struktur dan fungsi dinding sel
TRANSCRIPT
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