Welcome to Cells and Control!
In this chapter, we are going to explore two of the most important things a living organism does: growing and reacting. We will look at how one cell turns into billions, how your brain works like a super-fast computer, and how your eyes help you see the world.
Don't worry if some of the long names seem scary at first—we will break them down into simple pieces together!
1. Mitosis and the Cell Cycle
Every single person started as just one cell. To grow into an adult, that cell had to divide over and over again. This process is called mitosis.
What is Mitosis?
Mitosis is a type of cell division that produces two genetically identical daughter cells. This means the new cells have the exact same DNA as the original cell. These new cells are diploid, meaning they have a full set of chromosomes (in humans, that's 46).
The Stages of the Cell Cycle
Think of the cell cycle like a life cycle for a cell. It spends most of its time preparing and then a short time dividing.
1. Interphase: The "prep stage." The cell grows and copies its DNA so there is enough for two cells.
2. Prophase: The nucleus starts to break down and spindle fibres appear.
3. Metaphase: The chromosomes line up in the Middle of the cell.
4. Anaphase: The chromosomes are pulled Away to opposite ends of the cell.
5. Telophase: Two new nuclei form around the chromosomes.
6. Cytokinesis: The cell membrane pinches in to create two separate cells.
Memory Aid: Use the mnemonic I Play Music At Tea-time (Interphase, Prophase, Metaphase, Anaphase, Telophase).
Why is Mitosis important?
Organisms use mitosis for three main things:
- Growth: To make the organism bigger.
- Repair: To replace damaged or dead cells (like when a scab forms).
- Asexual Reproduction: Making offspring that are clones of the parent.
Quick Review: Mitosis makes 2 identical cells for growth and repair. If mitosis goes wrong and cells divide uncontrollably, it can lead to cancer.
Key Takeaway: Mitosis is the process of making identical copies of cells to help us grow and fix our bodies.
2. Growth in Animals and Plants
Growth isn't just about having more cells; it's about cells becoming specialized for different jobs.
Growth in Animals
In animals, growth happens through cell division (mitosis) and differentiation. Differentiation is when a "basic" cell changes to become a "specialized" cell, like a muscle cell or a nerve cell.
Growth in Plants
Plants are slightly different. They grow through:
- Cell division: Usually happens at the tips of roots and shoots.
- Elongation: This is where plant cells get longer by taking in water. Imagine a balloon stretching as you blow air into it!
- Differentiation: Creating specialized cells like root hair cells or xylem.
Monitoring Growth
Doctors use percentile charts to check if a baby is growing at the right rate. If a baby is on the 75th percentile for weight, it means that 75% of babies that age are lighter than them, and 25% are heavier. It’s all about making sure the growth follows a steady curve.
Key Takeaway: Animals grow mostly by cell division, while plants use cell division and elongation.
3. Stem Cells
Stem cells are special because they are undifferentiated. This means they haven't "decided" what kind of cell they want to be yet.
Types of Stem Cells
1. Embryonic Stem Cells: Found in early embryos. These are amazing because they can turn into any type of cell in the body.
2. Adult Stem Cells: Found in places like bone marrow. They are more limited and can only turn into a few types of cells (like blood cells).
3. Meristems (Plants): These are found in the tips of roots and shoots. They can produce any type of plant cell for the plant's entire life!
Stem Cells in Medicine
Scientists hope to use stem cells to cure diseases like paralysis or diabetes by replacing damaged cells.
Benefits: Can treat previously incurable diseases.
Risks: Stem cells could continue to divide and cause cancer (tumours), or the body might reject them as foreign tissue.
Key Takeaway: Stem cells are "blank" cells that can become specialized. Embryonic cells are the most powerful.
4. The Brain and Scanning (Biology Only)
The brain is the control centre of the body. It’s divided into different parts that do different jobs.
Parts of the Brain
- Cerebral Hemispheres: The big "wrinkly" part at the top. It controls things like memory, language, and our senses.
- Cerebellum: Found at the back. It controls balance and muscle coordination (like riding a bike).
- Medulla Oblongata: Found in the brain stem. It controls automatic things we don't think about, like breathing and heart rate.
Looking Inside: CT and PET Scans
Because the brain is protected by a thick skull and is very fragile, it is hard to study. We use technology to see inside:
- CT Scan: Uses X-rays to show the structure of the brain. It can show tumours or bleeding.
- PET Scan: Uses a radioactive tracer to show which parts of the brain are active in real-time.
Did you know? Treating brain injuries is very difficult because the brain doesn't repair itself very well, and surgery can often cause more damage to the delicate tissue.
Key Takeaway: Different parts of the brain control different functions. Scans help us see structure (CT) and activity (PET).
5. The Nervous System
The nervous system allows us to react to our surroundings very quickly using electrical impulses.
The Pathway of an Impulse
When something happens, your body follows a specific order:
Stimulus (e.g., a hot plate) → Receptor (skin) → Sensory Neurone → CNS (Brain/Spinal Cord) → Motor Neurone → Effector (Muscle) → Response (Pull hand away).
Structure of Neurones
Neurones are long cells that carry electrical signals.
- Dendrites/Dendrons: Receive signals from other cells.
- Axon: The long part that carries the impulse away.
- Myelin Sheath: A fatty layer that acts like insulation on a wire, making the signal travel much faster.
The Synapse
When two neurones meet, there is a tiny gap called a synapse. The electrical signal cannot jump the gap! Instead:
- The electrical impulse reaches the end of the first neurone.
- It triggers the release of chemicals called neurotransmitters.
- These chemicals diffuse across the gap.
- They bind to receptors on the next neurone, starting a new electrical impulse.
Reflex Arcs
A reflex is a fast, automatic response that protects the body. It bypasses the thinking part of the brain to save time. In a reflex arc, the signal goes through a relay neurone in the spinal cord instead of going all the way to the brain first.
Key Takeaway: Neurones carry electrical signals. Synapses use chemicals to bridge the gap. Reflexes are fast because they skip the brain.
6. The Eye (Biology Only)
The eye is a sensory receptor that detects light.
Parts of the Eye
- Cornea: The clear outer layer that starts bending (refracting) light.
- Iris: The coloured part that controls how much light enters by changing the size of the pupil.
- Lens: Fine-tunes the light to focus it perfectly on the back of the eye.
- Retina: The "screen" at the back containing rods (for dim light) and cones (for colour).
Vision Defects
1. Short-sightedness (Myopia): You can't see far away. The light focuses in front of the retina. Fixed with a concave lens.
2. Long-sightedness (Hyperopia): You can't see close up. The light focuses behind the retina. Fixed with a convex lens.
3. Cataracts: The lens becomes cloudy. Fixed by replacing the lens with an artificial one.
4. Colour Blindness: Some cone cells in the retina don't work properly. This cannot currently be cured.
Quick Review: Rods = Dark. Cones = Colour. (Think Cone for Colour!)
Key Takeaway: The eye focuses light onto the retina. Lenses can correct focus issues like short and long-sightedness.
Congratulations! You've finished the notes for Cells and Control. Take a break, then try some practice questions on the stages of mitosis or the path of a reflex arc!