Cells

• What are the distinguishing features of eukaryotic cells?


  ● Cytoplasm containing membrane-bound organelles

  ● So DNA enclosed in a nucleus
• 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 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 function of rER


  ● Ribosomes on surface synthesise proteins

  ● Proteins processed / folded / transported inside rER

  ● Proteins packaged into vesicles for transport eg. to Golgi apparatus
• Describe the function of sER


  ● Synthesises and processes lipids

  ● Eg. cholesterol and steroid hormones
• Describe the function of Golgi apparatus


  ● Modifies protein, eg. adds carbohydrates to produce glycoproteins

  ● Modifies lipids, eg. adds carbohydrates to make glycolipids

  ● Packages proteins / lipids into Golgi vesicles

  ● Produces lysosomes (a type of Golgi vesicle)
• Describe the function Golgi vesicles


  ● Transports proteins / lipids to their required destination

  ● Eg. moves to and fuses with cell-surface membrane
• Describe the function of lysosomes


  ● Release hydrolytic enzymes (lysozymes)

  ● To break down / hydrolyse pathogens or worn-out cell components
• Describe the function of mitochondria


  ● Site of aerobic respiration

  ● To produce ATP for energy release

  ● Eg. for protein synthesis / vesicle movement / active transport
• Describe the function of chloroplasts in plants and algae


  ● Absorbs light energy for photosynthesis

  ● To produce organic substances eg. carbohydrates / lipids
• 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 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
• Describe how eukaryotic cells are organised in complex multicellular organisms


  Tissue - Group of specialised cells with a similar structure working together to perform a specific function, often with the same origin.

  Organ - Aggregations of tissues performing specific functions.

  Organ system - Group of organs working together to perform specific functions.
• What are the distinguishing features of prokaryotic cells?


  ● Cytoplasm lacking membrane-bound organelles

  ● So genetic material not enclosed in a nucleus

  -Examples of prokaryotic organisms: bacteria and archaea (always unicellular)
• 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


  1. Nucleic acids surrounded by a capsid (protein coat)

  2. Attachment proteins allow attachment to specific host cells

  3. No cytoplasm, ribosomes, cell wall, cell-surface membrane etc.

  4. 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
• Describe how the size of an object viewed with an optical microscope can be measured


  1. 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
• Describe and explain the principles of cell fractionation and ultracentrifugation as used to separate cell components


  1. Homogenise tissue / use a blender

  ● Disrupts cell membrane, breaking open cells and releasing contents / organelles

  2. Place in a cold, isotonic, buffered solution

  ● Cold to reduce enzyme activity → so organelles not broken down / damaged

  ● Isotonic so water doesn't move in or out of organelles by osmosis → so they don't burst

  ● Buffered to keep pH constant → so enzymes don't denature

  3. Filter homogenate

  ● Remove large, unwanted debris eg. whole cells, connective tissue
• 4. Ultracentrifugation


  - separates organelles in order of density / mass

  ● Centrifuge homogenate in a tube at a high speed

  ● Remove pellet of heaviest organelle and respin supernatant at a higher speed

  ● Repeat at increasing speeds until separated out, each time pellet made of lighter organelles (nuclei → chloroplasts / mitochondria → lysosomes → ER → ribosomes)
• Describe the stages of the cell cycle in eukaryotic cells


  1. Interphase

  ● (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

  2. Mitosis

  ● Nucleus divides

  ● To produce 2 nuclei with identical copies of DNA produced by parent cell

  3. Cytokinesis

  ● Cytoplasm and cell membrane (normally) divide

  ● To form 2 new genetically identical daughter cells
• Describe the behaviour of chromosomes & role of spindle fibres in mitosis


  Stage 1 - Prophase

  ● 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

  Stage 2 - Metaphase

  ● Spindle fibres attach to chromosomes by their centromeres

  ● Chromosomes align along equator

  Stage 3 - Anaphase

  ● Spindle fibres shorten / contract

  ● Centromere divides

  ● Pulling chromatids (from each pair) to opposite poles of cell

  Stage 4 - Telophase

  ● 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 for...

  ● 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 mitosis
• Describe how prokaryotic cells replicate


  1. Replication of circular DNA

  2. Replication of plasmids

  3. Division of cytoplasm to produce 2 daughter cells

  ● Single copy of circular DNA

  ● Variable number of copies of plasmids
• Describe how viruses replicate


  1. Attachment proteins attach to complementary receptors on host cell

  2. Inject viral nucleic acid (DNA/RNA) into host cell

  3. Infected host cell replicates virus particles:

  a. Nucleic acid replicated

  b. Cell produces viral protein / capsid / enzymes

  c. Virus assembled then released
• RP2

  Preparation of stained squashes of cells from plant root tips; set-up and use of an optical microscope to identify the stages of mitosis in these stained squashes and calculation of a mitotic index.

  Formula: actual size = size of image / magnification
• Describe how to prepare squashes of cells from plant root tips


  1. Cut a thin slice of root tip (5mm from end) using scalpel and mount onto a slide

  2. Soak root tip in hydrochloric acid then rinse

  3. Stain for DNA eg. with toluidine blue

  4. Lower coverslip using a mounted needle at 45 o without trapping air bubbles

  5. Squash by firmly pressing down on glass slip but do not push sideways
• Common questions


  1. Why are root tips used?

  ● Where dividing cells are found / mitosis occurs

  2. Why is a stain used?

  ● To distinguish chromosomes

  ● Chromosomes not visible without stain

  3. Why squash / press down on cover slip?

  ● (Spreads out cells) to create a single layer of cells

  ● So light passes through to make chromosomes visible

  4. Why not push cover slip sideways?

  ● Avoid rolling cells together / breaking chromosomes

  5. Why soak roots in acid?

  ● Separate cells / cell walls

  ● To allow stain to diffuse into cells

  ● To allow cells to be more easily squashed

  ● To stop mitosis
• Describe how to set-up and use an optical microscope


  1. Clip slide onto stage and turn on light 2. Select lowest power objective lens (usually x 4)

  3 a. Use coarse focusing dial to move stage close to lens

  b. Turn coarse focusing dial to move stage away from lens until image comes into focus

  4. Adjust fine focusing dial to get clear image

  5. Swap to higher power objective lens, then refocus
• What are the rules of scientific drawing?


  ✓ Look similar to specimen / image

  ✓ No sketching / shading - only clear, continuous lines

  ✓ Include a magnification scale (eg. x 400)

  ✓ Label with straight, uncrossed lines
• Explain how the stages of mitosis can be identified


  Prophase -

  ● Chromosomes visible / distinct → because condensing

  ● But randomly arranged → because no spindle activity / not attached to spindle fibre

  Metaphase -

  ● Chromosomes lined up on equator → because attaching to spindle

  Anaphase -

  ● Chromatids (in two groups) at poles of spindle

  ● Chromatids V shaped → because being pulled apart at their centromeres by spindle fibres

  Telophase -

  ● Chromosomes in two sets, one at each pole
• What is a mitotic index?


  ● Proportion of cells undergoing mitosis (with visible chromosomes)

  ● Mitotic index = number of cells undergoing mitosis / total number of cells in sample
• Explain how to determine a reliable MI from observed squashes


  ● Count cells in mitosis in field of view

  ● Count only whole cells / only cells on top and right edges → standardise counting

  ● Divide this by total number of cells in field of view

  ● Repeat with many / at least 5 fields of view selected randomly → representative sample

  ● Calculate a reliable mean
• Suggest how to calculate the time cells are in a certain phase of mitosis


  1. Identify proportion of cells in named phase at any one time

  ● Number of cells in that phase / total number of cells observed

  2. Multiply by length of cell cycle