WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:06.831 Unit C B 2: Cells and control. 00:00:09.500 --> 00:00:15.141 Mitosis produces two daughter cells with the same sets of chromosomes as the parent cell. 00:00:15.291 --> 00:00:21.008 In diploid body cells, each daughter cell is also diploid: it has two sets of chromosomes. 00:00:23.667 --> 00:00:29.052 Mitosis is important for growth, repair and replacement of damaged or worn-out cells. 00:00:29.202 --> 00:00:36.837 It also allows asexual reproduction, which produces genetically identical offspring (clones), apart from mutations. 00:00:39.500 --> 00:00:43.571 The cell cycle includes interphase, mitosis and cytokinesis. 00:00:43.721 --> 00:00:48.797 Interphase is preparation for division, rather than a stage of mitosis itself. 00:00:51.467 --> 00:00:57.489 During interphase, the cell grows, increases its sub-cellular structures and copies its D N A. 00:00:57.639 --> 00:01:01.680 Each chromosome then consists of two identical sister chromatids. 00:01:04.333 --> 00:01:08.256 During prophase, chromosomes coil up (condense) and become visible. 00:01:08.406 --> 00:01:13.611 The membrane around the nucleus (nuclear envelope) breaks down and spindle fibres form. 00:01:16.267 --> 00:01:21.801 During metaphase, chromosomes line up at the centre of the cell and attach to spindle fibres. 00:01:24.467 --> 00:01:29.055 During anaphase, sister chromatids separate and move to opposite ends of the cell. 00:01:29.205 --> 00:01:32.266 Each separated chromatid is now a chromosome. 00:01:34.933 --> 00:01:41.183 During telophase, new nuclear envelopes form around the two sets of chromosomes, producing two nuclei. 00:01:43.833 --> 00:01:49.645 Cytokinesis divides the cytoplasm and cell membrane to produce two separate daughter cells. 00:01:49.795 --> 00:01:52.950 Plant cells form a new cell plate and wall between them. 00:01:53.100 --> 00:01:58.769 The cell cycle sequence: preparation, nuclear division, then separation into two cells. 00:02:03.433 --> 00:02:13.330 A human body cell with 46 chromosomes produces two daughter cells with 46 chromosomes each by mitosis; it does not halve the chromosome number. 00:02:16.000 --> 00:02:20.471 Cancer results from changes in cells that cause uncontrolled cell division. 00:02:20.621 --> 00:02:23.031 A mass of these cells can form a tumour. 00:02:25.700 --> 00:02:29.849 Growth is an increase in an organism’s size, such as mass or length. 00:02:29.999 --> 00:02:34.829 In animals, cell division and differentiation contribute to growth and development. 00:02:37.500 --> 00:02:44.561 Percentage growth equals (final size minus initial size) divided by initial size times 100. 00:02:44.711 --> 00:02:47.121 Use the same units for both measurements. 00:02:49.800 --> 00:03:00.315 An organism growing from 20 g to 25 g has grown by 5 g: its percentage growth is 5 divided by 20 times 100 equals 25 percent. 00:03:02.967 --> 00:03:10.070 A percentile chart compares a child’s growth measurement with those of children of the same age in a reference population. 00:03:10.220 --> 00:03:14.955 Read age on the horizontal axis and the measurement on the vertical axis. 00:03:17.633 --> 00:03:24.993 The 50th percentile is the median: about half of the reference population has a smaller measurement and half has a larger one. 00:03:27.667 --> 00:03:33.766 At the 75th percentile, about 75 percent of the reference population has a lower measurement. 00:03:33.916 --> 00:03:38.369 This does not mean that the child has completed 75 percent of their growth. 00:03:41.033 --> 00:03:43.508 Repeated measurements show a growth trend. 00:03:43.658 --> 00:03:52.211 Large changes across percentile lines can prompt further assessment; a single low or high percentile is not by itself a diagnosis. 00:03:54.867 --> 00:04:01.041 Differentiation is the process by which an unspecialised cell becomes specialised for a particular function. 00:04:01.191 --> 00:04:04.783 Changes in its shape and structures help it perform its job. 00:04:07.433 --> 00:04:12.797 Stem cells are unspecialised cells that can divide and produce cells that differentiate. 00:04:12.947 --> 00:04:15.870 They supply new cells for growth or replacement. 00:04:18.533 --> 00:04:25.337 Embryonic stem cells can differentiate into most types of body cell, giving them a wide range of potential uses. 00:04:28.000 --> 00:04:31.603 Adult stem cells can form a more limited range of cell types. 00:04:31.753 --> 00:04:38.451 Bone marrow stem cells, for example, produce different types of blood cell; they do not simply become any nearby cell. 00:04:41.133 --> 00:04:44.567 Stem-cell treatments may replace damaged cells or tissues. 00:04:44.717 --> 00:04:51.809 Blood-forming stem-cell transplants are an established example; many other proposed treatments remain under investigation. 00:04:54.467 --> 00:05:01.229 Risks include uncontrolled division and tumour formation, infection and rejection by the recipient’s immune system. 00:05:01.379 --> 00:05:04.216 The cells must be carefully controlled and tested. 00:05:06.867 --> 00:05:13.501 Using embryonic stem cells raises ethical questions because obtaining them may involve destroying an embryo. 00:05:13.651 --> 00:05:17.559 Consider potential medical benefits alongside these concerns. 00:05:20.233 --> 00:05:24.266 Plants grow through cell division, elongation and differentiation. 00:05:24.416 --> 00:05:29.961 Regions called meristems contain cells that divide, particularly near root and shoot tips. 00:05:32.633 --> 00:05:42.691 In a root tip, cells are produced in the zone of cell division (meristem), increase in length in the zone of elongation, then become specialised in the zone of differentiation. 00:05:45.367 --> 00:05:52.598 Meristem cells can produce cells that differentiate into different plant tissues, allowing continued growth throughout the plant’s life. 00:05:55.267 --> 00:05:59.558 Xylem vessel elements become hollow, dead cells joined into tubes. 00:05:59.708 --> 00:06:04.997 Their strengthened walls support the plant, and the tubes carry water and mineral ions upwards. 00:06:07.667 --> 00:06:12.259 Root hair cells have a long projection giving a large surface area for absorption. 00:06:12.409 --> 00:06:16.076 A thin wall gives a short distance for water movement into the cell. 00:06:18.733 --> 00:06:24.631 Water enters root hair cells by osmosis; mineral ions can enter by active transport. 00:06:24.781 --> 00:06:27.903 Link the specialised structure to the process it helps. 00:06:30.567 --> 00:06:35.272 The central nervous system (CNS) consists of the brain and spinal cord. 00:06:35.422 --> 00:06:40.031 Peripheral nerves connect the CNS with receptors and effectors around the body. 00:06:42.700 --> 00:06:47.327 A stimulus is a change in the environment, such as heat, light or pressure. 00:06:47.477 --> 00:06:51.447 A receptor detects the stimulus and starts an electrical impulse. 00:06:54.100 --> 00:06:58.139 Sensory neurones carry impulses from receptors towards the CNS. 00:06:58.289 --> 00:07:01.383 Relay neurones connect neurones within the CNS. 00:07:01.533 --> 00:07:05.178 Motor neurones carry impulses from the CNS to effectors. 00:07:07.833 --> 00:07:12.899 Effectors bring about a response: muscles contract and glands secrete substances. 00:07:13.049 --> 00:07:16.763 A receptor detects the change; an effector responds to it. 00:07:19.433 --> 00:07:23.056 Neurones are specialised for transmitting electrical impulses. 00:07:23.206 --> 00:07:28.688 Dendrites receive signals, and a long axon carries impulses towards axon terminals. 00:07:31.367 --> 00:07:38.267 In a sensory neurone, a long dendron carries impulses towards the cell body and an axon carries them away from it. 00:07:38.417 --> 00:07:43.121 Motor-neurone cell bodies lie at one end; not all neurones have the same shape. 00:07:45.800 --> 00:07:50.631 A myelin sheath is an insulating, fatty covering around many nerve fibres. 00:07:50.781 --> 00:07:53.627 It increases the speed of impulse transmission. 00:07:56.300 --> 00:08:00.668 At a synapse, neurones are separated by a small gap called the synaptic cleft. 00:08:00.818 --> 00:08:04.839 A similar chemical junction can connect a motor neurone to a muscle. 00:08:07.500 --> 00:08:14.730 When an impulse reaches the end of a neurone (the axon terminal), it causes a chemical called a neurotransmitter to be released. 00:08:14.880 --> 00:08:21.417 The chemical diffuses across the synaptic gap and binds to receptors on the next cell, starting a new response. 00:08:21.567 --> 00:08:27.135 An electrical signal triggers chemical release; neurotransmitter crosses the synaptic cleft. 00:08:31.800 --> 00:08:36.419 Across a synapse the signal is chemical; along a neurone it is electrical. 00:08:36.569 --> 00:08:42.433 Synapses transmit in one direction because release sites and receptors are on opposite sides. 00:08:45.100 --> 00:08:50.366 A reflex is a rapid, automatic response to a stimulus that helps protect the body. 00:08:50.516 --> 00:08:53.119 It does not require a conscious decision. 00:08:55.800 --> 00:09:04.417 In a spinal reflex: stimulus to receptor to sensory neurone to relay neurone in the spinal cord to motor neurone to effector to response. 00:09:04.567 --> 00:09:08.182 Follow the numbered pathway from stimulus to protective response. 00:09:12.833 --> 00:09:21.072 For example, heat is detected by receptors in the skin; impulses pass through the reflex arc and a muscle contracts to withdraw the hand. 00:09:23.733 --> 00:09:30.783 The brain can receive information about the stimulus, but the protective response starts without waiting for conscious processing. 00:09:30.933 --> 00:09:34.247 Reflex pathways include synapses between neurones. 00:09:36.900 --> 00:09:42.425 A voluntary response involves conscious processing, such as choosing to catch a falling ruler. 00:09:42.575 --> 00:09:48.185 A ruler-drop test estimates reaction time rather than directly measuring nerve-impulse speed. 00:09:50.867 --> 00:09:53.171 For a ruler-drop investigation, 00:09:53.321 --> 00:09:57.737 hold the ruler at a consistent starting position above the participant’s fingers, 00:09:57.887 --> 00:09:59.721 release it without warning, 00:09:59.871 --> 00:10:02.739 and record the distance fallen before it is caught. 00:10:05.400 --> 00:10:10.371 Use a supplied conversion table to convert the distance fallen to reaction time. 00:10:10.521 --> 00:10:13.760 A smaller distance indicates a shorter reaction time. 00:10:16.433 --> 00:10:23.076 Repeat trials, calculate a mean, and control factors such as the hand used, starting position and distractions. 00:10:23.226 --> 00:10:28.068 Practice can affect results; discuss anomalies and avoid giving advance cues. 00:10:30.733 --> 00:10:32.867 That completes Cells and control. 00:10:33.017 --> 00:10:36.362 Revisit the notes and test yourself on the revision website.