Cells and control

Overview

Welcome to your deep dive into Cells and Control. This topic forms the bedrock of your understanding of how complex organisms, including humans, grow, repair themselves, and respond to their environment. We will explore the elegant process of mitosis, where one cell becomes two identical daughters, and how these cells then differentiate to perform specialised jobs. We then shift to the high-speed world of the nervous system, looking at how electrical impulses allow for everything from conscious thought to life-saving reflexes. Expect to see a mix of questions: long-answer descriptions of processes like mitosis and synaptic transmission, data interpretation from growth charts, and evaluations of medical technologies like stem cell therapy. This topic is not just about memorising facts; it's about understanding the processes that keep you alive and functioning, moment by moment.

Key Concepts

Concept 1: The Cell Cycle and Mitosis

The cell cycle is the life story of a cell, and mitosis is the dramatic final chapter where it divides. It is not just cell division; it is a regulated process ensuring that new cells are perfect copies. The cycle has two main parts: Interphase (the longest stage) and Mitosis.

• Interphase: This is the cell's 'day job'. It grows, carries out its normal functions, and, crucially, replicates its DNA. Before division, each chromosome is copied, resulting in the classic 'X' shape you see in diagrams – two identical sister chromatids joined at the centromere.
• Mitosis: This is the division of the nucleus. Its purpose is to separate the copied chromosomes into two identical sets. The outcome is critical: two genetically identical diploid daughter cells. This phrase is gold in an exam.

Stages of Mitosis:

1. Prophase: Chromosomes condense (thicken and shorten) and become visible. The membrane around the nucleus breaks down.
2. Metaphase: The chromosomes line up at the equator (middle) of the cell.
3. Anaphase: The sister chromatids are pulled apart to opposite poles (ends) of the cell by spindle fibres.
4. Telophase: A new nuclear membrane forms around each set of chromosomes at the poles.
5. Cytokinesis: The cytoplasm and cell membrane divide to form two separate cells.

Importance of Mitosis:

• Growth: Increasing the number of cells in an organism.
• Repair: Replacing damaged or dead cells (e.g., healing a cut).
• Asexual Reproduction: In some organisms, creating a new, genetically identical individual.

Concept 2: Cell Differentiation and Stem Cells

An adult human is made of trillions of cells, but they all came from a single fertilised egg. Differentiation is the process by which these cells become specialised for a particular function. A nerve cell is different from a muscle cell because it has switched certain genes on and others off, changing its structure and capabilities.

Stem cells are the body's master cells. They are undifferentiated, meaning they have not yet specialised. They have two unique properties:

1. They can divide by mitosis to make more stem cells.
2. They can differentiate into any type of specialised cell.

Types of Stem Cells:

• Embryonic Stem Cells: Found in early-stage embryos. They are pluripotent, meaning they can become any type of cell in the body.
• Adult Stem Cells: Found in certain tissues like bone marrow. They are multipotent, meaning they can only differentiate into a limited range of cell types (e.g., bone marrow stem cells can become red blood cells, white blood cells, or platelets).

Therapeutic Uses: Stem cells offer the potential to treat diseases by replacing damaged cells. For example, they could be used to replace nerve cells in paralysis, insulin-producing cells in diabetes, or brain cells in Parkinson's disease. However, there are challenges:

• Ethical Issues: Using embryonic stem cells involves the destruction of an embryo, which some consider morally wrong.
• Rejection: The patient's immune system might recognise the transplanted cells as foreign and attack them.
• Control of Differentiation: Scientists need to ensure the stem cells differentiate into the correct cell type and do not form tumours.

Concept 3: The Nervous System and Synaptic Transmission

The nervous system is a high-speed communication network. It consists of:

• Central Nervous System (CNS): The brain and spinal cord.
• Peripheral Nervous System (PNS): Nerves that connect the CNS to the rest of the body.

Information is transmitted as electrical impulses along nerve cells called neurones. However, neurones do not touch. The gap between them is called a synapse. The electrical impulse cannot cross this gap.

**Synaptic Transmission:**This is a chemical process. It is a classic 5-mark exam question.

1. The electrical impulse arrives at the pre-synaptic terminal.
2. This triggers the release of neurotransmitters (chemical messengers) from vesicles.
3. The neurotransmitters diffuse across the synaptic cleft (the gap).
4. They bind to specific receptors on the post-synaptic membrane.
5. This binding generates a new electrical impulse in the next neurone.

This process ensures impulses travel in one direction only and allows for complex processing of information.

Concept 4: The Reflex Arc

A reflex is an involuntary, rapid response to a stimulus that serves to protect the body. The pathway of this response is the reflex arc. Crucially, it bypasses the conscious parts of the brain for maximum speed.

The Pathway:

1. Stimulus: A change in the environment (e.g., touching a hot object).
2. Receptor: Detects the stimulus (e.g., pain receptors in the skin).
3. Sensory Neurone: Transmits the impulse from the receptor to the CNS.
4. Relay Neurone: Transmits the impulse across the spinal cord, connecting the sensory and motor neurones.
5. Motor Neurone: Transmits the impulse from the CNS to the effector.
6. Effector: The muscle or gland that carries out the response (e.g., a muscle in the arm contracts).
7. Response: The action taken (e.g., pulling the hand away).

Examiners frequently ask for this pathway in order. Marks are awarded for getting the components correct and in the right sequence.

Mathematical/Scientific Relationships

Growth Charts

Percentile charts are used to monitor growth in children. They are a key data interpretation skill.

• Reading the Chart: To find a value, use a ruler. Draw a line up from the age on the x-axis to the percentile curve, and then a line across to the measurement (e.g., mass) on the y-axis.
• Understanding Percentiles: The 50th percentile is the median average. If a child is on the 25th percentile for mass, it means 25% of children their age are lighter, and 75% are heavier. It is a comparison, not a judgement of health.

Practical Applications

Required Practical: Reaction Time

This practical investigates factors affecting reaction time. A common method is the ruler drop test.

• Apparatus: 30 cm ruler, a partner, a table.
• Method:
  1. Person A sits with their forearm resting on the edge of a table.
  2. Person B holds the ruler vertically so the 0 cm mark is level with Person A's thumb and forefinger.
  3. Person B drops the ruler without warning.
  4. Person A catches the ruler as quickly as possible.
  5. The distance the ruler fell is recorded. This can be converted to a reaction time using a conversion table.
  6. Repeat multiple times to calculate a mean.
• Variables: The independent variable could be the effect of a distraction (e.g., listening to music) or a stimulant (e.g., caffeine, though not usually tested in schools for safety reasons).
• Common Errors: Not dropping the ruler from the same height each time; the person catching anticipating the drop.
• Exam Focus: Examiners may ask you to evaluate the method, suggest improvements, or analyse data from a similar experiment.

Podcast

This 10-minute podcast covers the core concepts, exam tips, and common mistakes for the Cells and Control topic. Listen to it on the go to reinforce your learning.