AS Biology; Transport in plants

  • Subject: Biology
  • Level: AS
  • Exam board: All boards
  • Author: Katie
  • Year created: 2010

AS OCR Biology

Slide titles;

- Xylem

- Phloem

- Movement in the roots

- Role of the Casparian Strip

- Movement of water up the stem

- Transpiration

- Loading Sucrose into the Sieve Tube element

- Translocation

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Slide 2

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Xylem
Xylem is a plant transport tissue that carries
water and minerals from the roots to the rest of
the plant. It is made of hollow columns of dead
cells lined end to end and reinforced with Lignin
It provides important support for the plant.
Ways in which Xylem is specially adapted;
·The walls of the vessels are lignified. The lignin
makes the walls practically impermeable,
extremely strong and stops them from
collapsing.
·The tubes are narrow so the water column
does not break easily and capillary action can be
effective.
·The side walls have bordered Pits (unlignified
areas, to allow movement of water from one
vessel to another)
The flow of water is not impeded because; there
·Lignin deposited in the walls in spiral patterns
are no end walls, no cell contents, no nucleus or
allows Xylem to stretch as the plant grows and cytoplasm, lignin prevents the walls from
enables the stem or branch to stretch. collapsing

Slide 3

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Phloem
Phloem is a transport tissue that carries the products of photosynthesis (e.g. sugars) to the
rest of the plant. It consists of sieve tube elements and companion cells.
Plasmodesma linking Sieve tube elements; living tubular cells
cytoplasm of companion connected end to end. The end cell walls have
cell and sieve tube element
perforations in them to make sieve plates.
Lacks a nucleus and most organelles so there is
more space for solutes to move. The Cell
walls are made of cellulose so water can more
vertically as well as laterally.
Companion cell since the sieve tube lacks
organelles , the companion cell has a nucleus,
mitochondria, ribosome's and enzymes control
the movements of solutes and provide ATP for
active transport in the sieve tube element.

Slide 4

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When reading
this it may be
helpful to look at
the cross section
diagram on slide
Movement in the Root
one
Water moves through the root hair cells and then
moves across into the Xylem in the centre of the root.
Water moves this way by Osmosis. Minerals are also
absorbed however they are absorbed by active
transport using ATP as they are pumped against the
con concentration gradient.
1. Water moves into the root hair cells because soil water
has a higher water potential than the cell sap.
2. Water moves across the cortex of the root either
through the Apoplastic pathway or the Symplastic
pathway. The water moves by osmosis as the root hair
cell will have a higher water potential than the cell
next to it.
3. When the water reaches the part of the cell called the
endodermis it encounters a thick waxy, impenetrable
The Vacuolar pathway is similar to the band called the Casparian strip. In order to cross the
Symplast pathway but the water is not endodermis the water that has been moving through
confined to the cytoplasm of the cells the cell wall now needs to move through the
and can enter and pass through the cytoplasm
4. Once it has moved across the endodermis it continues
vacuoles as well. down the water potential gradient until it reaches a pit
in the Xylem vessel where is enters the vessels and
moves up towards the leaves.

Slide 5

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Roles of the Casparian Strip
The Casparian strip blocks
the Apoplastic pathway
between the cortex and the
Xylem
·This ensures that water and
dissolved nitrate ions have
to pass into their cell
through the cell cytoplasm
and cell membranes
·Nitrate can be actively
transported from the
cytoplasm of the cortex cells
into the Xylem
·This lowers the water
potential of Xylem so water
from the cortex moves into
the Xylem by osmosis.
·Once the Xylem has
entered the Xylem it cannot
move back into the cortex as
the Apoplastic pathway is
blocked.

Slide 6

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Movement of water up the Stem
There are 3 ways; Root pressure, Transpiration Pull and Capillary action.
1) Root pressure; The endodermis moves minerals into the Xylem by active transport therefore
decreasing the water potential in the Xylem and forcing water to move by osmosis into and up
the Xylem.
2) Transpiration Pull; Water molecules are attracted to each other by forces of cohesion, These
cohesion forces hold the molecules together in a long column. As molecules are lost at the top of
the column, due to transpiration, the whole column is pulled up as one chain. The pull from
above creates tension in the column of water. This is the cohesion- tension theory in the
transpiration stream. This is also why Xylem cells must be strengthened with lignin ­ to prevent
them collapsing under the tension.
3) Capillary action; The same forces that hold water molecules together also attract the water
molecules to the side of the Xylem Vessel. This is called Adhesion. Because the Xylem vessels are
very narrow these forces of attraction can pull the water up the sides of the vessel.

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