SB2 · Cells and controlTopic 2 — Cells and control
Cell division, growth and the nervous system
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.
The brain and its investigation
The cerebral hemispheres are involved in conscious thought, memory, intelligence and movements you choose to make (voluntary movements). The cerebellum coordinates movement and balance. The medulla oblongata controls automatic functions, including breathing and heart rate.These regions have different roles; they are not stages in a signal pathway.
(Higher tier) The skull protects the brain but makes direct investigation difficult. CT scanning uses X-rays to build cross-sectional images of structures, helping locate injuries or tumours.
(Higher tier) PET scans detect radiation from a tracer to show activity in different brain regions. A tracer linked to glucose uptake shows where cells are using more glucose. CT mainly shows structure; PET can show where more chemical activity is taking place (metabolic activity).
(Higher tier) Brain and spinal-cord injuries can disrupt complex networks. Some nerve cells regenerate poorly, and surgery or drug treatment can harm nearby healthy tissue. Brain tumours can compress tissue; removing them may damage essential functions.
(Higher tier) Evidence from scans, electrical measurements and patients with damage helps link regions with functions. A correlation between brain activity and a task supports a link, but does not by itself prove a simple cause.
The eye, accommodation and defects
The cornea bends incoming light strongly. The lens changes shape to focus light onto the retina, where receptor cells convert light into signals carried by the optic nerve.
The iris changes pupil size. 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.
Rods are sensitive in dim light but do not provide colour vision. Cones work best in brighter light and enable colour vision. The fovea is rich in cones; the blind spot has no photoreceptors where the optic nerve leaves.
Focusing on objects at different distances is called accommodation. For nearby objects, ciliary muscles contract, suspensory ligaments loosen and the lens becomes thicker, bending light more strongly. For distant objects, the muscles relax, ligaments tighten and the lens becomes thinner.The suspensory ligaments slacken for near vision and tighten for distant vision.
In short-sightedness, distant objects focus in front of the retina; a diverging lens spreads the rays before they enter the eye. In long-sightedness, nearby objects would focus behind the retina; a converging lens brings the focus forward.
A cataract is a cloudy lens that scatters light and reduces vision; replacing it with an artificial lens can restore clear focusing. Colour blindness commonly results from an inherited defect in cone pigments and is not corrected with ordinary focusing lenses.
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