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BOT2603 EXAM PACK 2023
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BOT2603EXAM
PACK
2023
, BOT2603 Exam Prep
Unit 1: Water relations
Outline
1. Plant cells & water
2. Diffusion, osmosis & water potential
1) Plant cells & water
Organisms can’t survive without water. Water is largest part of most organisms. It makes up about 90% of every
organises. Used directly or indirectly in all metabolic processes. Actual water content varies according to tissue & cell
type & is dependent to some extent on environmental & physiological conditions.
Xerophytes grow in desert area’s (low level of water). Actual
content of xerophytes will be different to mesophytes or
hydrophytes. Desert plants could have water content of only 20%.
Some metabolic processes may be inactive when low on water,
once water restored, their processes can become active again.
Water content of plants are in flux, which is dependent on metabolic activities, status of water in surrounding air &
soil & lots of other factors. When water is available a plant can grow, irrespective of the environment. Physiological
processes like photosynthesis & respiration can decline when water levels are low. Hydration of tissues is essential
for cellular expansion during growth. (low water = poor growth)
Unique properties of water
• Thermal properties: Water stays in a liquid state over a range of temperatures at which most biological
reactions happen. It is also responsible for temperature regulation to ensure plants don’t cool down or heat
up too quickly. Specific heat is the thermal capacity of a substance or the amount of energy that can be
absorbed for a given temperature rise. The specific heat of water is 4.184 J𝑔−1 °𝐶 −1 (joules per gram per ℃)
• Solvent properties: Enables a wide range of solutes to be dissolved and distributed for growth. Hence, it is
regarded as a universal solvent.
• High polarity: Water molecule is a polar molecule, because it attracts other polar molecules & the structure
of a water molecule has strong inter-molecular interactions. The oxygen atom of water has a partial negative
charge, and the 2 hydrogen atoms share a partial positive charge. This enables for an attraction between the
oxygen and the 2 hydrogen atoms share a partial positive charge. This makes the attraction between the O &
H to form a hydrogen bond.
• High heat of vaporization: The amount of heat required to change 1 mole of liquid water to 1 mole of vapor
water is ± 44kJ𝑚𝑜𝑙 −1 at 25℃. The hydrogen bonding makes it hard for water molecules to escape their
liquid state, thus water requires high amounts of heat(temperature) to become a vapor. Due to this the
amount of energy required to break the hydrogen bond before water vaporates is quite big.
Cohesion & adhesion
Water has cohesion & adhesion properties. They can attract one another & can attract other substances.
• Cohesion: Forces which develop between molecules of the same substance/material.
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