Biology 1st Paper (Botany) · Classes 11-12 · Chapter 1 · 1.3

Plastids.

A carrot grows underground, yet it is orange. Leaves are green, petals are bright, and roots and stems buried in the soil are colourless. Who decides all these colours? One organelle in three forms: the plastid. Keep the three names and three jobs apart and the whole section is yours.

1. What a plastid is, and who has one

A plastid is the largest organelle in the cytoplasm, rich in stroma and grana and bound by a lipo-protein membrane. In 1883 W. Schimper was the first to notice the green plastid in plant cells, and he named it the chloroplast.

Not every living thing has plastids. The cells of fungi, bacteria and blue-green algae lack them. In blue-green algae the plasma membrane folds inwards to form thylakoids.

Plastids are mainly of three kinds: the leucoplast, the chromoplast and the chloroplast. The chloroplast is the most important of the three.

2. The leucoplast: colourless

Leucoplasts are colourless (leucos = colourless). They sit in organs where sunlight never arrives, such as roots and underground stems. Bring one into the light and it can change into a chromoplast, most often into a chloroplast.

By what they store, leucoplasts come in three kinds: the amyloplast stores starch, the elaioplast stores fat, and the aleuroplast (proteinoplast) stores protein. The main work of a leucoplast is to store food and to make starch from sugar.

3. The chromoplast: coloured

Chromoplasts are coloured (chrome = colour). The colour comes from pigments: carotene gives orange-red and xanthophyll gives yellow.

Chromoplasts lie in petals, coloured fruits and seeds, and in the carrot root. They probably arise from chloroplasts. Their job is to attract insects with colour, which helps pollination, and they also play a part in spreading fruits and seeds.

4. The chloroplast: the green kitchen

The chloroplast is the green plastid. It holds chlorophyll-a, chlorophyll-b, carotene and xanthophyll. It is called the “kitchen of the cell” or the “factory of sugar synthesis”, because it is the organelle of energy conversion.

A higher plant cell usually has 10-40 chloroplasts. Each is lens-shaped and usually 3-5 micron across.

In algae the shape varies a lot. Match each scientific name to its shape: Chlamydomonas is cup-shaped, Spirogyra spiral, Oedogonium net-like, Zygnema star-shaped, Ulothrix ribbon or ring, and Pithophora round.

Where does a new chloroplast come from? In lower plants an old chloroplast divides to give a new one. In higher plants it grows from a proplastid, a round body 0.5 micron across.

5. Inside the chloroplast

Start from the outside. The chloroplast is wrapped in a two-layer semipermeable membrane. It holds glycosyl glyceride instead of phospholipid, and that is the exceptional feature.

The watery, colloidal matrix inside the envelope is the stroma. It holds 70S ribosomes, DNA, RNA and the enzymes that make sugar, and the C3 or C4 cycle occurs right here.

The sac-like structures of the stroma are thylakoids, 100-300 Å wide. Their chamber holds chlorophyll-a and b, xanthophyll, carotene, lipid and enzymes. This whole set together is called a quantosome.

A stack of 10 to 100 thylakoids is a granum (plural grana). A granum measures 0.3-1.7 μm, and a chloroplast usually has 40-60 grana. Fine tubes join the thylakoids of two neighbouring grana: these are the stroma lamellae (singular lamella), and they too hold some chlorophyll.

The thylakoid membranes carry photosynthetic units and ATP-synthase. Each unit packs about 300-400 molecules of chlorophyll-a, b, carotene and xanthophyll.

A chloroplast has its own circular DNA (about 200 molecules may occur) and its own ribosomes. With these it can reproduce itself and make some of its own proteins.

Chemically, 80% of chloroplast protein is insoluble and builds membranes with lipid, while 20% is soluble enzyme. Of the chlorophyll, 75% is chlorophyll-a and 25% is chlorophyll-b.

The chloroplast has several jobs. It makes sugar in photosynthesis. It converts solar energy into biological energy. In photophosphorylation it turns ADP into ATP. It fixes CO₂ from the air into the quantosomes, so it takes CO₂ out of the air. And it helps in photorespiration and cytoplasmic inheritance.

Why this design? The envelope protects and controls what passes in and out. The stroma, thylakoids and photosynthetic units inside work together to make sugar. So the outer structure protects, and the inner structure produces.

6. The three plastids side by side

The three plastids are one thing in three forms. Drag one plastid into the light, then on into a coloured fruit.

The three differ in colour and pigment: the leucoplast is colourless and pigment-free, the chromoplast is coloured (carotene, xanthophyll), the chloroplast is green (chlorophyll). Light also changes them: in light a leucoplast becomes a chromoplast or chloroplast, in light a chloroplast becomes a chromoplast, and without light a chloroplast becomes a leucoplast. Tap any row of the grid, after first trying to recall it yourself.

Now a test without the grid: can you name the plastid from where it lies or what it does?

7. Check yourself

Three questions in the exam's style. Start with the first.

In the second, think through the reason.

In the last, watch the numbers and the names.

The whole section on seven cards.

Source: Biology 1st Paper (Botany), Classes 11-12 (NCTB), Chapter 1 “Cell and Its Structure”, section 1.3.