Module 4 Flashcards
What is a molecular clock used for?
- DNA & protein sequences change through mutation over time
- longer periods = more change
- we estimate mutation rate & therefore the time of the last common ancestor of extant species
- fossil record is used to “calibrate” molecular clock
ESTIMATES TIME OF DIVERGENCE OF PLANTS & ANIMALS
How many times has multicellularity evolved independently?
at least 6 times
fungi (x2)
animals
green algae
brown algae
red algae
Fossil evidence for multicellularity arose when?
about 600 mya
Propose possible steps leading to multicellularity
- aggregation of cells into a cluster
- intercellular communication within the cluster
- specialisation of some cells within the cluster (cooperation)
- organization of specialised cells into groups (tissues)
A TRANSITION TO MULTICELLULARITY ALSO RESULTS IN INDIVIDUAL CELLS LOSING THE ABILITY TO LIVE INDEPENDENTLY
Possible first multicellular animal
placazoans
What are consequences of multicellularity?
- allows for more specialized systems
- e.g. in volvox
- outer cells have coordinated flagella for movement
- outer cells create an inner space to protect reproductive cells
- big inner cells are specialized for reproduction
- leads to change in size too (not prey to certain predators)
- FUNCTIONAL SPECIALIZATION (e.g. cells work in unison e.g. beating of flagella)
What does multicellularity enable?
- cell specialization allows cells to adopt new functions
- integration & cooperation b/w cells allowing for development of tissues & organs
- structurally & functionally complex bodies
- creation of a stable internal env.
- increase in size
- more efficient gathering of resources & adapting to specific environments
How do multicellular organisms develop?
develop from zygote as a result of embryogenesis
- during embryogenesis there are multiple rounds of cell division producing specific cell types along major spatial axes
Describe the pattern of cell fate
it is highly ordered & reflects the position of cells in the developing embryo - instructive cues (cytoplasmic factors / cell signalling molecules
- change in cell potency
Each cell has the same set of genes so how do they become so different?
cell properties are determined by the subset of genes that are expressed - therefore, the specification of cell fate involves gene regulation
Which essential genes are expressed in every cell?
housekeeping genes
What is morphogenesis?
- process by which cells & tissues organize & arrange themselves to create the final form of the body
Outline key processes in morphogenesis
- division
- changing shape (expansion)
- moving (not seen in plant embryogenesis)
- adhering to one another (not seen in plant embryogenesis)
- death (apoptosis)
Define body plan
general structure of organism, arrangement of organs systems, integrated functioning of its parts
How can you categorize body plan in animals?
according to symmetry, body cavity structure, segmentation, type of appendages, & type of nervous system
Describe body plan in plants
- body plan is modular
- aerial structures (shoot)- subterranean structures (roots) have a modular arrangement of organs (phytomers / rhizomers)
- plants have a radial arrangement of tissue types
Difference in growth b/w plants & animals
animals = determinate growth (predetermined body form)
plants = flexible body form - most plant development occurs after embryogenesis
Challenges faced by multicellular organisms
- surface area to vol ratio of a multicellular organism is SMALL
- distance of internal cells to external env. is LARGE
SOLUTIONs to large size of cells of multicellular organisms
- close to external env. so diffusion occurs directly
- might have central cavity which brins ex env into the animal
- large surface area of exchange organs (long, flat, folded, branched)
- thin surface area with small diffusion distances –> ensures maximal rate of exchange
- circulatory system solves diffusion limit (movement of extracellular fluids around body – maintains high concentration gradients for diffusion)
Difference b/w transport systems of animals & plants
- circulatory systems of animals - active (pumps)
- transport system of plants - passive
What solves the diffusion limit?
- a circulatory system
- movement of extracellular fluids around the body to ensure exchanged substances from exchange organs reach cells of body (BULK FLOW)
- maintains high concentration gradients for diffusion
To maintain a high level of metabolism large multicellular organisms. . .
- need highly branched internal transport system
- rapid movement of exchange substances
- considerable force required to move fluids through these transport systems
Examples of cells in multicellular organisms communicating
- through use of INTERCELLULAR SIGNALLING
- conveying positional info during development
- maintaining a stable internal env (homeostasis)
- ensuring cells work in unison (beating of Volvox flagella)
- physical / chemical signals arising from external env.
chemical signals
can activate receptors on nearby cells (e.g. ligans) or secreted into bloodstream & activate cells throughout body (e.g. hormones)
electrical signals
passed long distances very rapidly via neurons to very specific targets
Adaptations of plants to a sedentary lifestyle
- have organ systems that allow them to capture limited resources (root + shoot system)
- grow continuously to exploit new areas & respond to environmental cues (primary vs secondary growth)
Adaptation of root system
primary vs lateral roots
- absorbing water & mineral nutrients
- anchorage, storage, transport, hormones
Adaptation of shoot system
- photosynthesis & gas exchange
- hormones
- reproduction
primary growth of plants
longitudinal
secondary growth
radially
What is continuous growth associated with in plants?
repeated formation of modules (phytomers)
– includes the leaf, axillary bud, & internode
Differences b/w plant & animal cells
- plants have large vacuole but animals lack vacuoles
- plants are immobile but animal cells are mobile
- plants have cell walls, animals do not
Describe the plant cell walls
semi-rigid cell wall composed of cellulose fibers
Describe the outside of animal cells
surrounded by an extracellular matrix with collagen & proteoglycans as major components
Functions of plant cell wall
- provides semi-rigid support
- provides a barrier to prevent infection
- contributes to plant form by growing as plant cell grows
Plant cell wall molecular structure
- primary cell wall is composed of polysaccharides, cellulose, hemicellulose, & pectin
- cellulose fibrils are formed via H bonds
- extremely high tensile strength
Where else is pectin found in plant cells?
a major component of the middle lamella
If plant cells are surrounded by a semi-rigid cell wall, how do they grow?
- cell takes up water
- vacuole expands
- increase in turgor pressure
- cell expansion resisted by cell wall
- increased turgor pressure triggers release of enzymes to soften wall
What is expansin?
allows slippage b/w cellulose microfibrils by interfering w/ non-covalent binding of cellulose & the glycans, hemicellulose, & pectin
binds to “tensioned” glycans & makes them “relaxed” & vice versa
Plane of cell division determines. . .
direction of tissue growth
Difference b/w primary & secondary cell walls
primary: semi-rigid & expansion is possible
SECRETION OF WALL MATERIAL AFTER EXPANSION –>
secondary: thick & rigid & unable to expand
composed of lignin + cellulose
Difference b/w primary & secondary cell walls
primary: semi-rigid & expansion is possible
SECRETION OF WALL MATERIAL AFTER EXPANSION –>
secondary: thick & rigid & unable to expand
composed of lignin + cellulose
Define tropism
growth towards or away from a stimulus - light, gravity, water, touch
Define phototropism
differential growth in response to light
Define heliotropism
growth that follows the path of the sun
How does auxin hormone regulate phototropism & gravitropism?
- during phototropism auxin accumulates on shaded side
- during gravitropism auxin accumulates on lower side
- in both cases auxin-dependent growth bends the plant
How does auxin control growth via expansin activity?
- auxin promotes the activity of a proton pump
- lower pH in the cell wall activates expansin
- expansin loosens up the cell wall & makes it more flexible
Early steps of plant embryogenesis
- Begin as zygote
- Undergoes first division which is asymmetric
- Daughter cells are diploid (apical and basal) –> two-cell stage
- Cell undergo more orientated cell division (embryo vs suspensor)–> octant stage
- More orientated cell divisions + cell expansion –> heart stage
- Further elongation of cotyledons & the main axis of the embryo –> mature embryo (contains shoot apical & root apical meristems)
Define determination
commitment to a particular cell fate before cellular characteristics become apparent (at the octant stage for plants)
Define differentiation
cells acquire specific functions & characteristics due to differential gene expression (at heart + embryo stage in plants)
Define morphogenesis
the process by which differentiating cells organize to form the tissues & organs of the body
Define growth
increase in size of the body & organs due to cell proliferation & cell enlargement
Steps of plant development
- determination
- differentiation
- morphogenesis
- growth
What are the 2 principle axes of plant body?
apical-basal: arrangement of tissues along the shoot-root axis
radial: concentric (circular) arrangement of tissues
during early embryogenesis, the plant body is mapped out along 2 principle axes
Post-embryonic devel. of plants (dif b/w plants & animals)
plants continually make new organs as they grow - leaves, roots, flowers
- these structures are NOT produced during embryogenesis but arise post-embryonically from meristems
- animals usually have them produced embryogenically
Shoot vs root apical meristems
- shoot apical meristem: generates aerial structures –> phytomer (leaf, internode, axillary bud)
- root apical meristem: generates subterranean structures –> roots
Apical meristems must be kept inactive . . .
prior to seed disposal (imagine a strawberry with tiny leaves coming out of the seeds)
What are the 3 primary meristems?
- protoderm
- ground meristem
- procambium
Features of shoot apical meristems
- associated with primary growth of shoot
- source of cells for primary tissues of shoot
- branches will form indirectly from activity of axillary buds
What are the 3 ‘zones’ in the root
zone of maturation
zone of elongation
zone of cell division
Features of root apical meristems
- associated with primary growth of root
- source of cell for primary tissues of root
- source of cells for root cap formation
- lateral roots arise from internal tissue at some distance from RAM (root apical meristem)
How is continuous activity of apical meristems maintained?
self-renewing undifferentiated cells (stem cells)
Purpose of secondary growth
increases plant thickness by producing wood & bark
What generates secondary growth?
lateral meristems called cambium
stem cells are also present in these lateral meristems
Tree rings are affected by??
seasonal environmental conditions produce annual rings of secondary growth (wood)
- sprint: water is plentiful so light ring
- summer: water less available so dark ring
- winter: no growth
3 main tissue systems in plants
- dermal tissue system: forms outer covering of plant
- ground tissue system: carries out photosynthesis, stores photosynthetic products, & helps support the plant
- vascular tissue system: conducts water & solutes throughout plant
Dermal tissue system
- forms epidermis which is usually a single layer of cells
- epidermal cells may differentiate into (stomata, trichomes (leaf/stem hairs), root hairs)
- epidermis of aerial structures (leaves & stem) have a waxy cuticle
Function of waxy cuticle
- limits water loss & is gas impermeable
- protects against physical damage, UV radiation, & pathogens
Ground tissue system
- located b/w dermal & vascular tissue –> represents bulk of plant body
- classified according to cell wall structure:
1. parenchyma
2. collenchyma
3. sclerenchyma
Describe parenchyma
thin cells walls
large vacuoles
photosynthesis, storage (proteins, starch, fats, oil) in seeds & roots, nutrient transport
Describe collenchyma
unevenly thickened cell walls
(bendy) support e.g. strings of celery
Describe sclerenchyma
thick cell walls w/ secondary cell walls
very rigid support
fiber cells
Vascular tissue system
conducting (transporting) tissue that forms a network throughout the plant
xylem vs phloem
xylem
carries water & mineral ions from roots to shoots
- 2 cell types (tracheids & vessel elements)
- mainly composed of dead cells w/ secondary cell walls
phloem
moves sugars & nutrients from shoots to roots (or other places)
- sieve tube element cells & companion cells
- composed of living cells
companion cells help to keep the simple sieve tube elements alive
Role of water in plants
- photosynthesis in leaves
- transporting solutes b/w plant organs
- cooling the plant
- structural support (turgor pressure)
Role of water in plants
- photosynthesis in leaves
- transporting solutes b/w plant organs
- cooling the plant
- structural support (turgor pressure)
Roots are a . . .
net source of water
net sink (??) of sugar
Shoot is a . . .
net sink of water
net sink for sugars
Examples of macronutrients needed by plants
nitrogen + phosphorus
Example of micronutrient
iron
Define water potential
tendency of a solution to take up water from pure water across a selectively permeable membrane
Golden rule about water potential
water ALWAYS moves across a selectively permeable membrane towards regions of LOWER (more negative) water potential
2 components of water potential
- solute potential - the greater the concentration of solutes, the lower the potential
- pressure potential - the greater the internal pressure, the higher the potential
In plant cells, turgor pressure is equivalent to. . .
pressure potential
Describe movement of water based on water potential
under low turgor pressure, water will enter a plant cell by osmosis due to a low solute potential
when turgor pressure balances solute potential, there is no net flow of water in/out of the cell
What does a reduction in turgor pressure lead to?
plant cells become less rigid (flaccid cells)
causes plants to wilt
apoplast (water uptake by cells of the root)
interconnected cell wall & intercellular spaces b/w cells - movement is rapid & unregulated
symplast
- interconnected cytoplasm via plasmodesmata
- movement is SLOW & regulated
- water & solutes gain entry into symplast by first crossing a membrane
Casparian strip
forms a (diffusion) barrier to the apoplastic flux, forcing ions to pass through the selectively permeable plasma membrane into the cytoplasm, rather than move along the cell wall
Explain the movement of water through the root
- water & solutes enter root by osmosis (move through symplast + apoplast)
- water & solutes in the apoplast forced into endodermal cells (symplast)
- water & solutes remain in symplast
- solutes are actively transported out of the cell into the apoplast & water follows passively by osmosis
adhesion
interaction b/w the water molecules & the xylem wall (capillary action)
Stomata
opening through epidermis to allow gas exchange; opening controlled by 2 guard cells that open when turgid
regulated by light, CO2, temp, & water availability
Explain the activity of stomata & guard cells in presence of light
- protons are pumped out
- ions (K+ & Cl-) enter, lowering solute potential
- H2O enters by osmosis, increasing turgor pressure
- pore opens
Explain the activity of stomata & guard cells in absence of light
- proton pumps becomes less active
- K+ & Cl- ions diffuse out passively
- H2O followed by osmosis
- pressure goes down
- pore closes
How is osmosis controlled by light?
- light activates photoreceptor
- signaling cascade activates proton pump (H+ ATPase)
- H+ pumped out of cell
- electrochemical gradient drives K+ ions in through potassium channel
- symport protein also imports Cl- ions along w/ H+ ions to maintain electrical balance
- intracellular K+ & Cl- go up, decreasing solute potential
- H2O enters via osmosis
