Biology by Bradford
B2.3.5Cells of all sizes
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Many human cells are 10 to 30 µm across, but sizes vary widely with function. The biggest by volume is about mm across, the longest stretch about m, and striated muscle fibres can be several centimetres long.
B2.3 Cell specialization: revision map
The subtopic in the middle and ten tasks round it: label four diagrams, complete three tables and fill 58 gaps. Every standard level statement has a box; the five boxes for higher level only are dashed and badged HL, and drop off for an SL class. Type into the blanks and check them, or print it on A4, Letter or A3 with an answer key.
Cell specialization
Cells · Form and functionHover over a box to see it. Untick it to leave it off this sheet and its printout.
NameB2.3.6Scaling up a cube
| Side / cm | Surface area / cm² | Volume / cm³ | SA:V |
|---|---|---|---|
| 1 | 6 | 1 | 6:1 |
| 2 | 8 | ||
| 3 | 54 | ||
| 4 | 1.5:1 |
As the cube grows, SA:V . Exchange happens across the surface but the need for it rises with volume, so cells stay or change shape. Cubes are a simple of a cell, but scale works on them the same way.
B2.3 examA section through a human egg cell shortly after ovulationHL
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B2.3.7More surface, same volumeHL
Red blood cells are discs, so no haemoglobin is far from the membrane. Cells lining the proximal convoluted tubule of the have on the side facing the filtrate and deep of the membrane on the side facing the blood, so they can reabsorb fast.
B2.3.9Cardiac and skeletal muscleHL
| Cardiac muscle cell | Striated muscle fibre | |
|---|---|---|
| Branching | ||
| Nuclei | ||
| Typical length | about 0.1 mm | up to several centimetres |
Both contain contractile . A striated muscle fibre forms when separate cells , so whether it counts as one cell is debated.
B2.3Cell specialization
B2.3.8The alveolar wallHL
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The flat cells are extremely , so gases have only a short distance to . The rounded cells discharge into the air space, stopping the alveoli sticking shut. One tissue needs two cell types because the job needs both adaptations.
B2.3.1From one cell to many
Fertilisation makes a single cell, the , which divides into a ball of cells. Signal molecules spread out from one region, setting up concentration across the embryo. Each cell switches particular on or off according to the level it meets, so cells in different places follow different pathways and become specialised: .
B2.3.2, B2.3.3Stem cells and niches
Stem cells can divide again and again, , and their daughter cells can along different pathways. In adults they live in niches, such as the and the base of each . Signals in a niche either keep its stem cells as they are or make them and specialise.
B2.3.4Degrees of potency
| Stem cell | Can become | Example |
|---|---|---|
| any type of cell, including those of the placenta | cells of the embryo in its first few divisions | |
| any type of cell in the body, but not the placenta | cells of the embryo a few days later | |
| a few closely related types | stem cells in adult |
B2.3.10Gametes comparedHL
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The cap at the front releases that digest a way through the layers round the egg. The egg cannot swim, but its large volume of stores nutrients for the early embryo. At fertilisation, release their contents, which harden the so that no second male gamete can get in.
Word bank 0.112:124273:16496acrosomebone marrowbranchedcortical granulecortical granulescytoplasmdifferentiatedifferentiationdiffuseeggendlesslyenzymesfallsflagellumflattenedfollicle cellfusegenesgradientshair follicleinvaginationslamellar bodymanymicrovillimitochondrionmodelMultipotentmyofibrilsnephronneuronnucleusoneplasma membranePluripotentproliferatered blood cellsmallspermsurfactantthinTotipotenttype I pneumocytetype II pneumocyteunbranchedunspecialisedwhite blood cellzona pellucidazygote
Answer key
Every box on the sheet with its gaps filled in, as the subtopic reads in full.
GapsFrom one cell to many
Fertilisation makes a single cell, the zygote, which divides into a ball of unspecialised cells. Signal molecules spread out from one region, setting up concentration gradients across the embryo. Each cell switches particular genes on or off according to the level it meets, so cells in different places follow different pathways and become specialised: differentiation.
GapsStem cells and niches
Stem cells can divide again and again, endlessly, and their daughter cells can differentiate along different pathways. In adults they live in niches, such as the bone marrow and the base of each hair follicle. Signals in a niche either keep its stem cells as they are or make them proliferate and specialise.
TableDegrees of potency
Stem cell Can become Example Totipotent any type of cell, including those of the placenta cells of the embryo in its first few divisions Pluripotent any type of cell in the body, but not the placenta cells of the embryo a few days later Multipotent a few closely related types stem cells in adult bone marrow LabelCells of all sizes
- egg
- sperm
- red blood cell
- white blood cell
- neuron
Many human cells are 10 to 30 µm across, but sizes vary widely with function. The biggest by volume is about 0.1 mm across, the longest stretch about 1 m, and striated muscle fibres can be several centimetres long.
TableScaling up a cube
Side / cm Surface area / cm² Volume / cm³ SA:V 1 6 1 6:1 2 24 8 3:1 3 54 27 2:1 4 96 64 1.5:1 As the cube grows, SA:V falls. Exchange happens across the surface but the need for it rises with volume, so cells stay small or change shape. Cubes are a simple model of a cell, but scale works on them the same way.
GapsMore surface, same volume HL
Red blood cells are flattened discs, so no haemoglobin is far from the membrane. Cells lining the proximal convoluted tubule of the nephron have microvilli on the side facing the filtrate and deep invaginations of the membrane on the side facing the blood, so they can reabsorb fast.
LabelThe alveolar wall HL
- type I pneumocyte
- type II pneumocyte
- lamellar body
The flat cells are extremely thin, so gases have only a short distance to diffuse. The rounded cells discharge surfactant into the air space, stopping the alveoli sticking shut. One tissue needs two cell types because the job needs both adaptations.
TableCardiac and skeletal muscle HL
Cardiac muscle cell Striated muscle fibre Branching branched unbranched Nuclei one many Typical length about 0.1 mm up to several centimetres Both contain contractile myofibrils. A striated muscle fibre forms when separate cells fuse, so whether it counts as one cell is debated.
LabelGametes compared HL
- acrosome
- nucleus
- mitochondrion
- flagellum
The cap at the front releases enzymes that digest a way through the layers round the egg. The egg cannot swim, but its large volume of cytoplasm stores nutrients for the early embryo. At fertilisation, cortical granules release their contents, which harden the zona pellucida so that no second male gamete can get in.
Exam diagramA section through a human egg cell shortly after ovulation HL
- follicle cell
- zona pellucida
- plasma membrane
- cortical granule