Chapter 3

How Cells Are Put Together

The Plasma Membrane

• “Sheet” of molecules that encases all living cells• Fluid mosaic model

– A model for the plasma membrane– The PM is composed of proteins, lipids and

carbohydrates all of which are embedded in the phosholipid bilayer and may be exposed to the cell exterior/interior

– The PM is fluid; lipids that make up the plasma membrane can move laterally

Phospholipid

Phospholipid Bilayer

Proteins Embedded in Lipid Bilayer

Fig. 4.7Nonpolar regions of the lipid bilayer lock proteins into membranes

Eukaryotic Cell

Plant Cell

The Nucleus

• Spherical structure ensheathed in the nuclear envelope (a double-layered membrane)

• Fx = Command center of the cell!!– 1. Directs protein synthesis– 2. Directs cell reproduction– 3. Stores genetic information (DNA)– 4. Site where DNA is converted to RNA

Nuclear Envelope

• Membrane that surrounds the nucleus

• Contains “openings” through which RNA (once it’s converted from DNA) exits

The NucleusAnimal

Plant

The Endomembrane System

• Route through which material is passed within the cell

• Main components include:– Nuclear envelope– Endoplasmic Reticulum– Golgi Complex– Vesicles– Cell membrane

Endoplasmic Reticulum• An extensive system of internal membranes that form channels and

interconnections; also vesicles• ER surface: cell makes proteins to be exported here• 2 types: Rough ER Smooth ER

•Heavily studded with ribosomes

•Fx = sites of protein synthesis

•RNA is converted to a polypeptide chain

•Surface ribosomes are scarce

•Rather, surface is embedded w/ enzymes that aid in manufacturing carbs and lipids

Endoplasmic ReticulumAnimal

Plant

Golgi Complex

• Sacs of flattened vesicles (“pancake-like”)

• Receives polypeptide chain from ER where it is modified (i.e. carb addition) and packaged into secretory vesicles

• Fx = packages protein for export from the cell

Golgi ComplexAnimal

Plant

Lysosomes

• Spherical organelles that arise from Golgi

• Contain digestive enzymes that break down macromolecules

• “Recycling centers” of the cell: they digest intracellular matter

Fig. 4.15The Endomembrane System

Additional Organelles

• Mitochondria

• Chloroplasts

Mitochondria

• Rod-shaped organelles that derived from a bacterium that invaded a host cell

• The powerhouse of the cell!• Only organelle other than the nucleus to house DNA

– Mitochondrial DNA (mtDNA) has unique genes in their DNA that are lacking in nuclear DNA

• Place where oxidative phosphorylation occurs– Energy (ATP) is extracted from food through a series of

chemical reactions

• Two-membrane structure: – Outer– Inner: forms folds (cristae) which encompass the matrix

Mitochondria

Animal

Plant

Chloroplast

• Unique to plants• Organelle in which photosynthesis occurs• Plant cells can contain one to several hundred• Two-membrane structure (outer and inner

membranes)• Larger size than mitochondria and more

complex inner membrane organization – Thylakoids: fused inner membranes that form stacks

in which photosynthesis occurs

Chloroplast

Plant

The Cytoskeleton

• Dense fiber structure• Fx: 1. Provide support to cellular structure

2. Anchors organelles to fixed locations• Composed of dynamic protein fibers that are embedded

to membrane proteins embedded in the PM– Microfilaments– Microtubules– Intermediate Filaments

Cytoskeletal Proteins

The Cytoskeleton, continued

Microfilaments• Fine, thread-like protein fibers• Composed of the contractile

protein actin, the most abundant cellular protein!!!

• Prevent excessive stretching of cells

• Form and dissolve readily ≡ major role in determining cell shape

• Roles in cellular movement include gliding and contraction (when in combination with myosin) http://www.biology.arizona.edu/cell_bio/tutorials/cytoskeleton/page1.html

The Cytoskeleton, continued

Microtubules• Composed of tubulin protein• Assembled by centrioles

– Located near the nuclear envelope in the cytoplasm at right angles to each other

• “Conveyor belts” of the cell• They move vesicles and

organelles (i.e. mitochondria) as well as chromosomes

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Microtubule.gif

Centrioles

The Cytoskeleton, continued

Intermediate Filaments• Filamentous fibers• ‘Intermediate’ in size • Ubiquitous throughout the body• Bundles of IFs are found in

characteristic ‘plaques’ that form in AD in brain

• Includes keratin

http://www.cytoskeleton.com/aif.htm

Fig. 4.23

Central

Vacuole

Plant

Central Vacuole

• Storage center

• Contains large amounts of water, ions, sugars and pigments

• Increases surface-to-volume ratio outside the vacuole – Applies pressure to plasma membrane– Plasma membrane expands → ↑ surface area

• ↑ surface area → MORE EFFICIENCY!!

Is there anything outside the plasma membrane??

Cell WallPlants

Cell Wall

• Present in plants and fungi – NOT ANIMAL CELLS• Composed of cellulose (polysaccharide)• Primary walls are laid while plant is growing • Individual cells are held together by the middle lamella• Secondary walls are present in some plants

– Inside the Primary walls of fully expanded, grown cells• Located OUTSIDE the plasma membrane• Fx: maintain cell shape

protectionprevents excessive water uptake

Fig. 4.25

Extracellular Matrix

Animal cell

Extracellular Matrix

• Present in ANIMAL cells• Mixture of glycoproteins, collagen, elastin, proteoglycans• Fx: serves as a protective layer for the cell

helps coordinate cell behavior• Attached to the plasma membrane by fibronectin

– Glycoprotein – Bind to integrins in the plasma membrane

Everyday Science

• Forensic Science uses mitochondrial DNA testing• mtDNA is maternally inherited• Why use mtDNA vs. nuclear DNA?

– mtDNA is more abundant – in every mitochondria in the cell

– nuclear DNA may degrade more easily as there is only one copy

• What is the drawback of mtDNA testing?• Relatives from the same mother can’t be excluded