🏷 CORE Biology (9221) Study Notes: Cell Structure

Hey there, future biologist! This chapter, "Cell Structure," is absolutely vital. Think of it as learning the alphabet before reading a book. Cells are the fundamental building blocks of all life! Understanding them is key to mastering the whole curriculum section on Organisation.

Don't worry if the names seem complex—we'll break down the cell into tiny, manageable parts, just like taking apart a Lego model!

1. The Basic Building Block: What is a Cell?

Every living thing, from the smallest bacteria to the biggest blue whale, is made of cells. A cell is the smallest unit of life that can carry out all life processes.

Analogy: The City

Imagine a cell as a bustling miniature city. It needs a border (cell membrane), a power supply (mitochondria), and a central library (nucleus) to function correctly.

2. The Two Main Types: Plant Cells vs. Animal Cells

In CORE Biology, the most important distinction is between the cells that make up animals (like you!) and the cells that make up plants (like a tree or a daisy). While they share many components, their unique parts allow them to perform very different jobs.

Key Similarities (Shared Components)

Both animal and plant cells have:

  • Nucleus (The control center)
  • Cytoplasm (The jelly filling)
  • Cell Membrane (The border fence)
  • Mitochondria (The power stations)

Key Differences (Plant Cells Only)

Plant cells have three extra structures that animal cells do not:

  1. Cell Wall: A strong outer layer for support.
  2. Large Permanent Vacuole: A big sack for storing water and keeping the cell firm (turgid).
  3. Chloroplasts: Structures containing chlorophyll for making food.

Quick Tip: If you see a square or fixed shape, and green parts, you are looking at a plant cell! Animal cells are usually more irregular and rounded.

❗ Common Mistake Alert: Students often confuse the cell wall and the cell membrane. Remember, the Cell Wall is the tough, outermost layer in plants only. The Cell Membrane is inside the wall (in plants) or the outermost layer (in animals), and it controls what enters and leaves.

3. Organelles: The Cell's Mini-Organs and Their Functions

The components inside the cell, each with a specific job, are called organelles. Let’s look at what each one does.

A. The Control Center and Workshop

1. The Nucleus
This is the largest organelle and the control center.

  • Structure: Usually spherical and found near the center.
  • Function: Contains the cell's genetic material (DNA) in the form of chromosomes. It controls all cell activities (like growth, reproduction, and repair).
Memory Aid: Think of the N-ucleus as the N-erve center or CEO of the cell.

2. The Cytoplasm
This is the filling of the cell.

  • Structure: A jelly-like substance (mostly water) that fills the cell and surrounds the organelles.
  • Function: This is where most of the cell's essential chemical reactions take place (like making proteins and breaking down sugars).
Analogy: It’s the factory floor where all the work happens.

B. Energy and Borders

3. The Cell Membrane (Part of ALL Cells)

  • Structure: A thin layer surrounding the cytoplasm.
  • Function: It acts as a partially permeable barrier (or semi-permeable). This means it controls which substances can enter and leave the cell (like water, oxygen, and waste products).
Analogy: The membrane is the security gate—it checks IDs and lets the right materials in and out.

4. Mitochondria (The Powerhouse)

  • Structure: Small, bean-shaped organelles.
  • Function: This is the site of aerobic respiration. Respiration is the process that uses glucose and oxygen to release energy (ATP) for the cell to use.
Did You Know? Cells that need lots of energy (like muscle cells or sperm cells) have hundreds or even thousands of mitochondria!

C. Plant Cell Specialties

5. The Cell Wall (Plants Only)

  • Structure: A rigid layer made mainly of cellulose, found outside the cell membrane.
  • Function: Provides mechanical strength and support, protecting the cell and giving it a fixed shape.

6. The Permanent Vacuole (Plants Only)

  • Structure: A large sac in the center filled with cell sap (water, sugars, and salts).
  • Function: Stores water and maintains internal pressure (turgor). When full of water, it pushes against the cell wall, keeping the plant stiff and upright.

7. Chloroplasts (Plants Only)

  • Structure: Oval organelles containing the green pigment chlorophyll.
  • Function: The site of photosynthesis. Chlorophyll captures light energy to turn carbon dioxide and water into glucose (food).
Analogy: They are the cell's tiny solar panels and kitchen all in one!

✅ Key Takeaway from Organelles

Every part of the cell has a specific role, and they all work together to keep the organism alive. If the mitochondria stop working, the cell loses its energy supply!

4. Structure and Function: Specialised Cells

Not all cells are simple "plant" or "animal" types. Multicellular organisms (like us!) require cells that are specially adapted—or specialised—to carry out particular tasks efficiently. The cell's structure dictates its function.

A. Adaptations in Animal Cells

1. Red Blood Cell

  • Job: To carry oxygen from the lungs to all other body cells.
  • Adaptation:
    1. It has a biconcave disc shape (curved inward on both sides), which increases its surface area for maximum oxygen absorption.
    2. It lacks a nucleus (in mammals), providing more space to pack in the oxygen-carrying protein, haemoglobin.

2. Sperm Cell

  • Job: To fertilize the female egg (ovum).
  • Adaptation:
    1. It has a long tail (flagellum) to swim quickly.
    2. It has many mitochondria in its mid-piece to provide the massive amount of energy (ATP) needed to power the tail.

3. Muscle Cell

  • Job: To contract (shorten) to cause movement.
  • Adaptation:
    1. They are long and cylindrical, which allows them to contract.
    2. They contain large numbers of mitochondria to provide the energy needed for constant movement and contraction.

B. Adaptations in Plant Cells

1. Root Hair Cell

  • Job: To absorb water and minerals from the soil.
  • Adaptation:
    1. It has a long extension, the root hair, which dramatically increases its surface area. This allows for faster absorption of water.
    2. It has a thin cell wall, allowing easier movement of water into the cell.

2. Xylem Vessel (Tissue example, but important adaptation)

  • Job: To transport water and minerals from the roots up to the leaves.
  • Adaptation:
    1. The cells join end-to-end to form continuous, hollow tubes.
    2. The cell walls are thick and strengthened with lignin to provide support and prevent the tubes from collapsing under pressure.

✅ Quick Review Box: Structure = Function

If a cell needs to move, it needs a tail and lots of mitochondria (Sperm).
If a cell needs to absorb something quickly, it needs a huge surface area (Root Hair Cell).
If a cell needs to hold lots of stuff, it sacrifices its nucleus (Red Blood Cell).