All organisms are composed of cells. Knowledge of their structure and function underpins much of biology. The fundamental differences between eukaryotic and prokaryotic cells are explored and provide useful biological background for the section on Infectious disease. Viruses are introduced as non-cellular structures, which gives candidates the opportunity to consider whether cells are a fundamental property of life.

The use of light microscopes is a fundamental skill that is developed in this section and applied throughout several other sections of the syllabus. Throughout the course, photomicrographs and electron micrographs from transmission and scanning electron microscopes should be studied.

Candidates will be expected to use the knowledge gained in this section to solve problems in familiar and unfamiliar contexts.

An understanding of the principles of microscopy shows why light and electron microscopes have been essential in improving our knowledge of cells.

a) compare the structure of typical animal and plant cells by making temporary preparations of living material and using photomicrographs

b) calculate the linear magnifications of drawings, photomicrographs and electron micrographs

c) use an eyepiece graticule and stage micrometer scale to measure cells and be familiar with units (millimetre, micrometre, nanometre) used in cell studies

d) explain and distinguish between resolution and magnification, with reference to light microscopy and electron microscopy

e) calculate actual sizes of specimens from drawings, photomicrographs and electron micrographs

The cell is the basic unit of all living organisms. The interrelationships between these cell structures show how cells function to transfer energy, produce biological molecules including proteins and exchange substances with their surroundings. Prokaryotic cells and eukaryotic cells share some features, but the differences between them illustrate the divide between these two cell types.

a) describe and interpret electron micrographs and drawings of typical animal and plant cells as seen with the electron microscope

b) recognise the following eukaryotic cell structures and outline their functions:

• cell surface membrane

• nucleus, nuclear envelope and nucleolus

• rough endoplasmic reticulum

• smooth endoplasmic reticulum

• Golgi body (Golgi apparatus or Golgi complex)

• mitochondria (including small circular DNA)

• ribosomes (80S in the cytoplasm and 70S in chloroplasts and mitochondria)

• lysosomes

• centrioles and microtubules

• chloroplasts (including small circular DNA)

• cell wall

• plasmodesmata

• large permanent vacuole and tonoplast of plant cells

c) state that ATP is produced in mitochondria and chloroplasts and outline the role of ATP in cells

d) outline key structural features of typical prokaryotic cells as seen in a typical bacterium (including: unicellular, 1–5μm diameter, peptidoglycan cell walls, lack of membrane-bound organelles, naked circular DNA, 70S ribosomes)

e) compare and contrast the structure of typical prokaryotic cells with typical eukaryotic cells (reference to mesosomes should not be included)

f) outline the key features of viruses as non-cellular structures (limited to protein coat and DNA/RNA)

This section introduces carbohydrates, proteins and lipids: organic molecules that are important in cells. Nucleic acids are covered in a separate section. Biological molecules are based on the versatile element carbon. This section explains how macromolecules, which have a great diversity of function in organisms, are assembled from smaller organic molecules such as glucose, amino acids, glycerol and fatty acids. Life as we know it would not be possible without water. Understanding the properties of this extraordinary molecule is an essential part of any study of biological molecules. The emphasis in this section is on the relationship between molecular structures and their functions. Some of these ideas are continued in other sections, for example, the functions of haemoglobin in gas transport in Transport of mammals, phospholipids in membranes in Cell membranes and transport and antibodies in Immunity. Candidates will be expected to use the knowledge gained in this section to solve problems in familiar and unfamiliar contexts.

Tests for biological molecules can be used in a variety of contexts, such as identifying the contents of mixtures of molecules and following the activity of digestive enzymes.

a) carry out tests for reducing sugars and non-reducing sugars, the iodine in potassium iodide solution test for starch, the emulsion test for lipids and the biuret test for proteins to identify the contents of solutions.

b) carry out a semi-quantitative Benedict’s test on a reducing sugar using dilution, standardising the test and using the results (colour standards or time to first colour change) to estimate the concentration .

Carbohydrates and lipids have important roles in the provision and storage of energy and for a variety of other functions such as providing barriers around cells: the phospholipid bilayer of all cell membranes and the cellulose cell walls of plant cells.

a) describe the ring forms of α-glucose and β-glucose.

b) define the terms monomer, polymer, macromolecule, monosaccharide, disaccharide and polysaccharide .

c) describe the formation of a glycosidic bond by condensation, with reference both to polysaccharides and to disaccharides, including sucrose .

d) describe the breakage of glycosidic bonds in polysaccharides and disaccharides by hydrolysis, with reference to the non-reducing sugar test.

e) describe the molecular structure of polysaccharides including starch (amylose and amylopectin), glycogen and cellulose and relate these structures to their functions in living organisms.

f) describe the molecular structure of a triglyceride with reference to the formation of ester bonds and relate the structure of triglycerides to their functions in living organisms .

g) describe the structure of a phospholipid and relate the structure of phospholipids to their functions in living organisms.

An understanding of protein structure and how it is related to function is central to many aspects of biology, such as enzymes, antibodies and muscle contraction. Globular and fibrous proteins play important roles in biological processes such as the transport of gases and providing support for tissues. Water is a special molecule with extraordinary properties that make life possible on this planet 150 million kilometres from the Sun.

a) describe the structure of an amino acid and the formation and breakage of a peptide bond.

b) explain the meaning of the terms primary structure, secondary structure, tertiary structure and quaternary structure of proteins and describe the types of bonding (hydrogen, ionic, disulfide and hydrophobic interactions) that hold these molecules in shape.

c) describe the molecular structure of haemoglobin as an example of a globular protein, and of collagen as an example of a fibrous protein and relate these structures to their functions (The importance of iron in the haemoglobin molecule should be emphasised. A haemoglobin molecule is composed of two alpha (α) chains and two beta (β) chains, although when describing the chains the terms α-globin and β-globin may be used. There should be a distinction between collagen molecules and collagen fibres).

d) explain how hydrogen bonding occurs between water molecules and relate the properties of water to its roles in living organisms (limited to solvent action, specific heat capacity and latent heat of vapourisation).

There are many different enzymes, each one specific to a particular reaction. This specificity is the key to understanding the efficient functioning of cells and living organisms

a) explain that enzymes are globular proteins that catalyse metabolic reactions

b) state that enzymes function inside cells (intracellular enzymes) and outside cells (extracellular enzymes)

c) explain the mode of action of enzymes in terms of an active site, enzyme/substrate complex, lowering of activation energy and enzyme specificity (the lock and key hypothesis and the induced fit hypothesis should be included)

d) investigate the progress of an enzyme-catalysed reaction by measuring rates of formation of products (for example, using catalase) or rates of disappearance of substrate (for example, using amylase)

Investigating the effects of factors on enzyme activity gives opportunities for planning and carrying out experiments under controlled conditions.

a) investigate and explain the effects of the following factors on the rate of enzyme-catalysed reactions:

• temperature

• pH (using buffer solutions)

• enzyme concentration

• substrate concentration

• inhibitor concentration

b) explain that the maximum rate of reaction (Vmax) is used to derive the Michaelis-Menten constant (Km) which is used to compare the affinity of different enzymes for their substrates

c) explain the effects of inhibitors, both competitive and noncompetitive, on the rate of enzyme activity

d) investigate and explain the effect of immobilising an enzyme in alginate on its activity as compared with its activity when free in solution

The fluid mosaic model introduced in 1972 describes the way in which biological molecules are arranged to form cell membranes. The model has stood the test of time as a way to visualise membrane structure and continues to be modified as understanding improves of the ways in which substances cross membranes, how cells interact and how cells respond to signals. The model also provides the basis for our understanding of passive and active movement between cells and their surroundings, cell to cell interactions and long distance cell signalling.

Investigating the effects of different factors on diffusion, osmosis and membrane permeability involves an understanding of the properties of phospholipids and proteins covered in the section on Biological molecules.

Candidates will be expected to use the knowledge gained in this section to solve problems in familiar and unfamiliar contexts.

The fluid mosaic model allows an understanding of how substances enter and exit cells by a variety of different mechanisms. Investigating the effect of increasing the size of model cells allows an understanding of the constraints of obtaining resources across the cell surface and moving substances out of cells.

a) describe and explain the processes of diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis (no calculations involving water potential will be set)

b) investigate diffusion and osmosis using plant tissue and nonliving materials, such as Visking tubing and agar

c) calculate surface areas and volumes of simple shapes (including cubes) to illustrate the principle that surface area to volume ratios decrease with increasing size

d) investigate the effect of changing surface area to volume ratio on diffusion using agar blocks of different sizes

e) investigate the effects of immersing plant tissues in solutions of different water potentials, using the results to estimate the water potential of the tissues

f) explain the movement of water between cells and solutions with different water potentials and explain the different effects on plant and animal cells

When body cells reach a certain size they divide into two. Nuclear division occurs first, followed by division of the cytoplasm. The mitotic cell cycle of eukaryotes involves DNA replication followed by nuclear division. This ensures the genetic uniformity of all daughter cells.

Candidates will be expected to use the knowledge gained in this section to solve problems in familiar and unfamiliar contexts.

DNA is replicated and passed to daughter cells. Stem cells in bone marrow and the skin continually divide by mitosis to provide a continuous supply of cells that differentiate into blood and skin cells.

a) describe the structure of a chromosome, limited to DNA, histone proteins, chromatids, centromere and telomeres

b) explain the importance of mitosis in the production of genetically identical cells, growth, cell replacement, repair of tissues and asexual reproduction

c) outline the cell cycle, including interphase (growth in G1 and G2 phases and DNA replication in S phase), mitosis and cytokinesis

d) outline the significance of telomeres in permitting continued replication and preventing the loss of genes

e) outline the significance of stem cells in cell replacement and tissue repair by mitosis and state that uncontrolled cell division can result in the formation of a tumour

The events that occur during mitosis can be followed by using a light microscope.

a) describe, with the aid of photomicrographs and diagrams, the behaviour of chromosomes in plant and animal cells during the mitotic cell cycle and the associated behaviour of the nuclear envelope, cell surface membrane and the spindle (names of the main stages of mitosis are expected)

b) observe and draw the mitotic stages visible in temporary root tip squash preparations and in prepared slides of root tips of species such as those of Vicia faba and Allium cepa