Most Common Topics on Leaving Cert Biology

Plant biology comes up more than any other topic on the recent Leaving Cert Biology papers. Across the 22 papers from 2015 to 2025, Higher and Ordinary Level together, it covers 62 of the 349 questions and 2,740 of the 13,280 printed marks, and it appears in every one of the 22. Six other topic areas also appear in all 22 papers, so most of the course recurred every year. The counts below therefore separate the strands by where in the paper they arrived and what they were worth there.

The papers this post counts

Our copy of the exam corpus covers the Higher and Ordinary Level Biology papers for the eleven years 2015 to 2025, question by question. That is 22 papers and 349 questions, 175 of them Higher Level and 174 Ordinary Level.

Each paper printed more marks than any candidate answered. A candidate answered 400 marks, made up of five Section A questions at 20 marks each, two Section B questions at 30, and four Section C questions at 60. The papers from 2015 to 2020 printed 570 marks across 15 questions. From 2021 they print 650 across 17. The two added questions widen the choice, and the Exam Material Archive flags papers from 2021 onwards as carrying assessment adjustments made after learning was disrupted during Covid. The 2021 Ordinary Level paper is the one exception at 590 marks across 16 questions.

Adding those up gives the 13,280 printed marks this post counts against: 2,840 in Section A, 1,980 in Section B and 8,460 in Section C.

Biology changes shape in 2027

These papers were set on the syllabus that finishes with the June 2026 exam. Students who started fifth year in September 2025 are assessed on the redeveloped Leaving Certificate Biology specification in June 2027, under Senior Cycle Redevelopment. Their assessment is a 300-mark written paper of 2 hours 30 minutes plus a 200-mark Biology in Practice Investigation, and Sections A, B and C are gone. Our post on getting a H1 in Leaving Cert Biology sets out both components.

So the frequency data here describes biology content on the old papers. The section splits describe a paper structure that ends with the 2026 exam.

How the topics are grouped

Every question in the corpus carries freeform tags written during extraction, at the grain of osmosis, xylem or sex linkage. Those tags are grouped into 14 strands taken from the Biology syllabus. The Biology topic pages use the same grouping, so each row in the table below is a page holding the questions it counts.

A biology question often covers more than one strand. Of the 349 questions, 170 sit in a single strand, 113 in two, 52 in three and 9 in four. Five cover none, and all five are Section A true-or-false or terminology round-ups that range across the whole course. Every strand count below therefore includes questions shared with another strand, which is why the mark shares total more than 100 percent.

Seven strands appeared in all 22 papers

The papers column asks whether a strand reached a paper at all, counting a paper once if it held one question or five. The marks column adds up the marks printed on those questions, and the share divides that by 13,280.

Thirteen of the fourteen strands appeared in all eleven years. Human reproduction is the exception at ten years. It appears in 16 of the 22 papers and is absent from 2022 at both levels, which is the only year in the sample with no question on it.

The strands separate much more by marks than by recurrence. Plant biology carries 2,740 printed marks and cell division carries 740, a gap of about four times, and both were on a paper in every one of the eleven years.

Where a topic appears decides what it is worth

The old papers ran three sections with different jobs. Section A held short questions at 20 marks, Section B held the practical questions at 30 marks, and Section C held the long questions at 60 marks. A strand's position in the table below therefore tells you what size of answer it has come with.

Columns A, B and C are the three sections, counting questions. The strand names are shortened here to their opening words, and the rows are sorted by Section C questions. They are the same fourteen strands, named in full and linked in the table above.

Food and biomolecules appeared in all 22 papers, and 25 of its 35 questions are in Section A, against 2 in Section C, so it arrived mostly as a 20-mark question. Cell structure behaves the same way, with 21 of its 38 questions in Section A and 4 in Section C.

Scientific method sits at the other extreme. It carries 45 of the 66 Section B questions and none at all in Section C, because Section B was the practicals section and the general experimental-design tags sit there.

Plant biology reaches all three sections, with 34 Section C questions, 14 in Section B and 14 in Section A. Three strands never appeared in Section B in these eleven years, those being human responses, human reproduction and cell division, so none of them had a mandatory practical attached under the old syllabus.

Section B is worth a closer look if you are on the new specification, since 30 of the 33 Higher Level Section B questions ask about a laboratory activity carried out during the course. Those numbers are in the H1 Biology post, which covers what the old practicals are worth against the new investigation.

Choice meant a candidate met only part of the paper

The printed-mark shares above count what was on the paper. In Section C a candidate answered four questions out of the six or seven printed, so a strand could appear on a paper and never reach that candidate's script.

The choice was wide enough that a dropped strand could usually be avoided in Section C. For twelve of the fourteen strands, every one of the 22 papers offered at least four Section C questions that avoided the strand completely. Plant biology, the largest strand in the table, was avoidable in Section C on 21 of the 22 papers. The exception is the 2020 Higher Level paper, where only three of the six Section C questions avoided plant biology, so four answers could not be picked without meeting it.

Human responses is the other strand that closed off a choice, on the 2018 Higher Level and 2019 Ordinary Level papers. Everything else in the table was avoidable in Section C every year in the sample.

Read the shares with that in mind. A share of printed marks describes the paper as set, and it overstates what a candidate who skipped a topic actually faced.

What carries into the new specification

The redeveloped specification is built from a unifying strand, Nature of Science, and three contextual strands, Organisation of Life, Structures and Processes of Life, and Interactions of Life. Those three are divided into fourteen numbered units.

Most of the content in the table above is named in those units. Biomolecules are unit 1.2, cells 1.3, genetic inheritance 1.4 and evolution 1.5. Enzymes are 2.1, photosynthesis and respiration 2.2, and cell division and protein synthesis 2.3. Ecology, ecosystems and biodiversity are 3.1, microorganisms and nutrient cycling 3.2, and genetic engineering 3.3.

Two of our strands have no single home in the new arrangement.

Plant biology is the larger case. The new specification has no plant unit. Plant material is distributed into three units of Strand 2, with plant adaptations for response in 2.4, flowers, pollination, seeds and dispersal in 2.5, and xylem, phloem, transpiration and stomata in 2.6. Human responses and human reproduction sit in those same units, 2.4 and 2.5, so plant and human material that the old syllabus separated is now taught side by side. Our Plant Biology strand groups the questions the way the old papers set them.

Scientific method is the smaller case. Its content maps to the unifying Nature of Science strand, which the specification describes as underpinning all three contextual strands rather than sitting beside them, and to the Biology in Practice Investigation.

One caution on all of this. Unit headings tell you a content area is still on the course. They say nothing about how deeply it is examined, which is set by the learning outcomes and by the SEC sample papers. A topic that carried a 60-mark question on an old paper has no claim on the same weight in 2027.

Frequency is a record, and prediction is a different thing

Every strand in the table remains examinable in any year. Eleven years of history sets no rule for the twelfth. The State Examinations Commission has warned candidates against any source that claims to know a paper in advance, and has said prediction claims are spurious however they were produced. The sourced quote and the date are in our post on whether Leaving Cert questions repeat, which also has the word-level comparison of Higher Level Biology questions from different years.

The one thing the table describes without any inference is the material you have to practise on. You have been working off these same papers, so the counts are the number of attempts available to you on each strand. Human reproduction offers 18 questions and 880 marks of it across eleven years, and plant biology offers 62 questions and 2,740 marks. Those totals are already fixed, whatever next June holds.

The dangerous use of a table like this is dropping whatever sits at the bottom of it. Human reproduction is the thinnest strand in the table at 18 questions, and it still carried a 60-mark Section C question on both 2025 papers.

How to use the counts

Pick one strand and answer its questions from several years in a row, closed book, marking each against the official scheme before you move on. That method is set out in our guide on studying with past papers. By the third or fourth question on one strand you are meeting the same content in a layout you have not seen before, which is the thing a single question cannot give you.

Start with the strand you have done least of, then use the section splits to choose the format. If you want long-question practice, plant biology, human responses and human nutrition appear between them in 68 of the 141 Section C questions. If you want experimental work for the investigation, scientific method and the study of life holds the 45 Section B questions on experimental design and enzymes, metabolism and membrane transport holds another 19.

The same counting exercise for another subject is in our post on the most common topics in Higher Level maths.

FAQ

What is the most common topic on Leaving Cert Biology? Across the 22 Higher and Ordinary Level papers from 2015 to 2025, plant biology carries the most questions and marks, at 62 of the 349 questions and 2,740 of the 13,280 printed marks. It appeared in all 22 papers, as did six other strands.

What comes up every year in Biology? Thirteen of the fourteen strands appeared in all eleven years from 2015 to 2025. Seven of them appeared in all 22 papers. Human reproduction appeared in ten of the eleven years, missing 2022 at both levels.

What is the least common topic? By marks it is cell division, at 740 of the 13,280 printed marks across 21 questions, and it still appeared in all eleven years. By papers it is human reproduction, in 16 of the 22.

Does this data still apply to the new Biology course? The content areas mostly carry over, and they are arranged differently. The new specification has no plant biology unit, spreading that material across response, reproduction, and transport and transfer. The section splits in this post describe the old paper structure, which ends with the 2026 exam.

Can I use this to predict the Biology paper? No. The whole course is examinable every year, and the State Examinations Commission warns against sources claiming advance knowledge of a paper. The honest use of a frequency table is to find the parts of the course you have the least practice on.

Why do the mark shares add to more than 100 percent? Because 174 of the 349 questions belong to two or more strands, and each strand is credited with the full marks of any question it appears in.

The Leaving Cert Biology topic pages hold every question counted here, sorted into these strands and then by year, each one sitting beside its marking scheme. Every strand also has a free PDF pack at both levels, and no account is needed for any of it. The past paper finder has the whole papers if you want to sit one under timed conditions.