Chapter 9 Flashcards
Plant biology
Gas exchange in leaves
Plants photosynthesise- require CO2 and water to produce carbohydrates and oxygen
- also require oxygen for cell respiration
- plants obtain gases needed through diffusion, mainly through leaves
- most leaves have tiny pores, usually located on their underside (stomata)
Stomata
Tiny pores where gases can diffuse in and out
- two guard cells that surround stoma control whether it’s open or closed, and so can control gas exchange
when stomata are open, gases can be exchanged, but water vapour can also escape from the leaves. This water must be replaced by water taken into the roots and carried through the plants to the leaves. This process is called transpiration and is the result of gas exchange in the leaves.
Stomata are open
- gases can be exchanged, but water vapour can also escape from leaves
- water must be replaced by water taken into roots and carried through plants to leaves- transpiration
- transpiration is the result of gas exchange in the leaves
Why is transpiration essential to plants?
It pulls water up from the root to:
- provide one of the raw materials for photosynthesis
- transport minerals to leaves for use in synthesis of important molecules
- cool the leaves
Transpirational pull
- it’s important for a plant to replace water that’s lost during transpiration - otherwise it’ll wilt and won’t photosynthesise efficiently
- loss of water vapour through stomata creates a transpirational pull
- helps to pull water from roots to the leaves
- structure of xylem and water’s adhesive and cohesive properties make this movement possible
Porous pot
- shows transpirational pull caused by evaporation of water from a porous pot
- water evaporates from surface of pot and leaves of twig pulling more water up from beaker
- a net upwards movement of coloured water in tube is observed
Potometer
- models transpiration
- inside surface of pot absorbs water, which then evaporates
- cohesion between water molecules causes water to be drawn into pot from potometer- similar manner to transpiration water movements
- similar changes in transpiration rates for both pot and plant can be observed for changing environmental conditions
- a useful model that demonstrates passive nature of transpiration
Xylem vessels
- in vascular plants
- transports water from roots to all other parts of plant
- long continuous tubes that run from roots through stems of plants
- walls of xylem vessels are thickened w/ lignin (a woody tissue), can be in form of a thickened wall, rings or spirals
- pressure inside is lower than atmospheric pressure- but they don’t break because support from lignin makes tubes v. strong and rigid
- when formed, they’re elongated living cells, but when mature, they’re no longer living- lose their cytoplasmic content and are converted to hollow tubes that transport water within plant
- they’re no longer living, so flow of water is driven by passive forces
Lignin
- woody tissue
- walls of xylem vessels are thickened w/ lignin
- prevents walls of xylem vessels from collapsing under pressure
- adds strength to woody material of older plants
Primary xylem vessels
- first xylem that forms from root or shoot tip
- walls contain v. little lignin in form of annular and spiral thickening
Process of transpiration
- Leaves lose water vapour through their stomata
- More water evaporates from mesophyll cell walls into intercellular spaces, replacing water vapour
- Water adheres strongly to cellulose in cell walls
- Loss of water vapour from mesophyll cells causes water to be drawn from neighbouring xylem vessels- movement of water occurs via small pores present in cellulose cell wall
- Hence, adhesive property of water, and evaporation, generates tension in leaf cell walls that generates transpiration pull
- This causes upward movement of water in xylem vessels to replenish water lost through transpiration
Cohesion
Water molecules are polar and stick to each other
- allows water to be moved up over long distances
Adhesion
- water molecules are polar
- polarity interacts w/ hydrophilic parts of xylem vessels
- interaction between water and wall of xylem vessel
Cohesion and adhesion in transpiration pull
Result of these forces is a continuous stream of water through plant
- supported by structure of xylem vessels
- due to their lignin deposition, xylem vessels can withstand great pressure
- also, transport water under tension without breaking
Root hair cells
- water and minerals, that the plant needs, are taken up by the roots of a plant from the soil
- root hair cells are tiny extensions on specialised root cells that take up water through osmosis
Water uptake through osmosis in root hairs
- Water enters root hairs by osmosis
- Water passes across root, from cell to cell by osmosis
- it also seeps between the cells - Water is drawn up by xylem vessels
- transpiration is constantly removing water from the top of them
Transpirational pull caused by transpiration sucks water from epidermal root hair cells deeper into root
- ultimately into xylem vessels
Once in root, water can move to xylem in 2 ways
- Through apoplast:
- water moves through cell walls of epidermal cells of the root - Through symplast:
- water moves through cytoplasm of epidermal cells of the root
Endodermis
An inner layer of cells that surrounds the core of the root and the vascular tissue
- from endodermis onwards all water flows through symplast pathway to reach xylem vessels
Casparian strip
- a band of suberin, waxy substance that is impermeable to water
- found in cell wall of endodermis of plant roots
Mineral uptake in the root
- conc. of minerals is higher in root hair cells than in soil surrounding root
- protein pumps in plasma membrane of root hair cells actively pump minerals into cytoplasm
- this causes absorption of water by osmosis
- hence, minerals are transported while dissolved in water to xylem vessels
Xerophytes
Plants that have adapted to thrive under dry conditions
- adapted to survive in environments where water is scarce
Halophytes
Plants that can survive in environments of high salinity
- plants that have adapted to living in or near the sea
- have evolved mechanisms to cope w/ high levels of salt
Adaptations of xerophytes
- Leaf size is reduced
- minimise SA through which water can be lost - Stems or leaves are succulent to store water (if these aren’t reduced to spines)
- Stomata are on the plant stem w/ few if any on the leaves
- Stomata open up only during the night when it’s cooler
- so there’s less evaporation - Leaf epidermal cells are coated in a waxy cuticles to reduce water loss
- Leaves rollup to increase level of humidity around stomata
- reduces gradient for loss of water vapour from leaves - Root systems may spread out wide
- forms an extensive network of shallow roots
- or go deep into the ground to absorb water
Crassulacean acid metabolism
- During the night, when stomata are open, CO2 is absorbed and converted into malic acid
- during the day, CO2 is released from the malic acid
- this can be used for carbon fixation in light-independent reactions
NB/ cacti have evolved this system
- because only having stomata open during the night would affect photosynthesis
- as plant would quickly run out of CO2
Adaptations of halophytes
- Leaves are reduced in size
- Water storage structures develop in the leaves
- Some plants have a thick cuticle and a thick epidermis
- Stomata are sunk into pits
- Some plants have structures to remove salt build-up
- In some plants, root cells actively pump excess sodium chloride ions out into surrounding soil/water
Rate of transpiration
Determined by measuring amount of water lost from a plant (or part of it) per unit time
Internal factors that affect rate of transpiration
Internal factors: factors directly related to the plant
- root to shoot ratio
- SA of leaves
- total no. of stomata per unit leaf area
- structure of leaf, e.g. presence of hair or thick waxy cuticle
External factors that affect rate of transpiration
External factors: factors linked to environment
- light
- wind
- temperature
- humidity
- water supply
Effect of light on plant transpiration
- as light intensity increases, rate of transpiration increases
- stomata are closed in the dark
- but, as light intensity increases, stomata open and allow water vapour to escape from air spaces of leaves
- hence, bright sunlight increases rate of transpiration
Effect of wind on plant transpiration
- increase in wind velocity increases rate of transpiration
Wind speed is low:
- air surrounding a leaf becomes increasingly humid
- reduces water vapour conc. gradient from intercellular spaces to surrounding air- reduces rate of transpiration
Wind speed is high:
- humid air is carried away faster and is replaced by drier air
- this is due to a steeper diffusion gradient of water vapour between air spaces in leaves and surrounding atmosphere
- increases rate of transpiration
- if wind speed gets too high, stomata close reducing rate of transpiration
Effect of temperature on plant transpiration
- as temp. increases, rate of transpiration also increases
Higher temp.:
- provides more energy for evaporation of water from cell surfaces
- decreases humidity of external atmosphere.
High temp.:
- if temp. gets too high ( beyond 35°C), rate of transpiration gradually falls
- due to inactivity of protoplasm and closure of stomata
Effect of humidity on plant transpiration
- higher the relative humidity of outside atmosphere, lower th rate of transpiration
- lower the relative humidity of outside atmosphere, the higher the rate of transpiration
Low humidity:
- air surrounding a leaf is dry
- gradient for diffusion of water vapour from air spaces within the leaf to the outside, is steep
- Hence, rate of transpiration is high
Opposite occurs when the level of humidity in the air is high.
Effect of water on plant transpiration
- deficiency of water in soil decreases rate of transpiration
- absorption of water by roots can’t keep up w/ rate of transpiration
- leaf cells lose their turgidity
- stomata close- the plant wilts
- immediately reduces rate of transpiration.
Humidity
Refers to % of water vapour present in the atmosphere
Potometer
A device used for measuring the rate of transpiration of a leafy plant shoot
How does a potometer work?
As water is lost by leaves, water is absorbed from cute end of the shoot due to transpiration stem
Two ways of collecting data from a potometer
- measure water absorbed over a fixed amount of time
- measure drop in mass over a fixed amount of time
Sieve tube elements
elongated living cells that form the phloem tissue
- no nucleus
- no tonoplast
- no ribosomes
Sieve tube
several sieve elements are connected end to end to form a sieve tube
Sieve plates
cross walls within the sieve tubes become perforated during development to give rise to sieve plates
Sources
Photosynthesising tissues and some plant organs that export sugars to other parts of the plant
Sinks
Plant organs that can’t produce sugars, but need them for respiration or storage
Translocaiton
The steady supply of the products of photosynthesis eg. carbohydrates and other solutes eg. minerals and AA
- based on principle that some areas of the plant have too much (source) while others have too little (sink)
- direction of flow of sap is from source to sink
Sap
Water containing carbohydrates, amino acids and plant hormones
Sources
- photosynthetic tissues eg. leaves and stems
- storage organs which are unloading their stores at the beginning of the growing season
eg. germinating seeds, potato tubers and bulbs
Sinks
- roots
- developing food stores eg. fruits, seeds or new leaves
Steps of translocation
- Sugars produced by sources are actively (uses ATP) loaded into sieve tubes by companion cells
- causes conc. of solute to build up in sieve tubes - Water enters sieve tubes by osmosis from neighbouring xylem vessels
- Water is incompressible and sieve elements have a rigid cell wall
- this inflow of water creates a great deal of internal pressure
- pressure causes movement of water and carbohydrates through pores of sieve plates, down tube to the sink
- pressure that drives this mass flow is called hydrostatic pressure. - At the sink, companion cells actively unload sieve tube
- some carbohydrates are converted into starch and stored- some are used by respiring cells
- as sugars leave sieve tube, conc. of solute decreases
- leads to water moving to neighbouring vessel by osmosis - loss of water from sieve tube leads to drop in hydrostatic pressure
- this is important, it allows transport along hydrostatic pressure gradients in sieve tubes
- as phloem sap flows from source to sink , it’s transported from region of high hydrostatic pressure to one of lower hydrostatic pressure
- this is referred to as pressure-flow mechanism
Phloem loading
Process by which soluble carbohydrates (sugars) enter the phloem
- requires active transport
- resulting high conc. of carbohydrates need to be contained in the sieve elements
- so they don’t affect osmotic balance of neighbouring cells
Phloem
Column of living cells w/ perforated walls between them
- transports sap from sources to sinks
- ensures that all parts of the plant can perform the functions of life
Flow of sap
Sap flows from an area w/ high hydrostatic pressure to an area w/ low hydrostatic pressure
Structure of phloem
- consists of living cells w/ reduced cytoplasm and no nucleus
- but, cells do have membranes to maintain high conc. of solutes
- companion cells perform many of the genetic and metabolic functions of the sieve elements or sieve tube cells
- hence, sieve tube cell can maintain membrane structure necessary for high solute conc.
- reduced cytoplasm increases volume of sap that can be transported by sieve cells
- sieve elements lose most of their cellular components, eg. nucleus, cytoskeleton, ribosomes and tonoplast as they mature
- produces a tube-like structure that allows sap to flow through easily