1.2 Cells Flashcards
Describe the main ideas of cell theory (3)
- All living organisms are made up of one or more cells
- Cells are the basic functioning unit in living organisms
- New cells are produced from pre-existing cells
What are the distinguishing features of eukaryotic cells?
- Cytoplasm containing membrane-bound organelles
- So DNA enclosed in a nucleus
Describe the general structure of eukaryotic cells:
Describe the structure of the cell-surface membrane
Describe the function of the cell-surface membrane
- Selectively permeable → enables control of passage of substances in / out of cell
- Molecules / receptors / antigens on surface → allow cell recognition / signalling
Describe the structure of the nucleus
Describe the function of the nucleus
- Holds / stores genetic information which codes for polypeptides (proteins)
- Site of DNA replication
- Site of transcription (part of protein synthesis), producing mRNA
- Nucleolus makes ribosomes / rRNA
Describe the structure of a ribosome
- Made of ribosomal RNA and protein (two subunits)
- Not a membrane-bound organelle
Describe the function of a ribosome
Site of protein synthesis (translation)
Describe the structure of rough (rER) & smooth endoplasmic reticulum (sER)
- Rough Endoplasmic Reticulum (RER) Surface covered in ribosomes
- Formed from continuous folds of membrane continuous with the nuclear envelope
- Processes proteins made by the ribosomes
- Smooth Endoplasmic Reticulum (ER) Does not have ribosomes on the surface, its function is distinct to the RER
- Involved in the production, processing and storage of lipids, carbohydrates and steroids
Describe the function of rER and sER
Describe the structure of Golgi apparatus and Golgi vesicles
Describe the function of Golgi apparatus and Golgi vesicles
Describe the structure of lysosomes
Describe the function of lysosomes
Describe the structure of mitochondria
- The site of aerobic respiration within eukaryotic cells, mitochondria are just visible with a light microscope
- Surrounded by double-membrane with the inner membrane folded to form cristae
- The matrix formed by the cristae contains enzymes needed for aerobic respiration, producing ATP
- Small circular pieces of DNA (mitochondrial DNA) and ribosomes are also found in the matrix (needed for replication)
Describe the function of mitochondria
Describe the structure of chloroplasts in plants and algae
- Larger than mitochondria, also surrounded by a double-membrane
- Membrane-bound compartments called thylakoids containing chlorophyll stack to form structures called grana
- Grana are joined together by lamellae (thin and flat thylakoid membranes)
- Chloroplasts are the site of photosynthesis:
- The light-dependent stage takes place in the thylakoids
- The light-independent stage (Calvin Cycle) takes place in the stroma
- Also contain small circular pieces of DNA and ribosomes used to synthesise proteins needed in chloroplast replication and photosynthesis
Describe the function of chloroplasts in plants and algae
Describe the structure of the cell wall in plants, algae and fungi
- Composed mainly of cellulose (a polysaccharide) in plants / algae
- Composed of chitin (a nitrogen-containing polysaccharide) in fungi
Describe the function of the cell wall in plants, algae and fungi
- Provides mechanical strength to cell
- So prevents cell changing shape or bursting under pressure due to osmosis
Describe the structure of the cell vacuole in plants
Describe the function of the cell vacuole in plants
- Maintains turgor pressure in cell (stopping plant wilting)
- Contains cell sap → stores sugars, amino acids, pigments and any waste chemicals
Define a Tissue
Group of specialised cells with a similar structure working together to perform a specific function, often with the same origin
Define an Organ
Aggregations of tissues performing specific functions.
Define an Organ System
Group of organs working together to perform specific functions.
How are microvilli adapted to their function?
- Cell membrane projections that increase the surface area for absorption
How are Cilia adapted to their function?
- Hair-like projections made from microtubules
- Allows the movement of substances over the cell surface
How is the Flagella adapted to its function?
- Similar in structure to cilia, made of longer microtubules
- Contract to provide cell movement for example in sperm cells
Define a Eukaryotic cell
- This is a cell that has a membrane bound nucleus and chromosomes
- The cell also posesses a variety of other membrane-bound organelles
How are muscle cells adapted to their function?
Function: Contraction for movement
- All muscle cells have layers of protein filaments in them, these layers can slide over each other causing muscle contraction
- Muscle cells have a high density of mitochondria to provide sufficient energy (via respiration) for muscle contraction
- Skeletal muscle cells fuse together during development to form multinucleated cells that contract in unison
How are sperm cells adapted to their function?
Function: Reproduction
- The head contains a nucleus that contains half the normal number of chromosomes (haploid, no chromosome pairs)
- The acrosome in the head contains digestive enzymes that can break down the outer layer of an egg cell so that the haploid nucleus can enter to fuse with the egg’s nucleus
- The mid-piece is packed with mitochondria to release energy (via respiration) for the tail movement
- The tail rotates, propelling the sperm cell forwards and allowing it to move towards the egg
How are root hair cells adapted to their function?
Function: Absorption of water and mineral ions from soil
- Root hair to increase surface area (SA) so the rate of water uptake by osmosis is greater (can absorb more water and ions than if SA were lower)
- Thinner walls than other plant cells so that water can move through easily (due to shorter diffusion distance)
- Permanent vacuole contains cell sap which is more concentrated than soil water, maintaining a water potential gradient
- Mitochondria for active transport of mineral ions
How are xylem cells adapted to their function?
Function: transport tissue for water and dissolved ions
- No top and bottom walls between cells to form continuous hollow tubes through which water is drawn upwards towards the leaves by transpiration
- Cells are essentially dead, without organelles or cytoplasm, to allow free movement of water
- Outer walls are thickened with a substance called lignin, strengthening the tubes, which helps support the plant
How are phloem cells adapted to their function?
- Made of living cells (as opposed to xylem vessels which are made of dead cells) which are supported by companion cells
- Cells are joined end-to-end and contain holes in the end cell walls (sieve plates) forming tubes which allow sugars and amino acids to flow easily through (by translocation)
- Cells also have very few subcellular structures to aid the flow of materials
What are the distinguishing features of prokaryotic cells?
- Cytoplasm lacking membrane-bound organelles
- So genetic material not enclosed in a nucleus
Give Examples of prokaryotic organisms
bacteria and archaea (always unicellular)
Describe the general structure of prokaryotic cells
Compare and contrast the structure of eukaryotic and prokaryotic cells
Explain why viruses are described as acellular and non-living
- Acellular - not made of cells, no cell membrane / cytoplasm / organelles
- Non-living - have no metabolism, cannot independently move / respire / replicate / excrete
Describe the general structure of a virus particle
- Nucleic acids surrounded by a capsid (protein coat)
- Attachment proteins allow attachment to specific host cells
- No cytoplasm, ribosomes, cell wall, cell-surface membrane etc.
- Some also surrounded by a lipid envelope eg. HIV
Describe the difference between magnification and resolution
- Magnification = number of times greater image is than size of the real (actual) object
- Magnification = size of image / size of real object
- Resolution = minimum distance apart 2 objects can be to be distinguished as separate objects
Compare the principles and limitations of optical microscopes, transmission electron microscopes and scanning electron microscopes
How do electron microscopes have a higher resolution that optical microscopes?
A beam of electrons has a much smaller wavelength than light, so an electron microscope can resolve (distinguish between) two objects that are extremely close together
What are the two types of electron microscopes?
- Transmission electron microscopes (TEMs)
- Scanning electron microscopes (SEMs)
What are the advantages of TEMs?
- They give high-resolution images (more detail)
- This allows the internal structures within cells (or even within organelles) to be seen
How does a Scanning electron microscope (SEM) work?
- SEMs scan a beam of electrons across the specimen
- This beam bounces off the surface of the specimen and the electrons are detected, forming an image
- This means SEMs can produce three-dimensional images that show the surface of specimens
What are the advantages of SEMs?
- They can be used on thick or 3-D specimens
- They allow the external, 3-D structure of specimens to be observed
What are the disadvantages of SEMs?
- They give lower resolution images (less detail) than TEMs
- They cannot be used to observe live specimens (unlike optical microscopes that can be used to observe live specimens)
- They do not produce a colour image (unlike optical microscopes that produce a colour image)
Define an artefact
Artefacts are visible details that aren’t part of the specimen being observed, such as air bubbles or fingerprints.
Students should be able to appreciate that there was a considerable period of time during which the scientific community distinguished between artefacts (eg. dust, air bubbles occurring during preparation) and cell organelles. To overcome this, scientists prepared specimens in different ways. If an object was seen with one technique but not another, it was more likely to be an artefact than an organelle.
Centimetre (cm) Standard form:
10-2m
millimetre (mm) Standard form:
10-3m
micrometre (µm) Standard form:
10-6m
nanometre (nm) Standard form:
10-9m
Describe how the size of an object viewed with an optical microscope can be
measured
Line up (scale of) eyepiece graticule with (scale of) stage micrometre
2. Calibrate eyepiece graticule - use stage micrometre to calculate size of divisions on eyepiece graticule
3. Take micrometre away and use graticule to measure how many divisions make up the object
4. Calculate size of object by multiplying number of divisions by size of division
5. Recalibrate eyepiece graticule at different magnifications
What is a graticule used for?
To take measurements of cells
How is a graticule used?
- A graticule is a small disc that has an engraved scale. It can be placed into the eyepiece of a microscope to act as a ruler in the field of view
- As a graticule has no fixed units it must be calibrated for the objective lens that is in use. This is done by using a scale engraved on a microscope slide (a stage micrometer)
- By using the two scales together the number of micrometers each graticule unit is worth can be worked out
- After this is known the graticule can be used as a ruler in the field of view
How do we calculate total magnification?
eyepiece lens magnification x objective lens magnification = total magnification
Light v Electron Microscope Table
How do we calculate magnification?
Describe and explain the principles of cell fractionation and ultracentrifugation as used to separate cell components
Define cell fractionation
The process of separating cell organelles from each other
This process involves breaking up a suitable sample of tissue and then centrifuging the mixture at different speeds
What are the three stages of cell fractionation?
- Homogenisation
- Filtration
- Ultracentrifugation
What is Homogenisation?
- Homogenisation is the biological term used to describe the breaking up of cells
- The sample of tissue (containing the cells to be broken up) must first be placed in a cold, isotonic buffer solution
Explain why it is not possible to determine the identity of the structures labelled X using an optical microscope (2)
- Resolution (too) low
- Because wavelength of light is (too) long
Describe how a sample of chloroplasts could be isolated from leaves (4)
- Grind/blend cells/tissue/leaves and filter
- In cold, same water potential/concentration, pH controlled solution (isotonic)
- Centrifuge/spin and remove nuclei/cell debris
- (Centrifuge/spin) at high(er) speed, chloroplasts settle out
Outline the role of organelles in the production, transport and release of proteins from eukaryotic cells. Do not include details of transcription and translation in your answer. (4)
- DNA in nucleus is code (for protein)
- Ribosomes/rough endoplasmic reticulum produce (protein)
- Mitochondria produce ATP (for protein synthesis)
- Golgi apparatus package/modify
- Vesicles transport
Give one advantage of viewing a biological specimen using a transmission electron microscope compared with using a scanning electron microscope.
- Higher resolution
- View internal structures
Name an organelle found in both a chloroplast and a prokaryotic cell (1)
(70S) Ribosome
Name the process by which prokaryotic cells divide (1)
Binary fission
Contrast how an optical microscope and a transmission electron microscope work and contrast the limitations of their use when studying cells. (6)
- TEM use electrons and optical use light
- TEM allows a greater resolution
- (So with TEM) smaller organelles / named cell structure can be observed
OR - greater detail in organelles / named cell structure can be observed
- TEM view only dead / dehydrated specimens and optical (can) view live specimens
- TEM does not show colour and optical (can)
- TEM requires thinner specimens
- TEM requires a more complex/time consuming preparation
- TEM focuses using magnets and optical uses (glass) lenses
Name two structures present in plant cells that are not present in animal cells (1)
- Chloroplasts / plastids
- Cell wall
- Cell vacuole
Ice-cold
Explain why the solution the biologist used was ice-cold, buffered and the
same water potential as the liver tissue (step 1)
Slows / stops enzyme activity to prevent digestion of organelles / mitochondria
Buffered
Explain why the solution the biologist used was ice-cold, buffered and the
same water potential as the liver tissue (step 1)
Maintains pH so that enzymes / proteins are not denatured;
Same water potential
Explain why the solution the biologist used was ice-cold, buffered and the
same water potential as the liver tissue (step 1)
Prevents osmosis so no lysis / shrinkage of organelles / mitochondria
Explain why the biologist used a blender and then filtered the mixture (steps 2 and 3). (2)
- Break open cells / homogenise / produce homogenate
- Remove unbroken cells / larger debris
The cell-surface membrane can be seen with a transmission electron
microscope but not with an optical microscope.
Explain why (1)
Electron microscope has higher resolution (than optical).
Name two structures in a eukaryotic cell that cannot be identified using an optical microscope. (2)
Mitochondrion / ribosome / endoplasmic reticulum / lysosome / cellsurface membrane.
Describe how you could make a temporary mount of a piece of plant tissue to observe the position of starch grains in the cells when using an optical (light) microscope. (4)
- Add drop of water to (glass) slide
- Obtain thin section (of plant tissue) and place on slide / float on drop of water
- Stain with / add iodine in potassium iodide
- Lower cover slip using mounted needle
A transmission electron microscope was used to produce the image in the figure above. Explain why (2)
- High resolution
- Can see internal structure of organelles
Describe the Interphase stage of the cell cycle in eukaryotic cells
- (S phase) DNA replicates semi-conservatively
- Leading to 2 chromatids (identical copies) joined at a centromere
- (G1/G2) number of organelles & volume of cytoplasm increases, protein synthesis
Describe the Mitosis stage of the cell cycle in eukaryotic cells
- Nucleus divides
- To produce 2 nuclei with identical copies of DNA produced by parent cell
Describe the Cytokinesis stage of the cell cycle in eukaryotic cells
- Cytoplasm and cell membrane (normally) divide
- To form 2 new genetically identical daughter cells
Describe the behaviour of chromosomes & role of spindle fibres in the Prophase (Stage 1) ?
- Chromosomes condense, becoming shorter / thicker (so visible)
- Appear as 2 sister chromatids joined by a centromere
- Nuclear envelope breaks down
- Centrioles move to opposite poles forming spindle network
Describe the behaviour of chromosomes & role of spindle fibres in the Metaphase (Stage 2) ?
- Spindle fibres attach to chromosomes by their centromeres
- Chromosomes align along equator
Describe the behaviour of chromosomes & role of spindle fibres in the Anaphase (Stage 3) ?
- Spindle fibres shorten / contract
- Centromere divides
- Pulling chromatids (from each pair) to opposite poles of cell
Describe the behaviour of chromosomes & role of spindle fibres in the Telophase (Stage 4) ?
- Chromosomes uncoil, becoming longer / thinner
- Nuclear envelopes reform = 2 nuclei
- Spindle fibres / centrioles break down
Why do some eukaryotic cells not undergo the cell cycle?
- Within multicellular organisms, not all cells retain the ability to divide (eg. neurons)
- Only cells that do retain this ability go through a cell cycle
Explain the importance of mitosis in the life of an organism
Parent cell divides to produce 2 genetically identical daughter cells fo
- Growth of multicellular organisms by increasing cell number
- Replacing cells to repair damaged tissues
- Asexual reproduction
Describe how tumours and cancers form
- Mutations in DNA / genes controlling mitosis can lead to uncontrolled cell division
- Tumour formed if this results in mass of abnormal cells
- Malignant tumour = cancerous, can spread (metastasis)
- Benign tumour = non-cancerous
Suggest how cancer treatments control rate of cell division
- Some disrupt spindle fibre activity / formation
- So chromosomes can’t attach to spindle by their centromere
- So chromatids can’t be separated to opposite poles (no anaphase)
- So prevents / slows mitosis
- Some prevent DNA replication during interphase
- So can’t make 2 copies of each chromosome (chromatids)
- So prevents / slows mitosis
More effective against cancer cells due to uncontrolled cell division but also disrupts cell cycle of rapidly dividing healthy cells.
Describe how prokaryotic cells replicate
- Replication of circular DNA
- Replication of plasmids
- Division of cytoplasm to produce 2 daughter cells
- Single copy of circular DNA
- Variable number of copies of plasmids
Describe how viruses replicate
Being non-living, viruses do not undergo cell division
- Attachment proteins attach to complementary receptors on host cell
- Inject viral nucleic acid (DNA/RNA) into host cell
- Infected host cell replicates virus particles:
- Nucleic acid replicated (Reverse transcriptase converts viral RNA into DNA)
- Cell produces viral protein / capsid / enzymes
- Virus assembled then released
Describe binary fission in bacteria (3)
- Replication of (circular) DNA
- Replication of plasmids
- Division of cytoplasm (to produce daughter cells)
Suggest and explain how two environmental variables could be changed to increase the growth rate of these cells. (4)
- Increased (concentration of) glucose - Increased respiration (2)
- Increased (concentration of) oxygen - Increased respiration
- Increased temperature Increased enzyme activity
- Increased (concentration of) phosphate - Increased ATP/DNA/RNA
- Increased (concentration of) nucleotides - Increased DNA synthesis
Name the fixed position occupied by a gene on a DNA molecule (1)
Locus/loci
What is a homologous pair of chromosomes? (1)
- (Two chromosomes that) carry the same genes
Describe two aseptic techniques she would have used when transferring a sample of broth culture on to an agar plate.
Explain why each was important. (4)
- Keep lid on Petri dish - To prevent unwanted bacteria contaminating the dish
- Wear gloves - To prevent contamination from bacteria on hands
- Flame the loop/Use sterile pipette - To maintain a pure culture of bacteria
Describe the appearance and behaviour of chromosomes during mitosis. (5)
- (During prophase)
- Chromosomes coil / condense / shorten / thicken / become visible
- (Chromosomes) appear as (two sister) chromatids joined at the centromere
- (During metaphase)
- Chromosomes line up on the equator / centre of the cell
- (Chromosomes) attached to spindle fibres
- By their centromere
- (During anaphase)
- The centromere splits / divides
- (Sister) chromatids / chromosomes are pulled to opposite poles / ends of the cell / separate
- (During telophase)
- Chromatids / chromosomes uncoil / unwind / become longer / thinner
Describe what occurs during prophase (2)
- Chromosomes coil / condense / shorten / thicken / become visible
- (Chromosomes) appear as (two sister) chromatids joined at the centromere
Describe what occurs during Metaphase (2)
- Chromosomes line up on the equator / centre of the cell
- (Chromosomes) attached to spindle fibres
- By their centromere
Describe what occurs during Anaphase (2)
- The centromere splits / divides
- (Sister) chromatids / chromosomes are pulled to opposite poles / ends of the cell / separate
Describe what occurs during Telophase (2)
- Chromatids / chromosomes uncoil / unwind / become longer / thinner.
What are the three stages of the cell cycle?
- interphase
- nuclear division (mitosis)
- cell division (cytokinesis)
What are the three phases of interphase?
- G1 phase
- S phase
- G2 phase
What occurs during the G1 phase of Interphase?
Cell grows and receives a signal to divide
What occurs during the S phase of Interphase?
Synthesis of new DNA
What occurs during the G2 phase of Interphase?
- Further cell growth
- Error checking of newly synthesised DNA
What is Mitosis?
Mitosis is the process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent nucleus
What are the stages of mitosis in order?
- Prophase
- Metaphase
- Anaphase
- Telophase
How can we recognise prophase on a diagram?
- Chromosomes are visible
- The nuclear envelope is breaking down
How can we recognise metaphase on a diagram?
Chromosomes are lined up along the middle of the cell
How can we recognise anaphase on a diagram?
Chromosomes are moving away from the middle of the cell, towards opposite poles
How can we recognise telophase on a diagram?
- Chromosomes have arrived at opposite poles of the cell
- Chromosomes begin to decondense
- The nuclear envelope is reforming
What is the mitotic index?
The mitotic index is the proportion of cells (in a group of cells or a sample of tissue) that are undergoing mitosis
How do we calculate the mitotic index?
mitotic index = number of cells with visible chromosomes ÷ total number of cells
Define metastasis
This is when malignant tumour cells can break off the tumour and travel through the blood and / or lymphatic system to form secondary growths in other parts of the body
Why is metastasis dangerous?
It can be very difficult to detect, locate and remove secondary cancers
What are carcinogens?
Any agents that may cause cancer (eg. UV light, tar in tobacco smoke and X-rays). If the agent causes cancer it is described as carcinogenic
What is an antigen? (2)
- Foreign protein / glycoprotein
- That stimulates an immune response leading to production of antibody / memory cells
How are cells identified by the immune system? (2)
- Each type of cell has specific molecules on its surface (cell-surface membrane / cell wall) that identify it
- Often proteins → have a specific tertiary structure (or glycoproteins / glycolipids)
What types of cells and molecules can the immune system identify?
- Pathogens (disease causing microorganisms) eg. viruses, fungi, bacteria
- Cells from other organisms of the same species (eg. organ transplants)
- Abnormal body cells eg. tumour cells or virus-infected cells
- Toxins (poisons) released by some bacteria
Describe phagocytosis of pathogens (non-specific immune response) (5)
- Phagocyte attracted by chemicals / recognises (foreign) antigens on pathogen
- Phagocyte engulfs pathogen by surrounding it with its cell membrane
- Pathogen contained in vesicle / phagosome in cytoplasm of phagocyte
- Lysosome fuses with phagosome and releases lysozymes (hydrolytic enzymes)
- Lysozymes hydrolyse / digest pathogen
Phagocytosis leads to presentation of antigens where antigens are displayed on the phagocyte cell-surface membrane, stimulating the specific immune response (cellular and humoral response).
Describe the response of T lymphocytes to a foreign antigen (the cellular response)
T lymphocytes recognise (antigens on surface of) antigen presenting cells eg. infected cells, phagocytes presenting antigens, transplanted cells, tumour cells etc
- Specific helper T cells with complementary receptors (on cell surface) bind to antigen on antigen-presenting cell → activated and divide by mitosis to form clones which stimulate:
- Cytotoxic T cells → kill infected cells / tumour cells (by producing perforin)
- Specific B cells (humoral response)
- Phagocytes → engulf pathogens by phagocytosis
Describe the response of B lymphocytes to a foreign antigen (the humoral response)
B lymphocytes can recognise free antigens eg. in blood or tissues, not just antigen presenting cells.
- Clonal selection:
- Specific B lymphocyte with complementary receptor (antibody on cell surface) binds to antigen
- This is then stimulated by helper T cells (which releases cytokines)
- So divides (rapidly) by mitosis to form clones
- Some differentiate into B plasma cells → secrete large amounts of (monoclonal) antibody
- Some differentiate into B memory cells → remain in blood for secondary immune response
A vaccine can be used to produce immunity to HPV. Describe how memory cells are important in this process (3)
- They bind to the antigen
- Divide by mitosis
- Produce antibodies which bind to antigen
- Destroys HPV
What are antibodies? (3)
- Quaternary structure proteins (4 polypeptide chains)
- Secreted by B lymphocytes eg. plasma cells in response to specific antigens
- Bind specifically to antigens forming antigen-antibody complexes
Describe the structure of an antibody
Explain how antibodies lead to the destruction of pathogens (4)
- Antibodies bind to antigens on pathogens forming an antigen-antibody complex
- Specific tertiary structure so binding site / variable region binds to complementary antigen
- Each antibody binds to 2 pathogens at a time causing agglutination (clumping) of pathogens
- Antibodies attract phagocytes
- Phagocytes bind to the antibodies and phagocytose many pathogens at once
Explain the differences between the primary & secondary immune response
- Primary - first exposure to antigen
- Antibodies produced slowly & at a lower conc.
- Takes time for specific B plasma cells to be stimulated to produce specific antibodies
- Memory cells produced
- Secondary - second exposure to antigen
- Antibodies produced faster & at a higher conc.
- B memory cells rapidly undergo mitosis to produce many plasma cells which produce specific antibodies
What is a vaccine? (2)
- Injection of antigens from attenuated (dead or weakened) pathogens
- Stimulating formation of memory cells
Explain how vaccines provide protection to individuals against disease (6)
- Specific B lymphocyte with complementary receptor binds to antigen
- Specific T helper cell binds to antigen-presenting cell and stimulates B cell
- B lymphocyte divides by mitosis to form clones
- Some differentiate into B plasma cells which release antibodies
- Some differentiate into B memory cells
- On secondary exposure to antigen, B memory cells rapidly divide by mitosis to produce B plasma cells
- These release antibodies faster and at a higher concentration
Explain how vaccines provide protections for populations against disease (2)
- Herd immunity - large proportion of population vaccinated, reducing spread of pathogen
- Large proportion of population immune so do not become ill from infection
- Fewer infected people to pass pathogen on / unvaccinated people less likely to come in contact with someone with disease
Describe the differences between active and passive immunity
Explain the effect of antigen variability on disease and disease prevention (2)
- Antigens on pathogens change shape / tertiary structure due to gene mutations (creating new strains)
- So no longer immune (from vaccine or prior infection)
- B memory cell receptors cannot bind to / recognise changed antigen on secondary exposure
- Specific antibodies not complementary / cannot bind to changed antigen
Examples: yearly new flu vaccines, no vaccine for HIV, catch a cold many times
Describe the structure of a HIV particle
Describe the replication of HIV in helper T cells
- HIV attachment proteins attach to receptors on helper T cell
- Lipid envelope fuses with cell-surface membrane, releasing capsid into cell
- Capsid uncoats, releasing RNA and reverse transcriptase
- Reverse transcriptase converts viral RNA to DNA
- Viral DNA inserted / incorporated into helper T cell DNA (may remain latent)
- Viral protein / capsid / enzymes are produced
- DNA transcribed into HIV mRNA
- HIV mRNA translated into new HIV proteins
- Virus particles assembled and released from cell (via budding)
Explain how HIV causes the symptoms of acquired immune deficiency syndrome (AIDS)
- HIV infects and kills helper T cells (host cell) as it multiplies rapidly
- So T helper cells can’t stimulate cytotoxic T cells, B cells and phagocytes
- So B plasma cells can’t release as many antibodies for agglutination & destruction of pathogens
- Immune system deteriorates → more susceptible to (opportunistic) infections
- Pathogens reproduce, release toxins and damage cells
Explain why antibiotics are ineffective against viruses
- Viruses do not have structures / processes that antibiotics inhibit
- Viruses do not have metabolic processes (eg. do not make protein) / ribosomes
- Viruses do not have bacterial enzymes / murein cell wall
What is a monoclonal antibody?
- Antibody produced from genetically identical / cloned B lymphocytes / plasma cells
- So have same tertiary structure
Explain how monoclonal antibodies can be used in medical treatments
- Monoclonal antibody has a specific tertiary structure / binding site / variable region
- Complementary to receptor / protein / antigen found only on a specific cell type (eg. cancer cell)
- Therapeutic drug attached to antibody
- Antibody binds to specific cell, forming antigen-antibody complex, delivering drug
Some monoclonal antibodies are also designed to block antigens / receptors on cells
Explain how monoclonal antibodies can be used in medical diagnosis
- Monoclonal antibody has a specific tertiary structure / binding site / variable region
- Complementary to specific receptor / protein / antigen associated with diagnosis
- Dye / stain / fluorescent marker attached to antibody
- Antibody binds to receptor / protein / antigen, forming antigen-antibody complex
Examples vary, eg. pregnancy tests. You’ll need to interpret information in the question on how these work.
Explain the use of antibodies in the ELISA (enzyme-linked immunosorbent assay) test to detect antigens
(Direct ELISA)
- Attach sample with potential antigens to well
- Add complementary monoclonal antibodies with enzymes attached → bind to antigens if present
- Wash well → remove unbound antibodies (to prevent false positive)
- Add substrate → enzymes create products that cause a colour change (positive result)
Explain the use of antibodies in the ELISA (enzyme-linked immunosorbent assay) test to detect antigens
(Sandwich ELISA)
- Attach specific monoclonal antibodies to well
- Add sample with potential antigens, then wash well
- Add complementary monoclonal antibodies with enzymes attached → bind to antigens if present
- Wash well → remove unbound antibodies (to prevent false positive)
- Add substrate → enzymes create products that cause a colour change (positive result)
Explain the use of antibodies in the ELISA test to detect antibodies
(Indirect ELISA)
- Attach specific antigens to well
- Add sample with potential antibodies, wash well
- Add complementary monoclonal antibodies with enzymes attached → bind to antibodies if present
- Wash well → remove unbound antibodies
- Add substrate → enzymes create products that cause a colour change (positive result)
Suggest the purpose of a control well in the ELISA test
- Compare to test to show only enzyme causes colour change
- Compare to test to show all unbound antibodies have been washed away
Discuss some general ethical issues associated with the use of vaccines and monoclonal antibodies
- Pre-clinical testing on / use of animals - potential stress / harm / mistreatment
- But animals not killed & helps produce new drugs to reduce human suffering
- Clinical trials on humans - potential harm / side-effects
- Vaccines - may continue high risk activities and still develop / pass on pathogen
- Use of drug - potentially dangerous side effects
Suggest some points to consider when evaluating methodology relating to the use of vaccines and monoclonal antibodies
- Was the sample size large enough to be representative?
- Were participants diverse in terms of age, sex, ethnicity and health status?
- Were placebo / control groups used for comparison?
- Was the duration of the study long enough to show long-term effects?
- Was the trial double-blind (neither doctor / patient knew who was given drug or placebo) to reduce bias?
Suggest some points to consider when evaluating evidence and data relating to the use of vaccines and monoclonal antibodies
- What side effects were observed, and how frequently did they occur?
- Was a statistical test used to see if there was a significant difference between start & final results?
- Was the standard deviation of final results large, showing some people did not benefit?
- Did standard deviations of start & final results overlap, showing there may not be a significant difference?
- What dosage was optimum? Does increasing dose increase effectiveness enough to justify extra cost?
- Was the cost of production & distribution low enough?
Describe how HIV is replicated (4)
- Attachment proteins attach to receptors on helper T cell/lymphocyte
- Nucleic acid/RNA enters cell
- Reverse transcriptase converts RNA to DNA
- Viral protein/capsid/enzymes produced
- Virus (particles) assembled and released (from cell)
Describe how a phagocyte destroys a pathogen present in the blood (3)
- Engulfs
- Forming vesicle/phagosome and fuses with lysosome
- hydrolytic Enzymes digest/hydrolyse (lysozymes)
Give two types of cell, other than pathogens, that can stimulate an immune response (2)
- (Cells from) other organisms/transplants
- Abnormal/cancer/tumour (cells)
- (Cells) infected by virus
What is the role of the disulfide bridge in forming the quaternary structure of an antibody? (1)
Joins two (different) polypeptides
Explain how HIV affects the production of antibodies when AIDS develops in a person (3)
- Less/no antibody produced
- (Because HIV) destroys helper T cells
- (So) few/no B cells activated / stimulated
- OR (So) few/no B cells undergo mitosis/differentiate/form plasma cells
What is a monoclonal antibody? (1)
- (Antibodies with the) same tertiary structure
- OR (Antibody produced from) identical/cloned plasma cells/B cells/B lymphocytes;
Describe the role of antibodies in producing a positive result in an ELISA test (4)
- (First) antibody binds/attaches /complementary (in shape) to antigen
- (Second) antibody with enzyme attached is added
- (Second) antibody attaches to antigen
- Accept (second) antibody attaches to (first) antibody
- (Substrate/solution added) and colour changes (Enzyme causes colour change)
Describe and explain the role of antibodies in stimulating phagocytosis (2)
- Bind to antigen
- (Antibodies) cause clumping/agglutination | Attract phagocytes
Describe how phagocytosis of a virus leads to presentation of its antigens (3)
- Phagosome / vesicle fuses with lysosome
- (Virus) destroyed by lysozymes / hydrolytic enzymes
- Peptides / antigen (from virus) are displayed on the cell membrane
Describe how presentation of a virus antigen leads to the secretion of an antibody against this virus antigen (3)
- Helper T cell / TH cell binds to the antigen (on the antigenpresenting cell / phagocyte)
- This helper T / TH cell stimulates a specific B cell
- B cell clones / divides by mitosis
- (Forms) plasma cells that release antibodies
What is an antigen? (2)
- Foreign protein
- (that) stimulates an immune response / production of antibody
What is an antibody? (2)
- A protein / immunoglobulin specific to an antigen
- Produced by B cells OR Secreted by plasma cells
When a vaccine is given to a person, it leads to the production of
antibodies against a disease-causing organism. Describe how (5)
- Vaccine contains antigen from pathogen
- Macrophage presents antigen on its surface
- T cell with complementary receptor protein binds to antigen
- T cell stimulates B cell
- (With) complementary antibody on its surface
- B cell secretes large amounts of antibody
- B cell divides to form clone all secreting / producing same antibody
Describe the difference between active and passive immunity (5)
- Active involves memory cells, passive does not
- Active involves production of antibody by plasma cells / memory cells
- Passive involves antibody introduced into body from outside / named source
- Active long term, because antibody produced in response to antigen
- Passive short term, because antibody (given) is broken down
- Active (can) take time to develop / work, passive fast acting
Give one advantage of viewing a biological specimen using a transmission electron microscope copared with using a scanning electron microscope (1)
- Higher resolution
- Can see internal structures
Describe how you could make a temporary mount of a piece of plant tissue to observe the position of starch grains in the cells when using an optical (light) microscope (4)
- Add drop of water to (glass) slide
- Obtain thin section (of plant tissue) and place on slide / float on drop of water
- Stain with / add iodine in potassium iodide
- Lower cover slip using mounted needle
Why do optical microscopes have a low resolution ? (1)
- (Because) wavelength of light not short enough / too long
What is the function of Chloroplast (2)
- Photosynthesis
- Uses light (energy)
- To produce carbohydrates / starch / glucose / sugars / ATP /
reduced NADP
Starting with some lettuce leaves, describe how you would obtain a sample of undamaged chloroplasts. Use your knowledge of cell fractionation and ultracentrifugation to answer this question (6)
- Chop up (accept any reference to crude breaking up);
- Cold;
- Buffer solution;
- Isotonic / same water potential;
- Filter and centrifuge filtrate;
- Centrifuge supernatant;
- At higher speed;
- Chloroplasts in (second) pellet;
Explain how HIV affects the production of antibodies when AIDS develops in a person (3)
- Less/no antibody produced
- (Because HIV) destroys helper T cells
- (So) few/no B cells activated / stimulated / (So) few/no B cells undergo mitosis/differentiate/form plasma cells
Describe the structure of HIV (4)
- RNA (as genetic material)
- Reverse transcriptase
- Capsid
- (Phospho)lipid (viral) envelope
- Attachment proteins
What are the events of Interphase?
- G1 - Cell grows and recieves a signal to divide
- S - Synthesis of new DNA
- G2 - Further cell growth + Error checking of newly synthesised DNA
Interphase = G1 + S + G2
Describe the fluid-mosaic model of membrane structure
- Molecules free to move laterally in phospholipid bilayer
- Many components - phospholipids, proteins, glycoproteins and glycolipids
Describe the arrangement of the components of a cell membrane
- Phospholipids form a bilayer - fatty acid tails face inwards, phosphate heads face outwards
- Intrinsic / integral proteins span bilayer eg. channel and carrier proteins
- Extrinsic / peripheral proteins on surface of membrane
- Glycolipids (lipids with polysaccharide chains attached) found on exterior surface
- Glycoproteins (proteins with polysaccharide chains attached) found on exterior surface
- Cholesterol (sometimes present) bonds to phospholipid hydrophobic fatty acid tails
Explain the arrangement of phospholipids in a cell membrane
- Bilayer, with water present on either side
- Hydrophobic fatty acid tails repelled from water so point away from water / to interior
- Hydrophilic phosphate heads attracted to water so point to water
Explain the role of cholesterol (sometimes present) in cell membranes
- Restricts movement of other molecules making up membrane
- So decreases fluidity (and permeability) / increases rigidity
Suggest how cell membranes are adapted for other functions
- Phospholipid bilayer is fluid → membrane can bend for vesicle formation / phagocytosis
- Glycoproteins / glycolipids act as receptors / antigens → involved in cell signalling / recognition
What is the purpose of the cell membrane (2)
- Controls the movement of substances in and out the cell
- So allows different conditions to be established inside and outside the cell
What are the components of the cell-surface membrane
- Intrinsic protiens - Spans the while width of the membrane
- Extrinsic protiens - Confined to the inner or outer surface of the membrane
- Glycoprotiens - Protiens with attatched carbohydrate chains
- Phospholipids
- Cholesterol
- Glycolipids
Describe how movement across membranes occurs by simple diffusion
- Lipid-soluble (non-polar) or very small substances eg. O2 , steroid hormones
- Move from an area of higher conc. to an area of lower conc. down a conc. gradient
- Across phospholipid bilayer
- Passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances)
Explain the limitations imposed by the nature of the phospholipid bilayer
- Restricts movement of water soluble (polar) & larger substances eg. Na + / glucose
- Due to hydrophobic fatty acid tails in interior of bilayer
Describe how movement across membranes occurs by facilitated diffusion
- Water-soluble (polar) / slightly larger substances
- Move down a concentration gradient
- Through specific channel / carrier proteins
- Passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances)
Explain the role of carrier and channel proteins in facilitated diffusion
- Shape / charge of protein determines which substances move
- Channel proteins facilitate diffusion of water-soluble substances
- Hydrophilic pore filled with water
- May be gated - can open / close
- Carrier proteins facilitate diffusion of (slightly larger) substances
- Complementary substance attaches to binding site
- Protein changes shape to transport substance
Describe how movement across membranes occurs by osmosis
- Water diffuses / moves
- From an area of high to low water potential (ψ) / down a water potential gradient
- Through a partially permeable membrane
- Passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances)
Define Water Potential
- Water potential is a measure of how likely water molecules are to move out of a solution.
- Pure (distilled) water has the maximum possible ψ (0 kPA), increasing solute concentration decreases ψ
Describe how movement across membranes occurs by active transport
- Substances move from area of lower to higher concentration / against a concentration gradient
- Requiring hydrolysis of ATP and specific carrier protein
Describe the role of carrier proteins and the importance of the hydrolysis of ATP in active transport
- Complementary substance binds to a receptor on specific carrier protein
- ATP binds, hydrolysed into ADP + Pi, releasing energy
- Carrier protein changes shape, releasing substance on side of higher concentration
- Pi released → protein returns to original shape
Describe how movement across membranes occurs by co-transport
- Two different substances bind to and move simultaneously via a co transporter protein (type of carrier protein)
- Movement of one substance against its concentration gradient is often
- coupled with the movement of another down its concentration gradient
Fick’s Law
Rate of diffusion is proportional to = SA x Concenration Gradient / Length of diffusion pathway
Explain the adaptations of some specialised cells in relation to the rate of transport across their internal and external membranes
- Large number of mitochondria → make more ATP by aerobic respiration for active transport
- More protein channels / carriers → for facilitated diffusion (or active transport - carrier proteins only)
- Membrane folded eg. microvilli in ileum → increase in surface area
Describe how surface area affects the rate of movement across cell membranes
Increasing surface area of membrane increases rate of movement
Describe how the number of channel or carrier proteins affects the rate of movement across cell membranes
- Increasing number of channel / carrier proteins increases rate of facilitated diffusion / active transport
Describe how differences in gradients of concentration affects the rate of movement across cell membranes
- Increasing concentration gradient increases rate of simple / facilitated diffusion and osmosis
- Increasing concentration gradient increases rate of facilitated diffusion - Until number of channel / carrier proteins becomes a limiting factor as all in use / saturated
Describe how Water Potential affects the rate of movement across cell membranes
Increasing water potential gradient increases rate of osmosis
Name and describe five ways substances can move across the cell-surface membrane into a cell (5)
- (Simple) diffusion of small/non-polar molecules down a concentration gradient
- Facilitated diffusion down a concentration gradient via protein carrier/channel
- Osmosis of water down a water potential gradient
- Active transport against a concentration gradient via protein carrier using ATP
- Co-transport of 2 different substances using a carrier protein
Explain the function of a membrane-bound ATP hydrolase enzyme (2)
- (ATP to ADP + Pi ) Releases energy
- (energy) allows ions to be moved against a concentration gradient OR (energy) allows active transport of ions
The movement of Na+ out of the cell allows the absorption of glucose into the cell lining the ileum.
Explain how (2)
- (Maintains/generates) a concentration/diffusion gradient for Na+ (from ileum into cell)
- Na+ moving (in) by co-transport, brings glucose with it
Describe and explain two features you would expect to find in a cell specialised for absorption (2)
- Folded membrane/microvilli so large surface area (for absorption)
- Large number of co transport/carrier/channel proteins so fast rate (of absorption)
- Large number of mitochondria so make (more) ATP (by respiration) to release energy via aerobic respiration
The movement of substances across cell membranes is affected by membrane structure. Describe how (5)
- Phospholipid (bilayer) allows movement/diffusion of nonpolar/lipid soluble substances
- Phospholipid (bilayer) prevents movement/diffusion of polar/
charged/lipid-insoluble substances - Carrier proteins allow active transport
- Channel/carrier proteins allow facilitated diffusion/co-transport
- Shape/charge of channel / carrier determines which substances move
- Number of channels/carriers determines how much movement
- Membrane surface area determines how much diffusion/movement
- Cholesterol affects fluidity/rigidity/permeability
Give two similarities in the movement of substances by diffusion and by osmosis (2)
- (Movement) down a gradient / from high concentration to low concentration
- Passive / not active processes
- Do not use energy from respiration / from ATP
Compare and contrast the processes by which water and inorganic ions enter cells (3)
- Comparison: both move down concentration gradient
- Comparison: both move through (protein) channels in membrane
- Contrast: ions can move against a concentration gradient by active transport
Contrast the processes of facilitated diffusion and active transport (3)
- Facilitated diffusion involves channel or carrier proteins whereas active transport only involves carrier proteins
- Facilitated diffusion does not use ATP / is passive whereas active transport uses ATP
- Facilitated diffusion takes place down a concentration gradient whereas active transport can occur against a concentration gradient
Describe how the human immunodeficiency virus (HIV) is replicated once
inside helper T cells (TH cells) (4)
- RNA converted into DNA using reverse transcriptase
- DNA incorporated/inserted into (helper T cell)
- DNA transcribed into (HIV m)RNA
- (HIV mRNA) translated into (new) HIV/viral proteins (for assembly into viral particles)
Give one example of using monoclonal antibodies in a medical treatment (1)
Targets/binds/carries drug/medicine to specific cells/antigens/receptors
Accept cancer/diseased cells (as a specific cell). Ignore medical diagnosis/pregnancy/ PSA/ELISA test
Name the main biological molecule in W and X
- W – (cell surface) membrane
- X – cell wall
- Y – capsule
- Z – flagellum
Phospholipids, murein
- b ) A section/slice (so nucleus in another part of cell) or the nucleus is not stained
- (Nucleus) not stained
- c ) s - vacuole t - chloroplast
A transmission electron microscope was used to produce the image in the figure above.
Explain why (2)
- W – chloroplast, photosynthesis;
- Z – nucleus, contains DNA / chromosomes / holds genetic information of cell
- High resolution; can see internal structures
Some liver tissue was ground, filtered and centrifuged to make a suspension of organelle D.
(i) Explain why the solution in which the liver tissue was ground should be ice-cold (1)
Slows enzymes / prevents enzymes being denatured / prevents / stops self-digestion
The ground liver was centrifuged at low speed. The pellet that formed at the bottom of the centrifuge tube was thrown away and the supernatant centrifuged again at higher speed. Explain why it was necessary to first centrifuge the ground liver at low speed in order to obtain a suspension of organelle D (2)
- To remove organelle C / nuclei;
- Which are larger / more dense
Describe and explain how cell fractionation and ultracentrifugation can be used to isolate mitochondria from a suspension of animal cells (5)
- Cell homogenisation to break open cells
- Filter to remove (large) debris / whole cells
- Use isotonic solution to prevent damage to mitochondria / organelles (preventing cells bursting or shrinking)
- Keep cold to prevent / reduce damage by enzymes / use buffer to prevent protein / enzyme denaturation
- Centrifuge (at lower speed / 1000 g) to separate nuclei / cell fragments / heavy organelles
- Re-spin (supernatant / after nuclei / pellet removed) at higher speed to get mitochondria in pellet / at bottom
Describe the principles and the limitations of using a transmission electron microscope to investigate cell structure (5)
- Principles:
- Electrons pass through / enter (thin) specimen;
- Denser parts absorb more electrons;
- (So) denser parts appear darker;
- Electrons have short wavelength so give high resolution
- Limitations:
- Cannot look at living material / Must be in a vacuum;
- Specimen must be (very) thin
- Artefacts present
- Complex staining method / complex / long preparation time;
- Image not in 3D / only 2D images produced.
The student cut thin sections of tissue to view with an optical microscope. Explain why it was important that the sections were thin (2)
- To allow (more) light through;
- A single / few layer(s) of cells to be viewed
- More / faster mitosis / division near tip / at 0.2 mm
- (Almost) no mitosis / division at / after 1.6 mm from tip
- (So) roots grow by mitosis / adding new cells to the tip
The scientists measured cell damage by measuring the activity of lysosomes. Give one function of lysosomes (1)
Break down cells / cell parts / toxins
Name two structures in a eukaryotic cell that cannot be identified using an optical microscope (1)
Mitochondrion / ribosome / endoplasmic reticulum / lysosome / cell-surface membrane
- Push hard – spread / squash tissue;
- Not push sideways – avoid rolling cells together / breaking chromosomes;
- A - prophase; chromosomes thickening / condensing / becoming visible
- anaphase; chromatids / chromosomes moving to opposite poles /
ends of spindles
Describe two events during interphase which prepare a cell for mitosis (2)
DNA replication; synthesis or proteins / build-up of energy stores / growth / increase in cytoplasm; replication of organelles
Describe what happens in metaphase (2)
- Spindle formed / chromosome / centromere / chromatids attaches to spindle
- Chromosomes / chromatids line up / move to middle / equator
(of cell)
Describe what happens in anaphase (2)
- Chromosome / centromere splits / chromatids pulled apart
- To (opposite) sides / poles / centrioles (of cell)
Cells lining the human intestine complete the cell cycle in a short time.
Explain the advantage of these cells completing the cell cycle in a short time (1)
Form / replace cells quickly / rapidly / divide / multiply / replicate rapidly
Give two ways in which pathogens can cause disease (2)
- (Releases) toxins
- Kills cells / tissues
- Water potential in (bacterial) cells higher (than in honey) / water potential in honey lower (than in bacterial cells)
- Water leaves bacteria / cells by osmosis
- (Loss of water) stops (metabolic) reactions
- Antibody has specific tertiary structure / binding site / variable region
- Complementary (shape / fit) to receptor protein / GF / binds to receptor protein / to GF;
- Prevents GF binding (to receptor)
Phagocytes and lysosomes are involved in destroying microorganisms.
Describe how (3)
- Phagocytes engulf pathogens / microorganisms
- Enclosed in a vacuole / vesicle / phagosome
- Lysosomes have enzymes (fuse with phagosome)
- That digest / hydrolyse molecules / proteins / lipids / microorganism
Describe how B-lymphocytes respond when they are stimulated by antigens (4)
- divide by mitosis / form clones
- produce plasma cells
- (plasma cells) make antibodies
- (plasma cells) produce memory cells;
Scientists use this antibody to detect an antigen on the bacterium that causes stomach ulcers. Explain why the antibody will only detect this antigen (3)
- Antibody / variable region has specific amino acid sequence / primary structure
- The shape / tertiary structure of the binding site is complementary to / fits / binds with these antigens
- Forms complex between antigen and antibody
What is vaccination? (2)
- Injection of antigens
- (Antigen from) attenuated pathogens
- Stimulates the formation of memory cells
- Removes unbound 2nd antibodies;
- Otherwise enzyme may be present / may get colour change / prevents false positive
Explain why there will be no colour change if mumps antibodies are not
present in the blood (2)
- No antibodies to bind (to antigen)
- Therefore 2nd antibody (with the enzyme) won’t bind / no enzyme / enzyme-carrying antibody present (after washing in step 4)
Describe how antibodies are produced in the body following a viral infection (6)
- virus contains antigen;
- virus engulfed by phagocyte
- presents antigen to B-cell;
- memory cells / B-cell becomes activated;
- (divides to) form clones;
- by mitosis;
- plasma cells produce antibodies;
- antibodies specific to antigen;
- correct reference to T-cells / cytokines;
People with AIDS die because they are unable to produce an immune response to pathogens (lines 2-4). Explain why this leads to death (3)
- Infected by / susceptible to (other) pathogen(s) / named disease caused by a pathogen (from environment)
- Pathogen(s) reproduce / cause diease (in host)
- Damage cells / tissues / organs
- Release toxins
Explain why each of the following means that a vaccine might not be effective against HIV.
(i) HIV rapidly enters host cells (lines 6-7) (2)
(ii) HIV shows a lot of antigenic variability (2)
- (HIV enters cells) before antibodies can bind to / destroy it
- Antibodies cannot enter cells (to destroy HIV) / stay in blood
- (ii) Antigen (on HIV) changes
- (Specific) antibody / receptor no longer binds to (new) antigen
So far, these types of vaccine have not been considered safe to use in a mass vaccination programme (lines 14-15).
Suggest why they have not been considered safe (3)
- Inactive virus may become active / viral transformation
- Attenuated virus might become harmful
- Non-pathogenic virus may mutate and harm cells
- Genetic information / protein (from HIV) may harm cells
- People (may) become / test HIV positive after vaccine use
- (Complementary) nucleotides/bases pair
- A to T and C to G
- DNA polymerase
- Nucleotides join together (to form new strand)/phosphodiester bonds form
- (Fat substitute) is a different/wrong shape/not complementary
- Unable to fit/bind to (active site of) lipase/no ES complex formed
What is the evidence from Figure 2 that a scanning electron microscope was
used to take this photograph? (1)
3D image
Y is a protein. One function of Y is to transport cellulose molecules across the
phospholipid bilayer. Using information from Figure 3, describe the other function of Y (2)
- (Y is) an enzyme/has active site/forms ES complex
- That makes cellulose/attaches substrate to cellulose/joins β glucose
What is the evidence in Figure 3 that the phospholipid bilayer shown is part of the cell-surface membrane? (1)
Cell wall forms outside cell-surface membrane/has cellulose on it (on the outside);
Name the products of the hydrolysis of sucrose (2)
- Glucose
- Fructose
- Make/use maltose solutions of known/different concentrations
(and carry out quantitative Benedict’s test on each); - (Use colorimeter to) measure colour/colorimeter value of each solution and plot calibration curve/graph described
- Find concentration of sample from calibration curve
- t-test, because comparing two means
Describe how the structures of starch and cellulose molecules are related to their functions (5)
The student produced the sucrose solutions with different concentrations from a concentrated sucrose solution. Name the method she would have used to produce these sucrose solutions (1)
Dilution Series
- (If) too much water the concentration of pigment (in solution) will be lower / solution will appear lighter / more light passes through (than expected)
- So results (from different temperatures) are comparable
Describe a method the student could have used to monitor the temperature of the water in each tube (1)
(Take) readings (during the experiment) using a (digital) thermometer / temperature sensor
- Damage to (cell surface) membrane
- (membrane) proteins denature
- Increased fluidity / damage to the phospholipid bilayer
- Produce known concentrations of protein
- Measure absorbance of each concentration / Measure each concentration with colorimeter
- Plot a graph of absorbance on y-axis against concentration (on x-axis) and draw curve
- Use absorbance of sample to find protein concentration from curve
Older people are more likely to suffer from infectious diseases.
Suggest how this may be linked to the decrease in the mean concentration of
protein in the blood as people get older (1)
Fewer antibodies
Describe how the student could use an eyepiece graticule to determine the mean diameter of stomata (3)
- Measure (each stoma) using eyepiece graticule
- Calibrate eyepiece graticule against stage micrometer
- Take a number of measurements (to calculate a mean)
- named organelle e.g. nucleus / nuclear envelope
- ref to large(r) size
- (Many mitochondria) release energy / ATP for movement of vesicles / synthesis of protein / active transport
- (Many Golgi) vesicles transport protein / glycoprotein / milk to cell membrane / out of cell
The scientist decided to use the ratio of surface area to mass, rather than the ratio of surface area to volume. He made this decision for practical reasons.
Suggest one practical advantage of measuring the masses of frog eggs, tadpoles and adults, compared with measuring their volumes (1)
Easier / quicker (to find mass) because irregular shapes
- . Where dividing cells are found / mitosis occurs
- Single / thin layer of cells / spread out cells so light passes through (making cells / nuclei visible)
Describe and explain what the student should have done when counting cells to make sure that the mitotic index he obtained for this root tip was accurate (2)
- Examine large number of fields of view / many
- To ensure representative sample
- . Method to deal with part cells shown at edge /count only whole cells
- To standardise counting
- Stops anaphase / cell division / mitosis
- (By) stopping / disrupting / spindle fibres forming / attaching / pulling
- Preventing separation of (sister) chromatids
- (So) no new cells added (to root tip)
- D - Thylakoid
- E - Starch Grain
