B2.3 Cell specialization
SL & HL+ AHL sectionsSL and HL, with AHL content · Cells · Theme B
Big questions
- How does one fertilised egg give rise to hundreds of very different cell types?
- Why are cells small, and how do specialised cells get around the limits of size?
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From zygote to specialized cells
A human body contains hundreds of different types of cell, yet all of them come from one cell. Fertilization is the fusion of a haploid sperm with a haploid egg, giving a single diploid cell, the zygote. The zygote divides repeatedly by mitosis to form an embryo. Because mitosis produces genetically identical cells, every cell of the early embryo has the same genes as the zygote, and at first all the cells are unspecialized.
Differentiation
As the embryo develops, groups of cells follow different developmental pathways and become specialized for particular functions: nerve cells, muscle cells, red blood cells and so on. This process is differentiation. It does not involve losing genes. Each cell type expresses (transcribes and translates) only some of its genes, for example the genes for haemoglobin in red blood cell precursors or for actin and myosin in muscle cells, while its other genes stay switched off.
How do cells know what to become?
In an early-stage embryo, some cells release signalling chemicals called morphogens. A morphogen diffuses away from its source, so a concentration gradient forms: high near the source, lower further away. Cells detect the concentration around them, and different concentrations switch on different genes, often genes for transcription factors that control many other genes. A cell’s position in the gradient therefore decides which pathway of differentiation it follows.
- Gradients set up the main body axes, such as head to tail (anterior to posterior). In fruit fly embryos, a gradient of the protein Bicoid from the head end helps determine where the head and thorax form.
- In vertebrate embryos, a gradient of the signalling protein Sonic hedgehog across the developing spinal cord makes cells at different distances from its source develop into different types of neuron.
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Text description of this animation
A schematic diagram in six steps. A sperm fuses with an egg to form a zygote, which divides by mitosis into an embryo of 16 unspecialized, genetically identical cells, drawn as a strip. Cells at one end release a morphogen, which forms a concentration gradient along the strip. Cells exposed to high, medium and low concentrations switch on different genes (A, B or C) and differentiate into nerve, muscle and epithelial cells, while one cell stays a stem cell. The last step shows the two properties of a stem cell: self-renewal and differentiation.
- Fertilization. A haploid sperm fuses with a haploid egg. Their nuclei combine to form a diploid zygote, a single unspecialized cell that contains the whole genome.
- Mitosis forms an embryo. The zygote divides repeatedly by mitosis (1 → 2 → 4 → 8 → 16 cells). Every cell in the early embryo has the same genes and all of the cells are still unspecialized. The embryo is drawn here as a strip of cells.
- A morphogen gradient forms. Cells at one end release a morphogen, a signalling chemical. It diffuses away from its source, so its concentration is high at that end and falls with distance. Each cell's position is signalled by the concentration it receives.
- Different genes are switched on. Every cell has genes A, B and C, but the morphogen concentration determines which one is expressed. High concentrations switch on gene A, medium concentrations gene B and low concentrations gene C. The cells now make different proteins.
- Cells differentiate. Expressing different genes, the cells develop along different pathways and become specialized, here as nerve, muscle and epithelial cells. One cell does not differentiate and stays a stem cell. No genes have been lost, only switched off.
- Properties of stem cells. A stem cell divides again and again (self-renewal). Often one daughter cell stays a stem cell while the other differentiates into a specialized cell, such as a muscle cell, or into other cell types. Stem cells can therefore both keep a supply of themselves and produce new specialized cells.
Stem cells and potency
A stem cell is an unspecialized cell with two key properties:
- Self-renewal: it can divide again and again, in principle without limit, to produce more stem cells.
- Differentiation along different pathways: its descendants can develop into more than one type of specialized cell.
Often a stem cell divides so that one daughter cell stays a stem cell and the other goes on to differentiate. This keeps a supply of stem cells while also producing new specialized cells. The cells of an early embryo are all stem cells; in an adult, only a small fraction of cells are stem cells, but they allow tissues to grow, be maintained and be repaired.
Levels of potency
Potency describes how many different cell types a stem cell can become. It narrows as an embryo develops.
| Potency | Can become | Examples |
|---|---|---|
| Totipotent | Any cell type, including the placenta and other membranes outside the embryo | The zygote and the cells of the first few divisions |
| Pluripotent | Any of the cell types of the body, but not the placenta | Embryonic stem cells from the inner cell mass of a blastocyst (about 5 days after fertilization) |
| Multipotent | A limited range of closely related cell types | Adult stem cells, such as those in bone marrow that make all the types of blood cell |
Cells in early-stage animal embryos are therefore totipotent, but they soon become pluripotent as the embryo forms a blastocyst, and stem cells in adult tissues are multipotent.
Totipotent any cell type, including the placenta
The zygote and the cells produced by its first few mitotic divisions can each give rise to a complete organism: every cell type of the body plus the placenta and other membranes outside the embryo.
Pluripotent any body cell type, but not the placenta
After about five days the embryo is a hollow blastocyst. Cells of the inner cell mass (embryonic stem cells) can become any of the cell types of the body, but no longer the placenta, which develops from the outer trophoblast.
Multipotent a few related cell types · niche: bone marrow
Haematopoietic stem cells in the red marrow of bones such as the pelvis, sternum and femur divide to replace themselves and to make all the types of blood cell, via progenitor cells (dashed arrows simplify this). They cannot make nerve or liver cells.
Multipotent niche: hair follicle
Stem cells in the bulge of each hair follicle stay inactive for long periods, then divide to start a new phase of hair growth. Their descendants move down to the bulb, where rapid divisions produce the hair. They also help to repair the epidermis after a wound.
Stem cell niches in adults
Adult stem cells are not scattered at random. Each type is found in a particular place in its tissue, called its stem cell niche. The niche is the microenvironment around the stem cells: neighbouring cells, the extracellular matrix and the blood supply. It provides the signals that control what the stem cells do. A niche can:
- maintain the stem cells, keeping them inactive (dormant) and undifferentiated for long periods; or
- promote proliferation and differentiation, making them divide rapidly and produce specialized cells when more are needed.
Two examples
| Niche | Stem cells | What they produce |
|---|---|---|
| Bone marrow, in the spongy bone of the pelvis, sternum, ribs, vertebrae and the ends of long bones such as the femur | Haematopoietic (blood-forming) stem cells | All types of blood cell: red blood cells, the white blood cells and the cells that make platelets. About two million red blood cells are made every second. A rich blood supply brings oxygen and nutrients. |
| Hair follicles in the skin | Stem cells in the bulge, a region of the follicle wall below the sebaceous gland | New hair at the start of each growth phase (their descendants divide rapidly in the bulb at the base of the follicle), and new epidermis when the skin is wounded |
Most of the time the hair follicle stem cells are inactive. Signals from cells at the base of the follicle activate them at the start of a new growth phase, which is why hair grows in cycles.
Cell size as an aspect of specialization
Human cells vary greatly in size, and size is part of how each type is specialized for its function. Most are between about 5 and 30 µm across, but there are striking exceptions.
| Cell | Approximate size | How size relates to function |
|---|---|---|
| Sperm (male gamete) | Head about 3 µm wide; whole cell 50–60 µm long | Small and streamlined, with very little cytoplasm, so it can swim fast to the egg |
| Red blood cell | About 7–8 µm across, 2 µm thick | Small enough to squeeze through capillaries; thin, so it has a large SA:V for rapid O₂ uptake and release |
| White blood cells | About 7–20 µm across | Lymphocytes are small when inactive but enlarge when activated, producing plasma cells with extra cytoplasm for making antibodies |
| Egg (female gamete) | About 110 µm across | Very large, with a big volume of cytoplasm containing food reserves for the early embryo |
| Motor neuron | Cell body 20–100 µm; axon up to about 1 m long | A single long axon carries impulses all the way from the spinal cord to a muscle, for example in the foot |
| Striated muscle fibre | 10–100 µm wide; millimetres to over 10 cm long | Very large and long, so that one fibre can contract along much of the length of a muscle, giving a strong pull |
- Red blood cellabout 7–8 µm across, 2 µm thick
- White blood cells7–20 µm across (lymphocytes smallest)
- Spermhead about 3 µm wide; 50–60 µm long
- Egg (secondary oocyte)about 110 µm across
- Motor neuroncell body 20–100 µm; axon up to about 1 m
- Striated muscle fibre10–100 µm wide; millimetres to over 10 cm long
width or diameter length · logarithmic scale: each grid line is 10 times the one before
Notice that the largest cells are large in only one or two dimensions. Neurons and muscle fibres are long but narrow, and the egg is large but does very little exchange with its surroundings for its size. The next section explains why.
Surface area-to-volume ratio limits cell size
Every cell exchanges materials with its surroundings across its plasma membrane: oxygen and nutrients enter, and carbon dioxide and other waste leave. Two quantities matter:
- the rate of exchange depends on the surface area of the membrane;
- the need for exchange depends on the volume of cytoplasm, where the metabolic reactions happen.
The maths
For a cube of side length L, the surface area is 6L² (six square faces) and the volume is L³, so
SA:V = 6L² ÷ L³ = 6 / L
| Side length | Surface area | Volume | SA:V |
|---|---|---|---|
| 1 µm | 6 µm² | 1 µm³ | 6 µm⁻¹ (6 : 1) |
| 2 µm | 24 µm² | 8 µm³ | 3 µm⁻¹ |
| 3 µm | 54 µm² | 27 µm³ | 2 µm⁻¹ |
| 10 µm | 600 µm² | 1,000 µm³ | 0.6 µm⁻¹ |
Doubling the side length multiplies the surface area by 4 but the volume by 8, so the SA:V ratio halves. Volume increases faster than surface area as any object grows.
Why this limits cell size
As a cell grows, each unit volume of cytoplasm is served by less membrane. Eventually materials cannot enter or leave fast enough to meet the cell’s needs, and distances from the membrane to the centre are longer. A cell that reaches a certain size therefore divides, restoring a high SA:V. Heat produced by metabolism is also lost across the surface, so the same reasoning applies to heat.
Cube, L = 4 µm
SA = 6 × L² = 6 × 4² = 96 µm²
V = L³ = 4³ = 64 µm³
SA:V = 96 ÷ 64 = 1.5 µm⁻¹
A cube of side 4 µm has 1.5 µm² of surface for every µm³ of volume.
Show data table
| Side, L (µm) | SA = 6L² (µm²) | V = L³ (µm³) | SA:V (µm⁻¹) |
|---|---|---|---|
| 1 | 6 | 1 | 6 |
| 2 | 24 | 8 | 3 |
| 3 | 54 | 27 | 2 |
| 4 | 96 | 64 | 1.5 |
| 5 | 150 | 125 | 1.2 |
| 6 | 216 | 216 | 1 |
| 7 | 294 | 343 | 0.857 |
| 8 | 384 | 512 | 0.75 |
| 9 | 486 | 729 | 0.667 |
| 10 | 600 | 1,000 | 0.6 |
A model: real cells are not cubes, spheres or plates, and their shapes are irregular. The scale factors work the same way for any shape: multiply every length by x and the surface area rises by x² but the volume by x³.
Text description of this simulation
A slider sets the side length L of a cube from 1 to 10 µm. The cube is drawn to scale, and the working is shown for its surface area (6L²), volume (L³) and surface area-to-volume ratio (6/L). A line graph plots SA:V against size: the ratio falls steeply at first and then more slowly as the cube gets bigger. Two other shapes with the same volume can be chosen: a sphere, which has the lowest SA:V of any shape, and a flattened cell eight times wider than it is thick, which has a higher SA:V. A second graph shows the surface area and volume of the cube: volume rises faster than surface area. SA:V falls below 1 µm⁻¹ when L exceeds 6 µm. The two curves can be plotted together, but area and volume have different units, so their crossing has no biological meaning. A button divides the cell into eight smaller cells of the same shape: the total volume is unchanged, but the total surface area and the SA:V ratio both double, which is why cells divide when they become large. A data table gives values for cubes of side 1 to 10 µm.
Cubes as a model
Cells are not cubes. The cube is a model: a deliberately simplified version of a complex system. It is useful because scale factors work in the same way for any shape: if every length is multiplied by a factor x, all areas are multiplied by x² and all volumes by x³. The simulator shows that a sphere and a flattened cell of the same volume give curves of the same shape, just higher or lower.
Adaptations that increase SA:V
Cells specialized for exchange, such as absorption, secretion or excretion, have shapes that give a large area of plasma membrane for their volume. For a given volume a sphere has the smallest surface area, so any departure from a sphere increases SA:V. Three common adaptations are:
- Flattening: a wide, thin cell has a much larger SA:V than a round one of the same volume, and no part of the cytoplasm is far from the surface. Erythrocytes (red blood cells) are flattened biconcave discs: about 7–8 µm across but only about 2 µm thick at the rim and about 1 µm at the centre. Type I pneumocytes are an extreme example (next section).
- Microvilli: finger-like projections of the plasma membrane on the exposed surface of an epithelial cell. Packed closely together they form a brush border.
- Invagination: inward folding of the plasma membrane to form deep folds, channels or tubules.
Proximal convoluted tubule cells
The cells lining the proximal convoluted tubule of the nephron reabsorb useful substances, such as glucose, amino acids, Na⁺ and water, from the filtrate. They show two of these adaptations:
- The surface facing the lumen has a dense brush border of microvilli, greatly increasing the area of membrane containing cotransporters and channels that take substances in from the filtrate.
- The opposite (basal) surface has deep invaginations of the plasma membrane, increasing the area for the Na⁺/K⁺ pumps and other transporters that move reabsorbed substances out of the cell towards the blood. Pumping Na⁺ out keeps the Na⁺ concentration inside the cell low, which drives sodium-dependent glucose cotransport at the lumen side (B2.1 Membranes and membrane transport). Many mitochondria lie between the folds, supplying ATP.
Flattening: erythrocyte
A red blood cell (volume about 90 µm³) has a surface area of about 140 µm², so SA:V ≈ 1.5 µm⁻¹. A sphere of the same volume would have only about 97 µm² (SA:V ≈ 1.1 µm⁻¹). No haemoglobin is more than about 1 µm from the surface, so O₂ loading and unloading is fast. Type I pneumocytes are an even more extreme example of flattening.
Microvilli and invagination: proximal convoluted tubule cell
Facing the filtrate, thousands of microvilli greatly increase the area of membrane carrying cotransporters for reabsorption. At the base, deep invaginations of the plasma membrane increase the area for Na⁺/K⁺ pumps and other transporters that move reabsorbed substances out towards the blood, with mitochondria packed between the folds to supply ATP.
Type I and type II pneumocytes
The wall of each alveolus in the lungs is an epithelium only one cell thick. It contains two cell types with very different adaptations.
Type I pneumocytes
- Extremely thin and flattened, with very little cytoplasm and few organelles. Together with the capillary wall, the distance from air to blood is only about 0.5 µm.
- They cover about 95% of the alveolar surface.
- Their thinness keeps the diffusion distance for oxygen and carbon dioxide as short as possible, so gas exchange is rapid.
Type II pneumocytes
- Rounded (cuboidal) cells with dense cytoplasm, microvilli on the surface facing the air, and many lamellar bodies: secretory vesicles that store surfactant.
- Lamellar bodies discharge surfactant by exocytosis into the thin film of fluid lining the alveolus. Surfactant, a mixture of phospholipids and proteins, forms a layer at the air–water surface and reduces surface tension. This prevents the walls of the alveoli sticking together and the alveoli collapsing when we breathe out.
- The moist lining also lets oxygen dissolve before it diffuses into the blood.
- They cover only about 5% of the surface but are more numerous than type I cells. They can also divide to replace damaged type I cells.
A tissue with more than one cell type
No single cell could be both extremely thin and full of the endoplasmic reticulum, Golgi apparatus and vesicles needed for secretion. The alveolar epithelium is therefore an example of a tissue in which more than one cell type is needed, because the overall function of the tissue requires different adaptations. For ventilation and the alveoli themselves, see B3.1 Gas exchange.
Cardiac muscle cells and striated muscle fibres
Both cardiac muscle and skeletal (striated) muscle contain myofibrils, bundles of actin and myosin filaments arranged in repeating sarcomeres that give the striped (striated) appearance and produce contraction (B3.3 Muscle and motility). Both also contain many mitochondria to supply ATP. Their other features differ.
| Feature | Cardiac muscle cells | Striated muscle fibres |
|---|---|---|
| Branching | Branched, each joined to several others | Unbranched cylinders |
| Junctions | Joined end to end by intercalated discs with gap junctions | No intercalated discs; fibres lie side by side in bundles |
| Length | About 100 µm (roughly 50–150 µm) | Often several centimetres; some over 10 cm |
| Nuclei | Usually one, in the centre | Many, at the edge just under the plasma membrane |
Cardiac muscle cells branched · one nucleus · about 100 µm long
Striated (skeletal) muscle fibre unbranched · many nuclei · mm to cm long
Suggesting reasons for the differences
- Branching and intercalated discs: impulses pass through gap junctions from cell to cell and spread in all directions through the branched network, so the wall of each heart chamber contracts almost at once and squeezes the blood out. The discs also hold the cells together as the heart beats continually throughout life.
- Length and number of nuclei: a skeletal fibre forms when many embryonic muscle cells fuse, so it can be long enough to pull along much of a muscle’s length and is controlled as one unit. Its many nuclei supply the instructions for making proteins throughout its large volume of cytoplasm.
Is a muscle fibre a cell?
- For: it is surrounded by one continuous plasma membrane, it has one continuous cytoplasm and it behaves as a single unit.
- Against: it has many nuclei, it is formed by the fusion of many cells and it is far larger than a typical cell. It does not fit the usual idea of a cell as having one nucleus.
Many biologists describe it as a syncytium, a structure formed by cell fusion. Either view can be argued, as long as the evidence is used.
Adaptations of human sperm and egg cells
Sperm must travel to the egg and get through its outer layers; the egg must support the early embryo. Their adaptations reflect these very different roles. (Gamete production is in D3.1 Reproduction.)
Sperm
- Acrosome: a cap-like vesicle over the front of the head containing hydrolytic enzymes. When a sperm reaches the egg the enzymes are released and digest a path through the zona pellucida.
- Haploid nucleus: 23 chromosomes, with the DNA very tightly packed so the head is small and streamlined.
- Centriole: at the base of the head; in humans it organizes the centrosome used for the zygote’s first mitotic spindle.
- Mid-piece packed with mitochondria, wound in a helix around the tail, producing ATP by aerobic respiration for swimming.
- Tail (flagellum): a core of microtubules in a 9 + 2 arrangement whose bending propels the sperm.
- Very little cytoplasm, so the cell is small and light. Millions of sperm are made every day.
Egg (secondary oocyte)
- Large, about 110 µm across, with a big volume of cytoplasm containing lipid droplets and other nutrients, plus mitochondria and ribosomes, to support the embryo during its early divisions.
- Haploid set of 23 chromosomes. At ovulation these are held at metaphase of the second meiotic division, which is completed only after a sperm enters.
- Cortical granules: vesicles just inside the plasma membrane. After one sperm has entered, they release their contents by exocytosis, hardening the zona pellucida so that no more sperm can get in (preventing polyspermy).
- Zona pellucida: a thick layer of glycoproteins around the egg. Sperm must bind to it and digest a way through it, so it protects the egg and restricts entry to sperm of the same species.
- Corona radiata: a layer of follicle cells from the ovary that surrounds the egg when it is released.
- Usually only one egg is released each month; it is not motile.
Sperm small · motile · haploid · millions made
Egg (secondary oocyte) large · not motile · haploid · one a month
Must-know definitions
- Differentiation
- The development of an unspecialized cell into a specialized cell type with a particular structure and function. It happens because the cell expresses a particular set of its genes; it keeps all its other genes, but they are not transcribed.
- Morphogen
- A signalling chemical that spreads from a source in an early embryo, forming a concentration gradient. Each cell's position along the gradient determines which genes it expresses, and so which pathway of differentiation it follows.
- Stem cell
- An unspecialized cell with two properties: it can divide again and again to produce more stem cells (self-renewal), and its descendants can differentiate along different pathways into specialized cell types.
- Totipotent
- Able to develop into any type of cell, including the cells of the placenta and other membranes outside the embryo. The zygote and the cells of the first few divisions are totipotent.
- Pluripotent
- Able to develop into any of the cell types of the body, but not the placenta. Embryonic stem cells from the inner cell mass of a blastocyst are pluripotent.
- Multipotent
- Able to develop into only a limited range of related cell types. Adult stem cells are multipotent, for example haematopoietic stem cells in bone marrow, which make all the types of blood cell.
- Stem cell niche
- The particular location in a tissue where adult stem cells are found. The surrounding cells, extracellular matrix and blood supply provide signals that either keep the stem cells inactive and undifferentiated or make them divide and differentiate.
- Surface area-to-volume ratio (SA:V)
- Surface area divided by volume. For a cube of side L it is 6L² ÷ L³ = 6/L, so it falls as size increases. The rate of exchange depends on surface area, but the need for exchange depends on volume.
- Microvilli and invaginationAHL
- Two ways of increasing the area of plasma membrane without making the cell bigger: microvilli are finger-like projections of the membrane; an invagination is an inward fold. Proximal convoluted tubule cells have both.
- Lamellar bodiesAHL
- Secretory vesicles in type II pneumocytes that store surfactant. They discharge it by exocytosis into the film of fluid lining the alveolus, where it reduces surface tension.
- Intercalated discAHL
- The junction where two cardiac muscle cells meet end to end. It holds the cells together and has gap junctions through which ions, and so electrical impulses, pass rapidly from cell to cell.
Common exam mistakes
Loses marks Differentiated cells lose the genes that they do not need.
Write instead Every cell produced by mitosis from the zygote keeps the full genome. Cells differ because they express different genes: the rest are present but not transcribed.
Loses marks Stem cells are only found in embryos.
Write instead Adults have stem cells too, in niches such as bone marrow and hair follicles. They are multipotent rather than pluripotent.
Loses marks Totipotent and pluripotent mean the same thing.
Write instead Totipotent cells (zygote, first few divisions) can form any cell type including the placenta; pluripotent cells (inner cell mass) can form any body cell type but not the placenta.
Loses marks As a cell grows, its surface area-to-volume ratio increases because its surface area increases.
Write instead Surface area does increase, but volume increases faster (with the cube of length rather than the square), so the SA:V ratio decreases.
Loses marks The surface area of a cube of side L is L² (or 4L²).
Write instead A cube has six square faces, so its surface area is 6L². A cube of side 3 µm has SA = 54 µm², V = 27 µm³ and SA:V = 2 µm⁻¹.
Loses marks Large cells exchange materials more slowly because diffusion is slower in them.
Write instead The rate of exchange per unit area of membrane is not lower. The problem is that a large cell has less surface area per unit of volume, so exchange cannot keep up with the needs of all the cytoplasm, and diffusion distances to the centre are longer.
Loses marks Type I pneumocytes secrete surfactant.
Write instead Type II pneumocytes secrete surfactant from lamellar bodies. Type I pneumocytes are extremely thin, so the diffusion distance for gases is short.
AHLLoses marks Surfactant increases surface tension so that the alveoli stay open.
Write instead Surfactant reduces surface tension in the fluid lining the alveoli, which stops the walls sticking together and the alveoli collapsing when we breathe out.
AHLLoses marks Cardiac muscle cells have many nuclei and skeletal muscle fibres have one.
Write instead It is the other way round: a cardiac muscle cell usually has one central nucleus, while a striated (skeletal) muscle fibre has many nuclei at its edge.
AHLLoses marks The acrosome provides the energy for the sperm to swim.
Write instead The acrosome contains enzymes that digest a path through the zona pellucida. The energy (ATP) comes from the mitochondria in the mid-piece.
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What happens between fertilization and the first specialized cells?
Fusion of a haploid sperm and egg gives a diploid zygote. It divides by mitosis to form an embryo of genetically identical, unspecialized cells, which then become specialized by differentiation.
How do morphogen gradients cause cells to differentiate differently?
A morphogen diffuses from a source, so its concentration falls with distance. Cells at different positions receive different concentrations, which switch on different genes; each cell then follows the pathway of differentiation set by those genes.
Does a differentiated cell lose genes?
No. It keeps the full genome but expresses only some genes (for example haemoglobin genes in red blood cell precursors). The others are not transcribed.
State the two properties of stem cells.
(1) Self-renewal — they can divide repeatedly, in principle endlessly, to make more stem cells. (2) They can differentiate along different pathways into specialized cell types.
Totipotent vs pluripotent vs multipotent
Totipotent — any cell type including the placenta (zygote, first few divisions). Pluripotent — any body cell type, not the placenta (embryonic stem cells of the inner cell mass). Multipotent — a few related types (adult stem cells, e.g. bone marrow).
What is a stem cell niche and what can it do?
The location in a tissue where adult stem cells are found. Signals from its cells, matrix and blood supply either maintain the stem cells (inactive, undifferentiated) or promote their proliferation and differentiation.
Name two adult stem cell niches and what their stem cells produce.
Bone marrow — haematopoietic stem cells make all blood cell types. Hair follicles — stem cells in the bulge produce new hair at each growth phase and help repair the epidermis.
Give approximate sizes of an egg, a sperm and a red blood cell.
Egg about 110 µm across. Sperm about 50–60 µm long with a head about 3 µm wide. Red blood cell about 7–8 µm across and 2 µm thick.
Which human cells can be very long, and why?
Motor neurons (axon up to about 1 m) carry impulses from the spinal cord to distant muscles. Striated muscle fibres (often several cm) contract along much of a muscle's length for a strong pull.
Calculate SA, V and SA:V for cubes of side 1, 2 and 4 µm.
Side 1: 6 µm², 1 µm³, 6 µm⁻¹. Side 2: 24 µm², 8 µm³, 3 µm⁻¹. Side 4: 96 µm², 64 µm³, 1.5 µm⁻¹. SA:V = 6/L, so doubling the side halves the ratio.
Why does SA:V limit the size of cells?
Exchange depends on surface area but the need for exchange depends on volume. Volume rises with the cube of length and area only with the square, so a growing cell has less membrane per unit of cytoplasm; it must divide before exchange becomes too slow.
Three adaptations that increase SA:V, with examples
Flattening — erythrocytes (biconcave discs) and type I pneumocytes. Microvilli — on the lumen side of proximal convoluted tubule cells. Invagination — infoldings of the basal membrane of proximal convoluted tubule cells.
Type I vs type II pneumocytes
Type I — extremely thin, cover about 95% of the surface, short diffusion distance for gas exchange. Type II — cuboidal, many lamellar bodies that secrete surfactant by exocytosis, reducing surface tension.
Why does the alveolar epithelium need two cell types?
Its overall function needs different adaptations that one cell could not have at once — extreme thinness for diffusion, and organelles and vesicles for secreting surfactant.
Compare cardiac muscle cells and striated muscle fibres.
Both have contractile myofibrils and many mitochondria. Cardiac — branched, intercalated discs, usually one central nucleus, about 100 µm long. Striated — unbranched, no discs, many nuclei at the edge, often several cm long.
Is a striated muscle fibre a cell?
For — one continuous plasma membrane and cytoplasm, acts as a unit. Against — many nuclei, formed by fusion of many cells, very large. Often called a syncytium.
List adaptations of a human sperm.
Acrosome with enzymes to digest the zona pellucida; small haploid nucleus; mid-piece of mitochondria making ATP; flagellum with 9 + 2 microtubules for swimming; little cytoplasm; produced in very large numbers.
List adaptations of a human egg.
Large (about 110 µm) with cytoplasm containing food reserves; haploid chromosomes; cortical granules that harden the zona pellucida after fertilization (preventing polyspermy); zona pellucida; corona radiata of follicle cells.
Last reviewed 19 October 2026. Spotted a mistake? Report it.