Topic 2: Cells and Control (Paper 1: Biology 1)
Welcome to the "Cells and Control" chapter! In this unit, we are going to explore how your body grows from a single tiny cell into a complex person with trillions of cells. We will also look at how your body stays in control and reacts to the world around it using your nervous system. Don't worry if it seems like a lot at first—we'll break it down step-by-step!
1. Cell Division: Mitosis
Most of the cells in your body need to make copies of themselves. This process is called mitosis. It is how you grow from a baby to an adult and how your body repairs itself when you get a cut.
The Cell Cycle
The cell cycle is the life of a cell. Most of the time, the cell is just doing its normal job and preparing to divide. This preparation stage is called Interphase. During Interphase, the cell DNA replicates (makes a copy of itself) so there is enough for two cells.
Once ready, the cell enters the four stages of mitosis. You can remember the order using the mnemonic: PMAT.
- Prophase: The nucleus starts to break down and the chromosomes (which contain your DNA) condense and become visible.
- Metaphase: The chromosomes line up in the Middle of the cell.
- Anaphase: The chromosome copies are pulled Away from each other to opposite ends of the cell.
- Telophase: Two new nuclei form around the two sets of chromosomes.
- Cytokinesis: This is the final "snapping" where the cell membrane pinches in to create two separate cells.
The Result of Mitosis
Mitosis produces two daughter cells. These cells are:
1. Genetically identical to each other and the parent cell.
2. Diploid (they have a full set of paired chromosomes—46 in humans).
Why is Mitosis Important?
Organisms use mitosis for three main things:
- Growth: Increasing the number of cells.
- Repair: Replacing damaged or dead cells.
- Asexual Reproduction: Making "clones" of the parent (common in plants and bacteria).
Cancer: When Control is Lost
Sometimes, a cell's DNA changes (mutates), and it forgets when to stop dividing. Cancer is the result of uncontrolled cell division. This can lead to a mass of abnormal cells called a tumour.
Quick Review: Mitosis creates two identical diploid cells for growth and repair. If it goes out of control, it causes cancer.
2. Growth in Organisms
Growth isn't just about having more cells; it's also about cells getting specialized for different jobs.
Growth in Animals vs. Plants
Animals and plants grow in slightly different ways:
- In Animals: Growth happens through cell division (mitosis) and differentiation (cells becoming specialized, like a skin cell or a muscle cell).
- In Plants: Growth happens through cell division, differentiation, and cell elongation. Elongation is where the cell actually gets longer/stretches.
Cell Differentiation
When you were just a tiny embryo, all your cells were the same. As you developed, they went through differentiation to become specialised cells (like neurones or red blood cells). This is vital because it allows different parts of your body to perform specific functions.
Monitoring Growth: Percentile Charts
Doctors use percentile charts to check if a baby is growing at a healthy rate.
Example: If a baby is on the 75th percentile for height, it means they are taller than 75% of babies their age, and 25% are taller than them. We only worry if a baby's growth pattern jumps across many percentile lines suddenly.
Key Takeaway: Animals grow by division and differentiation; plants add elongation. Differentiation creates specialized cells for specific "jobs."
3. Stem Cells
Think of stem cells as "blank slate" cells. They are undifferentiated cells that have the potential to become many different types of specialized cells.
Types of Stem Cells
- Embryonic Stem Cells: Found in early embryos. These are amazing because they can turn into any type of cell in the human body.
- Adult Stem Cells: Found in places like bone marrow. These are more limited; a bone marrow stem cell can usually only make different types of blood cells.
- Meristems (Plants): These are found in the tips of roots and shoots. Unlike humans, plants have stem cells that can differentiate into any plant cell for their entire life.
Stem Cells in Medicine
Scientists hope to use stem cells to treat diseases like paralysis or diabetes by replacing damaged cells.
Potential Benefits: Healing injuries that currently can't be fixed.
Potential Risks: Stem cells might continue to divide uncontrollably (causing cancer) or be rejected by the patient's immune system.
Did you know? Using embryonic stem cells is controversial because some people believe it is unethical to use cells from an embryo that could have become a person.
4. The Nervous System
Your nervous system allows you to react to your surroundings. It is made up of the Central Nervous System (CNS)—the brain and spinal cord—and a network of neurones (nerve cells).
Neurones: The Information Highway
There are three main types of neurones you need to know:
- Sensory Neurones: Carry electrical impulses from receptors (like your eyes or skin) to the CNS.
- Relay Neurones: Found in the CNS. they connect sensory neurones to motor neurones.
- Motor Neurones: Carry impulses from the CNS to effectors (muscles or glands) to take action.
The Structure of a Neurone
Neurones have special shapes to help them carry signals quickly:
- Axon: A long fiber that carries the impulse over long distances.
- Dendron/Dendrites: Branch-like structures that receive signals.
- Myelin Sheath: A fatty layer around the axon. It acts like the plastic insulation on an electrical wire, speeding up the impulse and stopping it from leaking out.
Synapses and Neurotransmitters
Neurones don't actually touch! There is a tiny gap between them called a synapse.
When an electrical impulse reaches the end of a neurone, it triggers the release of chemicals called neurotransmitters. These chemicals diffuse (float) across the gap and start a new electrical impulse in the next neurone.
The Reflex Arc
A reflex is an automatic, rapid response to danger (like touching a hot stove). It is fast because the signal bypasses the brain—it only goes to the spinal cord and back.
The pathway of a reflex:
Stimulus (Heat) → Receptor (Skin) → Sensory Neurone → Relay Neurone (in Spinal Cord) → Motor Neurone → Effector (Muscle) → Response (Move hand).
Common Mistake: Students often forget the order. Remember: Sensory → Relay → Motor (SRM).
Key Takeaway: The nervous system uses electrical impulses in neurones and chemical signals at synapses. Reflexes are fast because they don't involve the brain's conscious thought.