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NCERT Exemplar · Q7

Q.Acrosomal reaction of the sperm occurs due to:

(a) Its contact with zona pellucida of the ova
(b) Reactions within the uterine environment of the female
(c) Reactions within the epididymal environment of the male
(d) Androgens produced in the uterus
Meghalaya MboseMCQ· 1mImportance★★★★★
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The acrosomal reaction is triggered when the sperm head makes physical contact with the zona pellucida layer surrounding the ovum.

The acrosomal reaction is one of the most critical steps in fertilisation — the moment a sperm finally meets the egg and must break through its protective layers. To understand why it happens, you first need to picture the journey of the sperm.

After ejaculation, sperm travel through the female reproductive tract, undergoing a process called capacitation inside the uterus and fallopian tubes. Capacitation makes the sperm membrane more fluid and prepares it to fertilise the egg. But the actual trigger for the acrosomal reaction does not occur until the sperm reaches the ovum itself.

The ovum is surrounded by two main layers: an outer ring of cells called the corona radiata, and a thick, translucent glycoprotein coat called the zona pellucida. When the capacitated sperm makes its way through the corona radiata and its head touches the zona pellucida, that physical contact sets off the acrosomal reaction.

Note

The zona pellucida contains specific glycoproteins (like ZP3 in mammals) that act as receptors. When the sperm binds to these, it triggers an influx of calcium ions into the sperm head, which causes the acrosome to burst open.

The acrosome is a cap-like structure at the tip of the sperm head, filled with powerful hydrolytic enzymes (such as hyaluronidase and acrosin). Once the reaction is triggered, these enzymes are released, digesting the zona pellucida and allowing the sperm to tunnel through to the egg's plasma membrane. Without this contact-triggered release, the sperm simply cannot penetrate the ovum.

Now look at the other options to see why they are incorrect: …

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