against gradient? Why?

It is for this fact that the cell has a different composition intra and extracellular. Since the movement of particles is directed from the region of lower concentration towards more concentrated, the cell must expend energy transportation asset is carried out by transport proteins from the membrane. The ATP provides the energy required for this process to degrade to ADP and inorganic phosphate.

Transportation macromolecules or particles

The macromolecules or large particles are introduced or expelled from the cell by two mechanisms:

Endocytosis

The cellular endocytosis is the process by which the cell moves inside large molecules (macromolecules) or particles, encompassed by a invaginación its cytoplasmic membrane, forming a gall which then emerges from the cell wall and incorporates the cytoplasm. This gall, called endosome then merges with a lysosome to carry out the digestion of vesicular content.

There are two processes:

• Pinocitosis involves ingesting liquids and solutes through small blisters.

• Fagocitosis involves the ingestion of large particles that are covered in large blisters (phagosome) emanating from the cell membrane.

See animation

Exocytosis

It is the expulsion of substances such as insulin through the fusion of vesicles with the cell membrane.

The cell exocytosis is the process by which the vesicles in the cytoplasm merge with the cytoplasmic membrane, releasing their contents.

The exocytosis is observed in many different secretory cells, both in the function of excretion in the endocrine function.

Also involved in exocytosis the secretion of a neurotransmitter in the synaptic gap, to enable the spread of nerve impulse between neurons. The secretion chemical triggers a depolarization of the membrane potential, from the axon of the cell station towards dendrite (or elsewhere) of the recipient cell. This neurotransmitter is then retrieved by endocytosis to be reused. Without this process, there would be a failure in the transmission of nerve impulse between neurons.

Exocytosis from the Greek exo outside and citing cell. The exocytosis is the process by which cells expel a compound (usually a chemical messenger) to the exterior of its membrane.

The blisters clear release neurotransmitters that arrive to contain the potential for action to the button terminal. The depolarization of the membrane causes calcium channels open and calcium from outside (Ca2 ⁺⁾ pass inside. he calcium channels are very similar to sodium channel that changed the heartbeat of the frog Loewi, except that (as its name indicates) they are permeable to calcium rather than sodium. These calcium channel that open to allow a small amount of ions entering the axonal button, the amount of calcium ions in the interior is called calcium ion concentration [Ca ^{2 +]} _i. The ion concentration in the cytoplasm of axonal button is very small, around the 0.0002 mM 0.0002 mM ,But a few nanometers of the active zone presináptica calcium can reach inside the membrane levels as high as 0.3 mM 0.3 mM . Enough for the blisters unloaded their precious cargo in a process called exocytosis.

The membrane vesicle merges with the membrane of the active zone presináptica, allowing the neurotransmitter discharges into the groove. The exocytosis occurs at a tremendous speed, the time at which calcium ions penetrate the button at the time of discharge takes less than two milliseconds. This speed is partly due to calcium ions penetrate the active zone, exactly where the blisters await anchored. The mechanism by which the increase of Ca ^{2 +} triggers the process of exocytosis represents a stimulant gap of ignorance in neuroscience, as a result powerful lights have been focused on this process.

Because of its speed, it is assumed that the blisters are not only anchored off the active zone but somehow have already taken some preliminary steps and are ready to download. Se cree que in the process of anchoring specialized proteins (Snare) bind the vesicle with the active zone, to change the means by the chemical increase in [Ca ^{2 +]} _i modify the snare its formation and thus the dual layer Lip of the gallbladder and the membrane presináptica merges forming a pore that allows the neurotransmitter escape into the cleft. The continuous pore opening to the point where the gallbladder is totally incorporated in the membrane presináptica. At this point the process of exocytosis conclude endocytosis and starts, during which the vesicular membrane is recovered by the neuron.

Outline of synapses. The calcium channel, blue, button depolarization terminal allowing the neurotransmitter, acetylcholine in this case (Ach), is released and activate the freceptores in green.