OxfordAQA IGCSE Biology: Organisation (9201)

This guide covers Topic 1 Organisation, the first of six topics in OxfordAQA International GCSE Biology (9201), for teaching from September 2016, exams May/June 2018 onwards. The qualification is linear and untiered, and is largely co-teachable with OxfordAQA International GCSE Combined Science (9204).

Where this fits in 9201

Topic 1 establishes how living things are organised, from cells up to whole organ systems, before the syllabus moves into Bioenergetics, Ecology, Organisms’ interaction with the environment, Inheritance, and Variation and evolution. The organisational hierarchy introduced here (cells, tissues, organs, systems) is assumed knowledge in later topics covering specific body systems.

Syllabus coverage

OXFORDAQA INTERNATIONAL GCSE BIOLOGY (9201) — TOPIC 1 ORGANISATION

• 3.1.1 Cell structure — the sub-cellular structures of animal and plant (eukaryotic) cells, and of bacterial (prokaryotic) cells; relating the structure of a specialised cell to its function
• 3.1.2 Principles of organisation — how cells differentiate and are organised into tissues, tissues into organs, and organs into organ systems
• 3.1.3 Animal tissues, organs and systems — muscular, glandular and epithelial tissue; the stomach as an example organ; the digestive system as an example organ system (the glands, stomach, small intestine, liver and large intestine, without the digestive enzymes, which belong to the following topic)
• 3.1.4 Plant tissues, organs and systems — epidermal tissue, palisade and spongy mesophyll, xylem and phloem; stems, roots and leaves as plant organs
• 3.1.5 Transport in cells — diffusion, osmosis and active transport; surface area to volume ratio; how exchange surfaces such as the small intestine, lungs, roots and leaves are adapted for exchanging materials

How to approach it

This topic is the foundation for the whole syllabus, so treat the cell-to-system hierarchy (cell → tissue → organ → organ system) as a recurring structure to apply to every organ studied later, rather than a one-off definition to memorise. Practise labelling diagrams of animal and plant cells and stating the function of each named structure, since structure-to-function questions are the most common format in this topic. For transport in cells, be precise about which process is which — diffusion, osmosis and active transport are frequently confused — and practise explaining how surface area, diffusion distance, blood supply and ventilation each affect the rate of exchange at a named surface (small intestine, lungs, roots or leaves).

Official syllabus

OxfordAQA International GCSE Biology (9201) specification PDF, Version 4.3 — oxfordaqa.com.

Cell structure (3.1.1)

Most animal cells (eukaryotic) have a nucleus (controls the cell’s activities), cytoplasm (where most chemical reactions happen), a cell membrane (controls what passes into and out of the cell), mitochondria (where most energy is released in respiration) and ribosomes (where protein synthesis occurs).

Plant cells (eukaryotic) have all of the above, plus chloroplasts (absorb light energy to make food) and a permanent vacuole filled with cell sap. Plant and algal cells also have a cell wall made of cellulose, which strengthens the cell.

A bacterial cell (prokaryotic) has cytoplasm and a membrane surrounded by a cell wall; its genes are not in a distinct nucleus, and some genes are located in circular structures called plasmids.

Cells may be specialised to carry out a particular function — you should be able to relate the structure of a named cell type to its role in a tissue, organ or organism.

Principles of organisation (3.1.2)

Large multicellular organisms need systems for exchanging materials. During development, cells differentiate so they can perform different functions. A tissue is a group of cells with similar structure and function. Organs are made of tissues — one organ may contain several different tissues. Organ systems are groups of organs that work together to perform a particular function.

Animal tissues, organs and systems (3.1.3)

Examples of animal tissue: muscular tissue (contracts to bring about movement), glandular tissue (produces substances such as enzymes and hormones) and epithelial tissue (covers parts of the body).

The stomach is an example organ, containing muscular tissue (to move contents through the digestive system), glandular tissue (to produce digestive juices) and epithelial tissue (to cover the inside and outside of the stomach).

The digestive system is an example organ system: glands such as the pancreas and salivary glands produce digestive juices; the stomach and small intestine are where digestion occurs; the liver produces bile; the small intestine is where soluble food is absorbed; and the large intestine absorbs water from the undigested food, producing faeces. The digestive enzymes themselves, and the roles of amylase, protease and lipase, are covered in the Bioenergetics topic, not here.

Plant tissues, organs and systems (3.1.4)

Examples of plant tissue: epidermal tissue (covers the plant), palisade mesophyll (carries out photosynthesis), spongy mesophyll (has air spaces to allow gases to diffuse) and xylem and phloem (transport substances around the plant). Plant organs include stems, roots and leaves.

Transport in cells (3.1.5)

Diffusion is the net movement of particles (in solution or in a gas) from a region of higher concentration to a region of lower concentration — the greater the concentration difference, the faster the rate. Oxygen needed for respiration passes through cell membranes by diffusion.

Osmosis is the diffusion of water from a dilute to a more concentrated solution through a partially permeable membrane. Differences in concentration inside and outside a cell cause water to move by osmosis, and you should be familiar with the terms isotonic, hypotonic, hypertonic, turgor and plasmolysis.

Active transport moves substances against a concentration gradient, using energy from respiration — for example, root hair cells absorbing mineral ions from very dilute soil solutions, or sugar being absorbed against a gradient in the intestine or kidney tubule.

A single-celled organism has a large surface area to volume ratio, so all necessary exchange can occur across its surface membrane. As organisms get larger and more complex, exchanging materials becomes harder, so multicellular organisms develop specialised exchange surfaces. An effective exchange surface has a large surface area relative to the volume it serves, is thin (a short diffusion path), and — in animals — has an efficient blood supply and, for gas exchange, is ventilated. The small intestine and lungs in mammals, and the roots and leaves in plants, are all adapted in these ways for exchanging materials.

Worked example: comparing two exchange surfaces

Explain why both the small intestine and the lungs have a large internal surface area, but only the lungs need to be ventilated.

Both the small intestine and the lungs are exchange surfaces, so
both need a large surface area and a short diffusion path (thin
walls) to maximise the rate of diffusion, and both have a rich
blood supply to maintain a steep concentration gradient.
The lungs additionally need ventilation because they exchange a
gas (oxygen and carbon dioxide) with the air: without a constant
supply of fresh air, the air in the alveoli would reach
equilibrium with the blood and diffusion would stop.
The small intestine does not need ventilating because it exchanges
dissolved food molecules with the blood, not a gas with the air --
the concentration gradient is maintained by blood flow alone.

Common mistakes

Muddling diffusion, osmosis and active transport — only active transport requires energy from respiration, and only osmosis is specifically about the movement of water. Describing a cell wall as present in all cells (it is not present in animal cells). Naming an organ system’s organs without saying what the system as a whole does. Confusing surface area to volume ratio with surface area alone when explaining why smaller or more specialised structures exchange materials more efficiently.

Quick revision checklist

• Label the sub-cellular structures of an animal cell, a plant cell and a bacterial cell, and state the function of each.
• State the cell → tissue → organ → organ system hierarchy with an example at each level.
• Name the animal tissues found in the stomach and the plant tissues found in a leaf.
• Define diffusion, osmosis and active transport, and say which one requires energy from respiration.
• Explain how the small intestine, lungs, roots and leaves are each adapted as exchange surfaces.