BIOLOGY UNIT 3 REVIEW
Mark Trofimiuk
Croydon Secondary College
June 2012.
Contents
AREA OF STUDY 1: CHEMICAL NATURE OF THE CELL ........................................................................................................... 3
Atomic Structure and Bonding ..................................................................................................................................... 3
Water............................................................................................................................................................................ 4
Biomacromolecules ...................................................................................................................................................... 5
Structure and Function of Lipids .................................................................................................................................. 7
Structure and Function of Proteins .............................................................................................................................. 8
Structure and Function of DNA and RNA ................................................................................................................... 10
Molecules on the Move ................................................................................................................................................. 11
Plasma Membrane Structure ..................................................................................................................................... 11
Diffusion ..................................................................................................................................................................... 12
Osmosis ...................................................................................................................................................................... 12
Transport Through Membranes ................................................................................................................................. 13
Biochemical Pathways .................................................................................................................................................... 14
Cellular Metabolism ................................................................................................................................................... 14
Anabolic Reactions ..................................................................................................................................................... 14
Catabolic Reactions .................................................................................................................................................... 14
Biochemical Pathways ................................................................................................................................................ 14
Enzymes...................................................................................................................................................................... 14
Activation Energy ....................................................................................................................................................... 15
Photosynthesis ........................................................................................................................................................... 16
Cellular Respiration .................................................................................................................................................... 17
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AREA OF STUDY 2: DETECTING AND RESPONDING ............................................................................................................ 19
Principles of Homeostasis .......................................................................................................................................... 19
Hormones ................................................................................................................................................................... 20
Types of Hormones .................................................................................................................................................... 21
Nervous System.......................................................................................................................................................... 23
Communication Between Organisms......................................................................................................................... 26
Communication in Plants ........................................................................................................................................... 26
INVADERS ....................................................................................................................................................................... 28
Nature of Disease ....................................................................................................................................................... 28
Pathogens ................................................................................................................................................................... 28
Keeping Pathogens Out .............................................................................................................................................. 32
Barriers to Infection in Plants..................................................................................................................................... 32
DETECTING AND RESPONDING TO INVADERS ............................................................................................................... 33
Non-Specific Immune Response ................................................................................................................................ 34
Specific (Adaptive) Immune Response ....................................................................................................................... 36
Disorders of the Immune Response ........................................................................................................................... 41
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AREA OF STUDY 1: CHEMICAL NATURE OF THE CELL
Atomic Structure and Bonding
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Atoms are made up of a nucleus which contains protons (+) and neutrons (0).
Electrons (-) spin around the nucleus in paths called orbitals.
An atom is neutral if electrons = protons
The number of protons defines an element.
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Some atoms of an element can have more neutrons in their nucleus than others.
Such atoms are called isotopes of the element eg. Carbon-14
The nucleus of the isotope is unstable and breaks apart giving off energy,
which we call radiation.
When this happens we say that the element ‘decays’.
By giving off radiation, atoms reach a more stable state.
Radioactive isotopes are useful tools in the study of many areas of science,
including biochemical reactions and medical research because their presence
can be detected by the radiation they release.
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The number and arrangement of electrons in an atom’s
outer shell determine its chemical behaviour or reactivity.
Outer shell electrons are called valence electrons.
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Compounds are stable combinations of atoms of different elements that are held together by chemical bonds.
Atoms accept, give away or share their valence electrons with other atoms, thereby achieving chemical stability.
The nature of the bonds differs according to the type of atoms involved.
Ionic Compounds
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Salts are ionic compounds that dissolve in water as the bonds
holding the ions together weaken and break, releasing them.
These particles in solution are called electrolytes.
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Acids, Bases and Buffers
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An acid is a substance that produces hydrogen ions (H+) in solution.
The acidity of a solution is measured by its pH.
The lower the pH, the more acidic the solution.
A buffer is a substance that can react with an acid or a base and maintain a steady pH.
Molecular Compounds
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Instead of losing or gaining electrons, atoms of non-metals combine with other non-metal atoms by sharing pairs of
valence electrons, thus forming molecular compounds.
The bonds holding the atoms in these molecules together are covalent bonds.
Water
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In the water molecule, oxygen and the two hydrogens share outer electrons.
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Oxygen now has 8 outer electrons (stable)
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Each hydrogen now has 2 outer electrons (stable)
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Water molecules are polar and form hydrogen bonds with each other (cohesion).
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Water molecules can form hydrogen bonds with other molecules (adhesion).
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More substances dissolve in water than in any other substance.
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Polar substances dissolve in water and are Hydrophilic (water loving).
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Non-polar substances (eg. Oil, petrol) do not dissolve in water and are Hydrophobic.
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Most gases dissolve in water.
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Biomacromolecules
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Macromolecules are large molecules involved in many processes in the cell.
The four main groups of macromolecules are:
 CARBOHYDRATES
 LIPIDS (fats and oils)
 PROTEINS
 NUCLEIC ACIDS
Each of the macromolecules is made up of smaller components.
Question: Complete the following table:
MACRO-MOLECULE
MAIN
ELEMENTS
SUBUNITS
EXAMPLE
CARBOHYDRATE
LIPID
PROTEIN
NUCLEIC ACID
POLYMERISATION
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Polymers are large molecules made up of single units (monomers) joined together to form long chains
As each monomer is added to the polymer, a bond forms and releases water (condensation reaction)
The opposite happens when polymers are broken down. Water molecules are involved in the breaking of the bonds
between the monomers (hydrolysis reaction)
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Structure and Function of Carbohydrates
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each molecule consists of carbon, hydrogen and oxygen atoms in the ratio of 1:2:1, giving the general formula for
carbohydrates of nCH2O.
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carbohydrates are classified as:
monosaccharides (eg. glucose)
disaccharides (eg. sucrose)
and
polysaccharides (eg. cellulose, starch)
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Organisms use carbohydrates as an energy source (eg. starch and glycogen) and for structural components (eg. cellulose
and chitin).
Most animals do not have the enzymes to break down cellulose in their diet, and have to rely on bacteria in their gut to
do it for them.
Carbohydrate molecules can combine with other atoms or groups to form important compounds, eg. glycoproteins,
which are a combination of carbohydrate and protein molecules.
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Structure and Function of Lipids
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Lipids are a diverse group of molecules and include:
 fats and oils,
 terpenes,
 waxes,
 phospholipids,
 glycolipids and
 steroids
their various functions relate to their hydrophobic nature.
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In cells, lipids have three important functions:
 energy storage - they have twice the amount of energy as carbohydrates
 structural component of membranes
 specific biological functions, such as the transmission of chemical signals both within and between cells
(hormones).
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The fats and oils of plants and animals are typically composed of
triglyceride molecules – three fatty acid chains attached to a glycerol backbone.
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The fatty acids can be saturated or unsaturated
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Phospholipids form when a phosphate group is added to the glycerol backbone rather than a third fatty acid chain.
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Glycolipids form when a carbohydrate group attaches to the glycerol backbone rather than a third fatty acid chain.
They are vital for communication and detect, and bind with, signalling molecules.
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Cholesterol is a component of cell membranes and of myelin sheaths around nerve cells.
It belongs to the group of lipids collectively known as steroids.
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Structure and Function of Proteins
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The whole set of proteins produced by a cell is called its proteome and the study of proteomes is proteomics.
Proteins are large complex molecules and are the most important molecules in living organisms.
As enzymes they control the thousands of chemical reactions that maintain life processes.
This diversity of proteins can be explained by the way their subunits, the 20 amino acids, are sequenced in various
combinations (like arranging 20 kinds of beads in different ways to make different necklaces of different lengths and the
necklace chains can then be arranged differently in loops and folds to give each its characteristic features).
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Amino acids are small molecules that have the same basic structure:
 a central carbon atom
 a hydrogen atom
 a carboxyl acid group (COO- )
 an amine group (NH3+ ) and
 an R group.
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It is the difference in the R group that distinguishes one amino acid from another and gives them their particular chemical
properties.
Some of the R groups are non-polar, some are polar and others are charged.
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There are only 20 different amino acids found in the proteins of living organisms.
PROTEIN STRUCTURE
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Primary structure
DNA determines the sequence of amino acids in the polypeptide.
Secondary structure
various parts of the polypeptide undergo coiling and folding due to interactions between the various amino acids that are
present.
 Hydrogen bonding
 Ionic bonding
 Disulfide bridges
 Hydrophobic Interactions (van der Waals’ interactions)
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Tight coils are known as α-helices and the folding forms β-sheets.
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Tertiary structure
R groups attract similar R groups.
This causes the polypeptide chains to become folded, coiled or twisted into the protein’s functional shape or
conformation.
Protein molecules with the same sequence of amino acids will fold into the same shape.
A change to just one amino acid will alter the shape of the protein molecule and it may not function properly.
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Quaternary structure
Many large, complex protein molecules consist of two or more polypeptide chains.
Haemoglobin, for example, which carries oxygen in the blood, consists of four polypeptide chains. A variety of bonds
holds the polypeptide chains together and gives the overall shape to the molecule.
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The function of protein molecules may change as a result of a number of factors:
 misreading the DNA code for proteins
 high temperatures
 strong salty solutions or
 very acidic or alkaline conditions (pH).
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These conditions can denature or change the shape of the protein molecules.
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Structure and Function of DNA and RNA
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Nucleic acids store information in a chemical code that directs the machinery of the cell to produce proteins.
Nucleic acids DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are large, linear polymers.
A molecule of DNA is composed of two long strands of subunits called nucleotides, wound around each other to form the
familiar double helix.
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A nucleotide has three chemical parts:
a five carbon sugar (ribose in RNA and deoxyribose in DNA)
a negatively charged phosphate group
an organic nitrogen-containing compound called a base
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There are four kinds of nitrogenous bases in DNA:
 adenine (A)
 thymine (T)
 guanine (G)
 cytosine (C).
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In each nucleotide strand, the sugar molecule of one nucleotide binds to the phosphate group of the next nucleotide,
leaving the nitrogenous base sticking out from each sugar and opposite the nitrogenous base of the second strand.
Hydrogen bonds between the opposing pairs of nitrogenous bases hold the double helix together, much like the rungs of
a twisted ladder or a spiral staircase.
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The bonding of the nitrogen bases does not happen by chance:
 a purine bonds with a pyrimidine
 2 or 3 Hydrogen bonds are formed, so
 A bonds with T, and C bonds with G,
giving rise to the base-pairing rule.
DNA CODE
The code carried by the DNA is organised in triplets (three nucleotides) that determine the order in which the amino acids
are sequenced and this determines which protein is formed.
The parts of the DNA that code for proteins are called genes.
The total set of genes that each cell of an organism has is called its genome.
The study of these sets of genes and the way they interact with each other is called genomics.
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DNA v RNA
The difference between the deoxyribose sugar of the DNA and the ribose sugar of RNA is that ribose has one more
oxygen atom.
The nitrogenous base thymine is replaced by the base uracil (U) in RNA.
RNA is usually composed of a single chain of nucleotides and forms a single strand.
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FUNCTION OF RNA
The information on genes in the DNA that codes for making proteins is transferred to messenger RNA (mRNA).
The mRNA molecule carries the code out of the nucleus and into the cytoplasm to the ribosomes.
The ribosomes read the mRNA code three nucleotides at a time (in codons).
The ribosomes are composed of ribosomal RNA (rRNA) and protein.
The incoming amino acids are attached to transfer RNA (tRNA) molecules.
Each tRNA molecule has an anticodon that will bind with a complementary codon on mRNA.
This is how the ribosomes determine the correct amino acid to add to a growing protein chain.
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Molecules on the Move
Plasma Membrane Structure
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Phospholipid molecule:
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Phospholipid bilayer:
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The plasma membrane is described as selectively or differentially permeable – it allows some substances
through and not others.
Specialised protein molecules are also embedded in the bilayer in various patterns or ‘mosaic’.
Proteins and lipids can also flip around in the membrane.
The structure of the plasma membrane is understood as a ‘fluid mosaic model’
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Some protein molecules embedded in the plasma membrane carry a sugar molecule, and are called
glycoproteins.
Glycoproteins are often receptors and marker molecules that identify self and are important in cell
recognition.
The immune system can recognise invaders by the self or non-self glycoproteins they have.
Membrane proteins are important for the regulation of cell behaviour and the organisation of cells in tissues.
Proteins are also involved in cellular communication.
Receptor sites on the surface of membranes detect hormones and other chemical molecules to control the
transmission of messages within and between cells.
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Some substances (eg. H2O, CO2, O2)
can move easily through membranes.
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Other substances (eg. Glucose, ions) only move
through with help from transport proteins.
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Substances pass in & out of cells by 4 main processes:
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diffusion
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osmosis
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active transport
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endocytosis & exocytosis
Diffusion
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net movement of molecules is from a region of high concentration to a region where they are at lower concentration
(down the concentration gradient).
at equilibrium, the molecules still move, but there is no net change.
it is a passive process – no additional energy required.
Diffusion increases when:
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Concentration gradient is large
Heat is applied
Molecules are smaller
Movement occurs in gas
Osmosis
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Osmosis is a special case of diffusion
It is the net movement of water across a selectively-permeable membrane
Water moves from a dilute solution (high water concentration) to a strong solution (low water concentration) –
down its own concentration gradient
Osmosis is a passive process
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If the fluid surrounding the cell is more dilute than the cytoplasm (eg. fresh water) it is said to be hypotonic.
Water will diffuse into the cell until it bursts (animal cell) or pushes out against the cell wall in plant cells (turgor).
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If the fluid surrounding the cell is less dilute than the cytoplasm (eg. sea water) it is said to be hypertonic.
Water will diffuse out of the cell causing it to shrink (animal cell)
In plant cells the plasma membrane shrinks away from the cell wall (plasmolysis) and the cell becomes flaccid.
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If the fluid surrounding the cell is of equal concentration to the cytoplasm it is said to be isotonic.
Water will diffuse equally in both directions resulting in no net movement into or out of the cell.
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Transport Through Membranes
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Molecules that are insoluble in lipids (eg. charged ions and glucose) cannot readily pass through the membrane.
Certain proteins in the membrane help these molecules to cross.
The proteins span the membrane from one side to the other.
This process is called facilitated diffusion.
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Channel Proteins form a water-filled pore in the
membrane and allow certain ions to pass through.
The movement of ions is passive.
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Carrier Proteins combine with the diffusing
molecule or ion and then carry it across the
membrane to the other side.
The movement is passive.
Carrier Proteins are susceptible to poisons.
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Active transport
o is the net movement of dissolved substances
into or out of cells against the concentration gradient.
o requires energy
o involves a carrier protein
o enables cells to maintain internal concentrations different
from external surroundings.
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Endocytosis
The endocytic process that encloses solid material
such as food is called phagocytosis and the one that
encloses droplets of liquid is called pinocytosis.
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Exocytosis
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Biochemical Pathways
Cellular Metabolism
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The sum of the thousands of chemical reactions that occur constantly in each living cell is known as cellular metabolism.
Anabolic Reactions
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Chemical reactions in which atoms and molecules are joined
together to make more complex molecules are called anabolic reactions.
Energy is required to form new chemical bonds.
Reactions that require an input of energy are called endergonic reactions.
Catabolic Reactions
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Reactions that break down complex molecules into simpler molecules
are called catabolic reactions.
Reactions that release energy are said to be exergonic reactions.
Biochemical Pathways
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The products or outputs of the first step become the reactants or inputs in the next step until the final products are
reached.
Each step is regulated by a specific enzyme.
Cofactors may be involved.
ATP – the Energy Carrier
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Adenosine tri-phosphate (ATP) is able to release energy for cellular
metabolism by breaking the high energy bond holding the third
phosphate group to become adenosine di-phosphate (ADP).
Energy produced during catabolic reactions such as
cellular respiration and photosynthesis can be used to attach
a free phosphate to ADP to reform ATP.
ATP is able to ‘carry’ that energy to other sites in the cell.
Enzymes
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Enzymes are organic catalysts
They speed up and control reactions
Enzymes are not consumed in a reaction and they can be recycled.
Without enzymes the reactions that occur in living organisms would be so slow as to hardly proceed at all.
Enzyme specificity is at the heart of how enzymes control each step in a biochemical pathway.
Intracellular enzymes speed up and control metabolic reactions inside cells.
Extracellular enzymes are secreted from the cell and catalyse reactions outside the cell.
Normally, an enzyme is named by attaching the suffix ‘-ase’ to the name of the substrate on which it acts.
Thus, lipases act on lipids, proteases act on proteins, and nucleases act on nucleic acids.
Pepsin and trypsin and are proteases that are an exception to the naming rule.
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Activation Energy
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Most reactions need an input of energy before they proceed
– this is called the ‘activation energy’
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enzymes lower the activation energy needed for a reaction to
proceed
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The folding of an enzyme protein into its tertiary structure forms an actual groove or pocket that can
accommodate one or more particular substrate molecules and is called the active site.
An active site is highly specific for a particular substrate, which must be of a compatible shape for binding to occur.
This model of enzyme action is known as the lock and key model.
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The bonds that form between an enzyme and its substrate can also
modify the shape of the enzyme.
This interaction is called the induced-fit model of enzyme action
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Some enzymes are inactive until they bind with other molecules or ions that change the shape of the enzyme’s active site
Two classes of substance bind to enzymes to activate them: cofactors and coenzymes.
Cofactors are small inorganic substances, such as zinc ions and magnesium ions.
Coenzymes are non-protein organic substances that are required for enzyme activity.
Factors affecting enzyme activity
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Effect of temperature
Enzymes operating in the human body work best at temperatures
of about 37°C (their optimum temperature)
The enzymes of psychrophiles, microorganisms that live in near-freezing
environments, can operate at very low temperatures.
The enzymes of thermophiles, microorganisms that live in the
geothermal-heated areas of the sea floor, can operate best at
temperatures of about 95–105°C.
Activity gradually increases until the optimum temperature for
enzyme activity is reached.
As temperature continues to increase, enzymes become denatured so the reaction rate decreases.
Effect of Changing pH
The pH of the solution surrounding enzymes, whether it is acidic, basic or
neutral, can have a profound effect on the structure and activity of the
active site of an enzyme and its interactions with a substrate.
As a solution becomes more basic, proteins tend to lose hydrogen ions.
In acidic solutions, proteins gain hydrogen ions.
If the charges on the amino acids in a protein are changed, then the
bonds that maintain the three-dimensional structure of a protein can be
changed.
In some cases the protein shape can alter so much that the enzyme
becomes denatured and can no longer catalyse a reaction, or the
substrate may change shape so it no longer fits into an active site.
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Effect of Substrate and Enzyme Concentration
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The amount of substrate or enzyme present in a reaction mix can limit
the amount of product produced.
Increased amounts of substrate will result in more products being made
until all the enzyme molecules are working at their maximum capacity
When the amount of enzyme in a system is increased, then the yield of
product increases exponentially until the product starts to inhibit
enzyme action or the substrate is depleted.
Inhibiting the work of enzymes
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Other chemical substances can inhibit enzyme function by
binding to the active site of the enzyme (competitive inhibitors),
or by combining with another part of the enzyme in such a way
that the shape of the binding site is altered (non-competitive inhibitors).
Photosynthesis
Chloroplasts
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are found in green plant cells and some protists.
are the site of photosynthesis.
have an outer and an inner membrane.
Enclosed by the inner membrane is the stroma,
a gel-like matrix that is rich in enzymes.
Suspended in the stroma is a third membrane system called
the thylakoid membranes: these are flat, sac-like structures
that are called grana when grouped together into stacks.
Chloroplasts contain their own genetic material (DNA and RNA) and ribosomes.
Photosynthesis is summarised by the following equation:
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Although the overall equation of photosynthesis seems simple, it is a complex pathway with many steps.
These steps form two stages:
the first stage are the light-dependent reactions, which take place on the inner membranes of the chloroplast.
As suggested by their name, these reactions require light.
Chlorophyll traps light energy and uses it to produce ATP and to split water into hydrogen ions and oxygen gas.
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the second stage of photosynthesis does not require light, so it is referred to the light-independent reactions.
These reactions take place in the fluid matrix (stroma) of the chloroplast.
ATP made during the light-dependent stage provides the energy needed to combine carbon dioxide with hydrogen ions
(also from the light-dependent stage) to form the energy-rich molecule glucose, and water.
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Factors Affecting Photosynthesis
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any of the factors that influence photo synthesis
(such as light intensity, carbon dioxide level or temperature)
may limit the rate of photosynthesis.
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the rates of photosynthesis at different carbon dioxide
concentrations and increasing light intensity are shown in the graph.
Cellular Respiration
Glycolysis
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Takes place in the cytoplasm of the cell.
Does not require oxygen.
Glucose (C6)is broken down into two molecules of Pyruvate (2 x C3) in 10 reactions.
Produces 2 ATP molecules.
Aerobic Respiration
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Takes place in the mitochondrion of the cell.
Pyruvate undergoes a reaction to produce Acetyl CoA which enters the mitochondrion.
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The Kreb’s Cycle is a set of reactions that produces CO2, ATP and carrier molecules (NADH and FADH2).
The carrier molecules enter an electron transport system on the cristae (folded membranes).
Electrons are transferred through compounds called cytochromes to oxygen and produce water.
36 – 38 ATP molecules are also produced
Aerobic Respiration
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Takes place when oxygen is not present.
Occurs in two different forms:
o
alcohol fermentation
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Carried out by micro-organisms and yeast.
Produces alcohol (ethanol) and Carbon dioxide gas.
Used in brewing and bread-making.
lactic acid fermentation
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Carried out by animal cells.
Produces Lactic acid and 2 ATPs
When oxygen is again present Lactic acid can convert back to Pyruvate and enter the Kreb’s cycle.
Summary of Cellular Respiration:
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Overview of Photosynthesis and Cellular Respiration:
AREA OF STUDY 2: DETECTING AND RESPONDING
Principles of Homeostasis
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Production of a signal
Detection of the signal
Transfer or transduction of the signal
Response to the signal
Control of the signal by switching it off
External receptors (exteroceptors) >>
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Internal receptors (interoceptors)
Most cells in a multicellular organism are surrounded by tiny spaces
filled with interstitial fluid.
The interstitial fluid makes up the internal environment and needs
to be maintained within narrow limits of pH, solute concentrations
and temperature, for cells to operate at maximum efficiency.
THE STIMULUS-RESPONSE MODEL
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NEGATIVE FEEDBACK
POSITIVE FEEDBACK
 The response actually reinforces the original stimulus
 eg. tadpoles metamorphose into frogs as a result of
positive feedback on the production of the hormone thyroxine
(which is usually under negative feedback control)
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In childbirth, Oxytocin produced by the posterior pituitary gland
stimulates contraction of the uterus and also stimulates
the pituitary gland to produce even more of the hormone.
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Positive feedback is rare and is usually harmful
Hormones
CHEMICAL COMMUNICATION IN ANIMALS
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Most chemical communication in animals is carried out by hormones:
o They travel in the bloodstream
o Only specific target cells respond
o Need very small amounts of hormone
o Involved in homeostasis, growth, reproduction, energy and behaviour
 There are two main types of hormones:
o Steroid hormones – mostly the sex hormones eg. testosterone and oestrogen
o Polypeptide or protein hormones – eg. insulin, glucagon, growth hormone, thyroxine, adrenaline.
DETECTING THE SIGNAL
 Receptor proteins in target cells have a complementary shape to the hormone,
similar to the active site on an enzyme.
 Receptor proteins are found either on the surface of the cell or in the cytoplasm.
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SECOND MESSENGER SYSTEMS (used by protein hormones)
 Receptor proteins stimulate the production of a
small molecule (eg. cyclic AMP) which acts as a
second messenger in the cell and produces a response.
 One hormone molecule can stimulate the production
of many second messengers.
DIRECT SYSTEMS (used by steroid hormones)
 Steroid hormones are fat-soluble and can move through
the plasma membrane.
 They bind with a receptor protein in the cytoplasm.
 They then move into the nucleus where they turn genes
on or off and this affects the types of enzymes that are produced.
Types of Hormones
CONTROL OF BLOOD GLUCOSE LEVELS (BGL)
 Blood glucose levels are maintained within narrow tolerance limits by the action
of two antagonistic hormones: insulin and glucagon.
 Antagonistic hormones have opposing actions in a given situation.
 Both of these hormones are produced in the pancreas.
Insulin is produced in the beta(β) cells of the islets of Langerhans,
and glucagon is produced in the alpha(α) cells.
Insulin lowers the blood glucose levels by:
 Increasing uptake of glucose by cells
 Increasing conversion of glucose to glycogen
and fat
 Reducing the conversion of glycogen to glucose
Glucagon raises the blood glucose levels by:
 Inhibiting the production of insulin
 Increasing conversion of glycogen to glucose
 Formation of glucose from non-carbohydrate
sources (eg. Fats and proteins)
Hyperglycaemia occurs when BGL is high:
 glucose is excreted in the urine
 person produces large amounts of urine
 person drinks large amounts of water
Hypoglycaemia occurs when BGL is low:
 person feels tired and lacking energy
 Glucagon is secreted and the BGL is raised
to optimal levels
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Diabetes occurs in two main forms:
Type 1: insulin-dependent diabetes (Diabetes mellitus) where the pancreas is unable to secrete enough insulin
Type 2: adult-onset diabetes is caused by the adoption of a Western diet high in fats, sugars and processed foods, and a
lack of exercise.
THYROID HORMONES
Thyroid hormones, often called thyroxines, are secreted by the thyroid gland,
situated in the neck close to the larynx, or voice box.
Thyroxine is derived from the amino acid tyrosine and the element iodine.
It affects three main physiological processes:
o cellular differentiation,
o growth and
o metabolism.
There are very few cells that are not affected by these hormones.
Thyroxine production is controlled through two negative feedback loops.
Thyroxine inhibits the secretion of thyroid-stimulating hormone (TSH)
from the pituitary gland and also the secretion of thyrotrophin-releasing
hormone (TRH) from the hypothalamus.
Hypothyroidism
Hyperthyroidism
People suffering from too little
thyroxine (hypothyroidism)
often show puffiness around the eyes
and coarsening of the skin.
Protrusion of the eyeballs
(exophthalmia) is a symptom of
hyperthyroidism, when the thyroid
gland produces too much thyroxine.
Goitre
Reduced or impaired production of
thyroxine by the thyroid gland can
cause it to enlarge to form a goitre.
Hormones vary in the distance they travel:
Endocrine hormones are
distributed widely to target
cells throughout the body.
Paracrine hormones act
locally on neighbourhood
cells.
Autocrine hormones act on
the cell that secreted them
or cells of the same type.
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Contact-dependent
signalling requires cells to
be in direct contact.
Nervous System
A typical neurone consists of a cell body where the nucleus is located.
However, extending from this cell body are a variety of projections called
dendrites and axons.
The dendrites are branching extensions of the cytoplasm that
receive signals from adjacent neurones and conduct them towards
the cell body.
A single neurone may have many dendrites.
Extending from the cell body is (usually) a single axon.
Sensory neurones transmit messages from
receptor organs to the CNS.
Motor neurones transmit impulses from the
CNS to the effector organs.
Connector neurones (interconnecting) relay
messages between neurones.
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Axons transmit signals away from the cell body and are capable of transmitting them
to a dendrite on an adjoining cell.
Adjacent neurones do not actually touch each other.
Instead there is a gap between them called a synaptic cleft.
The connection between the axon of one neurone and the dendrite of
another is called a synapse.
The nervous system is organised into the:
 central nervous system (CNS) – the brain and spinal cord
 peripheral nervous system – extending out to the rest of the body.
The CNS coordinates the information received from stimuli and coordinates
the best response.
The electrical impulse
The cell membrane of an axon is polarised (i.e. there is a
potential difference in charge between the inside and the
outside of the cell).
An axon that is at rest (i.e. not transmitting an impulse) is
negatively charged inside relative to the outside.
This is called a resting potential.
When an electrical impulse arrives, the membrane suddenly
becomes permeable to sodium ions which, being about 10
times more concentrated on the outside than in, diffuse in
rapidly.
This depolarises the membrane, reversing the resting
potential.
The inside of the axon is now positive relative to the outside.
This is called an action potential and takes place in a
millisecond.
As the sodium ions enter, the potassium ions begin to leave –
the start of the recovery process.
The sodium–potassium pump mechanism restores
polarisation by pumping out sodium ions and pumping back
in potassium ions to achieve the distribution of ions that
normally exists when the axon is at rest, that is, its resting
potential.
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Neurotransmitters and the Synaptic Cleft
Neurones do not actually touch – there is a gap between them
known as a synapse.
The electrical message is transmitted to the next neurone through
the use of chemical messengers called neurotransmitters.
The end of the synaptic knob is called the presynaptic membrane,
the gap between the knob and the adjoining neurone is called the
synaptic cleft, and the membrane on the far side is called the
postsynaptic membrane.
The synaptic knob contains numerous mitochondria and
secretory vesicles that contain a neurotransmitter.
When an impulse arrives it causes calcium ions to diffuse in from
surrounding tissue fluid. These stimulate some of the secretory
vesicles to migrate towards the presynaptic membrane.
These vesicles merge with the membrane and release their
contents, via exocytosis, into the synaptic cleft.
The neurotransmitter then diffuses across the cleft to bind with specific protein receptors.
This causes protein channels to open up in the membrane, allowing sodium ions to diffuse from the cleft into the postsynaptic
neurone, causing partial depolarisation.
If sufficient channels are opened, an action potential will be initiated.
Once the neurotransmitter has activated the protein channels, it is important that it does not continue to stimulate the post
synaptic neurone. It is deactivated by enzymes and reabsorbed into the synaptic knob of the presynaptic neurone.
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A common neurotransmitter is acetylcholine. It is normally deactivated by the enzyme cholinesterase.
Many insecticides work by destroying the enzyme cholinesterase. In this way, the insect’s muscle cells receive continuous
signals, resulting in muscle spasms.
Paralysis ticks, tiger snakes and other venomous animals produce chemicals that block the production or action of
neurotransmitters at synapses.
Some painkilling tablets take effect because they block the transmission of impulses from pain receptors.
Naturally occurring endorphins in the CNS relieve pain when they are released at times of stress.
‘Recreational drugs’ used by humans affect signal transmission in the nervous system in some way.
Caffeine, amphetamines, ecstasy and cocaine all promote the release of noradrenaline, a hormone released in response to
stress. These drugs act as stimulants.
Opiate drugs, such as morphine, heroin and codeine, block nerve transmission in sensory pathways of the CNS that signal
pain. Thus, they are useful in medicine as effective pain-killers. Opiates, however, are highly addictive and their use is tightly
regulated and sometimes illegal.
Psychedelic drugs, such as mescaline and LSD (lysergic acid), block reabsorption of the neurotransmitter dopamine. This leads
to high levels of dopamine in an area of the brain called the nucleus accumbens, which causes their hallucinogenic effects.
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Communication Between Organisms
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Visual
Olfactory (Smell)
Chemical
o Pheromones – can be social or sexual
Nature of signal
Transduction Pathway
Speed of signal
Duration of Response
Electrical
Chemical
Action Potential
Hormone
Along nerves
Blood and tissue fluid
Very fast
Slow
Short
Long lasting
Communication in Plants
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Plants produce growth regulators or Plant Hormones
No specialised secretory organs or cells are involved
Most cells can secrete the hormones
Hormones travel from cell to cell by diffusion or through the xylem and phloem.
RESPONDING TO LIGHT
 Light is involved in triggering:
o Germination
o Seedling development
o Flowering and fruiting
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Phytochromes are enzymes that are only found in plants
They exist in two forms that interchange according to the light they are exposed to.
In far-red light the enzyme is inactive.
In red light the enzyme is activated and able to catalyse various reactions which then alters the cell’s activity.
Auxin (IAA)
Young leaves
& buds
Abscissic Acid
(ABA)
Gibberellins
Cytokinins
Ethene
(ethylene)
Strong promoter of
growth.Apical
dominance –
suppresses growth of
lateral buds.
Growth inhibitor.
Leaves
Old leaves
Causes stomatal
closure.
Delays leaf fall
Role unclear
Shoot
Delays leaf fall
Promote elongation
just below shoot tip.
Growth
Promotes leaf fall
Promote cell
division.
Found in ripe fruit.
Fruit
Induces fruit fall.
Fruit growth
Induces ripening.
Promotes seed
dormancy.
Seeds
Root tips
Cambium
activity
Breaks dormancy
Produced in root tips
& transported to
areas of growth.
Promotes cell
enlargement and
differentiation.
Seed germination.
Produced in root
tips & travel in
xylem to leaves.
Promotes cell
enlargement and
differentiation.
Page 27
INVADERS
Nature of Disease
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A disease is any condition that interferes with how an organism, or any part of it, functions.
Diseases are described as endemic if they are common in a population but at a low level.
If there is a sudden increase in the incidence of the disease in the population of a species, it is referred to as an outbreak.
Sometimes, outbreaks of disease spread rapidly across the world and these are referred to as pandemics.
Infectious diseases (eg. tuberculosis) are caused by an agent that can be transmitted from one organism to another.
Contagious infection transmitted by direct contact.
Non-infectious diseases cannot be transmitted directly from one person to another.
Pathogens differ in their disease-causing capacity or pathogenicity.
The intensity of the effect of the pathogen is called its virulence.
Individuals vary in their susceptibility to a pathogen – some have greater resistance than others.
Pathogens
Non-cellular Agents as Pathogens
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Viruses - composed of a protein coat and nucleic acid,
either DNA or RNA but never both.
They have no metabolic machinery for processes such
as cellular respiration.
They rely totally on the host cell to provide them with
materials for their reproduction.
Viroids are small infectious particles that consist of a
single circular RNA molecule.
They lack the protein envelope found in viruses and do
not produce any protein products.
Viroids infect plants, but not animals.
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Prions are small infectious proteins
transmissible spongiform encephalopathy (TSE) is fatal
by causing degeneration of the nervous system.
the brains of infected victims have a spongy appearance
due to large, intracellular vacuoles.
The most notorious TSE of late is mad cow disease
(bovine spongiform encephalopathy)
Human TSE, variant Creutzfeldt-Jakob disease (CJD), is
characterised by gradual loss of motor coordination,
dementia and paralysis, eventually leading to death.
Prions are unique among pathogenic agents as they do
not possess any genetic material, neither DNA nor RNA.
They exist in two forms: the normal PrPc form and the
disease-causing PrPSc form.
When a PrPSc molecule encounters an unaltered PrPc
form, it converts it to the harmful form, which in turn
converts others.
The PrPSc forms aggregate to form long filaments that
gradually damage neural tissue.
Cellular Agents as Pathogens
BACTERIA
Bacteria can be transmitted from one host to another in a number of ways:
 by direct contact; in food and water; and in droplets of moisture in the air.
 biting insects, such as mosquitoes and fleas, can also transfer bacteria on their biting parts.
Once inside the host the bacteria divide rapidly.
Some damage host tissues directly, while others produce powerful toxins - their own metabolic wastes – that disrupt the
functioning of cells; for example, the toxins produced by the diphtheria bacteria in the throat have their effect on tissues
throughout the body.
Other bacterial strains interfere with the host’s immune system, making the host susceptible to other pathogens.
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FUNGI
The fungal world includes
o mushrooms and toadstools
o unicellular yeasts and moulds.
Like bacteria, not all fungi cause disease.
Many fungi play important roles as decomposers in ecosystems, helping to recycle valuable nutrients back to the plants.
Fungi have also been used extensively in the production of foods, such as bread and alcohol.
Microscopic fungi are generally larger than bacteria and they are eukaryotes.
They reproduce using spores and possess cell walls, like plants. However, the fungal cell wall is made of a different
polysaccharide, chitin, rather than cellulose.
They do not possess chlorophyll and are heterotrophic.
The main body of a fungus consists of fine, thread-like filaments called hyphae, which are collectively called a mycelium.
These filaments secrete digestive enzymes and other chemicals (including some antibiotics) into their surroundings.
These enzymes break down the organic matter and the fungus absorbs the digested products through its cell membrane
These enzymes and other chemicals are the main cause of disease in animals.
Most fungal infections in animals are external, irritating and inflaming the skin and outer body.
Tinea and ringworm are two human fungal diseases.
Thrush (Candida albicans) is a fungal infection that can attack the human body internally.
PROTISTS
Protists are unicellular, eukaryotic organisms.
They reproduce both sexually and asexually.
We do not have effective vaccines against many of them and the drugs
we have are limited in their effectiveness.
In Sydney in 1998 there was an outbreak of Giardia Lamblia, a
flagellated protozoan that caused mild intestinal upsets, such as
diarrhoea, but also had more severe effects on the young or the
elderly.
It is often found in cattle or wild animals and usually leaves their body
in the form of a cyst in the faeces.
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Trypanosoma brucei
o
o
o
o
flagellated protozoan
Causes African sleeping sickness
Fatal when it reaches the brain
Life cycle involves different hosts
Plasmodium and the Anopheles mosquito:
o Malaria is caused by the small
protozoan, Plasmodium.
o Humans (or other hosts) become
infected when bitten by a female
Anopheles mosquito.
The Primary Host is defined as the host in which the adult phase of the parasite produces gametes (humans for malaria).
The Intermediate Host often acts as the vector for the parasite (female Anopheles mosquito for malaria).
ENDOPARASITES
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Life cycle of the blood fluke, Schistosoma japonicum,
which causes schistosomiasis requires a human primary host,
standing in water in which larvae swim, and an aquatic
snail (the secondary host).
The blood flukes reproduce sexually and, after being fertilised,
the eggs leave the human body in faeces and hatch into
ciliated swimming larvae that burrow into a snail and multiply
asexually.
In time, many fork-tailed larvae develop. These leave the snail and swim until they contact human skin, where they bore
inward and migrate to thin-walled intestinal veins, and the cycle begins anew.
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Some tapeworms parasitise humans, with pigs, freshwater fish or cattle as intermediate hosts.
Humans become infected when they eat pork, fish or beef that is raw or improperly cooked and contaminated with
tapeworm larvae.
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Hookworms are especially serious in impoverished parts of the tropics and subtropics.
Adult hookworms live in the small intestine of humans and other animals, where they feed on blood and other tissues.
Adult females, about a centimetre long, can release a thousand eggs a day.
These leave the body in faeces and then hatch into juveniles.
These juveniles may then penetrate the skin of a person walking barefoot.
Inside a host, the parasite travels through the bloodstream to the lungs, where it works its way into the air spaces.
After moving up the trachea, the parasite is swallowed.
Soon it is in the small intestine, where it may mature and live for several years.
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ECTOPARASITES
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Mostly arthropods: fleas, ticks, etc.
Do not cause very serious diseases.
Usually are vectors for endoparasites
Keeping Pathogens Out
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First Line of Defence (Innate):
Barriers to Infection in Plants
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Plants are subject to attack by a huge array of mites, insects, roundworms, fungi, bacteria and viruses
Fungal infections, such as rusts, are very significant in plants.
The hyphae of fungi can penetrate the external surface of the plant and extend into its xylem or phloem.
PHYSICAL BARRIERS
The first line of defence for a plant is to provide a barrier to infection by preventing penetration.
 This may be:
o a thick cuticle or waxy surface of leaves, or the thick bark of stems
o hairs and thorns can deter vectors of particular pathogens.
 Stomatal openings are weak spots as they offer an entry point.
 Many plants have hairs that guard these openings, or have sunken stomata in the leaf that make access difficult.
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CHEMICAL STRATEGIES
Plants possess a second line of defence to combat these pathogens.
Unlike animals, they do not possess a circulatory system that can efficiently transport their defence mechanism, so their
responses tend to be more localised.
Wounds can be quickly plugged by resin.
The plant may shed infected leaves and branches.
Plants can also secrete enzymes and antibiotic-like substances, called phytoalexins.
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DETECTING AND RESPONDING TO INVADERS
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THE LYMPHATIC SYSTEM
Consists of the lymphoid organs and transport vessels that carry a clear fluid called lymph.
The lymph bathes all body cells and drains into lymph vessels, which have valves that control one-way flow.
Unlike the blood circulatory system, there is no pump, and lymph relies on muscle contraction for movement.
Lymph vessels eventually join up with the circulatory system and drain into the bloodstream at the neck.
The blood and the lymph are the two main fluid systems that transport the agents of the immune system.
The lymph and blood have various types of white blood cells that detect foreign material, such as invading pathogens.
Various types of white blood cells, leucocytes, have specific roles to play in the second and third lines of defence.
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Non-Specific Immune Response
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T he second line of defence depends on recognising ‘self ’ from ‘non-self ’.
Invading organisms, such as bacteria or viruses, macromolecules and pollen grains, can all be antigens.
The antigen binds to a receptor site on a component of the immune system.
Antigens are usually proteins or carbohydrates.
In the case of proteins, only a small peptide length may be potentially antigenic.
A small part of a larger molecule that actually binds with the receptor site is called an epitope.
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Our body cells identify themselves to the immune system as ‘self ’ by
marker proteins on the surface of the plasma membrane.
The markers are expressed according to the coded information in genes.
The group of genes that determine these protein markers is called the
major histocompatibility complex (MHC).
Because these MHC markers are determined by the genetic make-up of an
individual, they are unique to that individual.
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ANTIGEN RECEPTORS
Two important membrane-bound receptor molecules for antigens are found in the immune system
o antibodies found on B lymphocytes and
o T-cell receptors found on T lymphocytes.
Each B or T lymphocyte carries about 100 000 receptors for a specific antigen.
Antibody molecules are glycoproteins (immunoglobulins) whose function is to
recognise antigen receptors on the surface of B lymphocytes and, when
secreted by B lymphocytes, to have particular effects in all body fluids.
They are effector molecules.
T-cell receptors (TCRs) are bound to the surface of T cells.
These receptors bind with small peptide lengths, epitopes, which are
displayed on the end of MHC marker molecules.
The binding of an antigen or epitope to a lymphocyte receptor is similar to
that of a substrate binding to an enzyme.
PHAGOCYTES – CELLS THAT ENGULF NON-SELF
The most abundant and first cells on the scene are the neutrophils, which have irregular, multi-lobed nuclei and a
granular cytoplasm.
Macrophages are larger cells, with a non-granular cytoplasm and horseshoe-shaped nucleus.
They are capable of engulfing and digesting a larger range of infectious agents.
They also double up as a cleaner for the body, cleaning up and removing damaged tissue.
Other types of white blood cells involved in non-specific defence include eosinophils and basophils, which, like
neutrophils, have a granular cytoplasm.
Eosinophils secrete powerful enzymes that are capable of producing holes in pathogens, such as blood flukes and
parasitic worms.
Basophils secrete powerful chemicals, including histamines, which cause the blood vessels in that area to dilate and make
their walls more permeable, so that blood plasma and white blood cells can reach the site of infection more easily.
Located in various tissues are mast cells. These cells are closely related to basophils and also secrete histamines.
Other cells called natural killer (NK) cells circulate around the body and destroy infected or abnormal cells by attacking
their cell membranes, causing the cells to burst.
FEVER
Macrophages secrete chemicals known as interleukins.
They send a message to the hypothalamus and set the body’s temperature at a higher point, about 39°C.
The higher temperature can restrict the functioning of many pathogens, making it easier for other components of our
immune system to act.
In addition, this chemical causes drowsiness, thus lowering general body activity and allowing more energy to be used for
destroying the pathogen and repairing damaged tissue.
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INFLAMMATION
Usually involves swelling, redness and pain.
Chemicals such as histamine, promote vasodilation (widening of
vessels, especially arterioles) in this region.
Blood transfers white blood cells and heat to the swollen areas and
they often become very warm.
Histamines change the permeability of blood vessels in the
inflamed area, making it easier for neutrophils, macrophages and
blood proteins to squeeze into the infected tissue.
This causes swelling and some pain, and reduces voluntary
movement in that area.
CHEMICALS THAT ASSIST THE IMMUNE RESPONSE
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The phagocytes are aided by a system of proteins found in the blood plasma called complement proteins secreted by
macrophages, monocytes and other body cells.
These proteins are normally in an inactive form but may become activated when they encounter a foreign body.
Activation of one protein stimulates the activation of many other proteins.
These proteins produce a range of effects for defending the body such as:
o causing the bacterial cell membrane to rupture (lysis)
o binding to the surface of pathogens, thus signalling to phagocytes to engulf them.
Complement proteins and lysozymes are extremely effective against bacteria, but have little effect on viruses.
Some cells infected by viruses secrete interferons which:
o induce resistance to viral infection in the surrounding cells
o are non-specific in their effects
o reduce or slow down the growth, reproduction or development of the virus.
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Specific (Adaptive) Immune Response
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The cells and processes involved in the adaptive immune response are specific and can detect and distinguish between
different types of invaders, attacking only those for which they are specifically designed.
The adaptive immune response not only has specificity but also memory.
This allows the immune system to mount an enhanced defence against a pathogen that infects the host for a second
time.
Lymphocytes are the key cells of the immune system, the body’s specific, third line of defence.
THE B LYMPHOCYTES – HUMORAL IMMUNITY
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The humoral immune response is brought about by B lymphocytes,
or B cells, which produce different types of protein that attack foreign
antigens.
These designer proteins, called antibodies, combine with specific
antigens and help remove them from the body.
They consist of four polypeptide chains arranged in the shape of a Y.
All antibodies are identical except for a region at the two ends of the Y
where two identical binding sites that match up, or are
complementary, to a particular antigen are found.
It is believed that they work with a lock and key system of similar to
enzymes binding with their substrate.
Antibodies are also known as immunoglobulins (Ig) and they occur in
several different classes.
B lymphocytes originate as stem cells and begin to differentiate within the bone marrow.
Each type of B cell divides rapidly to produce many identical cells that are clones of it, all producing the same antibody.
By the time we are born we have a large number of different types of B cells, each with a small number of clones that are
programmed to recognise a specific type of antigen circulating throughout our blood and lymphatic systems.
Each B cell produces a specific antibody, which moves to the plasma membrane of the cell, where it is embedded with the
two arms sticking out, exposing the antigen-binding sites.
Recognition of a specific antigen triggers an amazing response in the B cell, causing it to divide rapidly, forming two types
of B cells: plasma cells and memory cells.
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The plasma cells secrete up to 10 000 molecules of a specific antibody per second into the body fluids, potentially
neutralising the antigen that induced its production in the first place.
Most antibodies will only attack one antigen, but a few will attack a number of antigens if they are closely related, such as
the smallpox and cowpox virus.
The B cells that have been activated will then be in much greater numbers than others.
Thus, the antigen itself selects which of the millions of different B-cell clones becomes active and secretes antibodies.
B lymphocytes persist in the body in fairly large numbers for
some months or even years, not secreting antibodies but
still displaying them mounted on their plasma membrane.
Once activated by the revisiting antigen, the B memory cells
divide and rapidly produce large quantities of antibody,
destroying the pathogen before any symptoms of its
presence arise.
This is why most people only experience chickenpox or
measles once in their lives.
After an initial infection and bout of the disease, they
become immune to future infections.
T LYMPHOCYTES – THE CELL-MEDIATED IMMUNE RESPONSE
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T lymphocytes also originate in the bone marrow but mature in the thymus gland, where they become ‘programmed’ to
recognise the antigen of a specific pathogen.
T lymphocytes do not produce antibodies.
They are able to distinguish self from non-self.
T lymphocytes do not bind with antigens directly but bind with the antigens presented on the MHC markers.
T lymphocytes come in different forms, including:
o T-helper (TH),
o cytotoxic T (TC)
o and T-suppressor cells.
T-helper cells help other cells of the immune system by producing and secreting chemicals, including lymphokines and
interleukins, which induce any activated B or T cell to divide and give rise to large numbers of clones.
The human immunodeficiency virus (HIV) which causes the disease AIDS – acquired immunodeficiency syndrome – is
deadly because it disables the T-helper cells.
Cytotoxic T cells are amazing killers, which can eliminate infected body cells or tumour cells by releasing powerful
cytotoxins when they ‘touch’ a cell that carries an unrecognised antigen.
These chemicals include perforin, a protein that causes holes to form in the target cell’s membrane, and enzymes that
induce cell death. It is these cells that are chiefly responsible for fighting off invading viruses by killing off infected cells.
They can also cause problems for patients requiring organ transplants because they are the primary cause of transplant
tissue rejection as they destroy the transplanted cells directly.
Patients receiving transplants must take high levels of immunosuppressant drugs to help counteract this response so that
the new organ is not destroyed by the immune system.
T-suppressor cells play an important role in regulating the action of lymphocytes and suppressing the action of
phagocytes. In this way, they help prevent the immune system overreacting to a stimulus.
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PREVENTING INFECTION
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The easiest way to treat a disease is to prevent the pathogen from entering the body in the first place.
This can involve:
o quarantining infected organisms
o knowledge of the mode of transmission of a pathogen
o protection against mosquitoes biting
o knowledge of the requirements of a pathogen
o Applying an antiseptic agent to this area can help to kill off pathogens
o Using disinfectants to clean surfaces, such as bench tops (they are too strong to be used directly on skin).
ASSISTING IMMUNITY
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Once inside the body, pathogens are much harder to combat and our immune system may take some time to overcome
the invader.
Medical technology has now been developed to help combat the pathogen while the immune system is being activated.
This may involve:
o administering drugs that are capable of destroying the pathogen directly or by specifically targeting, by means
of chemotherapy, cells that may have been invaded by viruses.
o Antibiotics are a class of drugs that are active against bacteria and may work in a variety of ways against
different bacteria.
 They may break down the cell wall of a bacterium and destroying them.
 They may inhibit enzymes involved with vital metabolic pathways, such as cellular respiration.
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Some antibiotics are very specific in their action, only destroying specific bacterial strains.
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Broad-spectrum antibiotics, however, will act against a wide range of bacteria, including nonpathogenic strains.
ACQUIRED IMMUNITY – ACTIVE
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Vaccines have been developed which work by fooling the immune system.
The antigen of the pathogen itself, contained in the vaccine, stimulates the immune system.
Memory B or T cells are formed, which then sit waiting in the lymph nodes ready to detect and respond to attack the real
pathogen with these antigens as soon as it invades.
A vaccination gives an organism the experience of a particular pathogen’s antigens without the host actually developing
symptoms of the disease itself.
Early vaccines were made up simply of organisms that were similar to the pathogen itself and therefore contained similar
antigens.
Today, vaccines may involve injecting a dead version of the pathogen or a weakened (attenuated) strain that produces
only mild symptoms of disease.
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Disorders of the Immune Response
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Sometimes the system of self-recognition fails and the immune system starts to react against its own tissues.
This gives rise to what is called an autoimmune disease.
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Rheumatoid arthritis is a crippling autoimmune disease that results in chronic
inflammation of skeletal joints.
It arises when T and B cells respond to collagen in the joints which are now
detected as antigens.
Other defence responses, such as the activation of blood complement
proteins and inflammation, also occur, leading to joints filling with fluid,
making them very painful and difficult to move.
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Multiple sclerosis occurs when antibodies develop against the myelin sheath that insulates nerve axons. Sufferers of this
disorder have impaired nerve transmission and movement.
OVERREACTION OF THE IMMUNE SYSTEM
Asthma and hay fever are forms of allergies – exaggerated immune responses to the antigens of normally harmless
substances that may be found on some pollens, foods, cosmetics, drugs or animals.
Symptoms include inflammation and excessive mucus production.
In most cases, allergies are simply annoying but for some they can be life-threatening if they result in anaphylactic shock,
in which inflammatory responses race through the body, leading to constriction of the lung airways and loss of fluid from
capillaries into body tissues.
This latter response is due to high levels of histamines and results in a sudden drop in blood pressure, which may lead to
a heart attack.
Victims of anaphylactic shock need medical treatment urgently to counteract this exaggerated immune response.
IMMUNODEFICIENCY DISEASES
In some people, their immune response is not
sufficient to keep pathogens at bay.
Infection with HIV invades cells of the immune
system itself, rendering the victim vulnerable to a
range of pathogens that are usually easily defeated.
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