Cell Structure and Function
Explore the fundamental unit of life -- the cell. Learn about key organelles, the difference between prokaryotic and eukaryotic cells, and how plant and animal cells compare.
Prokaryotic vs Eukaryotic Cells
All living organisms are made up of cells. The cell theory states that all living things are composed of cells, cells are the basic unit of life, and new cells arise from pre-existing cells. There are two fundamental categories of cells.
Prokaryotic Cells
- No membrane-bound nucleus -- DNA is in a region called the nucleoid
- No membrane-bound organelles
- Smaller (1--10 µm)
- Have ribosomes (smaller, 70S)
- May have cell wall, flagella, plasmids
- Examples: bacteria, archaea
Eukaryotic Cells
- Membrane-bound nucleus containing DNA
- Membrane-bound organelles (mitochondria, ER, Golgi, etc.)
- Larger (10--100 µm)
- Have ribosomes (larger, 80S)
- Linear chromosomes with histones
- Examples: animal, plant, fungi, protist cells
Key distinction: The defining feature of eukaryotic cells is the membrane-bound nucleus. "Eu" means true and "karyon" means nucleus. Prokaryotic means "before nucleus".
Key Eukaryotic Organelles
Eukaryotic cells contain specialised membrane-bound structures called organelles, each with specific functions. Think of them as the "organs" of a cell.
Nucleus
Contains the cell's DNA (genetic material). Controls cell activities and division. Surrounded by a double membrane (nuclear envelope) with nuclear pores.
Mitochondria
The "powerhouses" of the cell. Carry out aerobic cellular respiration to produce ATP (energy). Have their own DNA and a double membrane with inner folds (cristae).
Endoplasmic Reticulum (ER)
Rough ER: studded with ribosomes, synthesises and transports proteins. Smooth ER: synthesises lipids and detoxifies chemicals.
Golgi Apparatus
Modifies, packages and sorts proteins and lipids for transport within the cell or secretion outside. Consists of stacked membrane sacs (cisternae).
Ribosomes
Sites of protein synthesis. Found free in cytoplasm or attached to rough ER. Made of RNA and protein. Present in all cells.
Cell Membrane
A selectively permeable phospholipid bilayer that controls what enters and leaves the cell. Contains embedded proteins and cholesterol.
Endomembrane system: The ER, Golgi apparatus, lysosomes and vesicles work together as a transport and processing network. Proteins move from ribosomes → rough ER → Golgi → cell membrane or lysosomes.
Plant Cells vs Animal Cells
Both plant and animal cells are eukaryotic and share many organelles. However, plant cells have additional structures that support photosynthesis and structural rigidity.
Comparison Table
| Feature | Animal Cell | Plant Cell |
|---|---|---|
| Cell wall | ✗ | ✓ (cellulose) |
| Chloroplasts | ✗ | ✓ |
| Large central vacuole | ✗ (small vacuoles) | ✓ |
| Nucleus | ✓ | ✓ |
| Mitochondria | ✓ | ✓ |
| Cell membrane | ✓ | ✓ |
Chloroplasts
Found only in plant cells. Site of photosynthesis -- converting light energy, CO2 and water into glucose and oxygen. Contain chlorophyll (green pigment) and have their own DNA.
Central Vacuole
A large membrane-bound sac in plant cells filled with cell sap. Maintains turgor pressure (rigidity) and stores water, nutrients and waste products.
Key Vocabulary
Organelle
A specialised structure within a cell that performs a specific function, often enclosed by its own membrane.
Eukaryotic
A cell that has a membrane-bound nucleus and membrane-bound organelles. Includes animal, plant, fungi and protist cells.
Selectively Permeable
A property of the cell membrane that allows certain substances to pass through while blocking others.
Cellular Respiration
The process that occurs mainly in mitochondria, breaking down glucose using oxygen to release energy (ATP), CO2 and water.
Worked Examples
A cell is observed under a microscope. It has a nucleus, mitochondria and chloroplasts but no cell wall. Identify the cell type.
Has a nucleus → eukaryotic (rules out prokaryotic).
Has chloroplasts → suggests a plant cell.
No cell wall → unusual for a plant cell. This is likely an algal protist cell (a eukaryotic single-celled organism that photosynthesises but lacks a cellulose cell wall).
Note: Some algae (e.g. Euglena) have chloroplasts but no rigid cell wall.
Explain why plant cells have both mitochondria and chloroplasts.
Chloroplasts carry out photosynthesis (producing glucose from light energy) -- but only when light is available.
Mitochondria carry out cellular respiration (breaking down glucose to release ATP) -- this occurs all the time, day and night.
Plant cells need mitochondria to convert the glucose produced by chloroplasts into usable energy (ATP).
Describe the pathway of a protein from synthesis to secretion.
1. Protein is synthesised by ribosomes on the rough endoplasmic reticulum (rough ER).
2. Protein is folded and transported within the rough ER in vesicles.
3. Vesicles transport the protein to the Golgi apparatus, where it is modified, sorted and packaged.
4. The Golgi releases the protein in a secretory vesicle that fuses with the cell membrane, releasing the protein outside (exocytosis).
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
The main difference between prokaryotic and eukaryotic cells is that eukaryotic cells have:
Question 2
Which organelle is known as the "powerhouse" of the cell?
Question 3
Which structure is found in plant cells but NOT in animal cells?
Question 4
The function of the Golgi apparatus is to:
Question 5
Which of the following is an example of a prokaryotic organism?
Key Concepts Summary
- ●Prokaryotic cells lack a membrane-bound nucleus; eukaryotic cells have one.
- ●Key organelles include the nucleus (DNA), mitochondria (energy), ER (synthesis) and Golgi (processing).
- ●Plant cells have cell walls, chloroplasts and a large central vacuole that animal cells lack.
- ●The cell membrane is selectively permeable, controlling the movement of substances in and out.
- ●Proteins follow the pathway: ribosomes → rough ER → Golgi apparatus → secretion.