1. cell biology

• Striated muscle fibres (fused muscle cells) are:

  • Longer than typical cells (up to 300 mm in length in comparison to a cardiac muscle cell which has a length of 100 - 150 µm)
  • Have multiple nuclei surrounded by a single membrane (sarcolemma)
• Giant Alga (e.g. Acetabularia):

  • Acetabularia can grow to heights of 100 mm, and yet consist of only one cell with a single nucleus
  • Acetabularia have a relatively complex structure. They are divided into three parts: rhizoid, stalk and cap
• Aseptate fungal hyphae:

  • Fungi have many long, narrow branches called hyphae
  • Hyphae have cell membranes, cell walls and some have septa
  • Aseptate fungal hyphae do not have septa, thus these cells are multinucleated with continuous cytoplasm
• Advantages of using adult stem cells

  • They can divide (endlessly) OR they can differentiate
  • They can be used to repair/regenerate (tissues)
  • Their use has fewer ethical objections (than embryonic stem cells)
  • The source of the stem cells is not killed OR they could not otherwise grow into a new human (unlike embryonic stem cells) OR they do not require the deaths of embryos
  • Adults are able to give (informed) consent for their stem cells to be used
  • There will be no rejection problems if the patient's own cells are used
  • There is less chance of cancer/(malignant) tumour developing (compared to embryonic stem cells)
  • Most adults tissues contain (some) stem cells
• Discrepancies to the cell theory are:

  • Striated muscle cell BECAUSE (they are) multinucleated / greater in length
  • Giant algae BECAUSE of their greater size / (they are) large
  • Aseptate fungi hyphae BECAUSE (there are) no dividing walls between cells / no septa /  multinucleated single cell
• Two therapeutic uses of stem cell therapy:

  • Stargardt’s macular dystrophy
  • Retina cells derived from embryonic stem cells are injected into the eye
  • Vision improves
  • Type I Diabetes / Leukaemia
  • Replacement of faulty islet cells derived from stem cells / adult stem cells returned to replace tissue lost during chemotherapy treatment for leukemia
• Disadvantages for using stem cell in medicine:

  • Ethical implications associated with research and treatment
  • Sourcing embryonic stem cells results in the destruction of the embryo
  • Possibility of side effects e.g. stem cells becoming tumours
  • Adult stem cells are difficult / painful to extract
• Using embryonic stem cells from embryos created by IVF is controversial because:

  • (Using embryonic stem cells results in the) destruction of (fertilised) embryos
  • Each (fertilised) embryo has the potential to become a human life OR embryo could become a foetus if placed in a womb
  • (This raises) religious objections OR ethical objections OR (a fertilised) embryo has the right to life
• Evidence from mitochondria that supports the endosymbiotic theory includes:

  • Mitochondria have circular DNA
  • Mitochondria contain small/70S ribosomes
  • Mitochondrial DNA contains no introns
  • No histones/proteins associated with mitochondrial DNA
  • Mitochondria have double membranes
  • Mitochondria reproduce by (a process similar to) binary fission/prokaryotic cell division
• Compare eukaryotic and prokaryotic cells:

  • Both (prokaryotic and eukaryotic cells) have cytoplasm
  • Both have a plasma/cell surface membrane
  • Both contain DNA/genetic material
  • Both prokaryotic and plant cells/fungal cells have cell walls
• Prokaryotic cells

  • Do not have a nucleus
  • DNA is naked/not associated with histones
  • Have a (single) circular chromosome
  • Do not have membrane-bound organelles
  • Have 70S ribosomes
  • Are smaller (1-10 µm)
  • Have a peptidoglycan cell wall
• Eukaryotic cells

  • Have a nucleus
  • DNA is associated with histones
  • Have (multiple) linear chromosomes
  • Have membrane-bound organelles
  • Have 80S ribosomes
  • Are larger (10-100 µm)
  • Do not have a peptidoglycan cell wall
• Cell wall types

  • Prokaryotic cells have peptidoglycan cell wall
  • Plant cells have cellulose cell wall
  • Fungal cells have chitin cell wall
  • Animal cells have no cell wall
• Compartmentalisation is advantageous because:

  • Substances that could cause harm to the cell can be trapped inside membrane-bound organelles e.g digestive enzymes
  • Enzymes and substrates for metabolic reactions can be localised increasing the rate of reaction
  • Optimal conditions for metabolic processes can be maintained e.g. pH
  • The organelles can be transported around the cell
• The evidence that has allowed the universal acceptance of cells coming from pre-existing cells is:

  • Pasteur’s swan-neck flask experiments
  • (That the) ultrastructure of cells has not been able to synthesized by humans
  • (That) growth is due to mitosis or meiosis
  • (That) viruses with a simpler structure can only be produced within a host cell
  • (The) universality of the genetic code OR the same 64 codons exist in all cells except for some rare variations
• Miller and Urey’s apparatus provided the evidence of the first cells by:

  • Producing steam / boiling water to reflect the early primordial soup
  • Mixing gases / methane, hydrogen & ammonia into the steam recreating the atmosphere
  • Adding electrical discharges to stimulate lightning
  • Cooling the mixture to represent the condensation of water in the atmosphere
  • Analysing the condensed mixture and finding traces of simple organic molecules found
• Differentiate facilitated diffusion and active transport:

  • Facilitated diffusion involves channel or carrier proteins; active transport only involves carrier proteins
  • Facilitated diffusion does not use ATP / is passive; active transport uses ATP
  • Facilitated diffusion takes place down/along a concentration gradient; active transport can occur against/ up a concentration gradient
• The Davson and Danielli model of membrane structure consisted of…

  • A phospholipid bilayer
  • Two layers of protein adjacent / next to the phospholipid bilayer
• Daveson and Danielli model misinterpretations

  • The dark lines were thought to be two protein layers
  • The light region (between the dark lines) were thought to be phospholipids
  • A single cell membrane
  • The micrograph actually shows the cell membranes of two adjacent cells

  
• The evidence that led to the Davson and Danielli model being rejected is...

  • Techniques such as freeze-fracturing showed differences between the interior and exterior of the membranes (not uniform as originally proposed)
  • Some membranes have different thicknesses
  • Different membranes had differing amounts of protein
  • Some membrane proteins had hydrophobic regions (from non-polar amino acids) which were likely to be located within the membrane
• The evidence that led to the Davson and Danielli model being rejected is... (2)

  • Presence of transmembrane proteins
  • Proteins were found that varied in size and shape (unlike the proposed structural proteins that formed a continuous layer)
  • Proteins were mobile within the membrane
• The structure of the membrane allows for the transport of proteins because:

  • Fluidity of membrane allowing shape change / invagination / formation of vesicles
  • It is an active process / requires ATP AND proteins in the membrane (to create vesicles)
  • Phospholipid bilayer enables the membrane to be fluid
  • Cholesterol / kinks in phospholipid tails affect the fluidity of the membrane
• Evidence to falsify the Davson-Danielli model of membrane structure:

  • Freeze-etched electron micrographs showed evidence of proteins extending into centre of membrane
  • Fluorescent markers attached to membrane proteins showed that the proteins in membranes move
  • Biochemical analysis of membranes showed proteins were globular / varied in size and therefore would be unlikely to form continuous membrane layers
  • Proteins contain hydrophobic regions which would orientate towards the centre of the membrane
• The effects of putting human heart tissue into hypotonic solution are: 

  • Hypotonic solution has fewer solute particles / lower osmolarity / lower solute concentration (than the tissue / cells / cytoplasm)
  • (Therefore) water moves into the cells / tissue by osmosis (out of the hypotonic solution)
  • Water moves from lower solute concentration to higher solute concentration / up the solute concentration gradient
  • Pressure inside cells increases / cell becomes swollen / cells burst
  • Volume of cytoplasm increases
• Cholesterol helps with the regulation of the membrane fluidity and permeability

  • Interaction between cholesterol and phospholipid tails stabilises the plasma membrane at higher temperatures by stopping the membrane from becoming too fluid
  • Cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them and causing phospholipids to pack more closely together
• At colder temperatures cholesterol increases the fluidity of the membrane, stopping it crystallizing and becoming too rigid

  • This occurs because cholesterol stops the phospholipid tails packing too closely together
  • The impermeability of the membrane to hydrophilic ions (e.g. sodium and hydrogen) is also reduced by cholesterol
• Factors that affect the rate of diffusion are:

  • Concentration gradient
  • Temperature
  • Surface area
  • Length/distance of the diffusion pathway
  • Properties of the molecules
• The function of five membrane proteins are:

  • Transport OR named example (e.g. channel, pumps, carriers, voltage-gated)
  • Receptors / binding site for hormone / neurotransmitters
  • Cell adhesion / anchor
  • Cell-to-cell recognition / communication
  • Immobilised enzymes
• The similarities in the passive transport of substances across membranes (using named examples) are: 

  • Diffusion, facilitated diffusion and osmosis are passive
  • (That) they all move molecules from high to low concentration / down a (concentration) gradient
  • (That) they do not require ATP
  • (That) they do not have protein pumps
  • Named examples of passive transport (e.g. potassium channels in axons, carbon dioxide exchange in alveoli, water uptake in root hair cells)
• Cyclins: A family of proteins that regulate the cell cycle.
• Proto-oncogene:

  • (Is) a gene that controls cell division / cell cycle
  • Can become cancer-causing after mutating (Oncogene)
• The discovery of the role cyclins:

  • Serendipitous / occurred by chance
  • Required (Tim Hunt and his team/) researchers to explain unexpected observations
  • Occurred whilst protein synthesis in sea urchin embryos/eggs was being studied
  • Required scientists to be aware of associated research being undertaken / cooperation between scientists
• The stages in the cell cycle include:

  • Interphase; which is when metabolic reactions occur / DNA replication / protein synthesis / organelle synthesis
  • (Interphase) consists of the G1, S and G2 phases
  • Mitosis; which includes prophase, metaphase, anaphase and telophase
  • (Mitosis) where the division of a nucleus occurs to produce two genetically identical daughter cells
  • Cytokinesis; where cell plate forms/ cleavage furrow forms/ creates new plasma membrane
• Similarities between cell division of prokaryotic and eukaryotic cells are:

  • Two daughter cells are produced
  • Daughter cells are genetically identical to the parent
  • Prokaryotes AND eukaryotic plant cells have cleavage furrows form during cytokinesis
• Differences between cell division of prokaryotic and eukaryotic cells are:

  • Eukaryotic cells have linear chromosomes being replicated; prokaryotes have circular chromosomes and plasmids
  • Eukaryotic cells will undergo mitosis; prokaryotes undergo binary fission
  • Eukaryotic cells nuclear membrane breaks down; prokaryotes do not have a nuclear membrane
• Chromosomes are:

  • Single, super coiled molecules of DNA
  • After DNA replication, chromosomes exist as two DNA molecules / two chromatids joined together OR after DNA replication, chromosomes are ‘X’ shapedChromatids are:

  Any two of the following:

  • Replicate chromosomes OR identical copies of a chromosome OR contain identical copies of DNA to the sister chromatid
  • (They are) joined at their centromere
  • (They) form following DNA replication OR are found in ‘X’ shaped chromosomes
• Chromosomes are:

  • Single, super coiled molecules of DNA
  • After DNA replication, chromosomes exist as two DNA molecules / two chromatids joined together OR after DNA replication, chromosomes are ‘X’ shaped

  Chromatids are:

  • Replicate chromosomes OR identical copies of a chromosome OR contain identical copies of DNA to the sister chromatid; [1 mark]
  • (They are) joined at their centromere
  • (They) form following DNA replication OR are found in ‘X’ shaped chromosomes
• Difference in benign and malignant tumours:

  • Benign tumours do not cause cancer; malignant tumours do
  • Benign tumours (usually) grow slowly; malignant tumours (usually) grow rapidly
  • Benign tumours do not invade other tissues and damage them; malignant tumours do OR unlike benign tumours, pieces can break off of malignant tumours to start new tumours elsewhere in the body OR benign tumours do not lead to metastasis, WHEREAS malignant tumours do
• Proto-oncogenes control cell division by:

  • Stimulating cell division/cell cycle by producing proteins that make cells divide
  • (A mutation in an oncogene can) cause the gene to become overactive OR stimulate the cell to divide uncontrollably, cause the cell to become ‘immortal’ (if nutrient supply is maintained) OR increase the rate of cell division
  • Mutated proto-oncogene is an oncogene
  • This can result in a tumour (forming)
• During cytokinesis in animal cells and plant cells. . .

  In animal cells:

  • The cell membrane is pulled inwards across the centre of the cell;
  • This separates/’pinches off’ the cytoplasm into two halves (each containing a new nucleus)

  In plant cells:

  • Vesicles fuse to extend the cell membranes across the cytoplasm
  • New cell walls develop (between these cell membranes)