Plant Biology - 9.1-9.4 Flashcards
functions of a leaf
- photosynthesis
- transpiration
- Guttation
- Storage of water
epidermis
covers leaves in a single layer - protects the leaf from physical damage and pathogens
cuticle
transparent, waxy, colorless
coats the epidermis to prevent water loss
stomata
( small holes) are located on the lower epidermis of the leaf. The stomata allow gases and water vapor into and out of the leaf
guard cells.
two bean shaped
controls stomata
palisade mesophyll
layer of elongated cells containing chloroplasts found just under the upper epidermis.
The majority of photosynthesis takes place within this area.
spongy mesophyll
contains air spaces in which gases circulate.
petiole
connects the blade with the stem.
vascular tissues
pass through with the xylem (water transport) positioned in the top section of the vein while the phloem (food transport) occupies the lower section.
Stem function
- connect leaves, roots and flowers
- transport water and minerals (via xylem)
- transport food (via phloem)
- provide support
- may be modified to store food eg. rhubarb
Cell turgor created by
osmotic movement of water into cells (non woody plants stems will
wilt if they have not been watered sufficiently)
vascular bundles
Around the edges of a dicotlyedonous stem
consisting of xylem and phloem cells
cambium cells
Separating the xylem and phloem
Woody stems
have secondary growth of vascular tissues (yearly growth rings)
Xylem cells
transport water (and minerals) up from the roots to the leaves.
They consist of continuous tubes (have no end walls) which have been thickened cellulose and hardened with lignin.
Phloem cells
transport sugars from the leaves (or other food storage organs) to the plant.
They consist of cells called sieve tubes which have a perforated end plate that separates each cell.
Phloem also contains small companion cells that seem to control the activity of the sieve tubes.
Xylem - specific structure
It is a tube composed of dead cells that are hollow to allow for the passive movement of water
in one direction only
The cell wall contains numerous pores (called pits), which enables water to be transferred between cells
Walls have thickened cellulose and are reinforced by lignin, so as to provide strength as water is transported under tension
The lignin reinforcement can be as spirals or as rings (annular)
tracheids and vessels.
Xylems can be composed of them
Tracheids
Tracheids are tapered cells that exchange water solely via pits, leading to a slower rate of water transfer. In ferns and conifers.
vessel elements
, the end walls have become fused to form a continuous tube, resulting in a faster rate of water transfer. In angiosperms only.
Transpiration =
loss of water vapour from the stems and leaves of plants
Over 90% of the water absorbed by a plant lost by transpiration.
Stomata are open to allow the movement of carbon dioxide into the leaf.
= inevitable consequence of gas exchange
transpiration stream
flow of water –>
- absorbed by root hairs by osmosis
- moving from cells to xylem by osmosis
- drawn up xylem by pressure from below and suction due to transpiration from above
- cohesion and adhesion means that water flows up xylem tubes
- water evaporated and lost through stomata (transpiration pull)
Evaporation via the Stomata
Water is lost from the leaves of the plant when it is converted into vapour (evaporation) and diffuses from the stomata
Stomata’s placement (light and temperature)
mostly on the underside of the leaf (high humidity) to limit transpiration and are usually closed at night.
Stomata guard cells can close during high evaporation conditions (hot dry days).
Stomata =
pores on the underside of the leaf which facilitate gas exchange (needed for photosynthesis)
transpiration pull in the stomata
- photosynthetic gas exchange requires stomata to be open, transpiration will be affected by the level of photosynthesis = transpiration is an inevitable consequence of gas exchange in the leaf
negative pressure creates a tension force in leaf cell walls which draws water from the xylem (transpiration pull) - water is pulled from the xylem under tension due to the adhesive attraction between water and the leaf cell walls
Regulating Water Loss - transpiration rate
amount of water lost from the leaves
transpiration rate process and regulation
regulated by the opening and closing of stomata
rates will be higher when stomatal pores are open than when they are closed
Other factors that will affect transpiration rates include humidity, temperature, light intensity and wind
Guard cells flank the stomata and can occlude the opening by becoming increasingly flaccid in response to cellular signals
When a plant begins to wilt from water stress, dehydrated mesophyll cells release the plant hormone abscisic acid (ABA)
Abscisic acid triggers the efflux of potassium from guard cells, decreasing water pressure within the cells (lose turgor)
A loss of turgor makes the stomatal pore close, as the guard cells become flaccid and block the opening
Cohesion in the xylem
= force of attraction between two particles of the same substance (e.g. between two water molecules)
Water molecules are polar and can form a type of intermolecular association called a hydrogen bond
This cohesive property causes water molecules to be dragged up the xylem towards the leaves in a continuous stream
Adhesion in the xylem
force of attraction between two particles of different substances (e.g. water molecule and xylem wall)
The xylem wall is also polar and hence can form intermolecular associations with water molecules
As water molecules move up the xylem via capillary action, they pull inward on the xylem walls to generate further tension
Factors affecting Transpiration - light
guard cells close stomata at night, so transpiration is greatest during the day
Factors affecting Transpiration - temperature
evaporation of water in the spongy mesophyll increases with temperature. When
temperature is very high then the stomata close when it is dry and hot.
Factors affecting Transpiration - humidity
water diffuses out of the leaf along the concentration gradient, The lower the
humidity (drier) it is outside, the faster the rate of transpiration
Factors affecting Transpiration - wind
wind blows away evaporated vapour thus decreasing the humidity. If wind speed becomes
too high then the stomata may close
epidermis
cover roots with a very thin cuticle, so not to restrict water entry
Roots
branching and young roots have root hairs to increase surface area. Much of the root contains a cortex of parenchyma cells with air spaces for the aeration of root tissue. Xylem and phloem cells distributed in a ring around the root.
Root functions
- anchor plant to ground
- absorb and transport water and minerals (via xylem)
- store food eg. potatoes, carrots, kumara, maple, deciduous trees
- transport stored food (via phloem)
Mineral uptake by roots
- some mineral ions diffuse in
- most absorbed using active transport (as the mineral nutrient concentration in roots may be 10,000 times more than in surrounding soil.)
- Root hairs have mitochondria and proton pumps. Most roots need an oxygen supply to produce ATP for active transport.
During transport throughout a plant, minerals can exit xylem and enter cells that require them.
Plants absorb minerals in ionic form: nitrate (NO3−), phosphate (HPO4−) and potassium ions (K+)