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Plants evolved from freshwater green algae; the earliest plants were prostrate
in habit, but later developed upright shoots with leaves.
Summary of Sections 1.1 and 1.2
Other developments that were important adaptations to life on land include:
reduced dependence on water for sexual reproduction; waterproof coverings
for shoots; roots; specialized cells for strengthening and for internal transport
of water and nutrients; increased size.
Summary of Sections 1.1 and 1.2
The relative dominance of gametophyte and sporophyte generations is used in
plant classification.
Summary of Sections 1.1 and 1.2
The concepts of isomorphic and heteromorphic alternation of generations
were revised and two theories1—1transformation and interpolation1—1were
described which could explain the way in which a dominant sporophyte
lifestyle may have originated.
Summary of Sections 1.1 and 1.2
Finally a number of other characteristics of life cycles were listed as an
introduction to more detailed study of ferns, mosses, club mosses and
flowering plants.
Summary of Sections 1.1 and 1.2
In bryophytes the dominant generation is the haploid gametophyte, whereas in
ferns and all other vascular plants it is the diploid sporophyte.
Summary of Section 1.3
In mosses, flask-like archegonia and club-shaped antheridia on the leafy
gametophyte produce, respectively, egg cells and motile sperm. After
fertilization, the zygote develops into a sporophyte, which remains attached to,
and dependent on, the gametophyte; it consists of a rooting base, stem and
capsule. Meiosis in the capsule results in formation of haploid spores, which are
released into the air and germinate to give a flat, delicate protonema (with cell
division in only one plane) from which the gametophyte develops (with cell
division in two planes).
Summary of Section 1.3
In most ferns, similar archegonia and antheridia, with egg cells and motile
sperm, are formed within a delicate green prothallus, from which the sporophyte
develops after fertilization. The sporophyte is soon independent. Spores are
produced within sporangia grouped into sori on leaves, and after release develop
into prothalli.
Summary of Section 1.3
Some ferns (all aquatic types) and some lycophytes (e.g. Selaginella) are
heterosporous, and the gametophyte generation is reduced. Separate micro-
(male) and mega- (female) gametophytes develop within, respectively,
micro- and megaspores, which are released from, respectively, micro- and
megasporangia. When spore walls rupture, the minute male gametophyte,
comprising a vegetative cell and an antheridium, releases sperm, which swim
to the archegonia exposed on the relatively massive, food-storing female
gametophyte.
Summary of Section 1.3
The reproductive systems of seed plants, especially flowering plants, are the
most perfectly adapted of all vascular plants to life on land. Adaptations
include retention and reduction of the gametophyte generation, more effective
fertilization mechanisms, the evolution of seeds, and also mechanisms and
structures assisting in seed dispersal.
Summary of Section 1.4
Angiosperm flowers include a number of structures (sepals, petals, stamens
and one or more carpels, which collectively form a gynoecium), some of
which may be highly coloured and which originate from modified leaves.
Summary of Section 1.4
The anthers of male stamens produce pollen after meiosis. The nucleus of an
initial microspore divides giving a generative and a vegetative cell (immature
male gametophyte). Pollen is dispersed at this stage. It matures and
germinates on a receptive stigma, producing a pollen tube, which grows
down the style, where the generative cell divides to give two male gametes.
Summary of Section 1.4
The female structure (ovule) develops within an ovary in the nucellus tissue.
One cell undergoes meiosis to give four megaspores, three of which abort.
The remaining megaspore becomes the embryo sac (female gametophyte),
within whose wall eight nuclei are produced by mitosis and seven weakly
defined cells form1—1three at each end and two as a binucleate cell in the
centre. The central cell of the three, located at the entrance1—1micropylar
end1—1of the embryo sac, is the egg cell.
Summary of Section 1.4
Double fertilization occurs after the pollen tube penetrates the embryo sac
and releases the male gametes. One male gamete fuses with the egg cell, and
the zygote develops into an embryo. The second male gamete fuses with the
central cell, and the resulting triploid cell divides to give nutritive endosperm
tissue.
Summary of Section 1.4
The ovule matures to give a seed, which contains the embryo, food stores and
a protective wall (seed coat) formed from the integuments of the ovule. The
ovary matures to form a fruit.
Summary of Section 1.4
The basic strengthening elements in plant cell walls are cellulose in living
cells, and cellulose and lignin and/or suberin in dead cells.
Summary of Section 1.5
Parenchyma is the basic cell type. Other living cells adapted to specific functions
are: storage parenchyma; palisade cells, which are rich in chloroplasts,
and important sites of photosynthesis; phloem sieve tubes and companion
cells, which carry the products of photosynthesis; transfer cells.
Summary of Section 1.5
Living cells with thickened walls include: epidermal cells, which may
additionally have an external cuticle plus waxes; collenchyma, which
have a support function in outer tissues of young and short-lived organs;
endodermal cells, where the wall thickening allows only selective water
and ion movement into internal tissues, hypodermal cells and exodermal
cells on the outside of older roots.
Summary of Section 1.5
Increase in size (length or width) of plant tissue takes place in primary or
secondary meristems. The cells of these tissues are small and rectangular,
and have thin walls. The cells of secondary meristems (cambium) are
arranged as tiers, with cell walls showing the lineage of the oldest cells
of the tissue. Cork cambium produces cork cells to the outside, which gives
rise to bark.
Summary of Section 1.5
Dead cells acting as support include sclerenchyma and sclereids, xylem
fibres, tracheids and vessels. Tracheids and vessels also conduct water and
minerals from roots to the aerial parts of plants.
Summary of Section 1.5
The molecular components of photosynthetic light reactions are arranged on
chloroplast membranes such that PSII is largely confined to appressed
lamellae inside grana and PSI to the non-appressed stroma lamellae.
Summary of Section 2.2
Information about molecular arrangements on chloroplast membranes can be
obtained using low-temperature fluorescence emission spectra,
immunohistochemistry and freeze-etch/freeze fracture electron microscopy of
particular membrane fractions.
Summary of Section 2.2
Shade plants are genetically or physiologically adapted to conditions of low
light, rich in far-red wavelengths, and to exposure to sunflecks. They harvest
light efficiently under these conditions and minimize damage due to overexcitation
of PSII.
Summary of Section 2.2
Shade-plant characteristics include thin leaves, low rates of leaf respiration,
large antenna (or light-harvesting) systems and a high ratio of PSII to PSI.
Protection during sunflecks is achieved by physiological and molecular
mechanisms (energy redistribution involving covalent modification and
migration of PSII components).
Summary of Section 2.2
Strong light (high PPFD) can damage leaves through the production of free
radicals (ROS). General protection against ROS may be through leaf
movements and a range of molecular mechanisms, which include the
xanthophyll cycle (preventing formation of ROS), enzymatic destruction of
ROS, and rapid destruction and repair of D1 protein in PSII.
Summary of Section 2.2
Damage due to strong light which is reversible (by repair mechanisms) is
called photoinhibition. Irreversible damage is called photo-oxidation. Shade
leaves have only very limited capacity for damage repair and are easily
photo-oxidized, whereas sun leaves have a high repair capacity (unless
stressed, e.g. by low temperatures) and do not usually become photooxidized.
Summary of Section 2.2
The Calvin or C3 cycle is the only pathway in eukaryotes that achieves a net
fixation of carbon. It has three stages (carboxylation, reduction and
regeneration) and uses three molecules of ATP and two of NADP.2H to fix
one molecule of CO2.
Summary of Section 2.3
Triose phosphate from the C3 cycle is used to synthesize sucrose in the
cytoplasm (for export) and starch in chloroplasts (for temporary storage). The
phosphate translocator plays a key role in balancing and controlling these two
processes.
Summary of Section 2.3
Rubisco is a huge and very abundant enzyme that catalyses the first reaction
of the C3 cycle, carboxylation of RuBP with CO2 to give the 3-carbon
intermediate, 3-phosphoglycerate (PGA). It also acts as an oxygenase,
combining RuBP with oxygen to give PGA and phosphoglycolate. CO2 and
O2 compete as substrates for Rubisco.
Summary of Section 2.3
For maximum efficiency and economy, the rate of the C3 cycle matches that
of the light reactions and the cycle responds sensitively to changes in light
flux.
Summary of Section 2.3
Gross control of the C3 cycle is set by the amounts of enzymes (especially
Rubisco). Coarse biochemical control, which starts or stops the cycle abruptly
in light or darkness, respectively, operates by regulating the activity of
Rubisco (via Rubisco activase) and other cycle enzymes (via the ferredoxin–
thioredoxin system). Fine biochemical controls operate via pH and the levels
of Mg2+, ATP and NADP.2H, all of which influence the activity of certain C3
cycle enzymes.
Summary of Section 2.3
The C2 cycle metabolizes phosphoglycolate, produced by the oxygenase
reaction of Rubisco with RuBP, returning 75% of the carbon to the Calvin
cycle as phosphoglycerate. In the process, O2 is taken up, CO2 is released and
ATP and reducing power are utilized. Photorespiration is an alternative name
for this cycle.
Summary of Section 2.4
Reactions of the C2 cycle occur in chloroplasts, peroxisomes and
mitochondria and involve oxidation, decarboxylation and amination/
deamination. Oxygen is used in peroxisomes; CO2 is released in
mitochondria; and there is cycling of nitrogen between all three organelles,
with NH3 passing from mitochondria to chloroplasts.
Summary of Section 2.4
The C2 cycle may play a role (a) in regulating the balance of O2 and CO2 in
the atmosphere (Tolbert’s view); and/or (b) by acting as a safety valve and
temporary utilization phase, which protects against photoinhibition by
utilizing ATP and reducing power when CO2 availability is low.
Summary of Section 2.4
CO2-concentrating systems raise [CO2] at the site of Rubisco and hence minimize
the oxygenase reaction and the C2 cycle. There are three types:
• In aquatic photosynthesizers (cyanobacteria, some algae and some aquatic
plants), inorganic carbon (Ci) pumps and carbonic anhydrase are used.
• In C4 plants, a combination of Kranz anatomy and the C4 cycle are involved.
CO2 is fixed initially into a 4-carbon acid in mesophyll cells and is released
in bundle sheath cells at the site of Rubisco. An extra two ATP molecules are
required to fix one molecule of CO2 in these plants, which are mostly grasses
and sedges from open tropical habitats. Their water use efficiency is better
than that of C3 plants.
• In CAM plants there is temporal separation of initial CO2 fixation and
operation of the C3 cycle. CO2 is fixed and stored in vacuoles as malic acid at
night and released for use by Rubisco during the day. Taxonomic and
structural diversity is much higher among CAM plants compared with C4
plants, and CAM is also a more flexible system, often operated facultatively.
CAM plants have very low transpiration ratios (high water-use efficiency)
because stomata can remain closed during the day. They include many desert
succulents.
Summary of Section 2.5
At the physiological level, net photosynthesis (and hence growth) may be
limited by the availability of light or CO2 or by temperature. Plants may be
genetically adapted to a limited range of environmental conditions or show
phenotypic plasticity, having a capacity to acclimate to a wide range of
conditions.
Summary of Section 2.6
Light compensation points indicate plant adaptation to prevailing light
intensity; CO2 compensation points indicate the efficiency of CO2-
concentrating mechanisms in strong light.
Summary of Section 2.6
Shade plants have lower light compensation points than sun plants because
they use light more efficiently at low PPFD and have low respiration rates.
The trade-off is that their NP is lower than that of sun plants at high PPFD.
Summary of Section 2.6
The low CO2 compensation point of plants with CO2-concentrating
mechanisms results from their suppression of the Rubisco oxygenase reaction
and the C2 cycle. C4 and CAM plants have higher NP than do C3 plants
when conditions favour the C2 cycle, but may perform less well than C3
plants in cool, low light conditions.
Summary of Section 2.6
Current atmospheric concentrations of O2 reduce NP significantly in C3
plants in conditions of high light and temperature, but have no such effect on
C4 and CAM plants.
Summary of Section 2.6
The optimum and limiting temperatures for photosynthesis vary depending
on the usual habitat of a plant or the conditions under which it was grown.
Summary of Section 2.6
Water taken up from the soil moves radially across roots to the xylem and
then, by mass flow, up to the leaves.
Summary of Section 3.2
Effective water uptake by roots requires constant root growth and close
contact with soil particles, which is facilitated by formation of a rhizosheath.
Most water uptake occurs via fine roots, which branch from the larger,
framework roots.
Summary of Section 3.2
Radial water transport across roots from epidermis to endodermis may
involve any or all three of the apoplast, symplast and transcellular pathways,
depending on plant species and environmental conditions.
Summary of Section 3.2
Water movement across the endodermis and, if present, the exodermis
necessarily occurs via cell membranes or plasmodesmata because of the
waterproof thickening on radial cell walls. Aquaporin availability may
control and fine-tune water flow across the cell membranes in roots.
Summary of Section 3.2
Whereas solute movement into the stele is essentially one way, water can
move both into and out of the stele across the endodermis.
Summary of Section 3.2
Deep-rooted plants may distribute a substantial quantity of water into the
surface soil by hydraulic lift.
Summary of Section 3.2
Water potential, Ψ, is a relative measure of the free energy of water and pure
water has a value of 0. Water moves down gradients of water potential at a
rate that depends on ΔΨ and hydraulic conductivity, Lp.
Summary of Section 3.3
Living cells usually have a negative water potential. Its value depends on
hydrostatic pressure above atmospheric (turgor, P) and solute concentration
(osmotic pressure, p), which depends in turn on the reflection coefficient σ
of membranes. In systems with a high solid:liquid ratio (e.g. soil), matric
pressure is also a contributor. Equation 3.3 summarizes the interactions.
Summary of Section 3.3
Turgor is essential for the expansion growth of plant cells and for support in
non-woody tissues.
Summary of Section 3.3
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