National 5 Biology


Cell Structure

  • The nucleus is found in all cells except bacterial cells. The nucleus controls cell activities such as cell division
  • Bacterial cells do not have a nucleus instead they have a loop of DNA plus some extra circular plasmids
  • All of the chemical reactions occur in the cytoplasm of each cell
  • The cell membrane controls what substances can enter and exit a cell
  • All cells except bacterial gain energy from aerobic respiration which occurs in the mitochondria
  • Cells make protein at ribosomes which are found in the cytoplasm
  • Plant, fungal and bacterial cells have a cell wall. This provides shape and support to the cells. Plant cell walls are made of cellulose. Bacterial and fungal cells have a different structure
  • Plant cells are capable of making their own food through photosynthesis which occurs in the chloroplast. Chloroplasts contain a pigment called chlorophyll which traps light energy from the sun
  • Plant and fungal cells contain a vacuole. This is a fluid filled sac which contains water, sugars and salts
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Transport Across Cell Membrane

  • The cell membrane consists of phospholipids and proteins and is selectively permeable
  • Passive transport occurs down a concentration gradient and does not require energy
  • Examples of passive transport are diffusion and osmosis
  • Diffusion is the movement of molecules down a concentration gradient from a higher to a lower concentration
  • Osmosis is the movement of water molecules from a higher water concentration to a lower water concentration through a selectively permeable membrane
  • Animal cells can burst or shrink when placed in different water solutions and plant cells can become turgid or plasmolysed when placed in different water solutions
  • Active transport requires energy for membrane proteins to move molecules and ions against the concentration gradient
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DNA and the Production of Proteins

  • Structure of DNA: double-stranded helix held by complementary base pairs
  • The four bases: adenine, cytosine, guanine and thymine (A, C, G and T) make up the genetic code. A is always paired with T and C is always paired with G
  • A gene is a section of DNA which codes for a protein
  • DNA carries the genetic information for making proteins
  • The base sequence determines amino acid sequence in proteins
  • Messenger RNA (mRNA) is a molecule, which carries a complementary copy of the genetic code from the DNA, in the nucleus, to a ribosome, where the protein is assembled from amino acids 
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  • The variety of protein shapes and functions arises from the sequence of amino acids. Proteins have many functions such as structural, enzymes, hormones, antibodies, and receptors
  • Enzymes function as biological catalysts are made by all living cells. They speed up cellular reactions and are unchanged in the process. The shape of the active site of an enzyme molecule is complementary to its specific substrate(s). Enzyme action results in product(s). Enzymes can be involved in degradation and synthesis reactions.
  • Each enzyme is most active in its optimum conditions. Enzymes and other proteins can be affected by temperature and pH. Enzymes can be denatured, resulting in a change in their shape which will affect the rate of reaction
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Genetic Engineering

  • Genetic information can be transferred from one cell to another by genetic engineering
  • Stages of genetic engineering:
    • Identify section of DNA that contains required gene from source chromosome
    • Extract required gene using enzymes
    • Extract plasmid from bacterial cell
    • Insert required gene into bacterial plasmid, seal with enzymes
    • Insert plasmid into host bacterial cell
    • Put cell in fermenter to reproduce and produce genetically modified cells
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  • The chemical energy stored in glucose must be released by all cells through a series of enzyme-controlled reactions called respiration
  • The energy released from the breakdown of glucose is used to generate ATP. The energy transferred by ATP can be used for cellular activities such as muscle cell contraction, cell division, protein synthesis and transmission of nerve impulses
  • Glucose is broken down to two molecules of pyruvate, releasing enough energy to yield two molecules of ATP. Furthur breakdown depends upon the presence of oxygen. If oxygen is present, aerobic respiration takes place, and each pyruvate is broken down to carbon dioxide and water, releasing enough energy to yield a large number of ATP molecules
  • In the absence of oxygen, the fermentation pathway takes place. In animal cells, the pyruvate molecules are converted to lactate and in plant and yeast cells they are converted to carbon dioxide and ethanol
  • The breakdown of each glucose molecule via the fermentation pathway yields only the initial two molecules of ATP
  • Respiration begins in the cytoplasm. The process of fermentation is completed in the cytoplasm whereas aerobic respiration is completed in the mitochondria
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Producing New Cells

  • Mitosis provides new cells for growth and repai of damaged cells and maintains the diploid chromosome complement
  • Stages of mitosis:
    • Before mitosis begins, the DNA is unwound and replicating
    • The chromosomes condense and appear as duplicated chrosomes made of two identical chromatids
    • The membrane around the nucleus disappears and the chromosomes line up along the equator of the cell
    • The chromatids are pulled apart by spindle fibres and move to opposite ends of the cell
    • The nucleus membrane reforms around each group of chromatids and the cytoplasm begins to divide
    • Two identical daughter cells are formed
  • Stem cells are found in animals
  • They are unspecialised cells, which can divide in order to self-renew or become different types of specialsised cell
  • Stem cells are involved in growth and repair
  • A heirarchy exists: cells, tissue, organs, organ system, organism
  • Systems are groups of organs, which work together
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Control and Communication - Nervous Control

  • Nervous system consists of central nervous system (CNS) and nerves 
  • CNS consists of brain and spinal cord
  • The brain contains: cerebrum (higher functions and conscious responses), cerebellum (co-ordination of movement and balance) and medulla (controls heart and breathing rate)
  • The cells that make up the nervous system are called neurons
  • Neurons are of three types: sensory, inter and motor
  • In a reflex arc:
    • Receptors detect sensory input/stimuli
    • An electrical impulse is sent up a sensory neuron, along an internueron and down a motor neuron to an effector
    • Chemicals transfer these messages between neurons, at synapses
    • An effector can be a muscle or gland
    • The effector brings about a response eg pulls hand away from flame
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Control and Communication - Hormonal Control

  • Endocrine gland releases hormones into the bloodstream
  • Hormones are chemical messengers
  • A target tissue has cells with complementary receptor proteins for specific hormones, so only that tissue will be affected by these hormones
  • An example of hormonal control is blood glucose regulation
  • When we eat: our blood glucose rises. This is detected by cells in the pancreas which responds by producing insulin. Insulin travels in the blood to the liver where it tells liver cells to absorb glucose from the blood and convert that glucose to glycogen to be stored for later
  • When we exercise or have not eaten in some time: our blood glucose falls. This is detected by cells in the pancreas which responds by producing glucagon. Glucagon travels in the blood to the liver where it tells liver cels to breakdown glycogen to glucose which is released back into the blood
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  • All body cells are diploid, except gametes, which are haploid
  • In animals:
    • Male gamete is sperm. This is made in the testes
    • Female gamete is the egg. This is made in the ovary
  • Sperm cells hav a flagellum to swim towards the egg for fertilisation. They have many mitochondria, which regenerate ATP for the sperm
  • Egg cells have lots of food in the cytoplasm to make new cells
  • In plants:
    • Male gamete is inside the pollen grain. This is made in the anther
    • Female gamete is the ovule. This is made in the plant ovary
  • Fertilisation is the fusion of the nuclei of the two haploid gametes to produce a diploid zygote, which divides to form an embryo
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Variation and Inheritance

  • Discrete variation is a type of variation that can divide the members of a species into two or more groups eg eye colour, tongue rolling
  • Continuous variation is a type of variation that varies in an uninterrupted manner from one extreme to another eg number of apples on a tree, hair colour
  • Continuous variation is contolled by the alleles of more than one gene and is said to show polygenic inheritance
  • Earlope attachment and cystic fibrosis are examples of single gene inheritance
  • Gene - Sequences of bases that codes for a protein / characteristic
  • Allele - Alternative versions of genes
  • Phenotype - The characteristics possessed by an organism
  • Genotype - The alleles possessed by an organism
  • Dominant - An allele that always shows its effect
  • Recessive - An allele that is masked by a dominant allele
  • Homozygous - Two identical alleles for a gene (eg bb, BB)
  • Heterozygous - Two different alleles for a gene (eg Bb)
  • Predicted phenotype ration among offspring are not always achieved because fertilisation is a random process, which involves the element of chance
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Transport Systems - Plants

  • Plant organs include roots, stems and leaves
  • The layers of a leaf include: upper epidermis, palisade mesophyll, spongy mesophyll, vein (consisting of xylem and phloem), lower epidermis, guard cells and stomata
  • Water and minerals enter the plant at root hair cells by osmosis
  • Water is transported up the stem in dead xylem vessels
  • In leaves, the water is either: used inside chloroplasts for photosynthesis, or diffuse out the leaf through the stomata in a process called transpiration
  • Xylem vessels are non-living, hollow tubes strengthening by rings of strong lignin
  • The rate of transpiration is increased by increasing wind speed, tempreature and surface area. It is decreased by increasing humidity
  • Sugar is transported up and down the plant in living phloem.
  • Phloem tissue is living tissue made of sieve tubes and companion cells. Sieve tubes are made up of columns of cells with perforated walls and a continuous cytoplasm. Sieve tubes lack nuclei and are controlled by neighbouring companion cells
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Transport Systems - Animals

  • In mammals, the blood contains plasma, red blood cells and white blood cells
  • The blood transports nutrients, oxygen and carbon dioxide
  • Red blood cells are specialised by having a biconcave shape, having no nucleus and containing haemoglobin. This allows them to transport oxygen efficiently in the form of oxyhaemoglobin
  • White blood cells are part of the immune system and are involved in destroying pathogens. There are two main types of cells involved
  • Phagocytes carry out phagocytosis by engulfing pathogens
  • Lymphocytes produce antibodies which destroy pathogens. Each antibody is specific to a particular pathogen
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Transport Systems - The Heart

  • The heart contains four chambers: the right and left atrium and the right and left ventricle.
  • Valves exist between the atria and ventricles, and between the ventricles and the arteries
  • Deoxygenated blood enters the right atrium of the heart at the vena cava. The blood is pumped into the right ventricle. Deoxygenated blood is pumped out of the heart through the pulmonary artery. Blood is sent to the lungs where the blood takes up oxygen and carbon dioxide is removed
  • Oxygenated blood is returned to the left atrium of the heart by the pulmonary vein. Blood is pumped into the left ventricle and then pumped out of the heart via the largest artery in the body, the aorta
  • The coronary arteries supply the heart muscle with oxygenated blood. If these vessels become blocked the person suffers a heart attack
  • Arteries have thick, muscular walls, a narrow central channel and carry blood under high pressure away from the heart
  • Veins have thinner walls, a wider channel and carry blood under low pressure back towards the heart. Veins contain valves to prevent the backflow of blood
  • Capillaries are thin walled and have a large surface area, forming networks at tissues and organs to allow efficient exchange of materials
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Absorption of Materials

  • Oxygen and nutrients from food must be absorbed into the bloodstream to be delivered to cells for respiration
  • Waste materials, such as carbon dioxide, must be removed from cells into the bloodstream
  • Tissues contain capillary networks to allow the exchange of materials at cellular level
  • Surfaces incolved in the absorption of materials have certain features in common: large surface area, thin walls, extensive blood supply. These increase the efficiency of absorption
  • Lungs are gas exchange organs. They consist of a large number of alveoli providing a large surface area. Oxygen and carbon dioxide are absorbed through the thin alveolar walls to or from the many blood capillaries
  • Nutrients from food are absorbed into the villi in the small intestine. The large number of thin walled villi provides a large surface area. Each villus contains a network of capillaries to absorb glucose and amino acids; and a lacteal to absorb fatty acids and glycerol
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  • A species is a group of organisms that can interbreed and produce fertile offspring
  • Biodiversity is the degree of variation that exists among all living organisms on Earth
  • All of the members of a single species that live within a geographical area are described as a population
  • Producers are plants that begin food chains by making energy from carbon dioxide and water. Organisms that obtain their energy by eating other organisms are called consumers
  • An animal that feeds only on plants is called a herbivore. An animal that feeds only on animals is called a carnivore and an animal that feeds on both plant and animals is called an omnivore. An animal that hunts, kills and eats other animals for food is known as a predator. Organisms that are killed by predators for food are known as prey
  • Food chains show a sequence of feeding relationships. Food webs are a network of food chains, showing how they all link together
  • An ecosystem consists of all the organisms (the community) living in a particular habitat and the non-living components which the organisms interact
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Ecosystems 2

  • A niche is the  role that an organism plays within a community. It relates to the resources it requires in its ecosystem, such as light and nutrient availability and its ineractions with other organisms in the community. It involves competition and predation and the conditions it can tolerate such as temperature
  • Competition in ecosystems occurs when resources are in short supply. Interspecific competitions occurs amongst individuals of different species for one or a few of the resources they require. Intraspecific competition occurs amongst individuals of the same species and is for all resources required. Intraspecific is therefore more intense than interspecific competition
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Distribution of Organisms

  • Competition for resources, disease, food availability, grazing and predation are biotic factors. Light intensity, moisture, pH and temperature are abiotic factors
  • When sampling or measuring abiotic factors there is always a risk that the sample is not representative of the ecosystem as a whole. This source of error can be minimised by taking many samples to get a reliable result
  • Quadrats and pitfall traps. Evaluation of limitations and sources of error in their use
    • Pitfall traps can be used to sample invertebrates living on the soil surface. Sources of error and how these can be minimised:
    • Traps not checked regularly and insects eat each other - check the trap regularly or use a solvent to kill insects
    • Trap is not level with soil surface so insects do not fall in trap - level the top of the trap with the soil surface
    • Trap is not camouflaged and so birds eat insects - loosely camouflage the top of the trap with a leaf or use a loose fitting lid
    • Trap is too shallow and invertebrates can climb out - use a deep and steep walled trap
  • Quadrats can be used to sample plants and slow moving animals
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  • Photosynthesis is a two-stage process:
  • Light reactions:
    • The light energy from the sun is trapped by chlorophyll in the chloroplasts and is converted into chemical energy which is used to generate ATP
    • Water is split to produce hydrogen and oxygen. Oxygen diffuses from the cell
  • Carbon fixation:
    • A series of enzyme-controlled reactions, which use hydrogen and ATP (produced by the light reactions) with carbon dioxide to produce sugar
    • The chemical energy in sugar is available for respiration or the sugar can be converted into other substances, such as starch (storage) and cellulose (structural)
  • Limiting factors
    • Carbon dioxide concentration
    • Light intensity
    • Temperature
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Energy In Ecosystems

  • In transfers from one level to the next in a food chain, the majority of the energy is lost as heat, movement or undigested materials. Only a very small quantity is used for growth and is therefore available at the next level in a food chain
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Food Production

  • Increasing human population requires an increased food yield. This can involve the use of fertilisers and pesticides. Fertilisers provide chemicals such as nitrates which increase crop yield. Plants and animals which reduce crop yield can be killed by pesticides
  • Nitrates dissolved in soil water are absorbed by plants. Nitrates are used to produce amino acids which are synthesised into plant proteins. Animals consume plants or other animals to obtain amino acids for protein synthesis. Fertilisers can be added to sil to increase the nitrate content of the soil
  • Fertilisers can leach into fresh water, adding extra, unwanted nitrates. This will increase algal populations which cause algal blooms. Algal blooms reduce light levels, killing aquatic plants. These dead plants, as well as dead algae, become food for bacteria which increase greatly in number. The bacteria use up large quantities of oxygen availability for other organisms
  • Indicator species are species that by their presence or absence indicate environmental quality/levels of pollution
  • Genetically modified (GM) crops can be used to reduce the use of fertilisers
  • Pesticides sprayed onto crops can accumulate in the bodies of organisms over time
  • As they are passed along food chains, toxicity increases and can reach lethal levels
  • The use of biological control and genetically modified (GM) crops as alternatives to the use of pesticides
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Evolution of Species

  • A mutation is a random change to genetic material. Mutations may be neutral, confer an advantage or a disadvantage to survival
  • Mutations are spontaneous and are the only source of new alleles. Environmental factors, such as radiation and some chemicals, can increase the rate of mutation
  • New alleles produced by a mutation can result in plants and animals becoming better adapted to their environment. Variation within a population makes it possible for a population to evolve over time in response to changing environmental conditions
  • Species produce more offspring than the enironment can sustain. Natural selection or survival of the fittest occurs when there are selection pressures. The best adapted individuals in a population survive to reproduce, passing on the favourable alleles that confer the selective advantage. These alleles increase in frequency within the population
  • Speciation occurs after part of a population becomes isolated by an isolation barrier, which can be geographical, ecological or behavioural. Different mutations occur in each sub-population. Natural selection selects for different mutations in each group, due to different selection pressures. Each sub-population evolves until they become so genetically different that they are two different species
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