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Questions on eukaryotic cell structure and organelles
The ultrastructure of eukaryotic cells: nucleus, nucleolus and nuclear envelope, rough and smooth endoplasmic reticulum, the Golgi apparatus and the route a secreted protein takes through it, mitochondria and chloroplasts, lysosomes, ribosomes at 80S and 70S, the wall, vacuole and plastids that distinguish a plant cell from an animal one, amyloplasts, pits and the middle lamella and how to recognise them in an electron micrograph, and the cytoskeleton of microfilaments, intermediate filaments and microtubules with the motor proteins that walk along it.
20 original questions · 61 marks · the eukaryotic cell structure and organelles notes · Cells, microscopy and biological organisation
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Compare the structure of a mitochondrion with the structure of a chloroplast.
Mark scheme
- B1 both are surrounded by two membranes, but a chloroplast has a separate internal thylakoid membrane system as well
- B1 the inner mitochondrial membrane is folded into cristae, whereas the chloroplast membranes form thylakoids stacked into grana
- B1 the fluid interior of a mitochondrion is the matrix, where the Krebs cycle runs, whereas that of a chloroplast is the stroma, holding the enzymes of the light-independent reactions
- B1 a chloroplast contains chlorophyll in its thylakoid membranes, whereas a mitochondrion contains no pigment
Explain why the enzymes of a lysosome must be kept behind a membrane, and explain how those enzymes reach the lysosome after they have been made.
Mark scheme
- B1 a lysosome holds hydrolytic enzymes, which digest worn-out organelles and material taken in by phagocytosis
- B1 loose in the cytoplasm those enzymes would hydrolyse the cell that made them, so they are released only where they are wanted
- B1 the enzymes are proteins, made on ribosomes attached to the rough endoplasmic reticulum
- B1 they are packaged at the Golgi apparatus, and a lysosome is simply a vesicle budded from it and kept rather than secreted
Describe the internal structure of a cilium, and describe how this structure produces bending.
Mark scheme
- B1 nine pairs of microtubules are arranged in a ring around two central microtubules, the 9 + 2 arrangement
- B1 dynein arms sit between each pair of microtubules and its neighbour
- B1 when the dynein arms use ATP to walk along the neighbouring pair, the two pairs slide against each other
- B1 because the pairs are anchored at the base, they cannot slide apart, and the sliding is converted into bending of the whole cilium
Compare a plant cell with an animal cell, referring to the cell wall, the permanent vacuole, plastids and centrioles.
Mark scheme
- B1 a plant cell has a cellulose cell wall outside its cell-surface membrane, whereas an animal cell has none
- B1 a plant cell has a large permanent vacuole bounded by a tonoplast, whereas an animal cell has at most small, temporary vacuoles
- B1 a plant cell exposed to light has chloroplasts, whereas an animal cell never does
- B1 an animal cell has centrioles that organise its spindle, whereas centrioles are absent from the cells of flowering plants
Describe the middle lamella and a pit in the walls of plant cells, giving the function of each.
Mark scheme
- B1 the middle lamella is the layer between the walls of two neighbouring cells, made mainly of pectins or calcium pectate
- B1 it cements, or holds, neighbouring cells together
- B1 a pit is a thin region of the wall where no secondary wall has been laid down, leaving only the primary wall and middle lamella, usually matched by a pit in the neighbouring cell
- B1 pits allow water and dissolved substances to pass between neighbouring cells, for example sideways between xylem vessels
Describe the structure of the Golgi apparatus, and describe what happens to a protein while it passes through it.
Mark scheme
- B1 a stack of flattened membrane sacs, usually four to eight, with vesicles budding from one face
- B1 vesicles carrying newly made protein arrive at one side and the contents move through the stack
- B1 the protein is modified, for example by having sugar groups added to make a glycoprotein, then sorted and packaged into a vesicle addressed to its destination
An electron micrograph of a cell from the adrenal gland shows very little rough endoplasmic reticulum but an unusually large amount of smooth endoplasmic reticulum. The adrenal gland releases steroid hormones. Suggest an explanation for these two observations.
An adrenal cortex cell in section, drawn rather than photographed. Almost all of the cytoplasm is smooth endoplasmic reticulum; the one studded patch sits at the right. Mark scheme
- B1 smooth endoplasmic reticulum is the site of lipid and steroid synthesis
- B1 a large area of smooth endoplasmic reticulum allows the cell to make steroid hormone in quantity
- B1 the cell exports very little protein, so few attached ribosomes and therefore little rough endoplasmic reticulum are needed
Describe the route taken by an extracellular enzyme from the ribosome where it is made to the outside of the cell, naming each structure it passes through in order.
Mark scheme
- B1 the protein is made on a ribosome attached to the rough endoplasmic reticulum and is threaded into the cisterna as the chain grows
- B1 a transport vesicle buds off the rough endoplasmic reticulum and carries the protein to the Golgi apparatus, where it is modified and packaged
- B1 a secretory vesicle buds off the Golgi, moves to the cell-surface membrane and fuses with it, releasing the enzyme outside the cell by exocytosis
Cells that secrete a lot of protein are crowded with mitochondria. Explain why a secretory cell needs them.
Mark scheme
- B1 secretion is active: forming each peptide bond, moving vesicles along the cytoskeleton and fusing them with the cell-surface membrane all require ATP
- B1 mitochondria are the site of aerobic respiration, which produces that ATP
- B1 the more protein the cell exports, the more ATP it spends, so a large number of mitochondria is needed to keep the supply level with the demand
Describe the structure of the nuclear envelope, and describe the role played by the nuclear pores.
Mark scheme
- B1 the nuclear envelope is a double membrane surrounding the nucleus, continuous with the rough endoplasmic reticulum, and perforated by nuclear pores
- B1 the pores are not plain holes but carry transport machinery of their own, so movement across them is controlled rather than free
- B1 they allow processed messenger RNA to leave the nucleus for the cytoplasm while the chromosomes stay inside, keeping transcription and translation in separate compartments
State what a ribosome is made of, state how many subunits it has, and state where 80S ribosomes are found in a eukaryotic cell.
Mark scheme
- B1 a ribosome is made of ribosomal RNA and protein
- B1 it consists of two subunits, which clamp round a strand of messenger RNA in order to translate it
- B1 80S ribosomes occur free in the cytoplasm and attached to the rough endoplasmic reticulum
Name the three types of protein fibre that make up the cytoskeleton, and name the protein each one is built from.
Mark scheme
- B1 microfilaments, built from the protein actin
- B1 intermediate filaments, built from tough rope-like proteins such as keratins
- B1 microtubules, built from the protein tubulin
Explain why the cellulose cell wall of a plant cell does not control which substances enter the cell, and explain what stops the cell bursting when water enters it by osmosis.
Mark scheme
- B1 the cell wall is freely permeable, so it is no barrier to water or dissolved substances; control of what enters is carried out by the cell-surface membrane, which lies inside the wall
- B1 the wall is strong in tension, so as water enters by osmosis and the cytoplasm presses outward, the wall resists and pushes back
- B1 the cell becomes turgid rather than bursting, because the wall's resistance stops the cell expanding any further once it is fully inflated
Explain the function of plasmodesmata, and explain why they are necessary given that a cellulose cell wall separates one plant cell from the next.
Mark scheme
- B1 plasmodesmata are narrow channels through the walls of neighbouring plant cells, lined with membrane, through which the cytoplasm is continuous from one cell to the next
- B1 a rigid cellulose wall would otherwise be a complete barrier between neighbouring cells, isolating each one's cytoplasm from its neighbour's
- B1 plasmodesmata let substances and signals pass directly from cell to cell without having to cross the wall or two separate cell-surface membranes
Compare the structure of an amyloplast with the structure of a chloroplast.
Mark scheme
- B1 both are plastids bounded by an envelope of two membranes
- B1 a chloroplast contains thylakoid membranes stacked into grana, whereas an amyloplast has no thylakoids or grana
- B1 a chloroplast contains chlorophyll and only small starch grains, whereas an amyloplast has no chlorophyll and is largely filled by one or more large starch grains
A cell from the cap of a root, seen with an electron microscope, contains several oval organelles. Each is bounded by two membranes, has no internal membranes, and is almost filled by large, pale, structureless grains. Identify these organelles and give two pieces of evidence from the description that rule out a chloroplast.
Mark scheme
- B1 they are amyloplasts, plastids that store starch
- B1 there are no internal membranes, so no thylakoids or grana, which a chloroplast always has
- B1 the cells come from the root cap, which receives no light, so chloroplasts would not be expected there
State one function of smooth endoplasmic reticulum, and state the one structural feature that distinguishes it from rough endoplasmic reticulum.
Mark scheme
- B1 synthesises lipids, phospholipids or steroids; storing calcium ions also accepted
- B1 smooth endoplasmic reticulum has no ribosomes attached to its surface, whereas rough endoplasmic reticulum does
Name the cytoskeletal fibre along which vesicles are carried through the cytoplasm, and name one of the motor proteins that does the carrying.
Mark scheme
- B1 a microtubule, the hollow tubulin cylinder that acts as the cell's rail network
- B1 kinesin or dynein, which walks along the microtubule using ATP
On an electron micrograph at a magnification of ×4500, an amyloplast from a potato tuber measures 36 mm long. Calculate its actual length in micrometres.
Mark scheme
- M1 actual size = image size divided by magnification, with 36 mm converted to 36 000 µm
- A1 36 000 ÷ 4500 = 8 µm
As a pear ripens, its flesh softens and its cells separate from one another more easily. Suggest an explanation.
Mark scheme
- B1 enzymes break down the pectins, or calcium pectate, of the middle lamella
- B1 the middle lamella holds neighbouring cells together, so once it is broken down the cells come apart and the tissue is softer
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