for AS Biology

1. CELL STRUCTURE for AS Biology

2. Sizes of cells What kind of scale is this? What are the effective limits of light microscopy? What are the effective limits of electron microscopy? What makes the difference?

3. Click here for animal cells Click here for prokaryotic cells Click here for plant cells

4. Prokaryotic cells • include the BACTERIA and the CYANOBACTERIA • are much smaller than eukaryotic cells (1 – 10 mm, as opposed to 10 – 100 mm) • lack a membrane-bound nucleus and other membrane-bound organelles such as mitochondria, chloroplasts, e.r. • have small (70s) ribosomes (eukaryotic cells have 80s ribosomes) • contain DNA in the form of a closed loop (the bacterial chromosome or nucleoid

5. Structure of bacterial cells

6. Flagella are long hollow threads made of a protein called flagellin, which rotate by means of a "motor" located just under the cytoplasmic membrane, and so propel the bacterium through a liquid medium. Bacteria may have one, a few, or many flagella in different positions on the cell. Bacterial flagella Peritrichous flagella of Salmonella Polar flagella of Vibrio cholerae Lophotrichous flagella of Spirillum

7. Bacterial flagella When flagella rotate anti-clockwise, their actions combine to propel the bacterium is a more or less straight line. In unfavourable conditions these ‘runs’ are more frequent. When flagella rotate clockwise, their separate actions cause the bacterium to tumble randomly. In favourable conditions ‘runs’ are shorter and ‘tumbling’ more frequent. For an animation of this behaviour, go to http://www.bact.wisc.edu/MicrotextBook/BacterialStructure/Flagella.html

8. Pili Pili are hollow, hairlike structures made of protein allow bacteria to attach to other cells. A specialized pilus, the sex pilus, allows the transfer of genetic material from one bacterial cell to another. Pili (sing., pilus) are also called fimbriae (sing., fimbria).  Pili of Escherichia coli

11. The cytoskeleton is a network of fibrous proteins in the cytoplasm, responsible for maintaining the shape of the cell as well as anchoring organelles, moving the cell and controlling internal movement of structures. Microtubules function in cell division and serve as a "temporary scaffolding" for other organelles. Actin microfilaments are thin threads that function in cell division and cell motility. Intermediate filaments are between the size of the microtubules and the actin filaments. Recall as much as you can about the structure and functions of the cytoskeleton, then click for a summary. How much did you recall? Read up again if necessary! Next Return

12. Intermediate filaments consist of fibrous proteins entwined like rope. They are important in maintaining the shape of cells, and in holding neighbouring cells together. Actin microfilaments consist of two chains of globular monomers entwined around each other (like two entwined strings of beads). With another protein called myosin, they are responsible for muscle contraction. Microtubules are hollow cylinders with walls made of a helix of tubulin dimers. They lengthen or shorten by adding or subtracting dimers. Microtubules form the spindle that moves chromosomes during cell division, and form ‘pathways’ for the movement of vesicles in cells. Special arrangements of microtubules in cilia and flagella can propel cells, or move substances over cells (next slide) Return

13. In cilia and flagella nine fused pairs of microtubules surround a central unfused pair. ‘Arms’ made of a protein called dynein ‘walk’ up adjacent microtubules, thus bending the cilium or flagellum. This electron micrograph shows a section of a cluster of cilia on the surface of an epithelial cell (e.g. in the trachea or oviduct). Return

14. Endoplasmic reticulum is a network of interconnected membranes involved in the synthesis, modification and transport of proteins and other cell products. Rough endoplasmic reticulum (RER) is so-named because of its rough appearance due to the numerous ribosomes that occur along the ER. Rough ER connects to the nuclear envelope through which the messenger RNA (mRNA) that is the blueprint for proteins travels to the ribosomes. The membranes and cisternae (cavities) of RER are more flattened than those of SER. Smooth ER lacks the ribosomes characteristic of Rough ER and is thought to be involved in transport and modification of lipids. Smooth ER is usually more tubular than RER. Cisternae

15. Rough endoplasmic reticulum is concerned with the synthesis and packaging of proteins, especially those which are to be secreted from the cell. This secretion occurs via the Golgi apparatus Golgi vesicles may contain proteins for secretion: these secretory vesicles fuse with the cell surface membrane and release their contents in the process called exocytosis. Other Golgi vesicles may contain proteins for use inside the cell: lysosomes are formed in this way. The transport vesicles fuse with the cis portion of the Golgi apparatus: proteins pass through the medial Golgi where they are modified, and are then released in Golgi vesicles from the trans Golgi. Golgi apparatus of a plant cell Next Proteins synthesised by ribosomes on the r.e.r. are ‘packaged’ in transport vesicles Return

16. Electron micrograph of rough endoplasmic reticulum Membrane of r.e.r. Polyribosomes: all of the ribosomes in any one cluster are ‘reading’ a single strand of messenger RNA Cisterna Ribosomes on e.r. surface Next Return

17. SMOOTH ENDOPLASMIC RETICULUM … is thought to be involved in modifying proteins made by the r.e.r., and possibly other products such as lipids. Cisterna of smooth e.r. A peroxisome: reactions that produce toxic peroxides (such as hydrogen peroxide, H2O2) occur here. Peroxisomes contain enzymes that quickly decompose the peroxides to harmless products. They are formed as very small vesicles (microbodies) budding from the e.r. Return

18. Membrane structure is based on the AMPHIPATHIC properties of PHOSPHOLIPIDS: This end of the molecule is charged and therefore ‘water-loving’ (HYDROPHILIC) This end of the molecule is uncharged and therefore ‘water-hating’ (HYDROPHOBIC) … so in water phospholipid molecules organise themselves with ‘heads’ outwards, facing the water, and ‘tails’ inwards away from the water ...

19. This gives rise to the PHOSPHOLIPID BILAYER that is the basis of all biological membranes: A single phospholipid molecule Hydrophobic centre Hydrophilic heads in contact with watery medium A pure phospholipid bilayer would be very fluid. In real membranes CHOLESTEROL molecules are inserted into the hydrophobic centre as ‘stiffening’: the more cholesterol a membrane contains, the less fluid it is.

20. INTRINSIC PROTEINS may be embedded in the bilayer, or may span it completely: Amino acids with hydrophilic side chains here Amino acids with hydrophobic side chains here Such protein molecules may be ‘hollow’, forming HYDROPHILIC CHANNELS through which water molecules and water-soluble substances may pass:

21. …whilst GLYCOPROTEINS are often important as RECEPTORS or MARKERS on the outer face of cell surface membranes. Glucose Na+ Membrane proteins also include membrane-bound ENZYMES. Other membrane proteins may act as TRANSPORT PROTEINS such as SYMPORTS ... Substrate Hormone molecule ‘recognising’ glycoprotein receptor Same membrane a millisecond later:

22. This is the FLUID MOSAIC MODEL of membrane structure.

23. Cytoplasm of cell A Membranes 1 cm Cytoplasm of cell B This false-colour electron micrograph shows two opposing nerve cell membranes at a synapse. Each membrane appears as a pair of blue- or mauve-filled ‘tramlines’. The magnification of the micrograph is x436,740. Estimate the thickness of a single membrane in nm. (NB: before you measure the on-screen thickness of the membranes, use the 1 cm arrow to find how much your monitor is magnifying or diminishing the micrograph, and allow for this in your calculations.) Return

24. Mitochondria More … Return …

25. This electron micrograph appears on the page at a magnification of x 80 000. The arrow beside it is 1 cm long when the page is seen at its natural size. • What is the actual length of the mitochondrion shown? • What metabolic process takes place in mitochondria? • What is the ENDOSYMBIOTIC THEORY of mitochondrial origins, and what is the evidence for it? • Outline the nature and the functions of the following: • mitochondrial matrix • cristae Return to start

26. Length of mitochondrion in e.m. • the exact length of the on-screen image will depend on your monitor, but you should have found it to be 90 - 95 mm; suppose you found it to be 95 mm • image length = 95 mm = 95,000 mm • if image is 80 000 x actual size, then actual length = 95,000 / 80,000 • 95,000/80,000 = 95/80 = 1.1875 = 1.2 mm to two significant figures

27. Cristae • infoldings of inner membrane, increasing surface area • site of ELECTRON TRANSPORT CHAIN, by which electrons removed during oxidation of food molecules are passed finally to oxygen, producing water • most of the useful energy liberated in respiration is made available here • details will be filled in later, in A2 Unit 4

28. Mitochondrial matrix • aqueous solution of enzymes and metabolites • site of KREBS (TCA) CYCLE, by which small molecules derived from anaerobic respiration in the cytosol are oxidised to CO2; the electrons removed are fed into the electron transport chain on the cristae

29. Endosymbiotic theory • Proposes that mitochondria (and chloroplasts) originated as aerobic prokaryotic cells engulfed by the anaerobic ancestors of eukaryotic cells • instead of being digested the engulfed cells survived inside the larger cells, which used the energy they released; eventually the relationship evolved to the state of mutual dependence Evidence: • mitochondria contain their own DNA (termed mDNA), which like bacterial DNA is organised in a single closed loop • mitochondria contain their own ribosomes, which are the 70s (bacterial) type rather than the larger ribosomes found in eukaryotes • the double membrane is consistent with an endosymbiotic origin: