Plant Biology - 9.1-9.4 Flashcards
(95 cards)
1
Q
functions of a leaf
A
- photosynthesis
- transpiration
- Guttation
- Storage of water
2
Q
epidermis
A
covers leaves in a single layer - protects the leaf from physical damage and pathogens
3
Q
cuticle
A
transparent, waxy, colorless
coats the epidermis to prevent water loss
4
Q
stomata
A
( small holes) are located on the lower epidermis of the leaf. The stomata allow gases and water vapor into and out of the leaf
5
Q
guard cells.
A
two bean shaped
controls stomata
6
Q
palisade mesophyll
A
layer of elongated cells containing chloroplasts found just under the upper epidermis.
The majority of photosynthesis takes place within this area.
7
Q
spongy mesophyll
A
contains air spaces in which gases circulate.
8
Q
petiole
A
connects the blade with the stem.
9
Q
vascular tissues
A
pass through with the xylem (water transport) positioned in the top section of the vein while the phloem (food transport) occupies the lower section.
10
Q
Stem function
A
- 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
11
Q
Cell turgor created by
A
osmotic movement of water into cells (non woody plants stems will
wilt if they have not been watered sufficiently)
12
Q
vascular bundles
A
Around the edges of a dicotlyedonous stem
consisting of xylem and phloem cells
13
Q
cambium cells
A
Separating the xylem and phloem
14
Q
Woody stems
A
have secondary growth of vascular tissues (yearly growth rings)
15
Q
Xylem cells
A
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.
16
Q
Phloem cells
A
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.
17
Q
Xylem - specific structure
A
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)
18
Q
tracheids and vessels.
A
Xylems can be composed of them
19
Q
Tracheids
A
Tracheids are tapered cells that exchange water solely via pits, leading to a slower rate of water transfer. In ferns and conifers.
20
Q
vessel elements
A
, the end walls have become fused to form a continuous tube, resulting in a faster rate of water transfer. In angiosperms only.
21
Q
Transpiration =
A
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
22
Q
transpiration stream
A
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)
23
Q
Evaporation via the Stomata
A
Water is lost from the leaves of the plant when it is converted into vapour (evaporation) and diffuses from the stomata
24
Q
Stomata’s placement (light and temperature)
A
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).
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Stomata =
pores on the underside of the leaf which facilitate gas exchange (needed for photosynthesis)
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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
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Regulating Water Loss - transpiration rate
amount of water lost from the leaves
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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
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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
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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
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Factors affecting Transpiration - light
guard cells close stomata at night, so transpiration is greatest during the day
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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.
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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
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Factors affecting Transpiration - wind
wind blows away evaporated vapour thus decreasing the humidity. If wind speed becomes
too high then the stomata may close
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epidermis
cover roots with a very thin cuticle, so not to restrict water entry
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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.
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Root functions
1. anchor plant to ground
2. absorb and transport water and minerals (via xylem)
3. store food eg. potatoes, carrots, kumara, maple, deciduous trees
4. transport stored food (via phloem)
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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+)
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Water uptake by roots
Water uptake occurs by osmosis into cells of the roots and root hairs.
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Water Movement
two main pathways that the water (and ions) takes through the root cells to the xylem tubes:
Most water travels through the spaces within the cells walls (called apoplastic pathway)
Some travels through the cytoplasm of cells via openings between cells called plasmodesma.
(called the symplastic pathway)
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apoplastic pathway
Most water travels through the spaces within the cells walls
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symplastic pathway
Some travels through the cytoplasm of cells via openings between cells called plasmodesma.
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Plant adaptations for water conservation - Xerophytes
= a plant which is able to survive in an ecosystem with little available water
eg cactus, succulents --> eg in deserts or alpine/arctic conditions (frozen water)
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Plant adaptations for Xerophytes
Adaptations of xerophytes incude:
Limiting water loss (transpiration) by
Waxy cuticle
Thin small leaves (reduces surface area)
Few stomata, sucken stomata, stomata open at night
CAM photosynthesis (opens stomata during cool nights)
Hairs on surface (reduce drying effect, increase humidity on leaf surface)
Curled leaves eg tussock
Storing water (succulence)
Succulent leaves and stems eg. cactis
Increased water uptake
Deep roots
Extensive root systems
Increased surface area ie. root hairs
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halophyte =
plant that grows in areas of high salinity
(Most plants can toleate up to about 3g/L of salt)
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Adaptation to saline environments by halophytes may be:
Salt avoidance eg. a plant completes its reproductive life cycle during periods (such as during the rainy season) when the salt concentration is low would be avoiding salt rather than tolerating it.
Salt tolerance : a plant species may maintain a 'normal' internal salt concentration by excreting excess salts,via active transprt, through its roots and leaves or by concentrating salts in leaves that later die and drop off.
Cellular sequestration – halophytes can sequester toxic ions and salts within the cell wall or vacuoles
Many have similar adaptations to xerophytes for water retention and storage.
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Climate change and salt tolerance
Sea level rising due to global warming and the saltification of soils due to use of fertilisers poses challenges for plant growers to develop crops that are salt tolerant.
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Models of Water Transport - Capillary Tubing:
Water has the capacity to flow along narrow spaces in opposition to external forces like gravity (capillary action)
This is due to a combination of surface tension (cohesive forces) and adhesion with the walls of the tube surface
The thinner the tube or the less dense the fluid, the higher the liquid will rise (xylem vessels are thin: 20 – 200 µm)
49
Models of Water Transport - Filter Paper:
Filter paper (or blotting paper) will absorb water due to both adhesive and cohesive properties
When placed perpendicular to a water source, the water will hence rise up along the length of the paper
This is comparable to the movement of water up a xylem (the paper and the xylem wall are both composed of cellulose)
50
Models of Water Transport - Porous Pots:
Porous pots are semi-permeable containers that allow for the free passage of certain small materials through pores
The loss of water from the pot is similar to the evaporative water loss that occurs in the leaves of plants
If the porous pot is attached by an airtight seal to a tube, the water loss creates a negative pressure that draws more liquid
51
Transport of Sugars by the Phloem (Translocation) - BASIC OVERVIEW
Sugars and amino acids are transported by a process called active translocation. Phloem cells have to use energy. Food is loaded into the phloem where photosynthesis is occurring and unloaded at sites such as storage organs, fruits and seeds.
Sieve elements are unable to sustain independent metabolic activity without the support of a companion cell because the sieve element cells have no nuclei and fewer organelles (to maximise flow rate)
The movement of sugars in the phloem is multi-directional and occurs by active transport.
52
Phloem structure - Sieve tubes
Sieve tubes are long and narrow cells that are connected together to form the sieve tube
Sieve elements are connected by sieve plates which are porous to enable flow between cells
Have no nuclei, reduced numbers of organelles and thick, rigid cell walls.
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Phloem structure - Companion Cells
Companion Cells provide metabolic support for sieve element cells and facilitate the loading and unloading of materials at source and sink
Have an infolding plasma membrane which increases SA:Vol ratio to allow for more material exchange and many mitochondria for the active transport of materials between the sieve tube and the source or sink
54
Water and sugar movement in phloem sieve tubes
Active transport is used to load organic compounds into phloem sieve tubes at the source.
The cytoplasm of the sieve tubes and companion cells are connected by channels called plasmodesmata. These channels allow the companion cells to regulate the uptake of the sugars.
High concentrations of solutes in the phloem at the source lead to water moving in by osmosis.
This creates a hydrostatic pressure that pushes the sugar along the sieve tube.
55
Aphids, radioactive carbon and phloem transport
Using aphids to measure rates of phloem transport.
1. A plant is grown in the lab and one leaf is exposed for a short time to CO2 containing the radioactive isotope 14C.
2. The 14CO2 will be taken and incorporated into glucose by the process of photosynthesis. Glucose is converted into sucrose for translocation via the phloem.
3. Aphids are encouraged to feed on the phloem in different locations of the stem at different times.
4. The phloem is then analysed for 14C content and the results can be used to calculate the rate at which
substances move through the phloem
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Plant Hormones
Auxins, Gibberillins, Cytokinins, Ethylene and Abscisic acid are different types of phytohormones (plant hormones) that are chemicals that control plant growth and reproduction.
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Auxins
regulation of plant growth. Auxins were initially isolated from human urine. Auxin means to “enlarge” or “increase”. They induce cell division, differentiation and elongation.
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Gibberillins
Gibberellins are plant growth regulators that facilitate cell elongation, help the plants to grow taller. They also play major roles in germination, elongation of the stem, fruit ripening and flowering
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Cytokinins
plant-specific chemical messengers (hormones) that play a central role in the regulation of the plant cell cycle and numerous developmental processes
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Ethylene
The ethylene in a plant growth regulator that acts as a trace level of entire plant life by regulating and stimulating the opening of flowers, fruit ripening and shedding of leaves.
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Abscisic acid
regulates numerous aspects of plant growth, development, and stress responses.
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Plant Life cycle
The main steps are growth of plant to maturity, flowering, egg/pollen production, pollination by wind/insects, seed production, seed dispersal (by wind, animals, water, self), seed germination, and seedling growth.
(REFER TO DIAGRAM)
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Flowers
REFER TO DIAGRAM
90% of plants are hermaphrodites: they have both male and female reproductive organs in the same flower. (eg. roses, daffodils, peas).
10% are monoecious having both male and female reproductive organs on the same plant (eg. corn, conifers) whereas dioecious plants have both male and female flowers on different plants (eg. kiwifruit, spinach, asparagus)
The male gametes (sperm) are inside pollen grains which develop in the anthers of the flower.
The female gametes (ova) develop inside ovules which are enclosed in the ovary at the base of the carpel.
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Pollination
Before fertilization can occur pollen must be transferred from on flower to another (called pollination). Flowers rely on wind or insects to transfer pollen.
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Cross pollination
between different flowers) is the norm. This is achieved by flowers producing pollen and ovum at different times. However self pollination can occur.
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Insect Pollination:
Most flowering plants use mutualistic relationships with pollinators in sexual reproduction. The plant benefits from pollination. The pollinator (bees, bats, birds) benefit by gaining nectar as food. 87% of the crop plants depend to some degree upon animal pollination, including bees and bats.
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Fertilisation
Once a pollen grain has landed on a stigma, a pollen grows out of the pollen grain and down the style to the ovary.
The sperm then travels down this pollen tube, enters the ovule and fuses with the ovum.
Fertilisation has occurred.
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Seed Dispersal (4)
Wind eg. dandelion, thistle, sycamore
Animals eg. bidibidi, hookgrass
Water eg. mangrove, coconut
Explosion (self) eg. peas, lupins, gorse
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Seeds and Fruit - Once fertilisation has occurred several changes take place:
The zygote starts dividing and develops into an embryo. This consist of a plumule (young
shoot), a radicle (young root) and one or two cotyledons (seed parts).
The ovule develops into the seed, including the testa (seed coat)
The ovary develops into the fruit. A fruit is any structure that surrounds the seed, protects it and helps it’s dispersal. eg. nuts, apples, pea-pods
zygote --> embryo --> ovule --> seed --> ovary --> fruit
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Germination - To germinate seeds need:
Water: seed absorbs water, swells, testa splits, enzymes activated
Oxygen: stored food converted to ATP using aerobic respiration. Energy used for growth.
Suitable temperature: this activates enzymes at an optimum temperature.
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Metabolic events during germination
Absorption of water allowing cells to become metabolically active
Gibberellin hormone produced in cotyledons
Gibberellin stimulates the production of amylase enzyme which converts starch into simple sugars
Glucose is used in aerobic respiration to produce ATP energy or is used to make cellulose and other substances needed for growth.
As soon as the leaves of the seedling reach light then photosynthesis can supply the seedling with food, substances and energy.
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Control of Flowering - gene expression
Flowers are the reproductive organs of angiospermophytes (flowering plants) and develop from the shoot apex. Changes in gene expression trigger the enlargement of the shoot apical meristem. This tissue then differentiates to form the different flower structures – sepals, petals, stamen and pistil
Flowering plants will typically come into bloom when a suitable pollinator is most abundant
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photoperiodism
The most common trigger for a change in gene expression is day/night length
i.e. the response to changes in day length by making appropriate physiological changes such as flowering and dropping leaves.
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Short-day plants
require a short day and long night
– i.e. they will produce flowers only if the photoperiod is less than a certain critical length.
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Long-day plants
These require a long day and a short night
– or, more correctly, they require a photoperiod that exceeds a certain critical length for flowering to take place.
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Day-neutral plants
The flowering of these plants is relatively unaffected by the amount of daylight per day – e.g. tomatoes.
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The Phytochrome System
The ability of plants to activate the photoperiod response – i.e. to respond to light – is controlled by a pigment called phytochrome.
Light is detected by the pigment phytochrome. ---> This exists in two forms corresponding the two wavelengths that the pigment absorbs: the forms are called P665 (or Pr) and P725 (or Pfr).
When Pr absorbs red light (in daylight), it is quickly changed to Pfr
The photoperiodic stimulus is detected by the leaves. ---> From the leaves a message passes to the buds to form flowers. This messenger acts like a hormone, and has been given the name florigen, although it has not been extracted and may be a mixture of already identified hormones.
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The Phytochrome System --> in sunlight vs night
Sunlight contains mostly red light. Phytochrome red (Pr) absorbs red light.
Night time has only far red light. Phytochrome far red (Pfr) absorbs far red light.
At night, Pfr is slowly converted into Pr . After a long night, lots of Pr will have accumulated (and small amount of Pfr remaining). After a short night only a small amount of Pr accumulates, (high Pfr remaining)
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The Phytochrome System - winter
when the night is long (eg. 14 hours) then Pr. accumulates to high levels, and this stimulates short-day plants to flower. (The response may also be due to low concentrations of Pfr.)
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The Phytochrome System - summer
when the night is short (eg. 10 hours) then Pr. will be low, and this stimulates long-day plants to flower. (The response may also be due to high concentrations of Pfr.)
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Auxin and Growth - tropism
growth response towards or away from an environmental stimulus coming from one direction.
towards = Postivite
away = Negative
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positively phototropic
If the shoot of a plant grows towards the light
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positively geotropic
If the root of a seedling grows down
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negative chemotropic
If the roots in the soil grow away from some copper pipes
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Auxin =
(called indoleacetic acid (IAA)) is the plant growth hormone that controls the tropic response
The auxin is produced in the growing tip of the shoot. (meristematic tissue)
It is water-soluble and diffuses through agar blocks, but will not pass through a sliver of mica glass
The auxin causes elongation of the cells, thus promoting upward growth.
If the light is shone on to a seedling from the side, auxin migrates to the dark side, the cells on the dark side grow elongated – so the shoot bends towards the light.
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In shoots, auxin increases the flexibility of the cell wall to promote plant growth via cell elongation
Auxin activates a proton pump in the plasma membrane which causes the secretion of H+ ions into the cell wall
The resultant decrease in pH causes cellulose fibres within the cell wall to loosen
Additionally, auxin upregulates gene expression of expansins, which similarly increases the elasticity of the cell wall
With the cell wall now more flexible, an influx of water (to be stored in the vacuole) causes the cell to increase in size
87
Effect of Auxin on Shoots, Buds and Roots
This depends on the concentration of auxin. As you can see in the graph:
Low concentration stimulate roots, while
high concentrations inhibits
Low concentrations stimulate the growth of
lateral buds, while high concentrations inhibit
Low concentrations do not stimulate the
stems, while high concentrations do
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Geotropism (gravitropism) in seedlings
Because of gravity, when the shoot and root emerge from a seed the concentration of auxin in the underside is greatest. Higher concentrations of auxin stimulate the shoot, so the underside elongates the most and causes the shoot to curve upwards.
Higher concentrations of auxin inhibit the root, so the topside grows faster and so the root curves down.
It has been shown that plant cells can tell up from down by using small starch grains (specialised plastids) as statoliths.
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Hydrotropism
This is very strong in roots. It is stronger than geotropism, as a root will grow upwards to the soil surface if that is where the only water is available.
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Thigmotropism
The tendrils of a climbing plant – such as a passion-fruit vine – bend round any object they touch. In this case growth is slowed down on the side that touches the object. Auxin migrates to the outside of the tendril, causing it to elongate.
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Chemotropism
Roots will often move towards certain chemicals in the soil, eg. magnesium, or away from others eg, copper. The growth of the pollen tube towards the ovary in the flower is an example of a chemotropism.
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Etiolation
If a plant has reduced light or is grown in the dark it becomes etiolated, i.e. the leaves are small and yellow and the stem becomes very long because Auxin is only in apical stem and none in laterals (side branches).
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apical meristem
Meristems generate new cells for growth of the plant.
The auxin from the top of the plant (called the apical meristem) is produced in relatively high concentrations, so the shoot is stimulated to grow.
As the auxin diffuses down the plant it becomes less concentrated, Less auxin makes it’s way to the lateral meristems (eg. lateral buds). This causes the triangular shape of trees such as conifers. The process is called apical dominance.
A germinating seed shows both negative geotropism and positive geotropism.
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Micro propagation of plants (Tissue culture)
Plants can also be grown from only one cell = Micro propagation
To get the cell to divide and differentiate requires:
Using stem apex cells (these are totipotent)
Treatment of the cell with hormones (auxin)
A nutrient agar gel.
Sterile conditions
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Micropropagation is used for:
The production of exact copies of plants that produce particularly good flowers, fruits, or have other desirable traits.
To quickly produce mature plants.
The production of multiples of plants in the absence of seeds or necessary pollinators to produce seeds.
The regeneration of whole plants from plant cells that have been genetically modified.
The production of plants in sterile containers that allows them to be moved with greatly reduced chances of transmitting diseases, pests, and pathogens.
The production of plants from seeds that otherwise have very low chances of germinating and growing, i.e.: orchids and pitcher plants.
To clean particular plants of viral and other infections and to quickly multiply these plants as 'cleaned stock' for horticulture and agriculture.
