Unit 2 Topic 4Ai Plant Structure and Function Flashcards
What is cellulose
- Main constituent of all plant cell walls
- Unbranched, straight polymer of B-Glucose by 1.4-glycosidic bonds by condensation
- Hydrogen bonds hold parallel cellulose molecules to form units called microfibrils
Describe the structure of cellulose plant cell wall (4)
- cellulose is an unbranched, straight polymer of B-Glucose joined by 1.4-glycosidic bonds
- cellulose molecules lie parallel with each other, linked by hydrogen bonds forming microfibrils
- which are arranged in layers
- microfibrils criss-cross with each other, forming matrix
Why are plant stems green in colour?
- have chlorophyll
- photosynthesis may still occur, not its main function
What are the two main functionsof plant stems
- providing support
- providing transport
Examples of plant stems providing support to the plant
- flowers: supported in way to attract pollinators
- leaves: attached and supported in the best position for obtaining maximum levels of sunlight for photosynthesis
What enables plant stems to provide support
- plant stems are strong but flexible to withstand forces of nature
How do plant stems provide transport for plants
- provide route for movement of materials around the plants
- leaves require water and mineral ions in order to photosynthesise and carry out other important functions
- transport route for substances to travel from root to leaf
- transport route for products of photosynthesis to be transported from the tissue when photosynthesis occurs to the tissue where they are neede (leaf to root or other parts)
What is the xylem tissue responsible for in terms of transports
- transporting water and minerals from the roots to the rest of the plant
What is the phloem responsible for in terms of transport
- sugars
- amino acids
- other organic compounds throughout the plant
Where is epidermis and what is its function
- located at the outer layer of the stem
- to protect the cells beneath it (doesn’t provide support)
What are the kinds of cells of the ground tissue system
- parenchyma cells
- collenchyma cells
- sclerenchyma cells
Describe structure of cell wall of xylem vessel
a) Xylem vessel cell walls consists of cellulose and lignin
b) Cellulose molecules are held by hydrogen bonds into cellulose microfibrils
c) Layers of cellulose microfibrils criss-cross at different angles, forming a mesh-like structure
d) Hemicellulose and lignin are deposited in the secondary cell wall
e) Cell wall is lignified in helical or spiral manner
f) Bordered pits, which are unlignified regions, are present between xylem vessels
Structure of cellulose vs structure of starch
Similarity:
- Both made of glucose
- Both have 1,4- Glycosidic bonds
Difference:
- Starch made of A-Glucose, Cellulose B-Glucose
- Amylopectin has some 1,6- Glycosidic bonds
- Starch composed of amylose and amylopectin, more than one type of molecule
- Cellulose is unbranched, amylopectin is branched
- All monomers same direction in starch, every other one inverted in cellulose
Explain how structure of cell walls enables to be strong and flexible
- Cellulose molecules are held by strong hydrogen bonds as cellulose microfibrils
- Flexible: microfibrils parallel to each other in layers for flexibility
- Strong: layers of cellulose microfibrils criss cross at different angles into a mesh structure for strength.
- Cellulose microfibrils are embedded in pectin
Features of parenchyma cells
- thin cell wall (no secondary cell wall but only primary)
- small, have large central vacuole
- living cells
- most of the stem consist of ground packing tissue consisting of parenchyma cells
- outer layer of the parenchyma cells may contain some chloroplast
- consists mainly undifferentiated cells
What is the function of parenchyma
main site for physiological / biochemical processes in plants
example: photosynthesis, food and mineral store
Features of collenchyma cells
- uneven thickening of cell wall (not have secondary cell wall)
- addition deposition of cellulose, pectin and hemicellulose at the corners to give collenchyma tissue its strength
- elongated, slightly larger than parenchyma cells
- living cells
Where is collenchyma located at
the periphery of stem beneath the epidermis
What is the function of collenchyma
flexible to provide support while remaining alive
- streches as plant grows
- provide flexibility to them
What are slerenchyma cells
- strong secondary cell walls made of cellulose microfibrils positioned at right angles to each other
What are the features of sclerenchyma cells
- cellulose can attach to water but cannot dissolve (have -OH group, but insoluble due to polymer)
- lignified cell wall (tensile strength to resist stretching, provide support)
- larger than parenchyma and colenchyma
- dead cells, no nucleus
What is sclerenchyma
- modified parenchyma tissue
- not involved in the transport of substances
Where is sclerenchyma found
vascular bundles in older stems in leaves
What is the function of sclerenchyma
support the increasing weight of the upper part of a developing plant
What can sclerenchyma cells develop into
- sclerenchyma fibers
- sclereids
What is sclerenchyma fibers
long cells found in bundles / cyliners around the outside of a stem / root
Explain lignification and sclerenchyma fibers
- lignin deposited on the cell walls in spiral or ring pattern ==> strong but flexible
- lignification causes cell contents to die ==> water cannot pass through lignin, cell cannot get nutrients
- cause fibres to become long and hollow tubes
- once cell wall is lignified => cell can no longer grow => plant growth is higher up in the stem
- strength is based on their length and degree of lignification
What are pits and its function
- thinner areas of sclerenchyma fibers cell wall without lignification
- allow the communication and exchange of fluid between the cells
Features of sclereids
- star-shaped cells / irregular
- completely impregnated with lignin
- very tough cells found in groups thorughout the cortex of the stem / individual in plant tissue
What is cortex
outermost part of the stem between the epidermis and vascular
List two main functions of xylem
- transport water and dissolved minerals from the roots to the photosynthetic parts of the plant
- provide structural support to the plant
What are the two main cell types within the xylem tissue
- tracheids
- xylem vessels / vessel elements
Properties of tracheids
- exists in all vascular plants
- long and thin
- tapered end walls (becoming smaller at the end)
- arranged end-to-end, connected by pits
- less efficient water transport
Properties of xylem vessels
- exists in certain vascular plants
- hollow and elongated
- empty lumen => less resistance
- perforated end-walls, end walls are fused together
- more efficient water transport
Explain why xylem vessels have more efficient water transport
- larger diameter
- perforated end walls
What is protoxylem
The first-formed primary xylem tissue during plant growth, developing early when the plant organ (stem or root) is still elongating
Properties of protoxylem
- cell walls not fully lignified = can stretch and grow
- cellulose microfibrils in the wall of xylem vessels are arranged vertically in the stem = increase strength of tube, allow tube to resist compression forces from the weight of the plant pressing down to it
How is metaxylem formed as the stem matures
- increasing deposition of lignin to the cell wall of the xylem tissue (cellulose not strong enough to resist the weight of the growing plant)
- cell becomes impermeable to water and other substances (can be used for transport)
- stronger and more supportive
- cause xylem cells to die and become hollow
- forming metaxylem
how do lignin make xylem stronger and more supportive
- holds the cellulose microfibrils together by keeping them in their parallel arrangement
- xylem does not collapse despite the decrease in the hydrostatic pressure inside the tube due to transpiration
Why do xylem cells die and become hollow due to lignin
- lignified cell walls cause it to become impermeable to substances
- blocking substance transport between xylem cells and outer environment
- cytoplasm, chloroplast starve = organelles to die = xylem cells die and become hollow
- forming a continuous column
What is metaxylem
- lignified xylem tissue consisting of mature xylem vessels
- end walls between xylem cells broken down ==> forming hollow tubes from the roots to the tip of stem and leaves
Features of metaxylem
- do not contain any cytoplasm or organelles (do not slow down the flow of water)
- do not have end plates (mass flow of water and dissolved solutes are not impeded)
How is mass flow made possible in metaxylem
- supported by cohesive (between water molecules) and adhesive (between water molecules and cellulose walls) forces
- small regions which are not lignified (pits, only cellulose): allow for the lateral movement of water and minerals between xylem vessels
What is phloem
- living tissue made of many phloem cells joined together to make very long tubes that run from the tip of the shoot to the end of the roots
- not lignified, no support function
Xylem in stem
- located towards the centre of the stem, forming central vascular bundle
- surrounded by phloem tissue
- often arranged or forming a ring or several ring within the stem
Phloem in stem
- found between the xylem and outermost layer of the stem
- forming ring around xylem
Xylem in roots
- typically located in the centre
- central vascular bundle
- radiate outward
- youngest xylem tissue towards the center, older towards the periphery
Phloem in roots
- arranged in ring surrounding the xylem
- allows for efficient transport of sugars and other organic compounds produced in the leaves to the roots for storage or growth
Xylem in leaves
- found in midrib and veins
- extending throughout leaf lamina
- provides structural support
- facilitates the transport of water and minerals absorbed by the roos to the leaf cells
Phloem in leaves
- located adjacent to the xylem in the veins
- transports organic compounds such as sugars produced during photosynthesis from the leaves to the other parts of the plants
Main function of phloem
transport organic compounds (surcrose, amino acids) from the sources to the sinks (depends on season) needed for growth or storage
(organic compound are dissolved in water to form sap)
What is translocation
Transport can occur both up and down the plant and is an active process
- movements of sugars and photoassimilates (products of photosynthesis) from the sources to sinks
What are the key cells in phloem tissue
- sieve tube element
- companion cells
What is sieve tube element
- many seive tube members
- living cells with no nucleus
- tonoplast, and some other organelles
- break down as the gaps in sieve plates are made, allows phloem sieve tubes to become filled with phloem sap
What is tonoplast
cyroplasmic membrane surrounding large vacuoles within plant cells
What is sieve plates
- walls between phloem cells are perforated
- creating specialised sieve plates
- allows phloem sap to flow through holes in the sieve plates
What is the function of companion cells in phloem tissue
- allow phloem cells to remain alive, supporting activity of phloem cells
- involved in loading and unloading sugars into phloem sieve tube elemnets
How does the features of companion cells support its function?
- infoldings in the cell membrane: increase surface area for transport sucrose into the cell cytoplasm
- mitochondria: undergo aerobic respiration to supply ATP for the active transport of organic compounds
- linked to sieve tube by plasmodesmata
What is plasmodesmata
cytoplasmic bridges between plant cells allowing communication between cells
What are other cells present in the phloem tissue that is not sieve tube element or companion cells
parenchyma (for storage), sclerenchyma fibers (strengthening)
What is cambium
- meristematic tissue
- layer of unspecialised cells between the xylem and phloem
- divide to give rise to more specialised cells that form both xylem and phloem
Structural support in small, non-woddy plants
- turgid parenchyma cells in the center of the stem provide most of the support
- sclerenchyma, collenchyma tissues also provide support
- can wilt if too much water is lost by transpirtion if they are young
Structural support in woody plants
- more lignin is deposited in the xylem tissue to increase support as they grow
How does tree truck provides support to the tree
- lignified xylem makes up most of the tree trunk
- living cells around the cambium (undergo celll division) are on the outside of the trunk
- new ring of vascular tissue is made every year
What are the two pathways for transport of water in roots
- apoplastic pathway (extracellular pathway)
- symplastic plathway (intracellular pathway)
can occur simultaneously
What is apoplastic pathway
- water moves from cell to cell along cell walls by diffusion
What is symplastic pathway
water moves from cell to cell through plasmodesmata and cytoplasm of cells by osmosis
What is transpiration
loss of water vapour from leaf surface due to evaporation
Outline the process of transpiration (5)
- water in mesophyll cells evaporate
- water vapour formed in the intercellular spaces diffuses out of the leaves through stomata
- water loss from mesophyll cells are replaced by water drawn from xylem vessels into mesophyll cells by osmosis
- creating a transpiration pull to draw water up
- cohesive force between water molecules and adhesive force between water molecules and xylem vessel cause water to moveup xylem vessel in a continuous flow
(past paper) Describe the arrangment of cellulose molecules in cell walls.
- cellulose molecules linked by hydrogen bonds
- microfibrils
- layers / sheets of microfibrils
- microfibrils arranged in mesh / criss-cross / at different angles to each other
What is the function of the large vacuole in plant cells
- storing water and minerals
- maintaining cell turbidity
What is the function of amyloplast
- stores starch in plants
- converts it back to glucose when the plant needs it
Name the organelles that only plant cells have
- chloroplast
- cell wall
- amyloplast
- tonoplast
- plasmodesma
- large vacuole
Compare and contrast xylem and sclerenchyma
Similarities
- both have pits, cellulose, dead hollow cells, secondary cell walls and lignin
Differences
- no end walls in xylem
Why do xylem have no organelles or cytoplasm
- creates more space inside the vessel for transporting water
What is the cell wall of xylem made of
cellulose and lignin
Compare and contrast parenchyma and xylem
Similarities
- they both have cell walls
Differences
- parenchyma cell wall is made of cellulose; xylem cell wall is made of cellulose and lignin
- no cytoplasm and vacuoles in xylem (a lot in parenchyma)
(past paper) Explain the importance of lignin in the transport of materials in xylem vessels (3)
- vessel are hollow tubes
- lignin needed to add strength to the vessel
- xylem involved in transport of water
- lignin needed to waterproof the vessels
Sclerenchyma or xylem vessels have absence of end walls between adjacent cells
xylem vessels only
Sclerenchyma or xylem vessels have cell membrane
not found in both
Sclerenchyma or xylem vessels have lignified cell walls
both
Sclerenchyma or xylem vessels have pits
both
(past paper) Compare and contrast the structures of phloem sieve tubes and xylem vessels (4)
Similarities
- both fibres contain cellulose in cell walls
- both have tubular structures
- both do not contain a nucleus
Differences
- phloem sive tubes have sieve plates / perforated end walls whereas xylem vessel have no end walls
- phloem sieve tubes contain cytoplasm and are not hollow, whereas xylem vessels do not contain cytoplasm and are hollow
- phloem sieve tubes contain no lignin / secondary thickening while xylem do
- phloem have plasmodesmata whereas xylem have pits
(past paper) Describe the role of phloem (2)
- transport / translocation of sucrose / amino acids
- from source to sink
(past paper) Explain with 2 reasons why plants need water (2)
- photosynthesis to keep cool
- as a transport medium
- support / turgor
- hydrolysis
(past paper) Explain how plants use inorganic ions (3)
Nitrates = amino acids = proteins like enzymes
magnesium = chlorophyll
calcium = calcium pectate in the middle lamella in cell wall
Outline the transpiration stream
- movement of water from soil through root hair cells (osmosis)
- across the root to the xylem, up the xylem
- across the leaf until water is lost by evaporation from leaf cells (in the form of water vapour)
- water vapour diffuses out of the stomata down a concentration gradient
What is transpiration pull
upward pulling force created due to transpiration on leaf surface that draws water up xylem vessels
What is root pressure
- absorption of water increases osmotic pressure in root cells
- root pressure pushes water and mineral ions upward in xylem vessels
What is the function of guard cells
regulate the amount of water lost through transpirtion by opening and closing the stomata
Why is transpiration more important at night
- light intensity is low, stomata closes to prevent water loss through transpiration
- temperature is lower, less steep concentration gradient
- increased humidity, less steep concentration gradient
- reduced airflow
Outline the translocation of organic solutes in phloem
1.sucrose secreted into phloem sieve tubes through companion cells at source by active transport
- increase sugar concentration in phloem sieve tube
- decrease in water potential in the source end of phloem sieve tube
- water is drawn from xylem to phloem sieve tube by osmosis
- entry of water increases hydrostatic pressure in the source end of phloem sieve tube
2.hydrostatic pressure
- hydrostatic pressure in source end of phloem sieve tube is larger than the sink end of the tube
- force mass flow of phloem sap along phloem sieve tube from the source towards the sink
3.sugar converted into starch for storage / consumed at sink
- sugar diffuses from phloem sieve tube to sink
- decreases in water potential in sink
- water is drawn from phloem sieve tube by osmosis
- water moves out by osmosis, decrease hydrostatic pressure in sink end of phloem sieve tube
Why do we use plant fibres to make products rather than oil-based plastics
- less fossil fuels are used
- plants can be replanted for the next generation
- plant fibres are biodegradable (can be broken down by microbes, minimising environmental pollution)
- extracting and processing oil is expensie and difficult compared to growing and processing plants (easier for developing countries)
What is the disadvantage of using plant fibres than plastic ones
- not as strong typically
Why do plant fibres have good tensile strength
- fibres are very long long sclerenchyma cells and xylem tissue
- exists in bundles of fibres which are stronger than individual cells
- fibres not easily broken by pulling (under tension)
- cellulose and lignified cellulose are not easiily broken down by chemicals or enzymes
What substances of the plant fibres can be dissolved or removed
- matrix of pectates
- other compounds around fibres (including lignin)
What is retting
process using natural decomposers to break down materials around fibres (maxtrix of pectates)
- can be replaced by manufacturing processes using chemicals and enzymes to speed up the process
What is the processing process of cotton
- produced in the form of almost pure fibers packed around seeds
- retting / other treatment are not needed
- cotton fibre cells must be processed (spinning) to increase strength of each fibres
What is spinning
pulls out short, single fibres and twists them together to form a long, continuous thread
- increases strength of each fibres
Comparison of cotton and oil-based polyesters
- cotton can absorb water
- more breathable and comfortable (?)
What is the composite material
- made of at least two materials combined together
- with different properties from either of the constituent materials
What is the composite material of wood
- lignified cellulose fibers embedded in hemicelluloses and lignin
What are the properties of wood
- cellulose fibres = very resistant to compression (squeezing by weight)
- flexible due to intermeshing cellulose fibres
- good insulator of heat
- sustainable resource if planted
- can be carbon neutral when wood is burned
What is the application of wood
resistant to compression
- weight-bearing in buildings
- supported columns, horizontal beams
Explain why no net carbon is released when wood is burned
- takes carbon as it grows
- releases carbon as it burns
What are the limitations of wood
- difficult to extract because the matrix around cellulose fibers contains a lot of lignin
- need to be soaked in very strong alkalis (caustic soda) to produce a pulp consisting of cellulose and lignified cellulose in water => pressed onto frames ==> dry to make paper
Describe an investigation to test tensile strength of different materials (5)
- use five types of plastic / fibres of equal length
- clamp the strips between two stands and add bubble wrap in a tray below the plastic strips
- add a 10g slotted weight loop in the middle of the strip, continue adding 10g slotted weight onto the hook until the plastic / fiber breaks
- record the total mass required to break
- repeat for each type of plastic / fibre for at least 3 times and calculate the mean
What are bioplastics
- made from biological polymers (starch, cellulose)
Examples of bioplastics
- polylactic acid (PLA)
- thermoplastic starch
- poly-3-hydroxybutyrate (PHB)
- cellulose-based plastics
Properties and uses of polylactic acid (PLA)
- mostly produced from maize / sugar cane
- similar properties to polyethene, but biodegradable
- computer casings, mobile phones, drinking cups
Properties and uses of thermoplastic starch
- mixture of starch extracted from potatoes and maize and other compounds like gelatine
- smooth and shiny plastic
- pharmaceutical industry to make capsules containing drugs
- easily swallowed and digested
Properties and uses of poly-3-hydroxybutyrate (PHB)
- made from products of sugar industry
- stiff biopolymer like polypropene
- ropes, bank notes, car components
Properties and uses of cellulose-based plastics
- wood pulp
- food wrappers
Advantages of bioplastics
- high sustainbility (replaced at sustainble rate)
- biodegradable (broken down by microorganisms, decrease amount of plastic pollution)
- burning bioplastics (release energy, avoid production of methane)
Disadvantages of bioplastic
- economic considerations (small cost advantages, technology still new)
- ethical consideration (limited food crops dilemma, not always have same useful properties)
What are the adaptaions of root hair cells
- long to increase SA for absorption (increase rate of transport)
- many mitochondria (aerobic respiration to produce ATP energy, active transport of mineral ions to root hair cells)
Absorption of plants
- water and mineral ions are absorbed through root hairs on the root and travel up the stem in xylem vessels
- mineral ions are absorbed by active transport
- water absorbed by osmosis
Solute concentration / water potential inside and outside root hair cells
inside root hair cells
- higher solute concentration
- lower water potential
outside root hair cells
- lower solute concentration
- higher water potential
Functions of water for plants
- photosynthesis
- transport medium for minerals (dissolve polar substances like mineral ions)
- maintain turgidity to support plants
- cooling effect (carries heat as it evaporates, decrease chance of enzyme denaturation)
- hydrolysis
How do water maintain turgidity to support plants
- water moves into vacuoles of cells by osmosis
- cytoplasm becomes pressed against cell walls, making them turgid
Cohesion of water and plants
- forces between water molecules
- allow for movement of water from roots to leaves of plants in transpiration stream due to transpiration
Adhesion of water and plants
- forces between water molecules and hydrophilic molecules
- water molecules attracted to cellulose in the cell wall of xylem vessels and this contributes to the formation of the transpiration stream
Why can’t plants grow in clay / sand
- no air space between clay / sand particles as they are very close to each other
- mitochondria inside root hair cells cannot receive oxygen from the atmosphere
- cannot undergo aerobic respiration to produce ATP energy
- less or no active transport of mineral ions into the root hair cells
- may die
What is overwatering and its consequences
- oxygen cannot reach the roots well because water occupies the spaces between soil, instead of oxygen, so less airspace is available
Function of Calcium ions
- synthesis of calcium pectate in the middle lamella of plant cell walls (holds microfibrils in cell walls together, holding neighboring plants cells together)
- important for the permeability of membranes
Functions of magnesium ions
- synthesis of chlorophyll (proves green color, capture light to convert co2 and water into glucose and oxygen in photosynthesis)
- activation of plant enzymes, synthesis of nucleic acids
What happens if there is deficiency of calcium ions
- growing points die back
- yellow and crinkly leaves / young leaves become yellow
What happens if there is deficiency of magnesium ion
- less chlorophyl produced, plant die as cannot photosynthesis
- older leaves develop yellow areas, stunted growth
Function of nitrate inos in plants
- synthesis of amino acids in protein, nucleic acid (DNA / RNA), chlorophyll, enzymes where nitrate ions are needed
- important for plant growth, production of fruit and seeds (ignored in pp)
What happens if there is a deficiency of nitrate ions
- older leaves yellow and die
- stunted growth
What happens if there is a deficiency of phosphate ion
purple patches on leaves
What happens if there is a deficiency of potassium
curled-up leaves with dark-colored edges
