CIE AS-Level Biology 9700 - Summarized Notes

Cell Structure

  • Microscopy:
    • Light Microscope:
      • Source of radiation: Light.
      • Wavelength of radiation: 400-700nm.
      • Max resolution: 200nm.
      • Lens: Glass.
      • Specimen: Alive.
      • Stains: Coloured dyes (easier to use).
      • Image: Coloured photomicrograph.
      • View: Eye piece.
    • Electron Microscope:
      • Source of radiation: Electrons.
      • Wavelength of radiation: ±0.005nm.
      • Max resolution: 0.1-0.5nm.
      • Lens: Electromagnet.
      • Specimen: Dead.
      • Stains: Heavy metal.
      • Image: Black and white electron micrograph.
      • View: Fluorescent screen.
      • Vacuum: Present to prevent electrons from colliding with air particles for a sharp image.
      • Reason for dead specimen: Water boils in RT in a vacuum.
  • Magnification: Number of times larger an image is compared to the real size of the object.
    • Depends on the power of the objective and eyepiece lens used.
    • Calculation:
  • Eyepiece graticule: Fitted into the eyepiece of the microscope and is used to measure objects.
    • Has no units.
    • Calibrated by the stage micrometer which has an accurate scale (in mm) and provides reference dimensions.
    • Conversions: 1mm = 1000 μm ; 1μm = 1000 nm
    • Calculation example:
      • 0.1mm of SM = 40 div of EG
      • 1 div of EG= 0.1/40
      • 0.0025mm*1000= 2.5μm
      • 2.5μm*4 equ of chloroplast width= 10μm
  • Resolution: Ability to distinguish between two separate points.
    • The amount of detail that can be seen- higher resolution, higher detail.
    • Limit of resolution: half the wavelength of radiation used to view specimen.
      • Electrons have extremely short wavelength.
      • They’re negatively charged, thus easily focused using electromagnets.
  • Electron Micrograph Observations:
    • Very small particles can be observed as the electrons are easily absorbed.
    • The parts of the specimen that appear darker in the final image are denser and absorb more electrons.
    • Due to higher resolution, the electron micrographs of plant and animal cells show most organelles.
  • Ultrastructure: The structure revealed by the electron micrograph.
  • Organelles: Functionally and structurally distinct part of a cell, usually membrane bound.
  • Generalized Animal Cell (20μm):
    • Cell Surface Membrane (7nm): a selectively permeable membrane in plant and animal cells that allows for the exchange of certain biological molecules and ions.
      • Extremely thin with trilaminar appearance
      • Comprised of phospholipid bilayers with hydrophilic phosphate heads facing the aqueous environment and hydrophobic tails facing each other.
      • Functions:
        • Barrier between cytoplasm and external environment
        • Cell signalling
        • Cell recognition (surface antigens)
        • Cell-to-cell adhesion
        • Site for enzyme catalyzed reactions
        • Anchoring the cytoskeleton
        • Selection of substances that enter/leave the cell
        • Formation of Hydrogen bonds with water for stability
    • Nucleus (10μm): the largest organelle surrounded by the double membraned nuclear envelope and is continuous with rough endoplasmic reticulum.
      • Nuclear pore: gaps in the nuclear envelope that allow exchange between the nucleus and cytoplasm.
        • Substances leaving: mRNA and ribosomes for protein synthesis.
        • Substances entering: protein to help make ribosome, nucleotide, ATP, & some hormones.
      • Chromosome: contains the hereditary material DNA that is organized into genes which controls the activities of the cell and inheritance.
      • Nucleolus (0.2-0.5μm): one or more found (nucleoli) containing DNA and RNA, functioning to make ribosomes.
    • Rough Endoplasmic Reticulum (RER): 80S ribosomes of the rough endoplasmic reticulum are sites for protein synthesis and produce the rough appearance. The R.E.R provides a pathway for transport of materials through cell.
      • Made of two-dimensional flattened sacs, which are membrane-enclosed structures.
      • Proteins made by ribosomes on RER enter sacs and move through them. Transport vesicles bud off from the RER and join forming the Golgi body.
    • Smooth Endoplasmic Reticulum (SER): site for lipid synthesis and steroids eg cholesterol and reproductive hormones.
      • Meshwork of tubular membrane vesicles with fluid filled sacs that have no ribosome on its surface
    • Golgi Body/Complex/Apparatus: stack of flattened sacs formed by transport vesicles which bud off of the RER, and broken down to form Golgi vesicles.
      • Collects, processes, modifies and sorts molecules that are ready for transport in Golgi vesicles to other parts of the cell or out of the cell by:
        • Secretion/exocytosis: fusion of vesicle with plasma membrane to release content.
      • Makes lysosomes, glycoproteins and functioning proteins.
    • Mitochondria (1μm): surrounded by mitochondrial envelope; provides energy for aerobic respiration, synthesizes lipids and is more in areas that require maximal energy.
      • Has a matrix that contains 70S ribosomes and circular DNA which is used to make some of the mitochondrion’s own proteins.
      • Cristae: folding of inner membrane that projects into interior solution, matrix.
      • Intermembrane space: space between the two membranes.
      • Porin: transport protein in outer membrane, forms wide aqueous channel allowing water-soluble molecules from cytoplasm to intermembrane space.
      • Inner membrane: selective barrier controlling entrance of ions and molecules into the matrix.
      • Role of adenosine triphosphate (ATP):
        • Made up of 3 phosphate groups, a nitrogenous base and a ribose sugar.
        • The energy carrying molecule produced in mitochondria that spreads to parts where needed.
        • Energy is released by breaking ATP to ADP, a reversible hydrolysis reaction.
    • Endosymbiont Theory: Mitochondrion and chloroplast were bacteria that now live inside larger cells of animals and plants, which is why chloroplast and mitochondrion have circular DNA.
    • Ribosomes: the site at which mRNA (transcribed from the nucleus) is translated into polypeptides with the help of tRNA, therefore help with protein synthesis.
      • 80S ribosomes: in the cytoplasm and R.E.R
      • 70S ribosomes: in chloroplast and mitochondria.
    • Lysosomes (0.1-0.5μm): a single membrane with no internal structure in animal cells. They contain digestive (hydrolytic) enzymes that’s kept separate from rest of cell to prevent damage.
      • Responsible for breakdown of unwanted structures eg old organelles or whole cells, in WBC to digest bacteria.
    • Microtubules: long hollow tubes that make up the cytoskeleton which helps determine cell shape.
      • Made up of alpha and beta tubulin that combine to form dimers.
      • Dimers join end to end to form protofilaments (polymerisation).
      • 13 protofilaments line up alongside each other in a ring to form a cylinder with a hollow center ie microtubule.
      • Forms an intracellular transport system by moving along secretary vesicles, organelles and cell components on its outer surface.
    • Centrosome: pair of centrioles at right angles that’s involved in nuclear division and act as MTOCs.
    • Centriole: formed by 9 triplets of microtubules. Microtubules extend from centriole and attach themselves to kinetochore of chromosomes, forming spindle fibres. Centrioles duplicate, and a pair of centrioles then move to opposite poles of the cell (2 centrosome regions), thus separating sister chromatids during nuclear division.
      • Centrioles at bases of cilia and flagella (basal bodies) act as MTOCs. Microtubules extending from basal bodies into cilia and flagella help with their beating movements.
  • A generalised plant cell (40 μm):
    • Chloroplasts (5-10μm): This cell structure is only found in plant cells in the palisade mesophyll, spongy mesophyll and surface of stem and carries out photosynthesis.
      • It has a double membrane and contains flattened sacs known as thylakoids.
      • Chlorophyll is embedded in thylakoid membranes.
      • Thylakoids stacked on top of each other to form grana.
      • Grana are linked by lamella. These structures are present in a matrix called the stroma.
      • Contains starch grains, circular DNA and 70S ribosomes.
    • Cell Wall (10 nm): rigid as it contains fibres of cellulose (polysaccharide).
      • Gives the cell its definite shape and prevents it from bursting (by osmosis), allowing turgidity.
      • May be reinforced by lignin for extra strength.
      • Freely permeable.
    • Plasmodesmata: pore-like structures found in cell walls that allow a link between neighbouring cells by fine threads of cytoplasm.
    • Large Vacuole and Tonoplast: surrounded by partially permeable tonoplast, has cell sap (fluid) that consists of enzymes, sugars, waste products, pigments, mineral salts, oxygen, C0_2 and regulates osmotic properties.
  • Eukaryote: organisms with a true nucleus and have membrane bound organelles eg animals, plants, fungi, protoctist.
  • Prokaryote: organisms that lack a nucleus and have simpler structure eg bacteria.
  • Viruses (20-300nm): non- cellular and are parasitic as they reproduce by infecting and taking over living cells. The virus DNA/ RNA hijacks the protein synthesising machinery of the host cell, which then helps to make new viral proteins to make capsid.
  • Comparing Eukaryotes with Prokaryotes:
    • AVARAGE DIAMETER OF CELL
      • Prokaryote: 0.5-5μm
      • Eukaryote: 40μm; 10k-100k times volume of prokaryote
    • DNA
      • Prokaryote: Circular, Lies free in the cytoplasm, Is naked
      • Eukaryote: Linear, Surrounded by nucleus, Associated with histone, forming chromosome
    • RIBOSOME
      • Prokaryote: 70S (20nm)
      • Eukaryote: 80S (25nm)
    • ER
      • Prokaryote: Absent
      • Eukaryote: Present, to which ribosome may be attached
    • ORGANELLES
      • Prokaryote: Very few, No membrane bound organelles unless formed by infoldings of plasma membrane
      • Eukaryote: Many, Single, double and no membrane bound organelles
    • CELL WALL
      • Prokaryote: Murein, a peptidoglycan (polysaccharide with amino acid)
      • Eukaryote: Cellulose and lignin in plants; Chitin (nitrogen containing polysaccharide) in fungi

Biological Molecules

  • Benedict’s test for reducing sugars: Equal volume of sample being tested and Benedict’s solution are mixed and heated in a water bath up to 95C, giving brick red.
  • Acid or enzyme hydrolysis followed by benedict’s test for non-reducing sugars: Hydrochloric acid is added to a the sample being tested in the ratio of 1:2 respectively and heated in a water bath for approximetly 2 minutes. A pinch of sodium hydroxide is added to make the solution alkaline. After this, benedicts test is carried out.
  • Biuret’s test used to detect the presence of proteins: Equal amounts of the sample and biurets solution are added together, giving purple colour over several minutes.
  • Emulsion test for lipids: The sample is added to 2cm3 of ethanol and mixed well until it dissolves (lipids are soluble in ethanol). This mixture is then placed into a test tube containing the same amount of water. A milky white emulsion will appear if lipids are present and remain clear if not.
  • Iodine test for the presence of starch: Iodine solution is orange-brown. Add a drop of iodine solution to the solid or liquid substance to be tested. A blue-black colour is quickly produced if starch is present.
  • Glucose: Molecular formula C6H{12}O_6. Energy source broken down during respiration. Monomer for Starch and Cellulose. Two kinds: α- glucose and β - glucose (differ in –OH group position in ring structures).
  • Monomer: Simple molecule (basic building block for polymer synthesis) joined by condensation reactions. Examples: monosaccharides, amino acids, nucleic acids.
  • Polymer: Giant molecule made from monomers, such as polysaccharides, proteins, nucleic acids.
  • Macromolecule: Large, complex molecules formed by polymerization of smaller monomers (e.g., polysaccharides, nucleic acids).
  • Monosaccharide: Molecule consisting a single sugar unit, the simplest form of carbohydrate and cannot be hydrolised further. General formula: (CH2O)n.
  • Disaccharide: Sugar molecule with two monosaccharides joined by a glycosidic bond.
  • Polysaccharide: Polymer with monosaccharide subunits joined by glycosidic bonds.
  • Glycosidic Bonds: Covalent bonds between constituent monomers, formed via condensation reaction (removal of water molecule) for polysaccharides and disaccharides like sucrose.
    • Separated by hydrolysis, breaking glycosidic bond between monomers (e.g., acid hydrolysis of sucrose).
  • Starch: Macromolecule in plant cells, made of amylose and amylopectin. Polysaccharides made from α glucose molecules, containing 1,4 glycosidic bonds. Amylopectin is branched (α 1,6 glycosidic bonds), amylose is helical. Starch is compact and stores energy.
  • Glycogen: Macromolecule for energy storage in animal cells, from α glucose molecules. Similar to amylopectin but more branched (more α 1,6 glycosidic bonds).
  • Cellulose: In plant cell walls, from βglucose units forming β-1,4 glycosidic bonds. Alternate β- glucose molecules are rotated 180 degrees in order to form these bonds.
    • Hydrogen bonds form between parallel cellulose molecules. 60-70 molecules cross-link into microfibrils, held together in fibres by hydrogen bonding.
    • Fibres increase tensile strength to withstand osmotic pressure, making the plant rigid and determine cell shape. They’re also freely permeable.
  • Triglyceride: Forms by condensation of 3 fatty acid chains and a glycerol molecule, forming an ester bond. Fatty acid chains are long hydrocarbon chains with a carboxylic head. Glycerol is an alcohol containing 3 OH groups.
    • Unsaturated fatty acids: contain c=c bonds that are easier to break and melt easily. More than one c=c is a polyunsatured fatty acid.
    • Saturated fatty acids: contain c-c bonds that are solids at room temperature.
  • Role of Triglyceride:
    • Better energy reserves than carbohydrates as more CH bonds
    • Acts as an insulator and provides buoyancy
    • A metabolic source of water as gives CO2 and H20 on oxidation in respiration
  • Phospholipid: The hydrophilic head contains a phosphate group and glycerol, the hydrophobic tail contains 2 fatty acid chains. This is due to the partial negative charge on the phosphate group that gets attracted to the partial positive charge on the hydrogen atom of the water molecule.
  • Proteins: Made of amino acids, differing only in R- groups/ variable side chains. Always contain an amine group (basic), carboxyl group (acidic), and a hydrogen atom attached to the central carbon atom.
    • A peptide bond is formed by condensation between 2 amino acids, forming a dipeptide. Many amino acids that join together by peptide bonds form a polypeptide.
    • Peptide bonds are broken when hydrolysed into amino acids, often occuring in the small intestine and stomach.
  • Primary structure: Sequence of amino acids in a polypeptide/protein. A slight change in the sequence of amino acids can affect the protein’s structure and function.
  • Secondary structure: Structure resulting from regular coiling or folding of the amino acid chain.
    • α- helix: The polypeptide chain twists into a regular spiral and is maintained by hydrogen bonds between the (-NH) group of one amino acid and the (CO-) group of another amino acid 4 spaces later in the polypeptide chain.
    • β- pleated sheet: The chain is not tightly coiled and lies in a looser, straighter shape.
  • Tertiary structure: Compact structure of a protein molecule resulting from the three-dimensional coiling of the already-folded chain of amino acids
    • Hydrogen bonds between wide varieties of R- groups (can be broken by PH and temperature changes)
    • Disulphide bridges between two cysteine molecules (can be broken by reducing agents)
    • Ionic bonds between R groups containing amine and carboxyl groups. (Can be broken by PH changes.)
    • Hydrophobic interactions between non polar R groups.
  • Quaternary structure: Three-dimensional arrangement of two or more polypeptides, or of a polypeptide and a non-protein component such as haem, in a protein molecule. The polypeptide chains are held together by bonds in the tertiary structure.
  • Globular proteins: Curl up into a spherical shape with their hydrophobic regions pointing into the centre of the molecule and hydrophilic regions pointing outwards. They are soluble in water eg enzymes and haemoglobin.
  • Fibrous proteins: Form long strands, are insoluble in water, and have structural roles eg collagen, hair, nails.
  • Haemoglobin: A globular protein that has a quaternary structure with 4 polypeptide chains, 2 α-globin and 2 β- globin chains. Each chain has one prosthetic haem group containing an iron atom that reversibly binds to an oxygen molecule. Oxyhaemoglobin is bright red, when the haem group is combined with oxygen, otherwise it’s purplish.
    • Sickle cell anemia: a genetic condition in which a polar amino acid, glutamic acid is substituted by non polar valine on the surface of the β chain in haemoglobin, making it insoluble.
  • Collagen: A fibrous protein that is present in the skin, bones, teeth, cartilage and walls of blood vessels. It is an important structural protein.
    • A collagen molecule has 3 polypeptide chains that are coiled in the shape of a stretched-out helix. The molecule has a compact structure and almost every 3rd amino acid is glycine, the smallest amino acid. Glycine is found on the insides of the strands and its small size allows the three strands to lie close together and form a tight coil. The 3 polypeptide strands are held together by hydrogen and covalent bonds.
    • Many of these collagen molecules lie side by side, linked to each other by covalent cross-links between the side chains of amino acids, forming fibrils, and many fibrils make up a fibre.
  • Hydrogen bonding: A water molecule contains two hydrogen atoms and one oxygen atom held together by hydrogen bonds.
  • Solvent: Water is an effective solvent because of its polarity and so can form electrostatic interactions with other polar molecules and ions. Thus it’s a transport medium and reagent for metabolic and other reactions in the cells of plants and animals.
  • High surface tension and cohesion: cohesion refers to the attraction of one water molecule to the other. Water molecules have strong cohesive forces due to hydrogen bonds, thus having high surface tension.
  • High specific heat capacity: the amount of heat energy required to raise the temperature of 1 kg of water by 1 °C. Water has high SPC due to its hydrogen bonds. Temperature within organisms remains constant compared to external temperature, and water bodies also have a slow change in temperature, providing stable aquatic habitats.
  • High latent heat of vaporization: measure of the heat energy needed to vaporise a liquid. Water has a high LHV due to its high SPC as H bonds need to be broken before water can be vapourised, cooling the surrounding environment. Sweating is a good cooling mechanism. However, a large amount of energy can be lost for little amount of water, thus dehydration is prevented eg in transpiration.
  • Density and freezing properties: ice is less dense than water and floats on it, insulating water and preventing it from freezing, preserving aquatic life underneath it. Changes in the density of water with temperature cause currents, which help to maintain the circulation of nutrients in the oceans.

Enzymes

  • An enzyme is a biological catalyst that accelerates metabolic reactions. Enzymes are globular proteins as they have a roughly spherical shape and are water soluble. Enzymes functioning inside a cell are intracellular, but those that are secreted by cells and catalyse reactions outside cells are described as extracellular.
  • Enzymes have specific active sites that are complementary to the shape of the substrate. The substrate is held in place at the active site by weak hydrogen and ionic bonds. The combined structure is called the enzyme-substrate complex.
  • Activation energy is the energy required in any chemical reaction to break the bonds in reactant molecules so that new bonds are formed to make the product. An enzyme lowers the activation energy required for the reaction. However, overall energy released during reaction is maintained.
  • Lock-and-key theory: the shape of the active site is very precise and substrates that are not complementary to the shape of the active site cannot bind. The enzyme- substrate complexes formed enable the reaction to take place more easily.
  • Induced fit theory: the enzyme’s active site is not initially an exact fit to the substrate molecule. However, the enzyme molecules are more flexible and can change shape slightly as the substrate enters the enzyme. This means that the enzyme molecule will undergo conformational changes as the substrate combines with enzyme’s active site, forming the enzyme-substrate complex.
  • Enzymes speed up the rate of a reaction by lowering the activation energy of a reaction, They do this by holding the substrate or substrates in such a way that their molecules can react more easily.
  • The effect that enzymes have on the rate of reactions can be measured in two ways:
    • By measuring the amount of product accumulated over a period of time. Rate of reaction = volume of product produced/ time eg enzyme catalase breaking hydrogen peroxide to H2O + o2
    • By measuring the rate at which the reactants disappear from the reaction mixture, the effect of the enzyme on the rate of reaction can be determined. Eg:
    • measuring the rate at which starch disappears when the enzyme amylase is added.
  • Initially, there’s a large number of substrates and every enzyme has a substrate in its active site. The rate at which the reaction occurs depends only on how many enzymes there are and the speed at which the enzyme can convert the substrate into product, release it, and then bind with another substrate.
  • However, overtime, there are fewer substrates to bind with enzymes; the reaction gets slower, until it eventually stops. The rate of an enzyme-controlled reaction is always fastest at the beginning.
  • Temperature: As the temperature increases, the kinetic energy and the enzyme activity increases as there’s more collisions until optimal temperature is reached (usually 40C). At optimal temperature, maximum rate of reaction is achieved. If the temperature continues to increase beyond optimal temperature, the rate of the reaction begins to decrease as more kinetic energy breaks the hydrogen bonds in the secondary and tertiary structure of enzyme. This changes the shape of the enzyme and its active site and causes the substrate to no longer fit. The enzyme is denatured.
  • pH: Any change in the pH value of the medium around the enzyme will cause ionic and hydrogen bonds to be damaged, this will change the 3-D shape of the enzyme and deform the active site. The substrate will therefore not be able to fit into active site so the reaction slows down or stops. The effects of pH is reversible within certain limits but if the pH is far from optimal value, the enzyme gets denatured.
  • Enzyme concentration: As the concentration of enzymes is increased, there are more available active sites for substrates to fit into. More enzyme-substrate complexes are formed, more products are formed and the rate of reaction is increased. The limiting factor is the enzyme concentration. Once all substrates have formed enzyme- substrate complexes, a further increase in concentration will have no effect on the rate of reaction. At this point, the limiting factor is the substrate concentration. During comparison, look at initial rate to ensure differences in reaction rate are caused only by differences in enzyme concentration.
  • Substrate concentration: As the concentration of the substrates increases, there are greater chances of collision with enzyme. More enzyme-substrate complexes are formed, more products are formed and the rate of reaction is increased. The limiting factor is the substrate concentration. Once all enzymes are occupied and working at maximum rate (vmax), a further increase in substrate concentration will have no effect on the rate of reaction. At this point, the limiting factor is the enzyme concentration.
  • Inhibitor concentration: Inhibitors interfere with enzyme activity and reduce the rate of an enzyme catalysed reaction. Therefore, as the concentration of inhibitors increases, the rate of reaction decreases.
    • Reversible competitive inhibitor: has a similar shape to the substrate and fits into the active site. This reduces the number of enzyme-substrate complexes formed and the rate of reaction decreases. It is said to be reversible because it can be reversed by increasing the concentration of the substrate.
    • The reversible non-competitive inhibitor: has a different shape to the substrate and fits into a site other than the active site. While the non-competitive inhibitor is bound, the tertiary structure of the entire enzyme is distorted, preventing the formation of enzyme-substrate complexes and decreasing the rate of reaction regardless of substrate concentration.
    • End-product inhibition: used to control metabolic reactions via non-competitive reversible inhibitors. As the enzyme converts substrate to product, it is slowed down because the end product binds to another part of the enzyme and prevents more substrate binding. However, the end-product can lose its attachment to the enzyme and go on to be used elsewhere, allowing the enzyme to reform into its active state.
  • Theoretical maximum rate velocity (Vmax): the reaction rate is measured at different substrate concentrations while keeping the enzyme concentration constant. As substrate concentration is increased, reaction rate rises until the reaction reaches its maximum rate.
  • Michaelis–Menten constant (Km): The substrate concentration that corresponds to half of Vmax is Km. Km measures the affinity of the enzyme for the substrate. The higher the affinity, the more likely the product will be formed when a substrate molecule enters the active site. The higher the affinity of the enzyme for the substrate, the lower the substrate concentration needed for this to happen. The higher the affinity, the lower the Km and the quicker the reaction will proceed to Vmax.
  • Enzyme immobilization:
    • The enzyme is mixed with a solution of sodium alginate.
    • Little droplets of this mixture are then added to a solution of calcium chloride.
    • The sodium alginate and calcium chloride instantly react to form jelly, which turns each droplet into a little bead. The jelly bead contains the enzyme.
      • Can reuse the enzyme as it is not mixed with the solution, and can keep the product enzyme free, thus preventing contamination.
      • More tolerant to PH changes as the enzyme molecules are held firmly in shape by the alginate beads, thus don’t denature easily.
      • More tolerant to temperature changes as parts of the molecules embedded in the beads are not fully exposed to temperature or pH changes.
      • Active site may be distorted by immobilizing
      • Substrate passes through matrix when immobilized
      • Some product is retained within matrix

Cell Membrane and Transport

  • Fluid mosaic model: individual phospholipid and protein molecules move around within their own monolayer. The word ‘mosaic’ describes the pattern produced by scattered protein molecule when the surface of the membrane is viewed from above.
  • Phospholipid bilayer: This provides the basic structure of membranes, it is selectively permeable and acts as a barrier to most water-soluble substances.
    • The more unsaturated the tails, the more fluid the membrane as unsaturated fatty acid tails are bent and therefore fit together more loosely.
    • The longer the tail, the less fluid the membrane.
  • Micelle: phospholipid molecules that arrange themselves in a spherical form in aqueous solutions.
  • Cholesterol: regulates the fluidity of a membranes. Its hydrophobic region prevents polar molecules from passing through the membrane eg in myelin sheath
    • At low temperatures: cholesterol increases the fluidity of the membrane, preventing it from becoming too rigid.
    • At higher temperatures: helps stabilize cells when the membrane could otherwise become too fluid.
    • Helps with mechanical stability
  • Glycolipids and glycoproteins: Carbohydrate chains that are attached to membrane protein (glycoprotein) and phospholipids (glycolipid) project out into the watery fluids surrounding the cell where they form hydrogen bonds to stabilize the membrane structure.
    • Carbohydrate chains act as receptors, mainly:
      • Signalling receptors: The receptors recognise messenger molecules like hormones and neurotransmitters. When the messenger molecule binds to the receptor, a series of chemical reactions is triggered inside the cell.
      • Endocytosis: These group of receptors bind to molecules that are to be engulfed by the cell surface membrane.
      • Cell adhesion: binding cells to other cells in tissues and organs. Some glycolipids and glycoproteins act as antigens, allowing cell–cell recognition.
  • Proteins: Transport proteins provide hydrophilic channels for ions and polar molecules. Enzymes catalyse the hydrolysis of molecules. Cytoskeleton made of protein filaments help maintain the shape of the cell.
    • Intrinsic/integral proteins: Proteins that are found embedded within the membrane. They may be found in the inner layer, the outer layer or, most commonly, spanning the whole membrane, known as transmembrane proteins.
    • Extrinsic/peripheral proteins: found on the inner or outer surface of the membrane. Many are bound to intrinsic proteins or to phospholipids.
    • Channel proteins: water-filled pores that allow charged substances, usually ions, to diffuse through the membrane. They have a fixed shape and can be gated to control ion exchange. This does not use ATP and is in facilitated diffusion.
    • Carrier proteins: can flip between two shapes, and is mainly in active transport where it uses ATP to change shape and carry ions/molecules up the concentration gradient. It is also involved in passive transport (facilitated diffusion) down the concentration gradient without the use of energy.
  • Cell surface receptors: These are present in membranes and bind with particular substances, eg: hormones which are chemical messengers which circulate in the blood but only bind to specific target cells.
  • Cell surface antigen: These act as cell identity markers. Each type of cell has its own antigen. This enables cells to recognise other cells and behave in an organised way.
  • Cell signalling: Cells communicate by sending and receiving signals.
    • A signal arrives at a specific protein receptor in a cell surface membrane that recognises the signal.
    • The signal brings about a conformational change in the shape of the receptor, spanning the membrane, and the message is passed to the inside of the cell (signal transduction).
    • Changing the shape of the receptor allows it to interact with the G protein, which brings about the release of a ‘second messenger’ (a small molecule which diffuses through the cell relaying the message).
    • The second messenger activates a cascade of enzyme catalysed reactions which brings about the required change.
    • This is an active process involving ATP use.
  • Diffusion: the net movement of molecules or ions from a region of high concentration to a region of low concentration. It is a passive process (molecules have natural kinetic energy). As a result of diffusion, molecules reach equilibrium.
    • Steeper concentration gradient, higher temperature and increased surface areas all increase rate of diffusion.
    • Non polar molecules can pass directly through the membrane eg steroid hormones
    • Gases can diffuse through the membrane directly
    • Water can diffuse through directly as it is a small molecule despite being polar.
  • Facilitated diffusion: Movement of molecules from a region of high concentration to a region of low concentration down a concentration gradient. The movement is passive, however, molecules go through transport proteins instead of passing through phospholipids. This allows for the passage of large polar ions and molecules eg glucose, amino acids, Na+, Cl-
  • Osmosis: the diffusion of water molecules from a region of higher water potential (ψ) (less negative) to a region of lower ψ (more negative) through a selectively permeable membrane.
    • ψ is the tendency of water to move out of a solution; pressure potential (ψp) on liquid increases ψ
    • Pure water has 0 ψ
    • Negative ψ means that solution has more solute than solvent, therefore solute potential (ψs) reduces ψ.
    • In red blood cells:
    • In plant cells: ψ = ψs + ψp
      • Protoplast: the living part of the cell inside the cell wall
      • In pure water: water enters the cell by osmosis, and the cell wall pushes back against the expanding protoplast, building up pressure rapidly, becoming turgid. This is the ψp, and it increases the ψ of the cell until equilibrium is reached.
      • In concentrated solution: water will leave the cell by osmosis. The protoplast gradually shrinks until it is exerting no pressure on the cell wall. The ψp = 0, so ψ = ψs. The protoplast continues to shrink and pulls away from the cell wall, so the cell is plasmolyzed. The point at which ψp has just reached 0 and plasmolysis is about to occur is referred to as incipient plasmolysis.
  • Active transport: Movement of substances from a region of low concentration to a region of high concentration against a concentration gradient. This occurs via specific carrier proteins for specific ions/molecules that use energy from ATP.
    • Na^+ – K^+ pump:
  • Exocytosis: This is the movement of substances out of the cell. A secretory vesicle from golgi body moves towards the plasma membrane with the help of cytoskeleton, using energy from ATP. The vesicles fuse with the cell surface membrane releasing the contents outside.
  • Endocytosis: involves the engulfing of the material by fusing with the plasma membrane to form an endocytic vacuole in the form of phagocytosis (bulk uptake of solids) or pinocytosis (bulk uptake of liquids) using ATP.

The Mitotic Cell Cycle

  • Chromosome: threadlike structure containing DNA and genes.
  • Chromatin: combination of DNA (acidic) wound around histones proteins (basic).
    • Heterochromatin: tightly coiled (condensed);
      • Most condensed at metaphase;
      • Densely stained.
    • Euchromatin: loosely coiled;
      • At interphase (between divisions);
      • Not as densely stained.
    • Main functions of chromosome in nuclear division:
    • Chromosome condensed, so DNA is tightly packed ∴ easier to separate chromatids at centromere into daughter cells.
    • Telomeres: repeated short base sequence at end of chromosome (by telomerase).
      • Ensures ends of DNA are included during DNA replication.
      • Copying enzyme can’t copy the end of DNA so pieces of info are lost; eventually including loss of vital genes.
      • Ageing: specialised cells don’t top up their telomeres after DNA replication, therefore causing loss of genes, DNA and cell death.
    • Centromere: region where chromatids are held together.
  • Nucleosome: DNA wrapped around histones making 1⅔ turns (147 base pairs)
    • 11nm wide, 6nm long;
      *