Cell structure

• System and specialised cells

  Specialised cells → tissues that perform specific function → organs made of several tissue types → organ systems
• Structure and function of the cell-surface membrane

  1. ‘Fluid mosaic’ phospholipid bilayer with extrinsic & intrinsic proteins embedded

  2. Isolates cytoplasm from extracellular environment

  3. Selectively permeable to regulate transport of substances

  4. Involved in cell signalling / cell recognition
• Role of cholesterol, glycoproteins & glycolipids in the cell-surface membrane

  1. Cholesterol: steroid molecule connects phospholipids & reduces fluidity

  2. Glycoproteins: cell signalling, cell recognition (antigens) & binding cells together

  3. Glycolipids: cell signalling & cell recognition
• Structure of the nucleus

  1. Surrounded by nuclear envelope, a semi-permeable double membrane

  2. Nuclear pores allow substances to enter/exit

  3. Dense nucleolus made of RNA & proteins assembles ribosomes
• Function of the nucleus

  1. Contains DNA coiled around chromatin into chromosomes

  2. Controls cellular processes: gene expression determines specialisation & site of mRNA transcription, mitosis, semiconservative replication
• Structure of a mitochondrion

  1. Surrounded by double membrane folded inner membrane forms cristae: site of electron transport chain

  2. Fluid matrix: contains mitochondrial DNA, respiratory enzymes, lipids, proteins
• Structure of a chloroplast

  1. Vesicular plastid with double membrane

  2. Thylakoids: flattened discs stack to form grana; contain photosystems with chlorophyll

  3. Intergranal lamellae: tubes attach thylakoids in adjacent grana

  4. Stroma: fluid-filled matrix
• Function of mitochondria and chloroplasts

  1. Mitochondria: site of aerobic respiration to produce ATP

  2. Chloroplasts: site of photosynthesis to convert solar energy to chemical energy
• Structure and function of the Golgi apparatus

  1. Planar stack of membrane-bound, flattened sacs

  2. Cis face aligns with rER

  3. Molecules are processed in cisternae

  4. Vesicles bud off trans face via exocytosis: modifies & packages proteins for export, synthesises glycoproteins
• Structure and function of a lysosome

  1. Sac surrounded by single membrane embedded H+ pump maintains acidic conditions

  2. Contains digestive hydrolase enzymes

  3. Glycoprotein coat protects cell interior: digests contents of phagosome, exocytosis of digestive enzymes
• Structure and function of a ribosome

  1. Formed of protein & rRNA

  2. Free in cytoplasm or attached to ER

  3. Site of protein synthesis via translation: large subunit joins amino acids, small subunit contains mRNA binding site
• Structure and function of the endoplasmic reticulum (ER)

  1. Cisternae: network of tubules & flattened sacs extends from cell membrane through cytoplasm & connects to nuclear envelope

  2. Rough ER: many ribosomes attached for protein synthesis & transport

  3. Smooth ER: lipid synthesis
• Structure of the cell wall

  1. Bacteria: Made of the polysaccharide murein

  2. Plants: Made of cellulose microfibrils, plasmodesmata
• State the functions of the cell wall

  1. Mechanical strength and support.

  2. Physical barrier against pathogens.

  3. 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

  1. Folded membrane or microvilli increase surface area e.g. for diffusion.

  2. Many mitochondria = large amounts of ATP for active transport.

  3. 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)
• 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
  • Cellulose cell walls
  • No capsule, no plasmids, always cytoskeleton
• Describe the structure of a viral particle

  1. Linear genetic material (DNA or RNA) & viral enzymes e.g. reverse transcriptase

  2. Surrounded by capsid (protein coat made of capsomeres)

  3. No cytoplasm
• Describe the structure of an enveloped virus

  1. Simple virus surrounded by matrix protein

  2. Matrix protein surrounded by envelope derived from cell membrane of host cell

  3. Attachment proteins on surface
• 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
• How to 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
• 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
• 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
• 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
• actual size = image size / magnification
• 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
• Order of sedimentation of organelles during differential centrifugation

  • Most dense → least dense: nucleus → mitochondria → lysosomes → RER → plasma membrane → SER → ribosomes
• Reasons for keeping fractionated cells in a cold, buffered, isotonic solution

  • Cold: slow action of hydrolase enzymes
  • Buffered: maintain constant pH
  • Isotonic: prevent osmotic lysis/ shrinking of organelles