Concepts: Mitochondria, plastids and other organelles

5.1 Mitochondria

Mitochondria are not easily visible unless specifically stained, and their number varies with the activity of the cell. They are typically sausage-shaped or cylindrical, about 0.2 to 1.0 micrometre in diameter and 1.0 to 4.1 micrometres long. Each mitochondrion has two membranes. The outer membrane is the continuous boundary, and the inner membrane folds inwards to form cristae, which increase the surface area. The two membranes separate the organelle into two compartments, with the inner one filled by a dense matrix.

Each membrane carries its own enzymes, and the matrix holds one circular DNA molecule, a little RNA and 70S ribosomes and the parts needed to make proteins. The mitochondrion is the site of aerobic respiration, and it produces cellular energy as ATP, which is why it is called the powerhouse of the cell. Mitochondria divide by fission, so they can increase in number inside a cell.

5.1.1 Plastids

Plastids occur in all plant cells and in euglenoids. They are large and easy to see, and they carry pigments or stored food. There are three types. Chloroplasts contain chlorophyll and carotenoids and trap light for photosynthesis. Chromoplasts hold fat-soluble carotenoids such as carotene and xanthophyll, which give a plant part its yellow, orange or red colour. Leucoplasts are colourless and store nutrients: amyloplasts store starch, elaioplasts store oils and fats, and aleuroplasts store proteins.

A chloroplast is also double-membrane bound, and its inner membrane is less permeable than the outer one. The space inside the inner membrane is the stroma. Flat membranous sacs called thylakoids lie in the stroma, and stacks of thylakoids are called grana; stroma lamellae connect the thylakoids of different grana. The membrane of a thylakoid encloses a lumen, and chlorophyll sits in the thylakoid membranes. The stroma carries the enzymes for making carbohydrates and proteins, along with small circular DNA and ribosomes. Chloroplast ribosomes are 70S, smaller than the 80S ribosomes in the cytoplasm.

5.1.2 Ribosomes

Ribosomes are granular structures seen under the electron microscope as dense particles, and George Palade observed them in 1953. They are made of RNA and protein and are not enclosed by a membrane. Eukaryotic ribosomes are 80S and prokaryotic ribosomes are 70S. Each ribosome has a larger and a smaller subunit: the 80S type has 60S and 40S subunits, and the 70S type has 50S and 30S subunits. The S stands for the Svedberg unit, a measure of sedimentation that reflects density and size.

5.1.3 Cytoskeleton, cilia and centrosome

The cytoskeleton is a network of protein filaments in the cytoplasm, made of microtubules, microfilaments and intermediate filaments. It gives mechanical support, keeps cell shape, helps motility and supports organelles.

Hair-like outgrowths of the membrane include cilia, which are short and row the cell or its surrounding fluid, and flagella, which are longer and drive the cell forward. A cilium or flagellum is covered by plasma membrane, and its core, the axoneme, has microtubules along its axis. The usual arrangement is nine doublets of peripheral microtubules around a pair of central microtubules, the 9+2 array. The central tubules are joined by bridges and wrapped in a central sheath, and radial spokes connect the sheath to the peripheral doublets. Basal bodies, which resemble centrioles, anchor the cilium or flagellum.

The centrosome usually contains two cylindrical centrioles set at right angles to each other, surrounded by pericentriolar material. Each centriole is built like a cartwheel from nine evenly spaced peripheral fibrils of tubulin, each fibril a triplet, with a protein hub in the centre joined to the triplets by radial spokes. In animal cells centrioles give rise to basal bodies of cilia and flagella and to the spindle fibres of the division apparatus.

5.1.4 Seeing organelles

Most organelles are too small to see with a light microscope, and the chapter's knowledge of them came from electron microscopy and from specific stains. Mitochondria become visible when a stain such as Janus green is used, which is why the exemplar asks about the stain. Ribosomes were first seen as dense particles under the electron microscope. Chloroplasts are large enough to be seen in an ordinary leaf peel, which is why they were described early. The practical point is that a structure's visibility depends on the tool and the stain, also on its size, and that each technique opened a different part of the cell to study. The number of mitochondria in a cell tracks its energy demand, so a muscle cell carries far more of them than a cell that does little work. Cristae are the reason a mitochondrion can make enough ATP for a busy cell.