A raw guava is hard, but a ripe one is soft enough to squash with a thumb…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A raw guava is hard, but a ripe one is soft enough to squash with a thumb. As fruit ripens, enzymes break down the glue between its cells. (a) Name the layer between the walls of neighbouring cells, and the compound it is mainly made of. (b) Why does the fruit soften when this layer breaks down? (c) Which strands, crossing the walls, would also be broken as the cells part?
Need a reminder? The key ideaShow
The cell wall is a non-living, rigid layer outside the plasma membrane of fungi and plants. It gives shape, protects from mechanical damage and infection, helps cell-to-cell interaction and keeps out unwanted large molecules. Algae: cellulose, galactans, mannans, calcium carbonate. Land plants: cellulose, hemicellulose, pectins, proteins. Primary wall: young, can grow. Secondary wall: on the inner side, towards the membrane. Middle lamella: calcium pectate, holds cells together. Plasmodesmata: strands joining the cytoplasm of neighbouring cells.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The layer. It is the middle lamella, made mainly of calcium pectate.
Step 2. Softening. The middle lamella holds neighbouring cells together. When it breaks down, the cells come apart and slide past one another, so the flesh is soft.
Step 3. The strands. The plasmodesmata, which join the cytoplasm of neighbouring cells, are broken as the cells part.
Answer: (a) the middle lamella, calcium pectate; (b) the cells are no longer glued together; (c) the plasmodesmata.
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