WJEC BY2 Notes for 2.3 (Part Two - Plants)

Hmm this took me way longer than it should have.. got caught up with my stats exam! Still need to do 2.4, 2.5 and 2.6 as well as some past paper by monday! Wish me luck!

Good luck with the biology exam on monday if you're taking it :)

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  • Created on: 19-05-12 16:17
Preview of WJEC BY2 Notes for 2.3 (Part Two - Plants)

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BY2.3 Part 2 ­ Plants
Xylem tissue transports water and mineral salts from the roots to the leaves.
Phloem tissue transports the soluble products of photosynthesis (sucrose and amino
acids) from the leaves to the other parts of the plant.
In each case the walls of tubes are thickened by addition of cellulose and lignin (a wood material)
Xylem is made up of four different types of cells:
vessels - the main conducting cells, contain pits so that water can travel sideways between
one xylem vessel and another (dead)
tracheids - also conduct water but are not as well adapted as vessels for this function,
joined together with vessel cells to form long narrow tubes, this is where the water is
transported (dead)
fibres - they have no role in water transport but function in support, they are long, narrow
cells with tapered ends (dead)
Xylem parenchyma - this acts as packing tissue, stores food and allow lateral transport of
materials through vascular tissue (living)
Vessels and tracheids are dead cells and they form a system of tubes through which water can
travel. The cells are dead because lignin has been deposited on the cellulose cell walls rendering
them impermeable to water and solutes. These cells also provide mechanical strength and support
to the plant.
Tracheid
Parenchyma
cell
Vessel
Fibre
Lignified wall
of vessel

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Stem Root
The distribution of xylem tissue differs in primary stems, leaves and roots:
In stems it occurs as part of the peripheral vascular bundles. This organisation gives flexible
support but also resistance to bending strain.
In leaves the arrangement of vascular tissues in the midrib and network of veins also gives
flexible strength and resistance to tearing strains.
In roots the central arrangement is ideal for resistance to vertical stresses (pull) and so helps
the anchorage of the plant.
1.…read more

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There are 3 main pathways through which the water can move through from the roots to the xylem:
Apoplastic ­ Goes through non-living parts of the cell (through cell wall) by diffusion/osmosis
Symplast ­ goes through living parts of the cells ( cytoplasm).…read more

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The casparian strip is made from water-proof suberin and is found the on radial, lower and upper
walls, but not the tangential walls.
Therefore, the water has to take the symplast pathway ­ this means the plant selectively take up
which ions enter and leave the xylem.
Once in the pericycle, the water retakes the apoplast pathway as the xylem lacks cell contents.
Salts are actively secreted into the vascular tissue (xylem) from the endodermal cells, requiring ATP.…read more

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Pressure increases as temperature rises
2. Metabolic inhibitors (e.g. cyanide) that inhibit respiration cause root pressure to cease ( stops
the salts being actively secreted)
3. A decrease in O2 availability results in a reduction of root pressure
The movement of water from the root to the leaf:
Water travels in the xylem up through the stem to the leaves, where most of it evaporates from the
internal leaf surface and passes out, as water vapour, into the atmosphere.…read more

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Temperature - a rise in temperature provides more kinetic energy for the movement of
water molecules. This additional energy increases the rate of evaporation of water from
the plant and speeds up the rate of diffusion of water vapour into the surrounding
atmosphere. The water potential of the atmosphere becomes lower as its temperature is
raised and it can hold more moisture.
Humidity ­ if the air outside of the leaf is very humid (i.e.…read more

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The reservoir is used to push the air bubble back to "0"
The air bubble moves along the capillary tube as water is absorbed by the shoot ­ measuring the
rate of transpiration
Plants can be classified on the basis of their structure in relation to the prevailing water supply, into
hydrophytes, mesophytes and xerophytes.…read more

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Hairs - stiff, interlocking hairs trap water vapour and reduce the water potential
gradient.
Thick cuticle - the cuticle is a waxy covering over the leaf surface which reduces
water loss. The thicker this cuticle the lower the rate of cuticular transpiration.…read more

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The main ones are companion cells and sieve tubes.
The sieve tubes are elongated and their ends are perforated with pores. These areas are called
sieve plates.
Cytoplasmic filaments/strands containing phloem protein extend from one sieve cells to the next
through the sieve plates. The sieve tubes do not possess a nucleus. Each sieve tube is connected to
a companion cell by numerous plasmodesmata.
Companion cells have many mitochondria used for active transport of sucrose into the Phloem.…read more

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Unlike water in the xylem, the contents of the phloem can move up or down a plant stem, often
simaultaneously (but not in the same sieve tube)
1) Photosynthesising cells (source cells) produce glucose which is converted into sucrose,
which is loaded into the phloem sieve tube by active transport (using ATP)
2) By increasing the level or solutes in the phloem, the water potential is lowered and water
moves in from the adjacent xylem by osmosis, down a water potential gradient
3) This raises…read more

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