Module 2 ✅

What is the cell cycle process?

• The cell spends most of its time in interphase, which consists of the G1, S and G2 phases

• The next section is mitosis, when the nucleus divides

• Then the cell undergoes cytokinesis, when the cell splits into two genetically identical daughter cells

What happens in the cell during the G1 phase

G1 (1st growth phase of interphase)- the cell’s contents (except for chromosomes) duplicate, and proteins are synthesised

What happens in the cell during the S phase?

S (synthesis phase of interphase)- the DNA in the nucleus replicates, so each chromosome then is made up of two identical sister chromatids joined by a centromere

What happens in the G2 phase?

G2 (2nd growth phase of interphase)- the cell continues to grow and prepares for cell division (eg. produces tubulin protein for spindle fibres)

How is the cell cycle regulated?

Errors that occur in replicated DNA during the cell cycle are found and fixed at checkpoints in the process:

1. G1 checkpoint- after G1, the cell size, growth factors, nutrients and chromosomes are checked- if damage is detected in DNA then the cell enters a resting phase, G0, until it is repaired

2. G2 checkpoint- after G2, the cell size and replicated DNA are checked for errors- cells with incorrectly replicated DNA enter G0

3. Metaphase checkpoint- ensures the chromosomes are correctly attached to the spindle fibres during metaphase, so that anaphase occurs properly

When possible enzymes will repair the error but sometimes the cell destroys itself to prevent passing on harmful mutations

• A cell can also enter G0 when it has become too differentiated to divide further

What happens in prophase during mitosis?

• Chromosomes condense (can be seen as two sister chromatids that are joined together at the centromere when stained)

• Two centrioles move towards opposite poles of the cell

• Spindle fibres (microtubules) begin to form at the centrioles

• The nuclear membrane breaks down

• The nucleolus disappears

What happens in metaphase during mitosis?

• Spindle fibres attach to the centromere of each chromosome

• This attaches the chromosomes to each centriole, and they line up along the equator

What happens in anaphase during mitosis?

• Spindle fibres contract, dividing each centromere in half

• The chromosomes split into their two daughter chromatids

• The chromatids move towards the poles, so that both poles have one half of each original chromosome

What happens in telophase during mitosis?

• Chromosomes (now made up of only one chromatid again) reach each pole and decondense

• Nuclear membranes reform around the groups of chromosomes

• Nucleoli reform

• Spindle fibres break down

What happens in cytokinesis during mitosis?

• In animal cells, the cell membrane pinches and cleaves, dividing the cell into two

• In plant cells, the cell membrane and cell wall form so that the two new cells still remain connected

Why is mitosis important?

• Growth- mitosis produces two daughter cells which are genetically identical to the parent cell, allowing organisms to grow

• Tissue repair- as cells are constantly dying they are replaced by identical cells

• Asexual reproduction- plants, fungi and some animal species can reproduce without a second parent using mitosis

What happens in prophase 1 in meiosis?

• Chromosomes condense (can be seen as two sister chromatids that are joined together at the centromere when stained)

• Two centrioles move towards opposite poles of the cell

• Spindle fibres (microtubules) begin to form at the centrioles

• The nuclear membrane breaks down

• The nucleolus disappears

• The chromosomes are arranged side by side in homologous pairs (forming bivalents)

• As the homologous chromosomes are very close together the crossing over of certain genes at chiasmata between chromosomes may occur- this produces genetic variation

Prophase 2 is the same as prophase in mitosis

What happens in metaphase 1 in meiosis?

• The bivalents (pairs of homologous chromosomes) line up at the equator of the cell

• Spindle fibres attach to the centromere of each chromosome

• The maternal and paternal chromosome in each bivalent line up on either side randomly in an independent assortment- this produces genetic variation

Metaphase 2 is the same as metaphase in mitosis

• There is also independent assortment in metaphase 2, as the chromosomes line up randomly (can have either sister chromatid facing each pole)

What happens in anaphase 1 in meiosis?

• Spindle fibres contract, pulling whole chromosomes to each pole

• The bivalents are split up, so both poles have a version of each chromosome (which still have two chromatids joined by a centromere)

• Meiosis 1 is called reduction division because the chromosome number of resulting cells is halved, due to anaphase 1

Anaphase 2 is the same as anaphase in mitosis

What happens in telophase 1 in meiosis?

• Chromosomes (still made up of two chromatids joined by a centromere) reach each pole

• Nuclear membranes reform around the groups of chromosomes

• Nucleoli reform

• Spindle fibres break down

Telophase 2 is the same as telophase in mitosis

Mitosis vs meiosis process

• There is an interphase stage before meiosis 1 but not before meiosis 2

• Meiosis 1 produces haploid cells, but with chromosomes that have two chromatids

• Meiosis 2 produces haploid cells where the chromosomes have one chromatid

Why is meiosis important?

Sexual reproduction:

• Meiosis produces cells with half the number of chromosomes as in a normal cell, which means it is important for producing gametes

• Gametes, sperm and egg cells, need to be haploid so that they produce a diploid cell when they fuse

Produces genetic variation:

• Crossing over allows the exchange of genes between maternal and paternal chromosomes in a bivalent

• Independent assortment means there are often millions of combinations of alleles that will be passed on as the chromosomes in a bivalent arrange randomly

How are erythrocytes specialised?

Red blood cells transport oxygen and carbon dioxide, so they:

• Contain haemoglobin that binds to oxygen

• Have no nucleus

• Are biconcave and flattened so they have a higher s.a and a short diffusion pathway

• Are small to fit through capillaries

• Contain carbonic anhydrase to transport CO2

How are neutrophils specialised?

White blood cells protect the body from pathogens, so they:

• Have lysosomes that contain digestive enzymes

• Have multi-lobed nuclei so they can fit through small gaps

• Specific receptors on surface

• Flexible shape

How are sperm cells specialised?

Sperm cells fertilise egg cells, so they:

• Have a flagellum for movement

• Have lots of mitochondria to release energy

• Are haploid so they produce a diploid cell after fertilisation

• Have acrosome enzymes at the head to penetrate the egg cell

How is squamous epithelium tissue specialised?

Squamous epithelium lines the lungs, so it:

• Is one cell thick to allow fast diffusion of oxygen into the blood

How is ciliated epithelium tissue specialised?

Ciliated epithelium lines the trachea and fallopian tubes, so it:

• Has cilia that beat in rhythm and sweep particles

• Move bacteria, mucus, etc in trachea to protect lungs

• Move egg cells in the fallopian tubes

• Contain goblet cells that release mucus to trap particles in trachea

How is cartilage tissue specialised?

Cartilage is a connective tissue, so it:

• Contains elastin + collagen

• Is firm but flexible

• Composed of chondrocyte cells in a matrix

How is muscle tissue specialised?

Muscle is used for movement, so it:

• Contains contractile proteins

• Can contract + lengthen to move bones

• Contain many mitochondria for energy

• Can be skeletal, cardiac or smooth

How are palisade cells specialised?

Palisade cells are the main site of photosynthesis, so they:

• Contain many chloroplasts

• Are closely packed

• Have thin cell walls for carbon dioxide diffusion

• Chloroplasts can move towards light

How are root hair cells specialised?

Root hair cells absorb water and mineral ions, so they:

• Have a high surface area to increase diffusion

• Have many mitochondria for active transport

How are guard cells specialised?

Pairs of guard cells form stomata, so they:

• Are very subject to turgidity changes so they can close + open stomata

• Thick inner cell wall

• Control gas exchange and water loss

How is epidermis tissue specialised?

Epidermis covers the surface of plants, so it:

• Has a thick, waxy cuticle to reduce water loss

• Contain stomata

• Clear + thin, so light can still get through for photosynthesis

How is xylem tissue specialised?

Xylem transports water + mineral ions, so it:

• Is made of dead cells hollowed into tubes

• Is strengthened by lignin to withstand the pressure of the transpiration stream

• Only allows movement from roots to shoots

How is phloem tissue specialised?

Phloem transports sucrose by translocation, so it:

• Is made up of living sieve and companion cells

• Has pores between cells to allow sucrose to move

• Has little cytoplasm to allow for more substances to be transported

• Allows movement in both directions

• Has many mitochondria in companion cells for active transport

• Has large vacuole in companion cells for storage

What is totipotency?

• Totipotent stem cells can differentiate into any cell type, and can produce a whole organism

• They are found in zygotes (up to 16 cells)

What is pluripotency?

• Pluripotent stem cells can differentiate into any cell type, but can’t produce a whole organism

• Found in early embryos and the meristem of plants- the apical meristem (cambium) at the tips of roots and shoots differentiates into xylem and phloem tissues

What is multipotency?

• Multipotent stem cells can only differentiate into a few cells within a tissue

• Adult stem cells found in bone marrow can only differentiate into different blood cells (erythrocytes and neutrophils)- they replace these constantly

How can stem cells be used in medicine?

Replacing:

• Damaged heart muscle tissue to treat heart disease

• Insulin-producing cells in the pancreas to treat type 1 diabetes

• Dead dopamine-providing cells in the brain to treat Parkinson’s

• Dead nerve cells in the brain to treat Alzheimer’s

• Damaged retinal cells in the eye to treat macular degeneration

• Blood cells to treat blood diseases like leukaemia

• Damaged cells in the spinal cord to treat paralysis

• Producing cell cultures for drug trials

• Studying developmental biology

differences between RNA and DNA nucleotides?

• RNA is a ribose sugar rather than deoxyribose

• the thymine base is replaced with uracil [still pyrimidine]

Structure of a nucleotide

What are the purine and pyrimidine bases?

Purine: A and G (double ring)

Pyrimidine: T, U and C (single ring)

How is the sugar-phosphate backbone formed?

Condensation reactions forming phosphodiester bonds between the phosphate group from one nucleotide and the pentose sugar of another

• The 5-carbon of one sugar is linked to its phosphate group, which is then linked up to the 3-carbon of the sugar from the next nucleotide

What are the structures of ATP and ADP?

• ATP and ADP (adenine triphosphate and adenine diphosphate) are phosphorylated nucleotides

• They are made up of adenine bonded to a ribose sugar and three (ATP) or two (ADP) phosphate groups

How is ATP used?

• synthesis - of large molecules

• transport - pumping molecules across cell membranes

• movement - protein fibres in muscle cells

• In cells, one of the phosphate groups in ATP is removed by a hydrolysis reaction, to produce ADP and release energy

• This energy is used in cellular processes, mainly synthesis, transport and movement

• ATP is resynthesized from ADP in concentration reactions

• ATP is relatively unstable, but it is a good immediate energy source

What properties of ATP make it a good immediate energy source?

• Small- easy to transport

• Water soluble- can be used in aqueous environments

• Phosphate group bond has energy- enough to be used in reactions, but not so much that energy is wasted

• Easily regenerated- the ATP → ADP hydrolysis reaction is easily reversed in a condensation reaction

What is the structure of DNA?

• Two polynucleotide strands connected by hydrogen bonds between bases, which are antiparallel (run in opposite directions- one is 3’ to 5’ and the other is 5’ to 3’),

• twisted into a double helix

• bases bind - complimentary base pairing [tea for 2]

How can we purify DNA and what does each step do?

1. Add detergent, - breaks down cell membrane releasing cell contents

2. Add salt- breaks hydrogen bonds between DNA and water

3. Add protease - protease enzymes break down the histones that hold DNA together, and the salt neutralises the charge of the phosphate backbone

4. Heat in a water bath- more KE provides a higher rate of reaction, and the heat denatures enzymes in the cells that would break down the DNA further

5. Add ice cold ethanol- DNA is not soluble in alcohol, so adding ice cold ethanol allows the DNA to precipitate on top of the solution. You can then take out this precipitate with a glass rod

What do we call the process by which DNA is copied and why?

Semi-conservative replication, because in each of the two DNA molecules produced, one of the polynucleotide strands comes from the original DNA molecule being copied, and the other strand is new

What is the process of semi-conservative replication?

• The DNA helix untwists

• DNA helicase breaks the hydrogen bonds between the bases, separating the two strands

• Complementary free nucleotides join up with the exposed bases on the strand, by hydrogen bonds

• DNA polymerase catalyses the condensation reactions of phosphodiester bonds between the bases to form a new strand

Why does the DNA replication process have to be very accurate?

• The daughter DNA molecules must be an exact copy of the parent DNA molecule, avoiding mutations where:

  • Bases are inserted into the complementary strand in the wrong order

  • An extra base is inserted by accident

  • A base is left out

• Mutations can have a big impact on the structures produced from the DNA, particularly for enzymes (proteins), which need a specific shape

What is a gene?

A sequence of bases in a DNA molecule that codes for a protein

• a gene determines the sequence of amino acids in a polypeptide (the primary structure of a protein)

what are the 4 properties of the genetic code?

• triplet code

• universal

• degenerate

• non-overlapping

What does a triplet code mean?

Each sequence of three bases in a gene codes for one amino acid (of twenty)- in translation this is called a codon

How is the genetic code non-overlapping?

Genes are read in discreet triplets (codons), they do not overlap

How is the genetic code degenerate?

Multiple triplets of bases (codons) can code for the same amino acid

How is the genetic code universal?

The triplets of bases (codons) code for the same amino acids in all organisms

What is the process of transcription?

• In the nucleus, part of a DNA molecule unwinds

• DNA helicase breaks the hydrogen bonds between bases to split it into two strands

• RNA polymerase moves along the template (sense) DNA strand, pairing the exposed bases with complementary ones, but replacing T with U

• The nucleotides join up by phosphodiester bonds to create a strand of mRNA

• The mRNA leaves the nucleus to a ribosome

• The DNA rejoins into a double helix

What is the process of translation?

• The mRNA strand joins with a ribosome (made of rRNA)

• At the first codon of mRNA, a tRNA molecule binds, which has a complementary anticodon

• This repeats for each codon

• Each tRNA molecule carries a specific amino acid, which join together by peptide bonds to form a polypeptide chain

Why is mRNA used in protein synthesis?

DNA is too big to fit through nuclear pores, but mRNA is small enough to be able to

the role of enzymes in catalysing reactions that affect metabolism at a cellular and whole organism level

• anabolic and catabolic reaction pathways

digestion of starch:

1. starch polymers are partially broken down into maltose by amylase [produced at salivary glands and pancreas

2. maltose is then broken down into glucose by maltase [in the small intestine]

What are intracellular enzymes? Include an example

• Enzymes that are produces and function inside cells

• Catalase is an intracellular enzyme that converts toxic hydrogen peroxide into water and oxygen

What are extracellular enzymes? Include two examples

• Enzymes that are secreted from cells and function outside them

• Trypsin is an extracellular enzyme which breaks down proteins into shorter peptide chains and amino acids

• Starch is too big of a molecule to enter cells, so the enzyme amylase is involved in its digestion by hydrolysis into simple sugars, excreted from the salivary glands and the pancreas

Compare the two models for enzyme action

Lock and key hypothesis:

• Enzymes have a highly specific 3D tertiary structure as they are globular proteins

• So each enzymes active site is complementary to its substrate; they fit together exactly to form enzyme-substrate complexes

• the substrate/substrates then react the products: enzyme product complex are released

• held in such a way that atom groups are close enough to react

• R groups within the active site interact with the substrate put a strain on the bonds

Induced fit hypothesis:

• The active site and the substrate are not exactly complementary in shape

• The enzyme and its active site can undergo slight conformational changes (changes in shape) to form enzyme-substrate complexes

• initial interaction between enzyme and substrate is relatively weak but the interactions rapidly induce changes in the enzymes tertiary structure that strengthen binding putting a strain ont he substrate molecule

• lowering the activation energy for the reaction

How does pH affect enzyme activity?

• Hydrogen and ionic bonds hold the tertiary structure of enzymes together

• At the optimum pH for an enzyme, it has the highest rate of reaction

• Outside of an enzyme’s optimum pH, the H⁺ ions (acidic) and OH^- ions (alkaline) can cause these bonds to break

• This alters the shape of the active site, which means enzyme-substrate complexes form less easily, or not at all, where the enzyme has denatured

This can be demonstrated using amylase, starch solution, iodine and buffer solutions at different pHs

How does temperature affect enzyme activity?

• At the optimum temperature for an enzyme, it has the highest rate of reaction

• Below this temperature, the molecules have less kinetic energy and move slower, so they collide less often

• Because they collide at lower temperatures, there is less activation energy, so there are fewer successful collisions

• Above the optimum temperature, the particles have more kinetic energy and vibrate more, putting strain on the bonds and causing them to break

• This changes the specific 3D tertiary structure of the enzyme, so it denatures

What is the temperature coefficient?

• The temperature coefficient, Q₁₀, for a biological reaction measures how much the rate of reaction increases with a 10°C increase in temperature

• Most enzymes have a Q₁₀ of 2

How do substrate concentration and enzyme concentration affect enzyme activity?

• As enzyme or substrate concentration increases, rate of reaction increases

• This is because there are more active sites available to form enzyme-substrate complexes in a given volume. so there is a higher collision rate

• This continues up until the point of saturation, where the enzyme and substrate concentrations are the same and so all the active sites are filled

• This point is the V_max , where the rate of reaction is at its maximum possible - all active sites are occupied no more enzyme substrate complexes are formed

What do competitive inhibitors do?

• Competitive inhibitors have a similar shape to the substrate

• This means they can bind with the active site of an enzyme before the substrate can, saturating the active sites

• The substrate then can’t bind and no reactions can occur (competing)

• however the Vmax does not change

• e.g. statins

What do non-competitive inhibitors do?

• Non-competitive inhibitors bind to the allosteric site, separate from the active site

• This binding alters the 3D tertiary structure of the enzyme as a whole, including the active site

• This change in shape mean the substrate can no longer bind so no reactions can occur

How can we compete with competitive inhibitors?

• Increasing substrate concentration means the substrate is more likely to collide with an active site, so it can reach the normal V max

• This doesn’t work for non-competitive inhibitors as they active site has changed shape

What are reversible and non-reversible inhibitors?

• Reversible inhibitors only form weak ionic or hydrogen bonds with enzymes, so they can be removed (temporary)

• Irreversible inhibitors form strong covalent bonds with enzymes, so they can’t be removed (permanent)

How can different inhibitors be used in medicine?

Different inhibitors can be used to treat diseases and conditions by inhibiting certain enzymes involved in them, eg.

• PPIs can treat long-term indigestion, blocking an enzyme system responsible for secreting H ions into the stomach

How can different inhibitors act as metabolic poisons?

Non-reversible inhibitors permanently inactivate enzymes, which can completely stop a biological reaction- some of these inhibitors are used as metabolic poisons as they prevent vital processes, eg.

• Cyanide is a non-reversible inhibitor of a mitochondrial enzyme in respiration

What is end-product inhibition?

When the product of an enzyme-controlled reaction or pathway inhibits the enzyme that produces it, controlling the process (negative feedback)

• excess products are not made and resources aren’t wasted

What are cofactors?

• Some enzymes are inactive until they combine with another compound that changes their 3D tertiary structure in order to bind with the substrate correctly

• They can be organic, inorganic or prosthetic groups

What are inorganic cofactors? Include an example

• Some enzymes need inorganic ions to function, which are acquired from minerals

• Eg. chloride ions act as a cofactor for amylase; in order to digest starch into maltose, chloride ions must be present

What are organic cofactors? Include an example

• Larger organic cofactors are known as coenzymes

• They can either temporarily or permanently bind to the enzyme

• Coenzymes are involved in carrying electrons or chemical groups between enzymes, especially in a metabolic pathway

• Coenzymes are derived from vitamins

• Eg. vitamin B3 is used for NAD and NADP, coenzymes that are involved in respiration and photosynthesis respectively

What are prosthetic group cofactors? Include an example

• Prosthetic groups are permanent cofactors, they are always bonded as part of the enzyme

• Eg. zinc ions act as the prosthetic group for carbonic anhydrase, which metabolises carbon dioxide

what is precursor activation

used with enzymes which:

• can cause damage within cells where they are released

• where enzyme action needs to be controlled and only activated under certain conditions

• need to undergo a change in shape, to be activated, can be done through the addition of a cofactor

• before the cofactor - apoenzyme

• after - holoenzyme

What are the two types of metabolic pathway?

Anabolic- builds up large molecules from smaller ones (exothermic)

Catabolic- breaks down large molecules into smaller ones (endothermic)

What are the 4 main biological roles of water?

• Habitat

• Solvent

• Coolant

• Transport medium

Why is water a polar molecule?

• The oxygen atom attracts the electrons more strongly than the hydrogen atoms

• This gives the oxygen a weak negative charge (δ^-) and the hydrogen a weak positive charge (δ⁺)

• This means water has a dipole

How do hydrogen bonds work in water and why are they useful?

• Weak hydrogen bonds form between the hydrogen and oxygen atoms of adjacent water molecules, due to it’s polarity

This means that water:

• Is a good solvent as it attracts other polar molecules

• Has a high specific heat capacity + latent heat of vaporisation

• Is less dense when it freezes

• Has a high cohesion to itself and high surface tension

• adhesive properties - water attracted to other materials

How do the properties of water relate to it’s role as a solvent, and what are the examples of it?

• Water is a polar molecule, so it attracts other polar molecules and dissolves them

• Eg. Water can carry mineral ions in plant xylem + blood plasma carries blood cells and other substances

How do the properties of water relate to it’s role as a coolant, and what are the examples of it?

• Water has a high specific heat capacity, so it can take in a lot of energy before changing temperature

• It also has a high latent heat of vaporisation, so it takes in a lot of energy when boiling

• Eg. Evaporation is used to cool down, by sweating or panting

How do the properties of water relate to it’s role as a habitat, and what are the examples of it?

• Water has a high specific heat capacity, so it can take in a lot of energy before changing temperature

• Freezes in a crystalline structure, so it is less dense when solid, meaning ice floats and can insulate water bodies

• Polar molecule, so it attracts other polar molecules and can dissolve them

• Eg. Creates a stable environment in ponds, with a constant temperature (for enzyme activity) and dissolved nutrients

How do the properties of water relate to it’s role as a transport molecule, and what are the examples of it?

• Water is a polar molecule, so it attracts other polar molecules and can dissolve them

• This also means it has high cohesion to itself and adhesion to surfaces, so can easily flow

• Eg. Water can carry mineral ions in plant xylem + blood plasma carries cells and dissolved substances

What chemical elements make up carbohydrates, lipids, proteins and nucleic acids?

C, H and O for carbohydrates

C, H and O for lipids

C, H, O, N and S for proteins

C, H, O, N and P for nucleic acids

What are the biological cations calcium, sodium, potassium, hydrogen and ammonium each used for?

Calcium (Ca 2+) - nerve impulse transmission and muscle contractions

Sodium (Na +) - nerve impulse transmission and kidney function

Potassium (K +) - nerve impulse transmission and stomatal opening

Hydrogen (H +) - catalysis of reactions and pH determination

Ammonium (NH4 +) - production of nitrate ions by bacteria

What are the biological anions nitrate, hydrogen carbonate, chloride, phosphate and hydroxide each used for?

Nitrate (NO3 -) - nitrogen supply to plants for amino acid and protein formation

Hydrogen carbonate (HCO3 -) - maintains blood pH

Chloride (Cl -) - balance sodium and potassium ions in cells

Phosphate (PO4 3-) - cell membrane formation and bone formation and nucleic acid and ATP formation

Hydroxide (OH -) - catalysis of reactions and pH determination

What are the structures of alpha and beta glucose?

Glucose is a hexose sugar with two isomers- they both have the formula C6H12O6

describe the condensation reactions between glucose molecules

• 2 hydroxyl groups react, new bonds are reformed

• between alpha glucose a 1,4 glycosidic bond forms

• one H2O is produced

What are the structures of ribose and deoxyribose?

Ribose and deoxyribose are pentose (5 carbon) sugars, with similar formulas except that deoxyribose has one less oxygen than ribose (lost from the second carbon)

What three properties do monosaccharides have in common?

• Soluble in water

• Sweet tasting

• Forms crystals

How can disaccharides and polysaccharides be formed and broken down?

They can be formed by condensation reactions- when two hydroxyl (OH) groups from different saccharides interact to produce a water molecule and a glycosidic bond between the two saccharides

• This can be catalysed by enzymes

They can be broken down by hydrolysis- when water is added to a di or polysaccharide, breaking the glycosidic bond to form a hydroxyl group on each saccharide

• This can be catalysed by (different) enzymes

• We use this to test for non reducing sugars

What are the three most common disaccharides made from?

Maltose- two alpha glucose molecules

Sucrose- glucose + fructose

Lactose- glucose + galactose

What are reducing sugars?

Reducing sugars can give away electrons via the oxidisation of a carbonyl (C=O) group

• This is why reducing sugars can be detected using Benedict’s solution- they reduce the soluble blue copper sulphate to insoluble brick-red copper oxide

• All monosaccharides and some disaccharides are reducing sugars- polysaccharides aren’t

How can we detect non-reducing sugars and why?

Non-reducing sugars (di or poly saccharides) must be broken down into their monosaccharides, which are always reducing sugars, to be detected using Benedict’s solution

• We do this by hydrolysis, where we heat the sample with hydrochloric acid to break the glycosidic bond, and then neutralise it

• Then we can test with Benedict’s solution to see whether reducing sugars were produced, and hence whether non reducing sugars were originally present

What is the structure of starch?

Starch is made from two different alpha glucose structures :

• Amylose (20%)- a straight chain linked by 1,4-glycosidic bonds- amylose curls into a helix shape which allows it to be more compact

• Amylopectin (80%)- a branched chain linked by 1,4 and 1,6-glycosidic bonds

What is starch used for and how is it well suited?

• Starch is the main carbohydrate store in plants

• Stored in the plastids- amyloplasts and chloroplasts

This because it is:

• Compact, so large quantities can be stored

• Insoluble, so it won’t change the water concentration in cells and affect osmosis

• Amylopectin (80%) is linked by some 1,6-glycosidic bonds, so it has many terminal glucose molecules that can be hydrolysed for respiration or added for storage

What is the structure of glycogen?

• Made up of alpha glucose molecules linked by 1,6 and 1,4-glycosidic bonds

• Glycogen has a similar structure to amylopectin but is more branched, because it has more 1,6-bonds

What is glycogen used for and how is it well suited?

• Glycogen is used for storage in animals

• Stored in liver and muscle cells

This because it is:

• Compact but relatively large, so large quantities can be stored (more 1,6-bonds means it is more compact than amylopectin)

• Insoluble, so it won’t change the water concentration in cells and affect osmosis

• Linked by many 1,6-glycosidic bonds so it has many terminal glucose molecules that can be hydrolysed for respiration or added for storage

What is the structure of cellulose?

• Made up of beta glucose molecules linked by 1,4-glycosidic bonds

• To bond together, every other beta glucose molecule is flipped

• This means that hydrogen bonds can form between strands, to create microfibrils

• These make up the cellulose fibres that link into a network

What is cellulose used for and how is it well suited?

• Cellulose makes up the majority of plant cell walls

This is because it is:

• Held together by many hydrogen bonds between strands, so it has a very high tensile strength and is able to withstand the pressure from turgidity of the cell

• Linked to other molecules like lignin, which increases the strength of the cell walls

• Permeable, so water and solutes can enter or leave the cell