WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:06.871 Unit S B 2: Cells and control. 00:00:09.533 --> 00:00:15.175 Mitosis produces two daughter cells with the same sets of chromosomes as the parent cell. 00:00:15.325 --> 00:00:21.041 In diploid body cells, each daughter cell is also diploid: it has two sets of chromosomes. 00:00:23.700 --> 00:00:29.085 Mitosis is important for growth, repair and replacement of damaged or worn-out cells. 00:00:29.235 --> 00:00:36.871 It also allows asexual reproduction, which produces genetically identical offspring (clones), apart from mutations. 00:00:39.533 --> 00:00:43.604 The cell cycle includes interphase, mitosis and cytokinesis. 00:00:43.754 --> 00:00:48.831 Interphase is preparation for division, rather than a stage of mitosis itself. 00:00:51.500 --> 00:00:57.523 During interphase, the cell grows, increases its sub-cellular structures and copies its D N A. 00:00:57.673 --> 00:01:01.722 Each chromosome then consists of two identical sister chromatids. 00:01:04.400 --> 00:01:08.333 During prophase, chromosomes coil up (condense) and become visible. 00:01:08.483 --> 00:01:13.687 The membrane around the nucleus (nuclear envelope) breaks down and spindle fibres form. 00:01:16.367 --> 00:01:21.901 During metaphase, chromosomes line up at the centre of the cell and attach to spindle fibres. 00:01:24.567 --> 00:01:29.155 During anaphase, sister chromatids separate and move to opposite ends of the cell. 00:01:29.305 --> 00:01:32.366 Each separated chromatid is now a chromosome. 00:01:35.033 --> 00:01:41.283 During telophase, new nuclear envelopes form around the two sets of chromosomes, producing two nuclei. 00:01:43.933 --> 00:01:49.745 Cytokinesis divides the cytoplasm and cell membrane to produce two separate daughter cells. 00:01:49.895 --> 00:01:53.050 Plant cells form a new cell plate and wall between them. 00:01:53.200 --> 00:01:58.869 The cell cycle sequence: preparation, nuclear division, then separation into two cells. 00:02:03.533 --> 00:02:13.430 A human body cell with 46 chromosomes produces two daughter cells with 46 chromosomes each by mitosis; it does not halve the chromosome number. 00:02:16.100 --> 00:02:20.579 Cancer results from changes in cells that cause uncontrolled cell division. 00:02:20.729 --> 00:02:23.140 A mass of these cells can form a tumour. 00:02:25.800 --> 00:02:29.949 Growth is an increase in an organism’s size, such as mass or length. 00:02:30.099 --> 00:02:34.929 In animals, cell division and differentiation contribute to growth and development. 00:02:37.600 --> 00:02:44.661 Percentage growth equals (final size minus initial size) divided by initial size times 100. 00:02:44.811 --> 00:02:47.221 Use the same units for both measurements. 00:02:49.900 --> 00:03:00.949 An organism growing from 20 grams to 25 grams has grown by 5 grams: its percentage growth is 5 divided by 20 times 100 equals 25 percent. 00:03:03.600 --> 00:03:10.703 A percentile chart compares a child’s growth measurement with those of children of the same age in a reference population. 00:03:10.853 --> 00:03:15.589 Read age on the horizontal axis and the measurement on the vertical axis. 00:03:18.267 --> 00:03:25.626 The 50th percentile is the median: about half of the reference population has a smaller measurement and half has a larger one. 00:03:28.300 --> 00:03:34.399 At the 75th percentile, about 75 percent of the reference population has a lower measurement. 00:03:34.549 --> 00:03:39.003 This does not mean that the child has completed 75 percent of their growth. 00:03:41.667 --> 00:03:44.141 Repeated measurements show a growth trend. 00:03:44.291 --> 00:03:52.844 Large changes across percentile lines can prompt further assessment; a single low or high percentile is not by itself a diagnosis. 00:03:55.500 --> 00:04:01.675 Differentiation is the process by which an unspecialised cell becomes specialised for a particular function. 00:04:01.825 --> 00:04:05.416 Changes in its shape and structures help it perform its job. 00:04:08.067 --> 00:04:13.431 Stem cells are unspecialised cells that can divide and produce cells that differentiate. 00:04:13.581 --> 00:04:16.503 They supply new cells for growth or replacement. 00:04:19.167 --> 00:04:25.970 Embryonic stem cells can differentiate into most types of body cell, giving them a wide range of potential uses. 00:04:28.633 --> 00:04:32.236 Adult stem cells can form a more limited range of cell types. 00:04:32.386 --> 00:04:39.084 Bone marrow stem cells, for example, produce different types of blood cell; they do not simply become any nearby cell. 00:04:41.767 --> 00:04:45.200 Stem-cell treatments may replace damaged cells or tissues. 00:04:45.350 --> 00:04:52.442 Blood-forming stem-cell transplants are an established example; many other proposed treatments remain under investigation. 00:04:55.100 --> 00:05:01.862 Risks include uncontrolled division and tumour formation, infection and rejection by the recipient’s immune system. 00:05:02.012 --> 00:05:04.849 The cells must be carefully controlled and tested. 00:05:07.500 --> 00:05:14.134 Using embryonic stem cells raises ethical questions because obtaining them may involve destroying an embryo. 00:05:14.284 --> 00:05:18.183 Consider potential medical benefits alongside these concerns. 00:05:20.833 --> 00:05:24.866 Plants grow through cell division, elongation and differentiation. 00:05:25.016 --> 00:05:30.561 Regions called meristems contain cells that divide, particularly near root and shoot tips. 00:05:33.233 --> 00:05:43.291 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. 00:05:45.967 --> 00:05:53.198 Meristem cells can produce cells that differentiate into different plant tissues, allowing continued growth throughout the plant’s life. 00:05:55.867 --> 00:06:00.158 Xylem vessel elements become hollow, dead cells joined into tubes. 00:06:00.308 --> 00:06:05.597 Their strengthened walls support the plant, and the tubes carry water and mineral ions upwards. 00:06:08.267 --> 00:06:12.859 Root hair cells have a long projection giving a large surface area for absorption. 00:06:13.009 --> 00:06:16.676 A thin wall gives a short distance for water movement into the cell. 00:06:19.333 --> 00:06:25.231 Water enters root hair cells by osmosis; mineral ions can enter by active transport. 00:06:25.381 --> 00:06:28.503 Link the specialised structure to the process it helps. 00:06:31.167 --> 00:06:35.872 The central nervous system (CNS) consists of the brain and spinal cord. 00:06:36.022 --> 00:06:40.631 Peripheral nerves connect the CNS with receptors and effectors around the body. 00:06:43.300 --> 00:06:47.927 A stimulus is a change in the environment, such as heat, light or pressure. 00:06:48.077 --> 00:06:52.047 A receptor detects the stimulus and starts an electrical impulse. 00:06:54.700 --> 00:06:58.739 Sensory neurones carry impulses from receptors towards the CNS. 00:06:58.889 --> 00:07:01.983 Relay neurones connect neurones within the CNS. 00:07:02.133 --> 00:07:05.778 Motor neurones carry impulses from the CNS to effectors. 00:07:08.433 --> 00:07:13.499 Effectors bring about a response: muscles contract and glands secrete substances. 00:07:13.649 --> 00:07:17.363 A receptor detects the change; an effector responds to it. 00:07:20.033 --> 00:07:23.656 Neurones are specialised for transmitting electrical impulses. 00:07:23.806 --> 00:07:29.288 Dendrites receive signals, and a long axon carries impulses towards axon terminals. 00:07:31.967 --> 00:07:38.867 In a sensory neurone, a long dendron carries impulses towards the cell body and an axon carries them away from it. 00:07:39.017 --> 00:07:43.721 Motor-neurone cell bodies lie at one end; not all neurones have the same shape. 00:07:46.400 --> 00:07:51.231 A myelin sheath is an insulating, fatty covering around many nerve fibres. 00:07:51.381 --> 00:07:54.227 It increases the speed of impulse transmission. 00:07:56.900 --> 00:08:01.268 At a synapse, neurones are separated by a small gap called the synaptic cleft. 00:08:01.418 --> 00:08:05.439 A similar chemical junction can connect a motor neurone to a muscle. 00:08:08.100 --> 00:08:15.330 When an impulse reaches the end of a neurone (the axon terminal), it causes a chemical called a neurotransmitter to be released. 00:08:15.480 --> 00:08:22.017 The chemical diffuses across the synaptic gap and binds to receptors on the next cell, starting a new response. 00:08:22.167 --> 00:08:27.735 An electrical signal triggers chemical release; neurotransmitter crosses the synaptic cleft. 00:08:32.400 --> 00:08:37.019 Across a synapse the signal is chemical; along a neurone it is electrical. 00:08:37.169 --> 00:08:43.033 Synapses transmit in one direction because release sites and receptors are on opposite sides. 00:08:45.700 --> 00:08:50.966 A reflex is a rapid, automatic response to a stimulus that helps protect the body. 00:08:51.116 --> 00:08:53.697 It does not require a conscious decision. 00:08:56.367 --> 00:09:04.983 In a spinal reflex: stimulus to receptor to sensory neurone to relay neurone in the spinal cord to motor neurone to effector to response. 00:09:05.133 --> 00:09:08.749 Follow the numbered pathway from stimulus to protective response. 00:09:13.400 --> 00:09:21.609 For example, heat is detected by receptors in the skin; impulses pass through the reflex arc and a muscle contracts to withdraw the hand. 00:09:24.267 --> 00:09:31.316 The brain can receive information about the stimulus, but the protective response starts without waiting for conscious processing. 00:09:31.466 --> 00:09:34.780 Reflex pathways include synapses between neurones. 00:09:37.433 --> 00:09:42.958 A voluntary response involves conscious processing, such as choosing to catch a falling ruler. 00:09:43.108 --> 00:09:48.718 A ruler-drop test estimates reaction time rather than directly measuring nerve-impulse speed. 00:09:51.400 --> 00:09:53.704 For a ruler-drop investigation, 00:09:53.854 --> 00:09:58.270 hold the ruler at a consistent starting position above the participant’s fingers, 00:09:58.420 --> 00:10:00.255 release it without warning, 00:10:00.405 --> 00:10:03.272 and record the distance fallen before it is caught. 00:10:05.933 --> 00:10:10.905 Use a supplied conversion table to convert the distance fallen to reaction time. 00:10:11.055 --> 00:10:14.293 A smaller distance indicates a shorter reaction time. 00:10:16.967 --> 00:10:23.609 Repeat trials, calculate a mean, and control factors such as the hand used, starting position and distractions. 00:10:23.759 --> 00:10:28.601 Practice can affect results; discuss anomalies and avoid giving advance cues. 00:10:31.267 --> 00:10:38.818 The cerebral hemispheres are involved in conscious thought, memory, intelligence and movements you choose to make (voluntary movements). 00:10:38.968 --> 00:10:41.911 The cerebellum coordinates movement and balance. 00:10:42.061 --> 00:10:47.328 The medulla oblongata controls automatic functions, including breathing and heart rate. 00:10:47.478 --> 00:10:51.885 These regions have different roles; they are not stages in a signal pathway. 00:10:56.567 --> 00:11:01.568 (Higher tier) The skull protects the brain but makes direct investigation difficult. 00:11:01.718 --> 00:11:08.564 C T scanning uses X-rays to build cross-sectional images of structures, helping locate injuries or tumours. 00:11:11.233 --> 00:11:17.920 (Higher tier) P E T scans detect radiation from a tracer to show activity in different brain regions. 00:11:18.070 --> 00:11:22.360 A tracer linked to glucose uptake shows where cells are using more glucose. 00:11:22.510 --> 00:11:29.879 C T mainly shows structure; P E T can show where more chemical activity is taking place (metabolic activity). 00:11:32.533 --> 00:11:37.217 (Higher tier) Brain and spinal-cord injuries can disrupt complex networks. 00:11:37.367 --> 00:11:43.222 Some nerve cells regenerate poorly, and surgery or drug treatment can harm nearby healthy tissue. 00:11:43.372 --> 00:11:48.203 Brain tumours can compress tissue; removing them may damage essential functions. 00:11:50.867 --> 00:11:57.724 (Higher tier) Evidence from scans, electrical measurements and patients with damage helps link regions with functions. 00:11:57.874 --> 00:12:04.337 A correlation between brain activity and a task supports a link, but does not by itself prove a simple cause. 00:12:07.000 --> 00:12:09.517 The cornea bends incoming light strongly. 00:12:09.667 --> 00:12:17.122 The lens changes shape to focus light onto the retina, where receptor cells convert light into signals carried by the optic nerve. 00:12:19.800 --> 00:12:22.147 The iris changes pupil size. 00:12:22.297 --> 00:12:32.610 In bright light its circular muscles contract and radial muscles relax, reducing the pupil opening; in dim light radial muscles contract and circular muscles relax. 00:12:35.267 --> 00:12:39.237 Rods are sensitive in dim light but do not provide colour vision. 00:12:39.387 --> 00:12:43.260 Cones work best in brighter light and enable colour vision. 00:12:43.410 --> 00:12:49.383 The fovea is rich in cones; the blind spot has no photoreceptors where the optic nerve leaves. 00:12:52.033 --> 00:12:56.151 Focusing on objects at different distances is called accommodation. 00:12:56.301 --> 00:13:04.191 For nearby objects, ciliary muscles contract, suspensory ligaments loosen and the lens becomes thicker, bending light more strongly. 00:13:04.341 --> 00:13:09.414 For distant objects, the muscles relax, ligaments tighten and the lens becomes thinner. 00:13:09.564 --> 00:13:14.150 The suspensory ligaments slacken for near vision and tighten for distant vision. 00:13:18.800 --> 00:13:26.511 In short-sightedness, distant objects focus in front of the retina; a diverging lens spreads the rays before they enter the eye. 00:13:26.661 --> 00:13:33.401 In long-sightedness, nearby objects would focus behind the retina; a converging lens brings the focus forward. 00:13:36.067 --> 00:13:44.086 A cataract is a cloudy lens that scatters light and reduces vision; replacing it with an artificial lens can restore clear focusing. 00:13:44.236 --> 00:13:51.594 Colour blindness commonly results from an inherited defect in cone pigments and is not corrected with ordinary focusing lenses. 00:13:54.267 --> 00:13:56.400 That completes Cells and control. 00:13:56.550 --> 00:13:59.895 Revisit the notes and test yourself on the revision website.