The cell theory
Robert Hooke (1665) observed and named "cells" in a thin slice of cork.
Anton von Leeuwenhoek first observed and described live cells.
Matthias Schleiden (1838, plants) and Theodor Schwann (1839, animals) proposed that all living things are made of cells.
Rudolf Virchow (1855) added that new cells arise only from pre-existing cells (omnis cellula-e cellula).
The modern cell theory states:
All living organisms are composed of cells and the products of cells.
All cells arise from pre-existing cells.
Prokaryotic and eukaryotic cells
Feature | Prokaryotic | Eukaryotic |
|---|---|---|
Examples | bacteria, cyanobacteria, mycoplasma | protists, fungi, plants, animals |
Nucleus | no membrane-bound nucleus; DNA lies in the nucleoid | true nucleus with a nuclear envelope |
Membrane-bound organelles | absent | present (mitochondria, ER, Golgi, etc.) |
Ribosomes | 70S | 80S in the cytoplasm (70S inside mitochondria and chloroplasts) |
Typical size | small (about 1–10 µm) | larger (about 10–100 µm) |
Many bacteria also have small circular DNA molecules called plasmids, which can carry genes such as those for antibiotic resistance. Bacterial cell walls are made of peptidoglycan, and in Gram staining these walls hold the stain differently (Gram-positive versus Gram-negative). Mycoplasma are the smallest known living cells and have no cell wall.
The cell membrane
The cell membrane is described by the fluid mosaic model (Singer and Nicolson, 1972). It is a lipid bilayer of phospholipids, with their hydrophilic heads facing outward and hydrophobic tails facing inward. Proteins are embedded in it or attached to it. The lipids can move sideways, which makes the membrane "fluid".
The membrane is selectively permeable. Substances cross it by:
passive transport, down a concentration gradient with no energy needed (diffusion; osmosis for water),
active transport, against a concentration gradient using ATP (for example, the Na⁺/K⁺ pump).
Cell wall
Plant cells and fungi have a rigid cell wall outside the membrane. In plants it is made mainly of cellulose, and in fungi of chitin. It gives shape, protection and support. Neighbouring plant cells are cemented together by the middle lamella (mainly calcium pectate) and connected by plasmodesmata.
Organelles and their functions
Organelle | Key feature | Function |
|---|---|---|
Nucleus | double membrane with nuclear pores; contains chromatin and the nucleolus | controls cell activities; stores DNA. The nucleolus makes rRNA |
Endoplasmic reticulum | network of membranes | RER (with ribosomes) makes proteins; SER makes lipids and steroids |
Golgi apparatus | stacks of flat cisternae | packages and modifies proteins; forms glycoproteins and glycolipids |
Lysosomes | membrane sacs of hydrolytic enzymes | intracellular digestion |
Mitochondria | double membrane; inner membrane folded into cristae; own DNA and 70S ribosomes | aerobic respiration and ATP production ("powerhouse of the cell") |
Plastids | double membrane | chloroplasts: photosynthesis; chromoplasts: colour; leucoplasts: storage |
Ribosomes | RNA + protein, no membrane | protein synthesis |
Vacuoles | single membrane (tonoplast) | storage; in plants, a large central vacuole maintains turgor |
Cytoskeleton | microtubules and microfilaments | support, shape, movement |
Centrosome | two centrioles (in animal cells) | organises spindle fibres during cell division |
Mitochondria and chloroplasts are semi-autonomous. They have their own DNA and ribosomes and can divide on their own. This supports the idea that they arose from bacteria that once lived inside other cells (the endosymbiotic hypothesis).
Plant cells and animal cells
Feature | Plant cell | Animal cell |
|---|---|---|
Cell wall | present | absent |
Plastids | present | absent |
Vacuole | one large central vacuole | small or absent |
Centrioles | absent in most higher plants | present |