Cell Structure

Introduction to the Cell

All living organisms, from the smallest bacterium to the largest whale, are composed of cells. The cell theory states that cells are the fundamental structural and functional units of life, and all cells arise from pre-existing cells. To observe these microscopic structures, we use a light microscope, which uses light and lenses to magnify an image.

The relationship between the size of the image, the actual size of the object, and the magnification is given by the formula:

Magnification = Image Size / Actual Size (often remembered as the I=AM triangle)

When performing calculations, it is crucial to use consistent SI units. Biologists commonly use micrometres (µm).

• 1 millimetre (mm) = 1000 micrometres (µm)
• 1 micrometre (µm) = 1000 nanometres (nm)

Example Calculation: A student observes a cell under a microscope. The image they see is 50 mm wide. The actual width of the cell is 100 µm. What is the magnification?

1. Convert units to be the same: Convert 50 mm to µm. 50 mm * 1000 = 50,000 µm.
2. Apply the formula: Magnification = 50,000 µm / 100 µm = 500.
3. State the answer: The magnification is x500.

The Structure of a Typical Animal Cell

Animal cells are the basic units of animal life. They contain several key components called organelles.

• Cell Surface Membrane: A partially permeable (or selectively permeable) barrier that surrounds the cell. It is made of lipids and proteins and controls the passage of substances into and out of the cell.
• Cytoplasm: A jelly-like substance, mostly water, that fills the cell. It is the site of many metabolic reactions, and it contains all the organelles.
• Nucleus: A large organelle containing the cell's genetic material, **DNA** (deoxyribonucleic acid), organised into chromosomes. The nucleus controls all the cell's activities, including growth and cell division.
• Mitochondria (singular: mitochondrion): These are the 'powerhouses' of the cell. They are the site of **aerobic respiration**, a chemical process that releases energy from glucose. This energy is stored in a molecule called ATP, which powers all other cellular activities.
• Ribosomes: Tiny structures, either free in the cytoplasm or attached to membranes, where **protein synthesis** occurs. They translate the genetic code from the nucleus into proteins.

The Structure of a Typical Plant Cell

Plant cells have all the organelles found in animal cells, plus three additional structures.

• Cell Wall: A rigid outer layer made of **cellulose**, located outside the cell membrane. It is **fully permeable**, allowing water and dissolved substances to pass through freely. Its main functions are to provide structural support, maintain a fixed shape, and prevent the cell from bursting when it takes in too much water.
• Chloroplasts: Small, disc-shaped organelles containing the green pigment **chlorophyll**. They are the site of **photosynthesis**, the process where light energy is used to convert carbon dioxide and water into glucose. These are only found in the parts of the plant exposed to light, like leaves and young stems.
• Large Central Vacuole: A large, permanent, fluid-filled sac that can occupy up to 90% of the cell's volume. It contains **cell sap** (a solution of sugars, salts, and amino acids) and is enclosed by a membrane called the **tonoplast**. Its main function is to maintain **turgor pressure** against the cell wall, keeping the cell firm and supporting the plant.

Common Misconception: Many students believe plant cells do not have mitochondria because they have chloroplasts. This is incorrect. Plant cells respire 24 hours a day to release energy for processes like active transport and growth, just like animal cells. Photosynthesis only occurs in the presence of light.

| Feature | Animal Cell | Plant Cell |

| -------------------- | ------------------ | ---------------------------------- |

| Cell Wall | Absent | Present (made of cellulose) |

| Shape | Irregular / Round | Fixed / Regular (often hexagonal) |

| Chloroplasts | Absent | Present (in photosynthetic parts) |

| Vacuole | Small, temporary | Large, permanent central vacuole |

| Carbohydrate Storage | Glycogen | Starch |

Levels of Organisation

In complex multicellular organisms, cells are organised into a hierarchy to perform functions efficiently.

1. Cells: The basic building blocks (e.g., muscle cell, palisade cell).
2. Tissues: A group of similar cells working together to perform a specific function (e.g., muscle tissue, xylem tissue).
3. Organs: A group of different tissues working together to perform a specific function (e.g., the heart, a leaf).
4. Organ Systems: A group of organs working together to perform a major function in the body (e.g., the circulatory system, the shoot system in a plant).
5. Organism: The entire living being.

Specialised Cells

Cells differentiate to become specialised, meaning they develop specific features (adaptations) to carry out a particular job.

• Red Blood Cells: Transport oxygen. **Adaptations:** Biconcave shape increases surface area for diffusion; no nucleus to maximise space for **haemoglobin**; flexible membrane to squeeze through narrow capillaries.
• Root Hair Cells (Plant): Absorb water and mineral ions from the soil. **Adaptations:** A long, thin extension (the 'hair') creates a large surface area; many mitochondria to provide energy for the active transport of mineral ions.
• Xylem Vessels (Plant): Transport water and support the plant. **Adaptations:** Form a continuous hollow tube (lignin kills the protoplasm, and cross-walls break down); walls are strengthened with a waterproof polymer called **lignin**, which provides support. This is crucial for plants in Pakistan's diverse climates, from the plains of Punjab to the mountainous north, ensuring water reaches the highest leaves.