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 intestineincreases 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 transport vescicles (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 tubulin dimers• 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

membrane

integrins – 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.