3.2 Cells
All life on Earth exists as cells. These have basic features in common. Differences between cells are due to the addition of extra features. This provides indirect evidence for evolution.
All cells arise from other cells, by binary fission in prokaryotic cells and by mitosis and meiosis in eukaryotic cells.
All cells have a cell-surface membrane and, in addition, eukaryotic cells have internal membranes. The basic structure of these membranes is the same and enables control of the passage of substances across exchange surfaces by passive or active transport.
Cell-surface membranes contain embedded proteins. Some of these are involved in cell signalling – communication between cells. Others act as antigens, allowing recognition of ‘self’ and ‘foreign’ cells by the immune system. Interactions between different types of cell are involved in disease, recovery from disease and prevention of symptoms occurring at a later date if exposed to the same antigen, or antigen-bearing pathogen.
Cell structure
The cell theory is a unifying concept in biology.
Structure of eukaryotic cells
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The structure of eukaryotic cells, restricted to the structure and function of:
In complex multicellular organisms, eukaryotic cells become specialised for specific functions. Specialised cells are organised into tissues, tissues into organs and organs into systems. Students should be able to apply their knowledge of these features in explaining adaptations of eukaryotic cells. |
Structure of prokaryotic cells and of viruses
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Prokaryotic cells are much smaller than eukaryotic cells. They also differ from eukaryotic cells in having:
In addition, many prokaryotic cells have:
Details of these structural differences are not required. Viruses are acellular and non-living. The structure of virus particles to include genetic material, capsid and attachment protein. |
Methods of studying cells
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The principles and limitations of optical microscopes, transmission electron microscopes and scanning electron microscopes. Measuring the size of an object viewed with an optical microscope. The difference between magnification and resolution. Use of the formula: Principles of cell fractionation and ultracentrifugation as used to separate cell components. Students should be able to appreciate that there was a considerable period of time during which the scientific community distinguished between artefacts and cell organelles. |
AT d, e and f Students could use iodine in potassium iodide solution to identify starch grains in plant cells. MS 1.8 |
All cells arise from other cells
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Within multicellular organisms, not all cells retain the ability to divide. Eukaryotic cells that do retain the ability to divide show a cell cycle.
The behaviour of chromosomes during interphase, prophase, metaphase, anaphase and telophase of mitosis. The role of spindle fibres attached to centromeres in the separation of chromatids. Division of the cytoplasm (cytokinesis) usually occurs, producing two new cells. Meiosis is covered in section 3.4.3 Students should be able to:
Mitosis is a controlled process. Uncontrolled cell division can lead to the formation of tumours and of cancers. Many cancer treatments are directed at controlling the rate of cell division. Binary fission in prokaryotic cells involves:
Being non-living, viruses do not undergo cell division. Following injection of their nucleic acid, the infected host cell replicates the virus particles. |
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Required practical 2: Preparation of stained squashes of cells from plant root tips; set-up and use of an optical microscope to identify the stages of mitosis in these stained squashes and calculation of a mitotic index. Students should measure the apparent size of cells in the root tip and calculate their actual size using the formula: |
AT d and e MS 0.3 Calculation of a mitotic index. MS 1.8 |
Transport across cell membranes
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The basic structure of all cell membranes, including cell-surface membranes and the membranes around the cell organelles of eukaryotes, is the same. The arrangement and any movement of phospholipids, proteins, glycoproteins and glycolipids in the fluid-mosaic model of membrane structure. Cholesterol may also be present in cell membranes where it restricts the movement of other molecules making up the membrane. Movement across membranes occurs by:
Cells may be adapted for rapid transport across their internal or external membranes by an increase in surface area of, or by an increase in the number of protein channels and carrier molecules in, their membranes. Students should be able to:
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Required practical 3: Production of a dilution series of a solute to produce a calibration curve with which to identify the water potential of plant tissue. Required practical 4: Investigation into the effect of a named variable on the permeability of cell-surface membranes. |
MS 3.2 Students could plot the data from their investigations in an appropriate format. MS 3.4 Students could determine the water potential of plant tissues using the intercept of a graph of, eg, water potential of solution against gain/loss of mass. |
Cell recognition and the immune system
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Each type of cell has specific molecules on its surface that identify it. These molecules include proteins and enable the immune system to identify:
Definition of antigen. The effect of antigen variability on disease and disease prevention. Phagocytosis of pathogens. The subsequent destruction of ingested pathogens by lysozymes. The response of T lymphocytes to a foreign antigen (the cellular response).
The response of B lymphocytes to a foreign antigen, clonal selection and the release of monoclonal antibodies (the humoral response).
The use of vaccines to provide protection for individuals and populations against disease. The concept of herd immunity. The differences between active and passive immunity. |
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Structure of the human immunodeficiency virus (HIV) and its replication in helper T cells. How HIV causes the symptoms of AIDS. Why antibiotics are ineffective against viruses. The use of monoclonal antibodies in:
Details of the commercial or scientific production of monoclonal antibodies are not required. Ethical issues associated with the use of vaccines and monoclonal antibodies. The use of antibodies in the ELISA test. Students should be able to:
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