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In cell-attached recording, the membrane under the patch pipette remains intact and the cytosol is retained. If bath voltage is zero, cell-interior voltage is $V_{\mathrm{cell}}$ and pipette voltage is $V_p$, the patch's inside-minus-outside membrane potential is $V_{\mathrm{cell}}-V_p$. Commanding $V_p$ alone does not determine that difference when $V_{\mathrm{cell}}$ is unknown or changing. This is especially relevant to single-channel current-voltage relations.

A chloroplast is a photosynthetic plastid in plant and algae biological cells. The green-plant chloroplast modeled here has a chloroplast envelope surrounding chloroplast stroma and internal thylakoids. Light-dependent reactions occur at the thylakoid membrane; the Calvin cycle operates in the chloroplast stroma.^[ref]

Cyanobacteria perform oxygenic photosynthesis without chloroplasts: the prokaryotic biological cell itself contains the photosynthetic machinery. Thus chloroplast identifies an organelle, while oxygenic photosynthesis identifies a process. Endosymbiotic theory explains their historical connection; organelle protein import explains how many nuclear-encoded proteins reach the modern organelle.

The useful feature is selective separation: enzymes and substrates can share an organelle without sharing a compartment. For a constructed accessibility example, put a soluble enzyme in chloroplast stroma and its substrate in thylakoid lumen. Sharing the same chloroplast does not permit direct contact; contact requires a transport or boundary-disruption step. Placing the substrate between adjacent thylakoids instead puts it on the stromal side. The visual stack alone cannot establish chemical accessibility.

For the simplest boundary-counting model, represent surrounding cytosol, chloroplast intermembrane space, chloroplast stroma and thylakoid lumen by four vertices in a chain. Its three edges represent the outer envelope, inner envelope and thylakoid membrane, respectively. A route from cytosol to chloroplast stroma crosses two barriers; extending it into thylakoid lumen adds a third. This follows by listing the edges along the chain. Counting the two leaflets of each lipid bilayer separately would answer a different question. These counts describe this idealized geometry, not the number of protein targeting signals or the transport itinerary of every protein.

The light-dependent reactions article develops ATP, NADPH and electrochemical gradient coupling, and the Calvin cycle article develops carbon fixation bookkeeping. Their different locations make the chloroplast interesting as an organized chemical workspace. Light-independent chemical reactions still depend on supplied carriers and regulation; the name does not prescribe nighttime operation.

Cotranslational protein translocation is passage of a growing polypeptide chain across a membrane during biological translation. SRP recognizes an exposed hydrophobic ER signal sequence. The SRP receptor enables handoff to the Sec61 complex. Biological translation can slow during delivery; the ribosome remains on the cytosol side.^[ref]

The useful engineering feature is coordination: a targeting instruction can become accessible while the rest of its cargo is still being made. Recognition, docking and crossing remain distinct operations. A defect in any one can prevent successful import even when the protein primary structure contains an appropriate signal.

This route is not universal. Many tail-anchored proteins use delivery after biological translation, and their membrane-spanning segment lies near the C-terminus. Thus protein targeting signals need not be at the N-terminus.^[ref]

The cytoplasm is the material inside a cell membrane but outside the cell nucleus, when one is present. It includes the cytosol, the organelles suspended in it, and other structures; it is not just a name for the liquid. In a prokaryote, which has no cell nucleus, the nucleoid and ribosomes are within the cytoplasm. See Clark, Douglas and Choi, Biology 2e, 4.3: cytoplasm and cytosol and Clark, Douglas and Choi, Biology 2e, 4.2: prokaryotic organization.

Try classifying four positions in the diagram. A point in the pale blue region belongs to both cytosol and cytoplasm. A point inside the mitochondrion belongs to an organelle included in the cytoplasm, but not to the surrounding cytosol. A point on a free ribosome belongs to the cytoplasm, but the ribosome itself is not cytosol. A point inside the cell nucleus belongs to neither cytoplasm nor cytosol. These distinctions prevent the misleading inference that everything outside the cell nucleus shares one undivided liquid environment.

The usefulness of the cytoplasm is this combination of shared surroundings and specialized structures: enzymes in the cytosol carry out many steps of metabolism, while organelles maintain their own internal conditions. A soluble protein made by a free ribosome may work in the cytosol or be transported elsewhere, including into the cell nucleus. Cellular organization therefore combines separation with exchange, rather than assigning each protein permanently to its place of synthesis. See Alberts et al., Molecular Biology of the Cell (2002): traffic between nucleus and cytosol.

The cytosol is the aqueous part of the cytoplasm surrounding its structures, including organelles. It contains water, ions and dissolved molecules, including many proteins; it is a crowded working environment rather than pure water. The liquid inside a cell nucleus or a membrane-bounded organelle is not the surrounding cytosol.

Cytosol dialysis is exchange between the cytosol and pipette solution through the opening used in conventional whole-cell recording. Diffusion can change ion concentrations and dilute soluble regulators of ion channels. A diminishing response can therefore reflect the recording's alteration of the biological cell, rather than the experimental treatment.

A direct motivating example is the 1988 article Muscarinic activation of ionic currents measured by a new whole-cell recording method. In rat lacrimal gland biological cells, the acetylcholine response in conventional whole-cell recording lasted about five minutes. With a pore-forming substance in the patch pipette instead of open break-in, the response remained undiminished after one hour. The readout was activation of calcium-activated potassium and ${\mathrm{Cl}}^{-}$ ion electric currents; preserving the response supported a role for retained cytoplasmic components. The authors also tested the substance extracellularly and observed reversible increases in cation electrical conductance. This comparison motivates perforated patch recording, but does not show that every intracellular constituent or every ion channel response is preserved. ^[ref]

Practical controls are matched pipette and bath compositions, recording elapsed time after break-in, time-matched untreated preparations, and tracking both patch-clamp series resistance and baseline leakage. Comparing conventional whole-cell recording with perforated patch recording can separate loss of soluble components from an intended treatment effect, but only if electrical access is characterized in both. These are controls suggested by the mechanisms above, not a new set of measured results.

The endoplasmic reticulum is a membrane-enclosed network of tubules and flattened sacs in a typical eukaryotic biological cell. Its continuous boundary encloses the ER lumen, separate from the cytosol, and connects with the outer nuclear envelope. Rough endoplasmic reticulum and smooth endoplasmic reticulum are regions of this network.^[ref]

Endosymbiosis is an ecological association in which one organism, the endosymbiont, lives inside another. The definition specifies location, not that both partners benefit, nor that the internal partner has become an organelle. An intracellular association can remain an association between distinct organisms. Endosymbiotic theory addresses a different question: how particular ancestral bacteria became inherited, functionally integrated organelles during evolution.

The striking consequence is that a eukaryotic biological cell can combine histories: its nuclear lineage and its mitochondria need not have the same ancestry. Mitochondria have bacterial ancestry related to Alphaproteobacteria; the plastids of plants and many algae, including chloroplasts, have ancestry within cyanobacteria. These are ancestral lineages, not a claim that a named living bacterial species is the literal ancestor. Present-day bacteria and organelles have both evolved since their shared ancestors. ^[ref]

The ER lumen is the internal aqueous compartment of the endoplasmic reticulum. A ribosome attached to its membrane sits on the cytosol side, not inside the ER lumen. This distinction separates the machinery making a protein from the compartment receiving it.^[ref]

The Golgi apparatus modifies and sorts cargo arriving from the endoplasmic reticulum for delivery to other cellular destinations. The eponym refers to Camillo Golgi.

Its usual stacked organization is polarized: cargo enters on the cis side, encounters medial processing regions, and leaves through the trans side and trans-Golgi network. Golgi cisternae enclose a Golgi lumen separated from the surrounding cytosol. Processing changes cargo; sorting selects its destination.^[ref]

This division of labor explains why reaching the Golgi apparatus does not imply being secreted: cargo can take different onward routes, while resident enzymes continue working there. The conventional route discussed here is part of secretory protein routing; many cytosol proteins never enter it.

A Golgi cisterna is a flattened membrane-enclosed sac. Stacking several sacs does not make their interiors one open space: membrane separates Golgi lumen from intervening cytosol.^[ref]

The Golgi lumen is the enclosed interior of a Golgi cisterna, distinct from the surrounding cytosol.

The membrane-sidedness rule in secretory protein routing still applies within the Golgi apparatus. For a local worked example, stipulate a membrane protein with a carbohydrate chain exposed to Golgi lumen and a tail exposed to cytosol. An enzyme confined to Golgi lumen can contact that chain, but cannot directly contact the cytosolic tail without a boundary-crossing step. A change in which Golgi cisterna contains the protein need not turn the protein over. Thus chemical accessibility and cargo location are different questions: moving cargo onward preserves its membrane sides under the budding and fusion operations described in secretory protein routing.

In signal transduction, inositol trisphosphate usually means inositol 1,4,5-trisphosphate: a soluble intracellular second messenger. An extracellular ligand, its cell-surface receptor, IP3 and the released calcium ions are distinct components. IP3 binds an IP3 receptor on the endoplasmic reticulum, promoting stored ${\mathrm{Ca}}^{2+}$ release into the cytosol.^[ref]

The useful separation is spatial: an extracellular message can control an internal store without entering it. The existing intracellular-injection experiment under metabotropic receptor tests bypassing reception; the Pancreatic IP3 store fractionation experiment below tests which internal compartment supplies the released calcium. Neither question is answered merely by observing a downstream membrane electric current.

An inside-out patch is an excised membrane patch whose cytosol side faces the bath. Bath changes can therefore test effects on the intracellular face of an ion channel, with the extracellular face exposed to pipette solution.

The lysosomal lumen is the aqueous interior of a lysosome, separated from cytosol by its surrounding lipid bilayer. A membrane pump couples ATP hydrolysis to inward transport of hydrogen ions, maintaining an acidic environment for acid hydrolases. Membrane transport proteins export digestion products, including amino acids and sugars, for reuse in metabolism. Compartmentalization combines restricted access with controlled exchange.^[ref]

A lysosome is an acidic digestive and recycling organelle: one surrounding lipid bilayer encloses a lysosomal lumen containing acid hydrolases. These enzymes break down proteins, nucleic acids, carbohydrates and lipids by hydrolysis. Cargo can originate outside the biological cell, entering through endocytosis, or within its own cytoplasm, delivered through autophagy. Digestion therefore serves both disposal and reuse.^[ref]

An endosome sorts endocytosed cargo: some returns to the cell membrane, while some proceeds toward lysosomes. An autophagosome instead encloses material already in the cytoplasm. Both routes bring cargo into contact with digestive enzymes; neither implies that every internalized molecule is destroyed.^[ref]

Enzyme supply is a different itinerary from cargo disposal. Secretory protein routing and protein targeting deliver newly made digestive enzymes through the endoplasmic reticulum, Golgi apparatus and trans-Golgi network toward lysosomes.^[ref] The existing membrane transport and differential centrifugation articles explain the boundary-crossing and separation principles used below.

A proteasome is a different digestive machine: a protein complex without a surrounding lipid bilayer, acting on individual proteins in the cytosol and cell nucleus. Many targets carry ubiquitin chains. A lysosome accepts a much broader range of cargo, including material delivered inside an autophagosome. Thus an unwanted soluble protein and an unwanted mitochondrion pose different disposal problems; a proteasome does not swallow an intact mitochondrion.^[ref]

A membrane potential can coexist with near electroneutrality in the bulk cytosol and external chemical solution. The small imbalance producing the voltage lies near the cell membrane; most positive and negative electric charges balance in the bulk.^[ref]

For a separate original capacitor model, choose a total membrane capacitance $C=5\mathrm{pF}$ and $V_m=-70\mathrm{mV}$. These are stipulated parameters, not measured properties of a particular biological cell, and this voltage is not asserted to equal the preceding $E_{\mathrm{K}}$. Model the separated electric charge relative to an uncharged reference byUsing the elementary charge $e=1.602176634\times 10^{-19}\mathrm{C}$, this representsexcess monovalent negative-charge equivalents inside. These can represent a deficit of positive ions, rather than the arrival of that many negative ions.

Independently choose an internal reservoir volume $\mathcal{V}=1\mathrm{pL}=10^{-12}\mathrm{L}$ containing $140\mathrm{mM}=0.140\mathrm{mol}{\mathrm{L}}^{-1}$ potassium, with balancing negative electric charge. With the Avogadro constant $N_A=6.02214076\times 10^{23}\mathrm{mo}{\mathrm{l}}^{-1}$, its potassium inventory isThus, if charging were entirely due to outward monovalent potassium transport, about one in $38600$ of these potassium ions would suffice. This is a comparison with the chosen potassium inventory, not a fraction of all charged particles or a measurement of spatial charge density. No shape, surface area or thickness has been inferred from the chosen capacitance and volume. The calculation illustrates why substantial voltage does not require a strongly charged bulk cytosol.

A membrane potential is an electric potential difference across a cell membrane. We use the conventionTaking the external electric potential as zero makes $V_m<0$ mean that the inside is at lower electric potential. “Negative inside” describes a difference, not an absolute electric potential, and does not imply that the entire cytosol has a large net negative electric charge.

Consider a cell membrane permeable only to potassium ions, with higher potassium concentration inside. Diffusion initially favors outward transport. If balancing negative ions cannot follow, that loss of positive electric charge produces a small inner excess of negative electric charge and an outer positive excess. The resulting electric field favors inward potassium transport. At the Nernst equilibrium potential, chemical and electrical contributions to electrochemical potential balance; ions still cross both ways, with zero net transport.^[ref]

In living biological cells, several ions can permeate and pumps maintain their concentration differences through active transport. Pumps transporting net electric charge also contribute electric current. Thus $V_m=E_{\mathrm{K}}$ is a special limit, not a universal rule; membrane potential need not always be negative.^[ref] The existing reversal potential example shows how opposing electric currents can balance without each ion reaching thermodynamic equilibrium. This makes selective permeability a way to convert chemical differences into electrical signals: changing the available ion channels changes which electrochemical gradients influence the voltage.

Membrane topology specifies the sidedness of a membrane-associated polypeptide chain: where its N-terminus, C-terminus and intervening protein domains lie relative to the lipid bilayer. Side-specific placement is essential to interpreting a cell-surface receptor.^[ref]

For an original worked example, stipulate a continuous polypeptide chain with its N-terminus outside a biological cell, exactly three complete crossings of the cell membrane, and no additional excursions into the lipid bilayer. Its segments lie, in order, outside, inside, outside, inside. A protein domain between the first and second crossings faces the cytosol; one between the second and third faces the exterior; the C-terminus faces the cytosol. Counting crossings determines these placements without specifying an atomic protein structure.

The proof is a side-switching argument. Assign exterior the sign $+1$ and interior $-1$. Each complete crossing multiplies the sign by $-1$, so after $m$ crossings,An odd number puts the termini on opposite sides, an even number on the same side. This assumes complete crossings of one boundary; a chain that enters and returns to the same side without crossing must not be counted as a complete crossing. Nor does the rule determine which side the N-terminus starts on.

As a constructed accessibility test, put an epitope on the outside segment between crossings two and three, and another on the inside segment between crossings one and two. Stipulate an intact cell membrane that excludes the test antibody and accessible epitopes that bind it. Applying the antibody outside can label only the first epitope. If both become accessible after opening the boundary, the difference tests sidedness under those assumptions. Failure to label an epitope alone is insufficient: it could also be inaccessible because of protein folding or poor antibody affinity. This is a proposed reasoning exercise, separate from the historical mixing experiment below.

A mitochondrion is an organelle with an outer mitochondrial membrane and an inner mitochondrial membrane. The latter encloses the mitochondrial matrix and folds into cristae; the intermembrane space lies on its opposite side. This architecture separates a chemical workspace from a membrane that couples electron transport to ATP formation.^[ref]

For an original location exercise, place soluble label A in the mitochondrial matrix and soluble label B in the interior of a crista. In the depicted geometry, B can reach the peripheral intermembrane space through the fold's opening without crossing a lipid bilayer. A must cross the inner mitochondrial membrane to reach either location. A fold projecting into the mitochondrial matrix therefore does not make its enclosed fluid part of the mitochondrial matrix. Following a continuous path through the colored regions proves the distinction; merely asking whether a label is “inside the mitochondrion” loses it.

The functional connection is equally spatial. The mitochondrial respiratory chain pumps protons from the mitochondrial matrix side across the inner mitochondrial membrane. Their return through ATP synthase can drive ATP formation. Oxidative phosphorylation develops the mechanism and chemiosmosis gives its free energy bookkeeping. Compartmentalization is useful because it lets a shared electrochemical gradient couple different proteins without direct transfer of a chemical intermediate between each pair.

Under aerobic conditions, mitochondria supply much of the ATP in many eukaryotic biological cells; the contribution varies with biological tissue, demand and oxygen supply. Glycolysis also makes ATP in the cytosol. Mature mammalian red blood cells lack mitochondria, and bacteria have no mitochondria at all. Neither all ATP production nor all biological cells fit the familiar mitochondrial picture.^[ref]

Mitochondria descend from a bacterial endosymbiosis and retain a genome and ribosomes, although many required proteins are encoded by nuclear genes. Endosymbiotic theory develops the ancestry argument; organelle protein import explains delivery of nuclear-encoded proteins. A functioning isolated mitochondrion demonstrates biochemical autonomy under supplied conditions, rather than independently establishing that ancestry.