Cell Structure and Specialisation

Lessons

Senior 4 · Biology

Cell Structure and Specialisation

5 lessons

Lesson 3 of 5

Ultrastructure: energy organelles, the cytoskeleton and the plant cell

25 min

Organelles that behave like bacteria

Mitochondria and chloroplasts are strange organelles. Each has its own small loop of DNA, its own ribosomes of the bacterial size, and it divides in two inside the cell much as a bacterium does. In 1967 the American biologist Lynn Margulis took up an older idea and backed it with evidence: both began as free-living bacteria that were engulfed by an ancestral eukaryotic cell and stayed, to the benefit of both. Her idea, the endosymbiotic theory, was widely doubted at first; DNA evidence has since made it the accepted explanation.

This lesson completes the tour of the eukaryotic cell: the organelles that supply energy, the protein framework that shapes and moves the cell, and the structures special to plant cells.

Definition

Mitochondrion (plural mitochondria): an organelle with a double membrane, the inner one folded into cristae, in which aerobic respiration produces ATP.

Chloroplast: an organelle of plants and algae with a double-membrane envelope, containing stacks of membrane sacs (thylakoids, stacked into grana) in a fluid stroma; the site of photosynthesis.

ATP (adenosine triphosphate): the molecule that carries energy from respiration to the processes that use it.

Cytoskeleton: a network of protein fibres in the cytoplasm (microfilaments, microtubules and intermediate filaments) that supports the cell and moves things within it.

Plasmodesma (plural plasmodesmata): a channel through the walls of two neighbouring plant cells, lined by membrane, that links their cytoplasm.

Mitochondria and chloroplasts

Feature	Mitochondrion	Chloroplast
Found in	Almost all eukaryotic cells	Green cells of plants and algae
Membranes	Double; inner folded into cristae	Double envelope, plus a third system of thylakoid membranes inside
Inner compartment	Matrix: enzymes of the Krebs cycle	Stroma: enzymes that fix carbon dioxide into sugar
Where energy is captured	On the cristae, where ATP is made (oxidative phosphorylation)	On the thylakoid membranes, which hold chlorophyll and capture light energy
Own DNA and ribosomes	Circular DNA; 70S ribosomes	Circular DNA; 70S ribosomes
Typical size	1–2 µm long	About 5 µm long

The folding of the inner membrane (cristae) and the stacking of thylakoids both pack a very large membrane area into a small organelle, so that many enzyme and pigment molecules can work at once. Cells that need a lot of energy, such as muscle, sperm and liver cells, have many mitochondria with densely packed cristae.

Worked example 1: evaluating the endosymbiotic theory

List the evidence that mitochondria and chloroplasts descend from bacteria, and say which piece is the strongest.

They have a double membrane, as expected if a bacterium (with its own membrane) was engulfed inside a vesicle of the host's membrane.
They contain circular DNA without histones, like bacterial DNA, separate from the nucleus.
Their ribosomes are 70S, like bacterial ribosomes, not 80S like those in the cytoplasm; antibiotics that block bacterial ribosomes also block mitochondrial protein synthesis.
They divide by binary fission; the cell cannot make new ones from scratch.
They are about the size of bacteria (1–5 µm).
Strongest: the DNA sequences. Mitochondrial genes are most similar to those of one group of bacteria (alpha-proteobacteria) and chloroplast genes to those of cyanobacteria. The other features are consistent with the theory; the sequence comparison points to the actual ancestors.

Transmission electron micrograph of a chloroplast in a leaf cell of Anemone, at ×12 000: a long, lens-shaped organelle filled with many parallel thylakoid membranes, darker where they are stacked into grana, with small black lipid droplets; the thick grey band along the top is the cell wall. The scale bar is 1 µm. Image: and3k and caper437, CC BY-SA 3.0, via Wikimedia Commons

Worked example 2: measuring a chloroplast

On a screen, the 1 µm scale bar of the chloroplast micrograph above measures 12 mm, and the chloroplast measures 70 mm long. Calculate (a) the magnification as displayed and (b) the length of the chloroplast.

(a) $12\,\mathrm{mm} = 12\,000\,\mu\mathrm{m}$ , so $M = \frac{12\,000}{1} = 12\,000$ , which agrees with the ×12 000 in the file's title.
(b) $A = \frac{70\,\mathrm{mm}}{12\,000} = 0.0058\,\mathrm{mm} = 5.8\,\mu\mathrm{m}$ .
That fits the typical length of a chloroplast (about 5 µm), several times longer than a typical mitochondrion.

The cytoskeleton, centrioles, cilia and flagella

Microfilaments (about 7 nm across, made of the protein actin) support the cell surface, change cell shape, and drive movements such as the crawling of white blood cells and the pinching in two of an animal cell after division.
Microtubules (about 25 nm, hollow tubes of the protein tubulin) act as tracks along which motor proteins carry vesicles and organelles, and form the spindle that separates chromosomes when a cell divides.
Intermediate filaments (about 10 nm) are tough ropes that resist pulling, for example in skin cells.
Centrioles are a pair of short cylinders, each of nine triplets of microtubules, at right angles to each other near the nucleus of animal cells. They help organise the spindle, and a centriole also forms the basal body at the base of each cilium or flagellum. Most flowering plants have none.
Cilia (many, short) and flagella (one or a few, long) are extensions of the cell surface containing nine pairs of microtubules around a central pair: the 9 + 2 arrangement. Motor proteins make the pairs slide, so the cilium bends. Cilia lining the trachea sweep mucus and trapped dust upward; a sperm swims with its flagellum.

Structures of the plant cell

Cell wall: layers of cellulose microfibrils set in a matrix, strong but fully permeable. Neighbouring walls are glued by the middle lamella (mainly calcium pectate). Some cells add a lignified secondary wall (Unit 5).
Plasmodesmata pass through the walls, so water, ions and small molecules can move from cytoplasm to cytoplasm.
Large permanent vacuole, surrounded by the tonoplast: holds cell sap (sugars, ions, pigments such as the red anthocyanins), and when full of water presses the cytoplasm against the wall, making the cell turgid and the plant firm.

Epidermis cells of the garden plant Tradescantia spathacea (Rhoeo) under a light microscope: each many-sided cell is filled almost entirely by its vacuole of pink-purple sap, coloured by anthocyanin pigments, inside thin clear cell walls. The field of view is about 450 µm across. Photo: Mnolf, CC BY-SA 3.0, via Wikimedia Commons

CrashCourse: "Plant Cells" (Crash Course Biology #6). Listen for the features that plant cells have and animal cells lack: the cell wall, the vacuole and chloroplasts.

Warning

Common mistakes

Saying mitochondria "make energy". Energy cannot be made: they transfer energy from glucose and other fuels to ATP.
Saying plant cells have chloroplasts instead of mitochondria. Plant cells have both: they respire all the time.
Writing that cilia contain "nine microtubules". There are nine pairs around a central pair.
Describing the cell wall as partially permeable. It is fully permeable; the cell surface membrane controls what enters.

Tip

Memory hook. "Double-membrane trio": the nucleus, mitochondria and chloroplasts are the only organelles with two membranes; the last two also carry their own DNA and 70S ribosomes, the clues to their bacterial past.

✦

Practice: energy organelles, cytoskeleton and plant cells

Explain how the structure of a mitochondrion is related to its function.
Give three similarities between mitochondria and chloroplasts.
State the role of microtubules in (a) cell division; (b) cilia.
A mitochondrion is 1.5 µm long. How long would it be in a micrograph at ×20 000? Give your answer in mm.
Cells lining the small intestine absorb digested food by active transport. Suggest why they contain many mitochondria.
Explain why a plant cell placed in water does not burst, while a red blood cell does.
Smoking paralyses the cilia lining the airways. Predict the effect on the lungs, and explain.

Show answersHide answers

The inner membrane is folded into cristae, giving a large surface area for the enzymes and carriers that make ATP; the matrix holds the enzymes of the Krebs cycle; a double membrane separates the reactions from the cytoplasm.
Any three: double membrane; own circular DNA; 70S ribosomes; divide by binary fission; involved in energy transfer (ATP); large internal membrane area.
(a) They form the spindle that pulls chromosomes apart; (b) nine pairs around a central pair slide against each other, bending the cilium so that it beats.
$I = A \times M = 1.5 \times 20\,000 = 30\,000\,\mu\mathrm{m} = 30\,\mathrm{mm}$ .
Active transport uses ATP, which comes mainly from aerobic respiration in mitochondria; many mitochondria supply enough ATP.
Water enters both by osmosis. The plant cell's strong cellulose wall resists the swelling, so the cell becomes turgid instead of bursting; the red blood cell has no wall, so its membrane stretches until it bursts.
Mucus and the dust and bacteria trapped in it are no longer swept up and out of the airways; it collects in the lungs, causing coughing ("smoker's cough") and more infections.

Quick check

Which feature of mitochondria is the strongest evidence that they descend from bacteria?

Key points

Mitochondria (cristae, matrix) make ATP by aerobic respiration; chloroplasts (envelope, stroma, thylakoids in grana) carry out photosynthesis.
Both have double membranes, circular DNA and 70S ribosomes and divide by fission: evidence for the endosymbiotic theory.
The cytoskeleton (microfilaments, microtubules, intermediate filaments) supports and moves; centrioles organise the spindle; cilia and flagella have a 9 + 2 structure.
Plant cells add a cellulose wall with plasmodesmata and a large vacuole bounded by the tonoplast.

Sources and credits

2.1.4: Eukaryotic Cells, 2.1.6: The Cytoskeleton and 2.1.7: Connections between Cells and Cellular Activities, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for mitochondria, chloroplasts, endosymbiosis, the cytoskeleton, cilia, the cell wall, vacuoles and plasmodesmata. Adapted by Ganzaa.
Symbiogenesis, Wikipedia contributors, licensed under CC BY-SA 4.0. Used for Margulis's endosymbiotic theory and its evidence. Adapted by Ganzaa.
Figure 1, the worked examples, practice and quick check: written by Ganzaa.
Images: and3k and caper437, CC BY-SA 3.0; Dartmouth Electron Microscope Facility, public domain; Mnolf, CC BY-SA 3.0; all via Wikimedia Commons.
Video: "Plant Cells: Crash Course Biology #6", CrashCourse, embedded from YouTube.

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

1Organelles that behave like bacteria

2Mitochondria and chloroplasts

3The cytoskeleton, centrioles, cilia and flagella

4Structures of the plant cell

Organelles that behave like bacteria

Mitochondria and chloroplasts

The cytoskeleton, centrioles, cilia and flagella

Structures of the plant cell