Biology

• Passive transport


  Exchange of substances without requiring metabolic energy from the cell
• Diffusion


  Net movement of particles from an area of higher concentration to an area of lower concentration (down their concentration gradient)
• Facilitated diffusion


  Net movement of particles down their concentration gradient across a partially permeable cell membrane via carrier or channel proteins
• Water potential

  Measure of the tendency of water molecules to move from one area to another area and describes the pressure created by these water molecules; the more dilute a solution, the higher (less negative) the water potential (Ѱ)
• Osmosis


  Net movement of water from an area of higher water potential to an area of lower water potential across a partially permeable membrane
• Factors affecting rate of diffusion

  1. Increasing number of channel & carrier proteins

  2. Increasing surface area of the cell membrane

  3. Reducing the diffusion distance

  4. Creating a steeper concentration gradient
• Active transport


  Movement of particles from an area of low concentration to an area of high concentration (against their concentration gradient) across a cell membrane, using ATP and carrier proteins
• Co-transport


  Transport of one substance coupled with the transport of another substance across a membrane
• Phagocytosis


  1. Pathogens engulfed by phagocytes using endocytosis

  2. Pathogens stored in the phagosome

  3. Lysosome fuses with phagosome releasing hydrolytic lysozyme enzymes

  4. Pathogen hydrolysed

  5. Some used by cell, some presented on cell surface to activate lymphocytes

  
• Antigens


  Any part of an organism/substance which is recognised as foreign by the immune system and goes on to trigger an immune response
• Components of the immune system

  • Phagocytes
  • Macrophages
  • Neutrophils
  • T cells
  • T helper cells
  • Cytotoxic T cells
  • T memory cells
  • B cells
  • Plasma cells
  • B memory cells
• Antibodies


  Proteins produced by lymphocytes in response to the presence of the corresponding antigen
• Antibodies

  Agglutinate pathogens by forming antigen-antibody complexes, leading to phagocytosis & neutralise toxins
• Cell-mediated immunity


  1. Antigen from pathogen displayed on cell surface of body cells or phagocytes after phagocytosis

  2. T cells with correct specific receptor bind with antigen and are activated

  3. They divide by mitosis (clonal expansion) and differentiate into T helper, cytotoxic and memory cells

  4. Cytotoxic T cells release perforin which destroys cell surface membranes, killing infected cells
• Humoral immunity


  1. Free antigen binds to complementary B cell receptor

  2. Pathogen endocytosed, antigen presented on plasma membrane

  3. T helper cell binds presented antigen, activating B cell (clonal selection) and stimulating B cell division by mitosis (clonal expansion)

  4. B cell differentiates to plasma and memory cells

  5. Plasma cells release antibodies, memory cells remain in blood stream and allow secondary immune response
• Primary immune response


  Initial immune response to a pathogen infection, slow and leading to low antibody concentration
• Secondary immune response


  More rapid and vigorous response leading to higher antibody concentration, due to presence of memory cells
• Vaccination


  Introduction into the body of a vaccine containing disease antigens, by injection or mouth, to induce artificial immunity
• Herd immunity


  Vaccination of a significant proportion of the population provides protection for individuals who have not developed immunity
• Pathogen may mutate so its antigens change suddenly (antigenic variability), making the vaccine ineffective
• Types of immunity


  • Natural active
  • Natural passive
  • Artificial active
  • Artificial passive
• HIV


  Replicates in T helper cells, causing symptoms of AIDS due to decreased Th cell count and compromised immune system
• Antibiotics kill bacteria by targeting bacteria specific enzymes or organelles, but are ineffective against viruses
• Monoclonal antibodies

  Drugs can be attached to them to ensure delivery to specific cell types, used for disease diagnosis, pregnancy testing, and measurement & diagnosis in ELISA test
• Ethical considerations for using monoclonal antibodies: treatment may cause death, use of animals for production may cause harm, human trials
• Single celled organisms can exchange oxygen and carbon dioxide directly through their plasma membrane via diffusion
• Insect gas exchange


  • Air enters via spiracles, travels through trachea and tracheoles, delivering oxygen directly to every tissue
• Fish gas exchange


  • Occurs via gills, orientation of gill filaments and lamellae ensures water flowing over them moves in opposite direction to blood flow (countercurrent flow), maintaining diffusion gradient
• Plant gas exchange


  • Occurs in leaves, stomata can open to allow gases diffuse in and out, mesophyll cells have large surface area for rapid diffusion
• Plant adaptations to limit water loss

  • Hairs, waxy cuticle, small leaves, sunken stomata, rolled leaves
• Insect adaptations to limit water loss

  • Controlling open and closing of spiracles, hair around spiracles, waterproof, waxy cuticle
• Human gas exchange system

  • Alveolar epithelium adapted by having large surface area, good blood supply, thin walls & elastic fibres
• Ventilation


  1. Inspiration: external intercostal muscles contract, rib cage moves up & out, diaphragm contracts, volume of thorax increases, pressure decreases so air forced in

  2. Expiration: internal intercostal muscles contract, ribs move down and inwards, diaphragm relaxes, volume of thorax decreases, air forced out
• Surface area to volume ratio

  The greater the size of an organism, the smaller its surface area: volume ratio, requiring specialised exchange surfaces and transport mechanisms
• Haemoglobin


  Protein in red blood cells that transports oxygen, made up of four polypeptide chains each containing a prosthetic haem group that binds one oxygen molecule
• Bohr effect


  Haemoglobin's oxygen binding affinity is inversely related to the concentration of carbon dioxide, causing the oxygen dissociation curve to shift
• Cardiac cycle


  Sequence of events that occur within one full beat of the heart
• Tissue fluid formation


  Arteriole: Hydrostatic pressure > water potential, Venule: Hydrostatic pressure < water potential, Remaining fluid returns to circulation via lymphatics system
• Digestive enzymes in mammals

  • Amylase
  • Membrane-bound disaccharidases (maltase, sucrase, lactase)
  • Lipase
  • Endopeptidases (pepsin, trypsin & chymotrypsin)
  • Exopeptidases
  • Membrane-bound dipeptidases
• Lipid digestion and absorption

  1. Bile salts emulsify lipids, increasing surface area for lipases

  2. Micelles formed, travel to ileum where broken down, releasing monoglyceride and fatty acids that diffuse into epithelial cells

  3. Moved to ER, combined to form triglycerides

  4. Moved to Golgi, cholesterol added to form chylomicrons

  5. Chylomicrons leave epithelial cells by exocytosis and move into lacteals