Multicellularity, Development, Reproduction

  • Describe the physiological challenges of and explain the adaptations for large cell size and multicellularity

  • Large cell size can lead to difficulties in nutrient and waste transport due to increased distance from the cell membrane to the interior, resulting in slower metabolic rates.

  • To adapt, larger cells often develop specialized structures such as organelles that compartmentalize functions, thereby enhancing efficiency in metabolic processes.

  • Multicellularity allows for division of labor among cells, which can lead to more efficient functioning as different cell types can specialize in various tasks such as nutrient absorption, waste elimination, and reproduction.

  • Additionally, multicellular organisms often evolve complex signaling mechanisms to coordinate activities among cells, ensuring that they work together effectively to maintain homeostasis.

  • 1→constraint on cell size

    • small size→rely on diffusion to bring nutrients and eliminate waste and as cells become larger it is harder for them to exchange nutreints and waste - surface-area-to-volume ratio

      • plasma surface area→cells only interface with environment to take up nutrients or eliminate waste

      • internal transport→as cells get larger, it takes longer to transport material inside of them because more distance to cover

      • eukaryotes

        • cell geometry→some cells are long and thin to increase surface area

        • reproduction→cell divides into two cells when it becomes too large

        • internal complexity→eukaryotic cells have internal organelles that compartmentalize and perform special tasks

        • multicellularity→more complex solution

  • 2→multicellularity for development and specialization

  • 3→reproduction in eukaryotes

  • Explain the roles of the five essential developmental processes in development of a multicellular organism

    • key problem→growth and development must be regulated

      • regulation

  • 5 developmental processes

    • cell proliferation→reproduction of new cells via mitosis and critical for adding new cells

    • programmed cell death→death of specific cells

    • cell movement or differential cell movement→movement to new locations in body - animal cells move while plant cells differentially expand

    • cell differentiation→process of becoming specific cell type

    • cell-cell interaction (induction)→becoming a specialized cell

      • morphogen→signaling molecule that causes cell to take certain identity; displays a concentration gradient in developing embryo; amount of morphogen a cell detects helps determine what cell will become

      • each of these other processes are a result OF induction

  • the timing of these developmental processes is highly regulated and together result in development of specified tissue types and morphogenesis (development of organisms' shape)

    • cell proliferation→mitosis to result in creation of two identical daughter cells

    • induction→cell to cell communication from surrounding cells alters gene expression into two genetically identical cells

    • cell differentiation→two distinct specialized cell types

  • Describe the major reproductive strategies of eukaryotes

    • fungi→equal or multiple fission, budding, spores, fragmentation

    • animals→budding, parthenogenesis, polyembryony (twins), fragmentation

    • plants→spores, fragmentation, vegetative growth, parthenogenesis, polyembryony

  • asexual reproduction

    • budding→molecular different at unicellular v multicellular level; grow mini-me and then it breaks off in controlled way

    • fragmentation→broken off and then grow back tissues and organs they lost

    • parthenogenesis→growth of unfertilized egg

    • polyembryony→fertilized egg splits to form genetically identical clones

    • vegetative growth→growth of new organism from meristematic cells without spores or gametes

  • sexual reproduction→combination of (usually haploid) reproductive cells from two individuals to form a new (usually diploid) unique offspring

  • Explain the trade-offs between asexual and sexual reproduction, and predict which replication mode is more likely in different environmental conditions

    • cost of sexual reproduction→requires time/energy/finding mate, only females can produce offspring, novel combination of genes that can be unpredictable

    • two-fold cost of sex→asexual reproduction produces twice as many offspring as sexual reproduction given the same resources

      • asexual reproduction will outcompete

      • asexual will also only produce identical individuals that can be disadvantage

  • Compare and contrast the three types of life cycles of eukaryotes

    • sexual reproduction→offspring are not identical to the parents, always involves two changes in ploidy (number of copies of each chromosome)

      • 1→meiosis that reduces ploidy by 1/2 from 2n to 1n (haploid to diploid)

      • 2→doubling from 1n to 2n (return to original ploidy) by fertilization or joining of gametes

    • sexual reproduction in animals of 3 steps

      • gametogenesis→making gametes

      • mating→getting gametes together

      • fertilization→fusing gametes into single cell called zygote

    • sexual reproduction always requires

    • gamete→haploid, cannot become own organism

    • spores→haploid, can become own organism, multicellular diploid stage

  • A: Haplontic life cycleThe mature, multicellular organism is haploid and produces haploid gametes via mitosis, which fuse into a diploid zygote. The zygote immediately undergoes meiosis to produce haploid spores, which develop into mature multicellular haploid individuals.

  • B: Diplontic life cycle. The mature, multicellular organism is diploid and produces haploid gametes via meiosis, which fuse into a diploid zygote. The zygote undergoes development into a mature multicellular diploid organism.

  • C: Alternation of Generations. There is a mature multicellular haploid stage and a mature multicellular diploid stage. The multicellular haploid stage (the gametophyte) produces gametes via mitosis which fuse to form a diploid zygote. The zygote develops into a mature multicellular diploid individual (the sporophyte), which produces haploid spores via meiosis. The haploid spores then develop into a mature multicellular haploid individual.