Membranes and transport

A thin double layer of fatty molecules surrounds each cell, and two kinds of protein let ions cross it: channels and pumps 1.

An open channel passes tens of millions of ions per second down their electrochemical gradient 1. A pump has two gates that never open together, so it moves ions uphill several orders of magnitude more slowly 1. Pumps that hydrolyse ATP are called primary; secondary pumps couple the downhill flow of one ion, often Na+, to the uphill transport of another substrate 1.

The sodium pump

A pump in the outer membrane uses energy from ATP to move sodium out of the cell and potassium in 1.

In each cycle of the Na+/K+-ATPase, ATP phosphorylates a conserved aspartate, Asp376 in ATP1A1, and the pump exports 3 Na+ and imports 2 K+, up to a hundred times a second 12.

Transport cycle of the Na+/K+-ATPasePlasma membrane
123456E1E1·3Na+E1P(3Na+)E2PE2P·2K+E2(2K+)
  1. E1E1·3Na+Na+/K+-ATPase (catalytic α, auxiliary β)Cytoplasmic gate open; 3 Na+ bind from the cytosol
  2. E1·3Na+E1P(3Na+)Na+/K+-ATPase (catalytic α, auxiliary β)ATP → ADP, Mg2+ATP phosphorylates the conserved aspartate; 3 Na+ occluded
  3. E1P(3Na+)E2PNa+/K+-ATPase (catalytic α, auxiliary β)Extracellular gate opens; affinity falls and 3 Na+ leave the cell
  4. E2PE2P·2K+Na+/K+-ATPase (catalytic α, auxiliary β)Ouabain and other cardiotonic steroids2 K+ bind from outside; K+ antagonises ouabain binding
  5. E2P·2K+E2(2K+)Na+/K+-ATPase (catalytic α, auxiliary β)H2O → phosphateExtracellular gate closes; aspartyl phosphate hydrolysed; 2 K+ occluded
  6. E2(2K+)E1Na+/K+-ATPase (catalytic α, auxiliary β)ATP bound, not hydrolysedNucleotide opens the cytoplasmic gate; 2 K+ released into the cytosol

The Na+ gradient drives secondary pumps such as intestinal SGLT1, which takes up each glucose or galactose with 2 Na+ 2. The kidneys consume 7–10% of the body’s oxygen uptake and spend two-thirds or more of their share on the pump 3. A textbook table of rodent tissues in the standard state attributes to the pump 50–60% of oxygen consumption in brain, 5–10% in liver and skeletal muscle and 1–5% in heart 4.

Membrane potential

The inside of a resting cell is electrically negative, because potassium ions, far more concentrated inside, diffuse out 4.

K+, the dominant cation inside cells, is about 10 to 30 times more concentrated inside than outside, where Na+ dominates 4. K+ usually lies close to electrochemical equilibrium, where the inside-negative potential balances its concentration gradient 4. Resting potentials typically lie between −10 and −100 mV, depending on the ion channels a cell expresses 5. In pancreatic β cells, rising glucose raises ATP and lowers ADP, which closes ATP-sensitive K+ channels; the membrane depolarises, Ca2+ enters and insulin is released 5.

Compartments

Membranes divide the cell interior into compartments, each with its own enzymes 6.

CompartmentMain processes
CytosolGlycolysis; pentose phosphate pathway, which supplies NADPH and ribose 5-phosphate 6
MitochondriaCitric acid cycle, β-oxidation and the respiratory chain, whose proton gradient drives ATP synthase 6; mitochondrial DNA encodes 13 respiratory-chain subunits 7
Endoplasmic reticulumSynthesis of secreted, membrane and lysosomal proteins on ribosomes docked by the signal recognition particle; start of N-glycosylation 6; ethanol oxidation by CYP2E1 8
Golgi apparatusN-glycan modification 6; the GNPTAB phosphotransferase synthesises the mannose 6-phosphate marker that sorts lysosomal enzymes to lysosomes 9
LysosomesAcidified by a V-type ATPase 2; degradation of endocytosed and autophagic material; enzyme defects cause storage diseases such as Gaucher disease 6
Peroxisomesβ-Oxidation of very-long-chain fatty acids, phytanate breakdown and early plasmalogen synthesis 6; biallelic variants in any of 13 PEX genes cause Zellweger spectrum disorder 10
NucleusTranscription, mRNA capping, polyadenylation and splicing; mRNA export through nuclear pores 6

From receptor to gene

Insulin binds a receptor on the cell surface, and a chain of reactions transmits the signal to genes in the nucleus 11.

Insulin signalling to FOXO1Plasma membrane, cytosol and nucleus of the hepatocyte
  1. Insulin bound to the insulin receptor (INSR)
  2. Tyrosine kinase domains of the INSR β subunitsATPTrans-autophosphorylation of the activation loop
  3. INSR phosphorylated at Tyr1158, Tyr1162 and Tyr1163
  4. Insulin receptor tyrosine kinaseATP
  5. Tyrosine-phosphorylated IRS proteins
  6. Phosphoinositide 3-kinase (p85 regulatory, p110 catalytic)ATPp85 binds phosphorylated IRS YXXM motifs; p110 phosphorylates PIP2 to PIP3
  7. PIP3 in the plasma membrane
  8. PDK1 (Thr308) and mTORC2 (Ser473)ATPPIP3 recruits AKT to the membrane; residues numbered as in AKT1
  9. AKT phosphorylated at Thr308 and Ser473
  10. AKTATPOther kinases can also phosphorylate these sites
  11. FOXO1 phosphorylated at Thr24, Ser256 and Ser319
  12. several stepsPhosphorylated FOXO1 is excluded from the nucleus
  13. Lower transcription of G6PC1 and PCK1

Through IRS proteins, phosphoinositide 3-kinase and PIP3, the receptor activates the kinase AKT 116. AKT phosphorylates the transcription factor FOXO1, which leaves the nucleus; transcription of the gluconeogenic genes G6PC1 and PCK1 falls 11122. Glucagon raises gluconeogenic gene transcription through cAMP, protein kinase A and the CREB–CBP–CRTC2 complex 12.