Cells and Control

What you'll learn

This topic covers how cells divide and how organisms coordinate their internal activities. You'll study mitosis and the cell cycle, growth and development in animals and plants, stem cells and their applications, and how the nervous system controls the body's responses to changes in the environment.

Key terms and definitions

Mitosis — cell division producing two genetically identical daughter cells for growth and repair

Differentiation — the process by which cells become specialised for particular functions

Stem cells — unspecialised cells capable of differentiating into different cell types

Synapse — the junction between two neurones where chemical transmission occurs

Reflex arc — the pathway of neurones involved in a reflex action, bypassing conscious control

Meristem — plant tissue containing stem cells, found in root and shoot tips

Centromere — the region where two sister chromatids join together during cell division

Mitotic index — the ratio of cells undergoing mitosis to total cells in a sample

Core concepts

The cell cycle and mitosis

The cell cycle is the sequence of events that takes place in a cell leading to its division. It consists of two main stages: interphase and mitosis.

Interphase occupies approximately 90% of the cell cycle and involves three sub-phases:

• G1 phase: cell grows, new organelles and proteins are synthesised
• S phase: DNA replication occurs, producing two identical copies of each chromosome
• G2 phase: further cell growth, checking and repair of newly copied DNA

Mitosis is nuclear division producing two genetically identical daughter cells. The stages are:

Prophase:

• Chromatin condenses into visible chromosomes, each consisting of two sister chromatids joined at the centromere
• Nuclear envelope breaks down
• Spindle fibres form from the centrioles

Metaphase:

• Chromosomes line up along the cell equator
• Spindle fibres attach to the centromere of each chromosome

Anaphase:

• Centromeres divide
• Sister chromatids separate and move to opposite poles of the cell
• Spindle fibres shorten

Telophase:

• Nuclear envelopes reform around each set of chromosomes
• Chromosomes uncoil and become chromatin again
• Spindle fibres break down

Cytokinesis follows mitosis, dividing the cytoplasm to form two separate daughter cells.

Calculating mitotic index

The mitotic index indicates how rapidly tissue is dividing and growing. It is calculated using:

Mitotic index = (number of cells in mitosis ÷ total number of cells) × 100

This calculation is commonly tested in exam questions requiring mathematical skills. Cancer cells typically have a higher mitotic index than normal tissue because they divide uncontrollably.

Growth in animals and plants

Animal growth occurs through:

• Cell division (mitosis) increasing cell number
• Cell differentiation producing specialised cells
• Cell elongation (limited compared to plants)

Animal cells differentiate early in development and most lose the ability to differentiate further. Growth is measured by:

• Increase in wet mass (total body weight)
• Increase in dry mass (more reliable, excluding water content)
• Increase in length/height

Plant growth differs significantly:

• Growth occurs primarily in meristems (root and shoot tips)
• Meristems contain stem cells that retain the ability to divide throughout the plant's life
• Cell elongation is the main mechanism of growth after division
• Most plant cells retain the ability to differentiate

Percentile growth charts are used to monitor human growth. They show how an individual's measurements compare to the population average:

• The 50th percentile represents the median
• Values between the 25th and 75th percentile are typical
• Measurements consistently below the 9th or above the 91st percentile may indicate growth disorders

Stem cells

Stem cells are unspecialised cells capable of dividing to produce more stem cells or differentiating into specialised cell types.

Types of stem cells:

Embryonic stem cells:

• Found in early-stage embryos
• Totipotent or pluripotent (can differentiate into any cell type)
• Potential to treat conditions like paralysis, diabetes, and Parkinson's disease
• Ethical concerns about embryo destruction

Adult stem cells:

• Found in specific locations (bone marrow, skin, brain)
• Multipotent (limited range of cell types)
• Bone marrow stem cells can produce blood cells
• Lower differentiation potential than embryonic stem cells
• Fewer ethical issues

Plant meristems:

• Found in root and shoot tips
• Retain ability to differentiate throughout the plant's life
• Used in plant cloning and tissue culture
• Can produce rare plants quickly or preserve endangered species

Therapeutic applications:

• Treatment of blood disorders using bone marrow transplants
• Potential regeneration of damaged tissues (heart, nerves, spinal cord)
• Drug testing on stem cell-derived tissues instead of animals
• Research into developmental biology

Ethical and social considerations:

• Embryonic stem cell research involves destroying embryos
• Religious and ethical objections to embryo use
• Potential for exploitation of egg donors
• Risk of tumour formation if differentiation is uncontrolled
• Access and cost of treatments

The brain and nervous system structure

The nervous system coordinates rapid responses to stimuli. It consists of:

Central nervous system (CNS):

• Brain
• Spinal cord

Peripheral nervous system:

• Sensory neurones (carry impulses from receptors to CNS)
• Motor neurones (carry impulses from CNS to effectors)

Brain structure and function:

The cerebral cortex (outer layer):

• Controls consciousness, intelligence, memory, language
• Divided into two hemispheres

The cerebellum (back of brain):

• Coordinates muscle movements and balance
• Controls fine motor skills

The medulla oblongata (brain stem):

• Controls unconscious activities (heart rate, breathing)
• Connects brain to spinal cord

The hypothalamus:

• Regulates body temperature
• Controls water balance and hormone release from pituitary gland

Studying brain function:

• MRI and CT scans show brain structure
• PET scans show brain activity
• Study of patients with brain damage reveals function
• Electrical stimulation during surgery
• Limitations: brain complexity, ethical issues with experimentation

The nervous response pathway

Receptors detect stimuli (changes in the environment):

• Light receptors (retina)
• Sound receptors (cochlea in ear)
• Touch, pressure, and temperature receptors (skin)
• Chemical receptors (taste buds, nose)

Sensory neurones transmit electrical impulses from receptors to the CNS.

Relay neurones in the CNS process information and coordinate responses.

Motor neurones transmit impulses from CNS to effectors.

Effectors carry out responses:

• Muscles contract
• Glands secrete hormones

Synapses and nerve impulse transmission

A synapse is the gap between two neurones. Electrical impulses cannot cross this gap directly.

Synaptic transmission:

1. Electrical impulse arrives at the end of the first neurone
2. Triggers release of neurotransmitter chemicals from vesicles
3. Neurotransmitters diffuse across the synaptic cleft
4. Bind to receptor proteins on the membrane of the next neurone
5. Stimulate a new electrical impulse in the second neurone
6. Neurotransmitters are then destroyed or reabsorbed

Function of synapses:

• Ensure impulses travel in one direction only
• Filter out weak stimuli (impulse must be strong enough to trigger sufficient neurotransmitter release)
• Allow multiple neurones to connect
• Allow summation of weak impulses

Drugs and synapses:

• Some drugs bind to receptors, blocking neurotransmitters (e.g., beta blockers)
• Others prevent neurotransmitter breakdown (e.g., ecstasy prevents serotonin reabsorption)

Reflex arcs

Reflexes are rapid, automatic responses that bypass conscious control. They protect the body from harm.

Reflex arc pathway:

1. Stimulus detected (e.g., hand touches hot object)
2. Receptor activated (temperature/pain receptors in skin)
3. Sensory neurone carries impulse to spinal cord
4. Relay neurone in spinal cord connects sensory to motor neurone
5. Motor neurone carries impulse to effector
6. Effector responds (muscle contracts, hand withdraws)

Examples:

• Pupil reflex (adjusting pupil size in response to light)
• Knee-jerk reflex (leg extension when patellar tendon tapped)
• Withdrawal reflex from painful stimuli

The response occurs before the brain is aware because the relay neurone connects directly to the motor neurone in the spinal cord, not requiring processing in the brain.

The eye and accommodation

The eye contains light receptors (photoreceptors) in the retina:

• Rods detect light intensity, used in dim light, black and white vision
• Cones detect colour, require bright light, concentrated in the fovea

Accommodation is the process of changing the lens shape to focus on near or distant objects.

Focusing on distant objects:

• Ciliary muscles relax
• Suspensory ligaments tighten
• Lens pulled thin
• Light refracted less

Focusing on near objects:

• Ciliary muscles contract
• Suspensory ligaments slacken
• Lens becomes fatter
• Light refracted more

Common eye defects:

Myopia (short-sightedness):

• Eyeball too long or lens too thick
• Distant objects focused in front of retina
• Corrected with concave (diverging) lenses

Hyperopia (long-sightedness):

• Eyeball too short or lens too thin
• Near objects focused behind retina
• Corrected with convex (converging) lenses

Red-green colour blindness:

• Genetic condition, sex-linked (more common in males)
• Lack of specific cone cells
• Cannot distinguish between red and green

Worked examples

Example 1: Calculating mitotic index

Question (4 marks): A student observes 200 cells in an onion root tip sample. 32 cells are undergoing mitosis. Calculate the mitotic index and explain what this tells us about the tissue.

Mark scheme answer:

Mitotic index = (32 ÷ 200) × 100 [1 mark for correct formula or working]

= 16% [1 mark for correct answer with unit]

This is a relatively high mitotic index [1 mark], indicating the tissue is growing rapidly / cells are dividing frequently / this is typical for a root tip/meristem [1 mark for appropriate explanation]

Example 2: Reflex arc sequence

Question (5 marks): Describe the pathway of a reflex arc when a person steps on a sharp object.

Mark scheme answer:

1. Sharp object (stimulus) detected by pain receptors in foot [1 mark]
2. Sensory neurone carries impulse to spinal cord/CNS [1 mark]
3. Relay neurone in spinal cord connects sensory and motor neurones [1 mark]
4. Motor neurone carries impulse to leg muscle (effector) [1 mark]
5. Muscle contracts, foot withdrawn from sharp object (response) [1 mark]

Award maximum 5 marks. Must be in logical sequence.

Example 3: Stem cell comparison

Question (6 marks): Compare embryonic stem cells and adult stem cells. Explain why there are ethical concerns about using embryonic stem cells.

Mark scheme answer:

Similarities/differences:

• Embryonic stem cells can differentiate into any/all cell types whereas adult stem cells differentiate into limited/specific cell types [1 mark]
• Both can divide to produce more stem cells / self-renew [1 mark]
• Embryonic stem cells found in early embryos; adult stem cells found in specific tissues like bone marrow [1 mark]

Ethical concerns:

• Using embryonic stem cells involves destroying embryos [1 mark]
• Some believe embryos have a right to life / religious objections [1 mark]
• Concerns about where embryos come from / potential exploitation of egg donors [1 mark]

Award maximum 6 marks total (3 for comparison, 3 for ethics)

Common mistakes and how to avoid them

• Confusing mitosis with meiosis: Mitosis produces two identical diploid cells for growth; meiosis produces four non-identical haploid gametes for reproduction. Only mitosis is required for Cells and Control.

• Wrong order of mitosis stages: Remember the sequence PMAT (Prophase, Metaphase, Anaphase, Telophase). Draw diagrams to visualise each stage rather than just memorising descriptions.

• Stating neurones carry signals, not impulses: Always write "electrical impulses" not just "signals" or "messages" — examiners want precise terminology.

• Missing the synapse in reflex arc descriptions: Students often jump from sensory neurone directly to motor neurone. Always include the relay neurone in the spinal cord and mention synapses between neurones.

• Confusing lens changes during accommodation: For near objects, ciliary muscles CONTRACT (not relax), suspensory ligaments SLACKEN (not tighten), and the lens becomes FATTER (not thinner). Make a diagram.

• Not showing working in mitotic index calculations: Even if your final answer is correct, you must show the formula and substitution to gain full marks.

Exam technique for "Cells and Control"

• "Describe" questions: State what happens in a logical sequence. For processes like mitosis or reflex arcs, marks are awarded for each correct stage in order. Use numbered points.

• "Explain" questions: Give reasons why something happens, not just what happens. Link cause and effect using "because," "therefore," or "this causes." For example: "The lens becomes fatter because the ciliary muscles contract, which slackens the suspensory ligaments."

• Calculation questions: Always show your working, include units, and give answers to the appropriate number of significant figures (usually 2 or 3 for GCSE). For mitotic index, remember to multiply by 100 for a percentage.

• 6-mark extended response questions: Use the mark scheme criteria: aim for a logical sequence, correct scientific terminology, detailed explanation, and correct spelling of scientific terms. Plan your answer in 30 seconds before writing.

Quick revision summary

Cells and Control covers cell division (mitosis), growth in animals and plants, stem cells, and nervous coordination. Mitosis produces identical cells through PMAT stages. Stem cells differentiate into specialised cells—embryonic are pluripotent, adult are multipotent. The nervous system coordinates responses via receptors, sensory neurones, CNS (brain/spinal cord), motor neurones, and effectors. Synapses use neurotransmitters for chemical transmission. Reflex arcs bypass conscious control for rapid responses. The eye accommodates by changing lens shape through ciliary muscle and suspensory ligament action.