BI110 Midterm 1

biology is

the study of life, primarily cellular life

what is life?

• property that distinguishes living and dead organisms and inanimate matter, manifested in functions such as metabolism, growth, reproduction, and response to stimuli

• a system in which proteins and nucleic acids interact in ways that allow the structure to grow and reproduce

how many different eukaryotic species are there on Earth?

5-10 million → 30,000 are discovered each year

how many different species of microbes are there?

1.6 million - 1 trillion

what are the characteristics of life?

• display order

• harness and utilize energy

• reproduce

• respond to stimuli

• exhibit homeostasis

• grow and develop

• evolve

how many characteristics of life are there?

7

display order

• all forms of life are arranged in highly ordered manners

• the cell being the fundamental unit that exhibits all properties of life

harness and utilize energy

• acquire energy from the environment and use it to maintain their highly ordered state

reproduce

• ability to make more of their own kind

• cells dividing into daughter cells

respond to stimuli

• make adjustments to their structure, function, and behaviours in response to the changes to their external environment

exhibit homeostasis

• regulate internal environment such that conditions remain relatively constant

evolve

• populations of living organisms adapt over generations to their environment

grow and develop

• increase their size by increasing the size and/or number of cells

the seven characteristics of life are emergent

• they emerge from simpler interactions that individually would not have the emergent property

• organisms are more than the sum of their parts

is a virus alive?

no

why are viruses not considered to be living?

• they cannot harness energy from the environment

• they lack cellular machinery and metabolism

• they cannot break down chemical compounds and transform energy on their own

• they require host cells in order to replicate as they take their genetic information

• they do not maintain homeostasis

when was the Earth and the rest of the solar system formed?

4.6 billion years ago

when did the first prokaryotes form?

3.5 billion years ago

when did oxygen become a significant part of the atmosphere?

2.5 billion years ago

when did the first eukaryotes form?

2 billion years ago

when did animals appear?

525 million years ago

when did dinosaurs go extinct?

65 million years ago

how long have humans been around?

150,000 years

what was found on Earth when it was first formed?

• hydrogen (H2)

• carbon dioxide (CO2)

• ammonia (NH4)

• methane (CH3)

• lots of water vapour

how long did it take for the Earth to cool down to temperatures to tolerate life?

500 million years

what are the biologically important macromolecules?

• nucleic acids

• proteins

• lipids

• carbohydrates

what is abiotic synthesis?

• the production of organic compounds in the absence of life

• if macromolecules are an absolute requirement of life, then simple forms of these molecules must have been produced early on in the absence of life

Oparin-Haldane Hypothesis

• from the 1920s

• organic molecules that formed the building blocks of macromolecules could have been formed abiotically given conditions of primitive Earth

• reducing atmosphere that lacked oxygen favoured the formation of organic molecules

• there was no ozone layer so the ultraviolet light and lightning provided energy for the formation of biologically important molecules

• highly reducing conditions are good for promoting the synthesis of large and complex molecules such as organic molecules

Miller-Urey Experiment

• simulated primitive Earth conditions

• demonstrated that abiotic synthesis is possible

• 15% of the carbon in the “atmosphere” was converted to organic compounds such as amino acids, urea, lactic acid ect

• added HCN (hydrogen cyanide) and CH2O (formaldehyde/methanol)

basic requirements of life

• biologically important molecules and macromolecules

• system for storing, replicating and passing on info

• ability to capture and use energy (transform)

• separate these systems and processes from surrounding environment in distinct compartments

polymers from monomers

• proteins and nucleic acids are polymers that were no formed by the Miller-Urey expt

• unlikely that polymers would form in aqueous environments; instead, it is thought that polymerization reactions could have occurred on solid surfaces

• example: RNA molecules can be formed spontaneously from simple precursors such as hot sand, clay and rock

why could a virus be considered to be living?

• they have the ability to store and transmit info

• has a plasma membrane that separates living material inside the cell from the non-living environment around it

• evolves over time

how does a clay surface catalyze polymerization?

• the charged microscopic layered structure of clay allows for the formation of relatively short polymers of protein and nucleic acid, that would be long enough to impart a specific function

• clays can also store potential energy- requiring polymerization reactions

• clay consists of very thin layers of minerals separated by layers of water only a few nanometers thick

• the layered structure is charged, allowing for molecular adhesion forces to bring monomers together in precise orientation that could more readily lead to polymer formation

the central dogma

• the flow of information from DNA to RNA is common to all forms of life

• information in DNA is used to synthesize proteins though an RNA intermediate

• all organisms contain DNA

• info in DNA copied onto molecules to RNA

how does the info stored in DNA direct the synthesis of proteins?

• central dogma - this s the basic flow of info in a cell

• RNA

• specialized molecular structures within the cell then read the RNA molecule to determine which building blocks to use to create a protein process called translation

• converts info stored in the language of nucleic acids to info in the language of proteins

• central dogma - pathway from DNA to RNA to protein

what does each step of information flow require?

• the involvement of a group of proteins called enzymes which catalyze the transcription of DNA to RNA and translation of RNA into proteins

what is the use of term “transcription”?

• for the generation of ribonucleic acid (RNA) from DNA to RNA

• emphasizes that DNA and RNA use the same language of nucleic acids

• DNA is the template for RNA

what is the use of the term “translation”

• for this step to indicate a change in the language used

• from nucleic acids to amino acids

what are ribozymes?

• ribonucleic acid (RNA) enzyme that catalyzes a chemical reaction

• RNA molecules that catalyze reactions

• discovered by Thomas Cech and Sidney Altman

• like the action of protein enzymes

• can catalyze reactions on precursor RNA molecules that lead to their own synthesis, as well as on unrelated RNA molecules

• increases the rate of phosphodiester bond cleavage and peptide bond synthesis

potential conformations of ribozymes/RNA

• RNA molecules are single-stranded, but not always elongated like mRNA

• some RNA molecules can fold up on themselves, and in some cases, take on elaborate, complex conformations

• the specific shapes they take fold into allow some of them to be able to act as catalysts

• not nearly as elaborate as proteins, but dozens of different ribozymes are now known

“RNA World” Hypothesis

• RNA was originally the primary substance of life

• DNA eventually replaced RNA as information storage molecule and proteins mostly replaced RNA as structural and catalytic molecules

possible scenarios for evolution of flow of information

• the first cells may have contained only RNA which was self replicating and catalyzed a small number of reactions for survival

• some RNA molecules evolved to catalyze the production of proteins

• the evolution of DNA followed next

• it is hypothesized that a small population of RNA molecules then evolved that could catalyze the formation of very short proteins before the development of ribosomes

• in contemporary organisms, ribosomes are required for protein synthesis

• cell that evolved the ability to use the information present in RNA to direct the synthesis of even small proteins would be a tremendous advantage bc proteins are more versatile than RNA

why are proteins more dominant structural and functional macromolecules of all cells?

• greater diversity

• much higher rate of catalysis than ribozymes

• proteins are evolutionarily advantageous compared to RNA

• aa of proteins interact chemically with each other in bonding arrangements which is not possible between nucleotides

why is DNA better than RNA as a repository of genetic information?

• DNA is double stranded and more stable than RNA and evolved better

• there are still many ribozymes that are relevant like crispr cas 9

the evolution of biological energy sources

• Life requires energy

• The earliest forms of life may have derived energy from geochemical activity at hydrothermal vents in ocean floor

  • Sources of energy in the vent environment:

  • H+ concentration gradient is used to produce ATP by ATP synthase

  • Oxidation-reduction reactions

today, how does Earth derive all energy?

• from photons coming from the sun,

redox reactions

• REDOX reactions-evolution of energy releasing electron transfer reactions in the earliest cells (protobionts).

• The fact that electron transfer systems are such common and indispensable mechanisms for releasing energy in living things today may be a relic of the evolution of life in an alkaline vent chemical environment.

what are organic molecules?

• complex molecules that contain the element carbon bonded with other elements

• Carbon is a versatile element that can form bonds with hydrogen, oxygen, and nitrogen, or other carbon atoms, to form huge carbon chains.

• Some small C-containing molecules such as CaCO3 (mineral) and CO2 (gas) are inorganic

inorganic molecules

• do not contain carbon

evolution of metabolism

• metabolic processes today catalyzed by enzymes

• they speed up rxns that can also take place on their own and are reversible

• Can work in forward (oxidative) and reverse (reductive) modes

• krebs cycle / Citric Acid Cycle is a fundamental metabolic pathway; one of the simplest set of metabolic reactions

• Experiments suggest that before enzymes evolved, reactions in the citric acid cycle could have been catalyzed by iron–sulfur (Fe–S) clusters, an abundant output product of alkaline hydrothermal vents.

the first cells

• Honeycombed, microscopic pores of hydrothermal vent chimneys provide a model for the evolution of early cells.

• Suggestion that earliest cell membranes were an inorganic casing composed of metal sulfides (NiS, FeS)

  • Would trap and concentrate organic molecules and allow complex chemical reactions

  • An environment conducive to an early metabolic process mimicking the reverse citric acid cycle

the first cells

• Lipids form liposomes (vesicles) spontaneously in aqueous environment

• Abiotically produced organic molecules trapped by a lipid membrane or membrane-like structure would form cell-like structure

• May have been precursors of cells

  • Membrane-defined compartment

  • Defined space, protected from external environment, for metabolic reactions to take place

Explain which properties of protobionts are associated with life.

ANSWER:   Their lipid bilayer membrane is selectively permeable. They undergo osmotic swelling/shrinking when placed in solutions of different solute concentrations. Some store energy as a membrane potential—a voltage across the surface—and some maintain simple reproduction. Furthermore, if enzymes are included in the solution from which the droplets self-assemble, some protobionts can carry out simple metabolism.

the tree of life fifty years ago

• All life was divided into two groups: eukaryotes and prokaryotes

• Eukaryotes—animals, plants, fungi

  •Organisms with cells that contain a nucleus and membrane-bound organelles (single-celled or multicellular)

• Prokaryotes—bacteria

  •Single-celled organisms of metabolic simplicity without a nucleus or membrane-bound organelles

the tree of life updated

• For centuries, organisms had been grouped based primarily on how they looked (morphology).

• A more objective approach to building phylogenetic trees was proposed: Use differences in DNA sequences.

  • Need to pick a gene that all organisms have

  • Gene needs to be relatively long in sequence

• Basically, if there aren’t many DNA sequence differences between two species, they’re probably closely related and likely diverged somewhat recently.

• Carl Woese suggested using ribosomal RNA (rRNA) genes to investigate organismal differences and develop a more accurate tree of life.

• His research team showed that life could be grouped into three domains:

  • Bacteria

  • Archaea

  • Eukarya

the three domains in the tree of life

• rRNA sequence comparisons suggest that Bacteria, Archaea, and Eukarya are three domains of living organisms, and that Eukarya are more closely related to Archaea than Bacteria.

• Despite of the morphological similarities of the bacteria and archaea, molecular evidence tells us that the eukarya and archaea are more closely related

• The three-domain tree of life showed that there were actually two major groups of organisms with prokaryotic cell structures: Bacteria and Archaea

impact of the three-domain tree of life

• showed the power of molecular data

• allowed the classification of microbes

• showed that archaeans are more closely related to eukaryotes than to bacteria

which domain tree is correct?

• Since it was first published, the three-domain tree has been well supported by many studies.

• In 2010, DNA sequencing of sediment samples from the Arctic Ocean led to the discovery of a new group of archaeans referred to as Asgard.

• Asgard archaeans contain genes previously thought to be present only in eukaryotes

• Analysis of Asgard DNA suggests that the three-domain tree of life may be incorrect.

• Eukaryotes may not have had an independent origin as the three-domain tree suggests.

  •May have evolved from within the Archaea.

• Growing expectation that the two domains of life (Bacteria and Archaea) view will be most widely accepted in future.

what is LUCA

• last universal common ancestor

• A common ancestor from which all present-day organisms are descended

likely features of LUCA

• Lived in the absence of O2 (anaerobic)

• Fixed CO2 into organic molecules (autotrophic!)

• Possessed a metabolic pathway similar to reverse citric acid cycle

• Depended on H2 as its source of H+ and electrons

• Converted N2 into ammonia

• Lived in hot environments

• Biochemistry depended on FeS

where did the oxygen come from in oxygenation of the atmosphere?

• cyanobacteria

• evolved ability to oxidize water instead of H2S (hydrogen sulfide) or FE2+ as the source of electrons for photosynthesis

• explosion of cynobacterial growth turned the planet green

• oxygenation lead to aerobic respiration

theory of endosymbiosis

• 2 bya

• Energy-transducing organelles, chloroplasts and mitochondria, thought to have been derived from free-living prokaryotic cells

• Mitochondria developed from ingested prokaryotes capable of using oxygen for aerobic respiration

• Chloroplasts developed from ingested photosynthetic prokaryote

• Prokaryotic ancestors of modern mitochondria and chloroplasts were engulfed by larger prokaryotic cells, in a symbiotic relationship

• The theory of endosymbiosis suggests that mitochondria and chloroplasts evolved from ingested prokaryotic cells

• All eukaryotic cells contain mitochondria, whereas only plants and algae contain both mitochondria and chloroplasts.

• Endosymbiosis therefore occurred in stages, with the evolution of mitochondria occurring first.

evidence supporting prokaryotic origin of mitochondria and chloroplasts

• Morphology—size and shape are similar to that of prokaryotic cells

• Reproduction—divide by binary fission in the same way as prokaryotic cells

• Genetic information—contain their own DNA with genes essential for organelle function

• Transcription and translation—contain complete separate set of transcription and translational machinery

• Electron transport—like prokaryotic cells, mitochondria and chloroplasts have electron transport chains

• DNA sequence analysis—has shown that these organelles belong on the bacterial branch of the tree of life

  • rRNA

what describes the emerging thinking about the tree of life?

• there are two domains of life: bacteria and archaea

what describes LUCA the best

• LUCA is thought to have lived without O2 and to have been autotrphic by fixing CO2 into organic molecules by using a pathway similar to a reverse krebs cycle

the endomembrane system

• another characteristic feature of eukaryotic cells

• When thinking abt other structures in eukaryotic cells, you can thinks of things in the endomembrane system which contains everything but chloroplast and mitochondria

• Collection of internal membranes dividing the cell into structurally and functionally distinct regions

• These regions include the nuclear envelope, the endoplasmic reticulum (ER), and the Golgi complex

• All inter-connected via vesicular trafficking pathway Proteins and lipids are moving thru these vesicles (interconnected)

horizontal gene transfer

• genetic inheritance between unrelated species

• more common than you think

• critical for endosymbiosis

• Some protein-coding genes once located in the chloroplast or mitochondrial genomes relocated to the nuclear genome.

• Following transcription of these genes, translation occurs in the cytosol before proteins are imported into the mitochondrion or chloroplast.

vertical gene transfer

• genetic inheritance from one generation to the next within a species

horizontal gene transfer and the tree of life

• A tree of life incorporating the endosymbiotic events from Bacteria to Eukarya.

• While the host eukaryotic cell and the nuclear genome are directly descended from the Archaea, the energy-transducing organelles the mitochondria and chloroplast are descended from free-living bacteria. 

• THERE ARE TWO DOMAINS OF LIFE

two major characteristics that distinguish eukaryotic cells form prokaryotic cells

• the separation of DNA and cytoplasm by a nuclear envelope

• the presence in the cytoplasm of membrane bound organelles with specialized functions: mitochondria, chloroplasts, the endoplasmic reticulum (ER) and the Golgi complex, among others

rise of eukaryotes is linked to increasing oxygen levels

• a type of sedimentary rock called banded iron is dated to determine when oxygen levels in the atmosphere started increasing

evolution of multicellular eukaryotes

• led to increased speciation

• multicellularity in the species of algae appears in the fossil record starting about 1.2 bya

• all cells are structurally and functionally autonomous

• some cells specialized in harvesting energy whereas some are related to motility of the organism

• Multicellular organisms contain cells that are structurally and functionally distinct

• Multicellular eukaryotes probably evolved by differentiation of cells of the same species that congregated into colonies.

• Colonies gave rise to division of labour leading to structural and functional distinction among cells.

what is a cell colony

• unlike a true multicellular organism, a cell colony is a group of cells that are all of one type

• there is no specialization in cell structure or function

the fossil record is incomplete

• only provides direct evidence abt what life was life millions of years ago

• tells us abt size and appearance of ancient animals and plants

• only reps a small record of the most successful of organisms

• soft bodied organisms with small geo distributions are underrepresent in fossil record

fossils for when organisms are buried by sediments of preserved in oxygen poor environments

• most fossils form in sedimentary rocks

• particles of sedimentary rock forming layers are called strata

• fossils usually preserve details of hard structures

  • bones

  • teeth

  • shells of animals

  • wood

  • leaves

  • pollen of plants

solving an energy crisis

• Ability of early eukaryotes to generate more energy led to remarkable changes—cells could become larger and more complex

• Eukaryotes contain several mitochondria, each converting energy into ATP

• A wider variety of genes could be supported that led to eukaryotic-specific traits: the cell cycle, sexual reproduction, phagocytosis, endomembrane trafficking, the nucleus, and multicellularity

• Increase in complexity comes about by being able to support a larger genome that codes for a greater number of proteins

Relative and Absolute Fossil Dating

• Relative ages of fossils are determined by the sedimentary stratum within which they are found.

• Absolute fossil ages are determined using radiometric dating, which determines how much of an unstable parent isotope has decayed to form another.

common attributes for all forms of cellular life

• 1.Lipid molecules form a membrane bilayer that defines the cell

• 2.A genetic system based on DNA

• 3.A system of information transfer: DNA to RNA to protein

• 4.A system of protein assembly using messenger RNA and transfer RNA using ribosomes to polymerize the amino acids into peptides

• 5.Reliance on proteins as the major structural and catalytic molecule

• 6.Use of ATP as the molecule of chemical energy

• 7.The breakdown of glucose by the metabolic pathway of glycolysis to generate ATP (chemiosmosis, substrate-level phosphorylation)

cell theory

• By the mid-19th century, microscopic observations had yielded three generalizations, which constitute the cell theory

• All organisms are composed of one or more cells

• The cell is the basic structural and functional unit of all living organisms

• Cells arise only from the division of preexisting cells.

many kinds of cells

• Unicellular organisms carry out all activities necessary for life

• In multicellular organisms, the activities of life are divided among numerous types of specialized cells

• Cells assume a wide variety of forms in different prokaryotes and eukaryotes

size of cells

• Most cells are too small to be seen by the unaided eye—ranging from about 0.5 μm (bacteria) to a few hundred micrometres (plant cells)

• To see cells and the structures within them we use two types of microscopes

  • Light microscopes use light to illuminate the specimen

  • Electron microscopes use electrons to illuminate the specimen

why are cells so small

• Cell size is limited by surface area-to-volume ratio

  • doubling diameter of cell increases its surface area by four times and increases volume by 8 times

• The volume of a cell determines the amount of chemical activity that can take place within the cell.

• Surface area determines the amount of substances that can be exchanged between a cell and the outside environment

the plasma membrane

• All cells are surrounded by the plasma membrane, a bilayer made of lipids with embedded protein molecules

• The lipid bilayer is a hydrophobic barrier to water-soluble substances

  • Selected water-soluble substances can penetrate cell membranes through transport protein channels

  • Selective transport of ions and water-soluble molecules maintain the specialized internal environments required for cellular life

• Water-soluble substances cannot pass through the phospholipid part of the membrane. Instead, they pass through protein channels in the membrane; two proteins that transport substances across the membrane are shown. Other types of proteins are also associated with the plasma membrane. (Inset) Electron micrograph showing the plasma membranes of two adjacent animal cells.

internal organization

• DNA molecules are concentrated in a central area of all cells; DNA stores hereditary information (genes)

  • Genes are segments of DNA that code for individual proteins

• The cytoplasm (between the plasma membrane and the central region) corresponds to the cytosol and cytoskeleton

• Cytosol is an aqueous solution containing ions, various organic molecules, and organelles

• The cytoskeleton maintains cell shape and plays key roles in cell division, chromosome segregation, and transportation within the cell

prokaryotic cell

• nucleoid region has no boundary membrane

• many bacteria and archaea sepcies contain few of any internal membranes

• Three shapes are common among bacterial prokaryotes: spherical, rodlike, and spiral

• For most species, the DNA (located in the nucleoid) is a single, circular molecule (the prokaryotic chromosome)

• Information from DNA is copied into messenger RNA (mRNA) molecules and carried to ribosomes in the cytoplasm, which assemble amino acids into proteins

• Prokaryotic cytoskeletons maintain cell shape and also function in cell division

• Many bacteria and archaeans move using long flagella—the bacterial flagellum rotates in a socket and pushes the cell through a liquid medium

• Hairlike pili attach the cell to surfaces or other cells—a special sex pilus joins bacteria during mating

• The plasma membrane is typically surrounded by a rigid external cell wall coated with polysaccharides (glycocalyx)

• When the glycocalyx is loosely associated with the cells, it is a slime layer; when it is firmly attached, it is a capsule (as in previous figure)

• The plasma membrane contains molecular systems that metabolize food molecules (or light energy) into the chemical energy of ATP

eukaryotes

• have a membrane bound compartment called the nucleus

• cytoplasm typically contains extensive membrane systems that form organelles

golgi complex and exocytosis

• The Golgi complex “sorts” proteins to ensure they are delivered to their final destination

• the golgi sorts proteins to make sure they get to their lysosome or plasma membrane

• proteins that are leaving the cell move through this pathway

• Proteins to be secreted from the cell are transported to the plasma membrane in secretory vesicles, which release their contents to the exterior by exocytosis

• there is an aq protein that is meant to be secreted

• the membrane of the vesicle fuses with the membrane of the plasma membrane

• membrane proteins of plasma membranes get there the same way

• The membrane of the vesicle fuses with the plasma membrane and becomes part of the plasma membrane

endocytosis

• Vesicles also form by the reverse process, endocytosis, which brings molecules into the cell from the exterior

• The plasma membrane forms a pocket, which bulges inward and pinches off into the cytoplasm as an endocytic vesicle

• Endocytic vesicles carry materials to the Golgi complex or other destinations such as lysosomes

lysosomes

• Lysosomes are small, membrane-bound compartments containing hydrolytic enzymes that digest complex molecules

• Cells recycle the constituents of these molecules

• Lysosomes are found in animals and plants

• Lysosomes are formed by budding from the Golgi complex; their hydrolytic enzymes are synthesized in the rough ER

• The pH within lysosomes is acidic (pH = 5), significantly lower than the pH of the cytosol (pH = 7.2)

• Lysosomal enzymes digest food molecules, worn out organelles (autophagy), and materials engulfed by phagocytes (cells of the immune system)

• In lysosomal storage diseases, one of the hydrolytic enzymes normally found in the lysosome is absent

mitochondria

• membrane bound organelles in which cellular respiration occurs

• two membranes - outer and inner

• outer covers

• inner is expanded by folds called cristae

• more SA important bc more capacity for CR

cellular respiration

• the process by which energy rich food molecules are broken down to water and CO2 by mitochondrial reactions, and energy is converted to ATP

• mito require oxygen for CR

• breathing in animals

the cytoskeleton

• The cytoskeleton is an interconnected system of protein fibres and tubes that extends throughout the cytoplasm

• The cytoskeleton maintains a cell’s characteristic shape and internal organization, and functions in movement

• The cytoskeleton of animal cells is comprised of microtubules, intermediate filaments, and microfilaments

microtubules

• Outer diameter of 25 nm

• Microtubules are assembled from dimers of α- and β-tubulin proteins

• Dimers are organized head-to-tail in each filament, giving the microtubule polarity (+ and – ends)

• Microtubules are dynamic structures, changing their lengths by the addition or removal of tubulin dimers

• Centrosome (or cell centre) serves as MTOC in many animal cells

  • Organizes microtubule arrays that are involved in various processes

  • mitosis, holding organelles in position, and others

  • Centrosomes are comprised of two short, barrel-shaped structures also formed from microtubules called centrioles

• Microtubules provide tracks along which vesicles move between the cell interior and the plasma membrane

• Microtubules separate and move chromosomes during cell division (mitosis), and are involved in moving some eukaryotic cells themselves

motor proteins

• Eukaryotic cell movements are generated by “motor” proteins that push or pull against microtubules (dyneins and kinesins) or microfilaments (myosins)

• One end of a motor protein is fixed to a cell structure such as a vesicle

• The other end has reactive groups that “walk” along a microtubule or microfilament, using ATP for energy

Intermediate Filaments

•Intermediate filaments are intermediate in size between microtubules and microfilaments (8-12 nm)

•Assembled from a large and varied group of intermediate filament proteins

•Intermediate filaments occur singly, in parallel bundles, and in interlinked networks

•Provides structural support in many cells and tissues, and are tissue-specific in their protein composition

Microfilaments

•Microfilaments are thin (5-7 nm diameter) protein fibres assembled from actin subunits

•Microfilaments have polarity (+ and – ends)

•Microfilaments have many structural and locomotor functions:

•Components of contractile elements in muscle fibres

•Involved in cytoplasmic streaming, which transports nutrients, proteins, and organelles in animal and plant cells, and is responsible for amoeboid movement

•Divide the cytoplasm when animal cells divide

Flagella and Cilia

•Flagella and cilia are elongated, motile structures that extend from the cell surface

•

•Cilia are shorter than flagella and occur in greater numbers

•

•Movements of a flagellum propel a cell through a watery medium, and cilia move fluids over the cell surface

Eukaryotic Flagellum

The relationship between the microtubules and the basal body of a flagellum. (b) Diagram of a flagellum in cross-section, showing the 9 + 2 system of microtubules. The spokes and connecting links hold the system together. (c) Electron micrograph of a flagellum in cross-section. Individual tubulin molecules are visible in the microtubule walls.