Increase Expansion: in Cell Size
Lets define some terms:
Division: Increase in Cell Number Growth: Increase in Cell Size and/or Cell Number
Gap #1 (Cell Expansion)
The Cell Cycle:
G1
telophase anaphase
Mitosis
Mitosis/Cytokinesis (Cell Division)
metaphase prophase
M
Interphase
S
Synthesis (DNA Replication)
G2
Gap #2 (Cell Expansion)
Early plants were unicellular, and several kinds of green algae still get along just fine as a single cell.
Some advantages to being unicellular: 1. EVERY cell is a stem cell and can contribute to the next generation 2. Extracellular resources (e.g., nutrients) are only 1 membrane away 3. All of the energy you make is yours to keep. Major disadvantage of being unicellular: 1. EVERY cell has to be good at EVERYTHING!
Unicellular Cell Cycle:
Expansion
G1
DNA Replication (synthesis)
M G2 S
Premitosis (G2) Mitosis (M)
usually very short in unicellular organisms
All the cells of unicellular organisms must be capable of making more copies of themselves
Every cell can potentially divide for ever, and is thus:
etc.
indeterminate
Every cell can do every function of that cell, and is thus:
undifferentiated
But, in some organisms, especially in multicellular organisms, not every cell is capable of dividing (i.e., they are determinate), nor is every cell able to do every function (i.e., they are differentiated). Instead, some cells differentiate to take on new functions.
For example, in this cyanobacterium, cells called heterocysts differentiate into cells that specialize in fixing nitrogen. These cells then no longer grow and divide, or contribute to the next generation. (These cells are terminally differentiated.)
Nostoc
(a cyanobacterium)
The secret to true, complex multicellularity is DIFFERENTIATION!
A differentiated cell has undergone a change in cell fate compared to the normal cells (=undifferentiated). In most cases, the process of differentiation leads to the cell leaving the cell cycle.
Multicellular Cell Cycle:
G1 Arrest or G0
G1
Cells that leave within the G1 phase are said to be G1 arrested, or more commonly in G0 (G-zero). While cells that exit in G2 are G2 arrested. MOST CELLS in complex multicellular organisms are in G0, though some cells types arrest in G2 instead. Typically, only very specialized cells are allowed to freely proceed through the cell cycle (stem cells)
M
G2 G2 Arrest
S
Why dont cells exit the cycle in S or M phase?
Cell populations and cell fate:
differentiation = an axis of developmental/functional specificity
from undifferentiated stem cells to differentiated specialized cells.
determination = an axis of growth/division potential
from indeterminate (forever dividing) to determinate (limited/no growth)
never divides again
terminally differentiated differentiated, and determinate
parenchyma cell phloem sieve element or xylem trachied
determination
Secondary meristems
root pericycle or stem vascular cambium
Primary meristems
protoderm, ground meristem and procambium
divides/grows forever
Apical meristems
SAM, RAM
can make any cell type
differentiation
terminally differentiated
filamentous forms are common in many branches of life, and are probably the first step towards multicellularity
Bacteria
Nostoc
(a cyanobacterium)
bikonts
Spirogyra
(a charophyte green algae)
unikonts
Neurospora
(a filamentous fungus)
sheet forms are also common in many branches of life, and represent a second dimension towards multicellularity
bikonts
thallus = the body of a multicellular algae
Coleochaete
(a charophyte green algae)
unikonts
Tricoplax (a primative animal)
The Heterocyst of Nostoc and Anabaena: A simple example of differentiation
Nostoc
(cyanobacteria)
Anabaena sphaerica
(cyanobacteria)
Undifferentiated, photosynthetic (CO2-fixing, O2-producing) cells
Heterocyst: Terminally differentiated, non-dividing, non-photosynthetic, N2-fixing cells
The photosynthetic cells are undifferentiated, and continue to divide at will. The only way that a heterocyst can form is through differentiation of one of the other cells! In response to a developmental signal, one cell in the chain (*) will begin to differentiate: stopping photosynthesis degrading the thylakoids, and then expressing the machinery necessary for N2-fixation.
*
*
*
*
This kind of cyanobacteria actually knows how to count: Heterocysts will only form ~7-9 cells away from the last heterocyst! Think about how this could happen
Complex multicellularity can observed as steps with higher levels of cell-cell adhesions:
These are examples of extant plants, not all of which are in the same lineage. A similar series can be for animals
Chlamydomonas (chlorophyte)
unicellular (0-D)
Spirogyra (zygmenophyte)
filamentous (1-D)
(each cell contacts 2 other cells)
Coleochaete
(coleochatophyte)
one cell layer thick (2-D)
(each cell contacts 4+ cells)
two cell layers thick (3-D)
Ulva sea lettuce
(ulvophyte)
Both layers function the same
Marchantia liverwort
(embryophyte)
many cell layers thick (3-D w/ differentiation) The different layers function differently!
At least SEVEN different eukaryotic lineages produced complex, macroscopic, multicellular organisms, each from different unicellular ancestors
Mus mouse (mammal)
Agaricus button (basidiomycete)
Physarum (myxogastrian)
Macrocystis kelp (phaeophycean)
Porphyra nori Ulva sea lettuce (ulvophyte) (floridiophyte)
Pinus White Pine (gymnosperm)
Metazoans
animals
Dikaryans
complex fungi
Mycetozoans
plasmodial slime molds
Phaeophyceans
brown sea weeds
Eurhodophytes
red sea weeds
Ulvophytes
green sea weeds
Embryophytes
(land plants)
Gloeobotrys (xanthophycean)
Monosiga (choanoflagellate)
Allomyces (chytridiomycete) Amoeba (lobosean)
Rhodella (rhodellophyte)
Chlorella (trebuxiophyte)
Chlorokybus (chlorokybophyte)
OPISTHOKONTA
CHROMALVEOLATA
Rhodoplantae
Viridiplantae
AMOEBOZOA
ARCHAEPLASTIDA
UNIKONTS
BIKONTS
many unicellular lineages not shown
Major groups of the Embryophytes
Land plants Red lines indicate major innovations. Width of triangles indicates approximate numbers of extant species.
Monocots
~65k
one cotyledon
Eudicots
tricolpates
~200k
Magnoliids
~10k
tricolpate pollen
Angiosperms
xylem vessels
Gymnosperms
fruit flowers
Spermatophytes cotyledons (2)
(ferns, horsetails, etc.)
Monilophytes
~20k
seeds pollen true wood (i.e., a bifacial cambium)
Euphyllophytes
true leaved plants
true leaves
(i.e., megaphylls)
(club mosses, quilworts, etc.)
Lycopodiophytes
Bryophytes
Mosses
~12k
Tracheophytes
vascular plants
Anthocerophytes
Hornworts
land plants
true roots true vascular system (i.e., tracheids, sieve elements) Marchantiophytes Liverworts
Embryophytes Other charophyte algae
land multicellular sporophyte embryo
Plant stem cell populations:
The extra-embryonic tissues of the seed will be a combination of any residual endosperm surrounded by maternal-derived tissues
Endosperm Maternal SAM Embryonic tissues RAM
Endosperm Zygote
3n 2n
SAM Embryonic tissues RAM
The zygote is totipotent, and contains the potential to make every kind of cell, including all of the embryonic stem cells.
(The endosperm can only make endosperm, and are unipotent stem cells.)
The embryonic stem cells are determinate, and only divide to make all the cells of the embryo, including the indeterminate cells of the SAM, RAM and the determinate cotyledon(s).
At this point, both meristems go dormant while the cotyledons typically begin to store nutrients (either by digesting the endosperm or importing from maternal tissues) and then dehydrate with the rest of the seed to await conditions for germination.
Why do we say eudicots now, instead of just dicots?
Two cotyledons is the ancestral state of seed plants!
Cucurbita cotyledons (2) (Eudicot) pumpkin Eudicots
tricolpates
Allium cotyledon (1) (Monocot) onion
Sequoiadendron cotyledons (3-5) (Cupressales) giant sequoia
~200k
Monocots
~65k
one cotyledon tricolpate pollen
Abies cotyledons (3) (Pinales) fir Pinus cotyledons (8) (Pinales) pine Pinales
Juniperus cotyledons (2) (Cupressales) juniper Cupressales
Magnoliids Laurus cotyledons (2) (Magnoliid) bay leaf
~10k
Amborella cotyledons (2) (basal angiosperm) Early-diverging Angiosperms
xylem vessels
variable # cotyledons
Angiosperms
Gnetales Welwitschia cotyledons (2) (Gnetales)
Ginkgo and Cycads keep their two cotyledons in the seed coat
Ginkgoales Cycadales
Gymnosperms
fruit flowers
seed plants
Spermatophytes seeds pollen
Major groups of the Spermatophytes (seed plants)
Red lines indicate major innovations. Width of triangles indicating approximate numbers of extant species.
two cotyledons
true wood (i.e., a bifacial cambium)
Plant stem cell populations:
Endosperm Maternal SAM Embryonic tissues RAM
SAM
Endosperm Maternal SAM Embryonic tissues RAM
SAM (shoot tissue) Embryonic tissues RAM (root tissues)
Half the cells produced by the SAM/RAM become the determinate primary meristems.
RAM
Fig 4.23c
Upon germination, the seed imbibes water and the meristems reactivate. The first response is to initiate a root and then to expand the hypocotyl (without cell division).
Following germination, the SAM and RAM are indeterminate, and potentially grow forever.
Fig 4.23a
Apical Meristems: SAM (Shoot Apical Meristem) is the source of the shoot and leaf tissues (the aerial organs) RAM (Root Apical Meristem) is the source of the root tissues (the subterranean organs)
SAM converted to floral meristem
lateral shoot meristem
SAM Embryonic tissue RAM
lateral root meristem
Lateral Meristems: Lateral Shoot Meristems: quiescent (dormant) pieces of meristem that are stored at the nodes. Create branches of the shoot. Lateral Root Meristems: Formed de novo from cells in the root cortex. Create lateral roots. Once they are formed, they are indistinguishable from the Apical meristems!
Plant stem cell populations:
Endosperm Maternal SAM Embryonic tissues RAM
SAM (shoot tissue) Embryonic tissues RAM (root tissues)
SAM protoderm ground procambium RAM
Following germination, the SAM and RAM are indeterminate, and potentially grow forever. Half the cells produced by the SAM/RAM become the determinate primary meristems. The determinate primary meristems: protoderm, ground, and procambium then make the remaining determinate cells of the plant organs. Some of the cells made by the primary meristems retain can some stem cell activity (secondary meristems), and some can de-differentiate back to limited stem cell activity, but most are fully differentiated and determinate cells.
Primary Meristems, are produced directly by the SAM or RAM, and are responsible for producing the three main tissue layers of plants.
Fig 4.22
Primary Meristems make three basic tissue systems:
Each of these tissues are made at different parts of the meristematic regions
Fig 4.23b
Dermal: the epidermal layer and some specialized cells
Ground: regular cells that do most of the major housekeeping tasks of plant life
Procambium: vascular cells that make-up the water and nutrient transport systems
Fig 7.8 Fig 4.3c
Dermal tissues:
epidermal cells are alive, but non-photosynthetic at maturity. They are elongated and often irregular in shape (especially in leaves) Their outer walls contain cutin, the waxy substance that forms the cuticle.
epidermal cell
hair cell (trichome or root hair)
guard cells (surrounding a stomata)
leaf epidermis
(Fig 4.16b)
root epidermis
hair cells are extensions out from the surface that serve roles in protection or secretion (trichomes), or in nutrient absorption (root hairs) guard cells come in pairs that surround and control the aperture of a pore in the epidermal surface (stomata)
Ground tissues:
ground tissues are relatively simple, having mainly one kind of cell, but they do most of the things that plants do. The major cell of the ground tissue is made from parenchyma cells, and aggregates of these cells make up the majority of the volume of parenchymal tissues such as the mesophyll of leaves, the cortex of stems and roots, and pith of the stems.
Stem cross-section, Fig 4.3a
Root cross-section, Fig 4.3b Leaf cross-section, Fig 4.3c
Vascular tissues:
vascular tissues carry water and nutrients through the plant organs, and some of the most differentiated cells of the plant occur within the vascular system.
xylem: water-transporting tissues made from cells that are dead phloem: sugar-transporting tissues made from cells that are barely alive
bundle sheath phloem sieve tube vascular bundle phloem companion cell phloem parenchyma
xylem tracheary element xylem parenchyma
Cell walls/ECMs are generally made from 3 components:
PLANTS
STRUCTURAL FIBERS
Cellulose
poly-(-(14)-glucose)
long polymers of -(1>4) linked Glucose
CROSSLINKERS
hemicelluloses
e.g., xyloglucan
intermediate-length polymers of -(1>4)-linked Glucose with -(1-6)-linked Xylose that hold the long fibers of cellulose together.
GEL/HYDRATED MATRIX
pectins
e.g., rhamnogalacturonan
complicated(!) polysaccharides mainly of galacturonic acid and rhamnose that serve to make a gel support system that holds everything in place.
Just a little glimpse of pectin think about this next time you eat jelly
O O
O O
O O
O
O
O
O
O O
O O O
O O O O O O
O O
O O O O
O O
O
RG-I
O O
O
O O
O O O O O
O O O O
O O
O O
O O
O O
O O O O O O
O O
O O
O O
O O
O O
O O O O
O O O O
O O
O
O
Ca2+
O
O
O
O
O
Ca2+
O O O
Ca2+
O O O O O O
O O O O O O O
O
O
O O O O O O
HGA
O
O
O
O
O
O
O
O
O
O
B
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O O
O
Two RG II monomers cross linked through a boron diester.
O O
O O
O
The role of pectin in the cell wall:
Jell-o with suspended fruit
Pectin gel with suspended cellulose fibers
Pectin provides a hydrated gel that can surround and suspend particles, restricting their movement. By holding and retaining water, it effectively works to control the pore size of the wall matrix. It also provides some strength to resist compressive forces, but has poor tensile strength.
True pectin is only found in streptophytes (land plants and related green algae). In other kinds of algae, pectin is replaced by different polysaccharides with the same role: e.g., Agar or Carrageenan (in red algae) or Alginates (in brown algae)
Cell walls/ECMs are generally made from 3 or 4 components:
PLANTS
STRUCTURAL FIBERS
Cellulose
poly-(-(14)-glucose)
long polymers of -(1>4) linked Glucose
CROSSLINKERS
hemicelluloses
e.g., xyloglucan
intermediate-length polymers of -(1>4)-linked Glucose with -(1-6)-linked Xylose that hold the long fibers of cellulose together.
GEL/HYDRATED MATRIX
pectins
e.g., rhamnogalacturonan
complicated! polysaccharides mainly of galacturonic acid and rhamnose that serve to make a gel support system that holds everything in place.
SUPERGLUE
lignin
aromatic polymers of phenolic compounds that serve as water-proofing and permanently stick everything together for added strength.
lignin is a polymer of monolignols e.g., coniferyl alcohol HO
A small fragment of a larger lignin polymer
OMe HO
8-5
Individual reactive sites (arrows) in one coniferyl alcohol molecule react with similar points in another molecule, creating dimers, trimers, etc. HO OMe O
3-cell junction
Primary wall Middle Lamina Plasma Membrane Cytosol
Primary walls:
Have cellulose, hemicelluose and pectin. Generally allow fast diffusion of nutrients and water through large pores. Are good for holding cells together, but not really good for structure If lignin is added, primary walls are much stronger.
3-cell junction
Primary wall Middle Lamina Plasma Membrane Cytosol
Primary walls:
Have cellulose, hemicelluose and pectin. Generally allow fast diffusion of nutrients and water through large pores. Are good for holding cells together, but not really good for structure If lignin is added, primary walls are much stronger.
Secondary walls:
Often very thick layers of mainly cellulose fibers laid down within the primary wall. Tend to restrict diffusion of nutrients and water because pores are smaller. Very good at creating very strong cells, and even tissue-scale structures. Most secondary walls are highly lignified, leading to even more strength.
peeled-back cell
Cytosol Plasma Membrane Secondary wall #2 Secondary wall #1 Primary wall Middle Lamina
Cells with extensive secondary walls and lignification serve as structural members to hold up plants
fig 4.7
In herbaceous plants, the plants are supported by: partially lignified parenchyma cell layers, the extra-thick walls of aggregated collenchyma cells, or the ...