2.1: Structures and Functions of Eukaryotic Cells

Eukaryotic Cell

• DNA contained in a membrane-bound nucleus

• cell membrane has phospholipid bilayer and embedded proteins

  • separate cells content from surrounding

• interior filled with jelly like cytoplasm

  • everything outside nucleus, but within cell membrane

• organelle = smaller part of a cell with a specific function

  • work together so the cell can perform all life functions

  • ex. nucleus, vacuoles, mitochondria

Nucleus

• where genetic info is stored and replicated

• DNA molecules combine with histone proteins to form a chromosome during cell division

• # of chromosomes varies per species

• in non dividing cells, chromatin represents unfolded strands of DNA

Chromosomes and DNA

• information handed down from parents to offspring

• 46 human chromosomes

  • 23 pairs

• each parent contributes a sex cell with ½ DNA

  • one chromosome of each pair

Structure of the Nucleus

• nucleoplasm = thick fluid that fills the nucleus

• nucleolus = denser region, contains RNA, proteins, chromatin

• nuclear envelope = double membrane with 2 phospholipid bilayers

  • separates nucleus from the rest of the cell

• nuclear pores = groups of proteins that form openings in the nuclear envelope

  • allow water and ions to flow through

Ribosomes

• small particle, consists of rRNA and protein

• free floating or attached to rough endoplasmic reticulum

• translate mRNA into chains of amino acids

• “make proteins”

• different structure than prokaryotes

Endoplasmic Reticulum

• connected to nuclear envelope

• complex of membrane bound tubules and sacs

Rough ER

• ER surface studded with ribosomes

• devoted to synthesis of proteins

• assemble proteins that are part of membranes or intended for export from cell

• proteins that function in cytosol are made by ribosomes that are freely suspended there

Smooth ER

• regions of the ER that have no bound ribosomes

• synthesizes lipids and lipid containing molecules (ex. phospholipids)

• in liver, detoxifies drugs and alcoho

• in testes and ovaries, produces testosterone and estrogen

Vacuoles and Vesicles

• vesicle = membrane bound sacs used to transport and storage of substances in the cell

  • form by pinching off from cell and organelle membranes

  • fuse with cell and organell membranes to release contents

  • animal cell has many small vesicles

• vacuole = storage of water, ions, sugars, amino acids, macromolecules

  • contains enzymes to break down macromolecules

  • controls turgor pressure

  • one large central vesicle in plants

Golgi Body

• cisternae = layers of membranous tubules

• collect and package materials (lipids and proteins) to be removed from the cell

• make and secrete carbohydrates

• dynamic structure

  • joins with vesicles at cis face (takes in vesicles-er)

  • pinches off vesicles at trans face (where contents are released)

Endomembrane System

• nuclear envelope, er, golgi body, vesicles

• transportation and producer processing

• organelles connected nuy transport vesicles or directly

1. on the surface of the ER, polypeptides are produced by bound ribosomes and extruded into the lumen rather than into cytosol

2. polypeptides travel through lumen to smooth ER where they are stored and processed

3. when ready for transport, pieces of smooth ER pinch off to form vesicles containing the protein

4. vesicles from smooth ER travel across the cell to the cis face of the golgi body where vesicles merge with membrane

5. vesicles release contents into interior of golgi body

6. in golgi body, some proteins stored, others modified

7. when modified are ready for transport, pieces of golgi body pinch off from trans face (vesicles)

8. vesicles transport proteins to cell membrane or other destinations

Lysosomes

• made in Golgi body

• membranous sacs filled with enzymes that catalyze hydrolysis reactions

• act as a garbage disposal or digester of old cell parts and bacterial foreign particles

Peroxisomes

• membrane-bound sac, dense crystal core

• contain oxidative enzymes that break down excess fatty acids and hydrogen peroxide

  • produce hydrogen peroxide, so contain catalase (H2O2→ H20 + O2)

• liver cells: synthesis of cholesterol and bile acids

• made in smooth ER

• in all eukaryotes

Mitochondria

• ‘power plants’

• make energy available to cells

• active cells have more mitochondria

• smooth outer membrane, folded inner membrane

• cristae = folds of the inner membrane

• matrix = fluid-filled space in inner layers

• contain some of their own DNA

• encodes some of their own proteins

• contains enzymes that help convert glucose → ATP

  • cellular respiration

  • waste products are CO2 and H2O

  • C6H12O6+6O2→6O2+6H2O+38ATP

Chloroplasts

• chlorophyll pigment

• absorbs light energy

• autotrophs = obtain energy from sun to make glucose by photosynthesis

• contain some of their own DNA

• double membrane

• stroma = thick liquid in inner membrane

• stroma surrounds system of flatten disks containing chlorophyll (thylakoids)

• granum = stack of thylakoids

Cell Wall

• rigid, semipermeable protective layer

  • plant cells, fungi, bacteria, algae, archaea

• consumption varies

  • combination of polysaccharides, glycoproteins, or both

• protection

  • barrier against viruses and water loss

• structure

  • turgor pressure = force exerted against cell wall as contents of cell push cell membrane against cell wall

• support

  • mechanical strength and support

  • direction of cell growth

Cytoskeleton

• cell skeleton = internal network of protein fibres

• extend throughout cytoplasm

  • structure, anchor membrane and organelles in place

• vesicles and other organelles move along fibres to other parts of cell

• pseudopods = appendages that enable the cell to propel itself through the fluid surrounding it

Centrosomes and Centrioles

• centriole = nine bundles of microtubukles

  • microtubules = hollow tubes that give organelles their shape, arranged in a ring

• centrosome = two centrioles at right angles

  • close to nucleus

  • formation of miotic spindle fibres

  • creates and organizes microtubules for cytoskeleton

Cilia and Flagella

• appendages that develop outside of some eukaryotic cells

• composed of an internal shaft made of microtubules

  • covered with outer membrane (continuous of cell membrane)

• flagella = one or two long appendages

  • like tails, whip like movement

  • ex. sperm, euglenoid

• cilia = shorter appendages

  • ex. paramecium, respiratory tract cells

Cytoplasm

• suspends organelles

• needed for reactions (H20)

• maintain shape, cell movement, material exchange

Fluid Mosaic Model

• phospholipid bilayer

• mixture of molecules

  • phospholipids

  • cholesterol

  • proteins

  • carbohydrates

Phospholipids

• “scaffolding”

• proteins and macromolecules are embedded

• held together by weak intermolecular forces, allow molecules in membrane to move

• if punctured or teared, molecules rearrange to seal rupture

• hydrophilic (polar) head group faces aqueous environment

• hydrophobic (2 non polar) tails face in

Phospholipid Regulation

• if too fluid: permits too many materials in

• if not fluid enough: prevents too many from entering

• room temp. viscosity should be like vegetable oil

Temperature

• too high, too fluid: not a barrier

• too low, solidifies to gel-like state

Presence of Double Bonds in Tails

• bonds form kinks in fatty acid tail

  • causes molecules to be less tightly packed together and more fluid

Fatty Acid Tail Length

• longer tails have more intermolecular attractions and are tightly packed together

  • less fluidity

• average length is 16 or 18 carbon atoms

Cholesterol

• maintains fluidity

• increased temperature, more intermolecular forces in membrane

  • lower fluidity

• lower temperature, breaks intermolecular forces in gel-like phospholipids

  • higher fluidity

Proteins

• stabilize cell membrane by linking with cytoskeleton of cell

• integral = embedded in membrane

  • transmembrane = on both sides of bilayer

• peripheral = loosely attached to outer regions of membrane or to integral proteins

• transport: across cell membrane

• reaction catalysis: carry out chemical reactions

• cell reactions: carbohydrate chains protrude from glycoproteins on outer layer of cell membrane

  • enables cell to recognize each other and identify harmful intruders (disease causing bacteria)

• signal reception and transduction: receptor proteins in membrane bind to signal molecules (hormones) and change shape

  • initiates cellular response, enables cells to receive and respond to signals from brain and organs

Carbohydrates

• structure: attached to proteins or phospholipids and protrude outside the cell

• function: allows other cells to recognize cell as belonging to the body instead of the intruder

• glycolipids = a membrane lipid bound to a carbohydrate

• glycoproteins = membrane protein bound to a carbohydrate