Mr Kidman's Classroom
Revise the key ideas
The cell cycle and mitosis
- Mitosis produces two daughter cells with the same sets of chromosomes as the parent cell. In diploid body cells, each daughter cell is also diploid: it has two sets of chromosomes.
- Mitosis is important for growth, repair and replacement of damaged or worn-out cells. It also allows asexual reproduction, which produces genetically identical offspring (clones), apart from mutations.
- The cell cycle includes interphase, mitosis and cytokinesis. Interphase is preparation for division, rather than a stage of mitosis itself.
- During interphase, the cell grows, increases its sub-cellular structures and copies its DNA. Each chromosome then consists of two identical sister chromatids.
- During prophase, chromosomes coil up (condense) and become visible. The membrane around the nucleus (nuclear envelope) breaks down and spindle fibres form.
- During metaphase, chromosomes line up at the centre of the cell and attach to spindle fibres.
- During anaphase, sister chromatids separate and move to opposite ends of the cell. Each separated chromatid is now a chromosome.
- During telophase, new nuclear envelopes form around the two sets of chromosomes, producing two nuclei.
- Cytokinesis divides the cytoplasm and cell membrane to produce two separate daughter cells. Plant cells form a new cell plate and wall between them.
The cell cycle sequence: preparation, nuclear division, then separation into two cells. - A human body cell with 46 chromosomes produces two daughter cells with 46 chromosomes each by mitosis; it does not halve the chromosome number.
- Cancer results from changes in cells that cause uncontrolled cell division. A mass of these cells can form a tumour.
Growth and interpreting growth charts
- Growth is an increase in an organism’s size, such as mass or length. In animals, cell division and differentiation contribute to growth and development.
- Percentage growth = (final size − initial size) ÷ initial size × 100. Use the same units for both measurements.
- An organism growing from 20 g to 25 g has grown by 5 g: its percentage growth is 5 ÷ 20 × 100 = 25%.
- A percentile chart compares a child’s growth measurement with those of children of the same age in a reference population. Read age on the horizontal axis and the measurement on the vertical axis.
- The 50th percentile is the median: about half of the reference population has a smaller measurement and half has a larger one.
- At the 75th percentile, about 75% of the reference population has a lower measurement. This does not mean that the child has completed 75% of their growth.
- Repeated measurements show a growth trend. Large changes across percentile lines can prompt further assessment; a single low or high percentile is not by itself a diagnosis.
Differentiation and stem cells
- Differentiation is the process by which an unspecialised cell becomes specialised for a particular function. Changes in its shape and structures help it perform its job.
- Stem cells are unspecialised cells that can divide and produce cells that differentiate. They supply new cells for growth or replacement.
- Embryonic stem cells can differentiate into most types of body cell, giving them a wide range of potential uses.
- Adult stem cells can form a more limited range of cell types. Bone marrow stem cells, for example, produce different types of blood cell; they do not simply become any nearby cell.
- Stem-cell treatments may replace damaged cells or tissues. Blood-forming stem-cell transplants are an established example; many other proposed treatments remain under investigation.
- Risks include uncontrolled division and tumour formation, infection and rejection by the recipient’s immune system. The cells must be carefully controlled and tested.
- Using embryonic stem cells raises ethical questions because obtaining them may involve destroying an embryo. Consider potential medical benefits alongside these concerns.
Plant growth and specialised cells
- Plants grow through cell division, elongation and differentiation. Regions called meristems contain cells that divide, particularly near root and shoot tips.
- 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.
- Meristem cells can produce cells that differentiate into different plant tissues, allowing continued growth throughout the plant’s life.
- Xylem vessel elements become hollow, dead cells joined into tubes. Their strengthened walls support the plant, and the tubes carry water and mineral ions upwards.
- Root hair cells have a long projection giving a large surface area for absorption. A thin wall gives a short distance for water movement into the cell.
- Water enters root hair cells by osmosis; mineral ions can enter by active transport. Link the specialised structure to the process it helps.
The nervous system and neurones
- The central nervous system (CNS) consists of the brain and spinal cord. Peripheral nerves connect the CNS with receptors and effectors around the body.
- A stimulus is a change in the environment, such as heat, light or pressure. A receptor detects the stimulus and starts an electrical impulse.
- Sensory neurones carry impulses from receptors towards the CNS. Relay neurones connect neurones within the CNS. Motor neurones carry impulses from the CNS to effectors.
- Effectors bring about a response: muscles contract and glands secrete substances. A receptor detects the change; an effector responds to it.
- Neurones are specialised for transmitting electrical impulses. Dendrites receive signals, and a long axon carries impulses towards axon terminals.
- In a sensory neurone, a long dendron carries impulses towards the cell body and an axon carries them away from it. Motor-neurone cell bodies lie at one end; not all neurones have the same shape.
- A myelin sheath is an insulating, fatty covering around many nerve fibres. It increases the speed of impulse transmission.
- At a synapse, neurones are separated by a small gap called the synaptic cleft. A similar chemical junction can connect a motor neurone to a muscle.
- When an impulse reaches the end of a neurone (the axon terminal), it causes a chemical called a neurotransmitter to be released. The chemical diffuses across the synaptic gap and binds to receptors on the next cell, starting a new response.
An electrical signal triggers chemical release; neurotransmitter crosses the synaptic cleft. - Across a synapse the signal is chemical; along a neurone it is electrical. Synapses transmit in one direction because release sites and receptors are on opposite sides.
Reflexes and reaction-time investigations
- A reflex is a rapid, automatic response to a stimulus that helps protect the body. It does not require a conscious decision.
- In a spinal reflex: stimulus → receptor → sensory neurone → relay neurone in the spinal cord → motor neurone → effector → response.
Follow the numbered pathway from stimulus to protective response. - For example, heat is detected by receptors in the skin; impulses pass through the reflex arc and a muscle contracts to withdraw the hand.
- The brain can receive information about the stimulus, but the protective response starts without waiting for conscious processing. Reflex pathways include synapses between neurones.
- A voluntary response involves conscious processing, such as choosing to catch a falling ruler. A ruler-drop test estimates reaction time rather than directly measuring nerve-impulse speed.
- For a ruler-drop investigation, hold the ruler at a consistent starting position above the participant’s fingers, release it without warning, and record the distance fallen before it is caught.
- Use a supplied conversion table to convert the distance fallen to reaction time. A smaller distance indicates a shorter reaction time.
- Repeat trials, calculate a mean, and control factors such as the hand used, starting position and distractions. Practice can affect results; discuss anomalies and avoid giving advance cues.
