Study Notes: 2.1 Cell Structure
Hey there! Welcome to the world of cells! This chapter is the foundation of all Biology. Think of cells as the tiny building blocks of life—everything you see, from the biggest tree to the smallest insect, is made of them. Understanding cell structure helps us explain how whole organisms work (which fits perfectly into our section, "Organisation of the organism"). Let's dive in!
Key Takeaway from the Introduction
The cell is the basic functional unit of all living organisms. Knowing their internal structures (organelles) explains the fundamental life processes (like respiration, nutrition, and growth).
Part 1: Comparing Animal and Plant Cells (Eukaryotes)
Animal and Plant cells are both eukaryotic, meaning they have a true nucleus and other membrane-bound structures. However, they have important differences that reflect their different lifestyles.
Imagine a cell is a small, bustling factory:
Structures Common to Both (The Essential Factory Staff)
These structures are found in nearly all eukaryotic cells, regardless of whether they are animal or plant.
- Cell Membrane (The Security Gate):
- Function: Controls which substances enter and leave the cell (it's partially permeable).
- Location: Outermost layer of the animal cell; just inside the cell wall in plant cells.
- Nucleus (The Control Centre/CEO’s Office):
- Function: Contains the genetic material (DNA) in the form of chromosomes. Controls all cell activities, including growth and reproduction.
- Cytoplasm (The Factory Floor):
- Function: A jelly-like substance where most chemical reactions of life (metabolism) happen. It holds the organelles in place.
- Mitochondria (The Power Plants):
- Function: Site of aerobic respiration, where energy is released from nutrient molecules (like glucose).
- Remember: Mitochondria = Mighty energy!
- Ribosomes (The Small Assembly Workers):
- Function: Site of protein synthesis (making proteins). They follow instructions from the nucleus.
Structures Unique to Plant Cells (The Plant Special Features)
These features give plants their rigid structure and ability to make their own food.
- Cell Wall (The Outer Fortification):
- Structure: Rigid outer layer, made mainly of cellulose.
- Function: Provides support and a fixed shape for the cell. Prevents the cell from bursting when it takes up too much water.
- Chloroplasts (The Solar Panels):
- Structure: Contain the green pigment chlorophyll.
- Function: Site of photosynthesis, using light energy to make food (glucose).
- Vacuole (The Large Water Storage Tank):
- Structure: A large sac filled with cell sap (water, sugars, and salts).
- Function: Stores substances and helps maintain turgor (pressure) to keep the plant stiff and upright. Animal cells may have small, temporary vacuoles, but not a single large permanent one.
Plant Cells HAVE: Cell Wall, Chloroplasts, Large Central Vacuole.
Animal Cells LACK: Cell Wall, Chloroplasts, Large Central Vacuole.
Part 2: The Bacterial Cell (Prokaryotes)
Bacteria are prokaryotes. This means they are much simpler and smaller than animal and plant cells. They are the original, minimalist life forms!
Bacterial Cell Structure (Core 2.1.2)
They share some basic features with eukaryotes, but crucially lack a nucleus and most other complex organelles.
- Cell Wall: Present, for protection and support, but chemically different from the plant cell wall.
- Cell Membrane & Cytoplasm: Present, fulfilling the same basic functions.
- Ribosomes: Present, used for protein synthesis.
- Circular DNA: The main genetic material. It is not enclosed in a nucleus—it floats freely in the cytoplasm.
- Plasmids: Small, extra rings of DNA. These often carry genes for useful characteristics, such as antibiotic resistance.
Bacteria's simplicity is why they are so useful in genetic modification—plasmids are easy to manipulate and replicate quickly!
Common Mistake Alert!
Don't confuse the bacterial cell wall with the plant cell wall. They are made of different materials and are evidence that bacteria do not belong to the Plant Kingdom.
Part 3: From Cell to Organism – The Levels of Organisation
Complex organisms, like humans and large plants, are built using a specific hierarchy, starting from the smallest unit.
Definition of Terms (Core 2.1.7)
- Cell: The fundamental unit of life (e.g., a muscle cell or a palisade cell).
- Tissue: A group of similar cells working together to perform a specific function.
Example: Muscle tissue contains many muscle cells working together to contract.
- Organ: A group of different tissues working together to perform a particular function.
Example: The stomach contains muscle tissue, epithelial tissue (lining), and nervous tissue.
- Organ System: A group of different organs working together to perform a major body function.
Example: The Digestive System includes the stomach, intestines, liver, etc.
- Organism: A fully functional living thing, made up of many different organ systems.
Example: A single human, dog, or daisy.
Can Tom Often Offer Orangutans?
Cell → Tissue → Organ → Organ System → Organism
Part 4: Specialised Cells and Their Functions
Not all cells are simple squares or spheres! Cells in multicellular organisms have unique shapes and internal structures that adapt them perfectly for their specific job. This is called specialisation.
Examples of Specialised Cells (Core 2.1.6)
1. Ciliated Cells
- Location: Lining the trachea and bronchi (airways).
- Adaptation: Have tiny hair-like structures called cilia on their surface.
- Function: The cilia sweep mucus (which traps dust and pathogens) out of the lungs and towards the throat to be swallowed. (This is the "movement of mucus" function).
- Analogy: They act like tiny synchronized brooms cleaning the airways.
2. Root Hair Cells
- Location: On the surface of plant roots.
- Adaptation: Have a long, thin extension (the root hair) giving them a very large surface area.
- Function: Maximises the absorption of water and mineral ions from the soil.
3. Palisade Mesophyll Cells
- Location: Near the top surface of a leaf.
- Adaptation: Packed with chloroplasts and positioned where they can absorb maximum sunlight.
- Function: Primary site of photosynthesis.
4. Neurones (Nerve Cells)
- Location: Nervous system (brain, spinal cord, nerves).
- Adaptation: Very long fibres (axons and dendrons).
- Function: Responsible for the rapid conduction of electrical impulses around the body.
5. Red Blood Cells
- Location: Circulatory system (blood).
- Adaptation: Biconcave disc shape (increases surface area) and lack a nucleus (to make more space for haemoglobin).
- Function: Transport oxygen using the protein haemoglobin.
6. Gametes (Sperm and Egg Cells)
- Function: Used for reproduction (fusion of nuclei in fertilisation).
- Sperm Cell Adaptation: Has a flagellum (tail) for motility and mitochondria to provide energy for swimming.
- Egg Cell Adaptation: Large size and contains energy stores (yolk) to nourish the early embryo.
Part 5: Cell Division
Where do new cells come from?
The syllabus requires you to State that new cells are produced by division of existing cells (Core 2.1.5).
- Cells never spontaneously appear. They must always originate from older cells through the process of cell division.
- In simple terms, one existing cell divides to create two new cells.
- This process is essential for:
- Growth of an organism.
- Repair of damaged tissues (like when you cut your finger).
- Replacement of old cells (like skin cells).
All living things share fundamental cell components (like the nucleus, cytoplasm, and cell membrane). The differences in organelles (e.g., chloroplasts, cell wall) allow organisms to be categorised and specialised for different roles. Cells are organised into a clear hierarchy to form complex organisms.