Biology Grade 12

Biology Past Papers 12th Class Lahore Board

Section A-(MCQs)

i) Synapse occur during:                                                                                       (Mark 1)

A. Pachytene
B. Leptotene
C. Zygotene
D. Diplotene
C. Zygotene

Section B-(Short Questions)

Q.2 i) Compare osmoconformers and osmoregulators.                  (Marks 2)




The animals which kept body fluid isotonic to the external environment are called osmoconformers.

The animals whose body fluid concentrations differ noticeably from the outside environment are called osmoregulation.

They do not require active transport to adjust their internal osmotic state.                                                              

They actively regulate body by discharging excess water in hypotonic and excrete salts in a hypertonic environment.


Section C-(Long Questions)

Q.5 a) Write a note on osmoregulation in marine animals.           (Marks 4)

Osmoregulation in marine animals:
    Most marine invertebrates are ''osmoconformers''.
1. Hagfishes:
    Among the vertebrates, hagfishes are isotonic with the surrounding sea's water.
2. Cartilaginous fishes:
    Most cartilaginous fishes maintain lower internal salt concentration than that of sea's water. Their kidneys for osmoregulation excrete salts through gills and also possess salt excreting organs such as rectal glands. These employ active transport mechanism to remove salt against osmotic gradient. Some fishes have relatively low salts in body fluids but have rendered these hypertonic to that of sea's water by retaining urea in adequate concentration because urea in high concentration is damaging, so these fishes retain another chemical trimethylamine oxide for the protection against urea.
3. Bony fishes:
   Bony fishes, the descendants of freshwater ancestors but later became marine constantly loss water from their hypotonic body fluids to hypertonic environments. These fishes have adapted to drink large amount of sea's water and excrete concentrated urine resulting in maximum salt excretion and minimum water loss.
    However, osmoregulation has enabled the animals and plants to distribute themselves in a wide range of habitats.

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