Cell ultrastucture

At today’s lecture we will

1. Outline the main features of prokaryotic cells.

2. Describe in detail the organelles of eukaryotic cells paying particular attention to their appearance, structure and function.

3. Revisit the preparative methods for cell organelle isolation in relation to some of their characteristics.

By the end of today’s lecture you should:

1) Understand the main differences between prokaryotes and eukaryotes.

2) Recognise and be able to describe the structure and function of the main organelles found in eukaryotic cells: nucleus, endoplasmic reticulum, Golgi body, mitochondria, chloroplasts, lysosomes, ribosomes, centrioles, microvilli, caveolae, microfilaments, microtubules, intermediate filaments, cilia, flagella, vacuoles.

3) Understand the theory of the endosymbiotic origin of mitochondria and chloroplasts.

Learning outcomes

Procaryotic and eucaryotic cells

• Procaryote = without a nucleus (anucleate)

• Eucaryote = with a nucleus (nucleate)

TEM of E.coli bacterium

Features of prokaryotic cells

• No membrane bound nucleus – nucleoid.• Circular DNA not associated with proteins – does not form

chromatin.• Plasmids – DNA exchange between bacterial cells.• Cytoplasm – all metabolic enzymes.• No double-membraned organelles.• Mesosome in some– respiration and photosynthesis.• Simple flagellum.• Small 70S ribosomes.• Cell membrane – phospholipids and proteins.• Cell wall – peptidoglycan (murein).• Capsule – polysaccharides.• Chlorophyll – in blue-green algae.

Protoctists: Acanthamoeba

Plants: bean plant leafFungi: Budding yeast

Animals: skin epithelial cell

Eukaryotic cells

 

Prokaryotic cells Eukaryotic cellsSmall cells(5 µm) Larger cells(>10 µm)

Unicellular Often multicellular

No nucleus Nucleus

No membrane –bound Membrane-bound

organelles organelles

Circular DNA Linear DNA(no proteins associated) (associated with proteins)

Small ribosomes Large ribosomes

(70S) (80S)

Simple cytoskeleton Developed cytoskeleton

Features of eukaryotic cells

Cytoplasm (cytosol) – soluble medium inside the cells

Compartmentalisation of the eukaryotic cell

Control system

Nucleus – the control centre of the cell

Structure and functions of the nucleus

• the largest organelle• surrounded by a double membrane nuclear envelope• nuclear pores - large holes made by proteins that

control the exit of molecules such as RNA from the nucleus.

• nucleoplasm – chromatin (a DNA/protein complex containing the genes); euchromatin = active DNA; heterochromatine=inactive DNA; chromosomes)

• nucleolus - a structure inside the nucleus, where ribosomes are assempled from rRNA and proteins.

• Function of nucleus: storage/control/organisation of the genetic material.

Ribosomes – the protein making machines of the cell

Structure and function of ribosomes• tiny organelles

• two subunits – small and large

• composed of protein and rRNA; produced in the nucleolus

• either attached to the RER(synthesis of proteins for export or membrane incorporation) or free (synthesis of proteins for cell’s own use); polysomes

• 80S eukaryotic ribosomes

• Function – to catalyse the synthesis of proteins based on the information carried in the mRNA

Production and distribution system

Structure and function of the ER• a series of membrane channels and sacs. • rough ER (RER) - numerous ribosomes

attached to its outer surface(rough appearance); smooth ER (SER) – no ribosomes

• Functions: RER – enzymatic processing, folding and transport of proteins produced by the ribosomes on RER surface(glycoproteins); membrane synthesis;

SER – synthesis, secretion and storage of non-protein products; enzymatic break down of chemicals (liver).

The Golgi apparatus

Structure and function of Golgi apparatus

• flattened membrane cisternae.

• Vesicles from the RER with proteins inside fuse with one side of the Golgi membranes; at the other side small vesicles bud off and move towards the cell membrane - maturation.

• Function – packaging, transport and processing (glycosylation and phosphorylation) of proteins from the RER; formation of lysosomes.

Vacuoles• Membrane-bound sacs containing water, salts and other solutes.

• plant cells - permanent vacuole filled with cell sap and surrounded by a tonoplast membrane; contains organic solutes and mineral ions; keeps the cell turgid.

• specialized vacuoles - feeding vacuoles; contractile vacuoles.

Lysosomes – the digestive system of cells

Small vesicles containing a cocktail of digestive enzymes; acidic pH inside.

Function - to break down unwanted chemicals, toxins, organelles or even whole cells

Energy acquisition and processing system

Mitochondria – the power station of the cell

Structure and functions of the mitochondria

• double membrane - simple outer membrane; inner membrane highly folded into cristae (large surface area).

• intermembrane space between the 2 membranes• matrix - the space enclosed by the inner membrane;

contains small circular strands of DNA; 70S ribosomes; calcium phosphate granules and enzymes

• inner membrane studded with stalked particles (the site of ATP synthesis)

• Function – production of ATP via aerobic respiration.

Chloroplasts – the light harvesting machine of green plants

Structure and functions of chloroplasts

• double membrane

• thylakoid membrane folded into thylakoid disks stacked into piles called grana; photosynthetic pigments (i.e.chlorophyl) in the grana; ATP producing enzyme in stalked particles

• stroma - the space between the inner membrane and the thylakoid; gel-like; contains starch grains, 70S ribosomes and circular DNA

• Function – harvest and convert light energy in the form of chemical energy of carbohydrates.

Origin of mitochondria and chloroplasts

Prokaryotic cells - 3.5 billion years old

Eukaryotic cells – 1billion years old

Endosymbiosis – the idea that organelles such as mitochondria and chloroplasts are derived from prokaryotic cells

Supported by the following observations:•organelles contain circular DNA, like bacteria cells. •organelles contain 70S ribosomes, like bacteria cells. •organelles have double membranes, as though a single-membrane cell had been engulfed and surrounded by a larger cell.

How did membraned cell organelles appear?

Structural support, movement and communication system

Cytoskeleton

• a network of protein fibres.

• used for support, transport and motility.

• attached to the cell membrane give the cell its shape and hold all the organelles in position.

Green – microtubulesRed – actin filamentsBlue - nucleus

Cytoskeleton elements

• Microfilaments (actin);

d≈6nm

• Intermediate filaments;

d≈8-10nm

• Microtubules (tubulin);

d≈25nm

Centrioles• paired cylinders• characteristic microtubule

arrangement (9 triplets)• involved in cell division

Flagella and cilia

• slender mobile extensions of the cell

• complex assembly of microtubules reminiscent to that of centrioles (9 doublets+2 single).

• involved in cell movement

Movement of cillia and flagella

Microvilli

• Small finger-like protrusions of the cell membrane that are not actively motile.

• Actin filaments embedded in the cytoplasm form the base of the micrivilli.

• Functions – surface area increase; absorption; secretion; sensory functions.

Caveolae• A tiny pit/invagination in

the cell surface. • Functions –

endocytosis; structural organisation in the cell membrane.

• Viruses and bacteria can hide in caveolae and escape being destroyed by the lysosomes upon entry in the cells .

Plant cell wall

• a thick layer outside the cell membrane

• a network of fibres made of cellulose; hemicellulose, pectin, lignin and other polysaccharides

• channels through plant cell walls called plasmodesmata link the cytoplasms of adjacent cells

• neighbouring cell walls are linked together by pectin – middle lamella

• Function - provides strength and rigidity for cells but is freely permeable to solutes (unlike membranes)

Sedimentation coefficient and density of macromolecules and organelles

Differential centrifugation revisited

Density gradient centrifugation- revisited

centrifugation

Summary• We looked at the differences between

prokaryotes and eukaryotes.

• We described the structure and function of the main cell organelles in eukaryotic cells and saw images of each of those.

• We discussed the origin of mitochondria and chloroplasts from prokaryotic cells based on structural similarities.

• We looked in more detail at the value of preparative methods for isolation of cell organelles to enable their further study.