Cell structure — IB Diploma Biology HL

Revision notes

9 short notes, in the order of the specification. Each one in short:

1. Cell theory and structures common to all cells

   Cells are the basic structural unit of all living organisms: every organism consists of one or more cells. Typical cells share three structures: DNA as the genetic material, a cytoplasm composed mainly of water, and a plasma membrane composed of lipids that encloses the cytoplasm. Each is needed for the cell to function and reproduce.

2. Microscopy skills

   Practical

   Microscopy skills include making temporary mounts, staining, focusing with the coarse and fine adjustments, measuring with a calibrated eyepiece graticule and taking photographs. Magnification is image size divided by actual size, so actual size equals image size divided by magnification. Always convert both measurements to the same unit, such as micrometres, before dividing.

3. Developments in microscopy

   Electron microscopes use a beam of electrons, which has a much shorter wavelength than light, so they have far higher resolution and reveal cell ultrastructure. Freeze fracture splits frozen membranes to show the proteins inside them, cryogenic electron microscopy reveals the 3D structure of proteins, and fluorescent stains and immunofluorescence show where specific molecules are inside cells.

4. Prokaryote and eukaryote cell structure

   Prokaryote cells have a cell wall, plasma membrane, cytoplasm, naked DNA in a loop and 70S ribosomes, but no nucleus; Gram-positive eubacteria such as Bacillus and Staphylococcus are the type studied. Eukaryote cells have a compartmentalised cytoplasm with 80S ribosomes, a nucleus with a double membrane and pores, membrane-bound organelles and a cytoskeleton.

5. Processes of life in unicellular organisms

   A unicellular organism must carry out all the functions of life within one cell: homeostasis, metabolism, nutrition, movement, excretion, growth, response to stimuli and reproduction. In Paramecium, for example, contractile vacuoles expel excess water for homeostasis, cilia provide movement, food vacuoles digest food, and the cell reproduces by dividing in two.

6. Animal, fungal and plant cells, and atypical cells

   Plant, fungal and animal cells differ in their cell walls, vacuoles, plastids, centrioles, cilia and flagella. Plants have cellulose walls, large permanent vacuoles and chloroplasts; fungi have chitin walls and no plastids; animals have no wall but have centrioles. Atypical cells include multinucleate aseptate fungal hyphae and skeletal muscle fibres, and red blood cells and sieve tubes without nuclei.

7. Micrographs, drawing and annotation

   In light and electron micrographs, cells are identified as prokaryote, plant or animal from features such as the nucleus, cell wall, vacuole and chloroplasts. In electron micrographs you should recognise structures including the mitochondrion, Golgi apparatus and rough and smooth endoplasmic reticulum. Drawings use clear pencil lines and ruled labels, and annotations state each structure's function.

8. Origin of eukaryotic cells by endosymbiosis

   HL only

   Endosymbiosis is the theory that mitochondria and chloroplasts evolved from free-living prokaryotes that were taken into another cell and survived inside it. All eukaryotes evolved from a common unicellular ancestor with a nucleus; mitochondria arose first, then chloroplasts in some lineages. Evidence includes their 70S ribosomes, naked circular DNA and ability to replicate.

9. Cell differentiation and the evolution of multicellularity

   HL only

   Cell differentiation is the process by which cells become specialised to form tissues in multicellular organisms. Almost every cell has the same genes, but different patterns of gene expression, often triggered by changes in the environment, make cells develop differently. Multicellularity has evolved repeatedly and has the advantages of allowing larger body size and cell specialisation.