phloem in a plant forms only a very this layer about the same thickness as a
piece of paper. Phloem tissue consists of sieve tubes, companions cells and
phloem parenchyma.? All phloem tissue is
living (unlike xylem) although the cytoplasm of the sieve tubes is highly
specialised and has a reduced number of cell organelles. The sieve tubes
consist of a column of cells formed end to end.? Between each cell the cell wall has a number of holes so that it
has the appearance of a sieve and this is known as the sieve plate.? The cytoplasm of the sieve tubes is modified
and contains no mitochondria.? Adjacent
to each sieve tube is a companion cell which has a very dense cytoplasm and
which supplies energy for the sieve tubes. The sieve tubes carry
sugar up and down the plant.? They are
loaded with sugars in the leaves and then the sugar moves in solution either up
or down the plant to where it is needed.Theories of Phloem Transport1. Pressure flow 2. Cytoplasmic streaming 3. Electro-osmotic flowNo one theory provides a totally satisfactory explanation to
flow. The most accepted theory is the pressure flow theory that
states that sugars are loaded into the phloem in an area of high concentration,
the source, and are then transported by mass flow to an area of low
concentration, the sink, where they are unloaded.? This theory allows for substances to move both up and down the
plant.? Movement of substances in the
phloem is an active process requiring ATP.Evidence for1.
The contents of the phloem have a positive pressure- they
exude fluid when cut and aphid stylets exude fluid when they penetrate the
phloem. 2.
Experiments have shown a concentration in the phloem contents
with the highest concentration near the source-analysis of exudates from aphid
stylets 3.
A physical model of this theory functions 4.
Viruses can be moved in the phloem.? This must be mass flow as they are nor in solution and are
therefore not able to move by diffusion.Evidence against1.Sugars
and amino acids have been found to move in different directions in the same
vascular bundle. 2.
Phloem transport may not occur in the direction of the deepest sink. 3.
The sieve plate is an impediment to mass flowExperiments used to investigate mass flowRadioactive tracers.? These
are introduced via radioactive carbon dioxide and photosynthesis and the path
traced by autoradiography.Ringing experiments.?
The
phloem is removed in a ring around the stem and this stops flow in the phloem. ?Shows that sugars, amino acids and salts are
transported in the phloem.Use of Aphids for sampling3.6????????? Exchange of Water and Ions in PlantsMost
questions in the exam ask about some, or all, of the following: ·
Root structure and function (particularly mineral
absorption)·
Stomata and transpiration (and factors affecting
transpiration)·
Features of xerophytes (plants that live in very dry
conditions)Root
structure and function:·
Root structure ? learn
the typical layout of tissues in roots (Support Booklet p.20) and how it
differs from stems.·
Root function:·
Water and minerals are absorbed through root hairs and pass
through the cells of the cortex. ·
These substances can move through the porous cell walls in
the cortex, rather like water soaking through paper, this is called the apoplast
pathway. ·
Water and minerals can also pass through the living part of
these cells (cell membrane, cytoplasm etc.) ? the symplast
pathway. ·
The cells of the cortex also contain large vacuoles, and
substances can pass through these (as well as the cytoplasm etc.) ? the vacuolar
pathway. ·
Between the cells of the cortex and the xylem and phloem is
a layer of cells called the endodermis. These cells have a special waterproof
layer in part of their cell walls, forming the Casparian strip.
This forces water and minerals to
take the symplast pathway through the endodermis.·
Because all cell membranes are selectively permeable, this
allows the cells of the endodermis to control
the amount of each mineral taken into the xylem: Substance Method of transport across endodermis Reason Water Osmosis Water is drawn up xylem in transpiration stream (see 3.7) Minerals at a higher
concentration in soil than plant cells Facilitated diffusion These can flow down
their concentration gradient into the plant Minerals at a lower concentration
in soil than plant cells Active transport (requires ATP) These must be moved against
their concentration gradient into the plant Toxins Transport blocked or inhibited Mechanism unknown (Water and minerals then pass
up the stem in the xylem – see 3.7 ? and enter the leaves)Stomata
and transpiration·
99% of the water that goes up the xylem evaporates into air
spaces in the leaves, and diffuses out through the stomata as
water vapour, this is transpiration. ·
Anything that affects the concentration gradient of water vapour from plant to air will
therefore affect the rate of transpiration: Factor Effect on rate of transpiration Reason Increased light intensity Increases Stomata open wider in light (see below) Increased humidity Decreases Decreased concentration gradient (humid air around leaves) Increased air movement Increases Increased concentration gradient (humid air around leaves
blown away) Increased temperature Increases More rapid evaporation from leaves Dry soil around roots or high salt
concentration (e.g. sea water) Decreases Decreased uptake of water into roots, therefore less
available in leaves (The rate of transpiration can be measured with a potometer).·
Clearly, stomata are very important in transpiration, as
most of the water vapour passes through them. They usually open in the light and close
in the dark; they also close when water supply to the roots is very poor.·
Stomatal opening is controlled by the two guard cells
which surround each stoma. The cell wall on the inner surface is much thicker
than on the outer surface. As these cells become turgid (swell) they bend outwards,
causing the stoma to open (you can demonstrate this by sticking sellotape on
one side of a sausage-shaped balloon then blowing it up, it bends away from the sellotape).·
There are two hypotheses to explain how guard cells change their shape:·
The potassium
movement hypothesis states that potassium ions (K+) are
pumped into the guard cells, by active
transport. This lowers their water potential, water flows in by osmosis,
the guard cells become turgid and stomata open. The reverse process closes
stomata. This hypothesis is the most widely accepted. ·
The starch-sugar
hypothesis states that there is a balance between sugars (soluble) and starch
(insoluble) controlled by two enzymes with different optimum pH’s. The enzyme
which converts starch into sugar has a high
optimum pH (alkaline), which is produced in the day, because acidic CO2
is used up in photosynthesis. Therefore, sugar accumulates, water potential
drops, water enters, cells become turgid, stomata open. The enzyme which
converts sugar to starch has a low
optimum pH (acidic), which is produced at night, because CO2 is
produced by respiration (no photosynthesis). Starch accumulates, but because
starch is insoluble water potential rises, water leaves, guard cells lose
turgidity, stomata close. This hypothesis is not widely accepted.Xerophytes·
These are plants that are adapted to live in very dry
conditions by having some, or all, of the following features:·
A very thick, waxy cuticle to reduce evaporation of water
through this part of the leaf (cuticular transpiration). ·
Stomata sunk into pits, which trap a layer of humid
around them, so reducing transpiration. ·
Hairs
around stomata, again trapping a layer of humid air. ·
Few, small leaves; often rolled into a tube. This reduces surface area for
transpiration, and humid air is also trapped inside the inrolled leaf. ·
Closing
stomata in the day, when it is hot, and opening them at night,
reducing evaporation. (such plants take in CO2 at night, store
it? as an organic acid and then break
the acid down in the day to release the CO2, internally, for
photosynthesis. This is called CAM
photosynthesis). ·
Storage
of water in thick stems and leaves (these plants are called succulents). ·
Deep, tap roots to draw up water from deep soil layers. ·
Roots very close to the soil surface, to absorb
condensation which forms at night.