Module 2

Eukaryotes:

• Complex (10 - 100 micrometers)                                                       - Large 80S ribosomes
• Animal & plant cells                                                                          - Many memrane-bound organelles
• Linear DNA - has proteins (true chromosomes)
• Nucleus present
• Cellulose cell wall (plant cells)/ Chitin cell wall (fungi)
• Flagella made of microtubule arranged in '9 + 2' formation

Prokaryotes:

• Single-celled organisms (e.g. bacteria)                                             - Small 70S ribosomes
• Smaller & simpler (diameter < 2micrometer)
• No internal membranes (no membrane bound organelles) 
• No nucleus - genetic material free in cytoplasm
• Circular DNA - doesnt have proteins (Naked DNA)
• Polysaccharide cell wall
• Flagella made of flagellin protein arranged in a helix

Nucleus:

• Controls cells activities (by controlling DNA transcription).
  
• Surrounded by nuclear envelope with pores - seperates contents of nucleus from rest of the cell & allows movement of substances between cytoplasm & nucleus.
• Nucleolus - makes ribosomes.
• Chromatin - made of DNA & protein.

Ribosomes:

• Site where proteins are made.
• Found in the cytoplasm & attached to RER.
• Made of proteins & RNA.
• Not membrane bound

Rough Endoplasmic Reticulum:

• Folds and processes proteins.
• System of membranes enclosing a fluid-filled space covered with ribosomes (involved in protein synthesis).

Smooth Endoplasmic Reticulum:

• Sythesises & processes lipids & steriods.
• No ribosomes attached.

Golgi Apparatus:

• Group of fluid-filled membrane bound flattened sacs called cristae.
• Processes & packages lipids & proteins from the SER & RER.
• Makes lysosomes.

Lysosome:

• Membrane bound round organelle.
• No clear internal structure.
• Contains digestive enzymes which digest invading cells & break down worn out cell components.
• Example: - Head of sperm contains lysosome called acrosome which digests a path to the ovum.

Mitochodrion:

• Oval shaped surrounded by a double membrane.
• Outer membrane is smooth.
• Inner membrane folds to form cristae (singular = crista).
• Matrix - soltuion containing enzymes involved in respiration.
• Site of aerobic respiration & where ATP is produced.

Plasma Membrane:

• Regulates the movement of substances in & out of the cell.          - Partially permeable.
• Regulates the internal environment of the cell.                             - Made of lipids, proteins & carbohydrates.
• Has receptor molecules which respond to chemicals (e.g. hormones).

Vesicles:

• Membrane bound fluid-filled sacs found in the cytoplasm.
• Formed by golgi apparatus and endolasmic reticulum.
• Transports substances in & out of cells & between organelles.

Animal cells only: 

• Centriole:
  • Hollow cylinders made of microtubules (straight hollow "scaffolding" structures made of tubulin).
  • Occur in pairs found at right angles to each other.
  • Involved in the formation of spindle fibres during nuclear division.
• Cilia:
  
  • Short hair like structures on the surface of some membranes.
  • '9+2' formation.
  • Move substances along the cell surface.
  • Undulopodium - longer cillium projects out of cell enabling it to move (e.g. sperm).
• Flagellum:
  • Longer projections, found in fewer numbers.
  • '9+2' formation.
  • Microtubules contract to make flagella move, they propel cells forward.

Plant cells only:

• Cell Wall:
  • Made up of cellulose & fully permeable.
  • Rigid - gives cell structure and support.
  • Prevents cell from bursting when water enters.
  • Links neighbouring cells by plasmodesmata (tiny holes with cytoplasm going through them).
• Vacuole:
  
  • Tonoplast (membrane) - controls movement of substances between vacuole & cytoplasm.
  • Cell sap - weak solution containing mineral salts, sugars, enzymes, waste products, pigments, O2+CO2...
  • Added support to the cell wall.
• Chloroplast:
  
  • Double membrane flattened structure containing chlorophyll pigment (green characteristic colour).
  • Thylakoid membranes stacked to form grana.
  • Grana linked by lamellae
  • Site of photosynthesis
  • Starch grains - glucose stored as starch granules used to form starch grains.

Cytoskeleton:

• Network of long thin protein fibres.Support ogranelles keeping them in position.
  • Microfilaments - give support & mechanical strength, keep cellls shape, cell movoment during cytokinesis.
  • Microtubules - made of tubulin, support/shape the cell, tracks along organelles move, form spindle fibres.
• Strengthen & maintain cell shape like scaffolding.
• Movement of materials within cell.
• Cause the whole cell to move.

Protein production:

• Free ribosomes - proteins stay in the cytoplasm.
• RER - proteins excreted or attached to the plasma membrane.
• RER folds and processes proteins.
• Proteins transported to the golgi apparatus.
• Proteins are further processed.
• Packaged and transported around the cell or excreted out the cell by vesicles.

Cell Wall:

• Made of peptidoglycan.
• Not fully permeable (no proteins/nucleic acids).

Ribosome:

• Smaller than in eukaryotes - 70s ribosomes

Circular DNA: (nucleiod)

• Closed loop.
• Also called naked DNA - because it doesnt have protein.

Glycogen Granules & Lipid Droplets: Food reserves.

Mesosome:

• Infolding of plasma membrane involded in cell division & formation of new cell walls.

Capsule:

• Protection - e.g. from white blood cells of the animal cells the bacterium infects.
• Prevent cell drying out.
• Help bacteria attach to other cells.

Plasmids:

• Small circular pieces of DNA - used in genetic engineering (insulin).
• Carry genes helping bacterium survive in unfavourable conditions (antibiotic resistance).

Flagellum:

• Does not have microtubules.                                  - Helps bacterium to move.

Pili:

• Attached to cell wall of capsule.                            - Helps bacteria to attach to tother surfaces/cells.
• Help bacteria to stick to one another when exchanging genetic material during reproduction.

- Magnification: How much bigger the image is than the specimen.

- Resolution: The ability to distinguish between two seperate points.

Magnification = Image size / Object size   =   I / AM.

Units:

• Milimetre:   mm      1cm = 10mm
• Micrometre: um       1mm = 1000um
• Nanometre:  nm      1um = 1000nm

Light Microscope:

• Uses light
• Resolution = 0.2 micrometres 
• Magnification = X1500
• Whole cell/tissue

Electron Microscopes:

• Uses electrons.
• Higher resolution so give more detailed images.
  • Transmission Electron Microscpe:
    • Uses electromagnets to focus beam of electrons which is transmitted through the specimen.
    • Specimen absorbs the electrons - denser parts absorb more.
    • Can only be used on thin specimens.
    • Magnification = X1,000,000
    • Resolution = 0.0002um
  • Scanning Electron Microscope:
    • Beam of electrons scanned across the specimen.
    • Electrons get knocked off from the specimen.
    • Gathered in a cathode ray tube to form an image.
    • 3D images showing surface of the specimen.
    • Magnification = X500,000
    • Resolution = 0.002um

Laser Scanning Confocal Microscope:

• Uses laser beams to scan a spemcimen.
• Specimen tagged with a fluorescent dye.
• The dye gives off light which is focused through a pinhole onto a detector.
• 3D image is generated.
• Out of focus light is blocked out by the pinhole producing a clearer image.
• Can look at different depths in thick specimens.

Staining:

• Light microscopes:
  • Eosin - stains cell cytoplasm
  • Methylene blue - stains DNA.
• Electron Microscopes:
  • Specimen is dipped in a heavy metal solution (e.g. lead).
  • Metal ions scatter electrons creating contrast.

Dry Mount:

• Thinly slice the specimen.
• Use tweezers to pick and place the specimen in the middle of the slide.
• Pop a cover slip on top.

Wet Mount:

• Pipette a drop of water onto the slide.
• Use tweezers to place the specimen on top of the drop.
• Stand the cover slip upright next to the water.
• Tilt & lower, covering the specimen.
• Prevent any air bubbles from forming.
• Place drop of stain next to the edge of the cover slip & a paper towel next to the opposite edge.
• The stain gets drawn under the slip across the specimen.
• Wet mounts are good for looking at tiny organisms that live in water.

• Hydrogen bonds give water a high specific heat capacity:
  • The energy needed to raise the temperature of 1g of a substance by 1 C.
  • Hydrogen bonds absorb a lot of energy so it takes a lot of energy to heat it.
  • Water doesnt experience rapid temperature changes making it a good habitat.
• Hydrogen bonds give water a high latent heat of evaporation:
  • A lot of energy is used when water evaporates.
  • Good for cooling (e.g. sweating).
• Water's polarity makes it very cohesive:
  • Cohesion - attraction between molecules of the same type.
  • Helps water to flow (transpiration stream) - good for transporting substances.
• Water's polarity makes it a good solvent:
  • Ions can dissolve in water as it is polar (completely surrounds the ion).
• Water is less dense when it's solid:
  • Water molecules are held further apart in ice making it less dense.
  • Water forms 4 hydrogen bonds forming a lattice shape when freezes.
  • Ice forms an insulating layer.

• Storage & transport of energy.
• Structural component.

Monosaccharides:

• Galactose - found in dairy products.
• Fructose - found in fruit necter/honey/vegetables.
• Glucose - hexose sugar (6Cs) C6H12O6
  • Alpha glucose:                                                       - Beta glucose:

• Energy source.
• Soluble therefore easily transported

• Ribose - pentose sugar (5Cs)

Disaccharides:

- 2 monosaccharides join during a condensation reaction.

- Water is eliminated & glycosidic bond is formed.

- Water added breaks glycosidic bond in hydrolysis reaction.

Sucrose - A glucose + fructose.

Lactose - B glucose + galactose.

Maltose - A glucose + A glucose.

• Broken down by maltase in hydrolysis.

Condensation Reaction: (A glucose + A glucose)

Polysaccharides:

• • More than 2 monosaccharides join together.
  • Insoluble - doesn.t affect water potential.
• Starch:
  • Main energy storage material in plants.
  • Made of A glucose.
    • Amylose - long unbranched chain of A glucose.
      • Arranged in a helix,coiled structure.
      • Compact - good for storage.
    • Amylopectin - long branched chain of A glucose.
      • Side branches allow enzymes to reach the glycosidic bonds easily releasing glucose quickly.
• Glycogen:
  • Main energy storage material in animals.
  • Shorter/ highly branched/ compact.
• Cellulose:
  
  • Structural support for cells - found in cell walls.
  • Long unbranched B glucose chains form hydrogen bonds with each other forming strong microfibrils.

Triglycerides:

- Soluble in organic solvents.

- Macromolecules mainly used as energy storage molecules in animals and plants.

- Made of 3 fatty acids & 1 glycerol molecule.

- Form ester bonds between each fatty acid & the glycerol molecule by condensation reaction.

- Esterification - synthesis of triglycerides.

- Bonds are broken by hydrolysis reaction.

- Saturated fatty acids - no double bonds.

- Unsaturated fatty acids - at least one double bond causing the chain to kink.

- Long hydrophobic hydrocarbon tails release a lot of energy when broken down.

- Insoluble droplets - hydrophobic tails face inwards sheilding themselves from water with their glycerol  heads so don't cause water to enter the cells by osmosis.

Phospholipids:

- Found in cell membranes - form the phospholipid bilayer.

- 1 fatty acid molecule replaced by phosphate group.

- Phosphate group is hydrophilic.

- Heads are hydrophilic tails are hydrophobic forming a double layer - heads face out to the water.

- Centre of the bilayer is hydrophobic - water soluble molecules cannot pass through.

Cholesterol:

- Hydrocarbon ring structure with hydrocarbon tail.

- Has a polar hydroxyl group (OH).

- Strengthen the cell membrane by interacting with the phospholipid bilayer.

- They bind to the hydrophobic tails packing them closer together making the membrane less fluid & more  rigid.

- Macromolecules made of amino acids.

- Dipeptide - 2 amino acids join together.

- Polypeptide - more than 2 amino acids join together.

- Proteins - made up of 1 or more polypeptides.

Amino acids:

• Carboxyl group (-COOH), amino group (-NH2) & variable group R attached to a carbon atom:

• Undergo condensation reaction forming peptide bonds:

Primary structure: Linear sequence of amino acids in a polypeptide chain held together by polypeptide bonds.

Secondary structure: Hydrogen bonds form between amino acids (amino & carboxyl groups).

• A helix.
• B pleated sheets.

Tertiary structure: Secondary structures coil/ fold further.

• 3D structure.
• Hyrdogen bonds - slightly positively charged hydrogen atoms and slightly negatively charged atoms on R groups.
• Disulfide bonds/ bridges - covalent bonds between 2 cystene sulfur atoms.
• Ionic bonds - between oppositely charged R groups.
• Hydrophobic/ hydrophilic interactions - hydrophobic R groups clump together pushing out hydrophilic R groups affecting how the protein folds into its structure.

Quaternary structure: The final 3D structure of proteins.

• Made of several different polypeptide chains.

Globular Proteins: Round & compact & soluble.

• Haemoglobin:
  • Carries oxygen in red blood cells.
  • Conjugated protein - prosthetic group attached (non - protein group).
  • Made of 2 A & 2 B polypeptide chains.
  • Each chain contains prosthetic group haem.
  • Haem - contains iron which oxygen binds to.
• Insulin:
  • Hormone secreted by pancreas.
  • Helps regulate blood glucose level.
  • Soluble - transported in the blood to  tissues.
  • Consists of 2 polypeptide chains held together by disulfide bonds.
• Amylase:
  • Enzyme - catalyses breakdown of starch in the digestive system.
  • Made of single amino acids chain.
  • Secondary structure contains both A helix & B pleated sheet.
  • Most enzymes are globular proteins.

• Insoluble
• Strong
• Structural proteins & unreactive.

Collagen:

• Found in animal connective tissues (bone/skin/muscle).
• Srong molecule.
• Minerals bind to the protein increasing rigidity.

Keratin:

• Found in external structures (skin/hair/nails)
• Can be flexible (skin) or hard & tough (nails/horns).

Elastin:

• Found in elastic connective tissue (skin/ large blood vessles/ ligaments).
• Elastic - allows tissues to return to original shape after getting stretched.

• Calcium (CA2+):
  • Transmission of nerve impulses.                        - Bone formation.
  • Cofactor for enzymes - blood clotting & muscle contraction.
  • Release of insulin from pancreas.
• Sodium (NA+):
  
  • Generating nerve impulses.     - Muscle contraction.  - Maintanence of pH.
  • Regulating fluid balance - osmotic pressure/water levels.
• Potassium (K+):
  • Generating nerve impulses.                      - Muscle contraction.
  • Active transport across the cell membrane.    - Opening of stomata.
  • Activate enzymes for photosynthesis.
• Hydrogen (H+):
  
  • Transport of O2 & CO2 in blood.               - Regulating blood pH.
    
  • Photosynthesis & respiration.
• Ammonium (NH4+):
  
  • Components of nitrogen cycle.
  • Components of amino acids - proteins.

• Nitrate (NO3-):
  • Component of nitrogen cycle.
  • Supplies nitrogen to plants forming amino acids & proteins.
• Hydrogencarbonate (HCO3-):
  • Buffer - maintain pH of blood.
  • Transport of CO2 in/out blood.
• Chloride (Cl-):
  • Chloride shift - maintain blood pH during gas exchange.
  • Cofactor for amylase.
  • Absorption of water & production of urine.
• Phosphate (PO4,3-):
  • Photosynthesis & respiration.
  • AMP/ ADP/ ATP/ phospholipids/ DNA/ RNA.
  • Strengthen bones.
• Hydroxide (OH-):
  • Regulating blood pH.

• Benedict's test:
  • Reducing sugars - all monosaccharides & some disaccharides.
    • Heat the sample with Benedict's reagent in water bath.
    • Positive test - coloured precipitate produced (blue to...).
  • Non-reducing sugars - e.g. sucrose
    • Heat sample with dilute HCl in water bath.
    • Neutralise with sodium hydrogencarbonate.
    • Benedict's test again:
      • Negative - stay blue (doesn't contain any sugar).
• Test strips - glucose
  • colour chamge = glucose present.
• Iodine test - starch
  • Add iodine dissolved in potassium iodide solution.
  • Present = brown/orange to blue/black.
• Biuret test - proteins
  • Add sodium hydroxide solution then copper (ll) sulfate solution.
  • present - blue to purple.                      - no protein - blue

Emulsion test - lipids:

• Shake with ethanol and pour solution into water.
• Lipid present - milky colour.
• No lipid - clear.

Colorimetry - determine glucose concentration:

• Benedicts test - remove the precipitate
• Use colorimeter
• calibrate curve with the results.

Biosensers - detect chemicals in a solution.

Chromatography - good for seperating & identifying compounds.

Paper chromatography - identify unknown amino acids.

Rf value = distance travelled by spot/ distance travelled by solvent.

• Used to make nucleic acids.
• made from pentose sugar (5C), nitrogenous base & phosphate group.
• Deoxyribonucleic acid (DNA):
  • Deoxyribose - pentose sugar.
  • DNA nucleotide consists of same sugar and phosphate group but different base:
    • Adenine          
    • Thymine
    • Cytosine              
    • Guanine
  • Purine base - 2 carbon-nitrogen rings joined together (adenine & guanine).
  • Pyrimidine base (smaller) - 1 carbon-nitrogen ring (cytosine & thymine).
  • Contains two polynucleotide chains (many nucleotides joined together).
• Ribonucleic acid (RNA):
  
  • Ribose sugar.
  • Uracil instead of thymine as a base.
  • Made up of single polynucleotide chain.

ADP & ATP are phosphorylated nucleotides.

Phosphorylate - add one or more phosphate group.

Adenosine diphosphate (ADP) - adenine base, ribose sugar & 2 phosphate groups.

Adenosine triphosphate (ATP) - adenine base, ribose sugar & 3 phosphate groups.

ATP - provides energy for chemical reactions.

Process:

Synthesised from ADP & inorganic phosphate (Pi).

Uses energy from an energy releasing reaction.

ADP forms phosphate bond forming ATP.

Energy is stored in the bond, releasing it when broken back down to ADP & Pi.

• Nucleotides join together to form polynucleotides.
  • Form phosphodiester bond between sugar and phosphate group.
  • Sugar-phosphate backbone - chain of sugars and phosphates.
• DNA double-helix - 2 twisted antiparellel polynucleotide strands.
  • Join together by hydrogen bonding between the bases - 2 between A+T, 3 between C+G.
  • Complementary base pairing - each base can only join with one particular base:
  • Purine pairs with pyramidine - adenine + thymine/ cytosine + guanine (AT Covent Garden).
• Precipitation reaction - purify DNA.
  • Create solution of detergent, salt & distilled water & add broken up cells.
  • Incubate in water bath (60C) then cool in ice bath.
  • Filter & add protease enzyme then cold ethanol.
  • White precipitate forms.
    • Detergent - breaks down cell membranes.
    • Salt - binds to DNA clumping it together.
    • Temperature - prevent enzyme from working properly and breaking down the DNA.

DNA replicates before cell division - each new cell has the full DNA.

• Semi-conservative replication - half of the strands from original DNA molecule:
  • DNA helicase breaks hydrogen bonds between the polynucleotide strands.
  • Helix unzips forming 2 silngle strands.
  • The original strand acts as a template for the new strand.
  • Free-floating DNA nucleotides join to the exposed bases by complementary base pairing on the original template strands.
  • DNA polymerase joins together nucleotides of the new strand forming sugar-phosphate backbone.
  • New & original strand form hydrogen bonds and twist forming a double helix.
• Accurate - genetic information is conserved each time DNA is replicated.
• Random spontaneous mutations occur.
• Mutation - change to the DNA base sequence.
  • Can alter the sequence of amino acids in a protein.
  • Causes abnormal protein to be produced - may function better/ not work at all.

Gene - sequence of DNA nucleotides that codes for a polypeptide.

Sequence of amino acids in a polypeptide forms the primary structure of a protein.

Different proteins have different number/ order of amino acids.

Order of nucleotide bases in the gene determine the order of amino acids in the protein.

Triplet code - sequence of three bases in a gene code for an amino acid.

Different sequences code for different amino acids.

Sequence of bases in a section of DNA is a template used to make proteins during protein synthesis.

DNA is copied into RNA for protein synthesis:

DNA molecules found in the nucleus but ribosomes (make proteins) found in cytoplasm.

DNA too large to move out the nucleus so a section is copied onto mRNA (transcription).

mRNA leaves the nucleus & joins with a ribosome in the cytoplasm to be used to synthesise a protein (translation).

Messenger RNA (mRNA):

• Made in the nucleus.
• Codon - three adjacent bases.
• Carries the genetic code from DNA in the nucleus to the cytoplasm where it's used to make a protein during translation.

Transfer RNA (tRNA):

• Found in the cytoplasm.
• Has an amino acid binding site & a sequence of 3 bases (anticodon).
• Carries the amino acids used to make proteins to the ribosome during translation.

Ribosomal RNA (rRNA):

• Forms the two subunits in ribosomes.
• The ribosome moves along the mRNA strand during protein synthesis.
• rRNA helps to catalyse the formation of peptide bonds between the amino acids.

Codons & anticodons - triplets.

Genetic code - the sequence of base triplets (codons) in DNA or mRNA which codes for specific amino acids.

Each base triplet is read in sequence.

Seperate from the triplet before & after it.

Genetic code is non-overlapping - base triplets don't share their bases.

Genetic code is degenerate:

- There are more combinations of triplets than there are amino acids.

- 20 amino acids - 64 possible triplets.

- Some amino acids coded for by more than one base triplet.

Start & stop codons - used to tell the cell when to start & stop protein synthesis (UAG - stop signal).

Found at the beginning & end of the gene.

Genetic code is universal - same base triplets code for the same amino acid in all living things.

First stage - Transcription:

An mRNA copy of a gene (section of DNA) is made in the nucleus.

1) RNA polymerase attaches to the DNA double-helix at the beginning of a gene.

2) The hydrogen bonds break seperating the strands & the DNA molecule uncoils at that point.

3) One of the strands is used as a template to make an mRNA copy.

4) RNA polymerase lines up free RNA nucleotides alongside the template strand by complementary base pairing.

5) Once paired up the RNA nucleotides are joined together forming an mRNA molecule.

6) RNA polymerase moves along the DNA assembling the mRNA strand.

7) Hydrogen bonds reform between the DNA strands once RNA polymerase has passed by, coiling back into  a double-helix.

8) RNA polymerase stops making mRNA, detaching from the DNA once stop codon has been reached.

9) mRNA moves out the nucleus through a nuclear pore attaching to a ribosome in the cytoplasm.

Second stage - Translation:

Occurs at the ribosomes in cytoplasm.

Amino acids are joined together forming a polypeptide chain (protein) following the sequence of codons carried by the mRNA.

• mRNA attaches to a ribosome
• tRNA molecules carry amino acids to the ribosome.
• tRNA molecule with an anticodon complementary to the start codon on the mRNA attaches to mRNA by complementary base pairing.
• Another tRNA molecule attaches to the next codon the same way on the mRNA.
• rRNA in the ribosome catalyses formation of peptide bonds between the 2 amino acids attached to tRNA molecules joining them together.
• The first tRNA molecule moves, leaving the amino acid.
• Third tRNA molecule binds to the next codon on the mRNA, its amino acid binds to the first two & the second tRNA moves away.
• Process continues until stop codon on mRNA molecule is reached.
• The polypeptide chain moves away from the ribosome & translation is complete.

• Enzymes - biological catalysts that increase the rate of a chemical reaction without being changed/ used up.
• Globular proteins - 3D shape.
• Convert substrate into product.
• Active site - specific shape only allowing certain substrate molecules to bind.
• Substrate temporarily binds to the active site before being converted into products.
• Enzyme - substrate complex:
  • Amino acids that make up the enzyme and active site have R-groups.
  • Form weak interactions with the substrate molecule forming enzyme-substrate complex.
• Induced fit model - enzyme changes shape slightly to allow the substrate to fit closely in the active site.
• Lock & key model - substrate binds to active site without the enzyme changing shape.
• Intracellular: catalyse reactions inside cells
  • Catalase - catalyses breakdown of hydrogen peroxide (toxic by-product) into O2 & H2O.
• Extracellular: catalyse reactions outside cells
  • Pepsin - breaks down proteins into amino acids.
  • Amylase - in saliva secreted by salivary glands. Catalyses hydrolysis of starch into maltose in the mouth.
  • Trypsin - produced in the pancreas, secreted into the small intestine.
    • Catalyses hydrolysis of peptide bonds, making large polypeptides small.

• Catabolic reactions: 1 substrate converted into 2 or more products.
  • Bond breaking in the substrate.
  • Substrate molecule binds to active site of the enzyme.
  • Converted into products by breaking bonds within it.
  • Product molecules released from active site of enzyme.
  • Example - break down of lipids into glycerol & fatty acids by lipase.
• Anabolic reactions: 2 substrate molecules joined to give 1 product.
  • 2 substrate molecules bind to active site of the enzyme.
  • Substrate molecules converted to a product by forming bonds between them.
  • Product molecule released from active site of the enzyme.
  • Example - DNA polymerase during DNA replication.
• Activation energy: - amount of energy needed for a reaction to occur.
  • Enzymes lower the activation energy needed increasing rate of reaction.
  • Formation of enzyme-substrate complex lowers the activation.
  • Anabolic reaction - substrates held closer together by enzyme, reducing repulsion allowing bond to form easily.
  • Catabolic reaction - fitting into active site puts strain on the bonds allowing molecule to break easily.

• Temperature:
  • Rate increases as temperature increases...:
    • Molecules have more kinetic energy and move faster.
    • Enzymes & substrates more likely to collide.
    • Energy of collisions increase so more successful collisions.
  • ... but up to a certain point:
    • Rise in temperature makes enzymes vibrate more.
    • Vibration breaks some bonds holding the enzyme in shape.
    • Active site changes shape - substrate can no longer fit.
    • Enzyme becomes denatured - no longer function as a catalyst.
  • Temperature coefficient (Q10): - shows how rate changes when temperature increases by 10C.
    • Before the optimum - value 2 = rate doubles, value 3 = rate trebles.
• pH:
  • Optimum pH value - usually pH 7 (pepsin - pH2).
  • H+ & OH- ions can interfere with ionic & hydrogen bonds holding enzymes tertiary structure in place.
  • Interference makes active site change shape, denaturing the enzyme.

• Enzyme concentration:
  • More enzyme molecules there are in a solution the more likely a substrate molecule will colide with it forming an enzyme-substrate complex.
  • Increasing enzyme concentration increases rate of reaction.
  • If the amount of substrate is limited, an increase in emzyme concentration will have no further affect on the rate.
• Substrate concentration:
  • Higher the substrate concentration faster the reaction.
  • More substrate molecules.
  • Collision with enzymes more likely.
  • More active sites will be used.
  • Only up until a saturation point - all the active sites become full.
  • Increasing substrate concentration will have no affect on the rate.
  • Substrate concentration decreases with time during a reaction.
  • If no other variables are changed, the rate of reaction will decrease over time.
  • Makes the initial rate of reaction the highest rate of reaction.

Cofactors & coenzymes essential for enzymes to work.

Cofactor - inorganic molecule (chloride & calcium ions).

Help enzyme and substrate bind together.

Don't directly participate in the reaction - aren't used up/ changed.

Example: Chloride ions (Cl-) cofactors for amylase.

Coezyme - organic complex molecule (coenzyme A).

Participate in the reaction and get changed.

Act as carriers - moving chemical groups between enzymes.

Continually recycled.

Prosthetic group - cofactor tightly bound to the enzyme.

Example - Zn2+ cofactors for carbonic anhydrase - permanent part of enzyme's active site.

Competitive inhibition: Reversible

Competitive inhibitor molecules have a similar shape to substrate molecules.

The inhibitor binds to active site of enzymes in direct competition substrate molecules.

Blocks the active site preventing substrate molecule to fit in it.

Inhibitor only binds to the active site briefly.

Effect of inhibitor not permanent.

Concentration of inhibitor greater than substrate - rate of reation decreases.

Can be reversed - increae substrate concentration.

Concentration of substrate greater than inhibitor at the start of the reaction - Inhibitor will not affect the rate.

• Non-competitive inhibition: Irreversible
• Inhibitor binding permanently to active site of enzyme:
  • Example - Penicillin antibiotic blocks active site of a bacterial enzyme, essential in making its cell wall.
• Inhibitor molecule binding permanently to enzyme away from its active site:
  • Binds to allosteric site of enzyme causing active site to change shape preventing substrate molecule from binding.
  • Don't compete - they are a different shape.
  • Increasing substrate concentration will have no affect on reaction rate - enzyme will still be inhibited.
• Inhibition irreversible - strong covalent bonds between inhibitor & enzyme.
• Inhibition reversible - weaker hydrogen & ionic bonds between inhibitor & enzyme.
• Non-competitive inhibition: Reversible (End-product inhibition)
• Metabolic pathways (series of connected metabolic reactions) regulated by end-product inhibition.
• Product of first reaction takes part in second reaction etc.
• Product inhibition - Enzyme inhibited by the productof the reaction they catalyse.
• End-product inhibition - final product in metabolic pathway inhibits enzyme acting earlier on in the pathway.
• Helps regulate the pathway & controlling amount of end-product produced.
• Reversible - level of product drops, inhibition decreases, enzyme starts to function again, more product made.

• Enzyme inhibition can help to protect cells.
• Sometimes enzymes synthesised as inactive precursors in metabolic pathways
• Prevent damage to cells - proteaseses snthesised as inactive precursors to stop damaging proteins in the cell.
• Part of precursor molecule inhibits its actions - once removed the enzyme becomes active again.
• Some medicinal drugs are enzyme inhibitors:
  • Antiviral drugs (stop HIV viruses) - e.g. reverse transcriptase inhibitors inhibit reverse transcriptase enzyme (catalyse replication of viral DNA).
    • Prevents the virus from replicating.
  • Antibiotics - e.g. penicillin inhibits tramspeptidase enzyme (catalyse formation of proteins in bacterial cell walls).
    • Weakens the cell wall preventing regulation of osmotic pressure resulting in cell bursting, killing the bacterium.
• Metabolic poisons interfere with metabolic reactions (enzyme inhibitors) causing damage/ illness/ death:
  • Cyanide - irreversible cytochrome c oxidase inhibitor (catalyses respiration reactions).
    • Cell can't respire so die.
  • Malonate - inhibits succinate dehydrogenase (catalyses respiration reactions).
  • Arsenic - inhibtis pyruvate dehydrogenase (catalyses respiration reactions).

• Plasma membranes: membranes at the surface of cells
  • Barrier between cell and its environment
  • Control which substances enter/ leave the cell.
  • Partially permeable - let some molecules through but not others.
  • Allow recognition by other cells & cell communication.
• Membranes within cells:
  • Barrier between organelle and cytoplasm - makes different functions more efficient.
  • Form vesicles to transport substances around the cell.
  • Partially permeable - control which substances enter/ leave the organelle.
  • Membranes within organelles - barriers between membrane contents & rest of the organelle (thylakoid membranes).
  • Site of chemical reactions - e.g. inner membrane of mitochondria contains enzymes for respiration.
• Fluid mosaic structure:
  • Phospholipid molecules - form fluid (phospholipids constantly moving) bilayer (double layer). (7nm thick)
  • Cholesterol molecules - within bilayer.   - Protein molecules - scattered through bilayer.
  • Glycoproteins - polysaccharide chain attached to protein.
  • Glycolipids - polysaccharide chain attached to lipid.

Phospholipids: barrier to dissolved substances

• Phospholipid head - hydrophilic (attracts water).
• Phospholipid tail - hydrophobic (repels water).
• Automatically arrange themselves into a bilayer - heads face out towards water on either sides.
• Centre of bilayer is hydrophobic - membrane stops water-soluble substances through it, acting as a barrier. (ions/polar molecules).
• Lipid-soluble sustances/ small molecules can pass through. (alcohol/ fat-soluble vitamins).

Cholesterol: gives membrane stability

• Type of lipid present in all cell membranes (except bacterial cell membranes).
• Fits between phospholipds - bind to hydrophobic phospholipid tails
• Cause them to pack more closely together making membrane less fluid & more rigid.
• Have hydrophobic & hydrophilic parts - makes sure ions/polar molecules don't get through.

Proteins: control what enters and leaves the cell

• Intrinsic proteins - transport proteins found embeded across the membrane.
• Extrinsic proteins - found embedded in one half of the membrane used for cell recognition with a glycoprotein attached.
• Have polar & non-polar parts - help to associate with the lipid bilayer.
• Carrier proteins & channel proteins.
• Can be enzymes help catalyse reactions, found on the cell membrane.

Glycoproteins & Glycolipids: receptors for messenger molecules

• Stabalise the membrane - form hydrogen bonds with surrounding water molecules.
• Sites where drugs/ hormones/ antibodies bind.
• Receptors for cell signalling.
• Antigens - cell surface molecules involved in the immune response.

• Cells communicate with each other using messenger molecules.
  • One cell releases a messenger molecule (e.g. hormone).
  • Travels in the blood to another cell.
  • Messenger molecule is detected by the cell  - binds to a receptor on its cell membrane.
• Cell membrane receptors:
  • Proteins in the cell membrane act as receptors - membrane-bound receptors.
  • Have specific shapes - only messenger molecule with complementary shape can bind.
  • Different cells have different receptors - responding to different messenger molecules.
  • Target cell - cell that responds to a particular messenger molecule.
  • Example: Glucagon is a hormone released when there isn't enough glucose in the blood. It binds to receptors on liver cells causing them to break down glycogen into glucose.
• Drugs also bind to cell membrane receptors:
  • Drugs work by binding to receptor molecules in cell membranes.
  • Trigger a response in the cell/ block the receptor preventing it from working.
  • Example: Antihistamines:
  • Cell damage causes the release of histamine. It binds to receptors causing inflammation. Antihistamines block histamine receptors preventing histamine from binding therefore stopping inflammation.

• Increasing temperature inceases membrane permeability:
  • Below 0C:
    • Phospholipids don't have enough energy - can't move much - packed closely together, rigid membrane.
    • Channel & carrier proteins in the membrane deform, increasing permeability of the membrane.
    • Ice crystals may form peircing the membrane making it highly permeable when it thaws.
  • 0C-45C:
    • Phospholipids can move around & aren't tightly packed together - partially permeable.
    • Temperature increases phospholipids move more, increasing permeability.
  • Above 45C:
    • Phospholipid bilayer starts to melt becoming more permeable (breal down).
    • Water expands inside cell putting pressure on the membrane.
    • Channel & carrier proteins deform unable to control substances entering/leaving the cell, increasing permeability.
• Changing the solvents affects membrane permeability:
  • Surrounding cells in solvents (ethanol) increases permeability - lipids dissolve, membrane loses structure.
  • Increasing concentration of solvent increases membrane permeability.
  • Some solvents increase permeability more than others e.g. ethanol more than methanol.

Diffusion: the net movement of particles from an area of higher concentration to an area of lower concentration.

• Molecules diffuse both ways - net movement to the area of lower concentration.
• Continues until particles are evenly distributed throughout.
• Concentration gradient - path from area of high concentration to low concentration.
• Particles diffuse down a concentration gradient.
• Diffusion is a passive process - no energy needed for it to occur.
• Small non-polar molecules (O2/CO2) are able to easily diffuse through spaces between phospholipids.
• Water is small enough to fit between phospholipids - diffuses across plasma membrane even though it's polar (osmosis).

Factors affecting rate of diffusion:

• Concentration gradient - higher it is the faster the rate of diffusion.
• Thickness of exchange surface - thinner the exchange surface the faster the rate of diffusion (shorter the distance of travel for particles).
• Temperature - warmer it is the faster the rate of diffusion, particles have more kinetic energy so move faster.

• Facilitated diffusion: (passive process - doesn't use energy)
  • Larger molecules (amino acids/ glucose), ions & polar molecules don't diffuse directly through the phospholipid bilayer.
  • Diffuse through carrier proteins/ channel proteins in plasma membrane by facilitated diffusion.
  • Moves particles down a concentration gradient.
    
  • Carrier proteins: move large molecules in/out of cell down a concentration gradient.
    • Different carrier proteins facilitate diffusion of different molecules.
    • Large molecule attaches to carrier protein in the membrane.
    • Protein changes shape using energy provided by ATP releasing the molecule on the opposite side of the membrane.
  • Channel proteins: form pores in the membrane for charged particles to diffuse through down a concentration gradient. Different channel proteins facilitate diffusion of different charged particles.
• Active transport: moves substances against a concentration gradient
  • Uses energy to move molecules/ ions across plasma membranes against a concentration gradient involving carrier proteins.
  • Molecule attaches to carrier protein which changes shape using energy provided by ATP releasing the molecule on the opposite side of the membrane.

• Endocytosis: Take in substances
  • Some molecules too large to be taken into the cell by carrier proteins.
  • Cell surrounds the substance with a section of its plasma membrane (invagination of the cell membrane).
  • Membrane pinches off to form vesicle inside the cell containing the ingested substance.
  • Endocytosis uses ATP for energy.
  • Phagocytosis - how a cell takes in solid material in bulk.
    • Example: Amoeba - how they feed by forming food vacuoles around the substance they're engulfing.
    • White blood cells (phagocytes) - how they engulf foreign cells (bacteria).
  • Pinocytosis - how a cell takes in liqiud material in bulk (forms pinocytic vesicles).
• Exocytosis: secrete substances
  • Substances (hormones/ lipids/ digestive enzymes) need to be released.
  • Vesicles containing the substance pinches off from golgi apparatus sacs & move towards the plasma membrane.
  • Vesicles fuse with the plasma membrane releasing the contents outside the cell.
  • Some substances are inserted directly into the plasma membrane (membrane proteins).
  • Exocytosis uses ATP for energy.
    • Example: Pancreas - release of digestive enzymes.

• Osmosis: diffusion of water molecules from an area of high water potential to an area of low water potential through a partially permeable membrane down a water potential gradient. (passive process).
• Water potential - the tendency of water molecules to move from one area to another.
• Dilute solutions have a high water potential, concentrated solutions have a low water potential.
• Pure water has the highest water potential (value of 0) - all solutions have a lower water potential than water.
• Animal cells: Hypotonic solution - solution with a higher water potential than the cell - net movement of water molecules into the cell - cell bursts.
  • Isotonic solution - solution with the same water potential as the cell - water molecules pass in/out of cell in equal amounts - cell stays the same.
  • Hypertonic solution - solution with a lower water potential than the cell - net movement of water molecule is out of cell - cell shrinks.
• Plant cell: Hypotonic solution - net movement of water is into cell - vacuole swells and pushes against the cell wall along with cytoplasm - cell becomes turgid (swollen).
  • Isotonic solution - water molecules move in/out of cell in equal amounts - cell stays the same.
  • Hypertonic solution - net movement of water is out of cell - cell becomes flaccid (limp) - cytoplasm and the membrane pull away from the cell wall - Plasmolysis.

The cell cycle is the process that all body cells in multicellular organisms use to grow and divide.

Starts - when a cell has been produced by cell division

Ends - by the cell dividing to produce two identical cells.

Cell consists of two phases:

Interphase - a period of cell growth & DNA replication (when cell grows and gets all the nutrients needed for the next stage, mitosis).

M phase - a period of cell division (cell splits into two).

M phase involves mitosis (nuclear division) & cytokinesis (cytoplasmic division).

Interphase (cell growth) is subdivided into 3 seperate growth stages (G1/ S/ G2).

Cell cycle regulated by checkpoints occuring at key points of the cycle to:

Prevent uncontrolled cell division leading to cancerous cell developing into a tumor.

Find & repair damage to the DNA (e.g. due to U.V. radiation).

Interphase: 95% of the cycle

• Gap phase 1 (G1):
  • Cells grow, increasing in size.
  • Gene transcription & production of RNA takes place.
  • Duplication of organelles & protein synthesis takes place making necessary proteins needed for DNA replication.
• G1 checkpoint - checks chemicals needed for replication are present & for any damage to the DNA before entering S phase.
• Synthesis phase (S):
  • Duplication of DNA occurs - new cells will have exact copies of DNA.
  • DNA that makes up each chromosome is copied.
  • The DNA copies remain attached to each other by centromeres.
  • The chromatids will be excact copies of each other with the same genes.
• Gap phase 2 (G2):
  • Further growth of the cell & proteins needed for cell division are made.
• G2 checkpoint - cell checks whether all the DNA has been replicated without any damage.

M phase: mitosis & cytokinesis.

• Asexual reproduction for some plants/ animals/ fungi - needed for cell growth/ repairing damaged tissues.
• Interphase: cell's DNA unravelled & replicated - double its genetic content.
  • Organelles replicated & ATP content increased.
• Prophase (picture): Chromosomes condense getting shorter/ fatter (now made of two chromatids).
  • Centrioles move to opposite ends of the cell forming spindle fibres (network of protein fibres).
  • Nuclear envelope breaks down, chromosomes lie free in the cytoplasm & nucleolus disappeaers.
• Metaphase (middle): Chromosomes line up along the equator of the cell & attach to the spindle fibres by the centromere.
  • Metaphase checkpoint - checks that all chromosomes are attached to the fibres.
• Anaphase (apart): Centromeres divide seperating each pair of sister chromatids.
  • Spindles contract, pulling chromatids to opposite ends of the cell.
• Telophase (two): Chromatids reach opposite poles of the cell & spindle ribres break down.
  • They uncoil & become long & thin again (decondense) - now called chromosomes.
  • A nuclear envelope forms around each group of chromosomes - 2 nuclei.
• Cytokinesis: Begins in anaphase & ends in telophase - seperate process to mitosis.
  • Cytoplasm divides & a cleavage furrow forms in animal cells to divide the cell membrane.
  • 2 genetically identical daughter cells produced.

• Takes place in all body cells apart from gametes or sex cells.
• 1 parent cell divides to give 2 new daughter cells (identical to each other/ parent cell).
• Parent cell has 46 chromosomes (diploid).
• Mitosis produces diploid cells.

Growth:

• Animals - growth of muscle/ bone.
• Plants - growth of roots/ leaves/ stem/ shoot/ fruit etc.
• Replacement of cells: e.g. bone marrow producing red blood cells by mitosis.

Asexual reproduction:

• Involves one parent cell - no mating/ fertilisation.
• Produces clones.
• Examples: Amoeba is a unicellular organism that reproduces asexually using mitosis.
• Plants - Strawberries/ potatoes.

Sexual reproduction - two gametes (egg & sperm cell) join together at fertilisation to form a zygote.

Zygote then divides & develops into a new organism.

Meiosis happens in the reproductive organs to produce gametes.

Involves reproduction division.

Cells that divide have the full number of chromosomes to start with but form cells with half the number of chromosomes (haploid).

Cells formed are genetically different as each new cell ends up with a different combination of chromosomes.

Homologous chromosome pairs:

• 46 chromosomes in total - 23 pairs.
• 1 chromosome in each pair come from mum & dad (2 no.1's, 2 no.2's...)
• The chromosomes that make up each pair are the same size & have the same genes, although they could have different versions of those genes (alleles).
• These pairs of chromosomes are called homologous pairs.

• Interphase: DNA unravels & replicates producing double-armed chromosomes (sister chromatids).
• Prophase 1: Chromosomes condense, getting shorter & fatter.
  • Arrange themselves into homologous pairs forming bivalents.
  • Each chromosome of homologous pair is made up of 2 sister chromatids.
  • A chromatid of 1 of the chromosomes in a pair crosses over with a chromatid of the other chromosome forming a chiasma. Sections get swapped which mixes up alleles.
  • Centrioles move to opposite ends of the cell forming spindle fibres & nuclear envelope breaks down.
• Metaphase 1: The bivalents line up at the equator of the cell & spindle fibres attach to their centromeres.
  • These pairs are arranged randomly (independent assortment of chromosomes).
• Anaphase 1: Spindle fibres contract seperating the pairs - one chromosome goes to each end of the cell.
• Telophase 1: Nuclear envelope forms around each group of chromosomes.
  • The chromosomes decondense.
• Cytokinesis 1: Division of cytoplasm occurs producing 2 haploid daughter cells.
  • Centrioles divide forming new spindle fibres ready for meiosis 2.

Prophase 2: The chromosomes condense becoming visible again.

Spindle fibres form and the nuclear envelope breaks down.

Metaphase 2: The chromosomes line up at the equator.

The chromatids of each chromosome are randomly assorted.

The way they're assorted will decide how the chromatids seperate during anaphase 2.

Anaphase 2: The centromeres are pulled apart which splits the chromosomes into its chromatids.

The chromatids are moved to opposite poles & become chromosomes.

Telophase 2: The chromosomes at the poles decondense & nuclear envelopes are formed around them seperating the chromosomes into seperate nuclei.

Cytokinesis 2: The 2 cells undergo cytokinesis forming 4 haploid daughter cells.

Each daughter cell contains 2 chromatids (now chromosomes).

Each chromatid came form one of the homologous pairs of chromosomes in the parent cell.

• Crossing over in prophase 1:
  • Homologous pairs of chromosomes pair up.
  • The chromatids twist around each other & sections of chromatids swap over at the chiasmata.
  • The chromatids still contain the same genes but now have a different combination of alleles.
  • Each of the 4 daughter cells formed contains chromatids with different alleles increasing genetic variation in offspring.
• Independent assortment of chromosomes:
  • Each homologous pair of chromosomes made from 1 maternal chromosome & 1 paternal chromosome.
  • When the homologous pairs line up in metaphase 1 & seperated in anaphase 1, it's completely random which chromosome from each pair ends up in which daughter cell.
  • The 4 daughter cells have completely different combinations of the maternal/ paternal chromosomes.
  • Independent assortment of chromosomes leads to genetic variation in offspring.
• Independent assortment of chromatids during metaphase 2:
  • Random arrangement of sister chromatids on the equator determines how they seperate in anaphase 2.
• Random mutation:
  • Changes in DNA which can occur in interphase.
  • Mutations increase genetic variation.

• Stem cells: unspecialised cells
  • They can develop into different types of cell.
  • Found in early embryos - can develop into any type of human cell.
  • Divide to become new cells which then become specialised (differentiation).
  • Adult stem cells used to replace damaged cells.
  • Can divide to produce more undifferentiated stem cells.
• Cells in bone marrow differentiate into blood cells:
  • Bones are living organs - containing nerves & blood vessels.
  • Main bones of the body have marrow in the centres.
  • Adult stem cells divide & differentiate to replace worn out blood cells - erythrocytes (red blood cells)/ neutrophils (white blood cells that help to fight infection).
• Cells in meristem differentiate into xylem & phloem:
  • In plants stem cells are foundin the meristems.
  • In the root and stem, stem cells of the vascular cambium divide & differentiate to become xylem & phloem sieve tubes.

Stem cells can develop into different specialised cell types - used to replace damaged tissues in diseases.

Alzheimer's disease:

With Alzheimer's the nerve cells in the brain die in increasing numbers.

This results in severe memory loss.

Researchers are hoping to use stem cells to regrow healthy nerve cells in peaople with Alzheimer's.

Parkinson's disease:

Patients with Parkinson's suffer from tremors that they can't control.

The disease causes the loss of a particular type of nerve cell found in the brain.

These cells release the chemical dopamine, which is needed to control movement.

Transplanted stem cells may help to regenerate the cells.

• Neutrophils: type of white blood cell - defend the body against disease.
  • Flexible shape - allows them to engulf foreign particles.
  • Contain many lysosomes in cytoplasm containing digestive enzymes - break down engulfede particles.
• Erythrocytes: red blood cells - carry oxygen in the blood.
  • Biconcave disc shape - large surface area for gas exchange.
  • No nucleus - more room for haemoglobin (protein carrying oxygen).
• Epithelial cells: cover surfaces of organs.
  • Cells joined by interlinking cell membranes & membrane at their base.
  • Ciliated epithelial - cilia ceat to move particles away (in airways).
  • Squamous epithelial - very thin to allow efficient gas diffusion (in lungs).
• Sperm cells:
  • Flagellum - swim to the egg.
  • Mitochondria - provide enrgy to swim.
  • Acrosome - contains digestive enzymes to enable sperm to penetrate the surface of the egg.

Palisade mesophyl cell: in leaves - do most of the photosynthesis.

• Contain many chloroplasts - absorb a lot of sunlight.
• Thin walls - carbon dioxide can easily diffuse into the cell.

Root hair cell: absorb water & mineral ions from the soil.

• Large surface area - absorption.
• Thin permeable cell wall - for entry of water & ions.
• Extra mitochondria - provide energy needed for active transport.

Gaurd cells: found in pairs.

• Gap between them - form a stoma.
• Tiny pores in the surface - gas exchange.
• Int the light - gaurd cells take up water becoming turgid.
• Thin outer walls & thickened inner walls force them to bend outwards opening it.

• Tissue - group of cells that are specialisde to work together to carry out a particular function.
• Can be more  than one cell type.
• Squamous epithelium: single layer of flat cells lining a surface.
  • Found in many places such as alveoli in lungs.
• Ciliated epithelium: layer of cells covered in cilia.
  • Found on surfaces where substances need to be moved - trachea, moving mucus along.
• Muscle tissue: made up of bundles of elongated cells - muscle fibres.
  • Smooth muscle - linind the stomach wall.
  • Cardiac tissue - found in the heart.
  • Skeletal muscle - which you use to move.
  • All are different structures.
• Cartilage: type of connective tissue.
  • Found in the joints.
  • Shapes & supports the ears, nose & windpipe.
  • Formed when chondroblast cells secrete an extracellular matrix (jelly like containing protein fibres) which they become trapped inside.

Plant tissue:

• Xylem tissue: plant tissue which transports water arpund the plant & provide support.
  • Contains hollow xylem vessel cells (dead) & parenchyma cells (living).
• Phloem tissue: transports sugasrs around the plant.
  • Arranged in tubes - made up of seive cells, companion cells & plant cells.
  • Seive cells have end walls (seive plates) with holes to allow sap move easily through them.
• Organ - group of different tissues that work together to perform a particular function.
• Example: Lungs - squamous epithelial tissue (alveoli) & ciliated epithelial tissue (bronchi) & elastic connective tissue & vascular tissue (blood vessels).
  • Leaves - palisade tissue (photosynthesis) & epidermal tissue (prevent water loss from leaf) & xylem & phloem tissues (veins).
• Organs work together to form organ systems - each system has a particular function.
  • Respiratory system - lungs/ trachea/ larynx/ nose/ mouth/ diaphragm.
  • Circulatory system - heart/ arteries/ veins/ capillaries.