Biology

Biology

Nerves and Hormones
1. The Nervous System
  1. Reflex actions are extra-rapid responses to stimuli. This process also involves the nervous system but bypasses the brain.
  2. Cells
    1. Most ANIMAL cells have a nucleus, cytoplasm and cell membrane. Light receptors have these cell components too.
      1. The human central nervous system (CNS) consists of the brain and spinal cord. When a receptor is stimulated it sends a signal along the nerve cells, also called NEURONES, to the brain. The brain then coordinates the response.
        Nerve Cells = Neurones
        They carry information as tiny electrical signals.
        There are 3 different types of neurones which each had a slightly different function....
        SENSORY neurones: Carry signals from receptors to the spinal cord and brain.
        RELAY neurones: Carry messages from one part of the CNS to another.
        MOTOR neurones: Carry signals from the CNS to effectors.
        This diagram shows a typical neurone (a motor neurone), which has tiny branches at each end and a long fibre that carries the signals.
  3. Receptors
    1. Receptors are found in: -Eyes -Ears -Tongue -Nose -Skin
      1. Receptors are groups of specialised cells that can detect changes in the environment called STIMULI.
      2. Each organ has receptors that are sensitive to particular kinds of stimuli. For instance...
        Skin = Touch, Pressure, Pain and Temperature
        Nose = Chemicals in the air
        Tongue = Chemicals in food
        Eyes = Light
        Ears = Sound and position of the head
  4. The nervous system allows the body to respond to changes in the environment in a process usually coordinated by the brain.
  5. Synapses
    1. When 2 neurones meet there is a tiny gap called a SYNAPSE.
      1. Signals cross this gap using chemicals. One neurone releases the chemical into the gap. The chemical diffuses across the gap and makes the next neurone transmit an electrical signal.
  6. Reflex Actions
    1. Sometimes a quick response is needed when a receptor is stimulated. One that does not involve the brain: this is a REFLEX ACTION.
      1. Reflex actions happen without us thinking. For example, you would pull your hand away from a hot flame without even thinking about it.
        Reflex Arc is the nerve pathway involved in a reflex action.
        Here's what happens:
        1. Receptor detects a stimulus - a chance in the environment.
        2. Sensory neurones sends impulses to relay neurone.
        3. Motor neurone sends impulses to effector.
        4. Effector produces a response.
        An effector is any part of the body that produces the response.
        Examples of effectors: -A muscle contracting -A gland releasing a hormone or other chemical
    2. In a simple reflex action: Stimulus -> Receptor -> Sensory neurone -> Relay neurone -> Motor neurone -> Effector
2. Control in the human body
  1. Hormones are chemical substances that regulate processes in the body. Hormones are released by glands and travel to their target organs in the bloodstream.
    1. Several hormones are involved in the female menstrual cycle.
    2. Hormones can be used to control human fertility and have advantages and disadvantages.
      1. Hormones regulate the functions of many cells and organs.
    3. The maintenance of a constant internal environment is called HOMEOSTASIS
  2. Water content in the body. Water in the body has to be controlled to protect cells from either too much water entering or leaving them. Water content is controlled by water loss from....
    1. the lungs - when we exhale
    2. the skin - through sweating
    3. passing urine - produced by the kidneys
    4. Ion (salts) content of the body. Ion levels are controlled to protect cells from too much water entering or leaving them. Ion content is controlled by loss of ions from...
      1. the skin - through sweating
      2. passing urine - produced by the kidneys
      3. Temperature of the body. This is controlled to maintain the temperature at which enzymes work best. Body temperature is controlled by sweating, shivering, and controlling blood flow to the skin.
    5. Blood sugar levels This is controlled to provide cells with a constant supply of energy. Blood sugar level is controlled by the release and storage of glucose controlled by insulin.
  3. Hormones in the menstrual cycle
    1. Several hormones control this cycle, which includes controlling the release of an egg each month from an ovary. Theses hormones are released by the ovaries and pituitary gland.
    2. Follicle stimulating hormone, FSH is released by the pituitary gland. FSH makes two things happen...
      1. It causes an egg to mature in an ovary.
      2. It stimulates the ovaries to releasee the hormone oestrogen.
        Oestrogen makes two things happen...
        It stops FSH being produced - so that only one egg matures in a cycle.
        It stimulates the pituitary gland to release luteinizing hormone (LH), which triggers ovulation (the release of the mature egg from the ovary).
3. Control in plants
  1. Plants produce hormones and respond to external stimuli, growing towards sources of water and light, which they need to survive.
    1. A tropism is a growth in response to a stimulus.
      1. Positive Tropism - the plant grows towards the stimulus. Negative tropism - the plant grows away from the stimulus.
        Phototropism is a tropism where LIGHT is the stimulus. A gravitropism (also called Geotropism) is a tropism where gravity is the stimulus.
    2. An auxin is a plant hormone produced in the stem tips and roots, which controls the direction of growth.
    3. Plant hormones are used in weed killers, rooting powder and to control fruit ripening.
  2. 1. Rooting powder: Contains growth hormones which make stem cutting quickly develop roots.
      1. The weed killer is absorbed in larger unties by the weeds than the beneficial plants, which stay healthy.
        Selective weed killers contain growth hormone that cause the weeds to grow quickly and then die.
        Weed killers can be useful for getting rid of dandelions in a lawn without killing the grass, or getting rid of thistles in a field whiteout killing the wheat.
        Using plant hormones
  3. Sensitivity in Plants
    2. The roots and shoots of a plant respond differently to the same stimuli. Shoot = positive phototropism and negative gravitropism. Root = negative phototropism and positive gravitropism
  4. Auxins
    1. Auxins are a family of hormones found in plants. They are mostly made in the tips of the shoots and roots, and can diffuse to other parts of the shoots or roots.
    2. Shoots and roots respond differently to high concentrations of auxin: Cells in shoots grow more. Cells in roots grow less.
    3. In a shoot, the shaded side contains more auxin. This means that the shaded side grows longer, causing the shoot to bend towards the light.
    4. In a root, the shaded side contains more auxin, but this time the shaded side grows less than the lit side. This causes the root to bend away from the light.
Plants
1. Photosynthesis
  3. Photosynthesis is the chemical change which happens in the leaves of green plants. It is the first step towards making food - not just for plants but ultimately every animal on the plant.
  4. Carbon dioxide and water are converted into glucose and oxygen.
    1. Some glucose is used for respiration, while some is converted into insoluble STARCH for storage.
      1. The stored starch can later be turned back into glucose and used in respiration. Oxygen is released as a by-product of photosynthesis.
  5. Photosynthesis takes place in leaf cells. These contain chloroplasts, which are tiny objects containing chlorophyll. The green substance, chlorophyll, absorbs light energy, which the reaction needs.
2. Factors limiting photosynthesis
  1. Without enough light, a plant cannot photosynthesis very quickly, even if there is plenty of water and carbon dioxide.
  2. Increasing light intensity will boost the speed of photosynthesis.
  3. If it gets too cold, the rate of photosynthesis will decrease. Plants cannot photosynthesise if it gets too hot.
  4. Sometimes photosynthesis is limited by the concentration of carbon dioxide in the air.
  5. Even if there is plenty of light, a plant cannot photosynthesise if there is insufficient carbon dioxide.
3. Plants and minerals,
  1. Plants need to take in a number of elements to stay alive. The most important are: Carbon, Hydrogen. Oxygen.
  2. Plants get hydrogen and oxygen from water in the soil, and carbon and oxygen from carbon dioxide and oxygen in the atmosphere. Wa†er and carbon dioxide are used to synthesise food during photosynthesis. Oxygen is used to release energy from food during respiration.
  3. Plants need a number of minerals for healthy growth. These are absorbed through the roots as mineral ions dissolved in the soil water.
    1. Two important mineral ions needed by plants are: Nitrate - for making amino acids, which are needed to make proteins. Magnesium - for making chlorophyll.
      1. Deficient in nitrate = will suffer from stunted growth Deficient in magnesium = leaves will turn yellow

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	Created by Jenny B over 8 years ago