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3 Phospholipid (phosphatidylcholine)

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Transport In, Through and Between Cells

The lipophilic cell membrane protects the cell interior from the extracellular fluid, which has a completely different composition p. 2). This is imperative for the creation and maintenance of a cell's internal environment by means of metabolic energy expenditure. Channels (pores), carriers, ion pumps (^ p.26ff.) and the process of cytosis (^ p. 28) allow transmembrane transport of selected substances. This includes the import and export of metabolic substrates and metabolites and the selective transport of ions used to create or modify the cell potential (^ p. 32), which plays an essential role in excitability of nerve and muscle cells. In addition, the effects of substances that readily penetrate the cell membrane in most cases (e.g., water and CO2) can be mitigated by selectively transporting certain other substances. This allows the cell to compensate for undesirable changes in the cell volume or pH of the cell interior.

Intracellular Transport

The cell interior is divided into different compartments by the organelle membranes. In some cases, very broad intracellular spaces must be crossed during transport. For this purpose, a variety of specific intracellular transport mechanisms exist, for example:

♦ Nuclear pores in the nuclear envelope provide the channels for RNA export out of the nucleus and protein import into it (^ p. 11 C);

♦ Protein transport from the rough endoplasmic reticulum to the Golgi complex (^ p. 13 F);

♦ Axonal transport in the nerve fibers, in which distances of up to 1 meter can be crossed (^ p. 42). These transport processes mainly take place along the filaments of the cytoskeleton. Example: while expending ATP, the microtubules set dynein-bound vesicles in motion in the one direction, and kinesin-bound vesicles in the other (^ p. 13 F).

Intracellular Transmembrane Transport

Main sites:

♦ Lysosomes: Uptake of H+ ions from the cyto-sol and release of metabolites such as amino acids into the cytosol (^ p. 12);

! Endoplasmic reticulum (ER): In addition to a translocator protein p. 10), the ER has two other proteins that transport Ca2+ (^ A). Ca2+ can be pumped from the cytosol into the ER by a Ca2+-ATPase called SERCA (sarcoplasmic endoplasmic reticulum Ca2+-transporting ATPase). The resulting Ca2+ stores can be released into the cytosol via a Ca2+ channel (ryanodine receptor, RyR) in response to a triggering signal (^ p. 36).

! Mitochondria: The outer membrane contains large pores called porins that render it permeable to small molecules (< 5 kDa), and the inner membrane has high concentrations of specific carriers and enzymes (^ B). Enzyme complexes of the respiratory chain transfer electrons (e-) from high to low energy levels, thereby pumping H+ ions from the matrix space into the intermembrane space (^ B1), resulting in the formation of an H+ ion gradient directed into the matrix. This not only drives ATP synthetase (ATP production; ^ B2), but also promotes the inflow of pyruvate- and anorganic phosphate, Pi- (symport; ^ B2b,c and p. 28). Ca2+ ions that regulate Ca2+-sensi-tive mitochondrial enzymes in muscle tissue can be pumped into the matrix space with ATP expenditure (^ B2), thereby allowing the mitochondria to form a sort of Ca2+ buffer space for protection against dangerously high concentrations of Ca2+ in the cytosol. The inside-negative membrane potential (caused by H+ release) drives the uptake of ADP3- in exchange for ATP4- (potential-driven transport; ^ B2a and p. 22).

Transport between Adjacent Cells

In the body, transport between adjacent cells occurs either via diffusion through the extracellular space (e.g., paracrine hormone effects) or through channel-like connecting structures (connexons) located within a so-called gap junction or nexus (^ C). A connexon is a hemi-channel formed by six connexin molecules (^ C2). One connexon docks with another con-nexon on an adjacent cell, thereby forming a common channel through which substances with molecular masses of up to around 1 kDa can pass. Since this applies not only for ions such as Ca2+, but also for a number of organic substances such as ATP, these types ofcells are

|— A. Ca2+ transport through the ER membrane

Ca2+-ATPase

Signal

(depolarization, hormon, etc.) Discharge

Ca2+ channel

Ca2+-ATPase

Ca2+ Storage

Signal

(depolarization, hormon, etc.) Discharge

Ca2+ channel

Cytosolic Ca2+ concentration

.— B. Mitochondrial transport

ATP synthetase

Ribosomes

Granules

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