cells

• Cell surface membrane

  • made mainly of lipids and proteins
  • regulates the movement of substances into and out of the cell
  • also has receptor molecules on it which allow it to respond to chemicals like hormones
• Nucleus

  • has a double membrane as has a nuclear envelope and contains pores
  • also contains chromosomes and one or more nucleolus
  • controls the cells activities
  • DNA contains instructions to make proteins
  • the pores allow substances to move between the nucleus and cytoplasm
  • the nucleolus makes ribosomes
• Mitochondrion

  • have a double membrane and are oval shaped
  • inner folded to form cristae and inside is matrix contained enzymes for respiration
  • site of aerobic respiration which produces ATP
  • found in large numbers in cells that are active and require lots of energy
• Chloroplast

  • found in plant and algal cells
  • has a double membrane and a small flattened structure
  • thylakoid membranes stacked to form grana which are linked by lamellae surrounded by a fluid called stroma
  • site of photosynthesis
• Golgi apparatus

  • a group of fluid-filled membrane-bound flattened sacs
  • produces lysosomes and vesicles
  • processes and packages new lipids and proteins
• Golgi vesicle

  • small fluid-filled sac surrounded by a membrane
  • stores lipids and proteins made by the Golgi apparatus and transports them out of the cell
• Lysosome

  • round organelle surrounded by a membrane
  • contains digestive enzymes called lysozymes
  • these can digest invading cells or break down worn out components of the cell
• Ribosome

  • very small organelle made up of proteins and RNA
  • either floats free in cytoplasm or attached to RER
  • site of protein synthesis
  • EC= 70s in mitochondria and chloroplasts and 80s in cytoplasm
  • PC= 70s in cytoplasm
• Rough endoplasmic reticulum

  • system of membranes enclosing a fluid-filled space
  • surface is covered in ribosomes
  • folds and processes proteins made at the ribosomes
• Smooth endoplasmic reticulum

  • similar to RER but with no ribosomes
  • synthesises and processes lipids
• Cell wall

  • plants and algae made of cellulose and fungi made of chitin
  • supports cells and prevents them from changing shape
• Cell vacuole

  • membrane-bound organelle found in plant cells and has a surrounding membrane called a tonoplast
  • contains cell sap= a weak solution of sugar and salts
  • helps to maintain pressure and keeps the cell rigid
• Prokaryotic cells

  • cytoplasm with 70s ribosomes, cell-surface membrane [same as EC] and murein cell wall
  • slime capsule: only found in some and helps to protect the bacteria from attack by immune cells
  • plasmids: small loops of DNA that can be passed between prokaryotes and found in variable numbers
  • circular DNA: no nucleus so floats freely in cytoplasm as one long coiled-up strand
  • flagellum: long hair like structure that rotates to make the cell move and found in variable numbers
• Binary fission

  • DNA loop replicated once and plasmid replicates many times
  • cell grows and DNA loops move to opposite poles
  • cytoplasm divides and two daughter cells are produced
  • each has one DNA loop and a variable number of plasmids
• Viruses

  • are acellular meaning unalive
  • contain a protein coat called a capsid with attachment proteins which allow the virus to attach to a host cell
  • inside contain a core of genetic material
• Viral replication

  1. virus attaches to host cell receptor proteins
  2. genetic material is released into host cell
  3. genetic material and proteins are replicated by host cell 'machinery'
  4. viral components assemble
  5. replicated viruses released from host cell
• Magnification


  how much bigger the image is than the specimen
• Resolution


  how well a microscope distinguishes between two points that are close together
• Optical microscopes

  • use light to form an image
  • cannot view ribosomes, endoplasmic reticulum and lysosomes
  • low resolution and magnification= light has long wavelengths
• Electron microscopes

  • use electrons to form an image
  • high resolution and magnification= electron has short wavelengths
  • can view many organelles
• Transmission electron microscopes

  • use electromagnetics to focus a beam of electrons to be transmitted through the specimen
  • denser parts absorb more electrons making them appear darker
  • high resolution so can see internal structures of organelles
  • specimen has to be in a vacuum meaning not living
  • can only be used on thin specimens
• Scanning electron microscope

  • scan a beam of electrons across the specimen which knocks off electrons from it that are used to form an image
  • show the surface of the specimen and can be shown in 3D
  • can be used on thick specimens
  • give a lower resolution than TEMs
  • specimen must be in a vacuum meaning non-living
• Temporary mount

  1. pipette a small drop of water onto the centre of slide
  2. place a thin section of specimen on top of water drop
  3. add drop of stain [eosin for cytoplasm and iodine for starch]
  4. add cover slip slowly and at an angle
• Artefacts


  things that you can see down the microscope that are not part of the cell or specimen eg air bubbles, fingerprints and dust
• Cell fractionation

  • Homogenisation: physically breaking up the cells
  • kept in an isotonic, cold and buffered solution
  • filtered through a gauze to remove any large debris
  • ultracentrifugation: spun at low speed in a centrifuge and heaviest organelles form at the bottom called pellet and lightest at the top called supernatant
  • process repeats at higher and higher speeds until all organelles are separated out
  • order of mass: nuclei, chloroplasts, mitochondria, lysosomes, endoplasmic reticulum and ribosomes
• Transport across cell membranes

  Cell membrane structure
• Phospholipids


  Form a bilayer with hydrophilic heads and hydrophobic tails, allowing lipid-soluble molecules to pass through
• Fluid-mosaic model


  Flexible model of cell membrane structure
• Proteins in cell membrane

  • Span across the membrane
  • Part of the membrane
  • Provide mechanical support
  • Form channels
  • Act as carrier proteins
  • Function as receptors
  • Embedded throughout the fluid-mosaic model
• Cholesterol


  Adds strength and rigidity to the membrane, restricts movement of other membrane molecules
• Glycoproteins and glycolipids


  Help stabilise the membrane, act as receptors, important for cell attachment and recognition
• Types of transport

  • Simple diffusion
  • Facilitated diffusion
  • Active transport
  • Osmosis
• Simple diffusion


  Movement of small, non-polar, lipid-soluble molecules down a concentration gradient
• Facilitated diffusion


  Uses carrier proteins to transport large molecules like glucose down a concentration gradient
• Active transport


  Uses energy to move molecules and ions against a concentration gradient
• Glucose absorption


  1. Sodium ions actively transported from lumen to epithelial cell

  2. Sodium ions diffuse into cell down concentration gradient

  3. Glucose diffuses into cell by co-transporter

  4. Glucose diffuses from cell to blood by facilitated diffusion
• Antigens


  Proteins or glycolipids on the surface of cells, can be 'self' or 'foreign'
• Phagocytosis

  1. Phagocyte recognises foreign antigen

  2. Cytoplasm engulfs pathogen

  3. Phagosome fuses with lysosome

  4. Lysozymes digest pathogen

  5. Phagocyte presents pathogen antigens
• Immune system cells

  • Phagocytes
  • Helper T cells
  • Cytotoxic T cells
  • B cells
  • Plasma cells
• Clonal selection


  Process where an activated B cell divides to form plasma cells secreting specific antibodies