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A-Level Biology is HARD, there’s no denying it! Trying to cram all the theory, memorising all the practicals and applying it all to exam techniques – don’t make it easy.
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We cover every A-Level Biology Specification
All living things are made up of three main types of biological molecules. By understanding more about these important molecules we can learn about how living organisms function at a molecular level.
The variety of life, both past and present, is extensive, but the biochemical basis of life is similar for all living things. Monomers are the smaller units from which larger molecules are made.
Monosaccharides are the monomers from which larger carbohydrates are made. Polysaccharides are formed by the condensation of many monomer units.
Triglycerides and phospholipids are two groups of lipid.Triglycerides are formed by the condensation of one molecule of glycerol and three molecules of fatty acid.
Amino acids are the monomers from which proteins are made. A condensation reaction between two amino acids forms a peptide bond. A functional protein may contain one or more polypeptides.
Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are important information-carrying molecules. In all living cells, DNA holds genetic information and RNA transfers genetic information from DNA to the ribosomes.
A single molecule of adenosine triphosphate (ATP) is a nucleotide derivative and is formed from a molecule of ribose, a molecule of adenine and three phosphate groups.
Water is a major component of cells. It has several properties that are important in biology.
Inorganic ions occur in solution in the cytoplasm and body fluids of organisms, some in high concentrations and others in very low concentrations.
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All life on Earth exists as cells. These have basic features in common and differences between cells are due to the addition of extra features.
In complex multicellular organisms, eukaryotic cells become specialised for specific functions. Specialised cells are organised into tissues, tissues into organs and organs into systems.
The structure of eukaryotic cells, restricted to the structure and function of intracellular organelles. These include the nucleus, ribosomes, chloroplasts, Golgi apparatus and others.
Viruses are acellular and non-living. The structure of virus particles includes genetic material, capsid and attachment protein.
Within multicellular organisms, not all cells retain the ability to divide. Eukaryotic cells that do retain the ability to divide show a cell cycle.
Being non-living, viruses do not undergo cell division. Following injection of their nucleic acid, the infected host cell replicates the virus particles.
Set-up and use of an optical microscope to identify the stages of mitosis in these stained squashes and calculation of a mitotic index.
The basic structure of all cell membranes, including cell-surface membranes and the membranes around the cell organelles of eukaryotes, is the same.
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.
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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 pathogens, cells from other organisms of the same species, abnormal body cells and toxins.
An antigen is any substance that when introduced into the blood or tissue induces the production of antibodies. Antigens are usually large complex molecules such as proteins or glycoproteins.
Phagocytosis is a process wherein a cell binds to the item it wants to engulf on the cell surface and draws the item inward while engulfing around it.
Adaptive immunity is immunity you develop over your lifetime. It can come from: a vaccine, exposure to an infection or disease
Vaccines are used to provide protection for individuals and populations against disease.
Human immunodeficiency virus (HIV) replicated in helper T cells. After infection HIV causes the symptoms of AIDS.
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The internal environment of a cell or organism is different from its external environment. The exchange of substances between the internal and external environments takes place at exchange surfaces. To truly enter or leave an organism, most substances must cross cell plasma membranes.
Changes to body shape and the development of systems in larger organisms as adaptations that facilitate exchange as this ratio reduces.
Ventilation and the exchange of gases occurs in the human lungs. The mechanism of breathing to include the role of the diaphragm and the antagonistic interaction between the external and internal intercostal muscles in bringing about pressure changes in the thoracic cavity.
During digestion, large biological molecules are hydrolysed to smaller molecules that can be absorbed across cell membranes.
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Genetic diversity within a species can be caused by gene mutation, chromosome mutation or random factors associated with meiosis and fertilisation. This genetic diversity is acted upon by natural selection, resulting in species becoming better adapted to their environment.
In the nucleus of eukaryotic cells, DNA molecules are very long, linear and associated with proteins, called histones. Together a DNA molecule and its associated proteins form a chromosome.
The genome is the complete set of genes in a cell and the proteome is the full range of proteins that a cell is able to produce. mRNA is used by the cell to produce proteins.
Gene mutations involve a change in the base sequence of chromosomes. They can arise spontaneously during DNA replication and include base deletion and base substitution.
Genetic diversity is a factor enabling natural selection to occur. Natural selection results in species that are better adapted to their environment. These adaptations may be anatomical, physiological or behavioural.
Two organisms belong to the same species if they are able to produce fertile offspring. A phylogenetic classification system attempts to arrange species into groups based on their evolutionary origins and relationships.
Biodiversity can relate to a range of habitats, from a small local habitat to the Earth. An index of diversity describes the relationship between the number of species in a community and the number of individuals in each species.
Alleles may be dominant, recessive or codominant. In a diploid organism, the alleles at a specific locus may be either homozygous or heterozygous.
Individuals within a population of a species may show a wide range of variation in phenotype. This is due to genetic and environmental factors. The primary source of genetic variation is mutation.
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Life depends on continuous transfers of energy. In photosynthesis, light is absorbed by chlorophyll and this is linked to the production of ATP. In respiration, various substances are used as respiratory substrates. The hydrolysis of these respiratory substrates is linked to the production of ATP.
Photosynthesis is the name of the process whereby organisms convert light energy from the sun into sugar and other organic molecules. They can then go on to use these substances as chemical energy, in the same way, that humans use food. Photosynthesis is essential for almost all life on earth.
Respiration is a set of metabolic reactions and processes that take place within the cells of organisms. It stores biochemical energy within adenosine triphosphate (ATP) molecules. There are two types of respiration, aerobic and anaerobic. Respiration is the process of making ATP rather than breaking it down.
In any ecosystem, plants synthesise organic compounds from atmospheric, or aquatic, carbon dioxide. Most of the sugars synthesised by plants are used by the plant as respiratory substrates. The rest are used to make other groups of biological molecules. These biological molecules form the biomass of the plants.
Nutrients are recycled within natural ecosystems, exemplified by the nitrogen cycle and the phosphorus cycle. Microorganisms play a vital role in recycling chemical elements such as phosphorus and nitrogen.
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A stimulus is a change in the internal or external environment. Living organisms need to be able to respond to these changes in order to survive.
Organisms increase their chance of survival by responding to changes in their environment. Taxes and kineses as simple responses that can maintain a mobile organism in a favourable environment.
A receptor detects a stimulus. A coordinator formulates a suitable response to a stimulus. An effector produces a response. Receptors are specific to one type of stimulus.
Cardiac muscle is myogenic, meaning it can contract and relax without receiving signals from the nervous system. Receptors pick up changes in the blood and transfer this to sensory nerves to alter heart rate.
Changes in membrane permeability lead to depolarisation and the generation of an action potential in nerve cells.
The junction where two neurones meet is known as a synapse. A synapse consists of a presynaptic knob, synaptic cleft and postsynaptic membrane ·
Skeletal muscles are stimulated to contract by nerves and act as effectors. Muscles act in antagonistic pairs against an incompressible skeleton.
Homeostasis is the maintenance of a stable internal environment. Homeostasis in mammals involves physiological control systems that maintain the internal environment within restricted limits.
Insulin is a hormone released from β-cells in the pancreas when blood glucose concentration rises in order to lower the concentration back to its optimum via negative feedback.
The kidneys are made if nephrons which help filter the blood. The blood undergoes ultrafiltration at the glomerulus due to the smaller diameter of the efferent arteriole than the afferent arteriole, creating high hydrostatic pressure.
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Cells are able to control their metabolic activities by regulating the transcription and translation of their genome. Although the cells within an organism carry the same coded genetic information, they translate only part of it. In multicellular organisms, this control of translation enables cells to have specialised functions, forming tissues and organs.
Determining the genome of simpler organisms allows the sequences of the proteins that derive from the genetic code (the proteome) of the organism to be determined. This may have many applications, including the identification of potential antigens for use in vaccine production.
Gene mutations might arise during DNA replication. They include addition, deletion, substitution, inversion, duplication and translocation of bases.
In eukaryotes, transcription of target genes can be stimulated or inhibited when specific transcriptional factors move from the cytoplasm into the nucleus. The role of the steroid hormone, oestrogen, in initiating transcription.
There are many factors that control the expression of genes and, thus, the phenotype of organisms. Some are external, environmental factors, others are internal factors. The expression of genes is not as simple as once thought, with epigenetic regulation of transcription being increasingly recognised as important.
The use of labelled DNA probes that can be used to screen patients for heritable conditions, drug responses or health risks.
The technique of genetic fingerprinting in analysing DNA fragments that have been cloned by PCR, and its use in determining genetic relationships and in determining the genetic variability within a population.
Recombinant DNA technology involves the transfer of fragments of DNA from one organism, or species, to another. Since the genetic code is universal, as are transcription and translation mechanisms, the transferred DNA can be translated within cells of the recipient (transgenic) organism.
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