Unit 3: The cell as the basic unit of life
Essential Biology notes · PTU syllabus (BMLS101-18)
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Unit summary
The cell is the basic unit of life. This unit covers cell theory, prokaryotic and eukaryotic cells, unicellular and multicellular organisms, the cell wall and membrane (fluid-mosaic model), membrane transport, exocytosis and endocytosis, and the structure and functions of cell organelles.
After this unit you can
- State cell theory and compare prokaryotic and eukaryotic cells
- Describe the fluid-mosaic model and membrane transport
- Explain exocytosis and endocytosis
- Describe the structure and functions of cell organelles
PTU syllabus topics
- Cell theory
- prokaryotic and eukaryotic cells
- unicellular and multicellular organisms
- ultrastructure — cell wall
- cell membrane (fluid-mosaic model)
- membrane transport
- exocytosis and endocytosis
- cell organelles (nucleus, mitochondria, plastids, endoplasmic reticulum, Golgi complex, lysosomes, microtubules, centriole, vacuole, cytoskeleton, cilia and flagella, ribosomes)
Nucleus
No true nucleus (nucleoid)
True nucleus with membrane
Organelles
No membrane-bound organelles
Mitochondria, ER, Golgi and more
Ribosomes
70S
80S in cytoplasm
Examples
Bacteria
Animals, plants, fungi
Topic 1
Cell theory
- Proposed by Schleiden (1838) and Schwann (1839), completed by Virchow (1855): all living things are made of cells; the cell is the basic unit of structure and function; all cells arise from pre-existing cells (omnis cellula e cellula).
Topic 2
Prokaryotic and eukaryotic cells
Nucleus
No true nucleus; DNA in nucleoid
True nucleus with nuclear membrane
Size
1–10 µm
10–100 µm
Organelles
No membrane-bound organelles
Mitochondria, ER, Golgi, etc.
Ribosomes
70S
80S (70S in mitochondria and chloroplasts)
DNA
Circular, plus plasmids
Linear, with histones
Examples
Bacteria
Animals, plants, fungi, protozoa
- Unicellular organisms (bacteria, Amoeba) do everything in one cell; multicellular organisms (humans) have specialised cells forming tissues and organs.
Topic 3
Cell wall and cell membrane
- Cell wall: rigid outer layer in plants (cellulose), fungi (chitin) and bacteria (peptidoglycan); absent in animal cells.
- Fluid-mosaic model (Singer and Nicolson, 1972): the plasma membrane is a phospholipid bilayer (hydrophilic heads outside, hydrophobic tails inside) with proteins floating in it like a mosaic; cholesterol adds stability; glycoproteins and glycolipids act in cell recognition.
Topic 4
Membrane transport
Simple diffusion
None; high to low concentration
O₂, CO₂
Facilitated diffusion
None; through carrier or channel proteins
Glucose into red cells
Osmosis
None; water across a semipermeable membrane towards higher solute
Water into root hairs
Active transport
ATP; against the gradient
Na⁺/K⁺ pump (3 Na⁺ out, 2 K⁺ in)
Example
Red cells in hypotonic solution swell and burst (haemolysis); in hypertonic solution they shrink (crenation); in isotonic 0.9% saline they stay normal.
Topic 5
Exocytosis and endocytosis
- Exocytosis: vesicles fuse with the membrane to release contents (hormones, enzymes).
- Endocytosis: the membrane engulfs material — phagocytosis (cell eating, by neutrophils and macrophages), pinocytosis (cell drinking) and receptor-mediated endocytosis (LDL cholesterol).
Topic 6
Cell organelles
Nucleus
Double membrane with pores, nucleolus, chromatin
Contains DNA; controls the cell
Mitochondria
Double membrane; inner folds (cristae)
Aerobic respiration, ATP — powerhouse
Plastids
Chloroplasts, chromoplasts, leucoplasts (plants)
Photosynthesis, pigments, storage
Endoplasmic reticulum
Rough (with ribosomes) and smooth
Protein synthesis; lipid synthesis and detoxification
Golgi complex
Stacked flattened sacs
Modifies, packages and secretes proteins
Lysosomes
Sacs of hydrolytic enzymes
Digestion; suicide bags
Ribosomes
rRNA and protein; 80S or 70S
Protein synthesis
Vacuole
Large in plants
Storage, turgor
Centrioles
Nine triplets of microtubules
Spindle formation in animal cells
Cytoskeleton
Microtubules, microfilaments, intermediate filaments
Shape, movement, transport
Cilia and flagella
9 + 2 microtubule arrangement
Movement
Key terms
- Prokaryote
- Cell without a true nucleus
- Fluid-mosaic model
- Model of the membrane as a lipid bilayer with mobile proteins
- Osmosis
- Movement of water across a semipermeable membrane
- Active transport
- Movement against a gradient using ATP
- Mitochondria
- Organelles producing ATP
Quick revision
- Cell theory: Schleiden, Schwann, Virchow.
- Prokaryotic vs eukaryotic; 70S vs 80S ribosomes.
- Fluid-mosaic model; diffusion, facilitated diffusion, osmosis, active transport.
- Exocytosis; phagocytosis, pinocytosis, receptor-mediated endocytosis.
- Functions of each organelle.
Important exam questions
Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).
Short-answer questions
- Q1.State cell theory.
- Q2.Give three differences between prokaryotic and eukaryotic cells.
- Q3.What is osmosis?
- Q4.What happens to red cells in a hypotonic solution?
- Q5.Why are lysosomes called suicide bags?
- Q6.What is the 9 + 2 arrangement?
Long-answer questions
- Q1.Compare prokaryotic and eukaryotic cells.
- Q2.Explain the fluid-mosaic model and membrane transport.
- Q3.Describe the structure and functions of cell organelles.
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