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.
• Identify different types of cells based on their structures.
• Explain how the structures of cells relate to their functions.
• Understand the three main points of the Cell Theory.

Lesson summary

This lesson introduces the fundamental concept that all living things, from the mighty Odum tree to the smallest housefly, and even ourselves, are made of cells. The cell is the basic building block of life. Understanding the cell is like understanding the alphabet before learning to read; it is essential for all other topics in Biology. In Ghana, this knowledge is directly applicable to understanding our health (e.g., how diseases like malaria affect our blood cells), agriculture (e.g., why plantain stems are strong), and even food preparation (e.g., the fermentation of kenkey by yeast cells).

Lesson notes

This section contains the core content for the lesson. A teacher can deliver this content using a lecture, interactive discussion, or by having students read and discuss sections. A. The Cell Theory The Cell Theory is a cornerstone of modern biology. It was developed over many years by scientists like Matthias Schleiden, Theodor Schwann, and Rudolf Virchow. It provides a universal framework for understanding life. The theory has three main points: All living organisms are composed of one or more cells. An organism can be unicellular (made of one cell), like an Amoeba or bacteria, or multicellular (made of many cells), like a human or a mango tree. The cell is the basic structural and functional unit of life. This means the cell is the smallest thing that can be considered "alive." All life processes, such as respiration, excretion, and reproduction, happen at the cellular level. All cells arise from pre-existing cells. This was a revolutionary idea by Rudolf Virchow (*Omnis cellula e cellula*). It means cells do not spontaneously appear; they are formed through the division of other cells. B. Classification of Cells: Prokaryotes vs. Eukaryotes The most fundamental way to classify cells is based on their internal complexity. Prokaryotic Cells (from 'pro' = before, 'karyon' = nucleus): These are simple cells that lack a true, membrane-bound nucleus. Their genetic material (a single circular chromosome) floats in the cytoplasm in a region called the nucleoid. They also lack other membrane-bound organelles like mitochondria or chloroplasts. They are typically very small. Example: Bacteria (e.g., *E. coli*, the bacteria that causes cholera). Eukaryotic Cells (from 'eu' = true, 'karyon' = nucleus): These are complex cells that have a true nucleus, which houses the cell's genetic material (DNA in linear chromosomes). They also contain many other membrane-bound organelles, each with a specific function. Example: Plant cells, animal cells, fungi (like yeast for baking bread), and protists (like Amoeba).

| Feature | Prokaryotic Cell | Eukaryotic Cell | | :--- | :--- | :--- | | True Nucleus | Absent | Present | | DNA Structure | Single, circular molecule in the nucleoid | Multiple, linear chromosomes inside the nucleus | | Membrane-bound Organelles | Absent | Present (Mitochondria, Golgi, ER, etc.) | | Ribosomes | Present (smaller size - 70S) | Present (larger size - 80S) | | Average Size | Very small (0.1–5.0 μm) | Larger (10–100 μm) | | Example Organisms | Bacteria, Archaea | Plants, Animals, Fungi, Protists | C. The Structure of Eukaryotic Cells: Plant vs. Animal Cells While both are eukaryotic, plant and animal cells have key differences related to their lifestyles. Plants are stationary and produce their own food, while animals are mobile and must consume food.

| Feature | Typical Animal Cell | Typical Plant Cell | | :--- | :--- | :--- | | Shape | Irregular / Flexible | Fixed / Regular (often hexagonal) | | Cell Wall | Absent | Present (made of cellulose) | | Chloroplasts | Absent | Present (for photosynthesis) | | Vacuole | Small, temporary vacuoles (if any) | Large, central permanent vacuole | | Centrioles | Present (involved in cell division) | Absent in most higher plants | | Energy Storage| Glycogen | Starch | D. The Organelles: The "Organs" of the Cell These are the specialized structures within a eukaryotic cell that perform specific jobs. The key is to understand how their structure helps them perform their function.

Analogy: Think of a cell as a whole school. Each part of the school has a specific job. Cell (Plasma) Membrane Structure: A thin, flexible barrier made of a phospholipid bilayer with embedded proteins. It is selectively permeable. Function: Controls what enters and leaves the cell. It protects the cell and helps in communication. Analogy: The school's gate and security guards. They control who comes in and who goes out. Cytoplasm Structure: The jelly-like substance (called cytosol) that fills the cell and surrounds the organelles. Function: It is the site of many metabolic reactions (like the first stage of respiration). It holds the organelles in place. Analogy: The school compound. Everything happens within the compound. Nucleus (Plural: Nuclei) Structure: A large, often spherical organelle surrounded by a double membrane called the nuclear envelope, which has pores. It contains the cell's genetic material (DNA) in the form of chromatin, and a dense region called the nucleolus. Function: Controls all the activities of the cell, like growth and metabolism. The nucleolus is where ribosomes are made. Analogy: The Headmaster's office. It holds all the important school records (DNA) and sends out instructions to run the school. Mitochondrion (Plural: Mitochondria) Structure: A sausage-shaped organelle with a double membrane. The inner membrane is highly folded into structures called cristae. Function: This is the "powerhouse" of the cell. It is the site of aerobic respiration, a process that converts glucose and oxygen into usable energy in the form of ATP. The folded cristae increase the surface area for these reactions to occur efficiently. Analogy: The VRA/ECG power plant for the school. It generates the electricity (energy/ATP) that the school needs to function. Ribosomes Structure: Tiny particles made of RNA and protein. They can be found free in the cytoplasm or attached to the Endoplasmic Reticulum. Function: Responsible for protein synthesis (making proteins) by reading instructions from the nucleus. Analogy: The school's kitchen or workshop where raw materials are assembled into finished products (proteins). Endoplasmic Reticulum (ER) Structure: A network of interconnected membranes (cisternae) that form channels throughout the cytoplasm. Rough ER (RER): Studded with ribosomes. Its function is to modify and transport proteins made by the attached ribosomes. Smooth ER (SER): Lacks ribosomes. Its function is to synthesize lipids, detoxify poisons, and store calcium. Analogy: The school's corridors and transport system, moving students (proteins) and materials around. Golgi Apparatus (or Golgi Complex/Body) Structure: A stack of flattened, membrane-bound sacs called cisternae. Function: It receives proteins and lipids from the ER, further modifies, sorts, and packages them into vesicles for transport to other parts of the cell or for secretion out of the cell. Analogy: The school's administration or packaging department. It takes finished work, checks it, puts it in envelopes (vesicles), and sends it to the correct destination. Lysosomes (Primarily in animal cells) Structure: Small, spherical vesicles containing powerful digestive enzymes. Function: They break down old organelles, food particles, and ingested viruses or bacteria. They are the "suicide bags" of the cell. Analogy: The school's waste disposal and sanitation crew. They break down and remove rubbish. Vacuole Structure: A fluid-filled sac. Function: In Plant Cells: There is a large central vacuole that stores water, nutrients, and waste products. It maintains turgor pressure against the cell wall, which helps support the plant. In Animal Cells: Vacuoles are small and temporary, used for storage or transport. Analogy (for plant cell): The school's large water tank (polytank). It provides structural support and stores essential water. Cell Wall (Plant cells, not animal cells) Structure: A rigid outer layer made of a tough carbohydrate called cellulose. It is fully permeable. Function: Provides structural support and protection to the plant cell. Prevents the cell from bursting when it takes in too much water. Analogy: The school's strong fence wall. It gives the school its shape and protects it. Chloroplasts (Plant cells, not animal cells) Structure: A double-membraned organelle containing stacks of thylakoids (called grana) where the green pigment chlorophyll is located. Function: The site of photosynthesis, the process where light energy is converted into chemical energy (glucose). Analogy: The school's farm or garden. It uses sunlight to produce food for the school community.

Evaluation guide

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