Three families of cell, three completely different problems. Getting oxygen across a barrier, generating force, and getting two cells to become one. In each case the structure of the cell is the solution written out in physical form.
📚 What you need to know
The alveolar epithelium is one cell thick, giving a short diffusion distance, and millions of alveoli give a large surface area.
Type I pneumocytes are extremely thin cells covering about 95% of the alveolar surface; they carry out gas exchange.
Type II pneumocytes are rounded cells with many secretory vesicles, covering about 5%; they secrete pulmonary surfactant.
Surfactant is amphipathic and forms a monolayer that reduces surface tension, maintaining alveolar shape and preventing the alveoli from collapsing or sticking together.
Striated muscle fibres are cell-like units with many nuclei, a sarcolemma with T-tubules, a sarcoplasmic reticulum storing calcium, and myofibrils of actin and myosin.
Cardiac muscle is myogenic, does not fatigue, has one nucleus per cell, and its branched cells are joined by intercalated discs.
Sperm cells have a haploid nucleus in a streamlined head, an acrosome of hydrolytic enzymes, many mitochondria and a flagellum.
Egg cells have a haploid nucleus, a zona pellucida that hardens to prevent polyspermy, cortical granules, follicle cells, and cytoplasm rich in nutrients.
Pneumocytes: the cells of the alveoli
Your lungs contain millions of alveoli, which collectively provide an enormous surface area for gas exchange by diffusion. The walls of each alveolus, called the alveolar epithelium, are only one cell thick, and the capillary walls beside them are also one cell thick. That leaves usually less than 0.5 µm between the air and the blood.
Two different cell types make up that epithelium, and they do completely different jobs.
The thinness of the type I cell is not incidental — it is the single feature that makes gas exchange fast enough to keep you alive.
Pulmonary surfactant
The inside of an alveolus is wet, and a wet surface has surface tension: the water molecules pull on each other and try to make the surface as small as possible. In a tiny air sac, that force is strong enough to pull the walls together and collapse it.
Surfactant is the solution. Its molecules have hydrophobic tails and hydrophilic heads, so they form a monolayer on the water surface with the tails facing the alveolar air. That gets between the water molecules and reduces surface tension, which:
maintains the shape of the alveoli
prevents the alveolar sacs from sticking together
therefore prevents the alveoli, and so the lungs, from collapsing
The layer of moisture also helps gas exchange directly: oxygen dissolves in it before diffusing into the blood, and carbon dioxide diffuses out from that moist surface before being breathed out.
You have met amphipathic molecules forming a monolayer before — it is exactly what phospholipids do at a water surface. Surfactant is doing the same physics for a completely different biological purpose. Spotting that link is the sort of thing that lifts an extended response.
Muscle: striated and cardiac
Muscles attached to the skeleton are skeletal muscles, and they are described as striated because they look stripy under a microscope. Cardiac muscle in the heart is a specialised kind of striated muscle; smooth muscle in blood vessels and organs is a third type.
Striated muscle cells are bundled into fibres, each surrounded by a single plasma membrane called the sarcolemma. These fibres are usually called cell-like units rather than cells, and the reason is worth knowing:
they contain many nuclei, not one
the cytoplasm is specialised and called sarcoplasm, packed with mitochondria and myofibrils
myofibrils are bundles of actin and myosin filaments, which slide past each other during contraction
a specialised endoplasmic reticulum, the sarcoplasmic reticulum, stores calcium and uses protein pumps to signal to every part of the fibre at once
the sarcolemma folds inwards as deep transverse tubules, or T-tubules, which run close to the sarcoplasmic reticulum
The intercalated discs are the key cardiac feature: they let the contraction signal spread from cell to cell so the whole chamber beats together.
Feature
Skeletal (striated) muscle
Cardiac muscle
Nuclei
Many per fibre
One per cell
Cell shape
Long, unbranched fibres
Shorter, branched cells forming a network
Striations
Clear and strong
Faint
Junctions between units
None — fibres run in parallel
Intercalated discs, which spread contraction quickly
Control
Contracts when stimulated by a nerve
Myogenic — contracts without external stimulation
Fatigue
Tires with sustained use
Does not tire; beats continuously for life
Mitochondria
Numerous
Very numerous, for continual ATP supply
Myogenic, precisely. Cardiac muscle contracts without any external stimulation from nerves or hormones, which lets the heart beat at its own regular interval. The nervous and endocrine systems can still change how long that interval is — they modify the rhythm rather than create it.
Gametes: sperm and egg
Sperm and ova are the textbook example of structure fitting function, because their functions are so specific. One has to travel and get in; the other has to be got into exactly once, and then support an embryo.
A layer of follicle cells surrounds the ovum outside the zona pellucida, nourishing and protecting it. It is not shown here to keep the diagram readable.
Feature
Where it is
Why it is there
Haploid nucleus
Both gametes
Two haploid nuclei fuse to form a diploid zygote
Acrosome
Sperm head
Contains hydrolytic enzymes to digest a path through the zona pellucida
Many mitochondria
Sperm midpiece
Release the energy needed to power the flagellum
Flagellum
Sperm tail
Made of protein microtubules; propels the cell towards the egg
Zona pellucida
Around the ovum
A jelly layer that hardens after fertilisation to prevent polyspermy
Cortical granules
Ovum cytoplasm
Vesicles that release enzymes into the zona pellucida to harden it
Follicle cells
Around the ovum
Nourish and protect the developing egg cell
Nutrient-rich cytoplasm
Ovum
Feeds the embryo in the days after fertilisation
Polyspermy is what happens if more than one sperm penetrates the ovum, and it disrupts embryo development. The egg has a two-stage defence: cortical granules release their enzymes into the zona pellucida, and the zona hardens so no further sperm can get through.
🧠
Sperm: three parts, three jobs
Head — carry the DNA and break in. Midpiece — make the ATP. Tail — do the swimming. If you can name the three parts, the adaptations follow from what each one is for.
Worked examples
WE 1
Explaining gas exchange adaptations
Explain how type I pneumocytes are adapted for efficient gas exchange. (3 marks)
Point 1: the structure
They are extremely thin and flattened cells.
Point 2: the effect
This gives a very short diffusion distance between the air in the alveolus and the blood in the capillary, usually under 0.5 µm.
Point 3: the scale
They cover about 95% of the alveolar surface, so almost the whole of that large surface area is available for diffusion.
thin cells over a huge area means a fast rate of diffusionFick’s law in disguise: large area, short distance, steep gradient
WE 2
Applying knowledge to a clinical case
Babies born very prematurely often have difficulty inflating their lungs. Suggest an explanation in terms of pneumocytes. (3 marks)
Point 1: what is missing
Their type II pneumocytes may not yet be producing enough pulmonary surfactant.
Point 2: the consequence
Without surfactant, the surface tension of the water lining the alveoli is not reduced.
Point 3: the effect
High surface tension pulls the alveolar walls together, so the alveoli stick shut and collapse, making the lungs very hard to inflate.
too little surfactant, so surface tension collapses the alveolithis really is why premature babies are given artificial surfactant
WE 3
Comparing two muscle types
Describe two ways in which cardiac muscle differs from skeletal muscle, and explain the advantage of each. (4 marks)
Difference 1
Cardiac muscle is myogenic, contracting without external stimulation, whereas skeletal muscle contracts only when stimulated by a nerve.
The advantage
The heart can beat at its own regular rhythm without needing a signal for every beat.
Difference 2
Cardiac muscle cells are branched and joined by intercalated discs, whereas skeletal fibres run in parallel and are not joined.
The advantage
Contraction spreads rapidly from cell to cell, so a whole chamber contracts together as one coordinated beat.
myogenic rhythm, and coordinated spread through the network“describe and explain” means every difference needs a consequence attached to it
💡 Exam tips
Learn the split: type I is thin and does gas exchange (95%); type II is rounded and secretes surfactant (5%).
Explain surfactant through surface tension, not just “stops the lungs collapsing”.
Use the word myogenic for cardiac muscle, and add that nerves only modify the rate.
Name intercalated discs and say what they do: spread the contraction quickly.
For gametes, pair each feature with a function in the same sentence — acrosome to digest through the zona pellucida.
Remember that the ovum finishes meiosis only after fertilisation.
⚠ Common mistakes
Swapping the pneumocyte types. Type I is the thin one; type II is the secretory one.
Saying surfactant “helps oxygen dissolve” and nothing else. Its main role is reducing surface tension.
Calling a muscle fibre a cell. It is a cell-like unit with many nuclei, which is why the distinction exists.
Confusing the sarcolemma with the sarcoplasmic reticulum. The sarcolemma is the membrane; the SR is the specialised ER inside.
Saying cardiac muscle “never needs nerves”. It beats without them, but nerves and hormones adjust the rate.
Putting the mitochondria in the sperm head. They are in the midpiece, next to the tail they power.
That completes Specialised Cells & Stem Cells. Notice the single idea running through all four pages: a cell’s structure is an answer to a question about its job. Stem cells have not been asked the question yet, differentiation is the moment they answer it, and every specialised cell here — flattened, folded, striped or streamlined — is that answer made physical.
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