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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Cytoplasm and Organelles
Cells are the basic building blocks of all living organisms. Inside each cell is a busy world of structures working together to keep the cell - and you - alive! The cytoplasm is like the "living room" of the cell where most cellular activities happen, while organelles are specialized structures that perform specific functions - like tiny organs inside the cell.
Key Definitions:
The cytoplasm is a gel-like substance made mostly of water (about 80%) that fills the space between the cell membrane and the nucleus. It's not just empty space - it's where most of the cell's chemical reactions happen! The cytoplasm contains:
The cytoplasm is crucial for cell survival because it:
Organelles are like tiny organs inside the cell, each with specific jobs. Let's explore the most important ones:
The powerhouse of the cell! Mitochondria produce energy through cellular respiration, converting glucose and oxygen into ATP (the cell's energy currency). They have their own DNA and can replicate independently.
Structure: Double membrane with inner folds called cristae that increase surface area for more energy production.
Found only in plant cells and algae. Chloroplasts capture light energy for photosynthesis, converting carbon dioxide and water into glucose and oxygen.
Structure: Double membrane containing stacks of disc-shaped structures called thylakoids where chlorophyll (the green pigment) is found.
The protein factories. Ribosomes assemble amino acids into proteins following instructions from DNA. They can be found free in the cytoplasm or attached to the endoplasmic reticulum.
Structure: Made of RNA and protein, they consist of two subunits that join together during protein synthesis.
A network of membranes that process and transport materials within the cell. There are two types:
The cell's packaging and distribution centre. It receives proteins from the ER, modifies them and packages them into vesicles for transport to their final destinations.
Structure: Stack of flattened membrane sacs (cisternae) with vesicles budding off the edges.
The control centre of the cell, containing DNA (genetic material) that directs all cell activities. The nucleus is surrounded by a double membrane called the nuclear envelope with pores that allow materials to pass through.
Inside the nucleus is the nucleolus, where ribosomes are assembled.
The cell's recycling centres. Lysosomes contain powerful digestive enzymes that break down waste materials, old cell parts and foreign particles.
They're like the cell's "stomach" and play a key role in defending against bacteria and viruses.
Membrane-bound sacs that store materials like water, nutrients, waste products and sometimes pigments.
Found in plant cells, fungi and bacteria (but not animal cells). The cell wall:
Plant and animal cells have many similarities but also key differences in their organelles:
| Feature | Plant Cells | Animal Cells |
|---|---|---|
| Cell Wall | Present (cellulose) | Absent |
| Chloroplasts | Present | Absent |
| Vacuole | One large central vacuole | Small, multiple vacuoles |
| Shape | Regular, fixed shape | Irregular shape |
Organelles don't work in isolation - they cooperate to keep the cell functioning properly. Here's an example of how they work together to make and export a protein:
Mitochondria have their own DNA, separate from the DNA in the nucleus! This supports the endosymbiotic theory, which suggests that mitochondria were once free-living bacteria that were engulfed by larger cells millions of years ago, forming a mutually beneficial relationship. This is why mitochondria can replicate independently within the cell.
Many organelles are too small to see with a light microscope. Scientists use electron microscopes, which have much higher magnification, to study cell structures in detail. Special staining techniques can also help make certain organelles more visible.
Different techniques allow us to see different organelles:
Think of the cell as a busy city, with each organelle playing a specific role:
Just as a city can't function if any essential service breaks down, cells need all their organelles working together to survive and function properly!
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