Xylem Phloem Osmosis Root hairs
Root pressure Transpiration
Active transport Stomata
Lenticel KEYWORDS Cohesion
Tension Adhesion
Dixon and Joly Mesophyll
Tubers Bulb Guard cells Diffusion
Why plants need transport?
Plants need to be able to transport water, carbon dioxide, oxygen and minerals.
Water uptake by roots
• Root hairs are found at the tip of
roots.
• Root hairs have thin walls (helps
them to absorb water)
• Root hairs occur in large numbers
(large surface area)
NOTE: root hairs absorb water by
osmosis.
Movement of water into xylem • Water diffuses from the root hair into the ground tissue.
• It then diffuses from ground tissue into the xylem in centre of root.
• Water moves up through the stem to leaves and flower.
Upward movement of water
Water moves up through the plant by two methods:
1) Root pressure
2) Transpiration
Root pressure
• As water enters a root by osmosis, the build up of water in the roots causes root pressure.
• Root pressure pushes water up through small plants.
NOTE: root pressure does not explain how water can reach great heights.
Transpiration
Transpiration is the loss of water vapour due to evaporation from the leaves of a plant.
Transpiration takes place through the stomata (openings on the under side of a leaf).
As each water molecule is pulled through the xylem cells by
osmosis, the next water molecule is pulled also.
Water is pulled through the plant by transpiration
What allows water to reach great heights in trees?
Cohesion Tension model • Allows large amounts of
water to move quickly
from roots to leaves in warm
conditions.
• Can move 220 litres in one hour
up through a plant.
• This model was put forward by
Henry Dixon and John Joly (Irish
Scientist in Trinity College)
Cohesion Tension Model
Cohesion: the sticking of water molecules to each other.
Adhesion: when different molecules stick together (water sticks to the wall of the xylem)
Cohesion Tension Model
Leaf
Root
1) Water evaporates from the xylem out through the stomata into the air.
2) During transpiration each water molecule is pulled through the xylem.
3) Due to cohesion the next water molecule is pulled along by the one in front of it.
4) Xylem are adapted for movement of water because they are narrow.
5) When the water column in the xylem is stretched it is said to be under tension.
Cohesion Tension Model
• The tension between the water molecules allow the water to be pulled up to a height of 145 metres.
• Stomata open in daylight hours allowing for transpiration to occur, the xylem vessel narrows.
• Stomata close in night time hours which causes the xylem to return to normal size.
NOTE: lignin prevents the xylem vessel from collapsing.
Transpiration - YouTube
Control of transpiration in leaves • If leaves lose to much water, it will wilt and die.
They control the rate of transpiration by the following:
1) Leaves have cuticle which does not allow water to pass through (cuticle thicker on upper surface as this side is exposed to the sun)
How to control rate of transpiration:
2) Stomata are located on lower surface of leaf to reduce water loss (more evaporation would occur on upper surface)
How to control rate of transpiration:
3) Each stomata is surrounded by two guard cells. The stomata can open or close by the guard cells changing shape.
Factors impacting stomata
The following environmental conditions affect stomata opening or closing:
1) Plants lose to much water
2) High temperatures
3) High wind
4) Carbon dioxide concentration
NOTE: Plants do not grow well in dry conditions because the stomata will remain closed for long periods.
How does CO2 concentration control stomata opening or closing?
High concentration of CO2:
• High levels of CO2 cause stomata to close.
• Photosynthesis rate decreases in the evening causing the build up of CO2.
sachin joshi _ botany student - YouTube
• Low levels of CO2 cause the stomata to open.
• When photosynthesis begins in the morning CO2 is absorbed by the mesophyll cells (ground tissue).
SUMMARY:
High concentration of CO2 Stomata close
Low concentration of CO2 Stomata open
How do stomata open or close:
• Guard cells open and close the stoma by changing shape
• When water enters the guard cell by osmosis, they swell and become turgid.
• This causes the guard cells to buckle outwards creating a gap between the guard cells.
Gas exchange in the leaf:
Carbon dioxide:
Stomata allows for gas exchange to be carried out.
Once in side the leaf the CO2 diffuses from the air spaces into the mesophyll.
NOTE: the air spaces increase the internal surface area which allows more CO2 to diffuse quickly into the mesophyll for photosynthesis
NOTE: there can be 50000 stomata per cm2 which increases the rate of gas exchange.
Gas exchange in the leaf
Oxygen:
Photosynthesis produces oxygen.
Oxygen diffuses from the mesophyll into the air spaces and out through the stomata.
Water vapour
Water vapour diffuses out from the leaf through the stomata (transpiration)
Gas exchange in stems
Lenticels are openings in the stems of plants that allow gas exchange.
• Oxygen diffuses inwards through the lenticel.
• Carbon dioxide and water vapour diffuses outwards through the lenticel
NOTE: Cells inside the stem need oxygen for respiration to occur.
Mineral uptake in leaves
• Plants require minerals to
function (calcium, magnesium)
• Minerals are absorbed from
the soil by root hairs in a
process called active transport
(requires energy)
• Root hairs have lots of
mitochondria to supply energy.
Food storage organs in plants
Modified roots:
• Dicot plants can have large tap roots to store food (starch).
• This food is used to produce flowers, seeds and fruits.
Example:
Carrots
Turnips
Modified stem
• Potato plants produce an underground stem to store food (starch).
• These swollen stems are called tubers.
Modified leaves
Onions, daffodils and tulips all produce a bulb.
What is a bulb? • A bulb is an underground stem that has swollen fleshy leaves
to store food. • The bulb is protected by a dry scaly leaf on the outside.
Dry scaly leaves
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