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, 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 𝑔J −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.
, Strong mutual attraction between water molecules is due to hydrogen bonding. Cohesion forces is also
responsible for tensile strength. Tensile strength is the max tension a continues column of any material can
withstand without breaking. Water drops are formed due to cohesion between water molecules that are
attracted to one another.
• Adhesion: Forces which develop among molecules of different material. Water can be attached to solid
surfaces, glass surfaces or any surface that is different to it. Adhesion forces are responsible for the capillary
rise of water in small-diameter conduits. Meniscus is the curved surface of a liquid inside a container that is
created by adhesion forces.
Cohesion & adhesion is responsible for maintaining the continuity of water columns in plants. Inside the plant water
molecules are attracted to each other due to cohesion & due to adhesion the water molecules are attached to the
plant wall.
Surface tension is formed on the surface of water. It arises from cohesive forces between water molecules, this is
because the interaction between water molecules is stronger than between water & air. The water molecules at the
surface are constantly pulled into the bulk water. From this the surface tends to shrink & behave more like an elastic
membrane. This surface tension allows small insects with little molecular wight to talk on water.
2) Diffusion, osmosis & water potential
Diffusion
It is the net movement of particles (molecules or ion) of a substance from one point in a system to another due to
differences in chemical potential of the substance between the two points.
SIMPLE: Diffusion occurs from an area where the chemical potential
of the substance is high, to an area where the chemical potential is
of a substance is lower.
, Fick’s 1st law of diffusion
It explains how particles under random thermal motion tend to spread from a region of higher concentration to a
region of lower concentration.
The rate of diffusion is proportional to both the surface area &
concentration difference & is inversely proportional to the thickness
of the membrane.
∆𝑐𝑠
Formula for diffusion: Js = -Ds∆𝑥
• J(g/c𝑚2 .sec): The diffusion flux: it is known as the amount of (M) of material flowing through a unit cross
section (S) of a barrier in unit time (t).
• D(𝑐𝑚2 /sec): The diffusion coefficient & the neg sign indicates that the flow is down on concentration
gradient.
• dc( g/c𝑚3 ): is the concentration gradient.
• X(cm): is the distance of movement perpendicular to the surface of the barrier.
Osmosis in plant cells
Osmosis is the movement of molecules through a selectively permeable membrane (not all things can go through)
from an area of high-water potential to an area of lower water potential.
Diffusion of water across a selectively permeable membrane.
A selectively permeable membrane allows free passage of water & other small molecules but prevents the
movement of large solute molecules.
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