Download Chapter-8 Transport in Plants

Chapter-8
Transport in Plants
Key Definition:
Transpiration:
Loss of water from leaves by evaporation from the surface of mesophyll cells followed by loss of
water vapor through the stomata
Translocation:
The movement of sucrose and amino acids in phloem, from the region of production (Source) to
region of storage (Sink).
Wilting: when the transpiration rate is faster than the rate of water absorption. The amount of
water in the plant keeps on decreasing. The water content of cells decreases and cells turn from
turgid to flaccid. The leaves shrink and the plant will eventually die
Structure of the Xylem Tissue:

The xylem vessels are long hollow tubes made of dead lignified cells

The xylem vessel is specialized to transport water and dissolved minerals from the root

These walls of the xylem vessel contain holes called pits through which water enters.

The xylem tissue is adapted to its functions in different ways.

Cell wall of the dead cells of the walls of it is made of lignin, which makes it stronger to
support the stem,

The tube is also very narrow, to make it easier for the water to be transported upwards.
Structure of the Phloem Tissue:

This is a long tube that runs along the sides of xylem tissue.

They form sieve plates, which have small holes on them

These cells are called sieve tube elements

They contain cytoplasm but not nucleus

They do not have lignin in their cell walls

Each sieve elements has a companion cell next to it

The companion cell does have a nucleus and also contain many other organelles.

The function of the phloem tissue is to transport glucose and amino acids from the leaves
and to all other parts of the plant, this is called translocation.
Vascular bundles:

A group of xylem vessels and phloem tubes are called as vascular bundles

In roots, vascular tissue is found at the centre, where as in a shoot vascular bundles are
arranged in a ring near the outside edge.

Vascular bundles are also found in leaves, they help to support the plants
Adaptation of plant in the absorption of water:

Plants absorb water through root hairs and transported up to leaves.

Water is transported through xylem vessels

There is a root cap which protects the root from growing through the soil

The root hair absorbs water and mineral ions from soil by osmosis

Water diffuses in to the root hair down its concentration gradient through partially
permeable membrane

Once the water reaches the xylem it moves up like a movement of water through the
straw when we suck it

The pressure at the top of the layer is lowered and the pressure at the bottom stays high

It happens because of transpiration
Transpiration:

Transpiration is the evaporation of water from a plant

The evaporation takes place from the leaves through the stomata

The lower epidermal layer has more stomata than the upper epidermal layer

The cells inside the leaf are covered with a thin film of moisture

This film of moisture evaporates from the cells and this water vapour diffuses out of the
leaf through the stomata.

Water from the xylem vessels in the leaf will travel to the cells by osmosis to replace it

This reduces the pressure at the top of the xylem vessels so the water flows up them

This process is called transpiration stream
Water potential gradient:

Water potential gradient in the leaf is caused by transpiration

This produces a pull from above, drawing water to the plant

Root hair cells have large surface area through which water is absorbed

Water vapour diffuses out of the stomata, which reduces the water potential inside the
leaf, so water evaporates from the surface of mesophyll cells
Conditions that effects the rate of transpiration:
Humidity: The higher the humidity the slower the transpiration.
Temperature: The higher the temperature the faster the transpiration.
Wind speed: the faster the wind the faster the transpiration.
Light: when light intensity is high, transpiration is high but, in the dark the stomata are closed,
the transpiration rate is very slow.
Translocation:

The transport of organic food such as sucrose and amino acids in the plant through the
phloem vessels.

Glucose is converted into another more complex sugar called sucrose.

Sucrose and amino acids are transported to every tissue of the plant, each cell use it in a
different way.

Root cells convert sucrose into glucose for respiration and store it.

Growing cells make cellulose for cell walls from sucrose and use the amino acids to
make proteins for growth.

And fruits use the sucrose to make the attractive scent and tasty nectar to attract insects.

The areas of the plant where sucrose is made are called sources, and where they are
delivered to and made use of is called sinks.

Many plants have time of the year where they become dormant

Dormant plants don’t photosynthesis, but survive on their stored starch, oils and other
materials

When the season changes they begin to grow again

So the stored materials are converted to sucrose and transported to the growing regions
Measuring the rate of transpiration:

It is not easy to measure the amount of water lost from the plant

But is easy to measure how fast the plant takes up water

The rate at which the plant takes up water depends on the rate of transpiration

The faster it transpires the faster it takes up water

A photometer is an apparatus which can be used to compare the rate of transpiration at
different conditions

By recording how fast the air/water meniscus moves along the capillary tube.