Transport in Plants

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Outer layer of cells. Prevent damage and water loss Outer layer of cells. Prevent damage and water loss. Waxy outer layer (cuticle). Transparent to allow light through. Internal tissue of leaf. Made of parenchyma cells. Large number of chloroplasts. Spherical, loosely arranged cells. Fewer chloroplasts Transport tissue. Made of sieve tubes and companion cells. Cross Sectional Slide Mainly lower epidermis. For gas exchange. Bordered by guard cells.

Feature Day Night Respiration occurring Photosynthesis occurring State of stomata Concentration of CO2 in leaf Concentration of CO2 in atmosphere Concentration of O2 in leaf Concentration of O2 in atmosphere Net Gas Exchange

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Transport Tissues

Vascular Bundles Vascular bundles are essentially ‘bundles’ of the transport tissues They are found throughout the plant They consist of: Xylem (water and minerals, dead) Phloem (sucrose, living) Cambium (unspecialised cells that become the other two tissue types)

What’s what?

Xylem Xylem starts off living tissue – protoxylem It is not fully lignified so is stretchy and grows Microfibrils line up vertically increasing strength (compression forces) As it ages, more lignin is laid down: cell dies 

Lignified tissue is called metaxylem Cell end walls break down leaving a hollow tube Water can move in and out via pits Lignified xylem is very strong, it offers support in woody plants. In non-woody plants, that support is from turgid parenchyma (not xylem).

Translocation of Water

Translocation of water Plants don’t have a heart, need to use physical processes to move substances around (translocate) Xylem is thin 0.01 – 0.2mm so they have a lot of resistance but plants still manage to move water at 8 mh-1 up to 100m up

Transpiration is water loss from the plant surface Water evaporates from the cell wall of the spongy mesophyll cells into the air spaces The vapour then moves out of the leaf through the stomata along a diffusion gradient Air movements will increase the rate at which it occurs An oak tree may transpire 600L in a day!

Demonstrating water loss from a plant

Transpiration stream The key to this is the water itself! As water is lost via transpiration, cohesion of the water molecules due to their _______ bonds (which exist because water is _______) gives the column of water tensile strength

Water molecules also adhere to the walls of the xylem vessels This adhesion, attraction between two unlike molecules, is strong enough to support the whole column of water in the xylem More water is continuously moved into the root hairs from the soil by osmosis

Uptake of Water

How / Where do plants take up water? Roots don’t! It is done via root hairs

Even fairly dry soil has a layer of water around the soil particles – soil water Younger parts of the roots have root hairs Root hairs make contact with the soil water allowing it to be taken up

Uptake of water from the soil into the roots works by osmosis Water moves down a concentration gradient It moves from the soil water into root hair The root hair is then more dilute than the cell next to it So water moves from the hair into the cell Which is then more dilute than the next cell…. So eventually it reaches the xylem

In reality there are three routes by which the water can move from root hair to xylem: Vacuolar pathway Symplast pathway Apoplast pathway

Uptake of water in roots

Root Pressure Transpiration is a passive process It seems this is not the only process occurring to move water in xylem Root pressure is an active process (uses energy) Cut off the top of a plant and it keeps pushing sap out from its roots!

Uptake of Minerals

Why do plants need minerals? Plants make their own carbohydrates But they also need molecules like fats and proteins To make these they need other substances which can be gained from minerals in the soil

Nitrogen Nitrogen enters the plant as Nitrate ions or Ammonium. Used to make: amino acids DNA Hormones Without nitrates old leaves turn yellow and die. Growth is stunted Also need calcium, magnesium and phosphate

Moving minerals around Minerals are in low amounts in the soil so have to be taken up by active transport (against the concentration gradient) Go via the apoplast pathway or symplast pathway then flow through the plasmodesmata in the cytoplasmic stream

Sugars (sucrose) and the Phloem 3 things from the worksheet Use of ringing experiments Use of aphid stylets Use of radioactive tracers Anatomy of Phloem Mass Flow Hypothesis