Overview of the cell structureOverview of the cell structure  

Readings and Objectives

• Reading– Russell: 2.5a, 21.2, 22– Cooper: Chapter 1

• Objectives– Cell Theory– Basic properties of cells– Cell size– Cell diversity– Prokaryotic vs eukaryotic cells

• Diversity• Structure

Properties of CellsProperties of CellsBasic properties of cells• Order and complexity

– surrounded by a semi-permeable membrane, similar in composition and function

– Similar chemical composition: similarity of structural patterns of macromolecules and their functions

– Similar biochemical and regulatory processes– hierarchal complexity from molecules to supramolecular

structures • Self replication by division: all cells come from existing cells by

division, process of division share similarities among cells

Properties of CellsProperties of CellsBasic properties of cells• Cells are small (see module 3)

– Varied between ~0.5 to a few 100 um (eg E.coli ~0.6 um, RBC 8 um, skin epithelial cells ~30 nm and ameoba ~500 um)

– Why cells are generally small? – Limiting factor: Surface to

volume ratio (S/V)– Surface is the portal of supply

for the volume of cell

Properties of CellsProperties of CellsBasic properties of cells• Cells are small (see module 3)

– S/V vs X in nonlinear– S/V exponentially reduced as

X increases• Surface (size of cell) reaches

a limit that cannot support the cell’s need (volume)

• Eukaryotes: have increased the S/V by extending internal membrane surfaces

Properties of CellsProperties of CellsProperties…• Interaction with the environment• All have transmittable genetic program

– DNA: similar structure and function, being inherited– Genes: units of genetic function, basic similarity– Evolution: plasticity of genetic information, functional divergence in

response to environmental cues• Energy biogenesis

• Similarity of metabolic pathways, eg. glycolytic pathway, Krebs cycle, adaptive variations on a general theme

• Use of ATP as the universal cellular energy currency Chemical and mechanical activity• Similar enzymes carry out metabolic reactions → energy • Use of energy to do mechanical work

Cell theoryCell theoryEarly observationsEarly observations• Robert Hooke (1665): first to observe unit structures in cork,

called them “cells”• Antone Van Leeuwenhoek: discovers microbial single “cells”• Matthias Schleiden and Theodor Schwann (1839):

Independently concurred that all living organisms,– All living organisms consist of unit structures called Cells

(The Cell theory)• Rudolf Virchow (1855): expanded the cell theory,

– Each cell is the result of division of previous cells• Modern cell biology addresses the question “how cells work”• To understand the molecular basis of cellular processes

Cell diversityCell diversity• Domains of life: cells are organized in three domains

• Procaryotes- Eubacteria and Arhaebacteria• Eucaryotes- Protista, Fungi, Plants and Animals• Similarities:Similarities: Procaryotes and eucaryotes

– Cell membrane

– energy metabolism

– Genetic code

• Differences: Differences: – Procaryotes are unicellular

– Except for many members of protista the rest of eukaryotes are multicellular

– Size: procaryotes 0.5-5 um, eucaryotes 5-500 um

– Membrane bound organelles present in eucaryotes but not procaryotes

– Reproduction: cell fission in procaryotes vs mitosis in eukarotes

Procaryotes: StructureProcaryotes: StructureBacterial cytoplasm is surrounded by a cell

membrane, a cell wall, and for some with a polysaccharide capsule

Cell wall: used for protection, shape, rigidity• Composed of peptidoglycan, a polysaccharide

of alternating acetlylated muramic acid and glucoseamine

• β14 glycosidic bond btw sugars• cross-linked by a short oligopeptide

Two types of bacteria based on cell wall structure

– Gram positive: multiple layers of peptidoglycan– Gram negative: few layers

of peoptidoglycan

G- bacteria

G+ bacteria

Procaryotes: StructureProcaryotes: Structure• Plasma membrane– Beneath the cell wall is

the plasma membrane, a phospholipid bilayer with associated proteins

– Steroid like molecules instead of cholesterol

Procaryotes: StructureProcaryotes: StructureCytoplasm: lacks membrane bound

organelles• Ribosomes

–70S; 3 rRNA + 55 proteins (in 2 subunits 50S and 30S)

–Protein synthesis• Chromosome: single supercoiled

chromosome that resides in a region called nucleoid (not membrane bound)

Procaryotes: StructureProcaryotes: Structure• Bacteria have appendages

with specific functions– Flagella, composed of flagellin

helps bacteria move– Fimbriae are small bristlelike

fibers that allow bacteria to attach themselves to surfaces

– Sex pili (F pilus) used for conjugation to transfer DNA from one bacterium to another

• Genes coding for F-pilus are on F plasmid

Procaryotes: reproductionProcaryotes: reproductionReproductionReproduction• Fission instead of mitosis• Chromosomal DNA

replicationCytokinesisCytokinesis• formation of divisome (several

proteins that tag the centre of cell)

• Cell wall peptidoglycan deposited

• Chromosomes separated

Eucaryotic cellsEucaryotic cellsDiversityDiversity• Protista: free-living marine

unicellular, some photosynthetic• e.g. Giardia lamblia,

dinoflagellates, Paramecium, Plasmodium

• Multicellular organisms– Fungi, Plants, Animals– Differentiation– Model organisms

(a) Saccharomyces cerevisiae (yeast)(b) Arabidopsis thaliana(c) Caenorhabditis elegans(d) Drosophila melanogaster(e) Mus musculus

Eucaryotic cells StructureEucaryotic cells StructureCharacteristics:• Typically 10-30 um• Separation of DNA and cytoplasm by nuclear envelope• Presence of membrane-bound compartments with specialized

functions: Mitochondria, chloroplasts, ER, Golgi complex• Highly specialized motor proteins

• Nuclear envelope and internal membranes

– Originated from cell membrane

– differentiated and acquired special functions

Eucaryotic cellsEucaryotic cells• organelles can be

divided into four categories– The nucleus and

ribosomes– endomembrane

system– energy-related

organelles– cytoskeleton

Can you meet these objectives?

• Describe the cell theory• Describe the basic properties of cells• Outline the major advances leading to modern

cell biology• Describe the properties of procaryotic cells• Contrast and compare prokaryotes and

eukaryotes• Discuss the cellular structure of prokaryotes