Topic 2 – Cells and control

Overview

Welcome to Topic 2: Cells and Control. This topic explores the fascinating mechanisms that allow a single fertilised egg to develop into a complex, multicellular organism, and how that organism coordinates its responses to the environment. Understanding cell division, stem cells, and the nervous system is fundamental to Biology because these processes underpin growth, repair, and survival.

Examiners frequently test this topic through a mix of short recall questions (like naming the stages of mitosis) and longer, application-based questions (such as evaluating the use of stem cells or explaining the pathway of a reflex arc). Synoptic links are common here, especially connecting cell division to cancer (Topic 1) or linking the nervous system to homeostasis (Topic 7).

Listen to the podcast episode below for a comprehensive overview of the key concepts and exam techniques.

Key Concepts

Concept 1: The Cell Cycle and Mitosis

Every living cell goes through a cycle of growth and division known as the cell cycle. The most significant part of this cycle for your exam is mitosis, which is the process where a single parent cell divides to produce two genetically identical daughter cells. This is crucial for growth, repairing damaged tissues, and asexual reproduction.

Before mitosis can occur, the cell must go through interphase. During interphase, the cell grows, synthesises new organelles (like mitochondria and ribosomes), and critically, replicates its DNA so that there are two copies of every chromosome.

Mitosis itself is divided into four main stages, which you must learn in order:

1. Prophase: The nucleus starts to break down and spindle fibres appear. The chromosomes condense and become visible.
2. Metaphase: The chromosomes line up along the equator (middle) of the cell.
3. Anaphase: The spindle fibres contract, pulling the sister chromatids to opposite poles (ends) of the cell.
4. Telophase: A new nuclear membrane forms around each set of chromosomes.

Following telophase, the cell membrane and cytoplasm divide to form two separate cells. This final step is called cytokinesis.

Why this works: By replicating the DNA first and then carefully separating the copies using spindle fibres, the cell ensures that each new daughter cell receives an exact, complete set of genetic instructions.

Concept 2: Growth and Differentiation

Growth in multicellular organisms involves three processes: cell division (mitosis), cell elongation (especially in plants, where cells expand to take up more space), and cell differentiation.

Differentiation is the process by which an unspecialised cell becomes specialised to perform a specific function. For example, a generic animal cell might differentiate into a red blood cell (losing its nucleus to carry more oxygen) or a muscle cell (developing lots of mitochondria for contraction).

Concept 3: Stem Cells

Stem cells are unspecialised cells that retain the ability to divide and differentiate into various cell types. They are essential for development and tissue repair.

• Embryonic stem cells: Found in early embryos. They are pluripotent, meaning they can differentiate into almost any type of cell in the body.
• Adult stem cells: Found in specific 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 typically only produce blood cells).
• Meristems (Plants): Found in the growing tips of roots and shoots. Unlike animal cells, plant meristem cells retain the ability to differentiate into any type of plant cell throughout the plant's entire life.

Exam Focus: You are often asked to evaluate the use of stem cells in medicine. While they offer potential cures for conditions like paralysis or diabetes, the use of embryonic stem cells raises ethical issues regarding the destruction of potential human life, and there are clinical risks such as the cells dividing uncontrollably to form tumours.

Concept 4: The Nervous System

The human nervous system allows us to detect changes in our environment (stimuli) and coordinate appropriate responses. It consists of the Central Nervous System (CNS) — the brain and spinal cord — and the peripheral nervous system (the nerves connecting the CNS to the rest of the body).

Information is transmitted as electrical impulses along specialised cells called neurones.

There are three main types of neurones you need to know:

1. Sensory neurones: Carry electrical impulses from receptors (e.g., in the skin or eyes) to the CNS.
2. Relay neurones: Found within the CNS, they connect sensory neurones to motor neurones.
3. Motor neurones: Carry electrical impulses from the CNS to effectors (muscles or glands), which carry out the response.

Neurones are adapted for their function. They have a long axon to carry impulses over long distances. The axon is often surrounded by a myelin sheath, which acts as an electrical insulator, speeding up the transmission of the impulse.

Concept 5: Synapses and Reflexes

Where two neurones meet, there is a tiny gap called a synapse. Electrical impulses cannot jump this gap. Instead, the signal is transmitted chemically.

When an electrical impulse reaches the end of the presynaptic neurone, it triggers the release of chemical messengers called neurotransmitters from vesicles. These neurotransmitters diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic neurone. This binding triggers a new electrical impulse in the next neurone.

Reflex arcs are automatic, rapid responses to stimuli that do not involve conscious thought from the brain. They are essential for protecting the body from harm.
The pathway is: Stimulus → Receptor → Sensory neurone → Relay neurone (in spinal cord) → Motor neurone → Effector → Response.

Mathematical/Scientific Relationships

Magnification Equation

Must memorise
ext{Magnification} = rac{ ext{Image Size}}{ ext{Actual Size}}

• Image Size: The size of the object as it appears in the drawing or photograph.
• Actual Size: The real-life size of the object.
• Note: Ensure both Image Size and Actual Size are in the same units before calculating. You will frequently need to convert between millimetres (mm) and micrometres (\mu m).
• 1 ext{ mm} = 1000 ext{ } \mu m

Percentile Charts

Examiners often test your ability to interpret growth charts (e.g., baby mass or head circumference) using percentiles. If a baby is on the 75th percentile for mass, it means they are heavier than 75% of babies of the same age, and lighter than 25%.

Practical Applications

Understanding the nervous system is directly applied in medical testing, such as testing reflex times to assess neurological health. Stem cell research is currently one of the most heavily funded areas of medical science, with ongoing trials for treating conditions like macular degeneration (blindness) and leukaemia.