Cell structure and division (Topic 2A)

• Define the terms eukaryotic and

  prokaryotic cell.


  Eukaryotic: DNA is contained in a

  nucleus, contains membrane-bound

  specialised organelles.

  Prokaryotic: DNA is ‘free’ in cytoplasm,

  no organelles e.g. bacteria & archaea
• State the relationship between a system

  and specialised cells.


  ~Specialised cells → tissues that perform

  specific function → organs made of

  several tissue types → organ systems
• Describe the structure and function of

  the cell-surface membrane.


  Fluid mosaic’ phospholipid bilayer with extrinsic &

  intrinsic proteins embedded

  ● Isolates cytoplasm from extracellular environment.

  ● Selectively permeable to regulate transport of

  substances.

  ● Involved in cell signalling / cell recognition
• Explain the role of cholesterol,

  glycoproteins & glycolipids in the cell￾surface membrane


  Cholesterol: steroid molecule connects

  phospholipids & reduces fluidity.

  Glycoproteins: cell signalling, cell recognition

  (antigens) & binding cells together.

  Glycolipids: cell signalling & cell recognition.
• Describe the structure of the nucleus.


  ● Surrounded by nuclear envelope, a

  semi-permeable double membrane.

  ● Nuclear pores allow substances to

  enter/exit.

  ● Dense nucleolus made of RNA & proteins

  assembles ribosomes.
• Describe the function of the nucleus


  ● Contains DNA coiled around chromatin into

  chromosomes.

  ● Controls cellular processes: gene

  expression determines specialisation & site

  of mRNA transcription, mitosis,

  semiconservative replication
• Describe the structure of a

  mitochondrion


  ● Surrounded by double membrane folded

  inner membrane forms cristae: site of

  electron transport chain

  ● Fluid matrix: contains mitochondrial DNA,

  respiratory enzymes, lipids, proteins.
• Describe the structure of a chloroplast


  ● Vesicular plastid with double membrane.

  ● Thylakoids: flattened discs stack to form

  grana; contain photosystems with chlorophyll.

  ● Intergranal lamellae: tubes attach thylakoids

  in adjacent grana.

  ● Stroma: fluid-filled matrix.
• State the function of mitochondria and

  chloroplasts


  ● Mitochondria: site of aerobic

  respiration to produce ATP.

  ● Chloroplasts: site of photosynthesis

  to convert solar energy to chemical

  energy
• Describe the structure and function of

  the Golgi apparatus.


  Planar stack of membrane-bound, flattened sacs

  cis face aligns with rER.

  Molecules are processed in cisternae

  vesicles bud off trans face via exocytosis:

  ● modifies & packages proteins for export

  ● synthesises glycoproteins
• Describe the structure and function of a

  lysosome


  Sac surrounded by single membrane embedded H+

  pump maintains acidic conditions

  contains digestive hydrolase enzymes

  glycoprotein coat protects cell interior:

  ● digests contents of phagosome

  ● exocytosis of digestive enzymes
• Describe the structure and function of a ribosome.

  Formed of protein & rRNA

  free in cytoplasm or attached to ER.

  ● Site of protein synthesis via translation:

  large subunit: joins amino acids

  small subunit: contains mRNA binding site
• Describe the structure and function of

  the endoplasmic reticulum (ER)


  Cisternae: network of tubules & flattened sacs

  extends from cell membrane through cytoplasm &

  connects to nuclear envelope:

  ● Rough ER: many ribosomes attached for protein

  synthesis & transport.

  ● Smooth ER: lipid synthesis
• ..Describe the structure of the cell wall

  ● Bacteria:

  Made of the polysaccharide murein.

  ● Plants:

  Made of cellulose microfibrils

  plasmodesmata allow molecules to pass between

  cells, middle lamella acts as boundary between

  adjacent cell walls.
• State the functions of the cell wall.


  ● Mechanical strength and support.

  ● Physical barrier against pathogens.

  ● Part of apoplast pathway (plants) to

  enable easy diffusion of water.
• Describe the structure and function of

  the cell vacuole in plants.


  Surrounded by single membrane: tonoplast

  contains cell sap: mineral ions, water, enzymes,

  soluble pigments.

  ● Controls turgor pressure.

  ● Absorbs and hydrolyses potentially harmful

  substances to detoxify cytoplasm
• Explain some common cell adaptations.

  ● Folded membrane or microvilli increase

  surface area e.g. for diffusion.

  ● Many mitochondria = large amounts of ATP for

  active transport.

  ● Walls one cell thick to reduce distance of

  diffusion pathway.
• State the role of plasmids in prokaryotes.


  .● Small ring of DNA that carries

  non-essential genes.

  ● Can be exchanged between bacterial

  cells via conjugation.
• State the role of flagella in prokaryotes


  Rotating tail propels (usually unicellular)

  organism.
• -State the role of the capsule in

  prokaryotes


  polysaccharide layer:

  ● Prevents desiccation.

  ● Acts as food reserve.

  ● Provides mechanical protection against

  phagocytosis & external chemicals.

  ● Sticks cells together
• Compare eukaryotic and prokaryotic

  cells

  both have:

  ● Cell membrane.

  ● Cytoplasm.

  ● Ribosomes (don’t count as an

  organelle since not membrane-bound)
• Contrast eukaryotic and prokaryotic

  cells.

  O

  
• Why are viruses referred to as ‘particles’

  instead of cells?

  Acellular & non-living: no cytoplasm,

  cannot self-reproduce, no metabolism.
• ,Describe the structure of a viral particle

  ● Linear genetic material (DNA or RNA) &

  viral enzymes e.g. reverse transcriptase.

  ● Surrounded by capsid (protein coat

  made of capsomeres).

  ● No cytoplasm.
• Describe the structure of an enveloped

  virus.


  ● Simple virus surrounded by matrix

  protein.

  ● Matrix protein surrounded by envelope

  derived from cell membrane of host cell.

  ● Attachment proteins on surface
• State the role of the capsid on viral

  particles

  ● Protect nucleic acid from degradation

  by restriction endonucleases.

  ● Surface sites enable viral particle to

  bind to & enter host cells or inject their

  genetic material.
• state the role of attachment proteins on

  viral particles


  Enable viral particle to bind to

  complementary sites on host cell : entry

  via endosymbiosis.
• Describe how optical microscopes work


  1. Lenses focus rays of light and magnify the

  view of a thin slice of specimen.

  2. Different structures absorb different amounts

  and wavelengths of light.

  3. Reflected light is transmitted to the observer

  via the objective lens and eyepiece.
• Outline how a student could prepare a

  temporary mount of tissue for an optical

  microscope.

  1. Obtain thin section of tissue e.g. using ultratome or

  by maceration.

  2. Place plant tissue in a drop of water.

  3. Stain tissue on a slide to make structures visible.

  4. Add coverslip using mounted needle at 45° to

  avoid trapping air bubbles
• Suggest the advantages and limitations

  of using an optical microscope.


  + colour image

  + can show living structures

  + affordable apparatus

  • 2D image
  • lower resolution than electron microscopes =

  cannot see ultrastructure
• Describe how a transmission electron

  microscope (TEM) works


  '1. Pass a high energy beam of electrons through

  thin slice of specimen.

  2. More dense structures appear darker since they

  absorb more electrons.

  3. Focus image onto fluorescent screen or

  photographic plate using magnetic lenses.
• Suggest the advantages and limitations

  of using a TEM


  + electrons have shorter wavelength than light = high resolution, so ultrastructure visible + high magnification (x 500000) - 2D image - requires a vacuum = cannot show living structures - extensive preparation may introduce artefacts - no colour image
• Describe how a scanning electron

  microscope (SEM) works

  1. Focus a beam of electrons onto a specimen’s

  surface using electromagnetic lenses.

  2. Reflected electrons hit a collecting device and

  are amplified to produce an image on a

  photographic plate
• Suggest the advantages and limitations

  of using an SEM.


  + 3D image

  + electrons have shorter wavelength than light = high

  resolution

  • requires a vacuum = cannot show living structures
  • no colour image
  • only shows outer surface
• .Define magnification and resolution


  Magnification: factor by which the

  image is larger than the actual specimen.

  Resolution: smallest separation

  distance at which 2 separate structures

  can be distinguished from one another
• Explain how to use an eyepiece graticule

  and stage micrometer to measure the

  size of a structure

  1. Place micrometer on stage to calibrate eyepiece graticule. /

  2. Line up scales on graticule and micrometer. Count how

  many graticule divisions are in 100μm on the micrometer.

  3. Length of 1 eyepiece division = 100μm / number of

  divisions

  4. Use calibrated values to calculate actual length of

  structures.
• State an equation to calculate the actual

  size of a structure from microscopy


  actual size = image size ÷ magnification
• Outline what happens during cell

  fractionation and ultracentrifugation


  1. Mince and homogenize tissue to break open cells &

  release organelles.

  2. Filter homogenate to remove debris.

  3. Perform differential centrifugation:

  1. Spin homogenate in centrifuge.

  b) The most dense organelles in the mixture form a pellet.

  c) Filter off the supernatant and spin again at a higher speed
• State the order of sedimentation of

  organelles during differential

  centrifugation.


  most dense → least dense

  nucleus → mitochondria → lysosomes →

  RER → plasma membrane → SER →

  ribosomes
• Explain why fractionated cells are kept in

  a cold, buffered, isotonic solution


  cold: slow action of hydrolase enzymes.

  buffered: maintain constant pH.

  isotonic: prevent osmotic lysis/ shrinking

  of organelles