Cells: The Living Units
The plasma membrane, transport, organelles, division and protein synthesis
By the end of this unit you can…
- ✓Describe the fluid mosaic model and the functions of membrane proteins and junctions
- ✓Compare passive and active transport, including osmosis and tonicity
- ✓Name each organelle and match it to its function
- ✓Describe interphase and the phases of mitosis
- ✓Explain DNA replication, transcription and translation
Key terms
Practice →1The cell: an overview
Cell theory has four ideas: (1) the cell is the smallest structural and functional unit of life; (2) an organism's activity depends on its cells; (3) the activity of a cell depends on its shape and organelles (complementarity); (4) cells only come from other cells.
The human body has roughly 30–50 trillion cells of about 200 types, but all share three main parts: the plasma membrane, the cytoplasm (with organelles) and the nucleus. Fluid inside cells is intracellular fluid (ICF); fluid outside — including interstitial fluid that bathes cells — is extracellular fluid (ECF).
2The plasma membrane
The plasma membrane follows the fluid mosaic model: a double layer (bilayer) of phospholipids with proteins floating in it like tiles in a moving mosaic.
- Phospholipids
- Polar heads face the watery ICF and ECF; nonpolar tails hide in the middle — a barrier to water-soluble substances.
- Cholesterol
- Wedged between phospholipids; stabilizes the membrane and keeps it flexible.
- Glycolipids
- Lipids with sugar groups on the outer surface.
- Integral proteins
- Span the membrane (transmembrane); most are channels or carriers for transport.
- Peripheral proteins
- Loosely attached to the inner or outer surface; enzymes, mechanical support, links to the cytoskeleton.
- Glycocalyx
- The fuzzy, sugar-rich “sugar coating” on the cell surface — a molecular ID tag that lets cells recognize each other (immune cells use it to spot foreign cells).
Membrane proteins perform six main jobs: transport, receptors for signal transduction, enzymatic activity, intercellular joining, cell–cell recognition and attachment to the cytoskeleton and extracellular matrix.
| Cell junction | How | Where / why |
|---|---|---|
| Tight junction | Integral proteins fuse adjacent membranes into a leak-proof seal | Intestinal lining — stops digestive enzymes leaking between cells |
| Desmosome | Linker proteins (cadherins) + plaques + intermediate filaments act like rivets | Skin and heart muscle — resist mechanical stress |
| Gap junction | Hollow channels (connexons) connect cytoplasm of neighbours | Heart and smooth muscle — ions pass so cells contract together |
3Passive transport
Passive processes need no ATP — substances move down their concentration gradient (from high to low) using their own kinetic energy.
- Simple diffusion
- Nonpolar, lipid-soluble molecules (O₂, CO₂, fat-soluble vitamins) pass straight through the lipid bilayer.
- Facilitated diffusion
- Polar or charged substances cross with the help of a protein: carrier-mediated (carrier changes shape, e.g. glucose) or channel-mediated (water-filled pore, e.g. ions). Carriers can become saturated.
- Osmosis
- Diffusion of a solvent (water) across a selectively permeable membrane — through the bilayer or through aquaporins — toward the side with more solute.
Tonicity is the ability of a solution to change the shape of cells by changing their water volume:
| Solution | Solutes outside vs inside | Water moves | Red blood cell |
|---|---|---|---|
| Isotonic | Same | No net movement | Keeps normal shape (0.9% saline) |
| Hypertonic | Higher outside | Out of the cell | Shrinks — crenation |
| Hypotonic | Lower outside | Into the cell | Swells and may burst — lysis (hemolysis) |
4Active & vesicular transport
Active processes use ATP (directly or indirectly) — usually to move solutes against their concentration gradient (low to high).
- Primary active transport
- Energy comes directly from ATP hydrolysis. The star example is the Na⁺–K⁺ pump (Na⁺–K⁺ ATPase): per ATP it pumps 3 Na⁺ out and 2 K⁺ in, keeping Na⁺ high outside and K⁺ high inside.
- Secondary active transport
- Uses the Na⁺ gradient created by the pump: Na⁺ leaking back in “drags” another substance along. Symport = same direction (Na⁺–glucose); antiport = opposite directions.
Vesicular transport moves large particles and fluids in membrane sacs, also using ATP:
- Endocytosis
- Into the cell. Phagocytosis (“cell eating” — macrophages engulf bacteria), pinocytosis (“cell drinking” — gulps of ECF), and receptor-mediated endocytosis (specific molecules bound to receptors in clathrin-coated pits — how cells take up cholesterol and iron).
- Exocytosis
- Out of the cell. A vesicle fuses with the plasma membrane and releases its contents — hormone secretion, neurotransmitter release, mucus.
- Transcytosis
- Moving substances into, across and out of a cell (e.g. across capillary walls).
5Resting membrane potential
All cells have a voltage across their membrane — the resting membrane potential, about −50 to −100 mV (inside negative). It exists mainly because K⁺ diffuses out through leak channels much more readily than Na⁺ diffuses in, leaving unbalanced negative charges (proteins) inside. The Na⁺–K⁺ pump maintains the gradients that create it.
6Cytoplasm & organelles
The cytoplasm is the cytosol (viscous fluid), inclusions (stored nutrients like glycogen granules and lipid droplets) and organelles.
| Organelle | Structure | Function |
|---|---|---|
| Mitochondria | Double membrane; inner folds = cristae; own DNA | Make most ATP by aerobic cellular respiration — “powerhouses” |
| Ribosomes | rRNA + protein; free or attached to ER | Protein synthesis. Free → cytosol proteins; bound → export/membrane proteins |
| Rough ER | Membranous sacs studded with ribosomes | Makes secreted and membrane proteins; makes phospholipids |
| Smooth ER | Tubules without ribosomes | Lipid & steroid synthesis, drug detoxification (liver), Ca²⁺ storage (muscle) |
| Golgi apparatus | Stacked flattened sacs | Modifies, sorts and packages proteins — the cell's post office |
| Lysosomes | Sacs of acid digestive enzymes | Digest worn organelles, bacteria; autolysis — “demolition crew” |
| Peroxisomes | Sacs of oxidases and catalase | Neutralize free radicals and toxins (alcohol); break down fatty acids |
| Cytoskeleton | Microfilaments, intermediate filaments, microtubules | Shape, support, movement of cell and organelles |
| Centrosome & centrioles | Pair of microtubule barrels near nucleus | Organize the mitotic spindle; form bases of cilia/flagella |
| Cytoskeleton element | Made of | Role |
|---|---|---|
| Microfilaments | Actin | Cell movement and shape changes; core of microvilli |
| Intermediate filaments | Tough fibrous proteins (e.g. keratin) | Resist tension; most stable |
| Microtubules | Tubulin | Organelle movement; spindle; cilia & flagella; shape |
Cell extensions: cilia (move substances across the cell surface, e.g. mucus in airways), a flagellum (propels the cell — only sperm in humans) and microvilli (finger-like folds that greatly increase surface area for absorption).
7The nucleus
- Nuclear envelope
- Double membrane perforated by nuclear pores that control what enters and leaves (mRNA out, proteins in).
- Nucleoli
- Dark-staining bodies where ribosomal subunits are assembled.
- Chromatin
- DNA wrapped around histone proteins (forming nucleosomes, like beads on a string). It condenses into chromosomes when a cell divides.
Most cells have one nucleus. Mature red blood cells have none (anucleate); skeletal muscle fibers and osteoclasts are multinucleate.
8The cell cycle & mitosis
- G₁ (gap 1)
- Cell grows and carries out normal functions; variable length. Cells that stop dividing enter G₀.
- S (synthetic)
- DNA replicates — each chromosome now has two identical sister chromatids.
- G₂ (gap 2)
- Brief; enzymes and proteins needed for division are made.
- 1Prophase — chromatin condenses into visible chromosomes (two sister chromatids joined at a centromere); nuclear envelope breaks down; the mitotic spindle forms and attaches to kinetochores.
- 2Metaphase — chromosomes line up at the cell's equator (the metaphase plate).
- 3Anaphase — centromeres split; sister chromatids are pulled to opposite poles. Shortest phase.
- 4Telophase — new nuclear envelopes form around each set; chromosomes uncoil; spindle breaks down.
- 5Cytokinesis — a contractile ring of actin forms a cleavage furrow and pinches the cell in two (begins in late anaphase).
Apoptosis is programmed cell death: unneeded, damaged or stressed cells neatly self-destruct and are eaten by phagocytes, without causing inflammation.
9DNA replication & protein synthesis
- 1Helicase unwinds the double helix and separates the strands at the replication fork.
- 2Each original strand serves as a template.
- 3DNA polymerase adds complementary nucleotides (A–T, G–C), working only in one direction: the leading strand is built continuously, the lagging strand in fragments joined by DNA ligase.
- 4Result: two DNA molecules, each with one old and one new strand — semiconservative replication.
A gene is a DNA segment carrying the instructions for one polypeptide. The code is read in three-base triplets; each corresponds to one amino acid.
- Transcription
- In the nucleus: RNA polymerase copies a gene's DNA into mRNA. The pre-mRNA is edited — noncoding introns are removed and exons spliced together.
- Translation
- In the cytoplasm on a ribosome: mRNA codons are read; tRNA molecules with matching anticodons bring the right amino acids, which are joined by peptide bonds.
- Codon
- Three-base sequence on mRNA. AUG is the start codon (methionine); UAA, UAG, UGA are stop codons.