Specialised cells: structure related to function – Cell Structure and Specialisation (Senior 4 Biology) | Ganzaa

Cell Structure and Specialisation

Lessons

Senior 4 · Biology

Cell Structure and Specialisation

5 lessons

Lesson 5 of 5

Specialised cells: structure related to function

25 min

One genome, many jobs

You began as a single cell, a fertilised egg. Every cell of your body descends from it by mitosis and carries the same genes. Yet a red blood cell, a nerve cell and a sperm cell look so different that they could belong to different organisms. The difference lies in which genes each cell switches on as it develops, and so which proteins and organelles it builds. The result is a cell whose shape and ultrastructure fit one particular job.

In S1 you described specialised cells from their outward shape. At A-level you must explain each feature in terms of the organelles of Lessons 2 and 3, and predict the structure of a cell from its function.

Definition

Specialised cell: a cell whose structure is adapted to carry out a particular function.

Differentiation: the process by which an unspecialised cell becomes specialised, by expressing (switching on) some genes and not others.

Stem cell: an unspecialised cell that can divide repeatedly and can give rise to specialised cells. Examples: cells in bone marrow that produce all the types of blood cell; meristem cells at the tips of roots and shoots.

Surface area to volume ratio: the surface area of a cell divided by its volume; the larger it is, the faster substances can be exchanged for each unit of volume.

Animal cells

Cell	Function	Adaptations, and why
Red blood cell (erythrocyte)	Carry oxygen	No nucleus or mitochondria, leaving room for more haemoglobin; biconcave shape gives a large surface area to volume ratio and short diffusion distance for oxygen; flexible, so it squeezes through capillaries narrower than itself
Sperm cell	Reach and fertilise an egg	Acrosome (a large lysosome made by the Golgi) holds enzymes that digest the layers around the egg; haploid nucleus; midpiece packed with mitochondria to supply ATP; flagellum with 9 + 2 microtubules for swimming; very little cytoplasm, so it is light
Neurone (nerve cell)	Carry electrical impulses	Long axon to carry impulses over long distances; many dendrites to receive signals from other cells; many mitochondria and vesicles at the nerve endings, where chemical transmitters are released
Ciliated epithelial cell (airways)	Move mucus	Many cilia on the free surface beat together; many mitochondria near their bases supply ATP for beating
Pancreatic acinar cell	Secrete digestive enzymes	Much rough ER, a large Golgi apparatus and many secretory vesicles (Lesson 2)
Neutrophil (white blood cell)	Engulf and digest bacteria	Flexible membrane and cytoskeleton for engulfing (phagocytosis); many lysosomes; lobed nucleus that helps it squeeze out of capillaries

Plant cells

Cell	Function	Adaptations, and why
Root hair cell	Absorb water and mineral ions	Long, thin extension greatly increases surface area; thin wall gives a short pathway; carrier proteins in the membrane and many mitochondria for active uptake of ions; no chloroplasts (underground)
Palisade mesophyll cell	Photosynthesis	Column-shaped, packed near the upper surface of the leaf; many chloroplasts, which can move towards the light; large vacuole pushes chloroplasts to the edge of the cell
Guard cell	Open and close stomata	Pairs around a pore; inner wall thicker than outer, so when the cells take in water and swell, they curve apart and the stoma opens; chloroplasts and many mitochondria supply ATP for pumping ions
Xylem vessel element	Carry water and ions; support	Dead and empty when mature; end walls lost, forming continuous tubes; walls thickened with lignin, which resists collapse
Phloem sieve tube element and companion cell	Carry sugars	Sieve tube: little cytoplasm, no nucleus, end walls perforated as sieve plates; companion cell: nucleus, many mitochondria and ribosomes, linked by many plasmodesmata, keeps the sieve tube alive and loads sugar

Worked example 1: how much do root hairs add?

A 10 mm length of root, radius 0.5 mm, carries 2000 root hairs, each a cylinder 10 µm in diameter and 1 mm long. Treating the root and hairs as cylinders (ignore the ends), find how many times the hairs multiply the absorbing surface.

Root surface without hairs: $2\pi r L = 2 \times 3.14 \times 0.5 \times 10 = 31.4\,\mathrm{mm^2}$ .
One hair: $10\,\mu\mathrm{m} = 0.01\,\mathrm{mm}$ , so its surface $= \pi d L = 3.14 \times 0.01 \times 1 = 0.0314\,\mathrm{mm^2}$ .
All hairs: $2000 \times 0.0314 = 62.8\,\mathrm{mm^2}$ . Total surface $= 31.4 + 62.8 = 94.2\,\mathrm{mm^2}$ .
Factor: $\frac{94.2}{31.4} = 3.0$ . The hairs triple the area in contact with soil water, although they add very little volume. (Real roots carry many more hairs, so the gain is often larger.)

Worked example 2: predicting structure from function

Brown fat cells in newborn babies release heat by respiring fats very fast. Cells of the adrenal cortex make steroid hormones. Predict one ultrastructural feature of each, and explain.

Brown fat cells: very many mitochondria with many cristae, because the heat comes from respiration in mitochondria; also many fat droplets as fuel.
Adrenal cortex cells: extensive smooth ER, because steroids are lipids made in the smooth ER (and many mitochondria, where some steps of steroid synthesis occur).
Method: identify the main product or process (heat from respiration; lipid synthesis), then the organelle responsible, then predict that the organelle is abundant.

Worked example 3: interpreting data

The table gives the percentage of cell volume taken up by mitochondria in three tissues (illustrative data).

Tissue	Mitochondria / % of cell volume
Heart muscle	35
Liver	20
Skin epidermis	5

Explain the differences.

Heart muscle contracts continuously throughout life, using ATP all the time, so it needs the most mitochondria for aerobic respiration.
Liver cells carry out many energy-using processes (making proteins, storing glycogen, detoxifying), so they need many mitochondria, though fewer than heart muscle.
Outer epidermis cells mainly form a protective layer and many are dying as they move to the surface; they use little ATP, so they have few mitochondria.

A section of human cerebellum stained with an antibody: three large, rounded nerve-cell bodies (Purkinje cells) with branches rising above them are wrapped in a brown web of fine axons from other neurones; the small blue dots below are the nuclei of many small cells. Image: GerryShaw, CC BY-SA 3.0, via Wikimedia Commons

FuseSchool: "Cell Differentiation". Note how cells with the same DNA become different by switching different genes on, and link this to the specialised cells in Figure 1.

Warning

Common mistakes

Writing that specialised cells have "different genes". They have the same genes; they express different ones.
Stating a feature without its reason. "Many mitochondria" earns little; "many mitochondria to release energy as ATP for swimming" earns the mark.
Saying red blood cells "have no organelles so they cannot respire". They respire anaerobically in the cytoplasm.
Describing xylem vessels as living cells. Mature xylem vessels are dead and hollow.

Tip

Exam technique. Use the three-step pattern: feature → what it does → how that helps the function. Example: "The midpiece contains many mitochondria, which release energy in aerobic respiration as ATP, which powers the flagellum so the sperm can swim to the egg."

✦

Practice: specialised cells

Define differentiation, and explain how cells with the same genes can have different structures.
Explain two ways in which the structure of a red blood cell is adapted to carrying oxygen.
The acrosome is described as a modified lysosome. Explain what this means for its contents and origin.
Explain how the uneven thickening of guard-cell walls opens the stoma.
A cube-shaped cell of side 2 µm and one of side 20 µm: calculate the surface area to volume ratio of each, and say which can exchange materials faster for its size.
Predict which organelles are abundant in a plasma cell, a white blood cell that secretes large amounts of antibodies (proteins).
Evaluate the statement: "A cell with no nucleus cannot be a living cell."

Show answersHide answers

Differentiation is the process by which an unspecialised cell becomes specialised. All cells have the same genes, but each type switches on (expresses) only some of them, so it makes different proteins and builds different structures.
Any two: no nucleus or mitochondria, so more room for haemoglobin; biconcave shape, giving a large surface area to volume ratio and a short diffusion distance; flexibility to pass through narrow capillaries.
Like a lysosome, it is a membrane-bound sac of hydrolytic (digestive) enzymes made by the Golgi apparatus; its enzymes digest a path through the layers around the egg.
When guard cells take in water and swell, the thin outer walls stretch more than the thick inner walls, so each cell bends outwards; the two cells curve apart and the pore between them opens.
Small: SA $= 6 \times 2^2 = 24\,\mu\mathrm{m^2}$ , V $= 2^3 = 8\,\mu\mathrm{m^3}$ , ratio 3. Large: SA $= 6 \times 20^2 = 2400\,\mu\mathrm{m^2}$ , V $= 20^3 = 8000\,\mu\mathrm{m^3}$ , ratio 0.3. The small cell exchanges materials ten times faster per unit volume.
Much rough ER, a large Golgi apparatus, many secretory vesicles, many mitochondria (and a large nucleolus to make ribosomes).
Partly true. Without a nucleus a cell cannot make new proteins or divide, so it cannot repair itself and has a limited life. But mature red blood cells and phloem sieve tubes have no nucleus and still carry out their functions for weeks or months (red blood cells about 120 days), supported in the sieve tube's case by a companion cell. Such cells are alive but cannot reproduce.

Quick check

Match each feature to the cell it belongs to.

Tap one on the left, then its partner on the right.

Key points

Specialised cells arise by differentiation: the same genes, but different genes switched on.
Structure fits function: red blood cells (no nucleus, biconcave), sperm (acrosome, mitochondria, flagellum), neurones (long axon), ciliated cells, secretory cells (rough ER, Golgi), phagocytes (lysosomes).
Plant examples: root hairs (surface area), palisade cells (chloroplasts), guard cells (uneven walls), xylem (dead, lignified tubes), phloem (sieve plates plus companion cells).
Explain every feature as feature → what it does → how it helps.

1One genome, many jobs

2Animal cells

3Plant cells

One genome, many jobs

Animal cells

Plant cells