CH2 - Eukaryotic Microbial Cells

Lecture 8

eukaryotic cells contain:

• membrane-enclosed nucleus

• organelles and structures including the Golgi complex, pigmented chloroplasts (phototrophic cells only), lysosomes, ER, microtubules, microfilaments

• some have motility (flagella or cilia)

• some have cell walls

• membranes contain sterols that lend structural strength

nucleus contains

the cell’s chromosomes

DNA is wound around

histones for compaction and transcriptional regulation

nucleus is enclosed by two membranes:

• inner membrane interacts with nucleoplasm

• outer membrane interacts with cytoplasm

• together, referred to as nuclear envelope

nuclear envelope contains

nuclear pores that allow transport of proteins and nucleic acids into and out of the nucleus

nucleolus =

site of ribosomal DNA synthesis; located within the nucleus

whereas prokaryotic cells are generally haploid, many microbial eukaryotes can exist in either

haploid or diploid states

• mitosis process is the same

mitosis process

• chromosomes are replicated

• nucleus is disassembled

• chromsomes are segregated into two sets

• nucleus is reassembled in each daughter cells

meiosis

specialized form of nuclear division that converts a diploid cell into four haploid cells via two cell divisions

meiosis is used to

form gametes (eggs and sperm in higher eukaryotes, spores or related reproductive structures in eukaryotic microbes)

meiosis process

• chromosomes are replicated

• first cell division segregates homologous chromosomes into separate cells

• second cell division is like mitosis (haploid cells divide) to form

mitochondria, hydrogenosomes, and chloroplasts

• all are organelles that specialize in energy metabolism

• have evolutionary roots within the bacteria

• provide ATP to the eukaryotic cells from either the oxidation or organic compounds or from light

mitochondria

responsible for respiration and oxidative phosphorylation in aerobic eukaryotes

mitochondria enclosed by double membrane

• outer membrane is relatively permeable and contains pores that allow the passage of small molecules

• inner membrane is less permeable; structure resembles cytoplasmic membrane of Bacteria

mitochondria contain folded internal membranes called

cristae

mitochondria cristae

• formed by invagination of the inner membrane

• contain the enzymes needed for respiration and ATP production

  • also contain transport proteins that regulate the passage of key molecules into and out of the matrix

innermost area of mitochondria

matrix

mitochondria matrix

contains enzymes for oxidation of organic compounds (e.g. enzymes of the citric acid = major pathway for the combustion of organic compounds to CO2)

hydrogenosomes

anaerobic eukaryotes (killed by O2) that utilize strictly fermentative metabolism (e.g., Trichomonas and some protists) lack mitochondria

some eukaryotes hydrogenosomes:

• similar size to mitochondria

• lack Cristal and citric acid enzymes

• major function is oxidation private to H2, CO2, and acetate

hydrogenosomes - because they are anoxic and cannot respire, they

cannot oxidize the acetate produced from pyruvate oxidation like mitochondria do

hydrogenosomes - acetate is therefore

excreted from the hydrogenosome into the cytoplasm

some anaerobic eukaryotes have

H2-consuming, methane-producing archaea in their cytoplasm

• these methanogens consume H2 and CO2 produced by hydrogenosome and combine them to form methane (CH4)

chloroplasts

• chlorophyll-containing organelle found in phototrophic microbial eukaryotes

• function to carry out photosynthesis

• relatively large, visible with light microscope

• number/cell varies with species

chloroplast structure

• double membrane; permeable outer membrane and less permeable inner membrane

• inner membrane surrounds the stroma (analogous to the matrix of mitochondria)

• stroma contains large amounts of RubisCO (key enzyme of the Calvin cycle = biosynthetic reactions by which phototrophs convert CO2 to organic compounds)

chloroplasts permeability of the outer membrane allows

glucose and ATP produced during photosynthesis to diffuse into the cytoplasm where they’re used in biosynthesis

chlorophyll and all other components needed for ATP synthesis are located in a series of flattened membrane discs called

thylakoids

thylakoid membrane is

highly impermeable; functions to form a proton motive force that results in ATP synthesis

endosymbiotic hypothesis:

mitochondria and chloroplasts descended from respiratory and phototrophic bacterial cells

by associating with nonphototrophic eukaryotic hosts, the hosts gained

a new form of energy metabolism and the symbiotic bacterial cells received a stable and supportive growth environment inside the host

over time, these free-living symbionts became part of eukaryotic cells - Evidence:

• mitochondria, hydrogenosomes, chloroplasts contain their own circular DNA genomes

• also contain their own ribosomes; rRNAs closely related to bacterial rRNAs

endoplasmic reticulum (ER)

network of membranes continuous with the nuclear membrane

two types of ER: rough and smooth

• rough contains attached ribosomes; smooth does not

• smooth ER participates in the synthesis of lipids and carbohydrate metabolism

• rough ER is a major glycoproteins and new membrane material

golgi complex

stacks of cisternae (membrane-bound sacs) that function in conversation with the ER

the golgi complex function

chemically modifies (e.g., glycolysation) and sorts ER products into those destined for secretion vs. those that will function in other membranous structures in the cell

lysosomes

• membrane-enclosed compartments

• contain digestive enzymes used for hydrolysis of food

• degrade and recycle damaged cell components

• separate cell’s lytic activities away from cytoplasm

• following degradation of macromolecules, resulting nutrients pass from the lysosome into the cytoplasm for use by cytoplasmic enzymes

cytoskeleton

internal structural support

microtubules

• hollow tubes 23 nm in diameter; composed of a- and B-tubulin

• function to maintain cell shape and motility; move chromosomes (mitosis) and organelles

microfilaments

• polymers of actin strands, 7 nm in diameter

• function to maintain and/or change cell shape; involved in amoeboid motility and cell division

intermediate filaments

• keratin protein fibers, 8-12 nm in diameter

• function to maintain cell shape and position organelles

flagella and cilia

• present on surface of many eukaryotic microbes

• function as organelles of motility that allow cells to move by swimming

eukaryotic flagella are structurally distinct from prokaryotic flagella

and do not rotate; instead use a whiplike motion

cilia are essentially

short flagella that beat in synchrony to propel the cell

eukaryotic flagella and cilia are structurally similar

• each contains a bundle of nine pairs of mircotubules surrounding a central pair of microtubules

• dynein is attached to the microtubules and uses ATP to drive motility

eukaryotic flagella and cilia movement

is the result of sliding of microtubules against one another in a direction toward or away from the base of a cell → confers a whiplike motion