IB Biology 2016 - Topic 1 - Cells

Draw and label a diagram of the ultrastructure of an exocrine pancreatic cell

*This is a liver cell, but almost everything is the same** nucleus-smaller area than cytoplasm, surrounded by double membrane with pores; mitochondrion-surrounded by double membrane; inner membrane has infoldings; rough endoplasmic reticulum-stacked tubules with small circles on outer surfaces; Golgi apparatus-curved stacked tubules, small vesicles near ends of sacs; ribosomes both attached to rER and free ribosomes in cytoplasm drawn and labelled (as small circles); vesicles (need to be added) which contain digestive enzymes; lysosome, nucleolus, nuclear envelope, nuclear pore and plasma membrane labelled.

Explain the role of protein pumps and ATP in active transport across membranes

Active transport is the passage of materials against a concentration gradient (from low to high); This process requires the use of protein pumps which use the energy from ATP to translocate the molecules against the concentration gradient; The hydrolysis of ATP causes a conformational change in the protein pump resulting in the forced movement of the substance; Protein pumps are specific for a given molecule, allowing for movement to be regulated (e.g. to maintain chemical or electrical gradients); e.g. Na+/K+ pump which is involved in the generation of nerve impulses; 3 sodium pumped out for every 2 potassium pumped in to the axon;

Describe the active transport of sodium and potassium ions in axons

Active transport using energy from ATP to pump K+ and Na+; Sodium potassium pump transports 3 sodium ions OUT of cell for every 2 potassium ions IN; sodium ions bind to interior of pump on inside of axon; ATP hydrolysis allow phosphate to bind to pump; causes conformational change (change in shape) of pump; releasing sodium outside the cell; 2 potassium bind to pump outside of the cell; causing the release of phosphate; causing a conformational change in the pump; releasing potassium inside the cell

What is the advantage of compartmentalisation in cells?

Allows enzymes to be more concentrated that if spread out in the cytoplasm; substances which could damage other cell parts can be kept in one place; e.g. lysosomes; pH and other conditions can be kept at an optimum for a particular process; organelles with their whole contents can be moved around the cell, using the cytoskeleton

List the functions of life in Paramecium and one named photosynthetic unicellular organism.

As MRSGREN. Other example is Chlorella which can also photosynthesise as it is a uniceull

State that bacterial cells divide by binary fission

Binary fission is a form of asexual reproduction and cell division used by prokaryotic organisms; It is not the same as mitosis, there is no condensation of genetic material and no spindle formation; circular DNA is copied and cells split forming two separate cells;

Annotate the diagram of the pancreatic cell (or other similar Eukaryote) with the functions of each named structure

Cell Membrane: Semi-permeable barrier that controls the entry and exit of substances Cytoplasm (cytosol): The fluid portion of the cytoplasm (does not include the organelles or other insoluble materials) Nucleus: Contains hereditary material (DNA) and thus controls cell activities (via transcription) and mitosis (via DNA replication) Nucleolus: Site of the production and assembly of ribosome components Ribosome: Complexes of RNA and protein that are responsible for polypeptide synthesis (eukaryotic ribosomes are larger than prokaryotes - 80S) Mitochondria: Site of aerobic respiration, which produces large quantities of chemical energy (ATP) from organic compounds Golgi Apparatus: An assembly of vesicles and folded membranes involved in the sorting, storing and modification of secretory products Lysosome: Site of hydrolysis/breakdown of macromolecules Vesicles: Filled with digestive enzymes for exocytosis (transport out of the cell) to be released outside of the blood (exocrine gland) Peroxisome: Catalyses breakdwon of toxic substances like hydrogen peroxide and other metabolites Centrioles: Microtubule-organising centres involved in cell division (mitosis / meiosis and cytokinesis) Endoplasmic Reticulum: A system of membranes involved in the transport of materials between organelles Rough ER: Studded with ribosomes and involved in the synthesis and transport of proteins destined for secretion (exocytosis) e.g. insulin Smooth ER: Involved in the synthesis and transport of lipids and steroids, as well as metabolism of carbohydrates

Annotate the diagram with the function of each of the named structures

Cell Wall: A rigid outer layer made of peptidoglycan that maintains shape and protects the cell from damage or bursting if internal pressure is high Cell Membrane: Partially permeable barrier that controls the entry and exit of substances Cytoplasm: Fluid component which contains the enzymes needed for all metabolic reactions Nucleoid: Region of the cytoplasm which contains the prokaryotic DNA Plasmid: Additional DNA molecule that can be transmitted between bacterial species by pili Ribosome: Complexes of RNA and protein that are responsible for polypeptide synthesis (prokaryotic ribosomes are smaller than eukaryotes - 70S) Slime Capsule: A thick polysaccharide layer used for protection against dessication (drying out) and phagocytosis Flagella (singular flagellum): Long, slender projection containing a motor protein which spins the flagella like a propellor, enabling movement Pili (singular pilus): Hair-like extensions found on bacteria which can serve one of two roles

Explain the role of interphase in mitosis

Cell cycle is the period between one cell divsion and the next. Interphase is an active period in the life of a cell where the following happen: Composed of G1, G2 and SS; G1: A checkpoint stage before DNA replication during which the cell grows; duplicates organelles e.g. mitochondria and extra cytoplasm including enzymes; synthesises proteins; and produces ATP for energy demands of cell division; S: The stage during which DNA is replicated G2: A checkpoint stage before division during which the copied DNA is checked for fidelity (mutations) and final metabolic reactions occur;

What is cholesterol? What properties does it have?Where is it found in membranes? Why?

Cholesterol is a type of lipid; steroids; most of a cholesterol molecule is hydrophobic so it is attracted to the hydrophobic hydrocarbon tails in the centre of the membrane; one end of the cholesterol molecule has a hydroxyl (-OH) group which is hydrophilic; This is attracted to the phosphate heads on the periphery of the membrane; Cholesterol molecules are therefore positioned between phospholipids in the membrane;

Outline eukaryotic cells structure

Compartmentalised with many membrane bound organelles and a true nucleus; contain many organelles; See annotated diagram card for more details.

Outline the structure and function of organelles within palisade mesophyll

Contains many chloroplasts, function is photosynthesis and producing organic molecules from carbon dioxide and water, using energy from light; Roughly cylindrical shape, greater area for photosynthesis, as light passes down through many chloroplasts; Cell wall; stops it bursting from water pressure; Mitochondria; energy for synthesis of chloroplasts; Large central vacuole; pushes chloroplasts to edges, so they are lined up in vertical columns;

Define diffusion and osmosis

Diffusion: The net movement of particles from a region of high concentration to a region of low concentration (down the concentration gradient) until equilibrium is reached (equal concentrations). Osmosis: The net movement of water molecules across a selectively-permeable (plasma) membrane from a region of high(er) water potential (low(er) solute concentration) to a region of low(er) water potential (high(er) solute concentration until equilibrium is reached.

List the different ways that molecule can pass through the plasma membrane

Diffusion; eg oxygen, water Osmosis; water only Facilitated diffusion - e.g. glucose, sodium Active transport - e.g. sodium, potassium

Explain the events of interphase and how you could identify it from microscope images of cells dividing

During interphase DNA replicates; sister chromatids are two identical DNA molecules held together by centromere; sister chromatids are separated during mitosis to form two genetically identical nuclei;

Describe the differences between light and electron microscopes?

Electron microscopes are higher resolution (can distinguish between two points which are closer together); resolve 200x better than light miscroscopes; 0.001 micrometers; higher magnification (over 100,000X); show much more details including cell organelles; e.g. ribosomes; They are much more expensive and difficult to use than light microscopes; require samples to be dead; do not show colour or depth of image

Outline how the invention of the electron microscope allowed our understanding of cell structure to develop

Electron microscopes invented in 1930s and used in 1950s; allowed for resolution 200x smaller objects than light microscopes; shorter wave length than light; can resolve down to 0.001 micrometers (1 nanonmeter); allowed internal structure of mitochondria and chloroplasts to be seen; allowed identification of ribosomes, endoplasmic reticulum and lysosomes; disproved many theories about cell structure; e.g. that grana in chloroplasts were droplets of chlorophyll;

State that multicellular organisms show emergent properties (properties that emerge from a combination of their cellular components).

Emergent properties arise from the interaction of component parts: the whole is greater than the sum of its parts Multicellular organisms are capable of completing functions that individual cells could not undertake - this is due to the interaction between cells producing new functions In multicellular organisms: Cells may group together to form tissues Organs are then formed from the functional grouping of multiple tissues Organs that interact may form organ systems capable of carrying out specific body functions Organ systems carry out the life functions required by an organism

Outline the mitotic index and how it can be used as a measure of cell division

Equal to number of cells in mitosis (prophase, metaphase, anaphase, telophase and cytokinesis) divided by the total number of cells; gives an indication of how many cells out of the total are in a state of cell division; high mitotic index in root tips; and other regions of cell division

Describe the structure and function of organelles within the exocrine glands of the pancreas

Exocrine glands produce and release substances through their plasma membrane; exocrine cells of the pancreas produce digestive enzymes and secrete them into the pancreatic duct; to help digestion in the small intestine; enzymes are proteins, so exocrine glands have large numbers of ribosomes and rough endoplasmic reticulum to synthesise them; large amounts of mitochondria to provide energy for protein synthesis

Discuss the ethics of the therapeutic use of stem cells from specially created embryos, from the umbilical cord blood of a new-born baby and from an adult's own tissues.

For: Cells do not feel pain; could be argued not to be a fully formed human; could save lives; cure diseases; reduced suffering; economic benefits of less sick people; will lead to medical advances and new treatments; transplants less likely to be rejected; can be created without fertilisation and made from adult cells and turned into stem cells Against: Creation and destruction of human embryos could be argued to be against the right to life; Embryonic cells could lead to tumours; More embryos are produced than are used so are 'killed' unnecessarily; Expensive, money could be used for other technologies.

What were the issues with the Davson and Danielli model? How was is falsified?

Freeze-etched electron micrographs; fracture occurs along lines of weakness including the centre of membranes; Globular structures scattered through freeze-etched images of the centre of membranes were interpreted as transmembrane proteins- not a layer of protein as proposed; Membrane proteins were found to be globular in structure; Parts were found to be hydrophobic; which would mean they would be in the centre of the membrane not on the outside as proposed; Fluorescent antibody tagging; showed proteins moved in the membrane; this fitted the fluid mosaic model of membrane structure; not the idea of a layer of protein;

What is difference between integral and peripheral proteins and what properties do they have?

Integral proteins are hydrophobic on at least part of their surface; therefore embedded in the hydrocarbon chains in the centre of the membrane; Many integral proteins are transmembrane; they extend across the membrane, with hydrophilic parts projecting through the regions of phosphate heads on either side; Peripheral proteins are hydrophilic on their surface, so are not embedded in the membrane; Most of them are attached to the surface of integral proteins; Some have a single hydrocarbon chain attached to them which is inserted into the membrane; anchoring the protein to the membrane surface;

How does the cell membrane form? How is it arranged? What properties do the components have that allows this? W

Made of phospholipids; which are amphipathic; made from a polar head, which is hydrophilic; as it contains phosphate; also contains two non-polar fatty acid tails; which are hydrophobic; Phospholipids spontaneously arrange in a bilayer; Hydrophobic tail regions face inwards and are shielded from the surrounding polar fluid; the hydrophilic phosphate in the head region associates with the cytosolic and extracellular environments as they contain water; Phospholipids are held together in a bilayer by hydrophobic interactions (weak associations); Hydrophilic and hydrophobic layers restrict entry and exit of substances; Phospholipids allow for membrane fluidity; Phospholipids with short or unsaturated fatty acids are more fluid; Phospholipids can move horizontally or occasionally laterally to increase fluidity; Fluidity allows for the breaking and remaking of membranes in exocytosis and endocytosis;

Discuss the evidence for the cell theory

Microscopes have increased man's ability to visualise tiny objects; All living things when viewed under a microscope have been found to be made of cells and cell products (e.g. hair) Experimental Evidence: Cells removed from tissues can survive independently for short periods of time; Nothing smaller than a cell has been found to be able to live independently; Experiments by Pasteur have demonstrated that cells cannot grow in sealed and sterile conditions;

Describe how cells move proteins and other substances around the cell

Outline: Many proteins, lipids, carbohydrates travel via diffusion; however this is slow over longer distances in the cell; e.g. when large amounts of proteins/enzymes/lipids/carbohydrates need to be moved; particularly for exocytosis; cells then package into vesicles; phospholipids in a single spherical (circular) layer; Details: proteins synthesised by rough endoplasmic reticulum; vesicles containing proteins pinch-off or bud-off; carried to the golgi apparatus; vesicles fuse with the flattened-sac membranes of the golgi; modification and processing of proteins to put them in their final form takes place; vesicles bud off again; travel to the plasma membrane; or other locations in cell; fuse with the membrane to release contents outside the cell

Outline Pasteur's experiments to disprove spontaneous generation

Pasteur showed that heating of a flask containing nutrient broth killed micro-organisms; he heated yeast and sugar to boiling; if the flask was kept sealed, no bacteria or fungi grew; if the flask had a curved neck (so no air would blow over it) meant no microbes grew; air containing bacteria/fungus was required to get into the flask for reproduction of bacteria/fungus; if he introduced air which was filtered using cotton wool, then no microbes grew; but when he put cotton wool filter into the flask, microbes grew within a few days; therefore, cells do not spontaneously generate from the air

Outline how vesicles are involved in producing more membrane in a growing cell

Phospholipids for the growing plasma membrane are synthesised next to the RER; they then join the RER; packaged into vesicles; along with proteins for the plasma membranes which have been produced by ribosomes; pinch off RER; vesicles travel directly to the plasma membrane; fuse with membrane; contents become part of membrane

What is the role of cholesterol in membranes?

Regulates fluidity; through regulating fluidity, it regulates permeability to solutes; Cholesterol disrupts the regular packing of the hydrocarbon tails of phospholipid molecules; so prevents them crystallizing and behaving as a solid; it also restricts molecular motion and therefore reduces the fluidity of the membrane; it reduces the permeability to hydrophilic particles such as sodium ions and hydrogen ions; cholesterol can help membranes to curve into a concave shape; which helps in the formation of vesicles during endocytosis;

Compare the relative sizes of molecules, cell membrane thickness, viruses, bacteria, organelles and cells, using appropriate SI units

Relative sizes in order from smallest to largest: A molecule = 1 nm (nano metre) Cell membrane thickness = 7.5 nm Virus = 100 nm (range: 20 - 200 nm) Bacteria = 1 - 5 um (micro metre) Organelles = <10 um Eukaryotic cells = <100 um

Draw and label a diagram of the ultrastructure of Escherichia coli (E. coli) as an example of a prokaryote

Remember that pili should be really short; flagellum should be more than half the length of the cell.; cell wall must be a clearly distinct structure from the cell membrane and have a little width to it which is even all around; no nucleus;

Identify the structures in an electron micrographs of a liver cell

Remember that rough E.R. has ribosomes (little dots) and is often next to the nucleus, but not always. Golgi apparatus won't have ribosomes and has a curved shape. Mitochondria have lines for their internal membranes.

Compare prokaryote and eukaryote cells

Similarities: ◾Both have a cell membrane ◾Both contain ribosomes ◾Both have DNA and cytoplasm

Describe prokaryotic cells structure

Simple cell structure without compartmentalisation and without a nucleus; Prokaryotic means before nucleus; small ribosomes (70s); loose DNA; in nucleoid region; cell wall containing peptidoglycan; See annotated diagram card for more details.

Describe how to draw the rough endoplasmic reticulum of a eukaryotic cell

Smooth would be without the ribosomes. Should show some connected flattened sacs (not shown on diagram) so make a few rows with some connections.

Outline the evidence against spontaneous generation

Spontaneous generation is a theory that says cells, or organism, can instantly come into being, with no pre-existing cell required; evidence against is on previous card about evidence for cells coming from pre-existing cells. *Repeat*

Explain how stem cells are used to treat Stargardt's disease and one other named condition.

Stargardt's disease is an inherited disease leading to degeneration of the cells of the retina; Leads to blindness; retinal epithelial cells can be made into stem cells; through exposure to a biochemical solution; Stem cells then replace damaged cells and make the pigment required for vision; Also used for treating type 1 diabetes; stem cells replace the damaged cells in the pancreas;

Explain endosymbiotic theory

States that: Mitochondria were once free-living prokaryotic organism; able to respire aerobically; larger prokaryotes could only respire anareobically; larger prokaryotes engulfed the smaller mitochondria; endocytosis; but didn't digest them; now able to live within the larger prokaryote; symbiotic relationship; mutualistic relationship; also explains the origins of chloroplasts which is the same; explains why mitochondria and chloroplasts have their own DNA; have their own ribosomes; can only come from replication of other chloroplasts/mitochondria; transcribe their own DNA and use the mRNA to synthesise some of their own proteins (not all though)

What are stem cells? Why are they necessary for embryonic development?

Stem cells are unspecialised cells that have two key qualities: 1. Self renewal- They can continuously divide and replicate 2. Pluripotency/Totipotency- They have the capacity to differentiate into specialised cell types This is necessary for a developing embryo as its cell must form different tissues for it to survive, e.g. heart and brain.

Outline the role of supercoiling in cell division

Supercoiling refers to coiling of chromosomes; condenses chromosomes; at start of mitosis (prophase); allows DNA to be arranged; histones allow condensation of DNA into a much smaller space in eukaryotes (only); allows for separation of chromsomes in an ordered manner in mitosis (lining up in metaphase and pulling apart in anaphase);

Outline the cell theory

The cell theory states that: 1. All living things are composed of cells (or cell products) 2. The cell is the smallest unit of life 3. Cells only arise from pre-existing cells

Define mitosis

The division of a diploid (2 sets of chromosomes) nucleus to form two genetically identical daughter nuclei

Explain passive transport across membranes

The plasma membrane is selectively permeable and does not allow all molecules to pass through; Substances that move along the concentration gradient (high to low) undergo passive transport and do not require the expenditure of energy (ATP); Simple diffusion; of small, non-polar (lipophilic) molecules can freely diffuse across the membrane; e.g. oxygen, water (although it is polar); Facilitated diffusion; water; through aquaporins (protein channels); Larger, polar substances (ions, macromolecules like glucose); that cannot freely diffuse and require the assistance of transport proteins (carrier proteins and channel proteins) to facilitate their movement (facilitated diffusion);

Calculate the linear magnification of drawings

To calculate the linear magnification of a drawing the following equation should be used: Magnification = Size of image (with ruler) ÷ Actual size of object (according to scale bar) To calculate the actual size of a magnified specimen the equation is simply re-arranged: Actual size = Size of image (with ruler) ÷ Magnification

What functions do membrane proteins have?

Transport: Protein channels (facilitated diffusion of polar molecules like glucose); and protein pumps for active transport of ions like sodium; Receptors: For Peptide-based hormones such as insulin; Cell Anchorage: Cytoskeleton attachments and extracellular matrix Cell recognition: MHC proteins and antigens for the immune system Intercellular joinings: Tight junctions between groups of cells in tissues and organs and plasmodesmata Enzymatic activity: Metabolic pathways (e.g. electron transport chain) in the cell membrane and in chloroplasts; immobilised enzymes in the small intestine;

State that unicellular organisms carry out all the functions of life

Unicellular organisms (such as amoeba, paramecium, euglena and bacterium) are the smallest organisms capable of independent life. MRSGREN: All living things share 7 basic characteristics: Movement: Living things show movement, either externally or internally Reproduction: Living things produce offspring, either sexually or asexually Sensitivity: Living things can respond to and interact with the environment Growth: Living things can grow or change size / shape Respiration: Living things use substances from the environment to make energy Excretion: Living things exhibit the removal of wastes Nutrition: Living things exchange materials and gases with the environment

Explain the events of anaphase and how you could identify it from microscope images of cells dividing

at start of anaphase centromeres divide separating sister chromatids; separated sister chromatids known as (single stranded) chromosomes; (identical sets of) chromosomes pulled to opposite poles; move by contraction of microtubules;

Outline the correlation between smoking and cancer

correlation is a relationship between two variables; smoking and cancer are correlated; as smoking increases, so does risk of developing cancer; results show a huge increase in death rate due to cancers of the mouth, oesophagus, pharynx, larynx and lungs in smokers vs non-smokers; but death rate due to other cancers is not significantly different; however, this is not causation; but we now have evidence that chemicals in smoke are carcinogenic and cause mutations in cells; therefore little doubt that smoking causes cancers

Explain how cells can become specialised to form specialised tissues

differentiation is development in different ways, to become specialised; cells carry out specialised functions; e.g. a liver cell is differentiated cell in humans; cells have all genes and could develop in any way; some genes are switched on but not others; position, hormones, cell to cell signals and chemicals determine how a cell develops; a group of differentiated cells is a tissue;

Explain the events of metaphase how you could identify it from microscope images of cells dividing

during metaphase all chromosomes lined up at equator separately;

How were models used to represent the membrane? What were the alternatives?

first model from Gorter and Grendel; suggested a bilayer; Davson and Danielli proposed layers of protein adjacent to the phospholipid bilayer on both sides of the membrane; Singer and Niolson proposed current fluid mosaic model; lipid bilayer with proteins as fluid parts of the membrane;

Describe how the fluidity of the membrane allows bulk transport (of lots of molecules at once) into and out of the cell

fluid mosaic model; membrane proteins can move; phospholipids can move in membrane; substances can enter the cell via endocytosis; and exit the cell via exocytosis; membrane can pinch off and form a vesicle; due to fluidity of membrane which allows this; during endocytosis; remaining unbroken; e.g. phagocytosis of bacteria by phagocytes; vesicles from golgi apparatus and endoplasmic reticulum can fuse with membrane; for exocytosis; secreting substances outside of the cell; also exocytosis in Paramecium to expel excess water

Explain the events of prophase and how you could identify it from microscope images of cells dividing

in prophase chromosomes shorten and become visible as double-stranded chromosomes; chromosomes condense by supercoiling; chromosomes attach to spindle microtubules at centromeres; chromosomes begin to move towards equator;

Describe facilitated diffusion of potassium in axons

movement of K+ down concentration gradient; passive; no energy required; uses carrier proteins for K+; normally channel is closed; voltage-gated; change in voltage around the channel causes it to open; when more positive charges inside the cell than outside; channel opens and K+ can flow through; aids in repolarisation of axon as positive potassium flow down concentration gradient out of cell

Discuss the origins of the first cells

must have been billions of years ago; must have arisen from non-living material; from production of more complex compound from simpler ones; amino acids and sugars from simpler compounds such as methane, hydrogen and ammonia; carbon based molecules must have assembled into more complex polymers; some kind of membrane structure must have formed; e.g. amphipathic structure like phospholipids; forming layer around genetic material; molecules must have been able to self-replicate; perhaps RNA like material; evidence that RNA can self-replicate; can act as a catalyst

Explain the events of telophase and cytokinesis and how you could identify them from microscope images of cells dividing

nuclear membrane forms around each set of chromosomes; in telophase; division of cytoplasm and formation of 2 new cells in cytokinesis

Outline how you could estimate the osmolarity of different tissues

osmolarity is the concentration of solutes in a solution; e.g. concentration of sodium chloride; have known concentrations of solute; bathe the tissue samples in different concentrations; hypotonic (more dilute, with less solutes and more water) up to hypertonic (more concentrated, with more solutes and less water); observe the change in mass after a sufficient time period e.g. 20mins; or change in length; change in diameter of cells; change in vacuole size (pla whether the cells burst or not (for animal cells e.g. red blood cells); concentration is equal (isotonic) when there is no change in mass/length/turgidity/bursting of cells

Describe how cytokinesis is different in plants than animal cells

process of cell division (the actual dividing into two) is called cytokinesis; in animal cells the plasma membrane is pulled inwards; forms cleavage furrow; formed by ring of protein in cell contracting; actin and myosin (like in muscle contraction); cell pinched into two by cleavage furrow; Whereas in plants (draw table): vesicles are moved to the equator; fuse with tubular structures; form two layers of membrane across the whole membrane; create two new cell membranes; pectins transported by exocytosis to deposit between the two cells; and cellulose deposited in middle part (lamella); forming two new cell walls

Explain the role of cyclins in the cell cycle

proteins that control the cell cycle; ensure tasks are carried out at the right time in the cell cycle; bind to cyclin-dependent kinases (CDKs); these become active and start attaching phosphate groups to other proteins; activating them; causing cell cycle to proceed; different cyclins trigger G1, S, G2 and Mitosis

Outline how the discovery of cyclins relied on serendipity

researching sea urchins; protein found that increased in concentration after fertilisation and then decreased (unlike others which continued to increase in concentration; found to rapidly increase and decrease at different phases of the cell cycle; breakdown occuring about 10mins after start of mitosis; named cyclins by Hunt; serendipity as he did not set out to discover cell-cycle regulation

Explain the importance of the surface area to volume ratio as a factor limiting cell size

small cells have larger ratio of surface area to volume (than larger cells): surface area must be large enough to absorb nutrients e.g. oxygen; surface area must be large enough to excrete waste products; e.g. CO2 need for materials is determined by (cell) volume; cell size is limited (by SA / vol ratio) so cells divide when they reach a certain size (there is a limit to their volume); this allows enough diffusion across the membrane to meet the needs of the cell.

Explain what stem cells are and how they are used therapeutically

stem cells are cells that retain the capacity to divide and have the ability to differentiate along different paths into all types of cells; stem cells are derived from human embryos, left over from IVF or the placenta (or umbilical cord); new techniques rely on replacing diseased cells with healthy ones; you need to identify desired type of stem cell and grow in culture; scientists could develop a biochemical solution that will cause cells to differentiate into desired cell type; and could develop a means of implanting cells into patient's own tissues so that they function with the body's natural cells; danger of rejection of cells therefore need to suppress immune system; must make sure new cells do not develop into cancerous tumours; eg nerve tissue; help repair catastrophic spinal injuries regain movement;

What are the exceptions to cell theory?

striated skeletal muscle fibres are not typical cells; they are multinucleate (have many nuclei per cell); aseptate fungal hyphae are not divided up into individual cells; have many nuclei without boundaries and continuous cytoplasm; giant algae are not differentiated but are larger than other cells; complex in form; unicellular organisms can be considered acellular; because they carry out all life functions; some tissues large amounts of extracellular material that is not a cell; eg mineral deposits in bone or xylem

Outline the role of mitosis

to reproduce cells which are genetically identical; therefore can carry out the same function; to replace damaged cells; cells which become old and need to be replaced; cells are on average 7-10 years old in the body; some cells replaced rapidly; e.g. liver cells every 500 days; blood cells every 4 months; requires an exact duplication method

Explain how tumours and cancer occur

tumours are abnormal groups of cells; can be benign, when they adhere to each other and do not invade other tissues; other tumours can detach and move elsewhere; known as malignant tumours; caused by mutations to DNA; to oncogenes; which are genes capable of leading to cancer; because they are involved in cell cycle regulation usually several mutations necessary; mutations are changes in the base sequence of genes; can be due to carcinogens (cancer causing substances; e.g. cigarette smoke, radiation, pollution, UV light; primary tumour is the intial cancerous cell dividing into a ball; secondary tumour is where cells detach and spread elsewhere in the body

Explain how vesicles are used to transport materials within a cell

vesicle is made by pinching off a piece of membrane; fluidity of membrane allows this; vesicles can be used to transport material around inside cells; proteins are transported in vesicles; from the rough endoplasmic reticulum to the Golgi apparatus; from the Golgi apparatus to the plasma membrane; formation of vesicle from plasma membrane allows material to be taken in; endocytosis is absorption of material using a vesicle; fusion of vesicle with plasma membrane allows material to be secreted; exocytosis is secretion of material using a vesicle; e.g. exocytosis of insulin and endocytosis (phagocytosis) of bacterial cells.

Describe the evidence from experiments which shows that cells can only be formed by division of pre-existing cells

when cells are killed by heating and the mixture kept in sealed flask, there is no further grow of cells; Pasteur showed that heating of a flask containing nutrient broth killed micro-organisms; if the flask was kept sealed, no bacteria or fungi grew; if the flask had a curved neck (so no air would blow over it) meant no microbes grew; air containing bacteria/fungus was required to get into the flask for reproduction of bacteria/fungus; cell division can be seen under the microscope and has never been seen without a pre-existing cell present; cells are so complex that no natural mechanism has been shown to reproduce all of their parts except for cell division; tracing this back, there must have been a cell (or precursor cell) that first divided, leading to all current cells