Chapter 6 a tour of the cell
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Chapter 6: A Tour of the Cell. Technology to study cells. light microscopes – pass visible light through specimen and lenses. magnification – ratio of image size to actual size. resolution – clarity of image; minimum distance between two distinguishable points.

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Chapter 6: A Tour of the Cell

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Chapter 6:A Tour of the Cell


Technology to study cells


light microscopes – pass visible light through specimen and lenses


magnification – ratio of image size to actual size


resolution – clarity of image; minimum distance between two distinguishable points


electron microscopes – focus beams of electrons through or onto specimen– resolution 100x better than light microscopes


scanning electron microscope (SEM) – studying external structures


transmission electron microscope (TEM) – studying internal structures


Cell Fractionation – take cells apart and isolate organelles


Uses a centrifuge; spin test tubes very fast, separates cell components by size and density


Surface are to volume ratio:– limits cell size because as cells get bigger, their volume increases faster than their surface area


– surface area important for transport of substances through the membrane

microvilli in intestine

increases surface area for absorption


All cells have:

  • cytosol– semifluid substance containing organelles and dissolved nutrients

  • plasma membrane – selective barrier

  • chromosomes – packaged DNA

  • ribosomes – make proteins


Prokaryotic Cells


Prokaryotic Cells

  • smaller than eukaryotic cells

  • no membrane-bound organelles

  • no nucleus (nucleoid – region containing prokaryotic DNA)

  • small ribosomes

  • circular DNA

  • plasmids


Bacterial conjugation using pili


Eukaryotic Cells


nucleus – contains DNA

  • nuclear envelope – double membrane that encloses nucleus

  • nuclear pores – holes in the nuclear envelope. Allow passage of large molecules.


  • chromosomes – made of chromatin, a complex of proteins and DNA

  • nucleolus – rRNA synthesized, ribosomes assembled


ribosomes – synthesize proteins

  • made of ribosomal RNA (rRNA) and protein

  • cells that synthesize many proteins have many ribosomes

  • either free-floating in cytosol (make proteins for cell’s use) or bound to rough ER (make proteins for secretion)


endomembrane system

  • more than half the total membrane of the cell

  • consists of membranous tubules and sacs (cisternae)

  • lumen – interior cavity of cisternae


smooth endoplasmic reticulum (ER)

  • no bound ribosomes

  • synthesizes lipids (phospholipids, oils, steroids)

  • stores calcium ions, especially in muscles (important to muscle contraction)


Enzymes detoxify drugs and poisons, especially in the liver

  • add hydroxyl groups to drugs; makes them more soluble

  • drug tolerance due to proliferation of smooth ER in addicts; higher doses required to achieve the same effect


rough endoplasmic reticulum (ER)

  • has bound ribosomes

  • continuous with nuclear envelope


  • helps in synthesis of secretory proteins (proteins made for secretion), especially glycoproteins – proteins that have carbohydrates on them

  • adds carbohydrates to glycoproteins, sends them in transportvescicles (sacs of membrane) to Golgi


  • Also the membrane factory of cell; makes new membrane for itself that becomes vescicles; these eventually become part of cell membrane


Golgi apparatus – products of ER modified, stored and then shipped

  • flattened sacs (cisternae)

  • cis face – receiving side

  • trans face – shipping side


  • vescicles from ER fuse with cis face and empy contents into lumen of cisternae

  • products of ER modified in Golgi:

    – modifies carbohydrates

    – alters protein structure


  • Golgi makes some macromolecules

  • products transferred from one cisternae to another, eventually arrive at trans face.

  • products sorted and “addressed” for where they will go


  • vescicles bud off trans face and carry contents to cell membrane for export or to different parts of the cell


Lysosomes – digest

  • membrane sac of hydrolytic enzymes

  • digests molecules and worn-out cell parts (autophagy)


phagocytosis – food particle engulfed by cell and contained in vescicle– vescicle merges with lysosome and is digested


Tay-Sachs disease – lysosomal disorder in humans, allows lipids to accumulate in cells.


Tay-Sachs

  • Lipids accumulate in nervous tissue

  • Degeneration of mental and physical abilities

  • Seizures, paralysis

  • Death before age 4

Cherry-red spot on retina identifies Tay-Sachs


Vacuoles

  • membrane-bound sacs

  • central vacuole – in plants, storage for nutrients and wastes, water

    – membrane: tonoplast


  • food vacuoles – formed by phagocytosis

  • contractile vacuoles – in protists, pump excess water out of cell


Mitochondria – make cell energy

  • change molecular energy to cellular energy; cell respiration


  • double membrane

  • outer membrane is smooth

  • Inner membrane has folds called cristae

  • intermembrane space – between outer and inner membrane

  • mitochondrial matrix – lumen within the inner membrane


Chloroplasts – make carbohydrates

  • a plastid (other plastids are amyloplasts (store starch in plants) and chromoplasts (contain pigments that color fruit and flowers)

  • contain pigment chlorophyll


  • double membrane

    – outer membrane smooth

    – inner membrane is stacks of sacs called

    thylakoids

  • a stack of thylakoids is a granum

  • fluid between granum and outer membrame is stroma


Peroxisomes

  • sac containing enzymes that transfer hydrogen to oxygen, producing H2O2

  • digestion of fats, detoxification of alcohol

  • not part of endomembrane system (lysosomes are)


Cytoskeleton

  • support, maintain cell shape

  • cell motility (movement): both movement of whole cell and parts of cell within.

  • motor proteins – help cytoskeleton accomplish movement


Microtubules

  • hollow tubes of 13 columns of tubulindimers

  • 25 nm

  • -tubulin and -tubulin

  • cell shape (reists compression), cilia and flagella, move chromosomes during cell division, organelle movement


Microfilaments (actin filaments)

  • 2 intertwined strands of actin

  • 7 nm

  • cell shape (resist force), muscle contraction, cytoplasmic streaming, pseudopodia in amoeboid movement


Intermediate filaments

  • thick cables of fibrous protein

  • 8-12 nm

  • fibrous Keratin protein

  • cell shape (resist force), anchorage of nucleus and organelles


Centosomes and centrioles

  • centrosome – region near nucleus where microtubules grow out from

  • centrioles – in animals, 9 sets of triplet microtubules that help organize mitotic spindle during cell division


Cillia and flagella

  • 9 + 2 arrangement of microtubules

  • dynein arms are motor proteins


flagella – a tail-like structure for cellular locomotion or moving liquid past cell– made of microtubules


cilia – a hair-like structure, for locomotion or moving liquid past cell


  • Dynein arms bend cilia and flagella

  • Dynein “walking”: arms of one microtubule grip adjacent doublet, push it up, release, then repeat


basal body – where cilium or flagellum is anchored to cell

  • – 9 sets of triplet microtubules (9 x 3)

  • – a basal body of a sperm flagellum enters egg and becomes a centriole


Extracellular components of plants


cell wall

  • made of cellulose microfibrils and proteins. Protects, maintains shape, prevents too much water

  • also in prokaryotes, fungi and some protists

  • primary cell wall – young cell wall. Thin and flexible

  • secondary cell well – in woody plants. Grown between membrane and primary wall.


  • middle lamella – between primary cell walls of adjacent cells. Rich in pectins (sticky polysaccharides). Glues cells together.

  • plasmodesmata


Animal Extracellular Matrix (ECM)

  • Mostly glycoproteins; mainly collagen fibers

  • Collagen embedded in a network of proteoglycans


fibronectin – another glycoprotein in the ECM that binds integrins on cell membraneintegrins – proteins that span the cell membrane and transmit info on changes outside the cell to the cytoplasm


  • Changes in ECM my trigger changes in cells.

  • Integrins help relay signals to and from cells

  • Play role in coordinating behavior of all cells in a tissue.


Intercellular junctions

  • Plasmodesmata (plants) – channels made by perforation in cell walls. Cytosol, water and nutrients passes through them, linking cells

  • tight junctions (animals) – membranes of cells tightly pressed together, bound by proteins. Prevents leakage.


tight juction


  • desmosomes(animals) – fasten cells together into a strong sheet. Like rivets.

  • gap junctions (animals) – channels between cells through which flow ions, sugars, other molecules. Useful in cell communication.


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