1

Cell Structure

Chapter 5

2

Cell Theory

• All organisms are composed of one or more cells.

• Cells are the smallest living units of all living organisms.

• Cells arise only by division of a previously existing cell.

3

Cell Characteristics• Genetic material

– single circular molecule of DNA in prokaryotes

– double helix located in nucleus in eukaryotes – nuclear envelope (double membrane

• Cytoplasm fills cell interior – – sugars, amino acids, – proteins - organelles

• Plasma membrane encloses – the cell – phospholipid bilayer

Phospholipid

Membraneproteins

4

Generalized Eukaryotic Cell

5

Cell Size

• Most cells are relatively small because as size increases, volume increases much more rapidly.

– longer diffusion time

6

Visualizing Cells

• Resolution - minimum distance two points can be apart and still be distinguished as two separate points

– Compound microscopes - magnify in stages using multiple lenses

– Transmission electron microscope - electrons transmitted through specimen

– Scanning electron microscope - electrons beamed onto surface of the specimen

7

Visualizing Cells

8

Prokaryotic Cells• Simplest organisms

– Cytoplasm is surrounded by plasma membrane and encased in a rigid cell wall composed of peptidoglycan.

no distinct interior compartments gram-positive – thick single layer wall that

retains a violet dye from Gram stain procedure gram-negative – multilayered wall does not

retain dye Susceptibility of bacteria to antibiotics

depends on cell wall structure.

9

Prokaryotic Cells

• Some use flagellum for locomotion– threadlike structures protruding from cell

surfaceBacterial cell wall

Flagellin

Rotarymotor

Sheath

10

Eukaryotic Cells

• Characterized by compartmentalization by an endomembrane system, and the presence of membrane-bound organelles.

– Central vacuole – plants, storage– Vesicles (smaller)– Chromosomes - DNA and protein– Cytoskeleton (internal protein scaffolding)– Cell walls – plants and fungi

11

12

13

Nucleus• Repository for genetic material• Directs activities of the cell• Usually single, some cells several, RBC none

– Nucleolus - region of intensive ribosomal RNA synthesis

• Surface of nucleus bound by two phospholipid bilayer membranes

– nuclear membrane– Nuclear pores – protein gatekeepers

Usually proteins going in and RNA going out

14

Nucleus

15

Chromosomes

• DNA of eukaryotes is divided into linear chromosomes.

– exist as strands of chromatin, except during cell division

– associated with packaging histones, packaging proteins

nucleosomes

16

Endomembrane System• Compartmentalizes cell, channeling passage

of molecules through cell’s interior.– Endoplasmic reticulum

Rough ER - studded with ribosomes Smooth ER - few ribosomes

17

Endoplasmic reticulum

• Largest internal membrane• Composed of Lipid bilayer• Serves as system of channels from the

nucleus• Functions in storage and secretion• Rough ER is “rough” because of associated

ribosomes (sites of protein synthesis• Smooth ER - lack associated ribosomes –

contained embedded enzymes, catalyze synthesis of carbohydrate and lipid molecules

18

Endomembrane System• Golgi apparatus

– collection of Golgi bodies collect, package, and distribute molecules

synthesized at one location in the cell and utilized at another location

Front - cis , Back – trans Cisternae – stacked membrane folds

19

Vesiclebuddingfrom roughendoplasmicreticulum

Fusionof vesiclewith Golgiapparatus

Migratingtransportvesicle

Protein

Proteins

Transportvesicle

Golgiapparatus

Cisternae

Ribosome

trans face

cis face

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

20

Endomembrane System• Vesicles

– Lysosomes - membrane-bound vesicles containing digestive enzymes – from Golgi

– Microbodies - enzyme-bearing, membrane-enclosed vesicles.

Peroxisomes - contain enzymes that catalyze the removal of electrons and associated hydrogen atoms

Peroxisome – named for hydrogen peroxide produced as a by-product

Enzyme breaks down to water and oxygen

21

Cytoplasm

PhagocytosisFood

vesicleGolgi

apparatus

Lysosomes

Plasmamembrane

Digestion ofphagocytizedfood particles

or cells

Endoplasmicreticulum

Transportvesicle

Old or damagedorganelle

Breakdownof old

organelleExtracellularfluid

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

22

Ribosomes

• Ribosomes are RNA-protein complexes composed of two subunits that join and attach to messenger RNA.

– site of protein synthesis– assembled in nucleoli

23

Organelles With DNA

• Mitochondria – bounded by exterior and interior

membranes– interior partitioned by cristae

• Chloroplasts– have enclosed internal compartments of

stacked grana, containing thylakoids– found in photosynthetic organisms

24

Mitochondria

A. "Powerhouse of the cell" - cellular metabolism

B. Structure- outer and inner membranes, cristae

C. Have their own DNA

25

Chloroplasts

•Chloroplasts are larger and more complex than mitochondria

•Grana – closed compartments of stacked membranes

•Thylakoids – disc shaped structure – light capturing pigment

•Stroma – fluid matrix

26

Endosymbiosis

• Endosymbiotic theory suggests engulfed prokaryotes provided hosts with advantages associated with specialized metabolic activities.

27

Theory of Endosymbiosis

28

Evidence for the endosymbiont theory is that mitochondria and chloroplasts:

- Are appropriate size to be descendants of eubacteria. - Have inner membranes similar to those on prokaryotic

plasma membranes. - Replicate by splitting, as in prokaryotes. - DNA is circular and different from the DNA of the cell's

nucleus. - Contain their own components for DNA transcription and

translation into proteins . - Have ribosomes similar to prokaryotic ribosomes. - Molecular systematics lend evidence to support this theory. - Many extant organisms are involved in endosymbiotic

relationships.

29

Cytoskeleton

• Network of protein fibers supporting cell shape and anchoring organelles

– Actin filaments cell movement

– Microtubules Hollow tubes Facilitate cell movement Centrioles – barrel shaped organelles occur in pairs – help assemble animal cell’s microtubules

– Intermediate filaments Stable - don’t break down

Actin

Microtubules

Intermediate filaments

30

Cytoskeleton

31

Plant Cells

• Central vacuole – often found in the center of a plant, and

serves as a storage facility for water and other materials

• Cell wall– primary walls – laid down while cell is

growing– middle lamella – glues cells together– secondary walls – inside the primary cell

walls after growth

32

Plant Cell

33

Animal Cells

• Animal cells lack cell walls.– form extracellular matrix

provides support, strength, and resilience

34

35

36