IB Cells Flashcards

Question

Principles of cell theory

Answer 1

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

Answer 2

Striated muscle fibres: Muscle cells fuse to form fibres that may be very long (>300mm) Consequently, they have multiple nuclei despite being surrounded by a single, continuous plasma membrane Challenges the idea that cells always function as autonomous units Aseptate fungal hyphae: Fungi may have filamentous structures called hyphae, which are separated into cells by internal walls called septa Some fungi are not partitioned by septa and hence have a continuous cytoplasm along the length of the hyphae Challenges the idea that living structures are composed of discrete cells Giant Algae Certain species of unicellular algae may grow to very large sizes (e.g. Acetabularia may exceed 7 cm in length) Challenges the idea that larger organisms are always made of many microscopic cells

Answer 3

Metabolism – Living things undertake essential chemical reactions Reproduction – Living things produce offspring, either sexually or asexually Sensitivity – Living things are responsive to internal and external stimuli Homeostasis – Living things maintain a stable internal environment Excretion – Living things exhibit the removal of waste products Nutrition – Living things exchange materials and gases with the environment Growth – Living things can move and change shape or size MR SHENG

Answer 4

Paramecia are surrounded by small hairs called cilia which allow it to move (responsiveness) Paramecia engulf food via a specialised membranous feeding groove called a cytostome (nutrition) Food particles are enclosed within small vacuoles that contain enzymes for digestion (metabolism) Solid wastes are removed via an anal pore, while liquid wastes are pumped out via contractile vacoules (excretion) Essential gases enter (e.g. O2) and exit (e.g. CO2) the cell via diffusion (homeostasis) Paramecia divide asexually (fission) although horizontal gene transfer can occur via conjugation (reproduction)

Answer 5

Scenedesmus exchange gases and other essential materials via diffusion (nutrition / excretion) Chlorophyll pigments allow organic molecules to be produced via photosynthesis (metabolism) Daughter cells form as non-motile autospores via the internal asexual division of the parent cell (reproduction) Scenedesmus may exist as unicells or form colonies for protection (responsiveness)

Answer 6

Cells need to produce chemical energy (via metabolism) to survive and this requires the exchange of materials with the environment As a cell grows, volume (units3) increases faster than surface area (units2), leading to a decreased SA:Vol ratio If metabolic rate exceeds the rate of exchange of vital materials and wastes (low SA:Vol ratio), the cell will eventually die. Hence growing cells tend to divide and remain small in order to maintain a high SA:Vol ratio suitable for survival Adaptation: Intestinal tissue of the digestive tract may form a ruffled structure (villi) to increase the surface area of the inner lining or Alveoli within the lung increase the total membrane surface

Answer 7

Magnification = Image size (with ruler) ÷ Actual size (according to scale bar)

Answer 8

Emergent properties arise when the interaction of individual component produce new functions

Answer 9

Differentiation is the process during development whereby newly formed cells become more specialised and distinct from one another as they mature All cells of an organism share an identical genome – each cell contains the entire set of genetic instructions for that organism. The activation of different instructions (genes) within a given cell by chemical signals will cause it to differentiate. (Some genes are expressed while other are not)

Answer 10

Within the nucleus of a eukaryotic cell, DNA is packaged with proteins to form chromatin Active genes are usually packaged in an expanded form called euchromatin that is accessible to transcriptional machinery Inactive genes are typically packaged in a more condensed form called heterochromatin (saves space, not transcribed)

Answer 11

Stem cells are unspecialised cells that have two key qualities: 1. Self Renewal – They can continuously divide and replicate 2. Potency – They have the capacity to differentiate into specialised cell types

Answer 12

Totipotent – Can form any cell type, as well as extra-embryonic (placental) tissue (e.g. zygote) Pluripotent – Can form any cell type (e.g. embryonic stem cells) Multipotent – Can differentiate into a number of closely related cell types (e.g. haematopoeitic adult stem cells) Unipotent – Can not differentiate, but are capable of self renewal (e.g. progenitor cells, muscle stem cells)

Answer 13

The use of biochemical solutions to trigger the differentiation of stem cells into the desired cell type Surgical implantation of cells into the patient’s own tissue Suppression of host immune system to prevent rejection of cells (if stem cells are from foreign source) Careful monitoring of new cells to ensure they do not become cancerous

Answer 14

1. Stargardt’s Disease An inherited form of juvenile macular degeneration that causes progressive vision loss to the point of blindness Caused by a gene mutation that impairs energy transport in retinal photoreceptor cells, causing them to degenerate Treated by replacing dead cells in the retina with functioning ones derived from stem cells 2. Parkinson’s Disease A degenerative disorder of the central nervous system caused by the death of dopamine-secreting cells in the midbrain Dopamine is a neurotransmitter responsible for transmitting signals involved in the production of smooth, purposeful movements Consequently, individuals with Parkinson’s disease typically exhibit tremors, rigidity, slowness of movement and postural instability Treated by replacing dead nerve cells with living, dopamine-producing ones

Answer 15

Stem cells can be derived from one of three sources: Embryos (may be specially created by therapeutic cloning) Umbilical cord blood or placenta of a new-born baby Certain adult tissues like the bone marrow (cells are not pluripotent)

Answer 16

Using multipotent adult tissue may be effective for certain conditions, but is limited in its scope of application Stem cells derived from umbilical cord blood need to be stored and preserved at cost, raising issues of availability and access The greatest yield of pluripotent stem cells comes from embryos, but requires the destruction of a potential living organism

Answer 17

Involves the creation of embryonic clones by fusing a diploid nucleus with an enucleated egg cell (therapeutic cloning) More embryos are created by this process than needed, raising ethical concerns about the excess embryos

Answer 18

Induce a change in the gene expression profile of a cell in order to transform it into a different cell type (transdifferentiation) Involves the use of oncogenic retroviruses and transgenes, increasing the risk of health consequences (i.e. cancer)

Answer 19

Eukaryotic cell (plant) = ~100 μm Eukaryotic cell (animal) = ~10 – 50 μm Organelle (e.g. mitochondrion) = ~1 – 10 μm Prokaryotic cell (bacteria) = ~1 – 5 μm Virus = ~100 nm

Answer 20

Use electromagnets to focus electrons resulting in significantly greater magnifications and resolutions relative to light microscopes Transmission electron microscopes (TEM) pass electrons through specimen to generate a cross-section Scanning electron microscopes (SEM) scatter electrons over a surface to differentiate depth and map in 3D

Answer 21

Use lenses to bend light and magnify images by a factor of roughly 100-fold

Answer 22

Light Microscopes Can be used to view living specimens in natural colour Chemical dyes and fluorescent labelling may be applied to resolve specific structures Electron Microscopes Can be used to view dead specimens in monochrome (although false colour rendering may be applied) Much higher resolution and magnification

Answer 23

Cytoplasm – internal fluid component of the cell Nucleoid – region of the cytoplasm where the DNA is located (DNA strand is circular and called a genophore) Plasmids – autonomous circular DNA molecules that may be transferred between bacteria (horizontal gene transfer) Ribosomes – complexes of RNA and protein that are responsible for polypeptide synthesis (prokaryote ribosome = 70S) Cell membrane – Semi-permeable and selective barrier surrounding the cell Cell wall – rigid outer covering made of peptidoglycan; maintains shape and prevents bursting (lysis) Slime capsule – a thick polysaccharide layer used for protection against dessication (drying out) and phagocytosis Flagella – Long, slender projections containing a motor protein that enables movement (singular: flagellum) Pili – Hair-like extensions that enable adherence to surfaces (attachment pili) or mediate bacterial conjugation (sex pili)

Answer 24

Binary fission is a form of asexual reproduction used by prokaryotic cells In the process of binary fission: The circular DNA is copied in response to a replication signal The two DNA loops attach to the membrane The membrane elongates and pinches off (cytokinesis), forming two cells

Answer 25

Answer 26

Answer 27

Consist of a polar head (hydrophilic) composed of a glycerol and a phosphate molecule Consist of two non-polar tails (hydrophobic) composed of fatty acid (hydrocarbon) chains

Answer 28

The bilayer is held together by weak hydrophobic interactions between the tails Hydrophilic / hydrophobic layers restrict the passage of many substances Individual phospholipids can move within the bilayer, allowing for membrane fluidity and flexibility This fluidity allows for the spontaneous breaking and reforming of membranes (endocytosis / exocyto

Answer 29

Junctions – Serve to connect and join two cells together Enzymes – Fixing to membranes localises metabolic pathways Transport – Responsible for facilitated diffusion and active transport Recognition – May function as markers for cellular identification Anchorage – Attachment points for cytoskeleton and extracellular matrix Transduction – Function as receptors for peptide hormones JET RAT

Answer 30

Integral proteins are permanently attached to the membrane and are typically transmembrane (they span across the bilayer) Peripheral proteins are temporarily attached by non-covalent interactions and associate with one surface of the membrane

Answer 31

Single helices / helical bundles Beta barrels (common in channel proteins)

Answer 32

Cholesterol functions to immobilise the outer surface of the membrane, reducing fluidity It makes the membrane less permeable to very small water-soluble molecules that would otherwise freely cross It functions to separate phospholipid tails and so prevent crystallisation of the membrane It helps secure peripheral proteins by forming high density lipid rafts capable of anchoring the protein At high temperatures it stabilises the membrane and raises the melting point At low temperatures it intercalates between the phospholipids and prevents clustering Cholesterol’s hydroxyl (-OH) group is hydrophilic and aligns towards the phosphate heads of phospholipids The remainder of the molecule (steroid ring and hydrocarbon tail) is hydrophobic and associates with the phospholipid tails

Answer 33

When viewed under a transmission electron microscope, membranes exhibit a characteristic 'trilaminar’ appearance Trilaminar = 3 layers (two dark outer layers and a lighter inner region) Danielli and Davson proposed a model whereby two layers of protein flanked a central phospholipid bilayer The model was described as a 'lipo-protein sandwich’, as the lipid layer was sandwiched between two protein layers The dark segments seen under electron microscope were identified (wrongly) as representing the two protein layers There were a number of problems with the lipo-protein sandwich model proposed by Davson and Danielli: -It assumed all membranes were of a uniform thickness and would have a constant lipid-protein ratio -It assumed all membranes would have symmetrical internal and external surfaces (i.e. not bifacial) -It did not account for the permeability of certain substances (did not recognise the need for hydrophilic pores) -The temperatures at which membranes solidified did not correlate with those expected under the proposed model

Answer 34

Membrane proteins were discovered to be insoluble in water (indicating hydrophobic surfaces) and varied in size Such proteins would not be able to form a uniform and continuous layer around the outer surface of a membrane Fluorescent antibody tagging of membrane proteins showed they were mobile and not fixed in place Membrane proteins from two different cells were tagged with red and green fluorescent markers respectively When the two cells were fused, the markers became mixed throughout the membrane of the fused cell This demonstrated that the membrane proteins could move and did not form a static layer (as per Davson-Danielli) Freeze fracturing was used to split open the membrane and revealed irregular rough surfaces within the membrane These rough surfaces were interpreted as being transmembrane proteins, demonstrating that proteins were not solely localised to the outside of the membrane structure New Model: In light of these limitations, a new model was proposed by Seymour Singer and Garth Nicolson in 1972 According to this model, proteins were embedded within the lipid bilayer rather than existing as separate layers This model, known as the fluid-mosaic model

Answer 35

Passive transport involves the movement of material along a concentration gradient (high concentration ⇒ low concentration) Because materials are moving down a concentration gradient, it does not require the expenditure of energy (ATP hydrolysis)

Answer 36

Active transport involves the movement of materials against a concentration gradient (low concentration ⇒ high concentration) Because materials are moving against the gradient, it requires the expenditure of energy (e.g. ATP hydrolysis)

Answer 37

There are two main types of active transport: Primary (direct) active transport – Involves the direct use of metabolic energy (e.g. ATP hydrolysis) to mediate transport Secondary (indirect) active transport – Involves coupling the molecule with another moving along an electrochemical gradient

Answer 38

There are three main types of passive transport: Simple diffusion – movement of small or lipophilic molecules (e.g. O2, CO2, etc.) Diffusion is the net movement of molecules from a region of high concentration to a region of low concentration This directional movement along a gradient is passive and will continue until molecules become evenly dispersed (equilibrium) Small and non-polar (lipophilic) molecules will be able to freely diffuse across cell membranes (e.g. O2, CO2, glycerol) Osmosis – movement of water molecules (dependent on solute concentrations) Osmosis is the net movement of water molecules across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration (until equilibrium is reached) Water is considered the universal solvent – it will associate with, and dissolve, polar or charged molecules (solutes) Because solutes cannot cross a cell membrane unaided, water will move to equalise the two solutions At a higher solute concentration there are less free water molecules in solution as water is associated with the solute Osmosis is essentially the diffusion of free water molecules and hence occurs from regions of low solute concentration Facilitated diffusion – movement of large or charged molecules via membrane proteins (e.g. ions, sucrose, etc.)

Answer 39

Temperature (affects kinetic energy of particles in solution) Molecular size (larger particles are subjected to greater resistance within a fluid medium) Steepness of gradient (rate of diffusion will be greater with a higher concentration gradient)

Answer 40

In hypertonic solutions, water will leave the cell causing it to shrivel (crenation) In hypotonic solutions, water will enter the cell causing it to swell and potentially burst (lysis) In plant tissues, the effects of uncontrolled osmosis are moderated by the presence of an inflexible cell wall In hypertonic solutions, the cytoplasm will shrink (plasmolysis) but the cell wall will maintain a structured shape In hypotonic solutions, the cytoplasm will expand but be unable to rupture within the constraints of the cell wall (turgor)

Answer 41

Facilitated diffusion is the passive movement of molecules across the cell membrane via the aid of a membrane protein It is utilised by molecules that are unable to freely cross the phospholipid bilayer (e.g. large, polar molecules and ions) This process is mediated by two distinct types of transport proteins – channel proteins and carrier proteins Carrier Proteins Integral glycoproteins which bind a solute and undergo a conformational change to translocate the solute across the membrane Carrier proteins will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction Carrier proteins may move molecules against concentration gradients in the presence of ATP (i.e. are used in active transport) Carrier proteins have a much slower rate of transport than channel proteins (by an order of ~1,000 molecules per second) Channel Proteins Integral lipoproteins which contain a pore via which ions may cross from one side of the membrane to the other Channel proteins are ion-selective and may be gated to regulate the passage of ions in response to certain stimuli Channel proteins only move molecules along a concentration gradient (i.e. are not used in active transport) Channel proteins have a much faster rate of transport than carrier proteins Potassium Channels The axons of nerve cells transmit electrical impulses by translocating ions to create a voltage difference across the membrane At rest, the sodium-potassium pump expels sodium ions from the nerve cell, while potassium ions are accumulated within When the neuron fires, these ions swap locations via facilitated diffusion via sodium and potassium channels

Answer 42

Active transport uses energy to move molecules against a concentration gradient This energy may either be generated by: The direct hydrolysis of ATP (primary active transport) Indirectly coupling transport with another molecule that is moving along its gradient (secondary active transport) Active transport involves the use of carrier proteins (called protein pumps due to their use of energy) A specific solute will bind to the protein pump on one side of the membrane The hydrolysis of ATP (to ADP + Pi) causes a conformational change in the protein pump The solute molecule is consequently translocated across the membrane (against the gradient) and released Sodium-Potassium Pump The axons of nerve cells transmit electrical impulses by translocating ions to create a voltage difference across the membrane At rest, the sodium-potassium pump expels sodium ions from the nerve cell, while potassium ions are accumulated within An integral protein that exchanges 3 sodium ions (moves out of cell) with two potassium ions (moves into cell) The process of ion exchange against the gradient is energy-dependent and involves a number of key steps: Three sodium ions bind to intracellular sites on the sodium-potassium pump A phosphate group is transferred to the pump via the hydrolysis of ATP The pump undergoes a conformational change, translocating sodium across the membrane The conformational change exposes two potassium binding sites on the extracellular surface of the pump The phosphate group is released which causes the pump to return to its original conformation This translocates the potassium across the membrane, completing the ion exchange

Answer 43

Endoplasmic Reticulum The endoplasmic reticulum is a membranous network that is responsible for synthesising secretory materials Rough ER is embedded with ribosomes and synthesises proteins destined for extracellular use Smooth ER is involved in lipid synthesis Materials are transported from the ER when the membrane bulges and then buds to create a vesicle surrounding the material Golgi Apparatus The vesicle is then transported to the Golgi apparatus and fuses to the internal (cis) face of the complex Materials move via vesicles from the internal cis face of the Golgi to the externally oriented trans face While within the Golgi apparatus, materials may be structurally modified (e.g. truncated, glycosylated, etc.) Material sorted within the Golgi apparatus will either be secreted externally or may be transported to the lysosome Plasma Membrane Vesicles containing materials destined for extracellular use will be transported to the plasma membrane The vesicle will fuse with the cell membrane and its materials will be expelled into the extracellular fluid Materials sorted by the Golgi apparatus may be either: Released immediately into the extracellular fluid or stored within an intracellular vesicle for a delayed release in response to a cellular signal

Answer 44

The process by which large substances (or bulk amounts of smaller substances) enter the cell without crossing the membrane An invagination of the membrane forms a flask-like depression which envelopes the extracellular material The invagination is then sealed off to form an intracellular vesicle containing the material There are two main types of endocytosis: Phagocytosis – The process by which solid substances are ingested (usually to be transported to the lysosome) Pinocytosis – The process by which liquids / dissolved substances are ingested (allows faster entry than via protein channels)

Answer 45

The process by which large substances (or bulk amounts of small substances) exit the cell without crossing the membrane Vesicles fuse with the plasma membrane, expelling their contents into the extracellular environment The process of exocytosis adds vesicular phospholipids to the cell membrane, replacing those lost when vesicles are formed via endocytosis

Answer 46

If the two molecules are transported in the same direction it is called symport If the two molecules are transported in opposite directions it is called antiport The sodium-potassium pump is an example of an antiporter as sodium and potassium are pumped in opposite directions This is primary active transport as both molecules are pumped against their gradient and require ATP hydrolysis Glucose uptake in the kidneys is an example of symport as its movement is coupled to the parallel transport of sodium This is secondary active transport as the sodium is moving passively down an electrochemical gradient

Answer 47

The theory that living cells arose from non-living matter is known as abiogenesis This process is theorised to have occurred over four key stages: There was non-living synthesis of simple organic molecules (from primordial inorganic molecules) These simple organic molecules became assembled into more complex polymers Certain polymers formed the capacity to self-replicate (enabling inheritance) These molecules became packaged into membranes with an internal chemistry different from their surroundings (protobionts)

Answer 48

Miller-Urey Experiment The non-living synthesis of simple organic molecules has been demonstrated by the Miller-Urey experiment Stanley Miller and Harold Urey recreated the postulated conditions of pre-biotic Earth using a closed system of flasks and tubes Water was boiled to vapour to reflect the high temperatures common to Earth’s original conditions The vapour was mixed with a variety of gases (including H2, CH4, NH3) to create a reducing atmosphere (no oxygen) This mixture was then exposed to an electrical discharge (simulating the effects of lightning as an energy source for reactions) The mixture was then allowed to cool (concentrating components) and left for a period of ~1 week After this time, the condensed mixture was analysed and found to contain traces of simple organic molecules

Answer 49

Cells can only be formed by the division of pre-existing cells (biogenesis) The chemical processes that contributed to the initial formation of biological life required specific conditions to proceedThis included a reducing atmosphere and high temperatures (>100ºC) or electrical discharges to catalyse chemical reactions. These conditions do not commonly exist on modern Earth and hence living cells cannot arise independently by abiogenesis

Answer 50

The law of biogenesis is largely attributed to Louis Pasteur, who demonstrated that emergent bacterial growth in nutrient broths was due to contamination by pre-existing cells Broths were stored in vessels that contained long tubings (swan neck ducts) that did not allow external dust particles to pass The broths were boiled to kill any micro-organisms present in the growth medium (sterilisation) Growth only occurred in the broth if the flask was broken open, exposing the contents to contaminants from the outside From this it was concluded that emergent bacterial growth came from external contaminants and did not spontaneously occur

Answer 51

Membranes (double membrane bound) Antibiotics (susceptibility) Division (mode of replication (fission-like process)) DNA (presence of own naked circular DNA) Ribosomes (size of 70S) MAD DR

Answer 52

an endosymbiont is a cell which lives inside another cell with mutual benefit Eukaryotic cells are believed to have evolved from early prokaryotes that were engulfed by phagocytosis The engulfed prokaryotic cell remained undigested as it contributed new functionality to the engulfing cell (e.g. photosynthesis) Over generations, the engulfed cell lost some of its independent utility and became a supplemental organelle

Answer 53

The cell cycle is an ordered set of events which culminates in the division of a cell into two daughter cells It can be roughly divided into two main phases: Interphase The stage in the development of a cell between two successive divisions This phase of the cell cycle is a continuum of three distinct stages: G1 – First intermediate gap stage in which the cell grows and prepares for DNA replication S – Synthesis stage in which DNA is replicated G2 – Second intermediate gap stage in which the cell finishes growing and prepares for cell division M phase The period of the cell cycle in which the cell and contents divide to create two genetically identical daughter cells This phase is comprised of two distinct stages: Mitosis – Nuclear division, whereby DNA (as condensed chromosomes) is separated into two identical nuclei Cytokinesis – Cytoplasmic division, whereby cellular contents are segregated and the cell splits into two

Answer 54

DNA replication – DNA is copied during the S phase of interphase Organelle duplication – Organelles must be duplicated for twin daughter cells Cell growth – Cytoplasmic volume must increase prior to division Transcription / translation – Key proteins and enzymes must be synthesised Obtain nutrients – Vital cellular materials must be present before division Respiration (cellular) – ATP production is needed to drive the division process DOCTOR

Answer 55

Chromatin: DNA is usually loosely packed within the nucleus as unravelled chromatin In this unravelled form, the DNA is accessible to transcriptional machinery and so genetic information can be translated DNA is organised as chromatin in all non-dividing cells and throughout the process of interphase Chromosome: DNA is temporarily packaged into a tightly wound and condensed chromosome prior to division (via supercoiling) In this condensed form, the DNA is able to be easily segregated however is inaccessible to transcriptional machinery DNA is organised as chromosomes during the process of mitosis (condense in prophase, decondense in telophase)

Answer 56

A chromosome is the condensed form of DNA which is visible during mitosis (via microscopy) As the DNA is replicated during the S phase of interphase, the chromosome will initially contain two identical DNA strands These genetically identical strands are called sister chromatids and are held together by a central region called the centromere

Answer 57

Cytokinesis is the process of cytoplasmic division, whereby the cell splits into two identical daughter cells

Answer 58

After anaphase, microtubule filaments form a concentric ring around the centre of the cell The microfilaments constrict to form a cleavage furrow, which deepens from the periphery towards the centre When the furrow meets in the centre, the cell becomes completely pinched off and two cells are formed

Answer 59

After anaphase, carbohydrate-rich vesicles form in a row at the centre of the cell (equatorial plane) The vesicles fuse together and an early cell plate begins to form within the middle of the cell The cell plate extends outwards and fuses with the cell wall, dividing the cell into two distinct daughter cells

Answer 60

Cells in mitosis / total number of cells

Answer 61

Cyclins are a family of regulatory proteins that control the progression of the cell cycle Cyclins activate cyclin dependent kinases (CDKs), which control cell cycle processes through phosphorylation When a cyclin and CDK form a complex, the complex will bind to a target protein and modify it via phosphorylation The phosphorylated target protein will trigger some specific event within the cell cycle (e.g. centrosome duplication, etc.) After the event has occurred, the cyclin is degraded and the CDK is rendered inactive again

Answer 62

Mutagens A mutagen is an agent that changes the genetic material of an organism (either acts on the DNA or the replicative machinery) Mutagens may be physical, chemical or biological in origin: Physical – Sources of radiation including X-rays (ionising), ultraviolet (UV) light and radioactive decay Chemical – DNA interacting substances including reactive oxygen species (ROS) and metals (e.g. arsenic) Biological – Viruses, certain bacteria and mobile genetic elements (transposons) Mutagens that lead to the formation of cancer are further classified as carcinogens Oncogenes An oncogene is a gene that has the potential to cause cancer Most cancers are caused by mutations to two basic classes of genes – proto-oncogenes and tumour suppressor genes Proto-oncogenes code for proteins that stimulate the cell cycle and promote cell growth and proliferation Tumour suppressor genes code for proteins that repress cell cycle progression and promote apoptosis When a proto-oncogene is mutated or subjected to increased expression it becomes a cancer-causing oncogene Tumour suppressor genes are sometimes referred to as anti-oncogenes, as their normal function prevents cancer

Answer 63

Tumour cells may either remain in their original location (benign) or spread and invade neighbouring tissue (malignant) Metastasis is the spread of cancer from one location (primary tumour) to another, forming a secondary tumour Secondary tumours are made up of the same type of cell as the primary tumour – this affects the type of treatment required E.g. If breast cancer spread to the liver, the patient has secondary breast cancer of the liver (treat with breast cancer drugs)

Answer 64

Tissue repair / replacement Damaged or aged cells replaced with identical healthy ones Organismal growth Multicellular organisms derive new cells via mitotis Asexual reproduction Vegetative propagation in plants occurs via mitotic division Development (of embryos) Zygotes undergo mitosis and differentiate to become embryos TOAD

Answer 65

Not all cells are continually replicating – some cells may enter into a non-dividing G0 stage These cells may either be dormant (quiescent) or ageing and deteriorating (senescent) Cells enter the G0 phase from the G1 phase; quiescent cells may re-enter G1 at a later time (senescent cells do not) Normally, cells will only divide a finite time before reaching senescence (a typical human cell will divide ~ 40 - 60 times) Specialised cells will often permanently enter G0, as differentiation has prevented their capacity for further division Neurons are examples of cells that have been arrested in a G0 state – these cells are amitotic (cannot divide)