Cells and Control

What you'll learn

This topic examines how organisms grow, repair damaged tissues and replace worn-out cells through controlled cell division. You will study the stages of the cell cycle including mitosis, how stem cells differentiate into specialised cells, and how the nervous system coordinates rapid responses to stimuli. Edexcel GCSE Biology papers regularly test mitosis sequencing, percentile chart interpretation and reflex arc diagrams.

Key terms and definitions

Mitosis — a type of cell division that produces two genetically identical daughter cells from one parent cell, used for growth and repair.

Cell cycle — the series of stages that a cell goes through involving growth, DNA replication and division to produce two daughter cells.

Stem cell — an undifferentiated cell capable of dividing repeatedly and differentiating into specialised cell types.

Differentiation — the process by which a cell becomes specialised for a particular function by developing specific structures and producing particular proteins.

Synapse — the junction between two neurones where chemical transmission of nerve impulses occurs via neurotransmitter molecules.

Reflex arc — the pathway taken by nerve impulses in a reflex action, from receptor through sensory neurone, relay neurone and motor neurone to effector.

Percentile — a value on a growth chart below which a given percentage of measurements fall; for example, the 50th percentile is the median.

Meristem — a region of plant tissue containing undifferentiated cells capable of continuous division, found at root and shoot tips.

Core concepts

The cell cycle and mitosis

The cell cycle consists of three main stages that allow organisms to grow and repair tissues:

Stage 1 — Interphase (Growth and DNA replication)

• Cell grows and increases the number of sub-cellular structures (ribosomes, mitochondria)
• DNA replicates to form two copies of each chromosome
• Longest phase of the cell cycle
• Cell remains functionally active during this period

Stage 2 — Mitosis (Nuclear division)

The chromosomes line up at the centre of the cell and separate into two identical sets. Mitosis involves four phases:

1. Prophase — chromosomes condense and become visible; nuclear membrane breaks down
2. Metaphase — chromosomes align at the cell equator; spindle fibres attach to centromeres
3. Anaphase — spindle fibres contract and pull sister chromatids to opposite poles of the cell
4. Telophase — nuclear membranes reform around each set of chromosomes; chromosomes uncoil

Stage 3 — Cytokinesis (Cell division)

• The cytoplasm and cell membrane divide to form two genetically identical daughter cells
• In animal cells, the membrane pinches inward
• In plant cells, a new cell wall forms down the middle

Edexcel exam questions frequently show diagrams of cells at different stages and ask you to identify the stage or place them in the correct sequence. Remember that chromosomes are only visible as distinct structures during mitosis itself, not during interphase.

Growth in organisms

Organisms grow through two distinct processes:

Cell division — mitosis increases the total number of cells in an organism.

Cell differentiation and elongation — cells become specialised for particular functions and some cells (particularly in plants) elongate significantly.

In animals, most differentiation occurs during embryonic development. Once specialised, most animal cells lose the ability to differentiate into other cell types. Plant cells retain the ability to differentiate throughout the plant's life.

Growth charts and percentiles

Growth is monitored using percentile charts that plot measurements (height, mass, head circumference) against age. Key points:

• The 50th percentile represents the median value — half of children measure above this line, half below
• A baby on the 25th percentile for mass is heavier than 25% of babies the same age
• Consistent tracking along a percentile is normal growth
• Crossing percentile lines significantly upward or downward may indicate health issues requiring investigation
• Genetic factors, nutrition and health conditions all affect growth patterns

Edexcel papers present real percentile charts and require interpretation of growth patterns, comparison of individuals, or identification of periods of rapid growth.

Stem cells and differentiation

Stem cells are unspecialised cells with two key properties:

1. Self-renewal — they can divide repeatedly to produce more stem cells
2. Potency — they can differentiate into specialised cell types

Types of stem cells in animals:

Embryonic stem cells

• Found in early-stage embryos
• Totipotent — can differentiate into any cell type in the body
• Can be grown in laboratories for research and potential medical treatments
• Ethical concerns exist because embryos are destroyed during extraction

Adult stem cells

• Found in specific locations (bone marrow, skin, intestinal lining)
• Multipotent — can only differentiate into certain cell types
• Bone marrow stem cells produce blood cells, for example
• Fewer ethical concerns but more limited applications

Plant stem cells (meristems)

Plants retain unspecialised cells in meristem tissue throughout their life:

• Located at root tips and shoot tips (apical meristems)
• Also found in cambium layer (lateral meristems)
• Meristem cells divide rapidly and can differentiate into any plant cell type
• Allows plants to grow continuously and replace damaged parts
• Used in horticulture to clone plants from cuttings

Medical applications of stem cells

Stem cell therapy treats diseases by replacing damaged or non-functional cells:

• Leukaemia treatment — bone marrow transplants replace cancerous blood-forming cells
• Potential treatments for paralysis, diabetes, Parkinson's disease
• Challenges include immune rejection, controlling differentiation, and tumour formation

The nervous system

The nervous system detects and responds to changes in the environment. It consists of:

Central Nervous System (CNS) — brain and spinal cord

Peripheral Nervous System — all other nerves in the body

The nervous system coordinates rapid responses using electrical impulses travelling along specialised cells called neurones:

Sensory neurones — carry impulses from receptors to the CNS; cell body in the middle of the axon

Relay neurones — connect sensory and motor neurones in the CNS; short branched structures

Motor neurones — carry impulses from CNS to effectors; cell body at one end

All neurones share common features:

• Long axon to transmit impulses over distance
• Dendrites to receive signals from other neurones
• Myelin sheath insulates the axon and speeds up transmission

Synapses and neurotransmission

Neurones do not physically touch. The gap between neurones is the synapse. Transmission across synapses occurs chemically:

1. Electrical impulse arrives at the end of the presynaptic neurone
2. Impulse triggers release of neurotransmitter molecules into the synaptic cleft
3. Neurotransmitters diffuse across the gap (approximately 20 nanometres)
4. Neurotransmitters bind to receptor proteins on the postsynaptic membrane
5. Binding triggers a new electrical impulse in the next neurone
6. Neurotransmitters are rapidly broken down or reabsorbed to prevent continuous stimulation

Synaptic transmission is unidirectional — impulses can only travel in one direction because neurotransmitter receptors are only present on the postsynaptic membrane.

Reflex actions

Reflexes are rapid, automatic responses to stimuli that do not require conscious thought. They protect the body from harm.

The pathway of a reflex is the reflex arc:

1. Receptor detects stimulus (e.g., thermoreceptors in skin detect heat)
2. Sensory neurone transmits impulse to CNS
3. Relay neurone in spinal cord or brain connects sensory to motor neurone
4. Motor neurone transmits impulse to effector
5. Effector (muscle or gland) produces response (e.g., muscle contracts to pull hand away)

The brain is not involved in processing the information, which makes reflexes much faster than conscious actions. The brain receives sensory information about the reflex after it has occurred.

Examples of reflex actions:

• Pupil reflex — pupils constrict in bright light to prevent retinal damage
• Knee-jerk reflex — leg extends when tendon below kneecap is tapped
• Dropping a hot object — hand muscles contract to release the object

Edexcel papers commonly present reflex arc diagrams with structures labelled A, B, C etc., requiring identification. Remember: stimulus → receptor → sensory → relay → motor → effector → response.

The brain structure and function

The brain coordinates complex behaviours and contains billions of interconnected neurones. Key regions tested in Edexcel GCSE Biology:

Cerebral cortex — outer layer responsible for consciousness, memory, language and intelligence

Cerebellum — coordinates muscle movement and maintains balance and posture

Medulla — controls unconscious activities (breathing rate, heart rate)

Hypothalamus — regulates body temperature and water balance

Studying the brain presents challenges:

• Encased in skull, making direct observation difficult
• Delicate tissue easily damaged during investigation
• Ethical issues with human experimentation
• Complexity and interconnectedness of regions

Modern techniques include MRI and CT scanning to observe brain activity non-invasively.

Worked examples

Question 1: A student observes dividing onion root tip cells under a microscope. In a sample of 50 cells, 42 cells are in interphase, 7 cells are in mitosis and 1 cell is in cytokinesis. Calculate the percentage of cells undergoing mitosis. (2 marks)

Answer:

• Number of cells in mitosis = 7
• Percentage = (7 ÷ 50) × 100 [1 mark]
• Percentage = 14% [1 mark]

Question 2: The diagram shows a percentile chart for boys aged 0-12 months. Boy A is on the 75th percentile for mass at 6 months. Explain what this means. (2 marks)

Answer:

• Boy A is heavier than 75% of boys the same age [1 mark]
• 25% of boys the same age are heavier than Boy A [1 mark]

(Alternative: Boy A's mass is in the top 25% for his age)

Question 3: Describe the pathway of a reflex arc when a person steps on a sharp object. (4 marks)

Answer:

• Pain receptors in the foot detect the stimulus [1 mark]
• Sensory neurone carries impulse to the spinal cord [1 mark]
• Relay neurone connects to motor neurone in the spinal cord [1 mark]
• Motor neurone carries impulse to leg muscles (effector) which contract to lift the foot [1 mark]

Common mistakes and how to avoid them

Mistake: Stating that mitosis produces four daughter cells or cells that are genetically different. Correction: Mitosis produces two genetically identical daughter cells. Meiosis produces four non-identical gametes — this is covered in a different topic.

Mistake: Confusing chromosomes with chromatids. Describing chromosomes as "splitting in half" during mitosis. Correction: Each chromosome replicates during interphase to form two sister chromatids joined at the centromere. During anaphase, these chromatids separate and move to opposite poles — they are then individual chromosomes.

Mistake: Claiming embryonic stem cells can only become blood cells or have limited differentiation potential. Correction: Embryonic stem cells can differentiate into any cell type in the body. Adult stem cells (such as those in bone marrow) have limited potential.

Mistake: Drawing arrows on reflex arc diagrams that go from motor neurone to sensory neurone, or showing impulses travelling in both directions. Correction: Nerve impulses travel in one direction only: receptor → sensory → relay → motor → effector. Synapses ensure unidirectional transmission.

Mistake: Describing percentile charts incorrectly, saying "90th percentile means 90% are smaller." Correction: The 90th percentile means the individual is larger than 90% of the reference population, with 10% larger than them.

Mistake: Stating that the brain is involved in reflex actions or that reflexes require conscious thought. Correction: Reflexes bypass the conscious brain. The relay neurone in the spinal cord connects directly from sensory to motor neurone, making the response rapid and automatic.

Exam technique for Cells and Control

Command words matter: "Describe" requires stating what happens; "Explain" requires giving reasons or mechanisms. For a 3-mark "explain" question on mitosis importance, state the outcome (two identical cells), the reason (for growth) and the consequence (maintains chromosome number).

Sequence questions: When ordering stages of mitosis or steps in a reflex arc, draw arrows on the diagram first. Examiners expect precise terminology — use "anaphase" not "the chromosomes split stage."

Calculation questions: Show working clearly for percentile or mitosis percentage calculations. Even if the final answer is wrong, method marks are available. Always include units and check whether the question asks for a percentage, ratio or decimal.

Diagram analysis: Reflex arc and neurone diagrams appear frequently. Learn the direction of impulse travel and the names of all structures. If asked to label, be specific: "motor neurone" scores marks; "nerve" does not.

Quick revision summary

The cell cycle consists of interphase (growth and DNA replication), mitosis (nuclear division producing two identical daughter cells) and cytokinesis (cell division). Stem cells are undifferentiated and can divide repeatedly — embryonic types can form any cell; adult types are limited. Growth is monitored using percentile charts. The nervous system uses electrical impulses in neurones (sensory, relay, motor) with chemical transmission at synapses. Reflex arcs provide rapid automatic responses: receptor → sensory → relay → motor → effector. The brain has specialised regions including cerebral cortex (consciousness), cerebellum (coordination) and medulla (unconscious control).