A Level Biology Revision Notes

Comprehensive A Level Biology revision notes providing information and assistance for all UK examination boards (AQA, OCR, Edexcel) as well as international curriculum (CIE).

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Michaelis-Menten Constant

Introduction We all know that enzymes are the natural catalysts that are present in all the living cells. They are necessary for all the metabolic reactions. Specific substrate molecules bind the enzyme at the active site and get converted into the products. Several models have been put forward to explain the enzyme kinetics. One of …

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Mitotic Cell Cycle

Introduction The multicellular organisms like humans are made up of millions of cells. These cells go through multiple stages during their life, collectively called cell cycle. Mitotic cycle is a type of cell cycle in which cell division takes place by the process of mitosis. Mitotic cell cycle is essential for making new daughter cells …

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Replication and Division of Nuclei and Cells

Introduction Cells are the basic unit of life. Recall that some organisms are constituted of only one cell while multicellular organisms have millions or even more cells. The division and replication of cell is necessary for the survival and continuity of life. In unicellular organisms, cell division is the process by which the organism replicates. …

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Skin Cells

Introduction Skin is not a tissue but an organ. It is the largest organ present in our body. Skin is made up of a large number of multiple types of cells. Each type of cell plays a significant role that collectively adds to the function of the skin. Skin not only acts as a barrier …

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Telomeres

Introduction Telomeres are distinctive cap-like structures present at the end of each strand of DNA. The name “Telomere” comes from the ancient Greek language; télos means ‘end’, and méros means ‘part’, so the literal meaning of telomere “ending part”. These are regions of repetitive nucleotide sequences that protect the DNA from damage. Almost all eukaryotes possess …

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Temporal and Spatial Summation

Introduction Neurons are continuously receiving from thousands of other neurons around it. However, whether these inputs are able to elicit an action potential or not depends on the summation of these inputs. Summation can be defined as a process by which the excitatory and inhibitory signals together are able to generate an action potential or …

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Repolarization

Introduction Repolarization is the phase that follows depolarization. During an action potential, the first stage is depolarization in which sodium ion channels open causing an influx of sodium ions into the neuron. This causes the membrane potential to reach approximately +40mV from a resting membrane potential of -70mV. At this membrane potential of about +40mV, …

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Neuromuscular Junction

Introduction The neuromuscular junctions play the important role of transferring action potentials to the skeletal muscle fibers. Fibers of the skeletal muscles are innervated by large, myelinated nerve fibers that originate in the large motor neurons in the anterior horn of the spinal cord. All of these fibers branch out to innervate anywhere from between …

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Nerve Impulse

Introduction Nerve impulse was discovered by British Scientist Lord Adrian in the 1930s. Owning to the importance of this discovery, he was awarded Noble Prize in 1932. Nerve Impulse is a major mode of signal transmission for the Nervous system. Neurons sense the changes in the environment and as a result, generate nerve impulses to …

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Myelinated and Unmyelinated Axons

Introduction Nerve fibers or axons are a part of the nervous system which receives environmental signals and produce a response. For response, the nervous system uses nerve fibers for transmitting nerve impulses to the target cells. These nerve fibers are of three types: Sensory neurons Interneurons Motor neurons These neurons are classified into two categories …

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