IB BIO ESSAYS

IB BIOLOGY ESSAYS List of Chapters (HL & SL) 1.Cell Biology 2.Molecular Biology 3.Genetics 4.Ecology 5.Evolution 6.Human Physiology 7.Nucleic Acids 8.Metabolism 9.Plant Biology 10.Genetics 11.Animal Physiology

1.1 Introduction to Cell 1.List the key statements of the cell theory. ●Living organisms are made up of one or more cells. ●There is no smaller unit of life than cells. ●Every new cell must come from a pre-existing cell. 2.Outline 3 exceptions to the cell theory. ●Giant algae can grow up to 10 cm in length for a single cell but only have 1 nucleus. ●Striated muscle cells are very long, up to 30 cm, and have multiple nuclei (multinucleate). ●Aseptate fungi hyphae have no cross walls and multiple nuclei. 3.State and define the 7 functions of life displayed by all living organisms. ●Metabolism is the sum of all chemical reactions within the living organism ●Reproduction is the production of offspring ●Homeostasis is the maintenance of the internal environment ●Nutrition is the consumption of materials for growth and repair. Nutrients are the source of energy or matter to build the organism ●Excretion is the removal of the waste products of metabolism ●Response is sensing and responding to external stimuli ●Growth is the increase in cell number or size 4.Explain how surface area to volume ratio limits the size of cells. ●The rate at which metabolic reactions occur increases as the cell volume increases. ●The rate at which raw materials enter the cell and waste materials exit increases as surface area increases. 2

●As the cell grows and size increases, the volume increases faster than surface area. Thus the surface area to volume ratio decreases. ●If a cell is too large, it cannot take in raw materials nor export waste products fast enough to function and it may overheat, thus a cell can only grow up to a certain size. 5.How does cell differentiation occur? ●Cell division ensures all cells in a multicellular organism are genetically identical, so every cell in the body has the full set of genes ●During the differentiation of a cell, only some of the genes are expressed, while others are not expressed. ●In each type of specialised cell, a unique subset of genes will be expressed. ●Gene expression results in proteins being synthesised which determines the function of the cell 6.Describe the characteristics of stem cells that make them potentially useful in medicine. [5m] ●Stem cells can continually divide to produce large numbers of identical cells. They can be used to replace or repair damaged cells. ●Stem cells are undifferentiated, thus have the ability to differentiate to produce a large variety of different tissues or organs. For example, skin stem cells can be grown to produce skin grafts for burn victims and pancreatic islet beta cells can be developed to treat type 1 diabetes 7.Describe Stargadt’s disease and how stem cells can be used to treat it. ●Stargadt’s disease is a genetic disease that causes macular degeneration in children and young adults. Photoreceptor cells in the retina become damaged and are progressively lost. Patients gradually lose their vision, usually resulting in blindness ●Embryonic stem cells can be used to treat Stargadt’s disease. Scientists obtained stem cells from days-old human embryos and made them develop into retina cells. These cells were then injected into patients’ eyes and helped improve vision 3

8.Describe Leukemia and how stem cells can be used to treat it. ●Leukemia is a type of cancer that causes a high number of abnormal white blood cells to be produced by the bone marrow ●Adult stem cells can be used to restore healthy bone marrow in leukemia patients. Fluid is taken from the patient’s bone marrow. Healthy blood stem cells from the bone marrow or donor are identified and frozen. The patient receives a high dose of chemotherapy to kill all the cancer cells in the bone marrow. The bone marrow is then unable to produce any more blood cells. The stored or donated blood stem cells are injected into the patient and enter the bone marrow via the bloodstream. The bone marrow regains its ability to produce healthy blood cells. 9.What are the ethical implications of using embryonic stem cells for therapeutic uses? FOR ●Early-stage embryos are just groups of cells that have yet to develop essential features of life ●Early-stage embryos lack a nervous system so do not feel pain or suffering ●Large number of embryos produced during IVF are not used for implantation; better to use as stem cells to treat illnesses AGAINST ●Embryo is human life at the earliest stage and is immoral if the embryo dies as a result of the procedure. ●IVF involves invasive surgical treatment for removal of eggs ●If women are paid for supplying eggs for IVF, could lead to exploitation of vulnerable groups of women 1.2 Ultrastructure of Cells 1.Define the term resolution. Distinguish the resolution of a light microscope (LM) and electron microscope (EM). ●Resolution is the smallest distance apart two objects can be to appear distinct. ●The resolution of an EM is much higher than a LM, at least 200 times higher. (1 nm compared to 200nm) 4

2.Draw a labelled diagram of Escherichia coli as an example of a prokaryote. [4m] 3.Draw a labelled diagram of a eukaryotic cell as seen in an electron micrograph. [4m] 5

4.Outline prokaryotic binary fission. ●The chromosome is replicated and each identical copy is moved to either end of the cell. The cell elongates. ●New cell wall forms and plasma membrane pinches in. ●Cross walls form 2 separate cells. ●The 2 new cells separate. 5.Outline the advantages of compartmentalisation in eukaryotic cells. ●Different metabolic processes can be separated. High concentrations of enzymes and substrates specific for each process can be achieved. ●Different metabolic processes may require different environments such as pH. ●These conditions can be optimised for each metabolic process while allowing both processes to occur in the same cell. ●Hydrolytic enzymes that can damage the cell can be separated (e.g lysozymes) ●Allows for a greater surface area for processes occurring within the cell membrane. For example, the production of ATP in the mitochondria via chemiosmosis. 1.3 Membrane Structure 1.Explain how phospholipids form a bilayer in an aqueous environment. ●Phospholipids are amphipathic - the phosphate head is hydrophilic and the fatty acid tails are hydrophobic. Thus, a phospholipid bilayer forms in water. The phosphate heads are attracted to the water, so are on the outside of the bilayer, in contact with the water. ●The fatty acid heads are repelled by water, thus face inwards of the bilayer, in contact with each other. The surface of the bilayer is hydrophilic and the inside of the bilayer is hydrophobic. 6

2.Outline the evidence that led to the Davson-Danielli model. ●Chemical analysis of the membranes showed that they were composed of phospholipid and protein. Evidence suggested that the plasma membranes of red blood cells have enough phospholipids in them to form an area twice as large as the area of the plasma membrane, suggesting a phospholipid bilayer, ●Electron micrographs showed the plasma membrane as two dark bands on the outside and a light band in between, proteins appear very dark and phospholipids appear light. This supported the Davson-Danielli model. 3.Outline evidence for the falsification of the Davson-Danielli model. ●Freeze fracture electron micrographs showed globular proteins present on the upper surface of the inner phospholipid layer. ●Proteins extracted from the plasma membrane were globular and varied in size and parts of their surface were hydrophobic, suggesting that proteins were embedded within the phospholipid bilayer and their hydrophobic regions could attract the fatty acid tails. ●This showed that proteins did not form layers on the surface of the plasma membrane. ●X-ray diffusion showed that at higher temperatures, the membrane behaved as liquid. ●Membrane proteins of two cells were lin