Module 3 - Multicellularity Flashcards
Evolution of multicellularity
Molecular clocks and fossil records are used to estimate divergence in evolution + when plants and animals diverged
Molecular clocks = used in fossil records to estimate time of divergence based on DNA sequences differences (takes certain amount of time for a base to mutate)
Mutation in genomes (1-2 bases only) can calculate time differences and common ancestors
Common Ancestor of Plants and Animal
Most likely to be a single-celled protist named Choanoflagellate (unicellular)
Evolution of Complex Multicellularity
Evolution of complex multicellular organisms has happened independently several times: fungi (2x), green-algae, red algae, brown algae and animals
Process of Multicellularity (transition)
- aggregation of cells into a cluster
- intercellular communication within the cluster
- specialisation of some cells within the cluster
- organised arrangement of cells into groups (forms tissues)
multicellularity must’ve required intercellular communication to coordinate cellular activity which allows for specialisation homeostasis, ensuring cells work in unison, ability to get signals from external cells
Types of intercellular communication
Chemical - activating nearby cells (e.g. ligands: which are local) or through bloodstream (e.g. hormones: which are systemic)
Electrical - activates very specific target by travelling long distance rapidly
Advantages of Multicellularity
- Increase in size (introduced new preys/predators)
- Cell specialisation (group of cells work in unison e.g. flagella for movement or cells dedicated to one task e.g. reproduction)
- Structural + Functionality complexity
- Creation of internal stable environment allowing exploitation of new environmental niches
- ALL LEADS UP TO ALLOWING COMPLEX TO USE ENERGY MORE EFFICIENTLY
Example of Multicellular advantage
Volvox is larger than Chlamydomonas making it less susceptible prey to Copepod (filter-feeders)
No need to switch cell types to perform tasks
Outside cells to provide movement
Outer cells create protection for inner cells
Protected inner cells for reproduction
Development of Multicellular organisms (animals)
Embryogenesis - process of multicellular complexes developing from zygotes
Zygote = single-celled
- specificity occurs from early on (plant + animals cells can be differed)
- multiple cell divisions of specialised cells occurs along a major spatial axis (radial = animals, apical/basal axes = plants)
GENE EXPRESSION DETERMINES CELL SPECIALISATION
Animal Embryogenesis
Gastrulation = cells folding inwards to form an internal cavity whilst others break off and move inside (endoderm folds inwards whilst mesoderm is inside components)
Animal germ layers:
Ectoderm - outer layer
Mesoderm - inside layer of zygote (in between ectoderm + endoderm)
Endoderm - inner layer of zygote
Tripoblastic = 3 germ layers (animal) Diploblastic = 2 germ layers (lacks mesoderm e.g. cnidarian)
Plant Embryogenesis
Plants do NOT undergo gastrulation and have no germ layers
Development is continuous from SHOOT APICAL MERISTEM and ROOT APICAL MERISTEM
3 tissues are present in plants:
Apical tissue
Basal Tissue
Radical Tissue
Requirements of Multicellularity
-Must have Barrier (to have internal environment and maintain homeostasis)
-Size limit
Too large/volume causes slow diffusion of materials
SA: Volume ratio limits cell size
When a cell increases in size, SA increases faster than volume
Ratio reduces as organism gets larger
Rate of diffusion of oxygen through water equation:
Time = (Distance^2) / 2 x 0.00001 cm2/sec
Allows all cells to be in certain proximity of external environment for diffusion (e.g. flatworms) or complexes that bring external environment into animal (e.g. hydra = hollow cavity or sponges = porous cavities)
-Must have internal transportation system
circulates extracellular fluids ensuring optimal gas exchange, waste removal, nutrient mobilisation, communication
-Communication
intercellular communication to coordinate cellular activities and respond to internal and external environment
(homeostasis, physical/chemical signals, cells working in unison, specialisation of cells)
Plant Growth Strategies (overcoming sedentary lifestyle)
-plants have continuous growth (allow to occupy new areas + responding to environmental cues)
primary growth - longitudinal (roots and shoots)
secondary growth - radial (thickness of stem)
-organ systems are suited to capture limited resources (large SA)
capturing sun (shoot system, leafs, stems, blades)
capturing water and nutrients (root system)
What is a Phytomer?
Functional unit that has continuous growth (shoot system) that consists of:
- leaf
- auxillary node
- internode
Plant Cell Wall Structure
- made up of polysaccharides, cellulose, pectin and hemicellulose
- has cellulose fibrils via hydrogen bonds (giving tensile support)
- has middle lamella which is in between cell walls, holding it together (majorly pectin)
Primary and Secondary Cell Walls
Primary: semi-rigid, selectively permeable membranes
allows for expansion which leads to
Secondary cell walls: made up of lignin (very hard and rigid), not permeable at all, unable to expand
(occurs when extra layers of cellulose is secreted from primary cell wall aka growth)
Plant Growth by Cell Expansion
- due to semi-rigid cell wall, water is able to enter (taken up by vacuole) which expands causing turgor pressure
- cell wall resists expansion
- with increased turgor pressure, this activates an enzyme to be release to soften walls and allow for expansion
Enzyme: Expansin is released where it bonds to non-covalent bonding between cellulose, hemicellulose, pectins and polysaccharides
this causes cellulose microfibrils to have ‘slippage’
allows other microfibrils to expand from ‘relaxed’ into ‘tensioned’ (cell wall would elongate)
Plant Growth by Orientated Cell Divisions
Orientated cell divisions allow for plant to control growth, determining direction of tissue growth
-longitudinal or radially
Plant Embryogenesis
- plant embryogenesis begins with a zygote and after cell divisions it divides into two daughter cells (asymmetric body)
- TWO CELL STAGE: top cell (smaller) is apical daughter cell and bottom cell is basal daughter cell
-OCTANT STAGE: more cell divisions cause apical daughter cell to become plant embryo and basal daughter cell forms suspensor (receives and gives signals/nutrients from mother)
determination stage occurs here where cells are determined their specialisations (commit to cell types before characteristics)
-HEART STAGE: changes from radial symmetry to lateral symmetry. plant embryo forms Cotyledon primordia (top sections of ‘heart’) whilst bottom area consists of tissues (epidermis, ground tissue, vasculartissue)
Differentiation occurs: cell types begin to show specialised characteristics and acquire specific functions due to gene expression
Morphogenesis occurs: groupings of cells to form tissues and organs
- TORPEDO STAGE: growth of cells (increase in cell size due to proliferation and enlargement)
- MATURE EMBRYO: through elongation of cotyledon and main axis of embryo, the ‘seed’ forms
Body Plan during Embryogenesis
Body plan is mapped our during early embryogenesis along two axis:
Apical-basal - arrangement of tissues along the shoot-root axis (lateral)
Radial - circular arrangement of tissues (dermal, ground and vascular)
Post-embryonic development
Primary growth is through the shoot apical meristem (aerial structures like phytomers)
Root apical meristem gives rise to subterrean structures (roots)
[apical meristems are formed during embryogenesis but are kept inactive until seed dispersal occurs]
Apical and Primary Meristems (Primary growth)
Shoot apical meristem (part of bottom section in embryo) gives rise to 3 other tissues (dermal, ground and vascular tissue)
Shoot apical meristem is source of cells for organ growth/development
Apical meristem continously grows due to presence of stem cells
Stem cells = undifferentiated, renewing cells
Lateral Meristems (secondary growth)
Secondary growth occurs in bark/wood parts of trees (related to thickness/wideness of plants)
Lateral meristems (cork cambium and secondary phloem) is found in older woody stems where is generates secondary growth causing secondary plant body to be formed
(stem cells are also present in lateral meristems)
-Secondary growth is occurs with vascular bundle where xylem forms wood growth
-outer layer of vascular bundle is bark (secondary phloem + cork cambium)
-relates to tree rings
(spring has more water = rings are dark and thick
summer has less water = rings are light and not as thick
winter = no water)
Plant Tissue Types
DERMAL (epidermal): forms epidermis cells (usually single cell layer)
differentiates into: stomata, trichomes (hairs), root epidermis, leaf epidermis, root hairs
GROUND: found inbetween on dermal and vascular tissues
is the bulk of plant body, ground tissues are categorised based on their cell wall structure
Parenchyma (most common): have large vacuoles and thin cell walls
Functions: photosynthesis, nutrient transport and storage
Collenchyme: elongated, thick, uneven cell walls
found in non-woody stems, growing organs and petioles
Function: mechanical support (sturdy but flexible)
Sclerenchyma: there are Fiber cells and Sclereid cells (they under go apoptosis), very thick, long circular, lignified
Fiber cells = woody/bark structural support
Sclereid cells = small bundles that are very durable (can be found in nuts), when clumped it forms gritty texture
VASCULAR: conducting/transporting tissue that forms a network throughout plant
Xylem: made up of lignified, dead cells, secondary cell walls
have Tracheids (long spindle cells with holes to allow water to move freely) and vessel elements (like pipeline for water)
Phloem: Made up of sieve tube elements and companion cells
Has Plasmodesmatas which are large holes that connect cytoplasms of sieve tubes
-causes inner cell components to be lost thus having companion cells to provide necessary nutrients/materials
Plant Cuticles
Waxy cuticle is on top of epidermis
- limits water loss and is impermeable
- protects against UV rays, pathogens and physical damage
Plant Organ: Leaf
-composed of dermal, ground and vascular tissues
-has determined growth (specific shape and size allowing for function
upper size = cuticle
lower size = stomatas)
-ground tissues in a leaf are either palisade mesophyll cells or spongy mesophyll cells (all have same function = photosynthesis)
Plant Tropism
Growth of plant towards or away from a stimulus
e.g. Heliotropism (follows path of sun like sunflowers)
Phototropism - auxin accumulates on shaded size which promotes growth causing stem to grow towards light
Gravitropism - auxin accumulates on lower side of a root which promotes growth causing root to grow against gravity
Growth is due to cell elongation and due to EXPANSIN
Auxin activates a proton pump which increases number of protons inside cell wall (increased conc)
Expansin depends on pH, thus with increased protons (more acidic), expansin is more active (disrupts microfibrils)
What is Ordovician Landscape?
Ordovician landscape is the predicted landscape of earth long ago where plants were small and always next to water bodies
Vascular plants developed 380 mya (provided structure + transport)
What is water require for in plants?
Photosynthesis, cooling of plant, transporting nutrients, structural support
Roots are the source for water
Shoots are the sink for water(where it is needed)
Macro/micronutrients that plants require
Macronutrients = Nitrogen and Phosphorus
Lack of nitrogen = required for synthesis of DNA and protein (needed for growth)
Lack of phosphorus = needed for synthesis of DNA and ATP and phospholipid production (plant becomes dark green = stress accumulation)
Micronutrient = Iron
Lack of iron = important in redox reactions + ribosomes causing plant to become yellow (affects synthesis of chlorophyll)
What is water potential?
Tendency of a solution to uptake water (pure water) across a partially permeable membrane
Water ALWAYS moves across partial permeable membrane towards a lower (NEGATIVE) water potential
Equation:
Water potential = Solute potential + Pressure potential
What is solute potential and pressure potential?
Solute potential - the greater the solute concentration, the lower water potential (water will move in)
Pressure potential - the greater the internal pressure, the higher water potential
What is turgor pressure?
Turgor pressure is the same as pressure potential
Low turgor pressure = water will move in plant by osmosis due to low solute potential
(water moves in as water is important for structure, it must maintain it’s turgor)
How is water taken up by roots?
Apoplast route - water moves through connect cell walls and intercellular spaces between cells
-rapid and unregulated route
Symplast route - water moves through cytoplasm via plasmodesmata
-slow and regulated route
Water reaches the CASPARIAN STRIP (cell wall layer, with lignin, found in between endodermis (tissue surrounding vascular bundle)
Casparian strip = diffusion barrier, enable selective uptake of solution (protects plant)
It is a protective barrier where solutes MUST go through the symplast route to get inside vascular bundle
When solution reaches vascular bundle, it is active transported out of the cell into the symplast, causing water to follow by osmosis
Water movement in xylem
Water forms a continuous stream in xylem vessels due to cohesion and adhesion
Cohesion - interaction of water molecules with each other (hydrogen bonds)
Adhesion - interaction of water molecules and xylem wall (capillary action)
Transpiration
Transpiration is the loss of water due to evaporation from leaf surface
(concentration of water vapour is less outside of plant, thus water vapour diffuses out through stomata)
Transpiration-Cohesion-Tension Mechanism
Transpiration generate surface tension in the apoplast of mesophyll cells
- this pulls water into apoplast from adjacent cells
- tension in mesophyll cells draws water out of the veins into apoplast mesophyll cells
- also causes water to enters roots by osmosis
Pressure potential in xylem
Gradient of negative pressure potential lifts water column by bulk flow
Pressure potential in xylem becomes more negative as you move up plant (roots –> outside air)
Stomata
Stomata is controlled by guard cells that become turgid when open and flaccid to close
Regulated by light, temperature, carbon dioxide and water availability
Stomata in the presence of light
Light activates photoreceptor providing protons to be pumped outside of stomata cell
Proton gradient allows potassium and chloride ions to be moved inside of cell
- Potassim diffuses in poassively
- Chloride is pumped in through the use of protons
Movements of ions decrease solute potential inside cell causing water to move in cell by osmosis
(guard cells become turgid and opens stomata)
Transport of Sugars in Plants
Translocation = movement of carbohydrates + other solutes through phloem
(some tissues can be a sink at one time but become source later on, e.g. carrot leaves are source of carbohydrates and stored in roots, then carrots become the source)
Mass Flow
Loading = sucrose + other solutes are actively transported into sieve tubes via companion cells
Sucrose accumulates in sieve tubes (decreases water potential), causing water to be drawn in by osmosis
Increase of water causes pressure potential to increase in sieve tubes
Sucrose moves down to less concentrated and into sink cells by active transport (causes increase of water potential in sieve tube) causing water to move out into xylem
Tapping the Sap
Using aphids stylet to tap into phloem of plants to get sap samples
Stylet is punctured into phloem and due to high pressure, sap flows into tube and into anus of bug
Stylet is snapped off and sap is continuously flows out
Tissue and Organ Definition
Tissue = group of similar cells that form a functional unit Organ = group of several tissues with a distinct function
Development Key Stages (animals) - Determination
First step: Determination - establishment of cells fate before development
Zygote undergoes grastulation (develop of grastula which is the primary germ cell layers)
-Ectoderm = forms external surfaces + nervous system (e.g. skin, nervous system)
-Mesoderm = internal tissues (e.g. muscles, blood vessels, connective tissue)
-Endoderm = is the alimentary canal and organs that branch off it (e.g. liver, pancreas, lining of gut, lungs)
Experiment to see when determination occurs
Trial A and B
A: early posterior tissue was taken and transplanted into another host anterior region. when embryo developed, tissue developed into anterior structures (indicating determination had not occured yet)
B: older embryo posterior tissue was used this time and when embryo developed, posterior structures grew in the place of anterior (indicates determination already occured)
Development Key Stages (animal) - Differentiation
Specialising of cells after determination (specialisation of cells occur due to specific gene expression)
Gene regulation = specification of cell fate
-essential genes (e.g. histones, ribosomes) are expressed in all genes
Development Key Stages (animal) - Morphogenesis and Growth
Process of differentiating cells are grouped and organised into different sections of body
Mechanisms occuring in morphogenesis include: Apoptosis (death of cells) Dividing Changing shape Moving Around Adhering to each other Formation of tissues/organs Breaking free of epithelial connections
Growth is the increase in size due to proliferation (many divisons) and cell enlargement
What are stem cells?
Stem cells are undifferentiated cells that can divide indefinitely (forms another stem cell and daughter cell)
They are found in regenerative tissues where new cells must be made to replace old cells (e.g. lining of gut)
(stem cells in lining of gut can differentiate into goblet cells, enterocytes (absorption) or enteroendocrine (releases hormones)
Cell Potency of stem cells
-Ability of stem cell to give arise to other types of cells
Totipotent - can give rise to ALL cell types in an organism (only found in zygote)
Pluripotent - can give rise to all cell types of body but NOT extraembryonic tissues (e.g. placenta = embryonic stem cells; iPS stem cells)
Multipotent - can give rise to specific several cell types (e.g. intestinal stem cells)
Unipotent - give rise to only one type of cell (e.g. skin cells)
iPS stem cells - induced pluripotent cells
- changing genes in a cell to make it another cell type
e. g. taking skin cells and reprogramming its genes back to undifferentiated state (expressing stem-cell key genes) then reintroduced into body to get wanted cell types
Animal Tissue Types - Nervous Tissue
Nervous Tissue
Function: gathering information from external/internal environment, processing, controlling physiology and behaviour of body
Neurons = transmits electrical signals
electrical signals can either form inhibitory or excitation electrical network
3 Types of neurons:
Sensory neurons - generates electrical signals (e.g. pain/pressure receptors on skin, stretch receptors in muscles, olfactory receptors)
Interneurons (relay neurons) - takes input from one neuron and delivers/output to another neuron
Motor neurons - receives signal from interneuron and output to muscles (effector)
Glial cells - cells found in neurons that provide nutritional/mechanical support (e.g. Schwann cells)
Animal Tissue Types - Epithelial Tissue
2D sheets of cells that line body surfaces, internal cavities and internal tubes
Function: barrier function between body and outside world and between compartments of body (e.g. Endothelia = lining of blood vessels and are mesodermal origin)
Structure:
-Apico-basally polarized
Apical side = microvilli side
Basal side - Extracellular matrix side (basal lamina)
-Tight Junctions (below microvilli)
prevent passage of small molecules between cells (must go through microvillli to enter cells)
-Adherens Junctions and Desmosomes
Function: mechanical support
Actin filaments inbetween epithelial cells with Adherens junctions/Desmosomes achoring them into the cell
-Gap junctions
connection of cytoplasm of different cells (cell communication)
-Cell-Matrix Junctions
Connecting the epithelical cell to underlying extracellular matrix (ECM protein) (basal lamina) by adhesion molecules (actin molecules joins to matrix)
Epithelia have many functions: glands (secreting enzymes)
intestine - absorption of nutrients
skin - protection
airways - beating to cilia to keep airways clear
