BIOLOGI SELStruktur dan Fungsi

Dinding Sel

Dr. José L. NavarroMedical Doctor

Specialist in Medical Microbiology and Family MedicineMadrid (Spain)

Malang, Indonesia

Remember the ‘selaput sel’

Cells are surrounded by a membrane (selaput) phospholipid bilayer with proteins separates the cell from the surrounding environment selective barrier for the import and export of materials

Prokaryotes DON’T contain internal membrane-limited subcompartments DNA is concentrated in the center, most enzymes and metabolites diffuse freely Metabolic reactions:

Some, including protein synthesis and anaerobic glycolysis, take place there;

Other reactions, such as the replication of DNA and production of ATP take place at the plasma membrane.

So, prokaryotic cells DON’T HAVE a nucleus and DON’T HAVE membranous organelles

Remember the ‘selaput sel’

Eukaryotes: Bigger than Prokaryotes

They need to be partitioned to allow chemical reactions. If not, chemical reactions would be limited.

These partitions are smaller subcom-partments termed organelles. Each organelle is surrounded by one or more biomembranes Each organelle contains a unique complement of proteins

some in its membrane some in its interior these proteins enable each organelle to carry out its characteristic

cellular functions. The cytoplasm is the part of the cell outside the nucleus (that is

the largest organelle), the nucleus. The cytosol, (aqueous part of the cytoplasm) outside all of the

organelles, also contains its proteins).

Remember the ‘selaput sel’

Membranes in the cell Cell membranes Organelles membranes

The total surface area of internal membranes is much higher than the plasma membrane

Both have a similar structure Both are composed by

Lipid bilayer Proteins

Two-dimensional lipid sheet, amphipatic hydrophilic faces and a hydrophobic core impermeable to water-soluble molecules and ions

Remember the ‘selaput sel’

Composition of cell membrane: Lipids

50% of mass 5 x 106 lipid molecules in 1µm x 1 µm area Types

Phosphoglycerides Sphingolipids Steroids

Proteins

Lipid composition determines physical characteristics

Proteins determine functional properties

Phospholipids

Lipid bilayer

All three classes of lipids are amphipathic molecules polar (hydrophilic) head hydrophobic tail

the tail groups to self-associate into a bilayer with the polar head groups oriented toward water

They differ in Chemical structures, Abundance, Functions.

Lipid bilayer

PhosphoglyceridesThe most abundant Derivatives of glycerol 3-phosphateComposition

Hydrophobic tail two fatty acyl chains esterified to the two hydroxyl groups

in glycerol phosphate.

A polar head group attached to the phosphate group.

Lipid bilayer

Phosphoglycerides Classified according to the nature of its head group

Phosphatidylcholines, the most abundant phospholipids in the plasma membrane, the

head group consists of choline, positively charged alcohol, esterified to the negatively

charged phosphate. Phosphoethanolamine, Phosphoserine, Phosphoinositol.

The negatively charged phosphate group and the positively charged groups or the hydroxyl groups on the head group interact strongly with water

Lipid bilayer

SphingolipidsDerived from sphingosine

amino alcohol with a long hydrocarbon chain contain a long-chain fatty acid attached to the

sphingosine amino group. Sphingomyelin

the most abundant sphingolipid phosphocholine is attached to the terminal

hydroxyl group

Sphingomielyn

*Ceramide: Sphingosine + Fatty acid (“X” is only one H)

Lipid bilayer

Steroids Cholesterol and its derivatives Four-ring hydrocarbon. Cholesterol

Important in eukariotic cells. The major steroidal constituent of animal tissues. Hydroxyl substituent on one ring. Amphipathic. Abundant in the plasma membranes of mammalian cells but

is absent from most prokaryotic cells.

Lipid bilayer

1. Lipid molecules are free to move rotating freely around their long axes. diffusing laterally within each leaflet. so, they act as a fluid

degree of fluidity depends on lipid composition, (cholesterol and sphingolipids decreases fluidity) tails structure (short -> more fluidity) (kinks -> more fluidity) temperature (more temperature -> more fluidity)

2. Lipid composition influences the physical properties of membranes a typical cell contains many different types of membranes, each one with unique properties depending on the particular mix

of lipids and proteins. related with thickness and curvature

Proteins

Classification 1. Integral membrane proteins or Transmembrane proteins

three segments: a. Cytosolic b. Exoplasmic c. Membrane-spanning

The cytosolic and exoplasmic domains have hydrophilic exterior surfaces

Membrane-spanning domain contains many hydrophobic amino acids

α-helices or β-strands Most are glycosylated with a complex sugar group in the exoplasmic

domains

Proteins

Classification2. Lipid-anchored membrane proteins

bound covalently to lipid molecules. the hydrophobic carbon chain of the attached lipid

is embedded in one leaflet of the membrane and anchors the protein to the membrane.

polypeptide chain does not enter the bilayer.

Proteins

Classification3. Peripheral membrane proteins

do not interact with the hydrophobic core usually bound to integral membrane proteins or lipid head groups. peripheral proteins are localized to either

the cytosolic or the exoplasmic face of the plasma membrane.

Functions

Universal functions1. Permeability barrier

prevents the entry of unwanted materials

prevents the exit needed metabolites

2. Bacterial, fungal, and plant cells are surrounded by a rigid cell wall

Functions3. Transport

Passive transport Simple diffusion: Uses no energy, O2, CO2, water, Slow and Non-

selective Facilitated diffusion: Uses no energy, usual in eukaryotas. Selective

Active transport: Uses energy Ion-coupled transport:

uses a previously stablished ion gradient

ABC transport: Specific binding proteins that transfer the substrate to a membrane-bound protein

Group translocation

Functions

Specific functions1. Anchor cells to extracellular matrix

Mixture of fibrous proteins and polysaccharides that provides a “bed” for other structures

2. Receptor proteins that bind specific signaling

3. Enzymes

Cell wallDinding Sel

Cell wall

Layer that surrounds some types of cells Outside the cell membrane Plants, bacteria, fungi, algae, and some

archaea. Animals and protozoa do not have cell

walls. Composition varies between species (can

also differ depending on cell type and developmental stage)

Cell wall in plants

Composition of the primary cell wall layers of cellulose

microfibrils embedded in a matrix composed by

pectins, polymer of D-galacturonic acid and other monosaccharides

hemicellulose, short, highly branched polymer of several five- and six-carbon

monosaccharides

Thin, extensible but hard Cellulose is synthesized in outer face of cell membrane

UDP-Glu, ADP-Glu from cytosol Pectins and hemicellulose are synthesized in Golgi

Cell wall in plants

The primary cell wall must be extensible to allow growth

When grouth stopsSometimes the primary cell wall stays with no

modificationsBut, more commonly, a secondary cell wall is

produced Lignin

Cell wall in bacteria

Composed by peptidoglycan (=mucopeptid, =murein) Peptidoglycan is a complex polymer

backbone: composed of alternating N-acetylglucosamine and N-acetylmuramic acid;

a set of identical tetrapeptide side chains attached to N-acetylmuramic acid;

a set of identical peptide cross-bridges The backbone is the same in all bacterial species The tetrapeptide side chains and the peptide cross-

bridges vary

Cell wall in bacteria

Gram-positive and Gram-negative bacteria Most bacteria are classified as according

to Gram staining procedure. Hans Christian Gram, 1844

Cell wall in bacteria

GRAM-POSITIVE CELL WALLS Special components

Teichoic acids 50% of the dry weight of the wall Polysaccharides of glycerol phosphate or ribitol

phosphate.There are two types of teichoic acids

Wall teichoic acid covalently linked to peptidoglycan

Lipoteichoic acids, or membrane teichoic acid, covalently linked to membrane lipid.

Cell wall in bacteria

GRAM-NEGATIVE CELL WALLS Three special components

LipoproteinOuter membraneLipopolysaccharide

Cell wall in bacteria

Outer membrane Chemically distinct from all other membranes Bilayered structure

Inner leaflet resembles in composition that of the cell membrane

Outer leaflet contains a distinctive component, a lipopolysaccharide

The leaflets of this membrane are asymmetrical It can exclude hydrophobic molecules

This is unusual in other biologic membranes Serves to protect the cell (Ex.: enteric bacteria from bile salts.

Porins special channels proteins:passive diffusion of compounds like sugars, amino

acids, and certain ions… and some atibiotics

Cell wall in bacteria

Lipopolysaccharide (LPS) 1. Lipid A

phosphorylated glucosamine disaccharide units to which are attached a number of long-chain fatty acids

2. Polysaccharide a core

ketodeoxyoctanoic acid (KDO) and a heptose a terminal series of repeat units

‘O Antigen’ The lipid A is embedded in the outer leaflet of the outer

membrane LPS is synthesized on the cytoplasmic membrane and

transported to its final exterior position. LPS is extremely toxic to animals: endotoxin O antigen is highly immunogenic

Cell wall in bacteria

Lipoprotein Cross-link the outer membrane and

peptidoglycan layers The lipid is inserted in the outer membrane. Stabilize the outer membrane Anchor it to the peptidoglycan layer.

Cell wall in bacteria

What happens if cell wall is removed from bacteria? In osmotically protective media,

Protoplasts from gram-positive Spheroplasts from gram-negative cells.

If such cells are able to grow and divide, they are called L forms.

Cell wall in Archaea

Differences with bacteria cell wall Archaeal DON’T HAVE peptidoglycan Variety in chemical composition

1. Many archaea have a wall with a single, thick, layer resembling that in gram-positive bacteria• Composition: heteropolysaccharides.

• Pseudomurein, • a peptidoglycan-like polymer • N -acetyltalosaminuronic acid instead of N -acetylmuramic acid, • β(1→3) glycosidic bonds instead of β(1→4) glycosidic bonds

2. Other archaea contain complex polysaccharides similar to the chondroitin sulfate of animal connective tissue.

3. Other have either a layer of glycoprotein or protein sometimes there are two layers

Cell wall in Fungi

Composition varies between different groups but basic design is: Fibres:

Chitin microfibrils β-(1,4)-linked N-acetylglucosamine

Mainly near to the cell membrane Synthesized at the plasma membrane

Glucans (glucose polymers) Proteins

Glycosilated, mainly with mannose (mannoproteins or mannans).

[A group of fungi called Oomycota contain cellulose instead of chitin]

Cell wall in Algae

Different composition depending on the type of algae Polysaccharides:

Cellulose in some Mannanes: some marine green algae and some red algae Xylanes Alginic Acid and alginates in brown algae Sulfonated polysaccharides: most algae; those common in red

algae include agarose, carrageenan, porphyran, furcelleran and funoran.

Others

Functions of cell wall

1. Prevention of swallowing or contraction because of osmolarity difference• Osmolarity is higher inside the cell:

• High concentration of small organic molecules (sugars, amino-acids, nucleotides)

• most of these metabolites are charged, so they also attract other ions• Macromolecules

• Macromolecules themselves contribute very little to osmolarity• But they are highly charged so they attract ions

2. Prevention of desiccation (drying up) of cells.3. Protection of the plasma membrane and internal structure of the

cell. Rigidity and strength, (kekakuan dan kekuatan) offering protection against mechanical stress

4. Helps in the transport of various substances.5. Limits the entry of large molecules that may be toxic