Transport in Plants

RevisionA

B

C

D

E

F

H G

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.

Transport tissue. Made of sieve tubes and companion cells.

Spherical, loosely arranged cells. Fewer chloroplasts

Mainly lower epidermis. For gas exchange. Bordered by guard cells.

Cross Sectional Slide

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

http://plantphys.info/plant_physiology/plantbasics1.shtml

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

soil particles

epiderm is endoderm ispericycle

xylem

cell wallcytoplasm

vacuole

root hair

symplast pathway (cytoplasms)

aoplast pathway (cell walls)

cortex

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