The Tour of the Cell

Chapter 6

The Fundamental Units of Life

• All living things composed of cells• Cell structure correlated to cell function• All cells descend from existing cells

Microscopy

• Light microscope = visible light through specimen magnified by lenses– Up to 1000X

• Electron microscopes (EMs)

• Scanning EM (SEM) focus beam of electrons onto surface 3-D image

• Transmission EM (TEM) • focus beam of electrons through specimen • internal structures

• Gills of fish yeast

• HIV

Cell Fractionation

centrifugeseparates cell

components

Homogenization

Homogenate

Differential centrifugation

Tissuecells

TECHNIQUE

Supernatant poured into next tube

TECHNIQUE (cont.)

Homogenate

Pellet

Supernatant

1000 g 10 min

20,000g 20 min

80,000g 60 min

150,000g 3 hr

Nuclei, debris mitochondria membranes ribosomes

• Prokaryotic cells= Archaea and Bacteria• No nucleus, no membrane-bounded organelles• DNA in nucleoid region

0.5 µm

Eukaryotic cells = Plants, Animals, Fungi, Protista•DNA in nucleus•Organelles•Membrane bounded

•Cytoplasm = fluid + organelles•Cytosol = fluid

Featured scientist: Robert Hooke 1635-1703

Best CLM of its time!Micrographia was a best seller

The famous slide:

. . . I could exceedingly plainly perceive it to be all perforated and porous, much like a Honey-comb, but that the pores of it were not regular. . . . these pores, or cells, . . . were indeed the first microscopical pores I ever saw, and perhaps, that were ever seen, for I had not met with any Writer or Person, that had made any mention of them before this. . .

1. The plasma membrane = selective barrier allows passage of oxygen, nutrients, waste etc

• Composed of phospholipid bilayer

Features of cells

2. Surface to Volume ratio high

•Small cells have greater surface area relative to volume

•Larger organisms do not have larger cells than smaller organisms

Human Rat

The Eukaryotic Cell

1. The Nucleus

Hela cells

A. Nuclear envelope (NE)

–Double membrane; each a bilayer–Pores regulate entry and exit of molecules

from nucleus

Nuclear lamina fibrous proteins maintain shape of nucleus

Lamin A and lamin B can bind histones – may have role in chromosome organization

B. Chromatin = DNA + proteins

•Chromosomes = strands of chromatin

C. Nucleolus –Assembles ribosomes

• D. Nucleoplasm– Viscous fluid of nucleus

2. Ribosomes: Protein Factories

• Assemble amino acids into polypeptides– cytosol (free ribosomes)– RER/NE (bound ribosomes)

3. The Endomembrane System

• Components– Nuclear envelope– Endoplasmic reticulum (ER)– Golgi apparatus– Lysosomes– Vacuoles– *Plasma membrane

A. The Endoplasmic Reticulum

• >half of total membrane• continuous with nuclear envelope– Smooth ER – lacks ribosomes

1.Synthesizes lipids2.Metabolizes carbohydrates3.Detoxifies poison4.Stores calcium

• Rough ER (RER)

– Ribosomes assemble proteins thread through ER lumen transport vesicles

– Membrane factory

B. The Golgi Apparatus

• flattened membranous sacs called cisternae• cis and trans face

trans face(“shipping” side of Golgi apparatus)

• Functions of the Golgi apparatus:– Modifies proteins from ER– Sorts and packages protein into transport vesicles

Golgi makes polysaccharides in plants

Smooth ER

Nucleus

Rough ER

Plasma membrane

cis Golgi

trans Golgi

Where do the vesicles go?

• Virtual cell

Note: Ribosome, RER, vesicle, Golgi

C. Lysosomes

• membranous sac of enzymes that digest macromolecules

• What do they do?recycle cell components (autophagy)get rid of phagocytosed invadersform food vacuoles

How do they work?

phagocytosis A cell engulfs another cell to form a food vacuole– lysosome fuses with food vacuole and digests

molecules

D. Vacuoles– Food vacuoles formed by phagocytosis– Contractile vacuoles

• freshwater protists• store or/and pump excess water out of cells

– Central vacuoles• found in many plant cells• hold organic compounds and water

4. Mitochondria

• cellular respiration generates ATP (energy)• contain mtDNA• all eukaryotic cells have mt– Some have 1, some 1000sOuter

membrane

Cristae

mitochondrion

Mitochondria

• outer membrane and inner membrane fold into cristae– large surface area for enzymes that synthesize ATP

5. Chloroplasts (plastid)• found in plants and algae• sites of photosynthesis– green pigment chlorophyll, enzymes, other

molecules

6. Peroxisomes

• detoxify

catalase2 H2O2 2H2O + O2

(toxic)

• Bioflix Tour of animal cell – the big picture• Note:– Sticky extracellular matrix– Plasma membrane– Cytoskeleton – Mitochondria- ATP, surface area– Nucleus and nuclear envelope with pores– DNA and protein wrappings, code for protein– Ribosome builds protein– Endomembrane system = RER and SER + Golgi

7. Cytoskeleton

• Network of protein fibers organize structures and activities in cell

• Anchors organelles• Maintains cell shape

Cytoskeleton

• interacts with motor proteins to transport cargo or for movement

10 µm

Column of tubulin dimers

Tubulin dimer

25 nm

Actin subunit

10 µm

7 nm

5 µm

Keratin proteinsFibrous subunit

(keratinscoiled together)

8–12 nm

• Vesicles in a plant cell• Golgi sorting and packaging

8. Centrosomes and Centrioles

• Centrosome– microtubule-

organizing center

Centrosome

Microtubule

Centrioles0.25 µm

Longitudinal section of one centriole

Cross sectionof the other centriole

– centrioles • animal cells only• centrosome has pair • each with 9 triplets of microtubules arranged in a ring

centrosome

9. Cilia and Flagella

• Locomotor appendages of some cells

• Movement pattern controlled by microtubules

• Example: paramecium, algae

10. Extracellular materials

• Cells secrete materials external to plasma membrane

• Harvard life of a cell – 3 min.• Can you find –

– Cell membrane– Cytoskeleton– Microtubule polymerization and depolymerization– A motor protein walking along the cytoskeleton– Lysosomes and mt– A centriole– Nuclear pores with mRNA leaving nucleus– RER– Ribosomes making proteins– Vesicles budding with cis face of Golgi– Proteins leaving the cell

A. Cell Walls of PlantsAlso, prokaryotes, fungi, some protists

• protects, maintains shape, prevents excessive uptake of water

• cellulose fibers + polysaccharides and protein

• Layers of cell wall

– Primary wall: thin – Middle lamella: between primary walls of adjacent

cells– Secondary wall (some cells): between plasma

membrane and primary cell wall

• Plasmodesmata -channels between adjacent plant cells for water, nutrients…..

B. Extracellular Matrix (ECM) of Animal Cells

• No cell walls • Functions :

Support, Adhesion, Movement, Regulation

• Structure– Glycoproteins: bind to receptor proteins in membrane

called integrins• Integrins “glue cytoskeleton to ECM

Collagen

Fibronectin

Plasma membrane

Proteoglycan complex

Integrins

CYTOPLASM

Micro-filaments

EXTRACELLULAR FLUID

Collagen in the ECM Collagen, fibronectin and laminin of cartilage

C. Intercellular Junctions

• Function – Adherance, communication through direct physical contact

• 4 Types:– Plasmodesmata- plant cells– Tight junctions– Desmosomes– Gap junctions

• Tight junction seals against fluid and ions

• Desmosome in cells that experiencemechanical stress (skin)

• Gap junction connects cytoplasm to allow small molecules, ions to pass from cell to cell – connexin protein

Tight junction

0.5 µm

1 µm

Desmosome

Gap junction

Extracellularmatrix 0.1

µm

Plasma membranesof adjacent cellsSpacebetweencells

Gapjunctions

Desmosome

Tight junction

The Cell: Living Unit Greater Than Sum of Its Parts

• integration of structures and organelles to function

• example, a macrophage’s ability to destroy bacteria involves coordinating cytoskeleton, lysosomes, and plasma membrane

5 µ m