BIOL 371: Theme 1 - Evolutionary Underpinnings of Plant and Animal Biology

organism

can consist of a single cell or multiple cells, all of its component parts work together to promote its survival

hierarchy of the living world

biosphere, ecosystem, community, population, organism, organ system, organ, tissue, cell, organelle, molecule

domains of life

bacteria, archaea, eukarya, defined by cell characteristics

bacteria

prokaryotic, unicellular, eg. cyanobacteria, proteobacteria

archaea

prokaryotic, unicellular, eg. crenarchaeota, euryarchaeota

eukarya

multicellular, complex, eg. animals, fungi, plants

prokaryotic cells

nucleoid, 70s ribosomes, no mitochondrion or chloroplasts but can do oxidative phosphorylation and photosynthesis, smaller cells, single, circular DNA

eukaryotic cells

has nucleus, membrane bound organelles, 80s ribosomes, mitochondrion, some have chloroplasts and plastids, endomembrane system, larger cells, many linear chromosomes

typical features - nucleus, endoplasmic reticulum, mitochondrion, centrioles, lysosomes, microtubules, vesicles, golgi complex, cytosol, plasma membrane, microfilaments

cytoskeleton

microtubules, intermediate filaments, microfilaments, cilia, flagella

protein fibre networks, support plasma membrane and organelles within cytoplasm, organelles can move around the cell, cell can control shape and movement, enables phagocytosis (engulfing food particles)

microtubule

hollow tube formed from tubulin dimers (a and B), acts as a railroad

intermediate filaments

strong fiber composed of proteins

microfilament

double helix of actin monomers, important in movement, intracellular transport, grabs myosin

cilia

9+2 arrangement of microtubules, cytoskeletal elements allowing cell to move or create currents, beat like oars, dynein motor proteins

flagella

9+2 arrangement of microtubules, cytoskeletal elements allowing cell to move or create currents, move like a whip, dynein motor proteins

cilia and flagella

cytoskeletal elements allowing cell to move or create currents

endomembrane system

collective term for nuclear envelope, lysosomes, golgi apparatus, vacuoles, endoplasmic reticulum

series of flattened sacs and tubes formed of lipid bilayer membranes, directly interconnected or connected by moving vesicles

compartmentalizes the interior of the cell, thus isolating incompatible biochemical processes, and to transfer products between compartments, greatly increases available surface area for synthesis

chromosomes in prokaryotes

genome is a single loop of DNA, good for rapid replication, but gene regulation is very simple

chromosomes in eukaryotes

genome divided by a number of linear chromosomes, allows for complex gene regulation and production of different tissue types

mitochondria

not found in prokaryotes, site of oxidative phosphorylation in eukaryote cells, folds (cristae) to increase surface area

chloroplasts

not found in prokaryotes, site of photosynthesis in eukaryote cells, a type of plastid, folds (granum, thylakoids) to increase surface area

sexual reproduction

eukaryotes, 2 haploid gametes from two parents to form a genetically different individual (vertical transmission), independent assortment and recombination generates genetic diversity

evidence for endosymbiotic origins (lateral transfer)

circular DNA, independent fission (eukaryotic cells cannot produce new mitochondria or plastids), 1-10 microns in size (like bacteria), double membrane, certain proteins specific to bacteria cell membrane are also in mito/chloro membranes, 70s ribosomes

primary endosymbiotic hypothesis

1. heterotrophic eukaryotes evolved first through union of ancestral archaeon with aerobic a-proteobacterium, which became mitochondrion

2. autotrophic eukaryotes evolved from heterotrophic eukaryotes through union with photosynthetic cyanobacterium, which became chloroplasts

2 alternative scenarios for evolution of final form of eukaryote cells

• ancestral archaean first evolved endomembrane system then entered symbiosis with a-proteobacterium, which became mitochondrion OR

• ancestral archaeon entered symbiosis with a-proteobacterium, which became mitochondrion - endomembrane system evolved subsequently

which came first - mitochondrion or the endomembrane system?

all-at-once model

the endosymbiotic event that produced the mitochondria also produced the endomembrane system

archaea can’t engage in phagocytosis —> outgrowth of archaean cell wall around adjacent symbiotic a-proteobacteria enclose them and they become mitochondria

alternatively, endomembrane system could arise from mitochondrial vacuoles

modern endosymbiosis

nitrogen-fixing organelle shown in Braarudospaera bigelowii (modern marine unicellular eukaryote alga), shown to originate from symbiotic nitrogen-fixing bacterium, ~100 Mya, host cell is eukaryotic - can engage in photosynthesis

secondary endosymbiosis

between 2 eukaryotes, symbiosis of heterotrophic eukaryote cell with an autotrophic eukaryote cell, 3 independent occurrences (3 major eukaryote taxa arose)

eukaryote cell size

most have diameters in the 1-100 um range, generally much larger than prokaryotic cells

cube-square relationship

surface area and volume for a solid do not increase linearly with an increase in linear dimensions

• surface area is proportional to length²

• volume is proportional to length³

• surface area is proportional to volume^(2/3)

exchange across membranes around or within a cell is by diffusion or active transport, both only work effectively over very short distances and rates are dependent on surface area

types of multicellularity

simple and complex

simple multicellularity

cell adhesion, cell-cell communication, structurally simple, no bulk flow, most cells are in direct contact with the environment

eg. volvox - constituent cells specialized as flagellated photosynthesizers or reproductive cells, slime molds - spend part of life cycle as unicellular amoebas, congregate and produce multicellularity structures under environmental stress

bulk flow

movement of fluids or gases through channels, rather than cell to cell

complex multicellularity

cells adhere, communicate, differentiate (different development) and specialize (different functions), formation of tissues

arose independently at least 6 times in the history of life, many unicellular eukaryotes have capacities needed for multicellularity (adhesion, communication)

symbiotic theory

theory on the origins of multicellular life, different unicellular species —> advantage in coming together to be an organism, no good evidence for this

syncytial theory

theory on the origins of multicellular life, no cytokinesis (just nuclear division) —> differentiated cells, not a great theory

colonial theory

theory on the origins of multicellular life, a colony of cells —> organism, specialization, gametes, best representation!

the great oxygenation event

rise in environmental oxygen levels due to photosynthesis, gave selective advantage to possession of mitochondria and aerobic respiration, which permitted multicellularity (cyanobacteria)

simple multicellularity appeared at the end, complex multicellularity appeared with subsequent rise in environmental O₂ levels

requirements for multicellularity

1. cells can adhere to one another

2. cells divide up tasks, specialize

3. cells learn to communicate with one another (affect one another’s behaviour, influence one another’s development, coordinate complex actions

eg. billion year old fossil shows evidence of cell to cell adhesion and cell differentiation

most evolutionary models begin with flagellated unicellular organisms (animals —> choanoflagellates)

why multicellularity?

unicellular organisms require metabolism, homeostasis, reproduction, repair, etc. with the resources of a single cell

single isolated cells must deal directly with their environment

unicellular cannot get very big (cube-square relationship)

selective advantages of multicellularity

division of labour (for processes at the same time)

economy of scale (working together, 1 vs. many)

increased size (avoid predation, storage, internal environment, motility, metabolism)

consequences of multicellularity

complexity —> predator/prey and host/parasite interactions

increased opportunity for diversity in form/functions and niches

challenges of being large and multicellular

intercellular communication —> diffusion, gap junctions, plasmodesmata, bulk flow, nerves, signalling molecules

cell adhesion —> cells in a multicellular body must stick together

cube-square relationship (since exchange takes place across surfaces, must create solutions to allow exchange and rapid transport)

structure and support - physical laws sets limits on size and performance, morphology reflects accommodation with these limits, challenges vary over animal body size range (~12 orders of magnitude)

metabolic rate

• O2 consumption increases with body size

• basal metabolic rate much lower for elephants than for mice

• larger animals have less surface area for dissipation of heat

• extensive folding to increase surface area for short range transport methods

  • larger bodies must have long range transport methods - bulk flow

gas exchange and surface area

highly folded cells to pack greater surface area into small volume, eg. gills, bear lamellae, composed of flattened epithelial cells

nutrient absorption and surface area

increase for absorption, within a small volume, eg. villi in small intestine

filtration and surface area

body’s combined capillary beds provide extremely large cumulative surface are for exchange between blood and tissues

homeostasis

defend cells against hostile environment, maintain stable internal environment

extracellular fluid component

all of the body’s water not found within cell plasma membranes, forms cells’ immediate environment

intracellular fluid component

all of the body’s water found within cells, liquid portion of cytoplasm

reproduction and growth and multicellularity

multicellular body must be able to produce new multicellular bodies, sex, fertilization, development, growth

development and differentiation require complex and flexible control systems (genetic network)

cell adhesion

cells must be attached to one another or to an acellular matrix, junctions

tight junctions

in animals, penetrate cell membranes of adjacent cells, fix cells in place, prevent movement of liquids between cells

anchoring junctions

in animals, of adjacent cells link to each other and to microfilaments (desmosomes) and intermediate fibres of cytoskeleton

gap junctions

form channels penetrating cell membranes of both cells, signalling molecules and water can be passed directly from cell to cell, cells can thus communicate with one another

plasmodesmata

found in plants, cells connected by small openings lined by extensions of the endoplasmic reticulum connecting the endomembrane systems of adjacent cells

tissues

group of similar cells and extracellular substances working together to carry out specific functions for the organism as a whole, requires that cells attach to one another and communicate, both plants and animals organized at this level

triploblasts

animals, 4 types of tissues, arising from 3 embryonic germ layers, specialized in structure and function

epithelial tissue

cover exposed surfaces, line internal passageways and chambers, produce glandular secretions

connective tissue

fill internal spaces, provide structural support, store energy, eg. bone, cartilage, blood

muscle tissue

contracts to produce active movement

neural tissue

conducts electrical impulses/action potentials, carries information

extracellular matrix and cells

a characteristic of animals, basic matrix is acellular, penetrated by network of collagen fibres, varies greatly among organisms and types of connective tissueco

collagen

an extracellular fibrous protein found in connective tissues, unique to animals