2. Why plants need a
transport system?
Plants cells need a regular supply
of oxygen and nutrients.
Their body is too large.
Diffusion from external
environment is not an option.
3. Xylem and phloem tissue in
roots, stems and leaves
Xylem: Water uptake near roots->Water enters xylem-> water moves
up xylem->Water moves from xylem to leaf cells
Phloem: This process is called translocation and involves the
movement of organic substances around the plant. It requires energy
to create a pressure difference and so is considered an active process.
Sucrose is loaded into the phloem at a source, usually a
photosynthesizing leaf. For this to occur, hydrogen ions are pumped
out of the companion cell using ATP. This creates a high concentration
of hydrogen ions outside the companion cell. Sucrose is loaded
(moved into companion cells) by active transport, against the
concentration gradient.
However, the protein carrier involved in the loading, has two sites, one
for sucrose and one for a hydrogen ion. When it is used to pump
sucrose into the companion cell, hydrogen will move in the opposite
direction, back down its concentration gradient. This is why a high
concentration of ions is needed outside the cell.
4. The sucrose can then diffuse down the concentration
gradient into the sieve tube element via the
plasmodesmata that connects the companion cell with the
sieve tube element. This lowers the water potential of the
sieve element so water enters by osmosis.
At another point sucrose will be unloaded from the
phloem into a sink (e.g. root). It is likely that the sucrose
moves out by diffusion and is then converted into another
substance to maintain a concentration gradient. Again,
water will follow by osmosis.
This loading and unloading results in the mass flow of
substances in the phloem. There is evidence to support
this theory; the rate of flow in the phloem is about 10,000
times faster than it would be if it was due only to
diffusion, the pH of the phloem sap is around 8 (it is
alkaline due to loss of hydrogen ions), and there is an
electrical potential difference across the cell surface
(negative inside due presumably to the loss of positively
charged ions).
5. Mechanism Of Water
Transport
The root hair cells have a concentrated cell sap vacuole which
means that the water potential is low in it and high in the soil,
osmosis takes place and water enters the cell.
Minerals are also present in the soil but in low concentration,
using active up take, the root hair cells takes the mineral ions in.
The mixture of mineral and water moves from the root hair cells
through the other cells by osmosis active uptake till it reaches
the xylem vessel in the root, it enters the xylem through pits.
The xylem vessel transports the water from the root to the stem
(forming the vascular bundle with the phloem) and upwards to
the leaves.
The water and dissolved minerals leave the xylem and get
absorbed by the cells in the leaves.
7. How Water Moves
Through The Xylem
In root hair cells, the mineral concentration is high, it
helps pushing the water towards the xylem and the stem.
Capillarity is a factor that helps in the movement of water
in the xylem vessels. The water molecules are attracted to
each other, as one moves upwards it pulls its
neighbouring molecule with it. The molecules are also
attracted to the walls of the xylem, the narrower the
xylem the easier it is for water to move.
Transpiration force is the most effective force that causes
water movement. In the leaf, the water evaporates and
leaves the plant through the stomata, one molecule
escapes pulling the other with it, and so on, creating a
suction force. You can think of it as using a straw to drink.
8. Transpiration
Transpiration is the evaporation of water from plants.
It occurs chiefly at the leaves while their stomata are
open for the passage of CO2 and
O2 during photosynthesis.
9. Environmental factors that
affect the rate of transpiration
1. Light
Plants transpire more rapidly in the light than in the dark. This is
largely because light stimulates the opening of the stomata
(mechanism). Light also speeds up transpiration by warming the
leaf.
2. Temperature
Plants transpire more rapidly at higher temperatures because
water evaporates more rapidly as the temperature rises. At
30 C, a leaf may transpire three times as fast as it does at
20 C.
3. Humidity
The rate of diffusion of any substance increases as the
difference in concentration of the substances in the two regions
increases.When the surrounding air is dry, diffusion of water out
of the leaf goes on more rapidly.
10. Environmental factors that
affect the rate of transpiration
3. Humidity
The rate of diffusion of any substance increases as the difference in
concentration of the substances in the two regions increases.When the
surrounding air is dry, diffusion of water out of the leaf goes on more
rapidly.
4. Wind
When there is no breeze, the air surrounding a leaf becomes increasingly
humid thus reducing the rate of transpiration. When a breeze is present,
the humid air is carried away and replaced by drier air.
5. Soil water
A plant cannot continue to transpire rapidly if its water loss is not made up
by replacement from the soil. When absorption of water by the roots fails
to keep up with the rate of transpiration, loss of turgor occurs, and the
stomata close. This immediately reduces the rate of transpiration (as well
as of photosynthesis). If the loss of turgor extends to the rest of the leaf and
stem, the plant wilts.
11. Companion cells
A cell with an unthickened cellulose wall and dense
cytoplasm that is found in close association with a
phloem sieve element to which it is directly linked via
many plasmodemata.
Companion cells probably provide ATP, proteins, and
other substances to the sieve-tube elements, whose
cytoplasm lacks many structures necessary for cell
maintenance.
12. Xerophyte
A type of plant that is well-adapted to water
shortages and exhibits adaptations that enable it to
store or conserve water. Xerophytes often live in
regions where evapotranspiration (the sum of
evaporation and plant transpiration) is greater than
precipitation for the region during all or part of the
growing season. Adaptations that xerophytes might
exhibit include succulent leaves and stems (to store
water), fewer stomata (to reduce water loss), and a
deep or widespread root system (to optimize water
uptake).
