Contraction

Role of Calcium Ions

When muscle cells are stimulated, there is an influx of calcium ions. The ions play an important role in initiating muscle contraction. The steps involved are:

Depolarisation

Muscle contraction is initiated when an action potential arrives at a neuromuscular junction from a motor neurone.
The action potential causes depolarisation of the sarcolemma.
Depolarisation spreads along the T tubules and into the sarcoplasm.

Influx of calcium ions

Depolarisation of the T tubules stimulates the sarcoplasmic reticulum (SR).
The SR releases Ca²⁺ ions into the sarcoplasm.

Tropomyosin

Ca²⁺ ions bind to a protein attached to tropomyosin.
- Tropomyosin is a protein that blocks the actin-myosin binding site.
Binding of Ca²⁺ ions causes the protein to change shape.
Altering the protein causes tropomyosin to be moved. The actin-myosin binding site is no longer blocked by tropomyosin.

Actin-myosin cross bridge

The myosin head can now bind to the actin filament.
The bond between actin and myosin is called the actin-myosin cross bridge.

ATP hydrolase

Ca²⁺ ions also activate ATP hydrolase.
ATP hydrolase is an enzyme that hydrolyses ATP to ADP and inorganic phosphate. This process releases energy that can power muscle contraction.

Actin-Myosin Cross Bridges

The influx of Ca²⁺ ions to the sarcoplasm allows myosin and actin filaments to bind, creating an actin-myosin cross bridge. The roles of the cross bridges in muscle contraction are:

Bending of myosin heads

When Ca²⁺ ions activate ATP hydrolase, ATP is hydrolysed and energy is released.
The energy released from this reaction causes the myosin head to bend.
The movement of the myosin head causes the actin filament to slide past the myosin filament.
The actin filament is pulled by the myosin head because of the actin-myosin cross bridge.

Breaking the cross bridge

After the actin filament has slid past the myosin filament, the actin-myosin cross bridge is broken. This is driven by energy from ATP.
The myosin head is no longer attached to the actin filament.

Forming a new cross bridge

The myosin head bends back to its original position after it is released from the actin binding site.
The myosin forms a new cross bridge with a binding site further along the actin filament.

Contraction

The cycle of forming and breaking actin-myosin cross bridges occurs quickly and continuously.
As actin filaments are pulled past the myosin filaments, the overall result is the shortening of the sarcomere.
Shortening of the sarcomere causes muscle contraction.

Halting Contraction

Muscle contraction is stopped when the muscle cells are no longer stimulated. The steps involved are:

Removal of calcium ions

If action potentials are no longer stimulating the muscle cells, the release of Ca²⁺ ions by the sarcoplasmic reticulum (SR) will stop.
The Ca²⁺ ions are transported back into the SR by active transport.

Movement of tropomyosin

Removal of Ca²⁺ ions means that the protein attached to tropomyosin undergoes a conformational change.
The protein changes shape. This causes tropomyosin to shift so that it is blocking the actin-myosin binding sites.
Myosin heads can no longer bind to actin filaments.

Sarcomere lengthens

Myosin heads can no longer bind to actin filaments.
The actin filaments return to their resting position.
The sarcomere lengthens again. The muscle is no longer contracting.

1Unity & Diversity - Molecules

1.1Water

1.1.1Water & Hydrogen Bonding

1.1.2IB Multiple Choice - Water & Hydrogen Bonding

1.1.3Extended Response - Water

1.2DNA Structure & Replication

1.2.1DNA

1.2.2RNA

1.2.3DNA Replication

1.2.4DNA Replication - Summary

1.2.5IB Multiple Choice - DNA Replication

1.3Transcription & Gene Expression

1.3.1Introduction to Transcription & Protein Synthesis

1.3.2Types of RNA

1.3.3Transcription

1.3.4RNA Processing in Eukaryotes

1.3.5IB Multiple Choice - Transcription

1.4Translation

1.4.1Translation

1.4.2Retroviruses

1.4.3Scientific Practices: Coronavirus

1.4.4IB Multiple Choice - Translation

2Unity & Diversity - Cells

2.1The Origin of Cells

2.1.1Origins - Endosymbiosis

2.2Introduction to Cells

2.2.1Introduction to Cells

2.2.2Cell Size & Surface Area

2.2.3Surface Area to Volume Ratio

2.2.4Eukaryotic vs Prokaryotic Cells

2.2.5Stem Cells

2.2.6Stem Cells in Disease

2.3Ultrastructure of Cells

2.3.1Golgi, Lysosomes & Ribosomes

2.3.2Cell Walls & Vacuole

2.3.3Prokaryotes

2.3.4Binary Fission

2.4Cell Division

2.4.1The Cell Cycle

2.4.2Mitosis

2.4.3Regulation of the Cell Cycle

2.4.4End of Topic Quiz - The Cell Cycle

2.4.5IB Multiple Choice - The Cell Cycle

2.4.6Extended Response - Mitosis

2.5Structure of DNA & RNA

2.5.1DNA & RNA

2.6DNA Replication, Transcription & Translation

2.6.1Protein Synthesis

2.6.2Transcription & Translation

2.6.3End of Topic Test - DNA, Genes & Protein Synthesis

2.6.4Exam-Style Question - Protein Synthesis

2.6.5End of Topic Test - Coronavirus Translation

2.6.6IB Multiple Choice - Transcription

2.6.7IB Multiple Choice - Translation

2.7Cell Respiration

2.7.1Overview of Respiration

2.7.2Anaerobic Respiration

2.7.3End of Topic Test - Anaerobic Respiration

2.7.4Respiration Experiments

2.7.5End of Topic Test - Respiration

2.7.6IB Multiple Choice - Respiration

2.8Photosynthesis

2.8.1Overview of Photosynthesis

2.9Viruses

2.9.1Viruses

3Unity & Diversity - Organisms

3.1Diversity of Organisms

3.1.1Chromosomal Theory of Inheritance

3.1.2Chromosomal Basis of Inherited Disorders

3.1.3IB Multiple Choice - Chromosomal Inheritance

3.2Evidence for Evolution

3.2.1Evidence for Evolution - Fossils & DNA

3.2.2Evidence for Evolution - Anatomy & Geography

3.2.3IB Multiple Choice - Evidence for Evolution

3.2.4Extended Response - DNA & Evolution

3.3Natural Selection

3.3.1Introduction

3.3.2Evolutionary Fitness

3.3.3Natural Selection & Variation

3.3.4End of Topic Quiz - Natural Selection

3.3.5IB Multiple Choice - Natural Selection

3.3.6Speciation

4Unity & Diversity - Ecosystems

4.1Classification

4.1.1Introduction to Biodiversity

4.1.2Keystone Species

4.1.3Courtship & Ancestry

4.1.4Classification

4.2Cladistics

4.2.1Evolutionary Relationships

4.2.2IB Multiple Choice - Species & Taxonomy

4.3Evolution & Speciation

4.3.1Evidence for Evolution - Fossils & DNA

4.3.2Evidence for Evolution - Anatomy & Geography

4.3.3IB Multiple Choice - Evidence for Evolution

4.3.4Extended Response - DNA & Evolution

4.3.5Populations

4.3.6Mutations, Genetic Drift, & Gene Flow

4.3.7Speciation

4.3.8Rate of Speciation

4.3.9Allopatric & Sympatric Speciation

4.4Conservation of Biodiversity

4.4.1Coming Soon!

5Form & Function - Molecules

5.1Carbohydrates & Lipids

5.1.1Monosaccharides

5.1.2Disaccharides & Polysaccharides

5.1.3Triglycerides & Phospholipids

5.1.4Cellulose

5.1.5Starch & Glycogen

5.2Proteins

5.2.1Structure & Functions of Proteins

6Form & Function - Cells

6.1Membranes & Membrane Transport

6.1.1Passive Movement Across Membranes

6.1.2Active Movement Across Membranes

6.1.3Endocytosis & Exocystosis

6.1.4Cell Membrane Structure & Permeability

6.2Organelles & Compartmentalization

6.2.1Eukaryotic Cells & Organelles

6.2.2Subcellular Components & Organelles

6.3Cell Specialization

6.3.1Cell Specialization

7Form & Function - Organisms

7.1Gas Exchange

7.1.1Lung Structure & Alveoli

7.1.2Ventilation

7.1.3Lung Disease

7.1.4Lung Disease Data

7.2Transport

7.2.1The Circulatory System

7.2.2The Heart

7.2.3Blood Vessels

7.2.4Cardiovascular Disease

7.2.5Xylem Structure

7.2.6Cohesion-Tension Theory

7.2.7Investigating Transpiration Rate

7.2.8Phloem Structure

7.2.9Translocation

7.3Muscle & Motility

7.3.1Skeletal Muscle

7.3.2Sliding Filament Theory

7.3.3Contraction

8Form & Function - Ecosystems

8.1Species, Communities & Ecosytems

8.1.1Population Dynamics

8.1.2Communities & Measuring Diversity

8.1.3Relationships Between Populations

8.1.4Overview of Ecosystems

8.1.5Nutrient Cycles

8.1.6Fertilisers

8.1.7Eutrophication

8.1.8Chi-Squared Test

8.2Energy Flow

8.2.1Energy

8.2.2Energy Flow

8.2.3Biomass & Food Chains

8.2.4Heterotrophs & Autotrophs

8.3Carbon Cycle

8.3.1Carbon Cycle

8.4Climate Change

8.4.1Carbon Dioxide

8.4.2The Greenhouse Effect

8.4.3Global Warming

8.4.4Maintaining Biodiversity

9Interaction & Interdependence - Molecules

9.1Enzymes

9.1.1Enzymes

9.1.2Factors Affecting Enzyme Activity

9.1.3Enzyme-Controlled Reactions

9.1.4End of Topic Test - Enzymes

9.2Metabolism

9.2.1Molecules to Metabolism

9.3Cell Respiration

9.3.1Glycolysis

9.3.2Aerobic Respiration

9.4Photosynthesis

9.4.1Light-Dependent Reaction

9.4.2Light-Independent Reaction

9.4.3End of Topic Test - Photosynthesis Reactions

9.4.4Limiting Factors

9.4.5Photosynthesis Experiments

9.4.6End of Topic Test - Photosynthesis

9.4.7End of Topic Test - Photosynthesis

10Interaction & Interdependence - Cells

10.1Chemical Signalling

10.1.1Coming Soon!

10.2Neural Signalling

10.2.1Medical Application

10.2.2Speed of Transmission

10.2.3Synapses

10.2.4End of Topic Test - Nervous Comm&Coord

10.2.5Neuron Structure

10.2.6Types of Synapse

10.2.7End of Topic Test - Neurones & Action Potentials

10.2.8Transmission of an Impulse

10.2.9End of Topic Test - Synapses

10.3Adaptation to Environment

10.3.1Coming Soon!

10.4Ecological Niches

10.4.1Coming Soon!

11Interaction & Interdependence - Organisms

11.1Integration of Body Systems

11.1.1Plant Tropisms

11.2Defence Against Disease

11.2.1Immune System

11.2.2The Immune Response

11.2.3Antibodies

11.2.4Primary & Secondary Response

11.2.5Vaccines

11.2.6HIV

11.2.7End of Topic Test - Immune System

12Interaction & Interdependence - Ecosystems

12.1Populations & Communities

12.1.1Coming Soon!

12.2Transfers of Energy & Matter

12.2.1Coming Soon!

13Continuity & Change - Molecules

13.1DNA Replication

13.1.1DNA Replication

13.2Protein Synthesis

13.2.1Protein Synthesis

13.2.2Transcription & Translation

13.2.3End of Topic Test - DNA, Genes & Protein Synthesis

13.3Mutation & Gene Editing

13.3.1Genes

13.3.2Mutations

13.3.3DNA Technology

13.3.4PCR & Gel Electrophoresis

13.3.5Genetic Modification

13.3.6Cloning

14Continuity & Change - Cells

14.1Gene Expression

14.1.1Introduction to Transcription & Protein Synthesis

14.1.2Types of RNA

14.1.3Transcription

14.1.4RNA Processing in Eukaryotes

14.1.5IB Multiple Choice - Transcription

14.2Water Potential

14.2.1Controlling Blood Water Potential

15Continuity & Change - Organisms

15.1Inheritance

15.1.1Non-Nuclear Inheritance

15.1.2Linked Genes

15.1.3IB Multiple Choice - Non-Mendelian Genetics

15.1.4Extended Response - Inheritance

15.1.5Introduction to Non-Mendelian Inheritance

15.1.6Chi-Squared Test

15.1.7End of Topic Quiz - Inheritance

15.1.8Sex-Linked Genes

15.1.9Grade 4-5 (Scientific Practices) - Inheritance

15.2Homeostasis

15.2.1Overview of Homeostasis

15.2.2Blood Glucose Concentration

15.2.3Controlling Blood Glucose Concentration

15.2.4End of Topic Test - Blood Glucose

15.2.5Type I & Type II Diabetes

15.2.6Human Reproductive Systems

15.2.7Hormonal Control of the Menstrual Cycle

15.2.8Kidneys & Osmoregulation

16Continuity & Change - Ecosystems

16.1Natural Selection

16.1.1Natural Selection

16.2Stability & Change

16.2.1Coming Soon!

16.3Climate Change

16.3.1Carbon Dioxide

16.3.2The Greenhouse Effect

16.3.3Global Warming

16.3.4Maintaining Biodiversity

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