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BLGY1115 > Plant Cells and Cell Walls > Flashcards

Plant Cells and Cell Walls Flashcards

1
Q

Cell Walls and Growth

A
  • plants have a very distinctive mode of growth
  • they are constantly producing new organs and extending their surface into air and soil
  • but there is no relative movement of cells in a developing plant organ
  • this means that there have to be specific mechanisms for cytokinesis, cell expansion and cell adhesion/separation
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2
Q

Cell Walls in Relation to the Whole Plant

A
  • cell walls are abundant
  • 70% of assimilated carbon from photosynthesis ends up in cell walls
  • cell walls are the most abundant renewable resource on earth
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3
Q

Cell Walls and Biofuels

A
  • cell wall biomass is a renewable resource

- they burn effectively

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4
Q

How many types of plant cell wall are there?

A

There are two types, primary cell walls and secondary cell walls
Secondary cell walls are found inside the primary cell wall

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5
Q

Primary Cell Walls

A
  • present in all cells
  • tensile properties so can resist stretching
  • resist internal turgor pressure to prevent bursting
  • extendable so the cell is able to expand
  • rigid so hold cell shape
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6
Q

Secondary Cell Walls

A
  • only present in certain cells
  • found in xylem and sclerenchyma fibres
  • resist compressive forces
  • prevent cell collapse
  • thick, made up of three layers, s1, s2, s3
  • don’t contain pectin
  • often lignified
  • cannot extend
  • all cells start with a primary cell wall and the secondary cell wall is deposited afterwards
  • the protoplast must contract to allow it to form
  • deposited after growth is complete
  • cells with secondary cell walls are often dead at maturity
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7
Q

Cell Wall Molecular Structure

A
  • polysaccharides including cellulose and pectins
  • carbon, hydrogen, oxygen
  • -no nitrogen
  • great complexity
  • more ways of linking than amino acids in proteins
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8
Q

Eukaryotic Cell Walls

A
  • fibrous composites
  • -fibres embedded in matrix
  • -components are largely polysaccharides
  • fibres give tensile strength and rigidity
  • matrix components connect spaces and fibres to control cell wall porosity
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9
Q

Cellulose

A
  • 1-4 glucan chains
  • straight chains hydrogen bonded together to form fibrils
  • pectins between cellulose
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10
Q

What polymers are found in cell walls?

A
  • 1/3 cellulose
  • 1/3 non cellulose non pectic matrix glycans
  • 1/3 pectins
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11
Q

What happens when you remove the cell wall from a plant cell?

A

-cell wall can be removed with cell wall degrading enzymes
-this forms a protoplast
-which is circular in shape
but if you leave a protoplast in an appropriate nutrient medium it will construct a new cell wall

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12
Q

Cell Elongation

A
  • cellulose matrix glycan network is stretched and microfibrils slide apart
  • extensibility of the cell wall is determined by cross links between the microfibrils
  • new cell walls material is added to maintain a constant thickness as the cell wall expands
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13
Q

What determines the direction of cell expansion in cell elongation?

A
  • the orientation of the cellulose microfibrils
  • elongation occurs at right angles to the microfibrils
  • transversely orientated microfibrils lead to cells expanding in a cylindrical shape
  • other cell wall factors control the extent of cell expansion
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14
Q

What are the two mechanisms of turgor driven cell growth?

A
  • Diffuse Growth

- Tip Growth

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15
Q

Diffuse Growth

A
  • every part of the cell expands by the same amount

- observed in adhered cells in multicellular organs

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16
Q

Tip Growth

A
  • most of the cell does not expand
  • the cell elongates at one end only
  • observed in root hairs and pollen tubes, i.e. single unadhered cells
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17
Q

Cytokinesis in Cell Walls

A
  • no contraction in the middle like in animal cells
  • new cell wall constructed across a cell leading to the formation of two daughter cells
  • the division constructed down the middle is called the cell plate
  • primary cell wall material is deposited on both sides forming the middle lamella
  • the middle lamella is the junction between adjacent plant cells
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18
Q

Function of the Microtubules of the Plant Cytoskeleton

A
  • orientation of cellulose microfibrils
  • separation of chromosomes in mitosis
  • orientation of cell plate before mitosis
  • construction of the cell plate
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19
Q

Cell Plate Formation

A
  • during cytokinesis, the cell plate is synthesised from the centre of the cell
  • the cell plate portions the cell into two daughter cells
  • as the cells grow, primary cell wall material is deposited on both sides of the plate which becomes the middle lamella
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20
Q

Intercellular Space Formation

A
  • a region of the primary cell wall is dismantled allowing a new middle lamella from cytokinesis to join with an existing middle lamella
  • they join in such a way that a space is created between the two cells
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21
Q

What causes cell adhesion in plants?

A

cytokinesis, when new plant cells are formed they are already adhered to the cells around them
plant cells have fixed neighbours for life as plant cells do not move relative to each other

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22
Q

Plasmodesmata

Structure

A
  • plasma membrane lined pores across cell walls connecting adjacent plant cells
  • contain endoplasmic ER
  • often grouped together in thinner regions of the cell wall called pit fields
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23
Q

Plasmodesmata

Function

A
  • transport small molecules
  • viruses can exploit them to move from cell to cell
  • there is evidence that they are used for movement of transcription factors and nucleic acid that can regulate gene expression and influence meristem development
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24
Q

Apoplast and Symplast

A
  • Plant tissues and organs can be viewed as constituting two continums separated by the cell plasma membrane
  • the apoplast consists of the cell wall and intercellular space outside the protoplast
  • the symplast is everything inside the protoplast - cytoplasm, ER, etc.
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25
Q

Are phloem and xylem transport symplastic or apoplastic?

A

phloem transport = symplastic

xylem transport = apoplastic

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26
Q

Plant Cell Types

A
  • 40 different cell types
  • specialised structures with diverse function
  • each with specific set of genes
  • all produced from meristematic cells
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27
Q

What tissues are part of the tissue system dermal?

A

epidermis

28
Q

What cells form the epidermis?

A
  • epidermal cells

e. g. stomatal guard cells and root hair cells

29
Q

What tissues are part of the tissue system ground?

A
  • parenchyma
  • collenchyma
  • sclerenchyma
30
Q

What cells form the parenchyma?

A

parenchyma cells

31
Q

What cells form the collenchyma?

A

collenchyma cells

32
Q

What cells form the sclerenchyma?

A
  • sclerenchyma fibres

- sclereids

33
Q

What tissues are part of the tissue system vascular?

A
  • xylem

- phloem

34
Q

What cells form the xylem?

A
  • vessel elements
  • tracheids
  • fibres
  • parenchyma
35
Q

What cells form the phloem?

A
  • sieve tube elements
  • companion cells
  • fibres
  • parenchyma cells
36
Q

Possible Cell Modifications

A
  • cell shape
  • cell wall thickening (even or uneven, primary or secondary)
  • secondary cell wall
  • cell separation
37
Q

Meristematic Cells

A
  • stem cells for all cell types
  • small (10-50μm)
  • isodiametic (same length in every dimension)
  • nucleus takes up most of the cell
  • no vacuole
  • thin primary cell wall
  • no intercellular space
38
Q

Parenchyma Cells

A
  • packing cells
  • large
  • vacuolated
  • thin primary cell walls
  • intercellular spaces
  • elongated
  • cylindrical
  • occur in ground and vascular tissue
39
Q

Collenchyma

A

-structural and supportive tissue in growing organs

40
Q

Collenchyma Cells

A
  • occur in bundles at corner of stems/petioles in the sub epidermal cortex
  • never occur singly
  • uneven thickening of the primary cell wall
  • no intercellular space
  • cells firmly adhere together
41
Q

Sclerenchyma

A

-structural and supportive tissue of non-extending organs

42
Q

Sclerenchyma Fibres

A
  • evenly deposited secondary cell walls
  • thick secondary cell wall
  • elongated
  • in bundles associated with vascular tissue
43
Q

Scelereids

A
  • ‘stone cells’
  • evenly deposited secondary cell wall
  • thick secondary cell wall
  • irregular shape
  • occur singly
  • capable of intrusive growth, into the gaps between other cells
44
Q

Vascular Tissues

A
  • xylem and phloem form continuous linked systems throughout the plant body
  • arrangements depend on the organ and species
45
Q

Xylem

A
  • water and mineral ion transport
  • apoplastic transport
  • ascent of xylem sap depends on transpiration and the physical properties of water as xylem cells are dead at maturity
  • adhesion of water to the hydrophilic surface of cells and cohesion of water molecules puts xylem cells under tension
46
Q

Tracheary Elements

A
  • vessel elements

- tracheids

47
Q

Vessel Elements

A
  • discontinuous secondary cell wall
  • lignified cell walls
  • dead at maturity
  • elongated
  • joined end to end
  • no end walls
  • water passes from cell to cell through the gap at the end
48
Q

Tracheids

A
  • discontinuous secondary cell wall
  • lignified cell walls
  • dead at maturity
  • tapered ends
  • -gaps in secondary cell walls in pit pairs
  • water passes from cell to cell through the primary cell wall at pits
49
Q

Tracheids

Simple Pits

A

narrow path for diffusion of water

straight edges of secondary cell wall

50
Q

Tracheids

Bordered Pits

A
  • larger area for diffusion of water

- secondary cell wall pulled back

51
Q

Trachery Element

Structure

A
  • a balance between the capacity to store water and the ability to avoid collapse
  • primary cell wall permeable to water
  • primary cell wall reinforced in places by a secondary cell wall which is impermeable
52
Q

Deposition of Secondary Cell Wall in Protoxylem a

A
  • annular (rings) and helical
  • to allow the cell to elongate in between the sections with secondary cell wall as secondary cell wall cant stretch and protoxylem is the first xylem of primary growth in axially extending organs
53
Q

Deposition of Secondary Cell Wall in Metaxylem a

A
  • scalariform (ladder), reticulate and pitted
  • higher proportion of primary cell wall is covered than in protoxylem
  • extra strength form more secondary cell wall allow the cells to be bigger without collapsing
  • more secondary cell wall is OK in metaxylem as it is the later xylem of primary and secondary growth so is found in non-elongating organs and the cells don’t need to be able to elongate
54
Q

Phloem

A
  • transport of photosynthetically produced sugars

- translocation - pressure flow along gradient from source cells to sink cells

55
Q

Phloem

Conducting Cells

A

-sieve tube elements and sieve cells

56
Q

Conducting Cells

A
  • parenchyma cells with modified plasmodesmata and pit fields
  • plasmodesmata are enlarged in sieve areas forming sieve pores
  • cell contents are under pressure so a secondary cell wall to prevent collapse is not necessary
  • retain cytoplasm
  • no nucleus so are regulated by companion cells that do have a nucleus
57
Q

Sieve Tube Elements

A

-sieve pores concentrated at end walls, called sieve plates

58
Q

Sieve Cell

A
  • tapered ends
  • no distinct end wall
  • sieve areas distributed over surface
59
Q

Developmental Plasticity

A
  • as a plant grows it can modify its development

- no precise, final fixed form to a plant

60
Q

Developmental Plasticity

Response to Environment

A
  • plant embryos are simple
  • all organs are produced post embryonically
  • meristem activity is iterative and open ended
  • meristem activity can respond to conditions e.g. light, wind
    e. g. flowering in Arabidopsis is promoted by long day conditions
61
Q

Developmental Plasticity

Growth Correlations and Developmental Arrest

A
  • morphogenetic activity restricted to meristem
  • growth is modular
  • meristems are influenced by activity of other meristems
  • coordination of growth and developmental signals throughout plant body e.g. apical dominance
62
Q

Apical Dominance

A
  • actively growing apex inhibits the growth of lateral meristem
  • when the apex is cut of, lateral buds grow
  • when the apex is cut off but replaced with a block of auxin, lateral bud growth is still inhibited
63
Q

Plant Hormones

Properties

A
  • small molecular weight compounds
  • several are weak acids
  • several have multiple forms
  • not produced in glands
  • all cells can produce hormones
  • can flow between organs
  • mediate the effects of the environment
  • have multiple functions
  • pleiotropic effects
64
Q

Developmental Plasticity

Cell Totipotency

A
  • capacity of individual cells to proliferate and produce and produce whole organisms
  • to express the genetic potential of the whole organism
  • plant cell differentiation is not irreversible
  • cells can enter new pathways when subject to a change in developmental context
  • cells maintain full genome
  • whole plants can be regenerated from parts of plants or isolated single cells
65
Q

Which cells are totipotent?

A

all mature and immature cells EXCEPT xylem, sclerenchyma and phloem sieve tube elements

66
Q

Experimental Expression of Totipotency

A
  • plant cell culture
  • somatic embryogenesis - formation of embryos from cultured cells
  • regeneration of plants from protoplasts
67
Q

Plant Cell Culture

A

1) excise tissue sections and keep sterile
2) keep alive with nutrients
3) induce cell division to form a callus
4) grow callus on solid or liquid medium
5) manipulate growth and differentiation by adding hormones (e.g. auxins, cytokinins)
6) conditioning medium containing plant growth factors (e.g. phytosulfokines)

BLGY1115 (10 decks)

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