Cells

• Eukaryotic cell


  DNA is contained in a nucleus, contains membrane-bound specialised organelles
• Prokaryotic cell


  DNA is 'free' in cytoplasm, no organelles e.g. bacteria & archaea
• Relationship between a system and specialised cells

  Specialised cells → tissues that perform specific function → organs made of several tissue types → organ systems
• 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
• Cholesterol


  Steroid molecule connects phospholipids & reduces fluidity
• Glycoproteins


  Cell signalling, cell recognition (antigens) & binding cells together
• Glycolipids


  Cell signalling & cell recognition
• 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
• 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
• Mitochondrion


  • Surrounded by double membrane folded inner membrane forms cristae: site of electron transport chain
  • Fluid matrix: contains mitochondrial DNA, respiratory enzymes, lipids, proteins
• 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
• Function of mitochondria


  Site of aerobic respiration to produce ATP
• Function of chloroplasts


  Site of photosynthesis to convert solar energy to chemical energy
• 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
• 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
• 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
• 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
• Cell wall (bacteria)


  • Made of the polysaccharide murein
• Cell wall (plants)


  • Made of cellulose microfibrils
  • Plasmodesmata allow molecules to pass between cells
  • Middle lamella acts as boundary between adjacent cell walls
• Functions of the cell wall


  • Mechanical strength and support
  • Physical barrier against pathogens
  • Part of apoplast pathway (plants) to enable easy diffusion of water
• 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
• 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
• Role of plasmids in prokaryotes


  • Small ring of DNA that carries non-essential genes
  • Can be exchanged between bacterial cells via conjugation
• Role of flagella in prokaryotes

  Rotating tail propels (usually unicellular) organism
• Role of the capsule in prokaryotes


  Polysaccharide layer: Prevents desiccation, Acts as food reserve, Provides mechanical protection against phagocytosis & external chemicals, Sticks cells together
• Similarities between eukaryotic and prokaryotic cells


  • Cell membrane
  • Cytoplasm
  • Ribosomes (don't count as an organelle since not membrane-bound)
• Differences between eukaryotic and prokaryotic cells


  • 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
  • Eukaryotic: larger cells & often multicellular, 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
• Viruses are referred to as 'particles' instead of cells because they lack the basic characteristics of cells, such as a cell membrane, cytoplasm, and the ability to carry out metabolic processes independently
• Prokaryotic cells


  • Small cells & always unicellular
  • No membrane-bound organelles & no nucleus
  • Circular DNA not associated with proteins
  • Small ribosomes (70S)
  • Binary fission - always asexual reproduction
  • Cellulose cell wall (plants)/ chitin (fungi)
  • Capsule, sometimes plasmids & cytoskeleton
• Eukaryotic cells


  • Larger cells & often multicellular
  • Always have organelles & nucleus
  • Linear chromosomes associated with histones
  • Larger ribosomes (80S)
  • Mitosis & meiosis - sexual and/or asexual
  • No cellulose cell wall, murein cell walls
  • No capsule, no plasmids, always cytoskeleton
• Viruses are referred to as 'particles' instead of cells because they are acellular & non-living: no cytoplasm, cannot self-reproduce, no metabolism
• Viral particle


  • Linear genetic material (DNA or RNA) & viral enzymes e.g. reverse transcriptase
  • Surrounded by capsid (protein coat made of capsomeres)
  • No cytoplasm
• Enveloped virus


  • Simple virus surrounded by matrix protein
  • Matrix protein surrounded by envelope derived from cell membrane of host cell
  • Attachment proteins on surface
• Capsid


  • Protects nucleic acid from degradation by restriction endonucleases
  • Surface sites enable viral particle to bind to & enter host cells or inject their genetic material
• Attachment proteins


  Enable viral particle to bind to complementary sites on host cell : entry via endosymbiosis
• 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
• Preparing 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
• Optical microscope


  • Colour image
  • Can show living structures
  • Affordable apparatus
• Optical microscope limitations


  • 2D image
  • Lower resolution than electron microscopes = cannot see ultrastructure
• 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