Cell structure

• important properties of water?

  • its a polar molecule
  • its a metabolite
  • it’s a solvent
  • it has a high specific heat capacity
  • large latent heat of evaporation
  • It has strong cohesion between water particles
• Why is water polar and what is its importance?

  Due to the uneven distribution of charge within the molecule

  important because - this results in hydrogen bonding between oxygen and hydrogen atoms causing the particles to stick together
• Why is water being a metabolite important?

  As it’s used in chemical reactions such as hydrolysis and condensation reactions
• Why is water being a solvent important?

  • as it allows gasses to readily diffuse aswell as enzymes and waste products such as urea
• Why is water having a high specific heat capacity important?

  as it minimises temperature fluctuations and acts as a buffer
• Importance of large latent Heat of evaporation?

  this means that when water evaporates it provides a cooling effect (sweating) with little water loss
• Eukaryotic cell

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

  DNA is 'free' in cytoplasm, no membrane bound 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 function?

  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:
  • 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
• 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
• 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
• Plasmids in prokaryotes

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


  Rotating tail propels (usually unicellular) organism
• 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.
• Viruses

  • Acellular & non-living
  • No cytoplasm
  • Cannot self-reproduce
  • No metabolism
• 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
• 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
• Capsid

  • Protect 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