Subcellular Structures and the Endosymbiotic Theory PDF

Cholesterol - another lipid in cell membranes Cholesterol (a sterol) Also an amphipathic molecule (OH group is weakly polar) By itself does not form bilayers Intercalates between phospholipids In membranes cholesterol acts as a ‘fluidity buffer’ Role of cholesterol in membrane Modulates m...

Cholesterol - another lipid in cell membranes Cholesterol (a sterol) Also an amphipathic molecule (OH group is weakly polar) By itself does not form bilayers Intercalates between phospholipids In membranes cholesterol acts as a ‘fluidity buffer’ Role of cholesterol in membrane Modulates membrane fluidity Cholesterol molecules are short and relatively rigid Fill spaces between phospholipids left by the kinks ! Makes the lipid bilayer more rigid and less permeable (A) The shape of a cholesterol molecule (B) How cholesterol fits into the gaps between phospholipid molecules in a lipid bilayer (C) Space-filling model of the bilayer Membrane proteins Different membranes have different functions (depends on cell type) Membranes proteins are inserted into the membrane as they are synthesized Two main classes of membrane proteins: integral proteins and peripheral proteins Another class are the anchored proteins – not embedded in the membrane but covalently linked to lipids or the carbohydrate portion of glycolipids Lipid fluid mosaic model of membrane structure Integral proteins span the membrane – ie inserted across the lipid bilayer (integral proteins are also amphipathic) Anchored proteins not embedded in membrane but linked to fatty acyl chains or glycolipids Peripheral proteins interact with the membrane through hydrophobic domains Proteins within the apolar core of the bilayer are very rare ! Phospholipid bilayer has fluid-like properties Lateral diffusion within the plane of the bilayer – constrained in 2D of the horizontal plane of bilayer Phospholipid flip-flop – movement of lipid from onen half of the bilayer to the other, extremely slow Fast axial rotation of lipid molecules about long axis  Protein Mobility: Fluorescence recovery after photobleaching (FRAP) technique An immobilised cell with fluorophore-labelled(fluorescent) membrane component, an intense pulse of laser light bleaches the signal from a small area. The bleached area recovers as bleached molecules laterally diffuse out and intact fluorescent molecules diffuse into it Fusion of cultured cells – showing lateral diffusion of membrane proteins Fluid mosaic model of membrane structure Model forms a basis of our present concept of biological membranes (and pictorial representations) The lipids and protein molecules are arranged in tightly packed, water excluding mosaic Apolar (uncharged) regions of the lipids and proteins interact in the core of the membrane while hydrophilic parts face the aqueous environments (outside and inside of the cell !) Individual components are free to move in plane of membrane (proteins moving in a sea of lipids !) Fluid mosaic model of membrane structure Organelles of the eukaryotic cells (animal, plants and fungi) intracellular machinery (specialized subunits within a cell that perform a specific function) abundance of subcellular organelles is a prominent feature of mammalian cells by EM Intracellular organelles can be broadly divided into two types derived from membranes bacteria-like organelles Nucleus – the information store Prominent feature of eukaryotic cells Enclosed by two concentric membranes forming the nuclear envelope Contains DNA (prokaryotes do no keep their DNA inside a nuclear envelope !). Eukaryotic DNA is associated with proteins called histones Distinctive feature of nuclear envelope is the presence of pore structures (formed by fusion of inner and outer nuclear membranes) Allow movement of proteins, small molecules, ions and mRNA between nucleus and cytoplasm The cell nucleus EM of cell nucleus Organisation of nuclear envelope The endoplasmic reticulum System of interconnected flattened cisternae (spaces or compartments) Connected to the nuclear membrane Covered with ribosomes (sites of protein synthesis) = rough endoplasmic reticulum; RER Smooth endoplasmic reticulum (SER) have no ribosomes attached to the membranes (lipid and CHO synthesis and storage) Secretory cells (release hormones or enzymes) have more RER than cells that are nonsecretory Golgi apparatus Stack of disc-shaped flattened cisternae Post-translational modification of proteins and packaging them into vesicles Vesicles may be secretory (fuse with surface membrane of the cell to release protein) or lysosomes (stay within the cell) Secretory vesicles Vesicles and lysosomes Vesicles are membranous sacs have roles in storage and intracellular transport Single membrane structures that are fluid filled Lysosomes are a specialised forms of vesicles that contain hydrolytic enzymes – breakdown waste materials in cells, including old and defunct organelles – aid cell renewal Breakdown of the cells own constituents by lysosomes is called autophagocytosis Also digest ‘invaders’ eg pathogens ingested by phagocytic cells of the immune system Bacteria-like organelles within the cell Mainly concerned with energy generation Mitochondria (in almost all eukaryotic cells) and chloroplasts (photosynthetic cells – green plant) Has been proposed that these organelles evolved from free-living bacteria that formed a symbiotic relationship with a primordial eukaryotic cell  Bacteria-like Organelles mitochondria and chloroplasts Derived from once primitive bacterial cells (oxygen respiring/photosynthetic bacteria) These were engulfed by some ancestor of present-day eukaryotic cells An ancient symbiotic relation Endosymbiotic theory Evolution of modern cells from cells & symbiotic bacteria The Endosymbiotic Theory Mitochondria and chloroplasts have striking similarities to bacteria Have their own DNA Use their own DNA to produce many proteins/enzymes Double membrane (evidence that each was ingested by a primitive host) These organelles reproduce like bacteria, replicating their own DNA and dividing Mitochondria Have own genetic material (DNA) and protein synthetic machinery (bacterial-like ribosomes) mtDNA encodes some 13 proteins (16,569 bp) but vast majority of mitochond. proteins are encoded by nuclear DNA Bound by a double membrane (inner and outer) Mitochondria undergo fission/fusion Mitochondria Break down fuel molecules (cellular respiration) Glucose Fatty acids Release energy ATP Chloroplasts derived from photosynthetic bacteria capture solar energy in chlorophyll molecules and perform photosynthesis produce energy-rich sugar molecules and release oxygen as by- product