2.1.1

Outline the cell theory.

2

Include the following.

• Living organisms are composed of cells.

• Cells are the smallest unit of life.

• Cells come from pre-existing cells.

2.1.2

Discuss the evidence for the cell theory.

3

TOK: The nature of scientific theories could be introduced here: the accumulation of evidence that allows a hypothesis to become a theory; whether a theory should be abandoned when there is evidence that it does not offer a full explanation; and what evidence is needed for a theory to be adopted or rejected.

2.1.3

State that unicellular organisms carry out all the functions of life.

1

Include metabolism, response, homeostasis, growth, reproduction and nutrition.

2.1.4

Compare the relative sizes of molecules, cell membrane thickness, viruses, bacteria, organelles and cells, using the appropriate SI unit.

3

Appreciation of relative size is required, such as molecules (1 nm), thickness of membranes (10 nm), viruses (100 nm), bacteria (1 µm), organelles (up to 10 µm), and most cells (up to 100 µm). The three-dimensional nature/shape of cells should be emphasized.

TOK: All the biological entities in the above list are beyond our ability to perceive directly. They must be observed through the use of technology such as the light microscope and the electron microscope. Is there any distinction to be drawn between knowledge claims dependent upon observations made directly with the senses and knowledge claims dependent upon observations assisted by technology?

2.1.5

Calculate the linear magnification of drawings and the actual size of specimens in images of known magnification.

2

Magnification could be stated (for example, ×250) or indicated by means of a scale bar, for example:

Aim 7: The size of objects in digital images of microscope fields could be analysed using graticule baselines and image-processing software.

2.1.6

Explain the importance of the surface area to volume ratio as a factor limiting cell size.

3

Mention the concept that the rate of heat production/waste production/resource consumption of a cell is a function of its volume, whereas the rate of exchange of materials and energy (heat) is a function of its surface area. Simple mathematical models involving cubes and the changes in the ratio that occur as the sides increase by one unit could be compared.

Aim 7: Data logging could be carried out to measure changes in conductivity in distilled water as salt diffuses out of salt–agar cubes of different dimensions.

2.1.7

State that multicellular organisms show emergent properties.

1

Emergent properties arise from the interaction of component parts: the whole is greater than the sum of its parts.

TOK: The concept of emergent properties has many implications in biology, and this is an opportunity to introduce them. Life itself can be viewed as an emergent property, and the nature of life could be discussed in the light of this, including differences between living and non-living things and problems about defining death in medical decisions.

2.1.8

Explain that cells in multicellular organisms differentiate to carry out specialized functions by expressing some of their genes but not others.

3

2.1.9

State that stem cells retain the capacity to divide and have the ability to differentiate along different pathways.

1

2.1.10

Outline one therapeutic use of stem cells.

2

This is an area of rapid development. In 2005, stem cells were used to restore the insulation tissue of neurons in laboratory rats, resulting in subsequent improvements in their mobility. Any example of the therapeutic use of stem cells in humans or other animals can be chosen.

Aim 8: There are ethical issues involved in stem cell research, whether humans or other animals are used. Use of embryonic stem cells involves the death of early-stage embryos, but if therapeutic cloning is successfully developed the suffering of patients with a wide variety of conditions could be reduced.

Int: Stem cell research has depended on the work of teams of scientists in many countries, who share results and so speed up the rate of progress. However, ethical concerns about the procedures have led to restrictions on research in some countries. National governments are influenced by local, cultural and religious traditions, which vary greatly, and these, therefore, have an impact on the work of scientists.

TOK: This is an opportunity to discuss balancing the huge opportunities of therapeutic cloning against the considerable risks—for example, stem cells developing into tumours.

Another issue is how the scientific community conveys information about its work to the wider community in such a way that informed decisions about research can be made.

2.2.1

Draw and label a diagram of the ultrastructure of Escherichia coli (E. coli) as an example of a prokaryote.

1

The diagram should show the cell wall, plasma membrane, cytoplasm, pili, flagella, ribosomes and nucleoid (region containing naked DNA).

Label this diagram.

Label answers

2.2.2

Annotate the diagram from 2.2.1 with the functions of each named structure.

2

2.2.3

Identify structures from 2.2.1 in electron micrographs of E. coli.

2

2.2.4

State that prokaryotic cells divide by binary fission.

1

2.3.1

Draw and label a diagram of the ultrastructure of a liver cell as an example of an animal cell.

1

The diagram should show free ribosomes, rough endoplasmic reticulum (rER), lysosome, Golgi apparatus, mitochondrion and nucleus. The term Golgi apparatus will be used in place of Golgi body, Golgi complex or dictyosome.

2.3.2

Annotate the diagram from 2.3.1 with the functions of each named structure.

2

2.3.3

Identify structures from 2.3.1 in electron micrographs of liver cells.

2

2.3.4

Compare prokaryotic and eukaryotic cells.

3

Differences should include:

• naked DNA versus DNA associated with proteins

• DNA in cytoplasm versus DNA enclosed in a nuclear envelope

• no mitochondria versus mitochondria

• 70S versus 80S ribosomes

• eukaryotic cells have internal membranes that compartmentalize their functions.

2.3.5

State three differences between plant and animal cells.

1

2.3.6

Outline two roles of extracellular components.

2

The plant cell wall maintains cell shape, prevents excessive water uptake, and holds the whole plant up against the force of gravity.

Animal cells secrete glycoproteins that form the extracellular matrix. This functions in support, adhesion and movement.

2.4.1

Draw and label a diagram to show the structure of membranes.

1

The diagram should show the phospholipid bilayer, cholesterol, glycoproteins, and integral and peripheral proteins. Use the term plasma membrane, not cell surface membrane, for the membrane surrounding the cytoplasm.

Integral proteins are embedded in the phospholipid of the membrane, whereas peripheral proteins are attached to its surface. Variations in composition related to the type of membrane are not required.

Aim 7: Data logging to measure the changes in membrane permeability using colorimeter probes can be used.

2.4.2

Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the structure of cell membranes.

3

2.4.3

List the functions of membrane proteins.

1

Include the following: hormone binding sites, immobilized enzymes, cell adhesion, cell-to-cell communication, channels for passive transport, and pumps for active transport.

2.4.4

Define diffusion and osmosis.

1

Diffusion is the passive movement of particles from a region of high concentration to a region of low concentration.

Osmosis is the passive movement of water molecules, across a partially permeable membrane, from a region of lower solute concentration to a region of higher solute concentration.

2.4.5

Explain passive transport across membranes by simple diffusion and facilitated diffusion.

3

2.4.6

Explain the role of protein pumps and ATP in active transport across membranes.

3

2.4.7

Explain how vesicles are used to transport materials within a cell between the rough endoplasmic reticulum, Golgi apparatus and plasma membrane.

3

2.4.8

Describe how the fluidity of the membrane allows it to change shape, break and re-form during endocytosis and exocytosis.

2

2.5.1

Outline the stages in the cell cycle, including interphase (G1, S, G2), mitosis and cytokinesis.

2

2.5.2

State that tumours (cancers) are the result of uncontrolled cell division and that these can occur in any organ or tissue.

1

2.5.3

State that interphase is an active period in the life of a cell when many metabolic reactions occur, including protein synthesis, DNA replication and an increase in the number of mitochondria and/or chloroplasts.

1

2.5.4

Describe the events that occur in the four phases of mitosis (prophase, metaphase, anaphase and telophase).

2

Include supercoiling of chromosomes, attachment of spindle microtubules to centromeres, splitting of centromeres, movement of sister chromosomes to opposite poles, and breakage and re-formation of nuclear membranes.

Textbooks vary in the use of the terms chromosome and chromatid. In this course, the two DNA molecules formed by DNA replication are considered to be sister chromatids until the splitting of the centromere at the start of anaphase; after this, they are individual chromosomes. The term kinetochore is not expected.

Aim 7: Students could determine mitotic index and fraction of cells in each phase of mitosis. Individual groups could paste data into a database. Pie charts could be constructed with a graphing computer program. If a graphing computer program is used in DCP for internal assessment, it should be according to the IA and ICT clarifications.

2.5.5

Explain how mitosis produces two genetically identical nuclei.

3

2.5.6

State that growth, embryonic development, tissue repair and asexual reproduction involve mitosis.

1