Cell structure

• 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
• Describe the structure and function of the cell-surface membrane

  ~Fuid mosaic phospholipid bilayer with extrinsic and 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 and glycolipids in the cell-surface membrane

  Cholesterol: steroid molecule connects phospholipids and reduces fluidity

  Glycoproteins: Cell signalling, cell recognition and binding cells together

  Glycolipids: cell signalling and 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 and proteins assembles ribosomes
• Describe the function of the nucleus

  ~Contains DNA coiled around chromatin into chromosomes

  ~Controls cellular processes: gene expression determines specialisation and 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 space: tubes attach to 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 and 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

  ~Contain 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 and 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 extend from cell membrane through cytoplasm and connects to nuclear envelope

  ~Rough ER: many ribosomes attached for protein synthesis and 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 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 of microvilli increase surface area

  ~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 organism
• State the role of the capsule in prokaryotes

  ~Prevents desiccation

  ~Acts as food reserve

  ~Provides mechanical protection against phagocytosis and external chemicals

  ~Sticks cells together
• Compare eukaryotic and prokaryotic cells

  Both have

  ~Cell membrane

  ~Cytoplasm

  ~Ribosomes
• Why are viruses referred to as particles instead of cells

  ~Acellular and non living

  ~No cytoplasm

  ~Cannot self-reproduce

  ~No metabolism
• Describe the structure of a viral particle

  ~Linear genetic material and viral enzymes

  ~Surrounded by capsid

  ~No cytoplasm
• Describe the structure of an enveloped virus

  ~Simple virus surrounded by matrix protein

  ~Matrix protein surrounded by envelope derived from cell membrane 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 and enter host cells or inject their genetic material
• State the role of attachment proteins on viral particles

  ~Enables viral particle to bind to complementary sites on host cell

  ~Entry via endosymbiosis
• Describe how optical microscopes work

  ~Lenses focus rays of light and magnify the view of a thin slice of specimen

  ~Different structures absorb different amounts and wavelengths of light

  ~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

  ~Obtain thin section of tissue

  ~Place plant tissue in a drop of water

  ~Stain tissue on a slide to make structures visible

  ~Add coverslip using mounted needle at 45°c to avoid trapping air bubbles
• Suggest the advantages and limitations of using an optical microscope

  Advantages

  ~Colour image

  ~Can show living structures

  ~Affordable apparatus

  Limitations

  ~2D image

  ~Lower resolution than electron microscopes
• Describe how a transmission electron microscope (TEM) works

  ~Pass a high energy beam of electrons through thin slice of specimen

  ~More dense structures appear darker since they absorb more electrons

  ~Focus image onto fluorescent screen or photographic plate using magnetic lenses
• Suggest the advantages and limitations of using a TEM

  Advantages

  ~Electrons have shorter wavelength than light = high resolution

  ~High magnification

  Limitations

  ~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

  ~Focus a beam of electrons onto a specimen's surface using electromagnetic lenses

  ~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

  Advantages

  ~3D image

  ~Electrons have shorter wavelength than light = high resolution

  Limitations

  ~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 two 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

  ~Place micrometer on stage to calibrate eyepiece graticule

  ~Line up scales on graticule and micrometer. Count how many graticule divisions are in 100μm on the micrometer

  ~Length of one eyepiece division = 100μm/number of divisions

  ~Use calibrated values to calculate actual length of structures
• Outline what happens during cell fractionation and ultracentrifugation


  ~Mince and homogenise tissue to break open cells and release organelles

  ~Filter homogenate to remove debris

  ~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
• 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