▶️ Answer/Explanation
(a)
- The surface area of a cell controls how quickly substances like nutrients and waste can cross the cell membrane.
- The volume of a cell relates to the level of metabolic activity, which produces waste and requires materials.
- As a cell grows, volume increases faster than surface area, causing the surface area to volume ratio (SA:V) to decrease.
- A lower SA:V ratio means that not enough substances can enter or leave the cell efficiently.
- This limits the maximum size of a cell.
- Cells may develop adaptations like microvilli or a flattened shape to increase surface area.
- When the size becomes too large, cells often divide to restore a more efficient SA:V ratio.
(b)
- Osmosis is the passive movement of water molecules across a selectively permeable membrane.
- It occurs when there is a difference in solute concentration on either side of the membrane.
- Water moves from the region of low solute concentration (high water concentration) to the region of high solute concentration (low water concentration).
- Osmosis continues until equilibrium is reached.
- This process does not require energy (ATP).
- In some cells, special protein channels called aquaporins help speed up water transport.
(c)
- The small intestine is adapted for efficient nutrient absorption.
- It is very long, which increases the time and area for absorption.
- The lining contains villi and microvilli, which increase surface area greatly.
- The walls contain smooth muscle for peristalsis, which moves food along.
- Digestive enzymes are secreted into it, and it also receives bile and pancreatic juice to aid digestion.
- The villi have a dense capillary network to quickly absorb and transport nutrients like glucose and amino acids into the bloodstream.
- Lacteals, part of the lymphatic system, absorb lipids.
Markscheme:
a.
• Surface area of the cell affects the rate of material exchange
• When the cell increases in size, so does its chemical activity/metabolism
• More substances need to be taken in / more waste products need to be excreted as cell grows
• As volume increases, surface area increases but not proportionally (SA:V ratio decreases)
• Substances won’t enter fast enough to supply cell volume needs
• Some cells have adaptations to increase surface area (microvilli, flattened shape)
• Cells tend to divide when reaching maximum size
b.
• Form of diffusion specifically for water molecules
• Occurs across selectively permeable membranes
• Water moves from low solute to high solute concentration (until equilibrium)
• Passive process (doesn’t require energy/ATP)
• May involve aquaporins (water channel proteins)
c.
• Long length maximizes absorption area
• Lined with smooth muscle (circular and longitudinal) for peristalsis
• Receives digestive enzymes from pancreas and bile from gall bladder
• Villi and microvilli greatly increase surface area for absorption
• Dense capillary network rapidly transports absorbed nutrients
• Lacteals absorb lipids into lymphatic system
Accept annotated diagrams as part of the explanation.
▶️ Answer/Explanation
(a)
- Simple diffusion is the passive movement of small or non-polar molecules (like oxygen or carbon dioxide) from a region of high concentration to low concentration without using energy (ATP).
- Facilitated diffusion is also passive, but it involves movement through protein channels or carriers, helping larger or polar molecules (like glucose) move down the concentration gradient.
- Osmosis is the passive diffusion of water molecules across a selectively permeable membrane, from low solute concentration to high solute concentration.
- Active transport uses energy from ATP to move substances against their concentration gradient, through protein pumps (e.g., sodium-potassium pump).
(b)
(c)
- Enzymes are biological catalysts that speed up digestion by lowering activation energy. They work by binding specific substrates to their active sites, leading to the breakdown of macromolecules like proteins, fats, and carbohydrates into smaller, absorbable units.
- In the pancreas, several enzymes are produced to assist in digestion:
- Amylase: Breaks down starch into maltose (a disaccharide).
- Lipase: Breaks down triglycerides into glycerol and fatty acids.
- Trypsin (a type of endopeptidase): Breaks internal peptide bonds in proteins, producing smaller polypeptides.
- These enzymes are secreted into the small intestine, where they function best at an alkaline pH (~8). Each enzyme is specific to its substrate and can be denatured by extreme pH or temperature changes.
Markscheme:
a. Types of membrane transport:
- Simple diffusion: Passive movement of molecules/ions along a concentration gradient (no energy required)
- Facilitated diffusion: Passive movement through protein channels along concentration gradient (no energy required)
- Osmosis: Passive movement of water across a membrane from low to high solute concentration
- Active transport: Movement against concentration gradient using membrane pumps and ATP energy
- Endocytosis/Exocytosis: Vesicle formation requiring energy to move large particles in/out of cells
- Chemiosmosis: Proton diffusion through ATP synthase to produce ATP (energy coupling)
Award marks for any four correct types with energy descriptions
b. Dipeptide structure requirements:
- Two amino acids shown with NH2/NH3+ end and COOH/COO– end
- Peptide bond between C=O and N-H groups correctly drawn
- Chiral carbons with H and R groups on each amino acid
- Clear labeling of peptide bond between C-terminal and N-terminal
c. Enzyme action in digestion:
General enzyme action:
- Catalyze/speed up chemical reactions by lowering activation energy
- Have specific active sites for substrate binding
- Involve molecular motion and substrate-active site collisions
- Break macromolecules into absorbable monomers
Pancreatic enzyme roles:
- Amylase: Breaks down starch → disaccharides/maltose
- Lipase: Hydrolyzes triglycerides → fatty acids + glycerol
- Endopeptidase: Cleaves peptide bonds within proteins
- Trypsin: (Alternative) Cleaves peptide bonds at specific amino acids
Physiological context:
- Secreted into alkaline small intestine lumen (pH ~8)
- Exhibit optimal activity at specific pH ranges
- Can denature at non-optimal pH conditions
Award [6 max] if fewer than two specific enzymes described
▶️ Answer/Explanation
(a)
Annotation should show:
- Hydrophilic (water-loving) head – labeled at the circular parts of phospholipids (facing outward to water).
- Hydrophobic (water-fearing) tails – labeled at the wavy lines (facing inward, away from water).
- Phospholipids are amphipathic because they have both a hydrophilic region (head) and a hydrophobic region (tails).
(b)
- Cholesterol reduces membrane fluidity and adds stability, especially at high temperatures.
- It also prevents the membrane from becoming too rigid at low temperatures.
- It helps maintain membrane flexibility and integrity.
(c)
- Mitochondria and chloroplasts have their own circular DNA, like prokaryotes.
- They have double membranes and replicate independently by binary fission.
- These features suggest they were once free-living prokaryotes engulfed by a larger cell.
Markscheme:
a
a. line to circle labelled phosphate (head) and (tail) labelled fatty acid/hydrocarbon/lipid (tail);
b. label hydrophilic/polar/attracted to water/ and hydrophobic/non polar/not attracted to water;
b
reduces fluidity of membrane / reduces permeability of membrane (to some molecules);
c
a. mitochondria/chloroplasts have their own DNA;
b. mitochondria can self-replicate/undergo a process like binary fission;
c. mitochondria/chloroplasts have double membranes;
d. mitochondria/chloroplasts have(70s) ribosomes;
e. mitochondria/chloroplasts are sensitive to antibiotics;
f. similar in size to bacteria