1. Microscopes
1.1. Magnification -the degree to which the size of an image is larger than the object itself Resolution - the degree to which it is possible to distinguish two different objects as separate. Specimen - the sample to be viewed under a microscope Staining - coloured chemicals binding to the specimen to allow it to be seen Sectioning - cutting thin sections of a specimen embedded in wax
1.2. Light microscope
1.2.1. How it works: -A number of lenses produce an image viewed at the eyepiece. -Light passes through a condenser lens before passing through the specimen. -Eyepiece lens is 10x magnification -Different objective lenses; x40, x100, x400, x1000 Advantages: - Cheap and easy to use - Can be used to view live specimen Disadvantages: - Low magnification (x1500) - Low resolution (200nm) Eyepiece gracticule and stage micrometer - The graticule is a ruler of arbitrary units in the eyepiece. - The stage micrometer is 1mm long and divided into 100 divisions of 10 um; calibrates the eyepiece graticule.
1.3. Electron microscope
1.3.1. How it works: - A beam of electrons are fired at the specimen - Magnets are used to focus the beam - The image is projected onto a screen as a micrograph Scanning electron microscope (SEM): - Beam of electrons directed onto the sample and bounced off - Produces a 3D view - x100,000 Transmission electron microscope (TEM): - Electron beam passes through the sample; easier through less dense parts - 2D image - x500,000 Advantages: - Resolution of 0.2nm; 1000x more than a light microscope - 3D images (SEM) Limitations: - Samples have to be in a vacuum; not live - Very expensive - Requires highly trained people to operate
2. Cells and organelles
2.1. Structure and function of organelles
2.1.1. Vesicle
2.1.1.1. Membrane-bound sacs
2.1.1.2. Carries substances around the cell.
2.1.2. Vacuole
2.1.2.1. Filled with water and solutes
2.1.2.2. Pushed cytoplasm against cell wall to keep cell turgid for support
2.1.3. Cell wall
2.1.3.1. Made of a sieve-like network of cellulose strands
2.1.3.2. Supports the cell
2.1.4. Nucleus
2.1.4.1. Surrounded by a nuclear envelope (two membranes with a fluid between) with nuclear pores. Contains a nucleolous
2.1.4.2. Contains genetic material - chromatin consisting of DNA and proteins. Nucleolus makes RNA and ribosomes
2.1.5. Endoplasmic reticulum
2.1.5.1. Flattened membrane-bound sacs continuous with the outer nuclear membrane. Rough ER has ribosomes; smooth ER does not.
2.1.5.2. Rough ER transports proteins made on ribsomes. Smooth ER makes lipids.
2.1.6. Golgi apparatus
2.1.6.1. Stack of membrane-bound, flattened sacs
2.1.6.2. Modifies proteins and packages them into vesicles
2.1.7. Mitochondria
2.1.7.1. Two membranes separated by fluid-filled space. Inner membrane folded to form cristae. Central part called the matrix
2.1.7.2. Produces ATP during respiration
2.1.8. Chloroplasts
2.1.8.1. Two membranes separated by a fluid filled space.
2.1.8.2. Site of photosynthesis
2.1.9. Lysosomes
2.1.9.1. Spherical membrane bound sacs
2.1.9.2. Contains digestive enzymes to break down material eg pathogens engulfed by white blood cells
2.1.10. Ribosomes
2.1.10.1. Consists of two subunits and are bound to ER or free in cytoplasm
2.1.10.2. Site of protein synthesis - mRNA is used to assemble proteins from amino acids.
2.1.11. Centrioles
2.1.11.1. Tubes of protein fibres. Found as a pair next to the nucleus.
2.1.11.2. Form spindle fibres to move chromosomes during nuclear division
2.2. Movement in cells
2.2.1. Cytoskeleton
2.2.1.1. -Made of protein fibres which provide an internal framework within a cell -Actin filaments move against each other and can move organelles around inside the cell -Microtublules are made of tubulin and can move a microorganism through a liquid -Proteins called microtubule motors present on microtubules can move cell contents along the fibres - eg. chromosones move during mitosis and vesicles move from the ER to the golgi apparatus.
2.2.2. Flagella (undulipodia) and cilia
2.2.2.1. - Both made up of a culinder of 9 microtubles arranged in a circle, and two microtubules in a central bundle. -Undulipodia occur in ones or twos on a cell; eg. tale of sperm - Cilia are shorter and occur in vast numbers on a cell; eg in ciliated epithelial tissue that wafts mucus up the respiratory tract -Both flagella and cillia use energy from ATP to move -Bacterial flagella have a different structure to undulipodia and are made of a spiral protein (flagellin) attached by a hook to a disc at the base. The disk rotates to spin the flagellem
2.3. Division of labour in protein synthesis
2.3.1. -The instructions to make protein (the gene) are found on a chromosome in the DNA in the nucleus -The nucleus copies the gene into an mRNA molecule -mRNA leaves the nuclear pore and attaches to a ribosome on the rough ER -The ribosomes reads the instruction and sythesises the protein -The protein is pinched off in a vesicle and transported to the golgi apparatus -The golgi apparatus modifies/packages the protein into another vesicle -The vesicle then fuses to the cell membrane and secretes the protein.
2.4. Prokaryote and Eukaryotes
2.4.1. Prokaryotes (bacteria)
2.4.1.1. - Only a cell surface membrane - Cell wall made of peptidoglycan -Surrounded by a capsule for protection -Small ribosomes -Single loop of DNA in cytoplasm -ATP produced in mesosomes; infolded regions of the cell membrane. -Some have flagella -May contain hair-like appendages called pili -Can carry DNA on plasmids
2.4.1.1.1. Harmful prokaryotes
2.4.1.1.2. Helpful prokaryotes
2.4.2. Eukaryotes
2.4.2.1. -Contain membrane-bound organelles (organelles have membranes) -Plants have cell wall made of cellulose - No capsule - Large ribosomes - Linear chromosomes in the nucleus -ATP produced in mitochondria - Some have undulipodia - No pili - No plasmids
3. Biological membranes
3.1. The role of membranes
3.1.1. -Separate cell contents from the outside environment -Separate organelles form cytoplasm -Cell recognition/signalling -Holding components of metabolic pathways in place -Regulating transport of substance in and out of the cell
3.2. The phospholipid bilayer
3.2.1. -Phospholipids consist of a hydrophilic phosphate head and two hydrophobic fatty acid tails -They are arranged in pairs in the membrane with the tails on the inside, held away from the water molecules surrounding the membrane -The membrane is stable as phosphate heads cannot easily pass through the hydrophobic region; though some phospholipids may 'flip-flop' from one monolayer to the other -Membranes that are permeable to water (all of them) and allow some molecules to pass through are described as partially permeable
3.3. Differentiation of the cell membrane
3.3.1. Other components are required within a membrane to function properly: -Plasma membrane of growing shoot cells contain receptors to detect molecules that regulate growth -Muscle cell membranes contain channels allowing rapid intake of glucose -internal membranes of chloroplasts contain chlorophyll -Plasma membrane of white blood cells contain proteins to recognise foreign cells and particles
3.4. The fluid mosaic model
3.4.1. -Lipid molecules within the membrane give fluidity whilst proteins give it a mosaic appearance -The phospholipid bilayer contains intrinsic proteins(goes through whole bilayer) and extrinsic proteins(partially embedded in bilayer) -Increasing temperature gives molecules more kinetic energy so increased movement of phospholipids makes membranes more leaky
3.4.1.1. Glycoproteins - Proteins with carbohydrate chains attached
3.4.1.1.1. -Involved in cell signalling -Hormone receptor sites on target cells -Allows recognition of cells as 'self' -Cell adhesion; holding cells together in tissues
3.4.1.2. Glycolipids - lipids with carbohydrate chains attached
3.4.1.2.1. -Involved in cell singalling
3.4.1.3. Cholesterol - lipid molecules
3.4.1.3.1. -Gives mechanical stability; fits between fatty acid tails to make the barrier more complete
3.4.1.4. Channel proteins - proteins forming a channel through the membrane
3.4.1.4.1. -Allow the movement of some substances (large/charged) through the bilayer
3.4.1.5. Carrier proteins - proteins that move substances from one side to the other
3.4.1.5.1. -Use ATP to actively transport ions in and out of a cell
3.4.1.6. Enzymes and coenzymes
3.4.1.6.1. -Catalyzes reactions that take place in membranes eg. photosynthesis on chloroplast membranes or respiration in mitochondrial membranes
3.5. Cell signalling
3.5.1. -Cell signalling is the communication between cells -Signals are detected by receptors which are complementary; usually proteins -Insulin receptor; binds insulin and leads to more glucose channels being present in the membrane; reduces blood glucose level -Some drugs block receptor sites -Viruses bind with receptors on plasma membranes and use them to enter a cell
3.6. Diffusion
3.6.1. -Diffusion is the net movement of particles from an area of high concentration to an area of low concentration, down a concentration gradient -It is a passive process and relies on kinetic energy of molecules; not ATP -When diffusion has taken place, molecules are evenly distributed; equilibrium with no net movement -Facilitated diffusion is still down a concentration gradient but requires proteins -Carrier proteins facilitate large molecules such as glucose/amino acids as they are a specific shape to fit the molecule before the 'flip-flop' motion moves it across the membrane -Channel proteins facilitate the diffusion of ions
3.6.1.1. Increasing diffusion rate
3.6.1.1.1. -Increase temperature; molecules have more kinetic energy -Increasing concentration gradient -Stirring; increases movement -Increasing surface area -Decrease distance -Smaller molecules
3.6.1.2. Molecules that cross membranes by diffusion
3.6.1.2.1. -Lipid-bases molecules; steroids -Small molecules/ions; CO2,O2,H20 pass between phospholipids -Charged/large particles; require channel/carrier proteins to facilitate diffusion as they don't fit through the bilayer
3.7. Active transport
3.7.1. -Active transport is the movement of molecules from an area of low concentration to an area of high concentration using energy (ATP) -Carrier proteins act like pumps as they are complementary shapes to the molecules to be transported -Active transport faster than diffusion -One way flow as ATP changes the shape of the carrier protein so it only fits one side of the membrane
3.8. Bulk transport
3.8.1. -Endocytosis; movement of molecules into the cell -Exocytosis; movement of molecules out the cell - Bulk transport requires ATP; it is used to move membranes around to form vesicles to be pinched off
3.9. Osmosis and water potential
3.9.1. -Water molecules move down a concentration gradient from an area of high water potential to an area of low water potential - A substance that can dissolve is a solute, the liquid it dissolves in is the solvent, together it is called a solution -The highest water potential exists in pure water with no solutes dissolved - water potential of 0kPa - As more solutes are dissolved, the water potential becomes more negative as the water molecules cluster around the dissolved solute; less 'free' water molecules to move around -Osmosis refers to the movement of water molecules by diffusion across a partially permeable membrane from an area of high to low water potential -Animal cells will burst if submerged in pure water as water molecules move into the cell which has more solutes. Plant cells have a cell wall so don't burst; they become turgid -Animal cells become crenated (wrinkly) if submerged in sugar solution as water moves out of cell. Plant cells become plasmolysed (cell membrane pulls away from cell wall as water leaves
4. Cell cycle, mitosis and meiosis
4.1. Cell cycle
4.1.1. -Consists of interphase and mitosis -Parent cells divide to form two genetically identical daughter cells; full set of chromosomes in both -Molecules of DNA are wrapped around histone proteins to form chromatin
4.1.1.1. Interphase: -S phase; DNA is wrapped around histone proteins to form chromatin which is supercoiled to form visible chromosomes. Each chromosome is copied to form two replicas held together at a centromere. The replicas are called sister chromatids. Proof reading enzymes ensure that there are no mutations in the replica DNA strands -G1 and G2; These are growth phases in which proteins, organelles and other cell components are made
4.2. Mitosis
4.2.1. Stages of mitosis
4.2.1.1. Prophase
4.2.1.1.1. -Nuclear envelope breaks down -Centriole divides into two -Centrioles move to opposite poles of the cell to form the spindle
4.2.1.2. Metaphase
4.2.1.2.1. -Chromosomes line up along the equator of the cell -Each chromosome becomes attached to a spindle thread by its centromere
4.2.1.3. Anaphase
4.2.1.3.1. -Replicated sister chromatids are separated when the centromere splits -Each sister chromatid (now individual chromomes) is pulled to opposite poles as spindle fibres shorten
4.2.1.4. Telophase
4.2.1.4.1. -New nuclear envelope forms around set of chromosomes -Spindle breaks down and disappears -Chromosomes uncoil
4.2.2. -Cytokinesis occurs after telophase; this is the process by which the cell splits into two daughter cells - In animal cells, cytokinesis starts from the outside; nipping in the cell membrane along a cleavage - In plants, cytokineses occurs along a cell plate inside the cell where the spindle equator was - In yeast, cytokinesis occurs by producing a bud that nips of the cell; called budding -In plants, only meristem cells undergo mitosis
4.2.3. Cloning and asexual reproduction
4.2.3.1. -Clones are genetically identical cells derived from one parent -Bacteria divide and form clones binary fission -Plants undergo asexual reproduction by vegatative propagation; specialised parts such as potato tubers can produce new genetically identical organisms -Cuttings from plants can grow into adults that are genetically identical to the parent; this is artificial cloning -Cloning of Dolly the sheep; nucleus from a body cell is transferred to an empty egg cell and the embryo is put into a surrogate mother; the calf will be genetically identical to the sheep from which the body cell was taken.
4.2.4. Stem cells
4.2.4.1. -Totipotent; a cell that can divide into any cell type eg. embryonic stem cells -Pluripotent; adult stem cells that can develop into multiple cell types eg. stem cells in bone marrow that develop into all blood and bone cells
4.3. Meiosis
4.3.1. - Meisois is the production of daughter cells that are genetically different from the parent cell; forms gametes -Cells produced by meiosis contain half the number of chromosomes of the parent cell -Normal adult cells contain two sets of chromosomes in homologous pairs; this makes them diploid. Each chromosome in a homologous pair contains the same genes but different alleles (versions of a gene eg blue eyes or brown eyes are both alleles of the eye colour gene) -During meiosis, one member of each homologous pair goes into each daughter cell, so they have half the number of chromosomes; haploid - This allows for genetic variation as the haploids aren't identical; each pair of homologous chromosomes separates independently of all others -When fertilisation occurs, two gametes fuse to form a zygote with a full set of chromosomes