IBDP Biology Chapter 9 Notes

plant biology


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In this unit, we will understand the transport mechanism inside a plant and study growth and photoperiodism. We start with the leaf structure. It comprises guard cells, stomata, spongy and palisade mesophyll, xylem, phloem and epidermis. In plants, water is transported with the help of the xylem’s tracheids and vessels. Guard cells regulate the opening and closing of stomata because of the turgidity and uneven thickening of the cell wall.

Fluids move in plants because of cohesion between the water molecules, adhesion between water molecules and xylem vessels and the tension created by all these phenomena leading to the formation of a water column. Water lost due to evaporation creates a pull in the water column in the vessels, which replaces the lost water by pulling water from the root cortex into the xylem.

Net we discuss Phloem, which is involved in the transport of organic molecules according to the pressure-flow hypothesis according to which sugar is loaded into the sieve tubes at leaves(SOURCE) and formation of concentration gradient allows forces water from surrounding cells to enter the sieve tubes cells as well which creates hydrostatic pressure. And water flows from the source (high hydrostatic pressure) to the reservoir. Then we discuss growth in plants, which is of indeterminate growth pattern. Meristematic tissue can give rise to derivatives like dermal, ground, and vascular tissues.

Plant hormones travel via phloem and affect the target cells. Phototropism is affected by Auxin. Flowers are the reproductive organs in angiosperms, and fertilisation occurs by pollination.