What you need to know
- 7.10(i)Structure of a mammalian skeletal muscle fibre.
Fast and slow fibres are 7B 3. The cross-bridge cycle is 7B 4. Names of extra proteins beyond actin, myosin and the membrane system are not required on this page.
By the end, you should be able to…
- Describe a fibre as a multinucleate cell containing myofibrilA long bundle of actin and myosin filaments inside a muscle fibre, made of repeating sarcomeres..
- Locate the sarcoplasmic reticulum, T-tubules and mitochondrionThe organelle where the link reaction, Krebs cycle and oxidative phosphorylation of aerobic respiration take place..
- Define a sarcomereThe unit of a myofibril between two Z lines. It shortens when the muscle contracts. as the repeating unit between Z lines.
Build your understanding
A fibre is one long cell
A skeletal muscle is made of many muscle fibres. Each fibre is a single, long, cylindrical cell with many nuclei. The cell surface membraneThe partially permeable membrane around a cell that controls which substances enter and leave. is the sarcolemmaThe cell surface membrane of a muscle fibre.. The cytoplasmEverything inside the cell surface membrane except the nucleus: the fluid cytosol and the organelles. is the sarcoplasmThe cytoplasm of a muscle fibre.. Inside it, myofibrilA long bundle of actin and myosin filaments inside a muscle fibre, made of repeating sarcomeres. run the length of the fibre. Each myofibril is a stack of repeating units called sarcomereThe unit of a myofibril between two Z lines. It shortens when the muscle contracts..
A fibre is one long cell (continued)
A sarcomere runs from one Z line to the next. Thin filaments of actinThe protein of the thin filaments in muscle. Myosin heads bind to it. and thick filaments of myosinThe protein of the thick filaments in muscle. Its heads bind to actin and pull it, using ATP. overlap inside it. You need those two filaments before the contraction lesson can explain how they slide. This page is the architecture, not yet the cycle.
Membranes that deliver the signal and the calcium
Transverse tubules, or T-tubules, are infoldings of the sarcolemma. They carry the electrical impulse into the interior of the fibre so the whole cross-section is reached together. The sarcoplasmic reticulum is a membrane store of calcium ions around the myofibrils. When the impulse arrives, calcium ions are released into the sarcoplasm.
mitochondrionThe organelle where the link reaction, Krebs cycle and oxidative phosphorylation of aerobic respiration take place. lie between myofibrils and supply ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. by aerobic respirationRespiration using oxygen: glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation. It releases much more ATP than anaerobic respiration.. glycogenThe storage polysaccharide in animals: highly branched chains of α-glucose, so glucose can be released quickly. granules are a fuel store. Neither structure contracts. They exist because contraction and pumping calcium both hydrolyse ATP.
Scale, without a micrograph claim
You may be shown a drawing or an electron micrograph and asked to label Z lines, or the region of thick filaments. A light microscope will not resolve the filaments. Do not describe a classroom slide of “meat” as proof of myosin heads.
Cardiac muscle and smooth muscle are not this fibre. Cardiac muscle is striated and myogenicDescribes heart muscle that contracts by itself, without needing nerve impulses., but its cells and the conduction system are 7B 5. Do not import intercalated discs unless a question gives them.
NOT: the fibre is not a bundle of cells, and the sarcoplasmic reticulum is not a bone.
- Fibre is the cell
- Myofibril is the thread of sarcomeres
- Sarcomere runs from Z line to Z line
- Calcium store and T-tubules are membranes
Two traps this diagram blocks
- Do not call the fibre a bundle of cells.
- Do not confuse the sarcoplasmic reticulum with a tendon.
From the diagram alone
Question
Give the locations of calcium ion storage and of the repeating contractile unit in a skeletal muscle fibre. [2]
Show a model answer
Calcium ions are stored in the sarcoplasmic reticulum. The sarcomere, from one Z line to the next, is the repeating contractile unit of the myofibril.
| Structure | What it is | Why it is there |
|---|---|---|
| Sarcolemma | Cell surface membrane | Excitation and T-tubules |
| Myofibril | Bundle of sarcomeres | The contractile thread |
| Sarcomere | Z line to Z line | Actin and myosin overlap |
| Sarcoplasmic reticulum | Membrane store | Releases Ca²⁺ |
| Mitochondrion | Aerobic organelle | Supplies ATP |
Make your reasoning visible
Fibre, not fibril
The fibre is the cell. The myofibrilA long bundle of actin and myosin filaments inside a muscle fibre, made of repeating sarcomeres. is the thread inside it.
Z to Z
If you define a sarcomereThe unit of a myofibril between two Z lines. It shortens when the muscle contracts., say between two Z lines.
Avoid this misconception
The claim: A muscle fibre is a bundle of cells, and the sarcoplasmic reticulum is a bone.
The correction: The fibre is one multinucleate cell. The sarcoplasmic reticulum is the internal membrane that stores calcium ions. A tendon, not this reticulum, joins muscle to bone.
Zoom in on a skeletal muscle
A skeletal muscle is built from smaller and smaller parts. Order the levels from the whole muscle down to the filaments that slide.
Order the steps
What to do
- Reorder the cards below: drag a card (on a touch screen, drag its number), tap a card then tap where it goes, or use the ↑ and ↓ arrows at its right.
- Press Check under the cards to mark each one green (right place) or red; the count shows how many are right.
- Stuck? Hint first outlines the first wrong place, then gives a clue, then places that step for you.
- Shuffle mixes the cards again; Start again returns them to the starting order. Keyboard: Space picks up, arrow keys move, Space drops.
Put the levels in order, from the largest to the smallest.
0 of 6 in the right place
T-tubules, the sarcoplasmic reticulum and mitochondria sit between the levels and are not ordered here. Their roles are in 7B 4.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Give two structural features of a skeletal muscle fibre.
Show suggested answer
Any two of: long cylinder, many nuclei, sarcolemma, myofibrils of sarcomeres, sarcoplasmic reticulum, T-tubules, many mitochondria.
Practice question
Retrieval and explanation · suggested answer
What is a sarcomere?
Show suggested answer
The repeating contractile unit of a myofibril, from one Z line to the next, containing actin and myosin.
Practice question
Retrieval and explanation · suggested answer
What is stored in the sarcoplasmic reticulum, and why does that matter?
Show suggested answer
Calcium ions. Their release into the sarcoplasm allows contraction to begin. The mechanism is the next contraction lesson.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A drawing shows a fibre with myofibrils but no mitochondria drawn. A student says it cannot be skeletal muscle. What can you say, and what can you not say?
Show a hint
Absence on a drawing is not absence in the cell.
Explore the reasoning
Skeletal fibres do contain mitochondria, because contraction needs ATP. A diagram that omits them may be simplified. You cannot conclude the fibre has none, unless the question states that they are absent. You can still identify myofibrils and sarcomeres if those are drawn.
Recall this lesson
Lesson recall questions
Question 1 of 5: Name the repeating contractile unit along a myofibril.
Skeletal muscle fibre types
Practice question
Retrieval and explanation · suggested answer
Name the repeating contractile unit along a myofibril.
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
A pulled muscle is not a labelled sarcomere
7B · What Is Muscle?The NHS calls a strain an overstretched or torn muscle. That tells you muscle is a tissue that can be injured. It does not describe multinucleate fibres, T-tubules…Specification 7.10(i)Read & use the scienceClose article
Why this matters
The NHS calls a strain an overstretched or torn muscle. That tells you muscle is a tissue that can be injured. It does not describe multinucleate fibres, T-tubules or Z lines. Keep the leaflet out of the structure answer. 7.10(i): know the fibre, the membrane system and the sarcomere. An injury, symptom or whole-body performance result is not a direct image of a sarcomere or cardiac conduction pathway. This is an extension to the lesson, not another fact to memorise.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.10(i) Structure of a mammalian skeletal muscle fibre.
Use the science
Why does “I strained a ligament, so my Z lines tore” mix two lessons?
Explore a possible response
A ligament joins bone to bone. Z lines are inside a muscle sarcomere. A strain, if that is the word the NHS page is using, refers to the muscle, and still does not identify the ultrastructure.
Explore the source
A muscle fibre has machinery within machinery
7B · What Is Muscle?Structural biologists study how ordered actin and myosin filaments make a repeating contractile unit. Their review follows the molecular interaction through the sarcomere rather than treating “muscle” as…Specification 7.10(i)Read & use the scienceClose article
Why this matters
Structural biologists study how ordered actin and myosin filaments make a repeating contractile unit. Their review follows the molecular interaction through the sarcomere rather than treating “muscle” as a single rope. That distinction helps with the scale of 7.10: a whole muscle contains fibres, each fibre contains myofibrils, and the myofibrils contain repeated sarcomeres. A strain diagnosed in a clinic is an injury to tissue; it is not a direct picture of a T-tubule or a Z line. Researchers need imaging and biochemical methods to connect damage at one scale with impaired movement at another.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.10(i) Structure of a mammalian skeletal muscle fibre.
Use the science
If a micrograph shows disrupted Z lines, what level of muscle organisation is directly observed, and what further evidence would show a whole-joint effect?
Explore a possible response
The micrograph observes sarcomere organisation inside a fibre. Measuring force or movement of the muscle and joint would be needed to show the larger functional effect.
Explore the source
- The actin–myosin interaction in musclePeer-reviewed review via NIH PubMed Central