Higher Biology

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Higher Biology

• Unit 1
• Cell Biology
• Cell structure in relation to function

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• In a multicellular (many-celled) organism the
  cells are organised into tissues.
• A tissue is a group of similar cells which work
  together to carry out a specific function.

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• Some tissues have only one type of cell (e.g.
  muscle). Other tissues have several types of
  cells (e.g. phloem contains sieve tubes and
  companion cells).

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• The structure of a cell is related to its
  function (what the cell does).
• In a unicellular (one-celled) organism (e.g.
  amoeba, paramecium, euglena or yeast) all the
  processes necessary for life are carried out in a
  single cell.

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Paramecium
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Euglena
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Root hair
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Leaf epidermis
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Phloem
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Xylem
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Leaf mesophyll
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Parenchyma
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Lining of kidney tubule
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Return
Lining of trachea
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Lining of trachea
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Lining of mouth
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Bone cell
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Fat cell
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Red blood cell
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Muscle Cell
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Nerve cell
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Cell Ultrastructure
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Cell Boundries

• Cell wall
• Outer boundary of plant cells
• Made of cellulose fibres in layers
• Strong, slightly elastic
• Absorbs water, providing a pathway for water
  movement through plant tissues.

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• Plasma membrane
• Forms the cell membrane and forms or surrounds
  all cell organelles.

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• Made of a double layer of phospholipid molecules
  with protein molecules embedded.
• Called fluid-mosaic model because
• Molecules move around like fluid
• Proteins form a pattern on surface (mosaic)
• Some protein molecules enclose a pore through
  which small molecules can pass in/out of the cell.

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Functions of the plasma membrane

• Molecules can enter or leave a cell, across the
  membrane, in 5 ways
• Diffusion
• Osmosis
• Endocytosis
• Exocytosis
• Active Transport

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Diffusion

• Movement of molecules of (gas or) liquid from an
  area of high concentration to an area of low
  concentration down a concentration gradient.

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• The concentration gradient is the difference in
  concentration between two areas.

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• Molecules cross the plasma membrane in two ways
• Through the phospholipid layer
• Through pores in the protein molecules

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Osmosis

• Diffusion of water molecules
• Through a selectively permeable membrane (e.g.
  plasma membrane)
• (S.P. Membrane is a membrane with pores which
  allows small molecules to pass but not large ones)

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• Water moves from a high water concentration to
  low water concentration
• HWC LWC

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Effects of osmosis on cells
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• Hypotonic higher water concentration
• Hypertonic lower water concentration
• Isotonic same water concentration

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• Turgid cell swollen with water
• Flaccid cell limp through loss of water
• Plasmolysed in Plant cells, water loss causes
  cytoplasm to shrink away from the cell wall.

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Endo- and Exo-cytosis

• Cells sometimes take in, or expel, large
  quantities of material by forming a pocket in
  the membrane.
• This is called endocytosis (taking material into
  the cell) or exocytosis (materials leave the
  cell).

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• An example of endocytosis is

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Phagocytosis
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• In this form of endocytosis the cell engulfs
  solid particles (e.g. amoeba) like eating a
  bacterium.

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Active Transport

• Movement of ions across the plasma membrane
  against the concentration gradient
• i.e. Low concentration ? High concentration

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• Energy is needed.
• Protein molecules transport ions across the
  membrane.

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IN
High conc. inside the cell
Low conc. outside the cell
Energy
Low conc. inside the cell
High conc. outside the cell
OUT
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Essay question

• Discuss the role of the plasma membrane under the
  following headings
• The structure of the plasma membrane (4 marks)
• The role of the plasma membrane in transport (6
  marks)

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• Discuss the role of the plasma membrane under the
  following headings
• The structure of the plasma membrane (4 marks)
• The role of the plasma membrane in transport (6
  marks)
• Plasma membrane
• Composed of phospholipid bilayer (two layers)
• Contains proteins.
• Some proteins form pores through the membrane.
• Described as a fluid mosaic
• Role of plasma membrane
• Diffusion movement of liquid/gas from area of
  high conc. to area of low conc.
• Osmosis movement of water from area of HWC to
  area of LWC.
• Endocytosis description of engulfing a large
  molecule.
• Phagocytosis is an example of endocytosis (eating
  bacteria).
• Active transport movement from area of low
  conc. to area of high conc.
• Active transport requires energy and a carrier
  protein.
• 1 mark for each bullet point. TOTAL 10 Marks

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Lives in the sea Lives in the sea Lives in fresh water Lives in fresh water Lives in fresh water
Ion Fresh water Sea Water Lobster Mussel Cray-fish Frog Fresh WaterMussel
Na 0.24 478.3 530.9 79 212 109 15.6
K 0.005 10.1 8.7 152 4.1 2.6 0.5
Ca 0.67 10.5 15.8 7.3 15.8 2.1 6.0
Mg 0.04 54.5 7.6 34 1.5 1.3 0.2
Cl 0.23 558.4 558.4 94 199 78 11.7
SO4 0.05 28.8 8.9 8.8 - - -
Concentrations in mM per kg
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• In the example above, the lobster actively
  transports sodium inwards (higher concentration
  in the body than the sea water), actively
  transports magnesium out (lower concentration in
  body than sea water), but does not regulate
  chloride (concentration equal).

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Higher Biology

• Unit 1
• Cell Biology
• Photosynthesis

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Absorption, reflection and transmission of light
by a leaf
12 of light reflected
Light shining on leaf (100 )
83 of light absorbed but only 4 of this is
used for photosynthesis
5 of light transmitted
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Light absorption by leaf pigments

• Leaves contain several coloured pigments of which
  chlorophyll is the most important.
• These pigments absorb light energy.

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Which wavelengths of light are used

• White light is made up of several different
  wavelengths of light from 400 nm to 700 nm.
• Normal spectrum of white light

violet blue green yellow orange red
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• Collect a leaf and cut into small pieces.
• Add some propanone and sand into a mortar and
  pestle.
• Grind this up, until the propanone turns green.
• Filter the mixture into a test tube.
• Hold the spectroscope up towards the test tube
  and look towards the light.

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violet blue green yellow orange red
Spectrum viewed through Chlorophyll
violet blue green yellow orange red
The blue and violet are no longer visible and
only some of the red is still seen. These have
been absorbed by the chlorophyll.
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• The main wavelengths absorbed are violet and
  blue, and some red.
• These are most important wavelengths for a plant
  in photosynthesis.

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Absorbtion and Action Spectra

• A leaf contains several pigments which can be
  separated by chromatography.
• The main pigments are
• Chlorophyll a (blue-green)
• Chlorophyll b (yellow-green)
• Carotene (yellow)
• Xanthophyll (yellow)

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• An absorption spectrum shows the absorption of
  light of each wavelength by each pigment.
• An action spectrum shows the rate of
  photosynthesis at each light wavelength.
• Comparison of absorption and action spectra
  reveals a close match this is good evidence for
  the importance of leaf pigments in photosynthesis.

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• The presence of several pigments increases the
  range of wavelengths the plant can make use of.

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Separation of photosynthetic pigments by thin
layer chromatography
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Name Rf value
Carotene
Chlorophyll a
Chlorophyll b
Xanthophyll
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Chloroplasts

• The main pigments (chlorophyll ab, carotene and
  xanthophyll) are contained in the chloroplasts.

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• Chloroplasts have
• A double plasma membrane
• A liquid stroma
• Stacks of flattened membrane bags called grana
  (singular granum) which contain chlorophyll
• Connecting tubes between grana called lamellae.

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Chemistry of photosynthesis

• Remember Standard Grade
• Carbon dioxide water light energy ? glucose
  oxygen
• This takes place in 2 main stages
• Photolysis (needs light)
• Carbon fixation (Calvin cycle)

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Photolysis

• Happens in the GRANA of the chloroplasts.
• Light energy is absorbed by chlorophyll and used
  to split water molecules into hydrogen and
  oxygen.
• Energy is released.

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WATER
Oxygen
Hydrogen
ENERGY
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• These products are treated like this
• OXYGEN released as a by-product.
• HYDROGEN attached to a hydrogen acceptor
  molecule (NADP) to form NADPH2
• ENERGY stored as ATP

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• The hydrogen and the ATP play an important part
  in the second stage of photosynthesis, called
  CARBON FIXATION.

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Quick Quiz
3
5
4
2
6
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Answers

1. Carbon dioxide water light energy ? glucose
   oxygen
2. Grana/Granum
3. Outer membrane
4. Lamellae
5. Inner membrane
6. Stroma
7. Grana
8. Water split to hydrogen oxygen, energy released
9. Picked up by NADP to form NADPH2/Picked up by
   hydrogen acceptor
10. In ATP

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Carbon Fixation

• Takes place in the STROMA of the chloroplast.
• Molecules of carbon dioxide diffuse into the
  chloroplasts where they attach to molecules of
  5-carbon Ribulose biphosphate (RuBP)

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• The resulting 6-carbon compound is unstable and
  breaks down into two molecules of 3-carbon
  glycerate-3-phosphate (GP).
• In the next step, GP is reduced to triose
  phosphate (3-carbon) by the addition of hydrogen
  (from the NADPH2) and energy (from ATP).

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CO2 (1C)
6C unstable
2 x GP (3C)
NADPH2
ATP
RuBP (5C)
NADP
ADP Pi
Triose phosphate (3C)
CALVIN CYCLE
Glucose
Complex carbohydrates other organic molecules
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• Triose phosphate has two possible fates
• Synthesis of glucose (6 carbon) which is then
  built up into other carbohydrates (e.g. starch
  and cellulose)
• Plants also use carbohydrates to make other
  organic molecules (e.g. proteins, fats and
  nucleic acids)

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• 2. Conversion to RuBP so that more carbon dioxide
  can be taken up.
• The cycle of reactions involved in carbon
  fixation is know as the calvin cycle.

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Limiting factors

• A limiting factor is a factor which slows down
  the process of photosynthesis if is in short
  supply.
• Limiting factors are light intensity , carbon
  dioxide concentration and temperature.

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B
Rate of photosynthesis
30C
B
20C
B
A
10C
Carbon dioxide concentration
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• At point A Rate of photosynthesis depends on
  carbon dioxide concentration, regardless of
  temperature.
• Carbon dioxide is the limiting factor.
• At point B Further increase in CO2 has no
  effect. The rate of photosynthesis is increased
  by raising the temperature.
• Temperature is the limiting factor

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Limiting factor either Light or Temp
Limiting factor either CO2 or Temp
Rate of photosynthesis
Rate of photosynthesis
Limiting factor Light intensity
Limiting factor CO2 conc.
Light intensity
Carbon dioxide concentration
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Aerobic Respiration

• Unit 1 Higher Biology

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Energy storage in the cell

• Chemical energy is stored in cells in molecules
  of ATP (Adenosine Tri-Phosphate).

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• (Pi Inorganic phosphate)
• In order to release the chemical energy, the bond
  attaching the third phosphate is broken (shown in
  red).

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ATP formation

• A molecule of ATP forms when a molecule of ADP
  (Adenosine Di-Phosphate) joins with an inorganic
  phosphate.
• The energy required to join the Pi to the ADP
  comes from the chemical energy released from the
  breakdown of glucose during respiration.

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• The conversion of ADP to ATP is called
  Phosphorylation.

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Summary
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Aerobic respiration S-Grade
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Chemistry of Aerobic respiration

• Aerobic respiration is the complete oxidation of
  molecules of glucose to release energy.
• Oxidation is the removal of hydrogen with the
  release of energy.

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• Aerobic respiration takes place in 3 stages
• Glycolysis
• Krebs Cycle
• Cytochrome system.

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Stage 1 Glycolysis
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• Glucose molecules (6 carbons) are broken down
  into 2 molecules of 3-carbon molecule called
  Pyruvic acid.
• Happen in the cytoplasm.
• No oxygen is required.

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• There is a net gain of 2 ATP molecules.
• Hydrogen is released and temporarily attached to
  a co-enzyme carrier molecule called NAD.
• NAD 2H NADH2 (reduced co-enzyme)

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Stage 2 The Krebs Cycle

• a.k.a. The Citric Acid cycle OR Tri-carboxylic
  acid (TCA) cycle.
• This takes place in the mitochondria.

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Mitochondria

• Mitochondria (singular mitochondrion) are
  sausage shaped organelles surrounded by a double
  plasma membrane.
• The centre of the called the Matrix and is filled
  with fluid.

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• The inner membrane is folded into cristae which
  provide a large surface area for the stalked
  particles on which the cytochrome system takes
  place.

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• The pyruvic acid diffuses from the cytoplasm into
  the mitochondrion.
• In the matrix, the pyruvic acid is converted to a
  2-carbon compound called acetyl Co-A, releases
  CO2 and hydrogen. The hydrogen is bound to NAD.

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• Acetyl Co-A now enters the Krebs cycle by
  combining with a 4-carbon compound to form
  6-carbon citric acid.
• Citric acid is then broken down in a series of
  oxidation reactions to the original 4-carbon
  compound and the cycle begins again.
• The Hydrogen which is released binds with NAD.

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Stage 3 Cytochrome system

• This takes place on the stalked particles on the
  cristae.
• Hydrogen is released from the NADH2 and passed
  along a chain of hydrogen carriers called the
  cytochrome system.

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• As each pair of hydrogen atoms are passed along
  the chain enough energy is released to make 3
  molecules of ATP. This is called oxidative
  phosphorylation.
• At the end of the chain the hydrogen combines
  with oxygen to form water.

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Production of ATP

• Complete oxidation of one molecule of glucose
  produces 38 molecules of ATP (36 from oxidative
  phoshorylation in the cytochrome system and 2
  from glycolysis).

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• As the organism respires it uses oxygen (and
  produces carbon dioxide which is absorbed by the
  sodium hydroxide). This causes the liquid level
  to rise and the syringe is used to find the
  volume of oxygen consumed.

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Anaerobic respiration

• Aerobic respiration only occurs if oxygen is
  available to accept the hydrogen at the end of
  the cytochrome system.
• If no oxygen is available anaerobic respiration
  occurs.

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• During anaerobic respiration Glycolysis occurs as
  normal, but there is no Krebs cycle.

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Oxygen available
Krebs cycle
Glucose
Pyruvic Acid
Oxygen not available
ANIMALS
PLANTS
Lactic Acid
Ethanol CO2
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Aerobic respiration Anaerobic respiration
Oxygen required?
Total ATP production (per glucose molecule)
Other products (plants)
Other products (animals)
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• Anaerobic respiration is animals occurs during
  heavy exercise. After exercise stops the lactic
  acid can be converted back to pyruvic acid by
  repaying the oxygen debt by breathing heavily.
  It is therefore REVERSIBLE.
• Anaerobic respiration is plants is IRREVERSIBLE
  as the CO2 diffuses out of the plants.

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Measuring the rate of aerobic respiration

• The rate of aerobic respiration can be measured
  using a respirometer.

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• Parts of this apparatus have the following
  purposes
• Glass beads They are a control to show how it
  would be with something that does not respire.
• Water bath keeps the test tubes at a constant
  temperature, as the volume of gas would increase
  if the temperature increased.

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• Syringe It measures the volume of oxygen used,
  by pushing down and returning the dye to the same
  level as before.
• Sodium hydroxide Absorbs carbon dioxide.

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Synthesis and release of proteins

• Higher Biology
• Unit 1

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Codons
Amino acids
Translation
Single stranded
Ribosomes
Uracil
Peptide bonds
tRNA
C,H,O,N
mRNA
Globular
Transcription
Proteins
Fibrous
Rough ER
synthesis
Genetic code
DNA Polymerase
Replication
Secretion
Golgi body
Double helix
Chromosomes
A,T,C,G
DNA
Base
Nucleotide
A-T C-G
Genes
Sugar
Phosphate
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Structure and variety of proteins

• Proteins are composed of the following elements
• Carbon, Hydrogen, Oxygen, Nitrogen, (often
  Sulphur) HONCS
• Atoms of these elements form amino acids (20
  different ones).
• Amino acids link by peptide bonds to form
  polypeptides.
• Polypeptides link up to form Proteins.

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Role of proteins
TYPE ROLE EXAMPLES
Globular Enzymes
Globular Structural
Globular Hormones
Globular Antibodies
Fibrous Make hair and nails
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Role of genes

• Chromosomes consist of many genes. These carry
  out their instructions by producing enzymes.
  Enzymes are made of protein.
• So, genes produce protein Like this

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Protein Synthesis

• Genes contain a chemical code.
• This code is part of a molecule of DNA
  (Deoxyribonucleic acid).
• The structure of DNA enables the correct amino
  acids to be assembled in the correct sequence to
  make a particular protein.

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Structure of DNA

• A DNA molecule is made of 2 chains of
  nucleotides.
• Each nucleotide contains
• A deoxyribose sugar molecule
• A phosphate molecule
• A base molecule

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• There are four different bases, and therefore
  four different nucleotides

Adenine nucleotide (A)
Guanine nucleotide (G)
Thymine nucleotide (T)
Cytosine nucleotide (C)
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• Nucleotides are linked together by means of
  chemical bonds between phosphate and sugar
  molecules called sugar-phosphate bonds

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• Two of these nucleotide chains are joined by
  means of hydrogen bonds between bases.
• ADENINE always bonds with THYMINE
• CYTOSINE always bonds with GUANINE

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• The two, linked, nucleotide chains are twisted
  into a coil called a double helix.

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• Each chromosome consists of one double-helix
  shaped molecule of DNA containing many thousands
  of base pairs.
• A gene is a section of DNA molecule whose base
  order forms the code to make one protein.

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Replication of DNA

• Chromosomes must be able to copy themselves so
  that cells retain the same genetic information
  after cell divisions.
• This copying of the DNA in the chromosomes is
  called replication.

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• Replication requires
• A DNA molecule
• unattached nucleotides of 4 types
• enzymes (DNA polymerase)
• energy (in the form of ATP)

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• Replication takes place in these stages
• DNA uncoils
• The hydrogen bonds between bases break (starting
  at the end like a zip)
• Free nucleotides attach to exposed bases
• Sugar-phosphate back bone reforms

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The Genetic Code

• Proteins are made of a long chain of amino acid
  molecules.
• A gene contains a code (order of DNA bases) to
  ensure that the amino acids are joined in the
  correct order to make a specific protein.
• The order of the bases is called the genetic
  code.

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• This is a triplet code because the sequence of
  three bases is needed to code for each amino
  acid.
• e.g.
• AAG codes for amino acid Phenylalanine
• GAC codes for amino acid Aspartic Acid
• GGA codes for amino acid Glycine.

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• So a DNA strand with base sequence
• Would code for (part of) protein
• (one protein molecule may be hundreds or
  thousands of amino acids long)

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RNA (ribonucleic acid)

• Protein synthesis takes place on the ribosomes in
  the cytoplasm.
• The instructions in the genetic code are carried
  from the DNA (in the nucleus) to the ribosomes by
  a molecule of messenger RNA (mRNA)

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• RNA differs from DNA in 3 ways
• RNA is single stranded
• RNA contains ribose sugar
• In RNA the base thymine is replaced by Uracil

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• There are two types of RNA
• Messenger RNA (mRNA) which carries the genetic
  code from the DNA in the nucleus to a ribosome in
  the cytoplasm.
• Transfer RNA (tRNA) which carries amino acids to
  the ribosomes for assembling into polypeptide
  chains.

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Transcription

• The piece of DNA containing the relevant gene
  uncoils and the base pairs separate.
• Complementary RNA nucleotides then attach to the
  exposed DNA bases.
• They link together (ribose-phosphate chemical
  bonds) to form a messenger RNA (mRNA) molecule.

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G
T
C
A
C
T
A
T
A
G
G
C
G
T
DNA strand
A
A
G
C
C
G
A
T
T
C
G
G
A
T
A
A
C
G
C
T
G
C
C
G
T
T
C
A
C
A
G
T
G
A
T
A
T
C
C
G
One gene
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Ribosomes and rough endoplasmic reticulum

• Ribosomes are
• Found in all cells
• Free in cytoplasm or attached to rough
  endoplasmic reticulum
• Spherical, with two halves
• Site of translation of mRNA into protein

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Fluid filled cavity between sheets
Ribosomes
Sheets of endoplasmic reticulum
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Assembling the protein translation

• In the cytoplasm are molecules of transfer RNA
  (tRNA).
• These are composed to one triplet of bases and an
  amino acid molecule.
• e.g. Alanine Leucine
• Codon triplet of bases on mRNA
• Anti-codon Complementary triplet of bases on tRNA

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Stage 1
Stage 2
Stage 3
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• The mRNA attaches to a ribosome.
• The ribosome moves along the mRNA with successive
  codons entering the active site.
• Here, a tRNA with the appropriate anti-codon is
  attached.
• Adjacent amino acids then link up by a peptide
  bond to form a polypeptide and eventually a
  protein.

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Secretion of proteins
Nucleus
Rough endoplasmic reticulum
Nuclear membrane
Pore
Ribosome
Golgi Apparatus
Vesicle
Cell membrane
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• The golgi apparatus is made up of many flattened
  fluid filled sacs.
• Vesicles containing newly made protein are
  pinched off the rough endoplasmic reticulum.
• These move towards the Golgi and use with the
  outermost sac.

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• The contents then move down through the golgi
  from sac to sac, becoming modified in the
  process.
• The finished product (e.g. glycoprotein), in a
  vesicle, leaves the golgi and moves to the cell
  membrane and discharges its contents out of the
  cell.

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Cellular Defence Mechanisms
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Virus Structure
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Reproduction of viruses

• Viruses can only reproduce inside the cells of a
  host organism.
• They use the hosts nucleotides for replication
  and the hosts amino acids to construct protein
  coats.

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Defence against viruses
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First line of defence

• Mechanisms by which our bodies attempt to prevent
  entry of harmful microbes.

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Second line of defence

• Mechanisms by which our bodies attempt to kill
  harmful microbes which have succeeded in entering.

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Immunity

• Immunity is the ability of an organism to resist
  disease. The blood is usually involved.
• There are two types
• Non-specific immunity (phagocytosis)
• Specific immunity (antibodies)

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1. Non-specific Immunity

• Provides protection against a wide range of
  invading microbes e.g. by phagocytosis carried
  out by white blood cells.

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• Read page 70 of Torrance and make your own notes
  on Phagocytosis.
• Then
• Prepare a 2-3 min illustrated presentation to
  give to the class on the topic, explaining
• How invading bacteria are detected
• How invading bacteria are engulfed
• What a lysosome is, what it contains and what it
  does
• What pus is

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Phagocytosis

• Phagocytosis is the process by which foreign
  bodies such as bacteria are engulfed and
  destroyed.
• Cells capable of phagocytosis are called
  phagocytes.

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• A phagocyte detects chemicals released by the
  bacterium and moves up a concentration gradient
  towards it.
• The phagocyte adheres to the bacterium and
  engulfs it into a vacuole formed by an infolding
  of the plasma membrane.

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• Lysosomes fuse with the vacuole and release their
  enzymes into it.
• The bacterium becomes digested and the breakdown
  products are absorbed by the phagocyte.

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• During infection hundreds of phagocytes migrate
  to the infected area and engulf many bacteria by
  phagocytosis.
• Dead bacteria and phagocytes often gather at a
  site of infection forming pus.

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2. Specific immunity

• A specific invading particle (e.g. A virus) is
  attacked by a specific defending chemical.

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• Antigen a complex molecule recognised by our
  body as alien (e.g. A virus coat particle).
• Antibody a chemical produced a lymphocyte white
  blood cell to destroy antigens.

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How antibodies work

• An antibody is a Y-shaped molecule

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• The binding sites on the arms attach to antigen
  molecules making them harmless

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• There are many types of lymphocyte, each type
  targeting one antigen with specific antibodies.

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Types of specific immunity

• Active immunity
• We produce our own antibodies by
• Suffering from the disease and retaining the
  antibodies in the blood (natural)
• Receiving a vaccine of treated antigen (e.g.
  Empty virus coats) which triggers antibody
  formation (artificial).

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• (b) Passive immunity
• We receive ready-made antibodies from
• Mother, across the placenta (natural)
• Another mammal (e.g. A horse) which has made the
  antibodies in response to treatment
  (artificial).

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Rejection of transplanted tissues

• Make your own notes from Page 73 of Torrance.

179
Cellular Defence Mechanisms in Plants

• Plants defend themselves from attack by
• Producing toxic compounds
• Isolating the infected area or infectious
  organism

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(a) Production of toxic compounds

• Cyanide
• Made by clover plants by a process called
  cyanogenesis.
• Cyanide works by blocking the cytochrome system
  of e.g. Slugs
• It is produced when non-toxic glycoside and an
  enzyme are mixed as a result of leaf damage.

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• (ii) Tannins
• Tannins are toxic to micro-organisms
• They defend by preventing pathogens e.g. Fungi
  from gaining access to the plant organ under
  attack.
• The tannins act as enzyme inhibitors.
• Therefore, they interfere with the invading
  pathogens metabolism and render it harmless.

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• (iii) Nicotine
• This is toxic chemical produced in the root cells
  of tobacco plants and transported to its leaves.
• Since it is poisonous it protects leaves against
  attack by herbivorous insects.
• Nicotine can be extracted from tobacco plants and
  used as an insecticide.

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(b) Isolation of the problem

• Insect galls
• When a parasite penetrates the cuticle of a leaf,
  the leaf produces a gall in response to a
  chemical stimulus.
• A gall is an abnormal swelling of plant tissue
  resulting from active division of cells at the
  site of the injury.

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• The combination of the extra layers of cells and
  rich deposits of tannin in a gall provides the
  plant with a protective barrier where the
  parasite can be isolated.

185

• (ii) Resin
• Resin is a sticky substance produced by many
  trees.
• When a plant becomes wounded by a pathogen the
  resin-secreting cells increase in activity,
  trapping pathogens.