Biology

• Units


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• Scanning electron microscopes (SEM)


  • Use electrons to scan the surface of a specimen
  • Cannot see internal structures
  • Lower resolution than TEM but much higher than optical microscopes
  • Create a 3D image in black and white
• Transmission electron microscopes (TEM)


  • Use an electron beam to view internal structure of organelles
  • Focus using magnets
  • Can only view dead specimens
  • Preparation is more complex than for optical microscopes
  • Specimens must be very thin
  • Highest resolution but black and white images
• Mitosis stages


  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase
• Prophase


  • Nuclear envelope breaks down
  • Centrioles begin to form the spindle but the chromosomes are not attached to it so are arranged randomly
  • Chromosomes condense and become compact
• Metaphase


  • Chromosomes attach to the spindle using the centromere
  • Chromosomes are lined up at the equator of the cell
• Anaphase


  • Chromosomes divide at the centromere into two sister chromatids
  • Spindle contracts and the chromatids are pulled to opposite poles of the cell
• Telophase


  • Two nuclear envelopes form around the new sets of chromosomes
  • The cytoplasm divides (this is called cytokinesis)
• Interphase


  1. G1 phase (cell growth and replication of organelles)

  2. S phase (DNA replication)

  3. G2 phase (cell growth, synthesis of proteins)
• Mitosis


  The cell divides to produce two genetically identical daughter cells
• Mitosis Stages (photo)


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• Fluid mosaic model


  Constant movement of phospholipids and the random arrangement of protein
• Haemoglobin


  Protein consisting of four polypeptide chains and four haem groups which contain iron ions
• Haemoglobin


  • Each haemoglobin protein can bind four oxygen molecules to form oxyhaemoglobin (the haem groups bind oxygen) in a reversible reaction
  • Found in red blood cells
• Oxyhaemoglobin dissociation curve


  1. Partial pressure of oxygen (po₂) in effect means 'concentration of oxygen'

  2. Haemoglobin has a higher affinity for oxygen at high po₂ (oxygen loads where there lots of oxygen e.g. at the alveoli)

  3. Haemoglobin has a lower affinity for oxygen at low po₂ (oxygen unloads where oxygen is low e.g at respiring tissues)

  4. Dissociation curve shows the percentage saturation of haemoglobin with oxygen at different po₂

  5. Binding of first oxygen molecule changes the quaternary structure of haemoglobin, another haem group is uncovered for the second oxygen molecule to bind to so it binds more easily
• The Bohr effect


  • The dissociation curve shifts to the right at higher partial pressures of carbon dioxide (PCO₂)
  • Oxygen unloading is faster when pCO₂ is high e.g. at respiring tissues where oxygen is needed for aerobic respiration
  • Increasing pCO₂ decreases haemoglobins affinity for oxygen by decreasing blood pH
• Animals with a high metabolic rate


  Have a haemoglobin dissociation curve that is shifted right compared to adult human curve, so oxygen can be unloaded more easily at respiring tissues for aerobic respiration
• Organisms living in habitats with low oxygen concentrations


  Have a haemoglobin dissociation curve that is shifted left compared to adult human curve, so oxygen is loaded more easily
• The heart


  • Four chambers with muscular walls - two atria, two ventricles
  • Right side pumps deoxygenated blood to the lungs, left side pumps oxygenated blood to the body
  • Walls of ventricles are thicker than walls of atria, more contraction force needed to get blood out of the heart
  • Wall of left ventricle is thicker than the right ventricle, blood must be under higher pressure to travel round the whole body so stronger contraction is needed
• Heart valves


  • Atrioventricular (AV) valves and semi-lunar (SL) valves prevent backflow of blood
  • Cords prevent AV valves being forced into atria
• Cardiac cycle


  1. Ventricles relax, atria contract (volume decreases, pressure increases), AV valves open, blood forced into ventricles

  2. Atria relax, ventricles contract (volume decreases, pressure increases), AV valves close, SL valves open, blood forced into aorta and pulmonary artery

  3. Atria still relaxed, ventricles relax, SL valves close, blood returns to the atria, volume and pressure increase gradually, AV valves open for passive flow from atria to ventricles
• Stroke volume


  Volume of blood pumped out of the left ventricle during one cardiac cycle
• Cardiac output


  Stroke volume x heart rate
• Arteries


  • Carry oxygenated blood from the heart to the body (except pulmonary artery)
  • Thick layer of muscle and elastic tissue allows stretch and recoil to maintain and withstand high blood pressure
  • Folded endothelium allows stretching
  • Coronary arteries supply the heart muscle with blood
• Arterioles


  • Carry oxygenated blood, contain muscle tissue which can contract or relax to control blood flow to different areas of the body
  • When the muscle tissue contracts the lumen constricts and blood flow to capillaries is reduced (vasoconstriction)
• Capillaries


  • Endothelium is one cell thick to give a short diffusion pathway for exchange, site of gas exchange
  • Networks of capillaries throughout tissues called capillary beds, large number to increase surface area for exchange
• Veins


  • Carry deoxygenated blood from the body back to the heart (except pulmonary vein)
  • Wider lumen and thinner walls (less muscle and elastic tissue) than arteries because blood pressure is lower
  • Blood under low pressure so valves prevent backflow
• Glucose absorption


  1. Sodium-potassium pump actively transports Na+ from epithelial cell to capillary, generating a Na+ concentration gradient from ileum to epithelial cell

  2. Na+ diffuses from ileum to epithelial cell through the co-transporter protein, bringing glucose with it against the glucose concentration gradient

  3. Glucose diffuses through a channel protein from the epithelial cell to the capillary down a concentration gradient
• Endopeptidases


  • Hydrolyse internal peptide bonds within polypeptide chains to create smaller chains, increasing the number of 'ends' for exopeptidases (e.g. trypsin, chymotrypsin, pepsin)
• Exopeptidases


  • Hydrolyse peptide bonds at the ends of polypeptide chains (remove one amino acid), e.g. membrane-bound dipeptidases at the ileum epithelium
• Inspiration


  Active process - external intercostal muscles and diaphragm contract, ribcage moves up and out, volume of thoracic cavity increases, pressure in thoracic cavity decreases below atmospheric pressure, air moves down a pressure gradient into the lungs
• Expiration


  Passive process - external intercostal muscles and diaphragm relax, ribcage moves down and in, volume of thoracic cavity decreases, pressure in thoracic cavity increases above atmospheric pressure, air moves down a pressure gradient out of the lungs
• Forced expiration is active because it requires contraction of the internal intercostal muscles
• Xerophytes


  • Live in dry, windy, warm conditions and need adaptations to conserve water
  • Adaptations reduce the water potential gradient between the inside and outside of the leaf
• Thoracic cavity


  • Ribs and external intercostal muscles
• Expiration


  1. Curved diaphragm

  2. External intercostal muscles and diaphragm contract

  3. Ribcage moves up and out

  4. Volume of thoracic cavity increases

  5. Pressure in thoracic cavity decreases below atmospheric pressure

  6. Air moves down a pressure gradient into the lungs
• Passive expiration(breathing out)


  1. External intercostal muscles and diaphragm relax

  2. Ribcage moves down and in

  3. Volume of thoracic cavity decreases

  4. Pressure in thoracic cavity increases above atmospheric pressure

  5. Air moves down a pressure gradient out of the lungs
• Xerophytic adaptations


  • Thick, waxy cuticle increases diffusion distance and reduces transpiration
  • Can have spines instead of leaves which reduces surface area: volume ratio e.g. cacti
  • Hairs around stomata to trap water vapour
  • Stomata sunken into pits to trap water vapour
  • Curled leaves to trap water vapour
  • Fewer stomata
• Evaporation of water from a plant is called transpiration
• Gas exchange in fish

  • Gills protected by a bony flap called the operculum
  • Gills have a large surface area for gas exchange
  • Gill filaments covered in very thin lamellae
  • Thin lamellae have a good blood supply and give a short diffusion pathway for gas exchange