Cell structure

• Define the terms eukaryotic and prokaryotic cells.

  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 a specific function -> organs made of several tissue types -> organ system
• Describe the structure and function of 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 function of the nucleus.

  • Contains DNA coiled around chromatin into chromosomes
  • Controls cellular processes: gene expression determines specialisation & site of mRNA transcription, mitosis, semi-conservative 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 the nucleus.

  • Surrounded by semipermeable double membrane.
  • Nuclear pores allow substances to enter/exit.
  • Dense nucleolus made of RNA & proteins assemble ribosomes

  
• Describe the structure of chloroplast.

  • Vesicular plastid with double membrane
  • Thylakoids: flattened discs stack to form grana; contain photo systems with chlorophyll.
  • Intergranal lamallae: tubes attach thylakoids in adjacent grana.
  • Stroma: fluid-filled matrix.

  
• State the function of mitochondria and chloroplasts.

  • Mitochondria: site if 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 rear.

  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:

  • Digestive contents of phagosome
  • Exocytosis of digestive enzymes

  
• Describe the structure a d 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 and 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 a 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 Tali propels (usually unicellular) organism.
• State the role of the capsule in prokaryotes.

  Polysaccharide layer:

  • Prevents desiccation
  • Acts as a food reserve
  • Provides mechanical protection against phagocytosis & external chemicals.
  • Sticks cells together.
• Compare eukaryotic and prokaryotic cells.

  Both have:

  • Cell membrane
  • Cytoplasm
  • Ribosomes (doesn't count as an organelle since not membrane-bound).
• Contrast eukaryotic and prokaryotic cells

  Eukaryotic:

  • Larger cells & often multi-cellular
  • Always have organelles & nucleus
  • Linear chromosomes associated with histones
  • Larger ribosomes (80S)
  • Mitosis & Meiosis - sexual and/or asexual
  • Cellulose cell wall (plants)/ chitin (fungi)
  • No capsule, no plasmids, always cytoskeleton

  Prokaryotic

  • Small cells & always unicellular
  • No membrane-bound organelles & no nucleus
  • Circular DNA not associated with proteins
  • Small ribosomes (70S)
  • Binary fission - always asexual reproduction
  • Murein cell walls
  • Capsule, sometimes plasmids & cytoskeleton
• 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 acids 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 tissue e.g. using ultratome or maceration.
  2. Place plant tissue in a drip 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 structure appear darker since they absorb more electrons.
  3. Focus image onto fluorescent screen or photographic plate using magnetic lenses
• Suggest the advantages and disadvantages 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 specimens surface using electromagnetic lenses.
  2. Reflected electrons hit a collecting device and amplified to produce an image on a photographic plate.
• Suggest the advantages and disadvantages 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 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:

  • Spin homogenate in centrifuge
  • The most dense organelles in the mixture form a pellet.
  • 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.