Biology 9700/21 — May/June 2023

Background Concept

Plant cells vary in structure according to their function and location. Mesophyll cells of a leaf are specialised for photosynthesis and have very characteristic features: many chloroplasts distributed around the cell periphery, one large central vacuole that pushes the cytoplasm and nucleus to the edge of the cell, and a thin cell wall. Stem cells (especially in non-photosynthetic tissues) lack this photosynthetic specialisation, so they typically have few or no chloroplasts, may contain several smaller vacuoles rather than a single large one, and the nucleus remains more centrally placed.

Understanding the Question

The candidate is shown a TEM of a cell from the stem of the aquatic plant Stuckenia pectinata. The task is to give one piece of evidence visible in the image that rules out a leaf mesophyll origin. The command word is state, so a single clear observation is required.

Approach

Examine Fig. 1.1 and compare what is visible with the expected organisation of a mesophyll cell. Look for: chloroplast density, vacuole arrangement (one big vs many small) and the position of the nucleus.

Step-by-Step Reasoning

The micrograph shows:

• A handful of chloroplasts (label G) — far fewer than the many chloroplasts a mesophyll cell would pack against its wall.
• Several small vacuolar spaces (D points to one) rather than a single large central vacuole that fills most of the cell.
• A centrally placed nucleus (B) and nucleolus (C), not pushed to the cell periphery by a large vacuole.

Any one of these observations is sufficient and is enough to show the cell is not a mesophyll cell.

Key Takeaways

• Cell structure reflects function: mesophyll cells are photosynthetic and have many chloroplasts and one large central vacuole.
• Stem cells (especially in non-green tissues) have a different ultrastructure: fewer chloroplasts, often several smaller vacuoles and a more central nucleus.
• A single well-chosen observation is enough to answer a "state" question.

Common Mistakes

• Stating a feature without comparing it explicitly to a mesophyll cell (e.g. "the cell has a nucleus").
• Claiming "no chloroplasts" if at least one is clearly visible in the image.
• Confusing the tonoplast with the cell surface membrane when describing vacuole boundaries.

Things to Be Careful About

• Read the question as "state evidence… NOT from the mesophyll", so a comparison must be implied even if only one feature is named.
• Look carefully at vacuole size and number — the cell shown has several small vacuoles, not one dominant central one.
• Nucleus position is a giveaway: a peripheral nucleus implies a large central vacuole (mesophyll); a central nucleus implies small/multiple vacuoles (not mesophyll).

Background Concept

Each membrane-bound organelle carries out a specific role that depends on its structure:

• The cell surface (plasma) membrane is a thin phospholipid bilayer across which O₂ and CO₂ diffuse freely; it also houses carrier proteins for active transport.
• The mitochondrial inner membrane has a large surface area and is also thin enough to permit gas diffusion, so it qualifies for gas exchange in the same way.
• The nucleolus, a dense region inside the nucleus, is the site where rRNA combines with proteins to assemble the large and small subunits of ribosomes (it does not build whole ribosomes).
• Several membranes — cell surface membrane, tonoplast, mitochondrial inner membrane and nuclear envelope — all carry transport proteins able to perform active transport of ions using ATP.
• The mitochondrion is the site of aerobic respiration: the link reaction and Krebs cycle in the matrix, and the electron transport chain / oxidative phosphorylation on the inner membrane.

Understanding the Question

The candidate is given a four-row table listing cellular functions and must (1) name the structure responsible, and (2) give the letter that labels that structure on Fig. 1.1. One mark is awarded per correct row.

Approach

For each function, recall the organelle that performs it, then match the organelle to the letter assigned in the micrograph. In Fig. 1.1: A = cell surface membrane, B = nucleus, C = nucleolus, D = vacuole, E = mitochondrion, F = other membrane, G = chloroplast.

Step-by-Step Reasoning

• Gas exchange — small, lipid-soluble gases (O₂, CO₂) diffuse across thin membranes. The cell surface membrane (A) is the principal site; the mitochondrial membrane (E) is also accepted.
• Subunits of ribosomes — assembled in the nucleolus (C).
• Active transport of ions — requires ATP and specific carrier proteins. The cell surface membrane (A) is the most frequently cited site, but the tonoplast (F), mitochondrial membrane (E) and nuclear envelope (B) are all acceptable.
• Aerobic respiration — the mitochondrion (E) is the only organelle that carries out the Krebs cycle and oxidative phosphorylation.

Key Takeaways

• The cell surface membrane is central to gas exchange and to active transport of ions.
• The nucleolus builds ribosomal subunits, not whole ribosomes.
• Mitochondria are the unique site of aerobic respiration in eukaryotes.
• Several different membranes can perform active transport.

Common Mistakes

• Writing "cell membrane" rather than the precise "cell surface membrane".
• Saying the nucleus (rather than the nucleolus) makes ribosomes.
• Listing a whole organelle such as "mitochondrion" for active transport without specifying the membrane involved.
• Confusing the cell wall with the cell surface membrane (the cell wall is fully permeable and not a site of selective transport).

Things to Be Careful About

• The mark scheme requires "cell surface membrane" (not "cell membrane") for full credit.
• The nucleolus produces ribosomal subunits, not whole ribosomes — these are joined together in the cytoplasm.
• For active transport, any membrane with the appropriate carrier protein is acceptable; the most common answer is the cell surface membrane (A).

Background Concept

Cells move material across the plasma membrane in bulk by endocytosis and exocytosis:

• Endocytosis — the plasma membrane invaginates around extracellular material and pinches off to form a vesicle inside the cell.
• Pinocytosis — a form of endocytosis in which small droplets of extracellular fluid (and any dissolved solutes) are taken up in small vesicles; sometimes called "cell drinking".
• Phagocytosis — endocytosis of large solid particles such as bacteria or cell debris; these end up in large phagosomes and is performed by specialised phagocytes. Phagocytosis is not the same as pinocytosis, even though both are forms of endocytosis.

Understanding the Question

In Fig. 1.2, label X points to a vesicle interacting with the cell surface membrane. The command word "name" requires a single term.

Approach

Look at the size of the vesicle at X and the direction of movement: a small vesicle being internalised from the cell surface membrane indicates endocytosis / pinocytosis. A large vesicle engulfing a whole cell or large particle would indicate phagocytosis.

Step-by-Step Reasoning

The vesicle at X is small, comparable with the other transport vesicles in the diagram. Material is being taken up into the cell (the arrow points inward). This is the textbook description of endocytosis; because the vesicle is small and carries fluid, the more specific term pinocytosis is also accepted. Phagocytosis is explicitly rejected by the mark scheme because the vesicles shown are too small to be phagosomes and the cell is not a phagocyte.

Key Takeaways

• Endocytosis is the bulk uptake of material by vesicle formation at the cell surface membrane.
• Pinocytosis = endocytosis of fluid in small vesicles; phagocytosis = endocytosis of large particles.
• The size of the vesicle and the cell type determine the correct name.

Common Mistakes

• Writing "phagocytosis" — rejected by the mark scheme because the vesicles are small, not large phagosomes, and the cell shown is not a phagocyte.
• Writing "active transport" or "diffusion" — these are not bulk transport and do not form vesicles.
• Writing "osmosis" — that refers specifically to water movement, not to vesicle formation.

Things to Be Careful About

• "Endocytosis" is the broadest correct answer; "pinocytosis" is also accepted.
• "Phagocytosis" must be avoided here — it is reserved for the uptake of large particles by phagocytes.

Background Concept

Proteins are polymers of amino acids joined by peptide bonds (a type of covalent bond). Hydrolysis is a chemical reaction in which a bond is broken by the addition of a water molecule: the –OH from water goes to one fragment, the –H to the other. Hydrolytic enzymes speed up hydrolysis. Enzymes that act on proteins are called proteases (or peptidases / proteolytic enzymes); they are one class of the acidic hydrolases stored in lysosomes (animal cells) and vacuoles (plant cells). Plant vacuoles are therefore often described as having a lysosome-like role: they can digest unwanted or damaged macromolecules, including proteins that have not folded correctly.

Understanding the Question

The stem tells the candidate that misfolded proteins reach the plant vacuole and are broken down there. The task is to explain how this breakdown happens — i.e. the chemistry and the enzymes involved. Three marks are available.

Approach

Construct a logical sequence: identify the type of reaction, name the enzymes, name the bond broken, and state the products.

Step-by-Step Reasoning

1. The vacuole contains hydrolytic enzymes, including proteases (peptidases).
2. These enzymes catalyse the hydrolysis of the misfolded proteins — water is added across the bond.
3. The bonds broken are the peptide bonds that link amino acids in the polypeptide chain.
4. The products are shorter peptides and individual amino acids that the cell can recycle.

The mark scheme accepts any three of: hydrolysis / use of water; proteases / peptidases; peptide bonds broken; products are peptides / amino acids. The term "hydrolytic enzymes" alone is sufficient for the first point if hydrolysis is implied.

Key Takeaways

• Protein breakdown in the vacuole is by hydrolysis, catalysed by proteases.
• The bonds cleaved are peptide bonds (NOT glycosidic, ester or phosphodiester bonds).
• Products are peptides and amino acids, which the cell can reuse.
• Vacuoles carry out lysosome-like digestive functions in plant cells.

Common Mistakes

• Writing that "bonds" (without specifying peptide bonds) are broken — the mark scheme insists on peptide bonds for full credit.
• Naming an incorrect bond (e.g. glycosidic, ester or phosphodiester) — these are explicitly rejected.
• Omitting any mention of hydrolysis or water.
• Saying "the protein is broken into amino acids only" — shorter peptides are also produced.

Things to Be Careful About

• The mark scheme requires the type of reaction (hydrolysis) AND/OR the name of the enzymes (proteases) AND the bond (peptide) AND the products (peptides / amino acids). Three of these four ideas are needed for three marks.
• "Hydrolytic enzymes" is an acceptable shorthand for proteases in the vacuole.
• Do not confuse protein breakdown with denaturation — denaturation is loss of 3-D shape, not peptide bond cleavage.