Edexcel IAS Biology Unit 1 Comprehensive Review

50 detailed Question and Answer flashcards reviewing Carbohydrates, Proteins, Lipids, Circulatory System, Transport, Enzymes, Genetic Code, Nucleic Acids, and Genetics based on Edexcel IAS Biology Unit 1 lecture notes.

What causes a water molecule to be dipolar in nature?

Oxygen is more electronegative than hydrogen, giving oxygen a partial negative charge ($\delta^-$) and hydrogen a partial positive charge ($\delta^+$).

What is the specific heat capacity of water, and why is it biologically important?

It is $4200\,\text{J/kg}$, meaning water requires a lot of heat energy to raise its temperature, which minimizes sudden temperature fluctuations in aquatic habitats and internal environments.

At what temperature is water at its maximum density, and why is this property significant for aquatic life?

Water is most dense at $4^\circ\text{C}$. As it cools from $4^\circ\text{C}$ to $0^\circ\text{C}$, it expands and ice floats on the surface, creating an insulating layer that prevents water below from freezing solid.

What is the general chemical formula for carbohydrates?

The general formula is $C_n(H_2O)_n$ or $(CH_2O)_n$.

How do $\alpha\text{-glucose}$ and $\beta\text{-glucose}$ differ in their molecular structures?

In $\alpha\text{-glucose}$, the hydroxyl group ($\text{-OH}$) attached to carbon-1 is positioned below the ring, whereas in $\beta\text{-glucose}$, the hydroxyl group on carbon-1 is positioned above the ring.

Which two monosaccharides condense to form lactose, and what bond links them?

$\alpha\text{-glucose}$ and $\beta\text{-galactose}$ condense to form lactose via a $1,4\text{-glycosidic bond}$.

What are the key structural differences between amylose and amylopectin in starch?

Amylose is an unbranched, spiraled helix containing only $1,4\text{-glycosidic bonds}$, whereas amylopectin is a branched molecule containing both $1,4\text{-}$ and $1,6\text{-glycosidic bonds}$.

How is glycogen structured to allow rapid energy release in animal cells?

Glycogen is highly branched with $1,6\text{-glycosidic bonds}$ occurring every 8 to 10 glucose residues, providing many terminal ends that can be rapidly hydrolyzed by enzymes.

How is a triglyceride synthesized?

Through condensation reactions between one glycerol molecule and three fatty acid molecules, forming three ester bonds and releasing three water molecules.

Why do saturated fatty acids have higher melting points than unsaturated fatty acids?

Saturated fatty acids have straight hydrocarbon chains with no double bonds, allowing them to pack closely together with strong intermolecular forces that require more heat energy to break.

How much energy do lipids yield per gram compared to carbohydrates and proteins?

Lipids yield $9\,\text{kcal/g}$, whereas carbohydrates and proteins yield only $4\,\text{kcal/g}$.

What four standard chemical groups are bonded to the central carbon atom of an amino acid?

An amine group ($\text{-NH}_2$), a carboxylic acid group ($\text{-COOH}$), a hydrogen atom ($\text{-H}$), and a variable side chain ($\text{R}$ group).

What defines the primary structure of a protein?

The specific sequence of amino acids in a polypeptide chain held together by covalent peptide bonds.

What hydrogen bonding pattern stabilizes an $\alpha\text{-helix}$ secondary structure?

Hydrogen bonds form between the hydrogen of an $\text{-N-H}$ group and the carbonyl oxygen of the $\text{C=O}$ group located four amino acids further along the chain.

What types of chemical interactions hold together the tertiary structure of a globular protein?

Hydrogen bonds, ionic bonds between ionized $\text{R}$ groups, disulfide bridges between cysteine residues, and hydrophobic/hydrophilic interactions.

Describe the quaternary structure and amino acid composition of collagen.

Collagen consists of three polypeptide $\alpha\text{-chains}$ wrapped in a tight triple helix, characterized by repeating sequences of glycine along with proline and hydroxyproline.

What structural component is present in haemoglobin but absent in collagen?

Haemoglobin contains non-protein prosthetic haem groups (each containing an iron ion $Fe^{2+}$), whereas collagen contains no prosthetic group.

Why do large multicellular organisms require a specialized circulatory system?

They have a small surface area-to-volume ratio and long diffusion distances, so simple diffusion alone is too slow to supply oxygen and nutrients to meet their high metabolic rates.

What defines an open circulatory system?

A system where blood is pumped by a heart into open body cavities (sinuses), directly bathing the organs under low pressure.

Why is the muscular wall of the left ventricle thicker than that of the right ventricle?

The left ventricle needs to generate higher pressure to pump blood around the entire systemic body circuit, whereas the right ventricle pumps blood under lower pressure to delicate lung tissue.

What structural feature of capillaries enables rapid exchange of materials?

Capillary walls are composed of a single layer of endothelial cells (one cell thick), providing an extremely short diffusion pathway.

Where are the bicuspid and tricuspid valves located in the mammalian heart?

The bicuspid (mitral) valve is between the left atrium and left ventricle; the tricuspid valve is between the right atrium and right ventricle.

What is meant by myogenic stimulation of the heart?

The heart muscle initiates its own electrical impulse and contraction without needing external nerve signals, originating at the sinoatrial node (SAN).

How is tissue fluid formed at the arterial end of capillaries?

High hydrostatic pressure at the arterial end forces water and small solute molecules out through the capillary walls into the surrounding tissue space.

What are the main enzymatic steps that convert fibrinogen to fibrin during blood clotting?

Thromboplastin, calcium ions ($Ca^{2+}$), and vitamin K convert prothrombin into active thrombin, which then acts as an enzyme to hydrolyze soluble fibrinogen into insoluble fibrin.

How does endothelial damage initiate atherosclerosis?

Endothelial damage triggers an inflammatory response, leading to white blood cell accumulation, fatty plaque buildup (atheroma), calcium deposition, and hardening/narrowing of the arterial lumen.

What is the Bohr effect?

High carbon dioxide concentrations lower blood pH, changing haemoglobin's shape and reducing its affinity for oxygen, causing oxygen to unload more readily at respiring tissues.

Why is fetal haemoglobin's oxygen dissociation curve shifted to the left of adult haemoglobin?

Fetal haemoglobin has a higher affinity for oxygen, enabling it to bind oxygen from maternal blood at the low partial pressures present in the placenta.

What is the purpose of the chloride shift in red blood cells?

When hydrogen carbonate ions ($HCO_3^-$) diffuse out of red blood cells into plasma, chloride ions ($Cl^-$) move in to balance the electrical charge across the membrane.

What is the formula used to calculate Body Mass Index (BMI)?

$$\text{BMI} = \frac{\text{body mass (kg)}}{\text{height}^2 (\text{m}^2)}$$

How do High-Density Lipoproteins (HDLs) differ structurally and functionally from Low-Density Lipoproteins (LDLs)?

HDLs contain unsaturated fats and a higher proportion of protein, carrying cholesterol from body tissues to the liver for removal; LDLs contain saturated fats and lower protein, depositing cholesterol on artery walls.

How do statins reduce the risk of cardiovascular disease?

Statins inhibit the enzyme HMG-CoA reductase in the liver, blocking blood cholesterol synthesis and reducing circulating LDL levels.

How do ACE inhibitors lower blood pressure?

They block Angiotensin Converting Enzyme, preventing the synthesis of angiotensin II, thereby reducing vasoconstriction and keeping arteries dilated.

According to the fluid-mosaic model, why is the cell membrane described as 'fluid' and 'mosaic'?

It is 'fluid' because individual phospholipid and protein molecules can move laterally within the layer, and 'mosaic' because proteins are scattered randomly throughout the lipid bilayer.

How does cholesterol affect plasma membrane fluidity across different temperatures?

At warm temperatures, cholesterol restrains phospholipid movement to prevent excessive fluidity; at cool temperatures, it prevents tight packing of fatty acid tails to maintain fluidity.

How does active transport differ from passive diffusion?

Active transport moves substances against a concentration gradient using carrier proteins and requires metabolic energy from ATP, whereas diffusion is passive down a gradient without ATP.

What is exocytosis?

The bulk transport process in which intracellular vesicles fuse with the cell surface membrane to secrete their contents outside the cell.

How do enzymes reduce the activation energy of a metabolic reaction?

Enzymes bind substrates at their active site to form enzyme-substrate complexes, distorting substrate chemical bonds and facilitating bond cleavage or formation.

How does the Induced Fit theory refine the Lock and Key model of enzyme action?

It proposes that the active site is flexible and changes shape slightly upon substrate interaction to fit tightly around the substrate, forming the active enzyme-substrate complex.

What is the formula for the temperature coefficient ($Q_{10}$), and what does a value of 2 indicate?

$Q_{10} = \frac{\text{rate of reaction at } (x+10)^\circ\text{C}}{\text{rate of reaction at } x^\circ\text{C}}$; a value of 2 means the reaction rate doubles for every $10^\circ\text{C}$ increase in temperature.

How does a non-competitive inhibitor reduce enzyme activity?

It binds to an allosteric site away from the active site, inducing a conformational change in the enzyme's 3D tertiary structure that alters the active site shape so the substrate can no longer bind.

What three main features define the nature of the genetic code?

It is a triplet code (3 bases code for 1 amino acid), non-overlapping (each base is read once as part of one codon), and degenerate (more than one codon can code for the same amino acid).

What role does RNA polymerase play in transcription?

It unwinds DNA and links RNA mononucleotides aligned with the antisense strand by forming phosphodiester bonds via condensation reactions to construct mRNA.

What molecular interaction occurs between mRNA and tRNA during translation?

Complementary base pairing forms hydrogen bonds between 3-base codons on mRNA and 3-base anticodons on tRNA molecules carrying specific amino acids.

How can a base substitution mutation alter enzyme function?

Changing a DNA base alters the mRNA codon, which can code for a different amino acid, changing the protein's primary structure, tertiary folding, and active site shape so the substrate cannot fit.

How did Meselson and Stahl's experiment disprove conservative DNA replication?

Bacteria grown in $^{15}\text{N}$ transferred to $^{14}\text{N}$ produced a single intermediate hybrid density band after one generation, ruling out conservative replication which would yield distinct heavy and light bands.

What pentose sugars and nitrogenous bases distinguish DNA from RNA mononucleotides?

DNA contains deoxyribose sugar and thymine base; RNA contains ribose sugar and uracil base.

What molecular defect causes cystic fibrosis?

A mutation in the CFTR gene on chromosome 7 leading to an absent or non-functional CFTR chloride channel protein on the apical membrane of epithelial cells.

How does faulty CFTR protein lead to thick, sticky mucus in cystic fibrosis patients?

Non-functional CFTR channels block chloride exit, while open sodium channels allow continuous sodium entry; water moves out of the mucus into epithelial cells by osmosis, dehydrating the mucus.

What is Chorionic Villus Sampling (CVS), and what advantage does it have over amniocentesis?

CVS isolates fetal placental tissue for genetic screening early in pregnancy (8-12 weeks), allowing earlier diagnostic results than amniocentesis (performed around 16 weeks).