A-LevelBiologyThe mitotic cell cycle9700/22 · May/June 2017 · Q415 Marks

Meristematic tissue is found in the growing regions of plants, such as root tips.

(a)(i)Fig. 4.1 summarises a cell cycle for a meristematic cell in the root tip. The two phases of this cell cycle are shown: • interphase, which is divided into the G₁, S and G₂ stages • cell division, which is divided into stages 1–5. [Figure 4.1] In Fig. 4.1, stage 2 is metaphase. The stages shown in Fig. 4.1 are listed below. Draw a circle around the stage in the cell cycle that is cytokinesis and describe what happens in this stage in the root tip meristematic cell. 1 2 3 4 5 G1 S G2[3]
(a)(ii)The stages shown in Fig. 4.1 are listed below. Draw a circle around the stage in the cell cycle in which the semi-conservative replication of DNA occurs and state what is meant by semi-conservative replication. 1 2 3 4 5 G1 S G2[3]
(b)Meristematic cells have a similar role to stem cells found in animals. Suggest the role of a meristematic cell and explain the features that help it to carry out its role.[3]
(c)(i)Meristematic regions in the plant can sometimes be described as strong sinks. State what is meant by a sink.[1]
(c)(ii)Suggest what is meant by a strong sink.[1]
(d)Describe and explain how sucrose is loaded into phloem sieve tubes.[4]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

[1] circle round 5;
[1] cell plate forms (across equator of cell) or cell wall / cellulose, laid down ; A cell wall forms (between the two)
[1] (so) cytoplasm divided (into two) ; R cytoplasm constricts / pinches in / cleavage furrow forms/cleavage forms (i.e. referring to animal cell)
[1] AVP; detail of cell plate formation e.g. ref. to vesicles transported to equator/involvement of cytoskeletal structures idea that organelles shared out
[1] circle round S ;
[1] synthesis of two identical DNA, molecules / double helices (from one);
[1] each new molecule consists of one, original / parental, strand, and one, new / newly synthesised, strand ; AW A daughter for newly synthesised
[1] form cells that can, differentiate / become specialised; A can develop into other cell types / totipotent A named examples
[1] for, cell replacement / tissue repair / growth ; R for cell growth I found in growing region
[1] divides to give continuous supply of, meristematic / stem, cells; A divides to forms more, meristematic / stem, cells
[1] (stem cells are) undifferentiated; A not specialised / unspecialised
[1] able to divide; must be in context of mitosis A able to undergo mitosis A can replicate I reproduce R uncontrolled division
[1] idea that genes not switched off; ora
[1] growing/storage, area / region / correct named part of plant; examples of part of plant that stores/growing root/shoot tip / bud / flower / maturing leaf / tuber / fruit / seed I where sucrose, used / used up I nutrients unqualified receives, assimilates / sucrose / AW ; A area where sucrose unloaded in context of, via phloem/from source/from leaf
[1] high requirement for assimilates ;
[1] fast growing / highly active; AW e.g. requires a lot of energy
[1] active process / uses ATP / requires energy;
[1] hydrogen ions, pumped / active transport / move(d), out of companion cells ; this also gets mp1 if stated as, pumped/active transport/move by proton pump
[1] into cell wall / apoplast; A intercellular space R pumped into phloem sieve tube
[1] hydrogen ion gradient builds up ;
[1] hydrogen ions re-enter companion cell, down gradient / AW ; A facilitated diffusion
[1] via, cotransport protein(s) / cotransporter(s) ; A carrier protein(s)
[1] cotransport sucrose (molecules); A secondary active transport A idea of bringing sucrose into companion cell if cotransport / er stated for previous mp R cotransport if into phloem sieve tube from companion cell
[1] transport of sucrose against gradient ;
[1] diffusion of sucrose into phloem sieve tube (element); R facilitated diffusion A moves from high to low concentration
[1] via plasmodesmata ;