When does transport become a problem that has been solved?
All processes in the body require the interaction of different cells. To do this, they must be able to communicate with one another: Cell communication takes place both electrochemically (i.e. via charges) and chemically via signal molecules (the latter is known as signal transduction). Mass transport is necessary for both processes. In addition to the non-directional and energy-independent forms of transport, diffusion and osmosis, there are also specific transport processes for which energy must be used. As a rule, transport in cells means that a biological membrane (either the outer shell of the cell or the shell of the cell compartments) has to be overcome.
Since biological membranes consist of lipid bilayers, only small lipophilic substances can move through them unhindered. All other substances (large molecules, hydrophilic or charged particles) require assistance from transmembrane proteins. The transmembrane proteins include the channel proteins, which protrude through the membrane in a tubular shape, and the so-called "carrier" transporters, which can move back and forth in the membrane.
Mass Transfer Basics
Almost all processes in biological systems take place in aqueous solution. In a solution, every single molecule or ion is surrounded by solvent. The amount of a solute can be expressed in several ways.
- Molarity (molar concentration): c = nsolute/ Vsolvent
- Unit: mol / L
- c = molar concentration, n = number of particles in moles, V = volume
- Molality: b = nsolute/ msolvent
- Has the same properties as the molar concentration
- Used for real mixes
Molarity is the normal molar concentration in particles per volume of solvent; molality is a concentration in particles per mass of solvent!
Solution behavior of molecules in multiphase systems
- Multi-phase system: Consists of several non-mixed layers (= phases) with different polarity, e.g. water (polar) / oil (non-polar)
- Concentration of dissolved molecules: greater in the phase whose polarity matches the polarity of the molecule better
- Polar molecules and ions: dissolve in polar solvents, e.g. water
- Non-polar molecules: dissolve in non-polar solvents, e.g. oil
- Amphiphilic molecules: Arrange themselves at the interface between two phases, so that the polar part of the molecule protrudes into the polar phase and the non-polar part into the non-polar phase
- Can be described using the Nernst distribution coefficient
Similia similibus solvuntur - like dissolves in like!
Diffusion is the self-mixing of a solution, i.e. the distribution of a dissolved substance in the solvent. It is therefore a phenomenon of mass transfer.
- Cause: Non-directional, random thermal particle movement
- 1. Fick's law: The particle flux is proportional to the size of the concentration gradient and is opposite to it. The molecules diffuse along the concentration gradient, the particle flux being proportional to the size of the concentration gradient
- Particles diffuse from the place of higher concentration to the place of lower concentration
- The greater the concentration gradient, the stronger the particle flow
- Nernst partition coefficient: Describes the solution of a substance in two immiscible phases (e.g. water and oil, or water and ether)
In a closed system, diffusion causes a complete reduction of concentration differences in the long term!
Diffusion through membranes
Diffusion not only occurs unhindered in solutions, but can also run through membranes if these are permeable, ie "permeable".
- Diffusion through a membrane: D = P × A × (Δc / d)
- Unit: mol / s
- D = diffusion speed, P = permeability coefficient, A = area of the membrane, Δc = change in concentration, d = layer thickness, Δc / d = concentration gradient
- Sample calculation
- A cell membrane has a permeability of 1 × 10−7cm / s for the tryptophan molecule
- What is the diffusion rate if the difference in concentration on both sides of the membrane is 500 μmol / cm3 is, the membrane has a size of 2 cm2 and its thickness is negligibly small?
- Wanted: diffusion rate D
- Given: permeability P, membrane area A, concentration difference Δc
- Inserting it into the formula for the diffusion rate results in D = P × A × Δc = 1 × 10−7cm / s × 2 cm2 X 500 µmol / cm3= 1 × 10−4 μmol / s
- Tryptophan is thus with a diffusion rate of 1 × 10−4 μmol / s transported through the membrane.
Even some substances for which a membrane is not permeable can "diffuse" through it if transporters such as carriers or channels help. In this case, one speaks of facilitated diffusion. With this type of diffusion, the number of transporters is decisive for the strength of the particle flow and the diffusion equation no longer applies!
- Osmosis: Diffusion phenomenon on semipermeable (i.e. partially permeable) membranes, in which a concentration balance can only be achieved by diffusion if water molecules flow from the place of lower concentration to the place of higher concentration of a dissolved substance
- Osmolarity / osmotic concentration: Osmotically effective number of particles based on the volume of a solution
- Formula: cosmotic = nosmotic / Vsolvent
- Unit: osmol / L
- cosmotic = osmotic concentration; nosmotic = Number of osmotically active particles in moles (i.e. number of dissolved particles that themselves cannot diffuse through the membrane); V = volume of the solution
- Formula: cosmotic = nosmotic / Vsolvent
- Osmolality: Osmotically effective number of particles in relation to the mass of a solution
- Formula: bosmotic = nosmotic/ msolvent
- Unit: osmol / kg
- bosmotic = Osmolality; nosmotic = Number of osmotically active particles in moles (i.e. number of dissolved particles that themselves cannot diffuse through the membrane); m = mass
- Formula: bosmotic = nosmotic/ msolvent
- Osmotic pressure: pressure of two solutions of different concentrations, which are separated by a semipermeable membrane
- Formula: posmotic = σ × c × R × T (This equation is also called Van't Hoff's law.)
- Reflection coefficient σ: Describes the selectivity of a membrane for a solute
- σ = 0: membrane is completely permeable
- σ = 1: membrane is completely impermeable
- Semipermeable membrane: Is water permeable for the solvent, but not for all dissolved substances (σ = 1)
Osmosis and cells
Biological membranes are semi-permeable, which is why osmosis is an important and recurring phenomenon in medicine. The flow of water molecules to locations with high concentrations of solutes has consequences for cells and entire tissues.
Edema is the accumulation of water in the tissue that occurs when fluid leaks from the capillaries into the interstitium. The flow of fluid is actually controlled by a balance between the hydrostatic pressure in the capillary vessels and the oncotic pressure between the vessel and the interstitium: While the hydrostatic pressure promotes the release of fluid into the interstitium, the oncotic pressure binds the fluid in the vessels. The causes of edema formation are therefore either too high hydrostatic pressure, too low oncotic pressure in the capillaries, or too high oncotic pressure in the tissue. See also: Edema and the structure and function of blood vessels.
The principles of osmosis also play a role in the excretion of fluids from the body. If too many osmotically active substances are dissolved in the urine (e.g. sugar), then there is a dilution due to increased flushing out of water. This phenomenon is known as a symptom of diabetes mellitus and is also used in therapy with diuretics.
Transport into or out of cells
Biological membranes are lipid bilayers, so chemically represent a very non-polar environment. According to the principles of solubility (see above), they are permeable to non-polar molecules such as carbon dioxide or steroid hormones, but not to charged ions, polar molecules such as sugar or particularly large ones Molecules. The cell depends on controlled transport processes for such substances. A distinction is made between membrane-displacing transports, in which parts of the cell membrane are also transported, and membrane-protein-mediated transports, in which the transports are carried out through specialized membrane proteins (e.g. channels).
In membrane-displacing transports, parts of the cell membrane or the membrane of cell organelles are pinched off as vesicles in which the substance to be transported is located. One also speaks of vesicular transport. Transport into cells is called endocytosis, and transport out of cells is called exocytosis. In transcytosis, substances are first absorbed by endocytosis, passed through the cell and then released again by exocytosis. These membrane-displacing forms of transport are controlled by a specific group of G proteins, so-called Rab proteins, which ensure that the vesicles get to the right place.
The incorporation of molecules into cells is called endocytosis. A distinction is made between pinocytosis, phagocytosis and receptor-mediated endocytosis. What all three have in common is that the particles to be taken up finally arrive in vesicles inside the cell. The mechanisms are very different in each case.
Phagocytosis can only be carried out by specialized so-called phagocytic cells, such as macrophages or dendritic cells. These have special opsonin receptors that can bind so-called opsonins. During phagocytosis, large molecules and even small cells can be ingested.
Pinocytosis refers to the uptake of medium-sized dissolved molecules. In contrast to phagocytosis and receptor-mediated endocytosis, no receptor is involved in pinocytosis: it takes place spontaneously and can be carried out by all cells.
- "Spontaneous" invagination of the cell membrane around the molecule to be absorbed
- Inside the cell, actin is involved in the invagination of the membrane and the formation of the vesicles (so-called actin remodeling)
- Constriction of the vesicle inwards
In receptor-mediated endocytosis, small to medium-sized parts are taken up into the cell with the help of a membrane-bound receptor. It plays an important role, for example, in cholesterol metabolism, in the absorption of LDL or in the absorption of transferrin-bound iron.
- Binding of the ligand to a specific receptor
- Lowering of the membrane, intracellular accumulation of clathrin molecules
- Everting the membrane around the ligand to be transported and its receptor
- Constriction of a vesicle inside the cell by the protein dynamin
- Fusion of the vesicle with the early endosome
- An acidic environment causes the ligand to detach from the receptor
- Release of the receptor towards the cell membrane ("recycling")
- Release of the
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