Cell Structure and Specialisation

Lessons

Senior 4 · Biology

Cell Structure and Specialisation

5 lessons

Lesson 2 of 5

Ultrastructure: the nucleus and the endomembrane system

25 min

Following a protein out of the cell

The cells of the pancreas pour out digestive enzymes, which are proteins, all day long. In the 1960s George Palade and his colleagues gave pancreatic tissue a short "pulse" of radioactive amino acids, then a "chase" of ordinary ones, and used electron microscopy to see where the radioactive protein was at each moment. Within minutes it was in the rough endoplasmic reticulum; a little later in the Golgi apparatus; later still in the secretory granules near the cell surface. Palade shared the 1974 Nobel Prize in Physiology or Medicine with Albert Claude and Christian de Duve, who discovered the lysosome, for their work on the structure and organisation of the cell.

This lesson describes the organelles that the electron microscope revealed inside eukaryotic cells, starting with the nucleus and the connected system of membranes that makes, packages and exports proteins.

Definition

Ultrastructure: the detailed structure of a cell as seen with an electron microscope.

Nuclear envelope: the double membrane around the nucleus, crossed by nuclear pores through which molecules such as messenger RNA leave the nucleus.

Ribosome: a small organelle (about 25 nm in eukaryotes) made of RNA and protein, without a membrane, where proteins are made. Eukaryotic cytoplasmic ribosomes are 80S.

Endoplasmic reticulum (ER): a network of flattened membrane sacs (cisternae) and tubes. Rough ER carries ribosomes; smooth ER has none.

Golgi apparatus: a stack of curved, flattened membrane sacs that modifies, sorts and packages proteins and lipids into vesicles.

Lysosome: a small vesicle, bounded by a single membrane, containing digestive (hydrolytic) enzymes.

The organelles and their functions

Organelle	Structure	Function
Nucleus	Double-membrane envelope with pores; chromatin (DNA wound around histone proteins); one or more nucleoli	Holds the genetic material; controls the cell by controlling which proteins are made; DNA is copied here before division
Nucleolus	Dense region in the nucleus, no membrane	Makes ribosomal RNA and assembles the subunits of ribosomes
Ribosomes	Two subunits of RNA and protein; free in the cytoplasm or attached to rough ER	Protein synthesis: free ribosomes make proteins used inside the cell; ribosomes on rough ER make proteins for membranes, lysosomes or export
Rough ER	Flattened sacs studded with ribosomes, continuous with the outer nuclear membrane	Proteins made by its ribosomes enter its space, are folded, and are carried in vesicles to the Golgi
Smooth ER	Network of tubes without ribosomes	Makes lipids and steroids (such as sex hormones); in liver cells, breaks down toxins; in muscle, stores calcium ions
Golgi apparatus	Stack of curved cisternae; vesicles arrive at one face and leave from the other	Modifies proteins (for example by adding sugar chains to make glycoproteins), sorts and packages them into secretory vesicles and lysosomes
Vesicles	Small membrane-bound sacs	Carry materials around the cell; secretory vesicles fuse with the cell surface membrane to release their contents (exocytosis)
Lysosomes	Single membrane around hydrolytic enzymes, made by the Golgi	Digest worn-out organelles, and bacteria taken in by white blood cells; can destroy the whole cell when it dies

Together the nuclear envelope, ER, Golgi apparatus, vesicles, lysosomes and cell surface membrane form the endomembrane system: membranes that are linked directly or by vesicles moving between them.

Worked example 1: a pulse-chase experiment

Pancreatic cells were given radioactive amino acids for 3 minutes, then ordinary ones. The table shows the percentage of the radioactivity found in each structure (illustrative data, following the pattern Palade's group found).

Time after pulse / min	Rough ER	Golgi apparatus	Secretory vesicles
3	85	5	2
20	30	50	12
40	15	25	50
120	10	10	60

Describe and explain the results.

At 3 minutes most radioactivity (85 %) is in the rough ER: the amino acids were built into new proteins by the ribosomes on the rough ER.
By 20 minutes the peak has moved to the Golgi apparatus (50 %), while the rough ER has fallen to 30 %: the proteins were carried from the ER to the Golgi in vesicles.
By 40 and 120 minutes most is in secretory vesicles (50 %, then 60 %): the Golgi packaged the proteins into vesicles ready for release.
Because the label moves in sequence, the evidence supports the pathway rough ER → Golgi → secretory vesicles → exocytosis. Using a short pulse is the key: it labels one "batch" of protein, which can then be followed.

Electron micrograph of pancreatic acinar cells, labelled: a cluster of tiny black granules near the middle of the cell are secretory vesicles packed with digestive enzymes, waiting to be released by exocytosis. Image: OpenStax Anatomy and Physiology (micrograph provided by the Regents of the University of Michigan Medical School), CC BY 3.0, via Wikimedia Commons

Worked example 2: matching cells to organelles

Predict which organelles will be especially abundant in (a) a pancreatic cell that secretes enzymes; (b) a cell in the testis that makes the steroid hormone testosterone; (c) a white blood cell that engulfs bacteria.

(a) Rough ER (to make large amounts of protein), Golgi apparatus and secretory vesicles (to package and export it), and mitochondria (to supply ATP for synthesis and exocytosis).
(b) Smooth ER, because steroids are lipids, and lipid synthesis takes place there.
(c) Lysosomes, whose hydrolytic enzymes digest the engulfed bacteria.
The principle: a cell's ultrastructure reflects what it does. You will use this again in Lesson 5.

Transmission electron micrograph of a Golgi apparatus: a curved stack of thin, dark, flattened sacs, with its cis face (where vesicles arrive) and trans face (where vesicles leave) labelled, and small round vesicles around it. The scale bar is 50 nm. Image: OpenStax Anatomy and Physiology (micrograph provided by the Regents of the University of Michigan Medical School), CC BY 3.0, via Wikimedia Commons

CrashCourse: "Eukaryopolis - The City of Animal Cells" (Crash Course Biology #4). Note how the video compares each organelle to part of a city, and check each comparison against the table above.

Warning

Common mistakes

Writing that ribosomes are made in the nucleus "by the DNA". Ribosome subunits are assembled in the nucleolus from RNA and protein.
Saying the Golgi apparatus makes proteins. Proteins are made on ribosomes; the Golgi modifies and packages them.
Giving lysosomes a double membrane. Only the nucleus, mitochondria and chloroplasts have double membranes.
Calling ribosomes membrane-bound organelles. They have no membrane, which is why prokaryotes have them too.

Tip

Memory hook for the secretory pathway: "Real Gifts Visit Mothers": Rough ER → Golgi → Vesicle → Membrane (exocytosis).

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Practice: the nucleus and the endomembrane system

Describe the structure of the nuclear envelope and explain the function of nuclear pores.
Distinguish between rough and smooth ER in structure and function.
Explain the role of the Golgi apparatus in making a glycoprotein that is secreted.
Why do lysosomes not digest the cell that contains them?
A ribosome 25 nm across measures 5 mm in a micrograph. Calculate the magnification.
Liver cells of a person who drinks a lot of alcohol have more smooth ER than normal. Suggest why.
In a pulse-chase experiment, why would a long pulse of radioactive amino acids (2 hours) make the results impossible to interpret?

Show answersHide answers

Two membranes (inner and outer, the outer continuous with the rough ER) with a space between; pores where the membranes join let mRNA and ribosome subunits out and proteins (such as enzymes for DNA copying) in.
Rough ER has ribosomes on its surface and processes proteins for secretion, membranes and lysosomes; smooth ER has no ribosomes and makes lipids and steroids, detoxifies, and stores calcium in muscle.
The protein arrives from the rough ER in vesicles; the Golgi adds carbohydrate chains, sorts the glycoprotein and packages it into secretory vesicles, which move to the cell surface membrane and fuse with it (exocytosis).
Their enzymes are kept inside a membrane, separate from the cytoplasm; (also, the enzymes work best at the acid pH inside the lysosome and poorly at the near-neutral pH of the cytoplasm).
$5\,\mathrm{mm} = 5\,000\,000\,\mathrm{nm}$ ; $M = \frac{5\,000\,000}{25} = 200\,000$ , so ×200 000.
Smooth ER in liver cells contains enzymes that break down toxins such as alcohol; repeated exposure leads the cells to make more smooth ER, to deal with the larger load.
A long pulse labels proteins made over the whole 2 hours, so at any later time labelled protein would be in every compartment at once; the "batch" could not be followed from one organelle to the next.

Quick check

Put the stages of the secretory pathway in order.

Tap the cards in the right order.

Key points

The nucleus has a double envelope with pores, chromatin and a nucleolus that makes ribosomes.
Rough ER (with ribosomes) processes proteins for export; smooth ER makes lipids and steroids and detoxifies.
The Golgi apparatus modifies, sorts and packages proteins into vesicles and makes lysosomes.
Secretory pathway: rough ER → vesicle → Golgi → secretory vesicle → exocytosis, shown by pulse-chase experiments.

Sources and credits

2.1.4: Eukaryotic Cells and 2.1.5: The Endomembrane System and Proteins, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for the nucleus, ribosomes, ER, Golgi apparatus, lysosomes and the endomembrane system. Adapted by Ganzaa.
George Emil Palade, Wikipedia contributors, licensed under CC BY-SA 4.0. Used for the pulse-chase experiments and the 1974 Nobel Prize. Adapted by Ganzaa.
Figure 1, the worked examples (the pulse-chase data are illustrative), practice and quick check: written by Ganzaa.
Micrographs: OpenStax Anatomy and Physiology (provided by the Regents of the University of Michigan Medical School), CC BY 3.0, via Wikimedia Commons.
Video: "Eukaryopolis - The City of Animal Cells: Crash Course Biology #4", CrashCourse, embedded from YouTube.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Following a protein out of the cell

2The organelles and their functions

Following a protein out of the cell

The organelles and their functions