---
title: "Functional Organization of the Eukaryotic Cell"
book: "University Biology — Year 1"
subject: biology
language: en
chapter: 6
exercises: 12
source: https://one-course.com/books/biology/3/en/chapter/6-functional-organization-of-the-eukaryotic-cell
---

# Chapter 6 — Functional Organization of the Eukaryotic Cell

A [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) of the pancreas makes, every day, about its own mass of digestive enzymes and pours them into a duct. The enzymes are proteins that would digest the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) that made them; so the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) builds them inside a sealed sheet of membrane, ships them through a second compartment where they are sorted and concentrated, stores them in granules, and releases them only at the signal of a meal. Every step happens in a different membrane-bounded space, and the traffic between the spaces is carried by vesicles. This chapter describes the compartments of the [eukaryotic cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-prokeuk), the [endomembrane system](#def-b1-eukaryotic-cell-endomembrane) through which proteins flow, the [organelles](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-prokeuk) that transform energy, the skeleton that holds the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell)’s shape and moves its parts, and the features that distinguish a plant [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) from an animal one.

## 6.1 Compartments

**Definition 6.1 (Organelle, cytosol).**

An *organelle* is a structure inside a [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) with a definite composition and function; the membrane-bounded [organelles](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-prokeuk) are compartments whose interior differs in composition from the *cytosol*, the aqueous solution of the [cytoplasm](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) outside them. Membrane-bounded compartments: the [nucleus](#def-b1-eukaryotic-cell-nucleus), the [endoplasmic reticulum](#def-b1-eukaryotic-cell-endomembrane), the [Golgi apparatus](#def-b1-eukaryotic-cell-endomembrane), [lysosomes](#def-b1-eukaryotic-cell-endomembrane), [peroxisomes](#def-b1-eukaryotic-cell-peroxisome), [mitochondria](#def-b1-eukaryotic-cell-mitochondrion), and in plants the [plastids](#def-b1-eukaryotic-cell-plastid) and the [vacuole](#def-b1-eukaryotic-cell-plantcell). [Organelles](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-prokeuk) without a membrane: ribosomes, the [centrosome](#def-b1-eukaryotic-cell-cytoskeleton), the [cytoskeleton](#def-b1-eukaryotic-cell-cytoskeleton).

![A generalised animal cell cut open. The nucleus at the centre; around it the rough endoplasmic reticulum continuous with the nuclear envelope, the smooth reticulum, the Golgi stack and its vesicles, mitochondria, lysosomes, and the centrosome from which the microtubules radiate.](https://one-course.com/images/onecourse/chapters/biology-3/b1-eukaryotic-cell/fig-10d713d49376.svg)

*A generalised animal [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) cut open. The [nucleus](#def-b1-eukaryotic-cell-nucleus) at the centre; around it the [rough endoplasmic reticulum](#def-b1-eukaryotic-cell-endomembrane) continuous with the [nuclear envelope](#def-b1-eukaryotic-cell-nucleus), the smooth reticulum, the [Golgi](#def-b1-eukaryotic-cell-endomembrane) stack and its vesicles, [mitochondria](#def-b1-eukaryotic-cell-mitochondrion), [lysosomes](#def-b1-eukaryotic-cell-endomembrane), and the [centrosome](#def-b1-eukaryotic-cell-cytoskeleton) from which the [microtubules](#def-b1-eukaryotic-cell-cytoskeleton) radiate.*

**Definition 6.2 (The nucleus).**

The *nucleus* holds the chromosomes as *chromatin*, DNA wound on histone proteins ([Chapter 17](https://one-course.com/books/biology/3/en/chapter/17-genomes-of-cells-and-viruses#ch-b1-genomes)). It is bounded by the *nuclear envelope*, two membranes, the outer one continuous with the [endoplasmic reticulum](#def-b1-eukaryotic-cell-endomembrane), pierced by *nuclear pores* through which RNA and proteins pass in both directions under control. The *nucleolus* is the region where ribosomal RNA is made and ribosomes are assembled. The nucleus separates transcription (inside) from translation (outside), a separation prokaryotes do not have.

**Proposition 6.3 (The compartments of a liver cell).**

In a hepatocyte of $5000\,\text{µ}\mathrm{m}^{3}$, the [cytosol](#def-b1-eukaryotic-cell-organelle) takes about half the volume, the [mitochondria](#def-b1-eukaryotic-cell-mitochondrion) a fifth, the [endoplasmic reticulum](#def-b1-eukaryotic-cell-endomembrane) a sixth, the [nucleus](#def-b1-eukaryotic-cell-nucleus) a sixteenth, the [Golgi](#def-b1-eukaryotic-cell-endomembrane), [lysosomes](#def-b1-eukaryotic-cell-endomembrane) and [peroxisomes](#def-b1-eukaryotic-cell-peroxisome) a few percent together. The membranes tell a different story: of the $110\,000\,\text{µ}\mathrm{m}^{2}$ of membrane in the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell), half is [endoplasmic reticulum](#def-b1-eukaryotic-cell-endomembrane), a third the inner membrane of the [mitochondria](#def-b1-eukaryotic-cell-mitochondrion), and only $2\,\%$ the plasma membrane. Most of a [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell)’s membrane is inside it.

![Where a liver cell’s membrane is. The plasma membrane, the only one visible from outside, is two percent of the total; the reticulum and the inner mitochondrial membrane, folded inside, are four fifths.](https://one-course.com/images/onecourse/chapters/biology-3/b1-eukaryotic-cell/fig-c9c5545b106e.svg)

*Where a liver [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell)’s membrane is. The plasma membrane, the only one visible from outside, is two percent of the total; the reticulum and the inner mitochondrial membrane, folded inside, are four fifths.*

## 6.2 The endomembrane system

**Definition 6.4 (Endoplasmic reticulum, Golgi apparatus, lysosome).**

The *endoplasmic reticulum* (ER) is a network of membrane sheets and tubes enclosing one continuous space, the lumen, and continuous with the [nuclear envelope](#def-b1-eukaryotic-cell-nucleus). Its *rough* part is studded with ribosomes that inject the proteins they make into the lumen or into the membrane; its *smooth* part makes lipids, stores calcium and detoxifies. The *Golgi apparatus* is a stack of flattened sacs (cisternae), with a receiving *cis* face near the ER and a shipping *trans* face, in which proteins from the ER are modified (sugars trimmed and added), sorted, and packed into vesicles for their destinations. *Lysosomes* are acidic vesicles (pH $\approx 5$) full of hydrolytic enzymes that digest material brought in from outside or worn-out [organelles](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-prokeuk). Together with the vesicles that move between them and the plasma membrane, these form the *endomembrane system*: one connected set of compartments whose lumens are topologically the outside of the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell).

**Proposition 6.5 (The secretory pathway).**

A protein destined for secretion, for the plasma membrane, or for a [lysosome](#def-b1-eukaryotic-cell-endomembrane) is synthesised by ribosomes bound to the rough ER, enters the ER lumen as it is made, folds there, travels in vesicles to the cis face of the [Golgi](#def-b1-eukaryotic-cell-endomembrane), crosses the stack while being modified, and [leaves](https://one-course.com/books/biology/3/en/chapter/3-functional-organization-of-a-flowering-plant#def-b1-flowering-plant-organization-organs) the trans face in a vesicle addressed to its destination: a secretory granule that fuses with the plasma membrane on a signal (*regulated exocytosis*), a vesicle that fuses continuously (*constitutive* exocytosis, which also delivers new membrane), or a [lysosome](#def-b1-eukaryotic-cell-endomembrane). Cytosolic and nuclear proteins are made on free ribosomes and never enter the system. What decides is a *signal sequence* at the start of the protein ([Chapter 19](https://one-course.com/books/biology/3/en/chapter/19-gene-expression-transcription-and-translation#ch-b1-gene-expression)).

**Evidence.** Palade (1960s) fed slices of guinea-pig pancreas radioactive amino acids for three minutes (the *pulse*), then unlabelled ones (the *chase*), and located the radioactivity by autoradiography of electron micrographs at intervals. At $3\,\mathrm{min}$ the silver grains lay over the rough ER; at $20\,\mathrm{min}$ over the [Golgi](#def-b1-eukaryotic-cell-endomembrane); at $40\,\mathrm{min}$ over condensing [vacuoles](#def-b1-eukaryotic-cell-plantcell) and new granules; at $2\,\mathrm{h}$ in the duct lumen. [Cell fractionation](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-fractionation) gave the same sequence chemically. Blobel (1970s) showed that secretory proteins made in a test tube without membranes are twenty residues longer than the secreted form and are not protected from added protease, whereas made with ER membranes present they are cleaved and protected: the extra residues are the signal that directs the growing chain into the ER. ∎

![Palade’s pulse-chase in the pancreas: where the labelled proteins are, as a function of time. They pass from the ER to the Golgi in a quarter of an hour, reach the granules within the hour, and leave the cell over the next two.](https://one-course.com/images/onecourse/chapters/biology-3/b1-eukaryotic-cell/fig-57118cfd5300.svg)

*Palade’s pulse-chase in the pancreas: where the labelled proteins are, as a function of time. They pass from the ER to the [Golgi](#def-b1-eukaryotic-cell-endomembrane) in a quarter of an hour, reach the granules within the hour, and leave the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) over the next two.*

![A pancreatic acinar cell in the transmission electron microscope: rough ER packed in parallel stacks around the nucleus at the base, the Golgi above it, and the dense secretory granules crowded toward the apex and the duct.](https://one-course.com/images/onecourse/chapters/biology-3/b1-eukaryotic-cell/img-fa6ca9b2eb22.jpg)

*A pancreatic acinar [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) in the transmission [electron microscope](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#prop-b1-cell-unit-of-life-microscopes): rough ER packed in parallel stacks around the [nucleus](#def-b1-eukaryotic-cell-nucleus) at the base, the [Golgi](#def-b1-eukaryotic-cell-endomembrane) above it, and the dense secretory granules crowded toward the apex and the duct.*

**Method 6.6 (Following a molecule through the cell).**

1. Label a short cohort: expose [cells](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) to a radioactive or fluorescent precursor for a few minutes (the pulse), then replace it by excess unlabelled precursor (the chase), so that only molecules made during the pulse carry the label.
2. Sample at intervals, fix, and locate the label: by autoradiography of sections, by fluorescence microscopy, or by fractionation and counting.
3. The compartment whose label peaks first is upstream; the sequence of peaks is the route; the intervals are the transit times.
4. Block a step (cold, an inhibitor of energy, a mutant) and see where the label accumulates: the compartment before the block.

**Definition 6.7 (Endocytosis).**

*Endocytosis* brings material in: the plasma membrane invaginates around it and pinches off a vesicle, which generally fuses with a [lysosome](#def-b1-eukaryotic-cell-endomembrane). *Phagocytosis* engulfs particles (a bacterium, a dead [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell)); *pinocytosis* takes in droplets of fluid; *receptor-mediated endocytosis* concentrates a particular molecule (cholesterol-carrying particles, iron-carrying transferrin) on receptors in coated pits before taking it in. Exocytosis adds membrane to the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) surface and endocytosis removes it; in a secreting [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) the two balance so that the surface stays constant while whole areas of membrane cycle through it every hour.

## 6.3 Mitochondria and plastids

**Definition 6.8 (Mitochondrion).**

A *mitochondrion* is an [organelle](#def-b1-eukaryotic-cell-organelle) $0.5\text{ to }1\,\text{µ}\mathrm{m}$ across and a few micrometres long, bounded by two membranes: a smooth outer membrane permeable to small molecules, and an inner membrane folded into *cristae* that is impermeable to ions and carries the respiratory chain and the ATP synthase ([Chapter 15](https://one-course.com/books/biology/3/en/chapter/15-cellular-respiration-and-fermentation#ch-b1-respiration-fermentation)). Inside is the *matrix*, holding the enzymes of the Krebs cycle, ribosomes of the bacterial type and several copies of a small circular DNA. Mitochondria divide by fission, move along the [cytoskeleton](#def-b1-eukaryotic-cell-cytoskeleton), and number from a few hundred to a few thousand per [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell).

**Definition 6.9 (Plastids).**

*Plastids* are the double-membraned [organelles](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-prokeuk) of plant [cells](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell): *chloroplasts*, in which a third system of membranes, the *thylakoids*, stacked in grana and bathed in the *stroma*, carries out photosynthesis ([Chapter 14](https://one-course.com/books/biology/3/en/chapter/14-photosynthesis-and-autotrophy#ch-b1-photosynthesis)); amyloplasts, which store starch; chromoplasts, which hold the pigments of fruits and petals. All derive from proplastids and, like [mitochondria](#def-b1-eukaryotic-cell-mitochondrion), contain their own circular DNA and bacterial-type ribosomes and divide by fission.

![A mitochondrion (left) and a chloroplast (right) in section under the electron microscope. Two envelope membranes each; inside, the folded inner membrane of the mitochondrion and the stacked thylakoids of the chloroplast — the membranes on which energy is transduced.](https://one-course.com/images/onecourse/chapters/biology-3/b1-eukaryotic-cell/img-8d044dc24442.jpg)

![A mitochondrion (left) and a chloroplast (right) in section under the electron microscope. Two envelope membranes each; inside, the folded inner membrane of the mitochondrion and the stacked thylakoids of the chloroplast — the membranes on which energy is transduced.](https://one-course.com/images/onecourse/chapters/biology-3/b1-eukaryotic-cell/img-5a448090e3dd.jpg)

*A [mitochondrion](#def-b1-eukaryotic-cell-mitochondrion) (left) and a [chloroplast](#def-b1-eukaryotic-cell-plastid) (right) in section under the [electron microscope](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#prop-b1-cell-unit-of-life-microscopes). Two envelope membranes each; inside, the folded inner membrane of the [mitochondrion](#def-b1-eukaryotic-cell-mitochondrion) and the stacked [thylakoids](#def-b1-eukaryotic-cell-plastid) of the [chloroplast](#def-b1-eukaryotic-cell-plastid) — the membranes on which energy is transduced.*

**Proposition 6.10 (Endosymbiotic origin).**

[Mitochondria](#def-b1-eukaryotic-cell-mitochondrion) and [plastids](#def-b1-eukaryotic-cell-plastid) descend from free-living bacteria engulfed by an ancestral [eukaryotic cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-prokeuk) and retained as symbionts: [mitochondria](#def-b1-eukaryotic-cell-mitochondrion) from an aerobic bacterium, [plastids](#def-b1-eukaryotic-cell-plastid) from a cyanobacterium.

**Evidence.** Both [organelles](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-prokeuk) have two membranes, the inner of bacterial composition and the outer resembling the host’s; a circular chromosome without histones; ribosomes of the bacterial size and sensitivity to antibiotics; a genetic code with bacterial variants; division by binary fission independent of the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell)’s; and their genes, when sequenced, group with those of specific bacterial lineages (alphaproteobacteria, cyanobacteria) rather than with the nuclear genes of the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) that houses them. Most of their original genes have since moved to the [nucleus](#def-b1-eukaryotic-cell-nucleus), and the [organelles](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-prokeuk) can no longer live alone. ∎

**Definition 6.11 (Peroxisome).**

A *peroxisome* is a small single-membraned [organelle](#def-b1-eukaryotic-cell-organelle) containing oxidative enzymes that transfer hydrogen from substrates (fatty acids, alcohol) to oxygen, producing hydrogen peroxide, and catalase, which destroys the peroxide on the spot. It confines a dangerous chemistry.

## 6.4 The cytoskeleton

**Definition 6.12 (Cytoskeleton).**

The *cytoskeleton* is a network of protein filaments that gives the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) its shape, anchors its [organelles](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-prokeuk), and moves them and the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) itself. Three kinds: *microfilaments* of actin ($7\,\mathrm{nm}$, two twisted strands of globular subunits), concentrated under the plasma membrane, responsible for [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) shape, crawling, the contractile ring of division and muscle contraction with myosin; *microtubules* ($25\,\mathrm{nm}$ hollow tubes of tubulin), radiating from the *centrosome*, tracks along which motor proteins (kinesin outward, dynein inward) carry vesicles and [organelles](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-prokeuk), the spindle of division, and the core of cilia and flagella; *intermediate filaments* ($10\,\mathrm{nm}$, rope-like, of keratins and related proteins), the mechanically resistant fibres that stiffen [cells](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) and anchor at desmosomes. Microfilaments and microtubules assemble and disassemble continuously from their subunit pools.

![The three filaments of the cytoskeleton, drawn to the same scale. Actin and tubulin polymerise reversibly from globular subunits; intermediate filaments are stable ropes.](https://one-course.com/images/onecourse/chapters/biology-3/b1-eukaryotic-cell/fig-411d0069b473.svg)

*The three filaments of the [cytoskeleton](#def-b1-eukaryotic-cell-cytoskeleton), drawn to the same scale. Actin and tubulin polymerise reversibly from globular subunits; [intermediate filaments](#def-b1-eukaryotic-cell-cytoskeleton) are stable ropes.*

**Example 6.13 (Moving a vesicle).**

A secretory vesicle leaving the [Golgi](#def-b1-eukaryotic-cell-endomembrane) is picked up by kinesin, a motor that walks along a [microtubule](#def-b1-eukaryotic-cell-cytoskeleton) toward the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell)’s periphery at about $1\,\text{µ}\mathrm{m}/\mathrm{s}$, spending one ATP per $8\,\mathrm{nm}$ step; in a [neuron](https://one-course.com/books/biology/3/en/chapter/4-animal-body-plans-and-tissues#def-b1-body-plans-tissues-nervous) the same motor carries vesicles the length of an axon a metre long — a journey of ten days. Near the surface, the vesicle passes to the actin cortex and to myosin for the last micrometre. Diffusion would take a vesicle of $100\,\mathrm{nm}$ about a minute to cross a [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) of $20\,\text{µ}\mathrm{m}$, and years to travel a metre: [organelles](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-prokeuk) are carried, not left to wander.

## 6.5 Plant and animal cells

**Definition 6.14 (Cell wall, vacuole, plasmodesmata).**

A plant [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) adds three structures to the common plan. The *cell wall*, outside the plasma membrane, is a composite of cellulose microfibrils in a matrix of other polysaccharides ([Chapter 10](https://one-course.com/books/biology/3/en/chapter/10-carbohydrates#ch-b1-carbohydrates)); it fixes the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell)’s shape, resists the pressure inside, and cements neighbouring [cells](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) together through a shared middle lamella. The *vacuole*, a single-membraned compartment holding up to $90\,\%$ of the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell)’s volume, stores water, ions, sugars, pigments and wastes; its osmotic pressure pushes the membrane against the wall, and this *turgor* is what holds a [leaf](https://one-course.com/books/biology/3/en/chapter/3-functional-organization-of-a-flowering-plant#def-b1-flowering-plant-organization-organs) up. *Plasmodesmata* are channels through the walls, lined by plasma membrane, through which the [cytosols](#def-b1-eukaryotic-cell-organelle) of neighbouring [cells](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) are continuous, so that a plant [tissue](https://one-course.com/books/biology/3/en/chapter/4-animal-body-plans-and-tissues#def-b1-body-plans-tissues-tissue) is in part one connected [cytoplasm](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) (the symplast).

![A plant cell: the wall outside the membrane, the vacuole filling most of the volume, chloroplasts and mitochondria in the thin layer of cytoplasm between them.](https://one-course.com/images/onecourse/chapters/biology-3/b1-eukaryotic-cell/fig-3339f485aadc.svg)

*A plant [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell): the wall outside the membrane, the [vacuole](#def-b1-eukaryotic-cell-plantcell) filling most of the volume, [chloroplasts](#def-b1-eukaryotic-cell-plastid) and [mitochondria](#def-b1-eukaryotic-cell-mitochondrion) in the thin layer of [cytoplasm](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) between them.*

**Proposition 6.15 (Plant and animal cell compared).**

Both have a [nucleus](#def-b1-eukaryotic-cell-nucleus), ER, [Golgi](#def-b1-eukaryotic-cell-endomembrane), [mitochondria](#def-b1-eukaryotic-cell-mitochondrion), [peroxisomes](#def-b1-eukaryotic-cell-peroxisome), ribosomes and a [cytoskeleton](#def-b1-eukaryotic-cell-cytoskeleton). The plant [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) has a cellulose wall, a large [vacuole](#def-b1-eukaryotic-cell-plantcell), [plastids](#def-b1-eukaryotic-cell-plastid) and [plasmodesmata](#def-b1-eukaryotic-cell-plantcell), and no centrioles or [lysosomes](#def-b1-eukaryotic-cell-endomembrane) (the [vacuole](#def-b1-eukaryotic-cell-plantcell) digests); it does not crawl or engulf, and divides by building a new wall across the middle. The animal [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) has no wall (and so can change shape, crawl and phagocytose), [lysosomes](#def-b1-eukaryotic-cell-endomembrane), a [centrosome](#def-b1-eukaryotic-cell-cytoskeleton) with centrioles, and communicates through junctions rather than [plasmodesmata](#def-b1-eukaryotic-cell-plantcell); it divides by pinching in two.

**Example 6.16 (Turgor).**

A [leaf](https://one-course.com/books/biology/3/en/chapter/3-functional-organization-of-a-flowering-plant#def-b1-flowering-plant-organization-organs) [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) whose [vacuole](#def-b1-eukaryotic-cell-plantcell) holds $0.3\,\mathrm{mol}/\mathrm{L}$ of solutes draws water in until the wall pushes back with a pressure of about $0.7\,\mathrm{MPa}$, seven atmospheres; the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) is then turgid and the [leaf](https://one-course.com/books/biology/3/en/chapter/3-functional-organization-of-a-flowering-plant#def-b1-flowering-plant-organization-organs) firm. Deprived of water, the [vacuole](#def-b1-eukaryotic-cell-plantcell) loses volume, the pressure falls to zero, and the [leaf](https://one-course.com/books/biology/3/en/chapter/3-functional-organization-of-a-flowering-plant#def-b1-flowering-plant-organization-organs) wilts — the wall unchanged, the pressure gone ([Chapter 7](https://one-course.com/books/biology/3/en/chapter/7-membranes-and-membrane-transport#ch-b1-membranes-transport)). An animal [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) in the same solution, having no wall, would simply burst.

## 6.6 Exercises

**Exercise 6.1 ★.**

List the membrane-bounded [organelles](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-prokeuk) of an animal [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) and give the main function of each in a few words.

**Solution of Exercise 6.1.**

[Nucleus](#def-b1-eukaryotic-cell-nucleus) (chromosomes, transcription); rough ER (secretory and membrane proteins); smooth ER (lipids, calcium, detoxification); [Golgi](#def-b1-eukaryotic-cell-endomembrane) (sorting, modification, packing); [lysosomes](#def-b1-eukaryotic-cell-endomembrane) (digestion); [peroxisomes](#def-b1-eukaryotic-cell-peroxisome) (oxidations, catalase); [mitochondria](#def-b1-eukaryotic-cell-mitochondrion) (respiration, ATP).

**Exercise 6.2 ★.**

Trace the path of a digestive enzyme from its synthesis to the duct, naming each compartment and how the protein passes between them.

**Solution of Exercise 6.2.**

Synthesised by ribosomes bound to the rough ER and threaded into its lumen ([signal sequence](#prop-b1-eukaryotic-cell-secretory)); vesicles bud from the ER and fuse with the cis [Golgi](#def-b1-eukaryotic-cell-endomembrane); the protein crosses the stack, is modified and sorted; it [leaves](https://one-course.com/books/biology/3/en/chapter/3-functional-organization-of-a-flowering-plant#def-b1-flowering-plant-organization-organs) the trans face in a condensing [vacuole](#def-b1-eukaryotic-cell-plantcell) that matures into a secretory granule; the granule fuses with the apical membrane on a signal and empties into the duct.

**Exercise 6.3 ★.**

From the membrane-budget figure, what fraction of the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell)’s membrane is mitochondrial (outer plus inner)? Why is the inner membrane so much larger than the outer?

**Solution of Exercise 6.3.**

$7 + 32 = 39\,\%$. The inner membrane is folded into [cristae](#def-b1-eukaryotic-cell-mitochondrion) because it carries the respiratory chain and ATP synthase, whose throughput is proportional to its area; the outer is a simple envelope.

**Exercise 6.4 ★.**

Give three structures a plant [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) has that an animal [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) lacks, and two the animal [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) has that the plant [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) lacks.

**Solution of Exercise 6.4.**

Plant: cellulose wall, large [vacuole](#def-b1-eukaryotic-cell-plantcell), [plastids](#def-b1-eukaryotic-cell-plastid) ([plasmodesmata](#def-b1-eukaryotic-cell-plantcell) too). Animal: [lysosomes](#def-b1-eukaryotic-cell-endomembrane), centrioles (and the ability to crawl and phagocytose).

**Exercise 6.5 ★★.**

From the pulse-chase figure, at what time does the label peak in the ER, in the [Golgi](#def-b1-eukaryotic-cell-endomembrane), and in the granules? Estimate the transit time from ER to granule and the time before half the label has been secreted.

**Solution of Exercise 6.5.**

ER at $3\,\mathrm{min}$ (already declining), [Golgi](#def-b1-eukaryotic-cell-endomembrane) at about $15\,\mathrm{min}$, granules at about $60\,\mathrm{min}$. Transit ER to granule about $45\,\mathrm{min}$; half secreted at about $110\,\mathrm{min}$.

**Exercise 6.6 ★★.**

A [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) is treated with a drug that depolymerises [microtubules](#def-b1-eukaryotic-cell-cytoskeleton). Predict the effects on the position of the [Golgi](#def-b1-eukaryotic-cell-endomembrane), on vesicle traffic, on [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) division and on the beating of cilia.

**Solution of Exercise 6.6.**

The [Golgi](#def-b1-eukaryotic-cell-endomembrane), held near the [centrosome](#def-b1-eukaryotic-cell-cytoskeleton) by dynein on [microtubules](#def-b1-eukaryotic-cell-cytoskeleton), fragments and disperses; vesicle traffic slows to diffusion and transport to the periphery stops; the spindle cannot form and [cells](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) arrest in mitosis; cilia, whose core is [microtubules](#def-b1-eukaryotic-cell-cytoskeleton), stop (existing cilia are stable, but they are not renewed).

**Exercise 6.7 ★★.**

Give four pieces of evidence for the bacterial origin of [mitochondria](#def-b1-eukaryotic-cell-mitochondrion), and explain why a [mitochondrion](#def-b1-eukaryotic-cell-mitochondrion) nevertheless cannot live outside a [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell).

**Solution of Exercise 6.7.**

Two membranes with a bacterial-type inner one; circular DNA without histones; bacterial-size ribosomes sensitive to antibiotics; division by fission; gene sequences grouping with alphaproteobacteria. It cannot live alone because most of its genes have moved to the [nucleus](#def-b1-eukaryotic-cell-nucleus): it imports most of its proteins from the [cytosol](#def-b1-eukaryotic-cell-organelle).

**Exercise 6.8 ★★.**

The lumen of the ER is “topologically outside the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell)”. Explain the phrase by following a membrane protein whose sugar chains face the lumen from the ER to the plasma membrane: which side do the sugars face at the end?

**Solution of Exercise 6.8.**

The sugars are added on the luminal side in the ER and [Golgi](#def-b1-eukaryotic-cell-endomembrane). Each vesicle fusion preserves sidedness: the luminal face of a vesicle becomes the luminal face of the next compartment, and when the vesicle fuses with the plasma membrane its lumen opens to the outside, so the sugars face the exterior. What is luminal is, topologically, outside.

**Exercise 6.9 ★★.**

A secretory [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) releases $2000\,$ granules of $1\,\text{µ}\mathrm{m}$ diameter per day through an apical surface of $40\,\text{µ}\mathrm{m}^{2}$. Compute the membrane added per day and how many times the apical surface must be recycled. What does this imply?

**Solution of Exercise 6.9.**

Each granule $\pi d^2 = 3.14\,\text{µ}\mathrm{m}^{2}$; per day $6280\,\text{µ}\mathrm{m}^{2}$, i.e. $157\,$ times the apical face. The [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) must retrieve membrane by [endocytosis](#def-b1-eukaryotic-cell-endocytosis) at the same rate, so the apical membrane turns over every nine minutes and the retrieved membrane is returned to the [Golgi](#def-b1-eukaryotic-cell-endomembrane) for re-use.

**Exercise 6.10 ★★★.**

In a test tube containing ribosomes, mRNA for a secretory protein and amino acids, the product is a chain of $320\,$ residues; with ER vesicles added, a chain of $300\,$ residues appears inside the vesicles and is not digested by a protease added afterwards. Interpret each observation, and say what a mutant lacking the first twenty residues would do.

**Solution of Exercise 6.10.**

The $320\,$-residue chain is the precursor with its $20\,$ residue [signal sequence](#prop-b1-eukaryotic-cell-secretory), which is normally cleaved on entry into the ER. With vesicles, the chain enters the lumen during synthesis, the signal is cleaved, and the membrane protects the product from the protease. The mutant lacking the signal is not recognised by the ER machinery: it is made on free ribosomes and stays in the [cytosol](#def-b1-eukaryotic-cell-organelle), full length, never secreted.

**Exercise 6.11 ★★★.**

A patient’s [cells](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) lack a lysosomal enzyme that degrades a lipid; the lipid accumulates in swollen [lysosomes](#def-b1-eukaryotic-cell-endomembrane) and the [cells](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) die. Explain the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) biology, say why the disease is worst in [cells](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) that renew their membranes fastest, and propose how an enzyme injected into the blood could reach the [lysosomes](#def-b1-eukaryotic-cell-endomembrane).

**Solution of Exercise 6.11.**

Membranes internalised by [endocytosis](#def-b1-eukaryotic-cell-endocytosis) and worn [organelles](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-prokeuk) are delivered to [lysosomes](#def-b1-eukaryotic-cell-endomembrane) for digestion; without the enzyme, the lipid they contain cannot be broken down and fills the [lysosomes](#def-b1-eukaryotic-cell-endomembrane), which swell until the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) fails. [Cells](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) with the fastest membrane turnover ([neurons](https://one-course.com/books/biology/3/en/chapter/4-animal-body-plans-and-tissues#def-b1-body-plans-tissues-nervous), which recycle synaptic vesicles continuously; macrophages) accumulate it fastest. An injected enzyme carrying the sugar signal that lysosomal enzymes bear can bind receptors at the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) surface and be taken up by receptor-mediated [endocytosis](#def-b1-eukaryotic-cell-endocytosis) into the [lysosomes](#def-b1-eukaryotic-cell-endomembrane) — the principle of enzyme replacement therapy.

**Exercise 6.12 ★★★.**

“The [eukaryotic cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-prokeuk) is a federation of former bacteria held together by a membrane system.” Discuss in a paragraph, weighing what the endosymbiotic theory explains against what it does not ([nucleus](#def-b1-eukaryotic-cell-nucleus), ER, [cytoskeleton](#def-b1-eukaryotic-cell-cytoskeleton)).

**Solution of Exercise 6.12.**

Endosymbiosis accounts well for [mitochondria](#def-b1-eukaryotic-cell-mitochondrion) and [plastids](#def-b1-eukaryotic-cell-plastid) (two membranes, own genome and ribosomes, fission, bacterial relatives), and thus for the eukaryote’s energy metabolism. It does not explain the [nucleus](#def-b1-eukaryotic-cell-nucleus), the ER and [Golgi](#def-b1-eukaryotic-cell-endomembrane), or the [cytoskeleton](#def-b1-eukaryotic-cell-cytoskeleton), which have no bacterial counterparts and appear to be inventions of the host lineage — infoldings of its membrane and new proteins. “Federation” is apt for the [organelles](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-prokeuk); “held together by a membrane system” points to the larger, unexplained part: a host [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) that was already eukaryotic in its architecture before it took the bacteria in.

## 6.7 Problem: The Acinar Cell of the Pancreas

**Problem 6.1.**

Weekend problem — a cell that secretes its own mass of protein every day: Palade’s clock, the membrane it must recycle, the signal that routes the protein, ending on the transit time of a digestive enzyme

A pancreatic acinar [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) is a cube of side $12\,\text{µ}\mathrm{m}$, with its apical face ($144\,\text{µ}\mathrm{m}^{2}$, on the duct) and its basal face on the blood. A human pancreas of $80\,\mathrm{g}$ holds about $8 \times 10^{10}$ such [cells](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) and secretes $10\,\mathrm{g}$ of enzyme protein per day. A secretory granule is a sphere $1\,\text{µ}\mathrm{m}$ in diameter holding protein at $200\,\mathrm{mg}/\mathrm{mL}$. Ribosomes add $5\,$ amino acids per second; a typical enzyme has $250\,$ residues of mean mass $110\,\mathrm{g}/\mathrm{mol}$.

**Part I — Palade’s clock.** The pulse-chase figure of the chapter gives the location of the label with time.

1. Why must the pulse be short and the chase contain a large excess of unlabelled amino acid?
2. Read off the time at which the label peaks in the ER, in the [Golgi](#def-b1-eukaryotic-cell-endomembrane) and in the granules.
3. Estimate the transit time from ER to [Golgi](#def-b1-eukaryotic-cell-endomembrane) and from [Golgi](#def-b1-eukaryotic-cell-endomembrane) to granule.
4. Where is the label at $60\,\mathrm{min}$ , and why has some of it already left the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) while most of it is still in granules?
5. A second experiment is done at $15\,{}^{\circ}\mathrm{C}$ : the label stays in the ER for over an hour. Interpret.

**Part II — The mass of protein.**

6. Compute the protein secreted per [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) per day, in picograms.
7. Compute the volume of one granule and the mass of protein it holds.
8. Compute the number of granules released per [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) per day, and per minute.
9. Compute the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) ’s volume and, with a density of $1.05\,\mathrm{g}/\mathrm{cm}^{3}$ , its mass. What fraction of its own mass does it secrete each day?
10. Compute the number of enzyme molecules in one granule (Avogadro’s number $6.0 \times 10^{23}$ ).
11. Compute the time a ribosome needs to make one enzyme molecule.
12. How many ribosomes must work continuously to make the daily output? Compare with the $4 \times 10^{6}$ ribosomes such a [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) contains.

**Part III — Membrane.**

13. Compute the membrane area of one granule.
14. Compute the membrane delivered to the apical face per day by exocytosis of the granules of question 8.
15. How many times per day is the area of the apical face added? What must the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) do with it, and by what process?
16. The granule membrane arrives from the [Golgi](#def-b1-eukaryotic-cell-endomembrane) , which receives it from the ER. If the ER of the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) has $20\,000\,\text{µ}\mathrm{m}^{2}$ of membrane, how many days’ worth of granule membrane is that? What does this say about recycling within the [endomembrane system](#def-b1-eukaryotic-cell-endomembrane) ?
17. The apical face carries $2000\,$ microvilli of $0.1\,\text{µ}\mathrm{m}$ diameter and $1\,\text{µ}\mathrm{m}$ length. By what factor do they enlarge the $144\,\text{µ}\mathrm{m}^{2}$ face?

**Part IV — The address on the protein.**

18. An enzyme’s mRNA translated in a test tube without membranes gives a chain $20\,$ residues longer than the enzyme found in the granules. Where are the extra residues and what are they?
19. With ER vesicles present, the product is the right length, inside the vesicles, and protected from a protease. Interpret.
20. The same experiment with the mRNA of a cytosolic enzyme gives a product outside the vesicles and unprotected. Conclude.
21. A mutation deletes the twenty residues. Predict where the enzyme ends up and what happens to the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) .
22. Another enzyme is found in the [lysosomes](#def-b1-eukaryotic-cell-endomembrane) rather than the granules. It carries the same kind of [signal sequence](#prop-b1-eukaryotic-cell-secretory) . Where along the pathway must the second decision be taken, and by what mechanism, in outline?
23. A [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) is treated with a drug that blocks ATP synthesis. Predict where the label of a pulse-chase accumulates and why (two reasons).
24. Combine questions 3 and 8: at a steady state, how many granules are “in transit” between ER and granule at any moment?
25. Summarise the result: the transit time from synthesis to granule, the time to secretion, the granules released per minute, and the fraction of its own mass the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) secretes daily.

**Solution of Problem 6.1.**

**1.** A short pulse labels a narrow cohort of molecules so their position marks one time; the excess cold amino acid stops further incorporation so the cohort is not smeared by later synthesis. **2.** ER: $3\,\mathrm{min}$ (earliest point); [Golgi](#def-b1-eukaryotic-cell-endomembrane) and condensing [vacuoles](#def-b1-eukaryotic-cell-plantcell): about $15\,\mathrm{min}$; granules: about $60\,\mathrm{min}$. **3.** ER to [Golgi](#def-b1-eukaryotic-cell-endomembrane) about $10\,\mathrm{min}$; [Golgi](#def-b1-eukaryotic-cell-endomembrane) to granule about $45\,\mathrm{min}$. **4.** Mostly in granules ($70\,\%$), a tenth in the [Golgi](#def-b1-eukaryotic-cell-endomembrane), a tenth already secreted: the constitutive path and the first granules formed release some, while most wait for a stimulus. **5.** Vesicle budding and transport require energy and fluid membranes; in the cold the ER-to-Golgi step stops and the protein accumulates upstream of the block. **6.** $10\,\mathrm{g}/8 \times 10^{10} = 1.25 \times 10^{-10}\,\mathrm{g} =
125\,\mathrm{pg}$ per [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) per day. **7.** $V = \frac{\pi}{6}(1)^3 = 0.52\,\text{µ}\mathrm{m}^{3} =
5.2 \times 10^{-13}\,\mathrm{mL}$; protein $1.05 \times 10^{-13}\,\mathrm{g} = 0.1\,\mathrm{pg}$. **8.** $125/0.1 = 1250$ granules per day, $0.87$ per minute. **9.** $V = 12^3 = 1728\,\text{µ}\mathrm{m}^{3} = 1.73 \times 10^{-9}\,\mathrm{cm}^{3}$; mass $1.8 \times 10^{-9}\,\mathrm{g}$ = $1800\,\mathrm{pg}$. Secretes $125/1800 = 7\%$ of its mass daily (the whole [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) is only $20\,\%$ protein, so a third of its protein content). **10.** Molar mass $250\times 110 = 27\,500\,\mathrm{g}/\mathrm{mol}$; $1.05 \times 10^{-13}\,\mathrm{g}/27500\times6 \times 10^{23} = 2.3 \times 10^{6}$ molecules. **11.** $250/5 = 50\,\mathrm{s}$. **12.** Molecules per day $1250\times2.3 \times 10^{6} = 2.9 \times 10^{9}$; one ribosome makes $86\,400/50 = 1728$ per day; needed: $1.7 \times 10^{6}$ ribosomes, about $40\,\%$ of the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell)’s. **13.** $\pi d^2 = 3.14\,\text{µ}\mathrm{m}^{2}$. **14.** $1250\times 3.14 = 3900\,\text{µ}\mathrm{m}^{2}$. **15.** $3900/144 = 27$ times: the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) retrieves the same area by [endocytosis](#def-b1-eukaryotic-cell-endocytosis) and returns it to the [Golgi](#def-b1-eukaryotic-cell-endomembrane). **16.** $20\,000/3900 = 5$ days: the ER could supply only five days of granule membrane if none returned; the membrane must recycle continuously. **17.** Each microvillus $\pi\times 0.1\times 1 =
0.31\,\text{µ}\mathrm{m}^{2}$; $630\,\text{µ}\mathrm{m}^{2}$ for 2000; factor $5.4$. **18.** At the beginning of the chain: the [signal sequence](#prop-b1-eukaryotic-cell-secretory), twenty mostly hydrophobic residues. **19.** The signal directed the growing chain into the vesicles, where a signal peptidase removed it; the membrane shields the protein from the protease. **20.** Only proteins with a signal enter the ER; a cytosolic protein lacks it and stays outside. **21.** The enzyme is made on free ribosomes and stays in the [cytosol](#def-b1-eukaryotic-cell-organelle); being a digestive enzyme (as an inactive precursor it does little harm, but if activated) it would digest the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell) from inside. **22.** In the trans [Golgi](#def-b1-eukaryotic-cell-endomembrane): the lysosomal enzyme carries a distinguishing mark (a phosphorylated sugar) recognised by a receptor that packs it into vesicles bound for the [lysosomes](#def-b1-eukaryotic-cell-endomembrane) rather than into secretory granules. **23.** In the ER (and the [Golgi](#def-b1-eukaryotic-cell-endomembrane)): vesicle formation, motor transport along [microtubules](#def-b1-eukaryotic-cell-cytoskeleton) and membrane fusion all consume ATP; and protein synthesis itself stops, so the pulse is not even completed. **24.** $45\,\mathrm{min}$ transit at $0.87$ granules per minute: about $40\,$ granules’ worth of protein in transit. **25.** Transit from synthesis to granule about $45\,\mathrm{min}$ ($55\,\mathrm{min}$ counting the ER), secretion after about two hours in the experiment (four hours or more in the living gland, since granules wait for a meal); $0.9\,$ granules released per minute; $7\,\%$ of the [cell](https://one-course.com/books/biology/3/en/chapter/5-the-cell-unit-of-life#def-b1-cell-unit-of-life-cell)’s mass per day.