Lesson Notes By Weeks and Term v4 - SHS 2

Lesson Video

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Performance objectives

• Describe the basic structure of prokaryotic and eukaryotic cells.
• Explain the cell theory and its relevance.
• Classify different types of cells.
• Relate cell structures to their functions.

Lesson summary

This lesson introduces the cell as the fundamental building block of all living things. Just as a house is built from individual blocks, all organisms, from the smallest bacterium to the largest dawadawa tree and even ourselves, are made of cells. Understanding the structure of a cell and the jobs (functions) of its parts is crucial to understanding life itself, including growth, disease, and how our bodies work. In Ghana, this knowledge helps us understand issues like crop health (why does our kontomire wilt?), common diseases like sickle cell anaemia, and even the process of brewing local drinks like pito.

Lesson notes

Part A: The Cell Theory The Cell Theory is a foundational principle in biology. After centuries of observations by scientists like Robert Hooke, Schleiden, Schwann, and Virchow, the scientific community agreed on three main ideas: All living organisms are composed of one or more cells. An organism can be single-celled (unicellular), like an Amoeba or a bacterium, or multi-celled (multicellular), like a human, a goat, or a mango tree. The cell is the basic unit of structure and function in organisms. It is the smallest unit of life that can carry out all life processes (e.g., respiration, excretion, nutrition). All cells arise from pre-existing cells. Cells do not just appear from nowhere; they are produced through the division of other cells (e.g., mitosis, meiosis). Part B: Classification of Cells - Prokaryotic vs. Eukaryotic All cells can be broadly classified into two main types based on their internal structure, particularly the presence or absence of a nucleus. Prokaryotic Cells: (from *pro* = "before", *karyon* = "nucleus") These are simple, small cells. They do not have a true nucleus. Their genetic material (a single circular chromosome) is located in a region called the nucleoid, which is not enclosed by a membrane. They lack membrane-bound organelles like mitochondria, chloroplasts, or endoplasmic reticulum. They have ribosomes, but they are smaller (70S type). Example: Bacteria (like *E. coli*), Archaea. Eukaryotic Cells: (from *eu* = "true", *karyon* = "nucleus") These are larger and more complex cells. They have a true, membrane-bound nucleus that houses the cell's genetic material (DNA in linear chromosomes). They contain numerous membrane-bound organelles, each with a specific function. They have larger ribosomes (80S type). Examples: Cells of plants (e.g., onion cell), animals (e.g., human cheek cell), fungi (e.g., yeast), and protists (e.g., Amoeba).

| Feature | Prokaryotic Cell (e.g., Bacterium) | Eukaryotic Cell (e.g., Animal Cell) | | :--- | :--- | :--- | | True Nucleus | Absent | Present | | Genetic Material | In a nucleoid region, not enclosed | Enclosed within a nuclear membrane | | Organelles | No membrane-bound organelles | Membrane-bound organelles present | | Size | Typically 0.1–5.0 µm | Typically 10–100 µm | | Ribosomes | Smaller (70S) | Larger (80S) | | Example | *Escherichia coli* (bacterium) | Human liver cell, Cassava leaf cell | Part C: Structure and Function of Eukaryotic Organelles

Imagine a cell is a small factory. Each machine and department has a specific job. These are the organelles. Cell (Plasma) Membrane: Structure: A thin, flexible barrier made of a phospholipid bilayer with embedded proteins. Function: Controls what enters and leaves the cell (it is selectively permeable). It also provides protection and cell-to-cell communication. *Analogy: The factory's security gate.* Cell Wall (in Plants, Fungi, Bacteria; NOT in Animals): Structure: A rigid outer layer outside the cell membrane. In plants, it is made of cellulose. Function: Provides structural support, protection, and prevents the cell from bursting when it takes in too much water (maintains turgor pressure). *Analogy: The factory's strong outer walls.* Nucleus: Structure: A large organelle surrounded by a double membrane (nuclear envelope) with pores. Contains the chromatin (DNA and proteins) and a dense region called the nucleolus. Function: Controls all the cell's activities by controlling protein synthesis. The nucleolus is where ribosomes are made. *Analogy: The CEO's office or control room.* Cytoplasm: Structure: The entire region between the nucleus and the cell membrane, including the jelly-like fluid (cytosol) and the organelles suspended within it. Function: Site of most metabolic reactions (like glycolysis). The cytosol holds organelles in place. *Analogy: The factory floor.* Mitochondrion (plural: Mitochondria): Structure: Bean-shaped organelle with an outer and a highly folded inner membrane. The folds are called cristae. Function: The site of aerobic cellular respiration. It breaks down glucose to produce ATP (adenosine triphosphate), the main energy currency of the cell. Structure-Function Link: The folded cristae drastically increase the surface area available for the chemical reactions of respiration, allowing for much more ATP production. *Analogy: The power plant or generator.* Chloroplast (in Plant cells and some Protists): Structure: An organelle with a double membrane, containing stacks of flattened sacs called grana (singular: granum). Each sac is a thylakoid. The fluid-filled space is the stroma. Thylakoids contain chlorophyll. Function: The site of photosynthesis, where light energy is converted into chemical energy (glucose). Structure-Function Link: The stacked grana provide a large surface area for trapping sunlight. *Analogy: Solar panels.* Ribosomes: Structure: Tiny particles made of RNA and protein. Found free in the cytoplasm or attached to the Endoplasmic Reticulum. Function: Site of protein synthesis (translates genetic code into proteins). *Analogy: The small machines that build the products.* Endoplasmic Reticulum (ER): Rough ER: A network of membranes studded with ribosomes. Its function is to synthesize and modify proteins that are often destined to be exported from the cell. Smooth ER: A network of tubular membranes without ribosomes. Its functions include synthesizing lipids, detoxification of drugs and poisons. *Analogy: The factory's assembly line.* Golgi Apparatus (or Golgi Complex/Body): Structure: A stack of flattened membrane-bound sacs called cisternae. Function: Modifies, sorts, and packages proteins and lipids received from the ER into vesicles for transport to other destinations. *Analogy: The packaging and shipping department.* Vacuole: Structure: A membrane-bound sac. Function: In plant cells, there is a large central vacuole that stores water, nutrients, and waste products. It is crucial for maintaining turgor pressure. In animal cells, vacuoles are small, temporary, and used for storage or transport. *Analogy: The warehouse or storage tank.* Lysosome (mainly in Animal cells): Structure: A small, spherical vesicle containing powerful digestive enzymes. Function: Breaks down worn-out organelles, food particles, and ingested viruses or bacteria. *Analogy: The waste disposal and recycling crew.* Part D: Comparing Plant and Animal Cells

| Feature | Typical Animal Cell | Typical Plant Cell | | :--- | :--- | :--- | | Shape | Irregular / round | Fixed / rectangular | | Cell Wall | Absent | Present (made of cellulose) | | Chloroplasts | Absent | Present | | Vacuole | Small, numerous, and temporary | Large, single, permanent central vacuole | | Lysosomes| Present | Usually absent or rare | | Centrioles | Present (involved in cell division) | Absent in higher plants |

Evaluation guide

• Demonstrate
• knowledge and
• understanding of Cell
• structure and