1.1 Introduction to Cells
Cell Theory
1.1.1 According to the cell theory, living organisms are composed of cells
Basis of Cell Theory
cells are the basic structural and functional units of all living things
new cells arise from pre-exisiting cells
cells are the smallest microscopic units of life
Atypical Cells
1.1.2 Application: Questioning the cell theory using atypical examples, including striated muscle, giant algae and aseptate fungal hyphae
Muscle Fibres
Typical Cells:
Cells are considered to be single units enclosed in a cell membrane containing a single nucleus.
Exception:
Skeletal muscles contain more than one nucleus (multinucleate) surrounded by a plasma membrane (sarcolemma)
Fungal Hyphae
Typical Cells:
Cells are usually microscopic with typical plant cells measuring 10-100 µm.
Exception:
Giant algae are atypical cells due to their size, these cells are 5-100 mm (5000 - 100 000 µm
Giant Algae
Typical Cells:
Cells are single units surrounded by a cell membrane and are often microscopic.
Exceptions:
Fungal hyphae are atypical as they are not a single unit, they are elongated with multiple nuclei and no end walls between cells
Magnification
1.1.3 Skill: Use of a light microscope to investigate the structure of cells and tissues, with drawing of cells.
1.1.4 Skill: Calculation of the magnification of drawings and the actual size of structures and ultrastructures shown in drawings or micrographs. (Scale bars are useful as a way of indicating actual size in drawings and micrographs).
Functions of Life
1.1.5 Organisms consisting of only one cell carry out all functions of life in that cell. (Students are expected to be able to name and briefly explain these functions of life: nutrition, metabolism, growth, response, excretion, homeostasis and reproduction).
1.1.6 Application: Investigation of functions of life in Paramecium and one named photosynthetic unicellular organism (Chlorella or Scenedesmus are suitable photosynthetic unicells, but Euglena should be avoided as it can feed heterotrophically).
Nutrition - exchange of materials and gases with the environment
Metabolism (e.g. respiration) - essential biochemical reactions
Growth - cells replicate to allow organisms to change size
Response - detection and response to both internal and external stimuli
Excretion - removal of metabolic waste products
Homeostasis - maintaining a stable internal environment
Reproduction - asexual or sexual production of offspring
The Paramecium to the left is a single-celled organism. It can be used to demonstrate the functions of life. Try the widget to drag and drop structures of Paramecium which relate to the functions of life.
Surface Area : Volume
1.1.7 Surface area to volume ratio is important in the limitation of cell size
What does this mean for Cells?
Small cells have a large surface area to volume ratio.
With a large surface area to volume ratio diffusion is sufficient to meet the demands of the volume (e.g. oxygen for respiration).
SA : Vol restricts cell size.
As cells increase in size they must divide to maintain a large surface area to volume ratio.
As objects increase in size, surface area and volume do not increase proportionally. Volume increases MORE than surface area, leading to a smaller surface area to volume ratio (SA : Vol).
Emergent Properties
1.1.8 Multicellular organisms have properties that emerge from the interaction of their cellular components.
Extension Website:
The ‘product’ is more complex than the sum of it’s ‘parts
individual cells can carry out basic functions, combining cells leads to much more complex functioning
e.g. the heart is composed of cardiac muscle cells and nerve cells. Individual cardiac cells can contract. Nerve cells carry impulses. However the combination of all cells results in a rhythmically contracting pump which can speed up or slow down depending on input received.
Which image illustrates emergent properties?
Image 2: Product of Interactions
Sum of interactions does not take into account that all components can interact. Product of interactions takes into account possible interactions of component parts and resultant emergent properties.
Differentiation
1.1.9 Specialised tissues can develop by cell differentiation in multicellular organisms
1.1.10 Differentiation involves the expression of some genes and not others in a cell’s genome
Gene Expression leads to differentiation and formation of specialised cells.
Cells contain the complete genome i.e. all genetic information.
Genes encode specific proteins which control cell properties.
Cell differentiation leading to specialisation is brought about by gene expression.
Example: in red blood cells the haemoglobin gene is expressed "switched-on" , other genes are not expressed.
Stem Cells
1.1.11 The capacity of stem cells to divide and differentiate along different pathways is necessary in embryonic development and also makes stem cells suitable for therapeutic uses.
1.1.12 Application: Use of stem cells to treat Stargardt’s disease and one other named condition.
1.1.13 Application: Ethics of the therapeutic use of stem cells from specially created embryos, from the umbilical cord blood of a new-born baby and from an adult’s own tissues.
Dividing and Differentiating
stem cells contain the full genome and have potential to differentiate into all cell types
stem cells have the capacity to divide indefinitely
differentiation and specialisation at embryonic level is determined by gene expression
stem cells are obtained from human embryos left over from IVF treatment, umbilical cord blood and also some adult tissues e.g. bone marrow.
Ethics of Stem Cell Treatment
Stem Cell Treatment
Stem cell therapy is based on replacing diseased cells with stem cells
Use in medical treatment
Diseased cells are destroyed e.g. chemotherapy and radiation therapy in leukaemia.
Desired type of stem cell is identified and grown in a chemical/ hormonal solution to induce differentiate into desired cell type.
Methods are under development to implant stem cells into host, currently they are injected into host.
Danger of rejection is counteracted with immunosuppressant drugs.
Potential of stem cells developing into cancerous tumours.
Extension "Guardian" Article:
Stem cell therapy success in treatment of sight loss from macular degeneration.