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Chapter 4

Eucaryotic Cell Structure and Function

The key to every biological problem must finally be sought in the cell

- E. B. Wilson

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1. An Overview of Eucaryotic Cell Structure membrane-delimited

nuclei membrane-bound

organelles that perform specific functions

more structurally complex than procaryotic cell

generally larger than procaryotic cell

Figure 4.3 The structure of two representative eukaryotic cells

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2. The Plasma Membrane and Membrane Structure the fluid mosaic model is based on

eucaryotic membranes major membrane lipids include

phosphoglycerides, sphingolipids and cholesterol

eucaryotic membranes contain microdomains called lipid rafts they are enriched for certain lipids and proteins they participate in a variety of cell processes

such as cell movement and transduction

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3. The Cytoplasmic Matrix, Microfilaments, Intermediate Filaments, and Microtubules cytoplasmic matrix

the many organelles of eucaryotic cells lie in the cytoplasmic matrix

provides complex environment required for cellular activities

cytoskeleton vast network of interconnected filaments within the

cytoplasmic matrix filaments that form the cytoskeleton:

microfilaments (4-7 nm), microtubules (25 nm), and intermediate filaments (8-10 nm)

plays role in both cell shape and cell movement

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Microfilaments

minute protein filaments, 4 to 7 nm in diameter

scattered within cytoplasmic matrix or organized into networks and parallel arrays

composed of actin protein involved in cell motion and shape changes

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Microtubules hollow cylinders

shaped like thin cylinders ~ 25 nm in diameter

composed of two kinds of protein subunits (α & β tubulins) maintain cell shape involved in cell or organelle

movement participate in intracellular

transport of substances

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Intermediate filaments

heterogeneous elements of the cytoskeleton

~10 nm in diameter role in cell is unclear

some shown to form nuclear laminaothers help link cells together to form

tissues

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4. Organelles of the Biosynthetic-Secretory and Endocytic Pathways cytoplasmic matrix is permeated with an

complex of membranous organelles and vesicles that move materials into the cell from the outside (endocytic) and from the cell to the outside & from location to location within the cell (biosynthetic-secretory)

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The Endoplasmic Reticulum

irregular network of branching and fusing membranous tubules and flattened sacs (cisternae – s., cisterna)

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Two types of endoplasmic reticulum (ER) rough ER

ribosomes attachedsynthesis of secreted proteins by ER-

associated ribosomes

smooth ERdevoid of ribosomessynthesis of lipids by ER-associated

enzymes

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Functions of ER

transports & synthesize proteins, lipids, and other materials within cell

major site of cell membrane synthesis

synthesis of lysosomes

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The Golgi Apparatus membranous organelle made of cisternae stacked

on each other dictyosomes: stacks of cisternae involved in modification, packaging, and

secretion of materials including proteins

movement of materials

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Lysosomes membrane-bound vesicles involved in intracellular digestion contain hydrolases, enzymes which

hydrolyze molecules and function best under slightly acidic conditions

maintain an acidic environment by pumping protons into their interior

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The Biosynthetic-Secretory Pathway used to move materials to lysosomes as

well as from the inside of the cell to either the cell membrane or cell exterior

proteins synthesized by ribosomes on rough ER released in small vesicles cis face of Golgi apparatus trans face of Golgi apparatus

transport vesicles released from trans face of Golgi

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after release some vesicles deliver their contents to lysosomes while others deliver to cell membrane

quality assurance mechanism unfolded or misfolded proteins are secreted

into cytosol, targeted for destruction by ubiquitin polypeptides

proteasomes destroy targeted proteins

The biosynthetic-secretory pathway...

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Proteasomes nonlysosomal protein

degradation system by the attachment of small ubiquitins in a ATP-dependent process

observed in eucaryotes, some bacteria, and many archaea

involved in producing peptides for antigen presentation during immunological responses

Figure 4.9 Proteasome degradation of proteins

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The Endocytic Pathway

endocytosisused to bring materials into the celluptake of solutes or particles by

enclosing them in vesicles or vacuoles pinched off from the plasma membrane

in most cases materials are then delivered to lysosome and destroyed

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Types of endocytosis phagocytosis

use of cell surface protrusions to surround and engulf particles

resulting vesicles called phagosomes clathrin-dependent endocytosis

involves membrane regions coated on cytoplasmic side with the protein clathrin (coated pits)

coated pits have external receptors that specifically bind macromolecules

pinching off of coated pits forms coated vesicles called receptor-mediated endocytosis

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caveolae–dependent endocytosis enriched in cholesterol and the membrane

protein caveolin when caveolae pinch off membrane are called

caveolar vesicles do not deliver their contents to lysosomes may play role in signal transduction, transport

of small as well as macromolecules

Types of endocytosis...

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Autophagy delivery of materials to be digested by

route that does not involve endocytosis involves digestion and recycling of

cytoplasmic components double membrane (may be from

endoplasmic reticulum) surrounds cell component forming an autophagosome

autophagosome fuses with late endosome which ultimately becomes a lysosome

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Figure 4.10 The endocytic pathway. Materials ingested by endocytic processes are delivered to lysosomes. The pathway to lysosomes differs, depending on the type of endocytosis. In addition, cell components are recycled when autophagosomes deliver them to lysosomes for digestion. This process is called autophagy.

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Once lysosome is formed...

digestion occurs without release of lysosome enzymes into cytoplasmic matrix

as contents are digested, products leave lysosome and can be used as nutrients

resulting lysosome called a residual body which can release contents to cell exterior by process called lysosome secretion

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Eucaryotic Ribosomes

80S in size a dimer of 60S +

40S subunits

may be attached to ER or free in cytoplasm

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Eucaryotic ribosomes… ER-associated ribosomes

synthesize integral membrane proteins synthesize proteins that are secreted

free ribosomes synthesize nonsecretory proteins and

nonmembrane proteins some proteins are inserted into organelles

polyribosomes (polysomes) complexes of mRNA with numerous ribosomes

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Mitochondria

site of tricarboxylic acid (TCA) cycle activity

site where ATP is generated by electron transport and oxidative phosphorylation

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Mitochondrial structure outer membrane

contains porins similar to the outer membrane of gram negative bacteria

inner membrane highly folded to form cristae (s., crista) contains enzymes & electron carriers for

electron transport and oxidative phosphorylation

small spheres (F1 particles = ATP synthase):

synthesize ATP during cellular respiration

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Mitochondrial structure... matrix

contains ribosomes, mitochondrial DNA, and large calcium phosphate granules

uses its DNA & ribosomes to synthesize its own proteins

contains enzymes of the tricarboxylic acid cycle and the β-oxidation pathway for fatty acids

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Figure 4.14 Mitochondrial structure

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7. Chloroplasts

type of plastid cytoplasmic pigment (chlorophyll)-containing

organelles observed in plants and algae

site of photosynthetic reactions and storage of food reserves

surrounded by double membrane

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Chloroplast structure

double membrane system encompasses chloroplast

stroma (matrix)contains DNA, ribosomes, lipid droplets,

starch granules, and thylakoidssite of dark reactions of photosynthesis

(formation of carbohydrates from water and carbon dioxide)

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Chloroplast structure... thylakoids

flattened, membrane-delimited sacs grana (s., granum) – stacks of thylakoids

site of light reactions (trapping of light energy to generate ATP, NADPH & oxygen)

algal chloroplasts many contain a pyrenoidparticipates in polysaccharide synthesis in

algae

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Figure 4.16 Chloroplast structure

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8. The Nucleus and Cell Division

nucleus

membrane-bound structure that houses genetic material of eucaryotic cell

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Nuclear Structure

chromatindense fibrous material within nucleuscontains DNAexists in a dispersed condition in non-

dividing cells, but condenses to form chromosomes during cell division

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Nuclear structure...

nuclear envelopedouble membrane structure that

delimits nucleus

penetrated by nuclear pores pores allow materials to be transported into

or out of nucleus

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The Nucleolus

1 nucleolus/nucleus not membrane enclosed

granular & fibrillar regions

important in ribosome synthesis directs synthesis and

processing of rRNA directs assembly of rRNA

and ribosomal proteins to form ribosomes

No

Nu

20× 40×

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Mitosis and Meiosis

mitosisone component of cell cycle

the process of nuclear division and chromosome distribution in eucaryotic cells

distributes DNA to 2 new nucleiploidy (number of sets of chromosomes)

of daughter cells is the same as the mother cell

after mitosis, a diploid organism remains diploid

38Figure 4.17 The eucaryotic cell cycle

period of cell growth

gap 1 period (G1): synthesis of RNA, ribosomes, and cytoplasmic constituents

G2: preparation for mitosis and cell division

synthesis period (S): rapid synthesis & doubling of nuclear DNA and histones

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Mitosis and meiosis...

meiosiscomplex, two-stage process of nuclear

divisionnumber of chromosomes in the resulting

daughter cells is reduced by 1/2: diploid haploid

haploid cells act as gametes and fuse to reform diploid organisms

40Figure 4.19 Generalized eucaryotic life cycle

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9. External Cell Coverings cell wall

rigid covering variable make-up

algae cell walls contain cellulose, pectin, or silica ... fungal cell walls – chitin, cellulose, or glucan

pellicle relatively rigid layer of components just

beneath plasma membrane common in protozoa and some alage not as strong or rigid as cell wall provides characteristic shape to cell

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10. Cilia and Flagella

cilia (s., cilium)5-20 μm longbeat with two phases, working like oars

flagella (s., flagellum)100-200 μm longmove in undulating fashion (helical

waves)

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11. Comparison of Procaryotic and Eucaryotic Cells

Figure 4.24 Comparison of procaryotic and eucayotic cell structure

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The molecular unity of procaryotes and eucaryotes

same basic chemical composition

same genetic code

same basic metabolic processes