Transport (biology)
| Parent |
| Biological process |
| Subordinate |
|
Intracellular transport, transmembrane transport, extracellular transport, secretion, transepithelial transport |
| Gene Ontology |
|---|
| QuickGO |
The transport of substances, energy and information is the prerequisite for living beings to coordinate and maintain their complex metabolism and other life processes.
Need for mass transport
All organisms are in a lively exchange of substances with their environment:
- On the one hand, they have to absorb substances from the environment that are used as building or energy materials directly or after being adapted to their own needs.
- On the other hand, they release substances into the environment and thereby change the environment for themselves and for other organisms ( excretion of toxins and waste, mucilage, protective substances, defense substances, decomposition substances to dissolve minerals or organic material).
But material flows can also be found within an organism:
- Substances have to be transported from the place of their origin (place of synthesis) to the place of consumption or storage.
- Within a cell, there is also an exchange of substances between the cell organelles and the cytosol .
Substances are also exchanged between the individuals of a population (e.g. pheromones ) and between the populations of an ecosystem (e.g. food , pollen ).
In many cases the material flows are linked to form a cycle (e.g. global carbon cycle ).
Cellular transport systems
Diffusion : The free, unhindered distribution of molecules and ions in a room depends only on the temperature and the concentration gradient . It occurs inside a cell or outside the cell in the so-called intercellular areas . Examples of this are the distribution of carbon dioxide in the intercellular areas of the sponge tissue of a leaf or the diffusion of transmitter molecules through the synaptic gap .
Cell walls , cell membranes and organelle membranes initially represent a diffusion barrier. Membrane transport is the exchange of substances across this barrier. Permeation is diffusion through these boundary layers
Transmembrane transport
Cell walls are omnipermeable , they allow all molecules and ions, except for the macromolecules, to pass.
Biomembranes are selectively permeable (selection of substances), they are only permeable to small molecules such as water, carbon dioxide or oxygen , which diffuse through the membrane due to irregularities in the lipid bilayer. Since larger molecules such as glucose or amino acids can only pass through the membrane with difficulty, transport mechanisms are necessary to facilitate absorption or to build up a concentration gradient as short-term energy storage.
In contrast to diffusion , protein transport is an enzymatic process, i.e., since only a limited number of transport proteins are available, the process activity increases hyperbolically until it has reached a saturation value that cannot be exceeded.
In addition, these transport systems are highly selective, they can recognize certain molecules from the environment and select them in a targeted manner. They can even differentiate between stereoisomers.
The transport through a membrane is usually carried out by integral proteins that extend from one side of the membrane to the other. They form a channel that is internally hydrophilic and, due to its diameter and certain charge, is selective for certain molecules or ions.
These channels are usually only opened on the basis of a signal (hormones, transmitters, action potential, mechanical deformation) so that the transmembrane transport can be controlled as required.
Passive transportation
In the case of passive transport , the migration of molecules or ions takes place due to a concentration gradient; in the case of ions, the membrane potential can also play a role. There are aquaporins in the cell membrane for the rapid transport of water into the interior of the cell .
- Ion channels:
- They facilitate the diffusion of ions through the membrane according to the concentration gradient and do not require any energy. Examples: Sodium and potassium ion channels in nerve cells, calcium ion channels in nerve cells and muscles, chloride channels in nerve cells and secretory cells. The ion channels are opened either by changing the membrane potential, by a transmitter or by Ca 2+ .
- Carrier:
- Carriers (carrier proteins) are transmembrane proteins that work in a similar way to enzymes : first, the molecule to be transported binds to a specific substrate binding site on the carrier protein, and a carrier-substrate complex is formed. This complex changes its conformation and the molecule is released again on the other side of the membrane.
Cotransport : Some carriers carry binding sites for different substrates. They are only transported when they are all occupied. Symport is the transport of all substrates in the same direction, with the antiport the substrates are transported in the opposite direction. (If it is only a substrate, it is also called Uniport .)
- Glucose Transporter:
- In bacteria and mammals (liver, β-cells of the pancreas), passive transport through the channel protein is facilitated by the fact that the glucose inside is immediately phosphorylated to glucose-6-phosphate so that it no longer diffuses back through the channel and the glucose concentration inside the cell remains low. The glucose-6-phosphate can be processed further in the glycolysis immediately. The phosphate comes from phosphoenolpyruvate (PEP), the precursor to pyruvic acid , the end product of glycolysis .
Active transportation
- active primary transport - ion pumps
- They increase the concentration differences between the compartments of a cell by transporting ions from the side of the lower to the side of the higher concentration. The energy comes from the hydrolysis of ATP to ADP and phosphate. Examples: ATPases (e.g. sodium-potassium ion pump )
In addition to ion pumps, there are also ATP-dependent transport systems for molecules. Example: Transport of peptides into the endoplasmic reticulum .
- active secondary transport
- If the energy of the concentration gradient is not used to build up ATP, but to transport another ion, it is called secondary transport. Here, too, there is Symport (the substances are transported in the same direction) and Antiport (transport in the opposite direction).
- Permeases in bacteria
- Permeases are channel proteins that actively transport molecules or ions across the cell membrane. They are ATP-independent and take their energy from concentration gradients or the membrane potential.
| Art | Cargo | Transport direction | driving force |
|---|---|---|---|
| Symport | Lactose and protons | inside | Membrane potential and pH gradient |
| Symport | Lactate and protons | inside | pH gradient |
| Antiport | Sodium cations and protons | H + inwards, Na + outwards | pH gradient |
| Uniport | Lysine cations | inside | Membrane potential |
Glucose is imported by the transmembrane enzyme II C, which phosphorylates glucose as it passes through and thus keeps the concentration of free glucose inside the cell low. The maltose import by the maltose permease is dependent on ATP.
Vesicle systems (membrane-displacing transport)
Macromolecules and larger food particles can no longer pass through the membrane by means of transport proteins. They are transported with the help of a vesicle system (Latin: vesica , the bladder). Inside the cell, these vesicles are commonly called endosomes.
Vesicles are formed when part of the membrane turns inside or out and then constricts as a closed hollow sphere. Substances and particles of the milieu are included. Conversely, a vesicle can re-fuse with the membrane and release its contents.
| export | import |
|---|---|
 Exocytosis |
 Endocytosis |
 Constriction of a vesicle |
 Fusion of a vesicle |
With endocytosis (Greek kytos , the cell; Greek endon , inside) substances or particles are imported into the cell. If it concerns food particles, one speaks of phagocytosis (gr. Phagos , eater). If water is absorbed with substances dissolved in it, this is called pinocytosis (Gr. Pino , drink, suck).
In receptor-mediated endocytosis, only certain molecules are selected for uptake. Example: cholesterol intake. Certain areas of the cell membrane are lined on the outside with cholesterol receptors and on the inside with proteins (coated pits, spiked pits ). If all the receptors are occupied, the membrane turns inside out , forming so-called spiked vesicles (coated vesicles).
Phagocytes are specialized scavenger cells of the immune system , which through phagocytosis ingest cell debris, pathogens and foreign proteins, even asbestos fibers (see macrophages , microphages ).
With exocytosis (Gr. Exo , outside, outside, outside) secretions or non-usable substances are excreted.
Examples:
- Amoebas take in food through pseudopodia.
- At the blink of an eye, ciliata swirl food particles into a receiving vacuole. This separates from the cell pharynx. As this vacuole travels through the cell, the food is digested. At the cell extractor, this vacuole fuses again with the cell membrane, releasing the indigestible components to the outside.
- In the eukaryotes , the nuclear membrane and endoplasmic reticulum (ER) on the one hand and the Golgi apparatus on the other hand form their own compartmental systems that are connected to one another via vesicles. For example, proteins are synthesized in the rough ER, packed in vesicles and transported to the dictyosomes of the Golgi apparatus. There the vesicles merge with the Golgi cisterns. In the cisterns, these proteins are further modified, packed in vesicles again and passed on to other cisterns. As the primary lysosomes , the Golgi vesicles eventually contain digestive enzymes. They fuse with the vacuoles of phagocytosis, the phagosomes , to form secondary lysosomes. Golgi vesicles with secretions migrate to the cell membrane, fuse with it and release their contents to the outside.
- Cytopempsis (gr. Pempsis , broadcast) is the passage of phagosomes through a cell and their transmission to the neighboring cell without their content being changed. It occurs in the epithelial cells of the vessels and in the epithelial cells of the intestine.
Cytoskeleton
The cytoskeleton consists of microtubules, actin filaments and intermediate filaments. In addition to maintaining and changing the shape of a cell, it is primarily used within the cell to transport chromosomes during cell division and to transport cell organelles, vesicles and macromolecules.
organism
Cell connections
- First, these cell-cell connections connect cells to form a tissue. In closing tissues, they seal the organ from the outside world. In the intestinal epithelium , however, they also enable the absorption of water and ions.
- Gap junctions only occur in the Eumetazoa .
In order to allow the exchange of substances between the cells of a tissue, the gap is (engl. Gap (Engl.) Between the cells through pores 1.2 to 2 nm diameter, bridged junction connection). These pores are formed by 6 radially arranged proteins, they are located in the membranes of both cells and are exactly opposite each other, so that a channel is created through both cell membranes. They are omnipermeable and allow both ions and uncharged molecules to pass according to the concentration gradient. However, they can also be selectively permeable and act as electrical synapses in the heart and nerve tissue to enable the transmission of action potential .
- In the cell walls of neighboring plant cells there are pores with a diameter of 20 to 40 nm, through which there is direct contact between the cytosol of the two cells. In this way, the cytosol of all cells in a plant tissue forms a coherent unit, the symplast . The plasmodesmata are traversed by desmotubuli , massive proteins that connect the endoplasmic reticulum of the two cells to form a unit. The transverse walls of the sieve tube , lines of the plants for assimilates , and the pits of the plant cells contain a particularly large number of plasmodesmata .
- In plants, the local transport of water and the nutrient salts and organic substances dissolved in it takes place in the roots via the apoplast, the coherent system of intercellular structures and cavities in the cell walls, by diffusion. Since roots actively absorb ions from the environment, the concentration in the apoplast increases and water flows in through osmosis . In the endodermis , the Caspary's strips are impermeable to water due to suberin deposits in the cell walls. Here ions are actively and selectively transported in the symplast.
Flagella and eyelashes
With the help of flagella and eyelashes ( cilia ), currents can be generated, with the help of which food particles are transported to the location of phagocytosis .
Examples:
- In the Monosiga collar flagellum cell , the base of the flagellum is surrounded by a collar of fine rods that are formed by the cell membrane. This wreath of chopsticks works like a trap: the whip blow creates a flow of water through the collar. Food particles stick to the mucus-coated collar sticks and travel with the mucus to the base of the collar, where they are phagocytosed. Collar flagellum cells are also found as specialized cells in sponges . There is also a constant flow of water from the outside through pores into the central space and from there through the upper opening back to the outside.
- In the paramecium Paramecium eyelashes make the mouth box for the transportation of food particles in the Empfangsvakuole.
- The mucous membrane of the respiratory tract is made up of two types of cells: goblet cells produce a film of mucus in which dust particles and pathogens get caught. This film is transported by the cells with cilia towards the oral and nasal cavities.
- The egg cell is transported in the fallopian tube through its ciliated epithelium.
Long-distance transport - organ systems
The larger a multicellular organism, the worse the cells inside are supplied by diffusion and cellular transport mechanisms alone. Our own transport facilities ensure a quick and even distribution of substances and heat. The larger the organism, the more branched these transport systems are.
Transport must be possible in any position regardless of gravity.
Animals
Digestive tract
As the food particles increase in size, it is necessary to break them up mechanically and, in the case of the larger multicellular cells, chemically in the digestive tract . The further transport of the food takes place via the longitudinal and circular muscles through peristalsis . The nutrient building blocks and other smaller molecules that are developed through digestion diffuse through the intestinal wall or are transported through the cell membranes into the body cavity by their own transport proteins.
blood circulation
Blood and lymph transport numerous substances ( nutrients , antibodies, waste materials, hormones ) and heat. Water, in which the substances are either dissolved or bound to carrier molecules, serves as a means of transport.
The blood is pumped around the body through a special organ, the heart . In vertebrates, vascular muscles and adjacent skeletal muscles support this pumping activity.
Insects and mollusks have an open blood circulation , vertebrates and annelids have a closed one. The blood is carried through the body in a closed vascular system. In the organs, the blood vessels branch out into narrow, thin-walled capillaries. This is where the exchange of substances with the adjacent tissue takes place through diffusion.
Gas transport
The gas exchange between the environment and the organism takes place either through the skin ( amphibians and worms living in water), through the gills ( fish , amphibian larvae, water snails , crabs) or through the lungs (animals living on land).
Various mechanisms have been developed to always supply the gills with fresh water: fish suck in fresh water through their mouths and expel it again via the gills, crabs generate a constant stream of water with their hind legs, the sessile tube worms move their gills through the water .
In order to supply the lungs with fresh air, a negative pressure is created by expanding the chest cavity with the diaphragm and intercostal muscles , which sucks in the air. By narrowing the chest area, the air enriched with carbon dioxide is pressed out again.
Transport of breathing gases in the body:
- Oxygen (humans): The oxygen diffuses through the alveoli into the blood. Only 3% dissolve physically, the remaining 97% are bound to the hemoglobin of the erythrocytes and thus distributed throughout the body. In the capillaries the oxygen is released again, in the muscles it is taken over by the myoglobin .
- Carbon dioxide (humans): Carbon dioxide reaches the blood through diffusion, is partially physically dissolved there (10%), reacts with water to form hydrogen carbonate and remains in the plasma (45%) or is converted into hydrogen carbonate (35%) or hemoglobin by the erythrocytes bound (10%) transported.
- Tracheal system : Since the open blood circulation system of the insects would not transport the breathing gases fast enough for the high demand of the flight muscles, they have their own tube system for the gas transport, the tracheal system. The ventilation is done by contraction and expansion of the abdomen.
Other transport systems
Further transport systems are the kidney tubules , ureters , vas deferens , bile ducts , milk ducts of the mammary glands and ducts of the pancreas , the sebum glands and the sweat glands of the skin.
Nervous system
The nervous system of the animals is an organ system for the reception, transmission, processing and storage of information. The basis is the transport of ions through the membrane of the nerve cells and of transmitter substances through the synaptic gap. Plant cells are also able to develop action potentials . However, it takes longer to develop and is not transmitted through its own nervous system.
plants
The long-distance transport of water and the nutrient salts dissolved in it takes place in the trachea and tracheids of the xylem , assimilates are transported in the sieve tubes of the phloem .
- Transpiration current
- In the transpiration flow, water and the ions dissolved in it, as well as organic compounds and alkaloids of the root metabolism, are transported up through the vessels of the xylem.
- Since the vessels are very narrow, the capillary force ensures that the water can rise up to 80 centimeters. The root pressure is a result of the active, ATP- consuming, ion transport in the roots, it allows the water column to rise up to 10 meters. The perspiration suction is created by the evaporation of water through the leaves. Since there is a cohesive column of water that extends to the roots and adheres to the vessel walls by adhesion, the water can be lifted up to a height of 120 meters.
- Assimilate stream
- The assimilates ( mono- and disaccharides ) are transported downwards in the sieve tubes . Their cytoplasm forms a coherent symplast, as their transverse walls (the sieve plates ) have numerous pores that are permeated by plasmodesma. The transport takes place along a strong osmotic gradient. The sieve tube cells receive the assimilates by active, ATP-consuming transmembrane transport from the surrounding escort cells (transfer cells ). Their plasmalemma is strongly folded to increase the surface area (similar to the intestinal villi ) so that there is space for many transport proteins. At the place of consumption or storage, the assimilates are actively pumped out of the sieve tubes again.
- Gas transport
- In plants, gas is transported solely by diffusion through stomata or lenticels and the intercellular system that runs through the entire plant. These cavities are particularly pronounced in the sponge tissue of the leaf. In marsh and aquatic plants , the gas exchange is promoted, especially in the submerged parts of the plant, by aerenchymes (ventilation tissue, star parenchyma in rushes ) and lacunae . In marsh plants (e.g. mangroves ), respiratory roots improve gas exchange with the air.
Ecosystems
Energy flow
The establishment and maintenance of structures makes a constant absorption of energy necessary for living beings . This energy is passed on through the food chains in the form of high-energy nutrients. Ultimately, this energy comes from solar radiation, which is used by phototrophic organisms and stored as chemical energy in nutrients. With every metabolic process, energy is lost in the form of heat movement.
Material flow
A constant exchange of nutrients, minerals and gases (O 2 , CO 2 ) takes place within an ecosystem, and these are conducted in a cycle.
The primary producers (photoautrotrophic plants , eukaryotic unicellular cells and chemoautotrophic bacteria ) build organic substances from inorganic substances during assimilation (biology) , which they use themselves as building and energy substances. The organic substances (real nutrients: proteins , carbohydrates , fats , DNA ) are therefore carriers of the energy and structure of life, which are created during the assimilation from the low-energy inorganic substances (mineral salts, CO 2 , H 2 O, incorrectly also referred to as "nutrients “Of the plants ). Destructors break down the organic matter again and lead it back to its inorganic state: the cycle is closed. Animals take part in the cycle of matter as heterotrophic consumers by absorbing organic nutrients, partly breathing them in and converting them back into inorganic substances or passing them on to the destructive or other consumers.
A self-sufficient ecosystem could also exist without animals as consumers, but not without destructors. The role of animals in the flow of matter in the ecosystem is based on the rapid comminution of organic material in nutrition, so that fungi and bacteria as destructive agents can remineralize more quickly.
Examples:
 |
 |
 |
| Carbon-oxygen cycle under aerobic conditions:
This biotic cycle is coupled with the abiotic carbon cycle : part of the CO 2 sediments in the water as calcium carbonate . Limestone weathered again. |
Carbon-oxygen cycle under anaerobic conditions. | Nitrogen cycle : nitrogen fixation , ammonification , nitrification and denitrification are carried out by bacteria. The nitrifying agents are strictly aerobic, denitrification takes place under anaerobic conditions. |
| Legend: | ||
Symbioses
In symbioses , substances are exchanged particularly effectively between the partners. A distinction is made between ectosymbioses and endosymbioses . With these, one partner lives inside the other. Examples of this are the metabolic symbioses of corals and radiolarians with green algae or cyanobacteria : The photosynthetically active partner receives carbon dioxide from the animal's cell respiration. Photosynthesis creates oxygen that is available to the animal for dissimilation. In addition, the autotrophic partners assimilate nitrogen and pass on the nitrogen-containing organic compounds to the animal. Green algae use nitrate as a source of nitrogen, cyanobacteria use elemental nitrogen .
The material cycle can also be disturbed
- Degradation is slower under anaerobic conditions. This means that dead, organic material can accumulate, which is transformed into peat , coal or petroleum over the course of geological times - the inorganic raw materials (especially CO 2 ) bound in it are thus withdrawn from the cycle. As a result, the O 2 gas formed during production has been greatly enriched in the atmosphere. Our current O 2 content in the atmosphere comes from the fossilization of dead biomass, and when these substances are burned, not only CO 2 is formed, but O 2 is also consumed.
- Dissolved minerals are constantly exchanged between ecosystems by water currents. The intensive fertilization in agriculture in particular contributes to these flows and disrupts ecological material cycles.
Geophysical transport
Materials and substances that can serve as the basis of life for living beings are added to or removed from an ecosystem through geophysical processes.
Circulation in the lake
A stagnant body of water is heated by the sun's rays from above. If it is deep enough, like a lake , a pronounced temperature stratification occurs. The warm surface water ( epilimnion ) floats on the cold deep water ( hypolimnion ). The two bodies of water are separated from each other by the thermocline ( metalimnion with thermocline ). If the stratification is stable, only the epilimnion can be mixed by the wind and supplied with oxygen for the animals or carbon dioxide for photoautotrophic organisms from the atmosphere. Dead organic material sinks to the bottom and is remineralized there by aerobic destructors. But since there is no exchange between hypo- and epilimnion, the nutrient salts accumulate at the bottom, while the upper layer is depleted of nutrient salts, which limits the growth of primary producers there.
If the surface water cools down to the temperature of the deep water, the thermocline collapses and the entire body of water can be mixed. The nutrient salts are transported by convection into the upper water layers and are thus available again to the photoautotrophic organisms, the oxygen-rich surface water is transported downwards, so that oxygen is again available to the destructors and animals of the deep water.
Circulation in the sea
Similar to a lake, a thermocline forms in the oceans that only allow the surface water to mix. Constant winds, Coriolis force , tides as well as temperature and salinity gradients create a local and global, horizontal and vertical flow system that extends beyond the thermocline. The Gulf Stream is part of this system. On the surface, it transports warm water to the north and thus ensures a moderate climate in these latitudes . The eel larvae hatched in the Sargasso Sea are transported through the Gulf Stream to Northern and Western Europe, where they swim upriver to their spawning areas.
The evaporation increases the salinity of the Gulf Stream and in the north the water is cooled. This increases the density of the water and it sinks east of Greenland to the bottom. As cold, salty deep water, it flows south again and enriches itself with nutrient salts. In the buoyancy zones of Peru- , Benguela and Canary stream this nutrient salts provide a high biomass production.
The increased melting of the ice caps of the North Pole and Greenland would lead to a lowering of the salt content and thus to the rupture of the Gulf Stream in the event of global warming . This would lead to an ice age-like cooling in Europe.
Shipments through rivers
In contrast to a lake, a river represents an open ecosystem: Due to the erosion in the upper reaches , the water is enriched with nutrient salts ( carbonates , sulfates , iron ions), which are transported downstream and used by primary producers and consumers . In the middle and lower reaches , on the one hand, there is erosion in the bank area on the impact slope , as a result of which organic material ( detritus ) gets into the river, which is remineralised by the destructive elements and is therefore available to the producers. On the other hand, sandbanks and silt banks build up, which represent special small biotopes. Delta areas can bring about an increase in land through embankments ( Euphrates and Tigris , Mississippi River ). Dead organic material from the living beings of the river or the bank area reaches the lower reaches in large quantities, so that this area represents a nutrient-rich ( eutrophic ) ecosystem.
Regular ( Nile ) or occasional floods ( Oderbruch ) supply the floodplain with inorganic and organic nutrients and thus create the basis for increased biomass production on land.
Wind displacements
As in the case of rivers, the wind also transports organic and inorganic material over a large area. This means that humus can be lost in one area and enriched again in other areas.
Living beings use the wind as a means of transport: wind pollinators ( spruce , fir , pine , grass ) allow their pollen to be transported. Mushrooms use the wind to spread their spores . Many flowering plants allow their seeds to be spread by the wind as flight fruits ( dandelion , willow , maple ). Young spiders sitting on a thread can be transported to other regions by the wind.
The transport of flight animals can also lead to the spread of their species (colonization of the Pacific island world by birds and insects, migration direction of the migratory locust ). However, these animals can also be blown out to sea, which can lead to their death. Therefore, one often finds flightless insects or birds ( dronts ) on small islands .
Animals as transport systems
Animals are also used as transport systems by other living things:
- Animal pollination : insects ( bees , bumblebees ), birds ( hummingbirds ), bats , primates transport pollen from one flower to another.
- Seed distribution : Burdock fruits stick to the fur or plumage. Seed kernels are indigestible and are excreted by fruit-eating birds, bats and primates far away from the mother plant. (See the particular distribution of violet seeds by ants .)
- Cattle egrets use elephants as a viewing platform: the hunt for insects and small animals is made easier by the fact that the prey animals are scared off by the elephants.
- Ship owners attach themselves to sharks , so that they can participate in falling prey at any time.
- Many animals with intensive brood care ( nestling ) feed their young and partners during the breeding season and rearing.
- Some animals without a nest transport brood and young animals on their backs ( midwife toads , spiders , primates ), in their bodies ( mouthbrooders ) or in their own brood pouches ( seahorses , marsupials )
- Parent animals transport young animals in their mouths to a safe place.
- Male toads allow themselves to be carried by the females for a long time during the spawning season.
literature
- Anita Roth-Nebelsick: The principles of plant-based water pipes . In: Biology in Our Time. 36 (2), pp. 110-118 (2006), ISSN 0045-205X