Q.The embryo sac of an Angiosperm is made up of

Step 1 — Where the acetyl group comes from

Pyruvate produced by glycolysis enters the mitochondrial matrix and undergoes oxidative decarboxylation by the pyruvate dehydrogenase complex:

Acetyl CoA is the fuel that feeds the TCA cycle.

Step 2 — The first reaction of the TCA cycle

The cycle begins when this 2‑carbon acetyl group condenses with a 4‑carbon acid, oxaloacetic acid (OAA), together with a molecule of water, to yield the 6‑carbon citric acid:

The carbon arithmetic $2+4=6$ is the quickest way to remember it.

Step 3 — Why the other options are the trap

All three distractors are intermediates of the very same cycle — which is exactly why they are tempting — but none is the acceptor: …

Step 1 — Where is the filiform apparatus?

A typical mature angiosperm embryo sac is 7-celled, 8-nucleate:

• Micropylar end — the egg apparatus: one egg cell flanked by two synergids.
• Centre — the large central cell with two polar nuclei.
• Chalazal end — three antipodal cells.

The synergids carry special thickenings of the cell wall at their micropylar tip; this structure is the filiform apparatus.

Step 2 — What does it do?

After the pollen tube grows down the style and enters the ovule through the micropyle, it must find the embryo sac. The filiform apparatus:

1. Greatly increases the surface area of the synergid at the micropylar tip,
2. Is the site from which chemotropic attractants are released, and
3. Thereby guides the pollen tube into one of the synergids, where it bursts and discharges its two male gametes. …

Step 1 — Where in meiosis this structure forms.

Prophase I of meiosis is subdivided into five stages:

1. Leptotene — chromosomes become gradually visible as long threads.
2. Zygotene — homologous chromosomes start pairing together, a process called synapsis, accompanied by the formation of the synaptonemal complex between them.
3. Pachytene — crossing over occurs between non-sister chromatids of the homologues, catalysed by recombinase; recombination nodules appear.
4. Diplotene — the synaptonemal complex dissolves and the homologues separate except at the chiasmata, the X-shaped sites of crossing over.
5. Diakinesis — chromosomes fully condense; terminalisation of chiasmata.

Step 2 — Name the structure formed by synapsis.

The complex formed by a pair of synapsed homologous chromosomes is called a bivalent — that is the definition asked for. It is also called a tetrad, because by this stage each homologue has already replicated into two sister chromatids:

The bivalents become clearly visible under the microscope at pachytene.

Step 3 — Eliminate the other options. …

Step 1 — Megasporogenesis and megagametogenesis. The functional megaspore (haploid) divides mitotically three times without cytokinesis:

This free-nuclear stage gives the characteristic 8 nuclei, four at each pole.

Step 2 — Cell organisation. Cell walls then lay down around the nuclei:

• Egg apparatus at the micropylar end — 1 egg cell + 2 synergids (with the filiform apparatus) = 3 cells
• Antipodals at the chalazal end = 3 cells …

The key is understanding what the Primary Endosperm Nucleus (PEN) is and how its ploidy relates to the gamete's chromosome number. In angiosperms, the PEN forms from the fusion of two polar nuclei (each 1n) with one sperm nucleus (1n) — a process called triple fusion. That makes the PEN triploid (3n). The gamete is haploid (n), so if the gamete has 17 chromosomes, n = 17, and the PEN has 3 × 17 = 51 chromosomes.

A common mistake is to confuse the PEN with the zygote (which is 2n) or to think the endosperm is always diploid. In flowering plants, the endosperm is triploid because of double fertilisation.

1. Identify the ploidy of the gamete. The problem says the gamete has 17 chromosomes. A gamete is haploid, so the haploid number is n = 17.

2. Determine the ploidy of the PEN. The Primary Endosperm Nucleus forms when two polar nuclei (each n) fuse with one sperm nucleus (n). That gives: n + n + n = 3n. So the PEN is triploid. …

Step 1 — How a pollen grain develops.

A microspore (the young pollen grain) is haploid. It undergoes an unequal mitotic division to give:

• a large vegetative (tube) cell — irregular nucleus, abundant food reserve; it later forms the pollen tube; and
• a small generative cell — dense cytoplasm, spindle-shaped, floating within the cytoplasm of the vegetative cell.

At this point the grain is at the 2-celled stage.

Step 2 — The two possible fates.

• In about 60% of angiosperms, the grain is shed at the 2-celled stage; the generative cell divides only afterwards, inside the growing pollen tube.
• In the remaining 40%, the generative cell divides mitotically before shedding, producing the two male gametes. The grain is then shed at the 3-celled stage.

Step 3 — Name the three cells. …