LIFESCI 1 Lecture 5: Week 5

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Life Sciences

From Unicellular to Simple Multicellular to Complex Multicellular Simple Multicellularity (Hollow Balls, Filaments, Sheets) ● Cell adhesion ● Little communication or transfer of resources between cells ● Little to no differentiation of specialized cell types ○ Most cells retain all function, including reproduction ○ Not susceptible to predation ● Every cells is in contact with the external environment ● What are the advantages of simple multicellularity? ○ Helps avoid predation ■ Aggregation, predators cannot focus on one of them ○ Anchoring and structure ○ Cooperative feeding ● Disadvantages? ○ Diffusion limitations ○ Death of some specialized cells can harm entire organism ○ Cooperation of non-reproductive cells (CANCER) ● Coenocytic organization: one large cell with many nuclei Complex Multicellularity ● Common characteristics of complex multicellular organisms ○ Highly developed mechanisms for adhesion between cells ○ Specialized structures for cell communication ○ Cell differentiation, organisation of tissues and organs ■ Cells must: ● Stick together ● Communication with one another ○ Plants: plasmodesmata ○ Animals: gap junctions ● Participate in a network of genetic interaction for cell division (growth) and differentiation ○ Plants: growth confined to cells at tips of stems and roots (meristems) ○ Animals: developing cells can migrate relative to each other ○ A small subset of cells contribute to reproduction (gametes) ○ Cell or tissue loss can be lethal for the organism ○ Presence of both interior and exterior cells (3D) ● Evolved 6 different times ○ Animals ○ Fungi (twice) ○ Red algae ○ Land plants ○ Brown algae (green algae is paraphyletic) ○ Earliest fossil record of complex multicellularity, soft bodied animals, 575 million years ago Evolutionary Adaptations ● Unicellular → multicellular ● Diffusion → bulk transport The acquisition of traits necessary for complex multicellularity occurred independently in plants and animals ● Cell adhesion molecules → a mechanism of communication → cell differentiation and bulk transport What is the major event that facilitated the evolution of diverse, complex multicellular organisms on Earth? ● Increase in atmospheric oxygen ○ More O2 → greater size → cell differentiation → bulk transport → even greater size → postive feedback loop of increasing size and complexity ● Complex multicellularity spread through the oceans only as the oxygen content of the atmosphere and oceans increased ● Organism radiated on land later, as one line of green algae adapted photosynthesis on land Complexity and Gene Regulation ● Cell differentiation and gene regulation allows for many complex phenotypes from relatively few protein-coding genes ● evolutionary-developmental biology (evo-devo) ○ Study of the evolutionary roles played by developmental genes Plants ● Human use of plants ○ Agriculture: artificial selection ○ Bio-prospecting → medicinal uses ○ Fuel sources- wood and coal ● Ecological roles ○ Produce oxygen ○ Provide habitat ○ Provide food--base of trophic pyramid ■ Primary producers that fix inorganic carbon ○ Fertilize soil (via decomposers) ○ Protect against erosion--hold soil together ○ Retain moisture in soil ○ Buffer local climate ● Evolution of terrestrial plants ○ Water to land transition ■ First terrestrial colonizer (470 MYA) ● Trace fossils suggest animals (arthropods) first colonized land (450 MYA) ■ Able to grow with tissues exposed to air ■ Dominant primary producers in terrestrial food chains ○ Three trends ■ Non-vascular plants ○ Molecular phylogenies shows all land plants form a monophyletic group. This suggests that there was a single transition from aquatic to terrestrial ○ Challenges ■ Water loss ■ Gas exchange ■ Transport ■ Gravity ■ Reproduction ■ Dispersal ● Typical features of a plant cell ○ Plant cell wall is a rigid barrier composed of polysaccharides ○ Vacuoles exert hydrostatic pressure on the cell walls and provide rigidity to the plant cell ○ Waxy cuticle is waxy layer that prevents water loss from stems and leaves ○ Stomata have pores that allow gas exchange in photosynthetic tissues ■ CO2 uptake in leaves ■ Light stimulate stomata to open (except for CAM) ■ High levels of CO2 inside the leaf cause stomata to close ■ Low water supply → guard cells dehydrate and shrink → stomata close ■ Evaporation from stomata creates negative pressure in leaves, pulling the water up from roots through xylem ● Avoiding desiccation ○ CAM photosynthesis ■ Open stomata at night and store CO2 ■ Photosynthesize during the day with stomata closed ■ Adaptation to arid conditions ■ Common in cactus and succulents ○ C4 photosynthesis ■ Stomata open during the day ■ Suppress photorespiration, accumulating CO2 in bundle sheath cells ■ Adaptation to warm, sunny conditions ■ Common in grasses like corn and sugarcane ■ TAKES MORE ENERGY ● Vascular Plant Stems ○ Xylem: water ○ Phloem ■ Transport from source to sink ■ Most photosynthesis occurs in the leaves ● Source of fixed C ● Growth Orientation ○ The hormone AUXIN stimulates cell growth in shoots inhibits cell growth in roots ○ Phototropism: grows toward
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