Cell: The Unit of Life: Class 11 Biology · Chapter 8

This chapter opens Unit 3, Cell: Structure and Functions. The chapters before it catalogued how varied living things are in form and body plan. Underneath that variety lies one shared plan: every organism is built of cells. Chapter 8 takes the cell apart and names its parts, Chapter 9 asks which molecules those parts are made of, and Chapter 10 follows a cell as it grows and divides.

There is a second way into biology. Grind up a tissue, list the elements and compounds in it, and then ask what each compound does: which enzyme digests starch, which protein recognises a germ, which molecules are involved in a memory. Many such reactions can even be run in a test tube, in a cell-free system. Explaining digestion, defence or disease in terms of molecules, with the methods of physics and chemistry, is called reductionist biology. It carries a built-in puzzle: single reactions work in a test tube, yet the full set of life's activities shows up only while the cell's organisation is intact. That is why we begin with the whole cell.

Put a neem leaf and a pebble side by side. Both are made of atoms, yet only the leaf grows, repairs itself and comes from a living parent. The difference lies not in some special substance but in organisation: the leaf is built of cells (कोशिका). Some organisms, such as the green alga Chlamydomonas in a village pond, are a single cell and are called unicellular. Others, like a neem tree or you, are multicellular: vast numbers of cells sharing the work of staying alive.

8.1 What is a Cell?

Consider what one Chlamydomonas cell manages. It swims, makes its own food, takes in salts, gets rid of wastes and divides to make new individuals, all without help from any other cell. Now try the reverse and take a cell apart. Isolated mitochondria in a test tube keep releasing energy for a while, but neither they nor any other fragment can feed, grow and reproduce on its own. The complete cell is the smallest package that can live independently. That is why we call it the basic structural and functional unit of every living organism.

Most cells are far too small for the naked eye, so their discovery followed the history of lenses (Fig. 8.1). In the 1670s the Dutch lens-maker Antonie van Leeuwenhoek became the first person to see and describe living cells, in drops of pond water.11Robert Hooke had used the word "cell" in 1665 for the tiny empty boxes he saw in a thin slice of cork; those were only the walls of dead cells. In 1831 the Scottish botanist Robert Brown described the nucleus. From then on each better microscope showed more, and the electron microscope finally exposed the fine structures you will meet in this chapter.

8.2 Cell Theory

The theory grew out of two parallel surveys. The German botanist Matthias Schleiden looked at plant after plant and in 1838 concluded that each one is a community of cells of different kinds, organised into tissues. A year later his countryman, the zoologist Theodor Schwann, carried the idea over to animals. Animal cells, he saw, have no wall; what bounds them is a thin outer layer, today called the plasma membrane, and comparing them with plant tissue convinced him that a wall is something only plant cells have. Joining the two kingdoms, Schwann proposed that every plant and animal body is built from cells and from substances that cells produce (Fig. 8.2).

The cell theory of Schleiden and Schwann left one question open: where do new cells come from? Many biologists of the day imagined cells forming freshly out of a structureless fluid. In 1855 the German physician Rudolf Virchow argued that cells divide, so that every cell comes from a cell that existed before it, an idea he summed up as Omnis cellula-e cellula. Virchow's addition completed the theory in the form we use today.

8.3 An Overview of Cell

In Class 9 you probably looked at onion peel and cheek cells under a microscope. Picture the two side by side. At the edge they differ: a box-like onion cell is fenced by a stiff cell wall, with its cell membrane pressed against the inside of the wall, while a flat cheek cell has nothing outside its membrane. In the middle they agree: each has a dense, membrane-wrapped nucleus, home of the chromosomes and therefore of the cell's DNA.

Whether or not the DNA sits inside a membrane-bound nucleus divides all cells into two camps. Cells that have one are eukaryotic (Greek eu, true, and karyon, kernel); cells without one are prokaryotic (pro, before). In both camps a semi-fluid matrix, the cytoplasm, fills the cell. The cytoplasm is where most of the cell's activity happens: thousands of chemical reactions that never stop while the cell is alive, and that keep it in the living state.

Organelles come in two kinds. Most are wrapped in membranes, and these occur only in eukaryotic cells: besides the nucleus, the list includes mitochondria, vacuoles, lysosomes, microbodies, the Golgi complex and the endoplasmic reticulum (ER). A prokaryotic cell has none of them. A few organelles have no membrane at all. The centrosome, which organises cell division in animal cells, is one. Ribosomes are the other, and they are the one organelle every cell has, prokaryote or eukaryote; in a eukaryotic cell you meet them in four places: free in the cytoplasm, on the rough ER, and inside both mitochondria and chloroplasts.

Cells differ enormously in size (Fig. 8.1). The smallest cells known, the Mycoplasma, are only about 0.3 µm long22Some printings of the NCERT text show this as 0.3 mm; the correct value is 0.3 µm, three-tenths of a micrometre., and bacteria measure a few micrometres (NCERT quotes 3–5 µm). In our own body a red blood cell is about 7.0 µm across, while nerve cells rank among the longest cells known. If we count only cells that exist on their own, the record-holder is the egg of an ostrich.

Shapes vary just as widely: disc-like, polygonal, columnar, cuboidal, thread-like or irregular. A cell's shape is seldom an accident. It usually suits the job the cell does (Fig. 8.3).

8.4 Prokaryotic Cells

Bacteria, blue-green algae (cyanobacteria), the wall-less Mycoplasma and the related PPLO (pleuro-pneumonia-like organisms) make up the prokaryotes. Compared with eukaryotic cells they are usually small and they reproduce fast: in good conditions some bacteria divide every 20 minutes. Their forms are varied, yet most bacteria fit one of four basic shapes (Fig. 8.4): the rod-like bacillus, the spherical coccus, the comma-shaped vibrio and the spiral spirillum.

Under the variety, every prokaryotic cell is organised in the same basic way (Fig. 8.5):

• No true nucleus exists. The genomic DNA, a single circular chromosome, lies naked in the cytoplasm without any nuclear membrane.
• Many bacteria also carry small circles of DNA outside the genomic DNA, called plasmids. Genes on a plasmid give the bacterium extra traits; resistance to antibiotics is a common example.
• A cell wall surrounds the cell membrane in all prokaryotes except Mycoplasma, and cytoplasm, a semi-fluid matrix, fills the cell.
• Apart from ribosomes, none of the organelles of eukaryotic cells is present. Instead prokaryotes have inclusions, stores of reserve material, and a mesosome, an infolding of the cell membrane that is typical of them.

In Class 12 you will see plasmids put to work: biotechnologists use them to carry foreign DNA into bacteria, and to track which bacteria have taken that DNA up (transformation).

8.4.1 Cell Envelope and its Modifications

A bacterium is wrapped in three layers bound so tightly that they behave as one protective coat, the chemically complex cell envelope. Peel it from the outside and you meet the glycocalyx, then the cell wall, then the plasma membrane. The layers differ in chemistry and in their individual jobs; together they shield the cell.

Differences in this envelope show up in a staining test devised by the Danish scientist Hans Christian Gram. Bacteria that keep Gram's stain are called Gram-positive; those that lose it are Gram-negative. The test is still one of the first steps in identifying bacteria in a hospital laboratory.

The glycocalyx differs from one bacterium to another in what it is made of and how thick it is.

Why does a bacterium in pond water not swell and burst, or shrivel and collapse? Its cell wall is strong enough to resist, and it also gives the cell its rod, sphere or spiral shape. The plasma membrane inside the wall is built much like the membranes of eukaryotic cells; it is the selectively permeable layer through which the cell deals with its surroundings.

A mesosome has a surprising number of jobs: building (it helps form the cell wall), copying (it helps replicate the DNA and pass a copy to each daughter cell), energy and export (respiration and secretion) and space (extra membrane area, and so more room for enzymes).33Many cell biologists now think that much of what was seen as mesosomes in electron micrographs was created while the samples were being prepared. For your exams, learn mesosomes as described here. Cyanobacteria have a second kind of inward membrane extension, the chromatophore, which carries their pigments.

Some bacteria swim and some do not. Swimmers use flagella, thin filaments growing out from the cell wall, and species differ in how many they have and where: one at an end, a tuft, or many all over. Each flagellum has three parts: a basal body set in the envelope like a motor mount, a short curved hook that acts as a joint, and the filament, the longest part, trailing outside. Two other surface structures have nothing to do with movement. Pili are long tubes of a special protein. Fimbriae are much shorter bristles sprouting from the surface; in some bacteria they anchor the cell to stones in a stream or to the tissues of a host.

8.4.2 Ribosomes and Inclusion Bodies

Proteins are made on ribosomes, and a prokaryote's ribosomes are associated with its plasma membrane. Each is a tiny particle, roughly 15 nm by 20 nm, assembled from a 50S and a 30S subunit into a 70S ribosome (the unit S is explained in Section 8.5.6). When protein is needed fast, one messenger RNA (mRNA) is read by many ribosomes at once, strung along it like beads. This string is a polysome (polyribosome), and every ribosome on it translates the mRNA into a protein.

What about stores? A prokaryote keeps its reserves as inclusion bodies: granules of phosphate, cyanophycean granules (in cyanobacteria) or granules of glycogen, lying free in the cytoplasm with no membrane around them. Some photosynthetic prokaryotes, the blue-green algae and the purple and green bacteria, also carry gas vacuoles that keep them afloat.

8.5 Eukaryotic Cells

What unites a yeast cell, an Amoeba, a leaf cell and your own cells? All are eukaryotic, and their key design feature is compartmentalisation: membrane-bound organelles divide the cytoplasm into separate rooms, each with its own chemistry, so that very different reactions can run side by side. Protists, fungi, plants and animals all build their cells this way. Their DNA is packed into chromosomes inside an organised nucleus with a nuclear envelope, and they have a range of complex structures for movement and for holding their shape (the cytoskeleton).

Within this common plan, plant and animal cells differ in a few telling ways. Only plant cells have a cell wall, plastids and one large central vacuole. Centrioles, in turn, are found in animal cells but are missing from almost all plant cells. Table 8.1 sets the three main cell types side by side; the explorer below lets you take each one apart.

8.5.1 Cell Membrane

Chemists worked out what the membrane is made of before anyone could see it. Their favourite material was the human red blood cell: easy to collect, and with no internal membranes, it yields pure plasma membrane. The analyses showed mostly lipids and proteins. Only when electron microscopes came into use in the 1950s could the membrane's structure be studied in detail.

Picture a phospholipid, the main membrane lipid, as a head with two tails. The head is polar and likes water; the hydrocarbon tails are non-polar and avoid it. In a membrane these molecules form two sheets, tails meeting in the middle and heads facing the watery fluid on each side, so the tails stay away from water: a lipid bilayer. Molecules of cholesterol are tucked in among the phospholipids.

Membranes also contain plenty of protein and some carbohydrate, though the balance differs between cell types; a human red blood cell's membrane has roughly 52 % protein against 40 % lipid. Biochemists sort membrane proteins by how hard they are to pull out. Integral proteins are partly or completely buried in the bilayer and are difficult to remove; peripheral proteins sit on its surface and come away easily.

The picture that tied these findings together came from S. J. Singer and G. L. Nicolson in 1972: the fluid mosaic model (Fig. 8.7). Its key idea is that the lipid bilayer is quasi-fluid. Proteins are not fixed in place but drift sideways through it, like icebergs in a sea of lipid, and the ease of this movement is the membrane's fluidity. Fluidity matters: growth, cell division, secretion, endocytosis and the making of junctions between cells all need a membrane that can flow and reseal.

Traffic control is a membrane's most important job, and it is selective: some molecules get through, others do not (the membrane is selectively permeable). Most traffic runs downhill. A substance moves from where it is more concentrated to where it is less, down its concentration gradient, and the cell spends no energy on it; this is passive transport. Small uncharged (neutral) solutes such as oxygen simply dissolve through the lipid bilayer: simple diffusion. Water obeys the same downhill rule, and its diffusion across a membrane has its own name, osmosis.

Glucose is different. Being polar, it cannot dissolve in the oily, non-polar core of the bilayer, so it crosses only with the help of a carrier protein in the membrane; as long as it moves downhill, the transport is still passive. Uphill movement, from lower concentration to higher, is another matter. It is called active transport, and the cell pays for it with energy from ATP. The textbook example is the $\mathrm{N}\mathrm{a}{\phantom{X}}^{+}$/$\mathrm{K}{\phantom{X}}^{+}$ pump, which keeps sodium low and potassium high inside your cells.44Each cycle of the pump uses one ATP to push three sodium ions out and bring two potassium ions in.

8.5.2 Cell Wall

Plants and fungi add one more layer outside the plasma membrane: a rigid, non-living cell wall. It fixes the cell's shape and guards it against knocks and infection. It also keeps out unwanted large molecules, while still letting neighbouring cells interact with each other.

What a wall is made of depends on the organism. Algal walls contain cellulose, galactans and mannans, often with minerals such as calcium carbonate. In other plants the wall is made of cellulose, hemicellulose, pectins and proteins. (Fungal walls, as you saw in Chapter 2, are made largely of chitin.)

Build the wall between two neighbouring plant cells from the middle outwards (Fig. 8.9). The first layer is the middle lamella, made mainly of calcium pectate, the glue between the cells. On each side of it a young cell lays down a thin primary wall that can stretch and grow with the cell. That growth slows as the cell matures, and a secondary wall is then added on the inner side, next to the membrane. Through all these layers run fine channels, the plasmodesmata, linking the cytoplasm of one cell with the next.

8.5.3 Endomembrane System

Think of a factory where one workshop makes parts, the next finishes them and a third packs and ships them. Inside a eukaryotic cell the ER, the Golgi complex, lysosomes and vacuoles form such a chain: each looks and works differently, but their jobs are coordinated and material flows between them. Together they are the endomembrane system. Mitochondria, chloroplasts and peroxisomes also have membranes, but their work is not coordinated with this chain, so they are not counted in it. Fig. 8.10 follows one protein along the chain.

8.5.3.1 The Endoplasmic Reticulum (ER)

The name says it: endoplasmic, within the cytoplasm, and reticulum, a little net. Under the electron microscope the endoplasmic reticulum appears as a network of fine tubes and flat sacs threading through the cell. Its membranes divide the cell's interior into two spaces: the luminal compartment inside the ER, and everything outside it, the cytoplasm, which is the extra-luminal compartment.

The ER comes in two forms. Where ribosomes stud its outer surface it looks grainy and is called rough ER (RER); without ribosomes it looks smooth and is smooth ER (SER). RER is extensive in cells busy making and secreting proteins, such as the pancreatic cells that release digestive enzymes, and it joins up with the outer membrane of the nuclear envelope. SER is the cell's main site of lipid synthesis. In animal cells the lipid-like steroid hormones are made in SER, so it is plentiful in hormone-making cells of the adrenal glands and the gonads.

8.5.3.2 Golgi apparatus

This organelle carries the name of the Italian scientist Camillo Golgi, who in 1898, staining nerve cells with silver salts, spotted dark net-like bodies beside the nucleus. Look closely and you see a pile of flat, disc-shaped sacs, the cisternae (each 0.5–1.0 µm across), lying parallel like a stack of plates; how many there are varies. The stack curves around the nucleus, so it has two faces that look and work differently yet are connected: the bulging (convex) cis or forming face and the hollow (concave) trans or maturing face.

Its main job is packing and dispatch. Material arrives from the ER in small vesicles that fuse with the cis face, which is why the Golgi always sits close to the ER. The material works its way through the stack towards the trans face. On the way many proteins made on the ER's ribosomes are modified, and this is where most glycoproteins and glycolipids are formed. At the trans face the finished products leave in new vesicles, addressed either to places inside the cell or to the outside (secretion).

8.5.3.3 Lysosomes

A lysosome is the cell's digestive sac: a small membrane-bound vesicle that the Golgi packs with enzymes. Lysosomes isolated from cells contain hydrolases of nearly every kind (lipases, proteases, carbohydrases and others), so between them they can break down all four major classes of large molecules: carbohydrates, proteins, lipids and nucleic acids. The enzymes work best in acid, and the lysosome keeps its inside at an acidic pH.

8.5.3.4 Vacuoles

Not every space in a cell is busy with chemistry. A vacuole is a membrane-bound space used mainly for storage: water, sap, excretory products and other substances the cell has no use for. Its single bounding membrane is the tonoplast. A plant cell's vacuole can be enormous, up to 90 per cent of the cell's volume, squeezing the cytoplasm into a thin layer against the wall.

The tonoplast does real work. In plant cells it moves many ions and other materials into the vacuole against their concentration gradients, so they become far more concentrated in the sap than in the cytoplasm. Other vacuoles have other jobs. The contractile vacuole of Amoeba collects surplus water and pumps it out, which matters for osmoregulation and excretion. Many cells, especially among protists, form food vacuoles by engulfing food particles.

8.5.4 Mitochondria

Without a special stain, mitochondria (singular mitochondrion) barely show up under a light microscope. How many a cell holds depends on how hard it works, from a few to thousands in a busy heart-muscle cell, and their shape and size vary too. A typical one is a sausage or cylinder 0.2–1.0 µm wide (about 0.5 µm on average) and 1.0–4.1 µm long.

Two membranes wrap each mitochondrion (Fig. 8.11). The smooth outer membrane is its continuous boundary. The inner membrane is folded into cristae (singular crista) that reach into the interior and multiply its surface area. The membranes create two watery compartments: a narrow outer compartment between them (the intermembrane space), and the inner compartment, filled with a dense, uniform fluid, the matrix. Each membrane carries its own set of enzymes for the organelle's work.

The mitochondrion's job is aerobic respiration: it releases the energy in food and traps it as ATP, the cell's energy currency, which is why mitochondria are nicknamed the powerhouses of the cell. Unusually, a mitochondrion can make some of its own proteins: its matrix contains one circular DNA molecule, a few RNA molecules, 70S ribosomes and the rest of the protein-making kit. New mitochondria arise when existing ones split in two (fission).

8.5.5 Plastids

The green of a leaf, the orange of a carrot and the white of a potato all trace back to plastids, organelles found in every plant cell and in euglenoids. Being large, they are easy to see under a microscope. They are classified by their pigments into chloroplasts, chromoplasts and leucoplasts (Table 8.2). The colourless leucoplasts, of many shapes and sizes, are food stores: amyloplasts hold starch (as in the potato), elaioplasts hold oils and fats, and aleuroplasts hold proteins.

In a green plant, the mesophyll cells of the leaves hold most of the chloroplasts, 20–40 in each cell, whereas the alga Chlamydomonas makes do with a single one. A chloroplast is typically 5–10 µm long and 2–4 µm wide, and its shape may be lens-like, oval, spherical, disc-like or even a ribbon.

Inside, a chloroplast is a set of membranes within membranes (Fig. 8.12). Two envelope membranes surround it, as in a mitochondrion, the inner one being the less permeable, and they enclose a fluid-filled space, the stroma. In the stroma lie flattened membrane sacs, the thylakoids. These are piled up like rotis in a box to form grana (singular granum), and flat membrane tubules called stroma lamellae link the thylakoids of different grana. Each thylakoid's membrane encloses its own small space, the lumen.

The chlorophyll pigments sit in the thylakoids, where light energy is trapped; the grana are the site of the light reactions. The stroma holds the enzymes that make carbohydrates and proteins (the dark reactions take place here), along with small double-stranded circular DNA molecules and ribosomes. Chloroplast ribosomes are 70S, smaller than the 80S ribosomes of the cytoplasm.

8.5.6 Ribosomes

Ribosomes have no membrane: each is a granule of ribonucleic acid (RNA) and protein. George Palade first described them in 1953, as dense particles in electron micrographs.

Every ribosome has two subunits, a larger and a smaller one (Fig. 8.13). Eukaryotic ribosomes are 80S, with subunits of 60S and 40S; prokaryotic ribosomes are 70S, with subunits of 50S and 30S. The S stands for the Svedberg unit, the unit of the sedimentation coefficient: a measure of how fast a particle settles when it is spun at very high speed. Since it depends on a particle's density as well as its size, it is only an indirect measure of size.55One Svedberg unit is $1\times {10}^{-13}\mathrm{s}$. It is named after Theodor Svedberg, who built the first ultracentrifuges.

8.5.7 Cytoskeleton

A cell is not a loose bag of parts. A web of protein filaments, the cytoskeleton, runs through its cytoplasm, built from three kinds of filament: microtubules, microfilaments and intermediate filaments.66Microfilaments are about 7 nm thick, intermediate filaments about 10 nm and microtubules about 25 nm. These values are for interest only. Like the poles, ropes and pegs of a tent, they give the cell mechanical support and keep its shape, and they also make movement possible. Microtubules form the core of cilia, flagella and centrioles, which come next.

8.5.8 Cilia and Flagella

Many eukaryotic cells move, or move the fluid around them, with hair-like outgrowths covered by the cell membrane. Short, numerous ones that beat like oars are cilia (singular cilium): they row a Paramecium through water, and in your windpipe they sweep mucus upwards. Longer ones, usually one or two per cell, are flagella (singular flagellum); they drive a cell such as a human sperm. Bacterial flagella share the name but not the build (Section 8.4.1).

Cut one across and look with an electron microscope. Inside the membrane lies the core, the axoneme: a bundle of microtubules running along the length. Read the cross-section from the outside in (Fig. 8.14):

• Nine outer doublets. Nine pairs of fused microtubules form a ring just under the membrane; each doublet is tied to the next by a linker (an interdoublet bridge).
• A central pair. Two single microtubules run down the middle, joined by a bridge and enclosed by a central sheath.
• Nine radial spokes. One spoke runs from the central sheath to one tubule of every doublet, so there are nine in all.

Nine doublets around two singles: this is the 9 + 2 array. At its base, every cilium and flagellum grows from a basal body, a structure built like a centriole.

8.5.9 Centrosome and Centrioles

The centrosome is an organelle that usually holds a pair of short cylinders, the centrioles, set at right angles to each other in a shapeless cloud of pericentriolar material (Fig. 8.15). Slice a centriole across and you see a cartwheel. Its rim is nine evenly spaced peripheral fibrils of the protein tubulin, each a triplet of microtubules linked to the next. At the proximal (near) end, a protein hub sits at the centre, joined to the triplets by protein radial spokes. There is no central pair, which is one way to tell a centriole from an axoneme.

Centrioles have two jobs. They form the basal bodies from which cilia and flagella grow, and in animal cells they give rise to the spindle fibres of the spindle apparatus that separates chromosomes during cell division (Chapter 10).

8.5.10 Nucleus

The nucleus is the longest-known organelle: Robert Brown described it in 1831. Its stainable contents got their name later, when Walther Flemming called the material that takes up basic dyes chromatin (Greek chroma, colour).

Between divisions a cell is in interphase. An interphase nucleus holds three things: chromatin, spread out as a loose, elaborate network of very long nucleoprotein fibres; a nuclear matrix; and one or more round nucleoli (singular nucleolus) (Fig. 8.16).

Its boundary is the nuclear envelope, and the electron microscope shows that it is double: two parallel membranes with a gap of 10–50 nm, the perinuclear space, between them. The envelope separates the contents of the nucleus from the cytoplasm, but it is not sealed. In many places its two membranes fuse, leaving minute nuclear pores, through which RNA and protein molecules pass, into and out of the nucleus. On the cytoplasm side, the outer membrane usually runs on into the endoplasmic reticulum, and like rough ER it carries ribosomes.

The nucleolus has no membrane; its material merges into the surrounding nucleoplasm (the nuclear matrix, which also holds the chromatin). It is where ribosomal RNA (rRNA) is made at a high rate. A cell that makes a lot of protein needs many ribosomes, so it has larger and more numerous nucleoli.

Chromatin and chromosomes

Chromatin is a mixture of DNA, RNA and two kinds of protein: basic proteins called histones, and other, non-histone proteins. When a cell divides, the loose chromatin coils up into distinct chromosomes, and for a while these replace the familiar nucleus. A single human cell carries roughly two metres of DNA, shared among its 46 chromosomes (23 pairs); how such a long thread is packed is a Class 12 topic.

Look at any chromosome in a dividing cell (the only time chromosomes are visible) and you will find a narrow waist, the primary constriction or centromere. It holds the chromosome's two chromatids together, and disc-shaped kinetochores sit on its sides (Fig. 8.17). Where along the chromosome the centromere lies gives four types:

• Metacentric: centromere in the middle, so the two arms are equal.
• Sub-metacentric: centromere a little away from the middle, giving one shorter and one longer arm.
• Acrocentric: centromere close to one end, giving one very short and one very long arm.
• Telocentric: centromere right at the end of the chromosome.

Some chromosomes show a second pinch as well. This secondary constriction does not take up stain and always appears at the same spot, so the piece beyond it looks like a separate small knob: the satellite.

8.5.11 Microbodies

Both plant and animal cells also contain many microbodies: minute vesicles, each with a single membrane and filled with enzymes. Peroxisomes, which break down hydrogen peroxide, and the glyoxysomes of germinating oily seeds, which help turn stored fat into sugar, are two examples.