Topic 7: Run for your life

What is skeletal muscle?

* The type of muscle you can move

Flexor muscle

* Bends the limb when it contracts

Extensor muscle

* Straightens the limb when it contracts

Tendon

* Attaches muscle to bone

Structure and function of tendons

* Allow movement of limbs
* Lengths of strong connective tissue (collagen)
* Flexible but do not stretch when a muscle pulls on a bone (inelastic)
* Able to resist high forces

Ligaments

* Connects bone to bone

Structure and function of ligament

* Holds the skeleton together and keeps it stable
* Cords made out of connective tissue, collagen and some elastin fibres

Antagonistic Muscles

* Muscles that come in pairs to work together on a bone
* One extensor and one flexor

Why are antagonistic muscles needed?

* Muscles can only contract to pull not push. Therefore they can only move in one direction without a pair of muscles working together

Antagonistic muscle action

* As one muscle pulls on a joint, the other must pull in the opposite direction

Raising the lower arm

* Bicep contracts, tricep relaxes
* Bone cannot be stretched so arm flexes around the joint
* The bicep is the flexor
* Tricep is brought to its full length so it can contract again to move the arm back down

Lowering the lower arm

* Tricep contracts, bicep relaxes
* Bone cannot be stretched so arm flexes around the joint
* The tricep is an extensor
* Bicep is stretched so it can contract to move the arm back up again

What are skeletal muscles made out of?

Muscle fibres

Muscle fibres

* Large bundles of long cells that make up skeletal muscles
* High specialised cell like units

Structure of a muscle fibre

* Contains an organised arrangement of contractile proteins in the cytoplasm (myofibrils)
* surrounded by a cell surface membrane (sarcolemma)
* Multi-nucleated

Why are muscle fibres multi-nucleated?

* So there are enough proteins synthesised in every part of the cell
* Proteins don’t have to be transported (because that would waste energy)
* Muscle fibres are centimeters long!

Special names for muscle fibre parts

* Cell surface membrane= Sarcolemma
* Cytoplasm= Sarcoplasm
* Endoplasmic reticulum= Sarcoplasmic reticulum (SR)

What are transverse (T) tubules (the T system)?

* Deep tube-like projections that fold in from the sarcolemma’s outer surface and stick to sarcoplasm
* Run close to the sarcoplasmic reticulum and help spread electrical impulses throughout the muscle fibre

What does the sarcoplasm contain?

* Mitochondria and myofibrils

Role of mitochondria in the sarcoplasm

* Carry out aerobic respiration to generate the ATP required for muscle contraction

What is the sarcoplasmic reticulum (SR)

* Network of internal membranes that run through the sarcoplasm
* Stores and releases calcium ions for muscle contraction
* Membrane of SR contains protein pumps that transport calcium ions into the lumen of the SR

The ultrastructure of skeletal muscle and of a section of muscle fibre

Myofibrils

* Long cylindrical organelles made of two types of protein filament
* Thick and thin filaments are arranged in a particular order, creating different types of bands and lines
* Located in sarcoplasm

Thick filaments

* Made of myosin
* A bands

Thin filaments

* Made of actin
* I bands

Myofibril parts?

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Fast twitch muscle fibres (full list)

* Contract rapidly
* Large amount of calcium ions present to stimulate contraction
* Rely of anaerobic respiration for ATP supply
* Fatigue quickly due to the lactate produced
* Quick energy release
* Fewer capillaries
* Lower fat stores
* Not alot of mitochondria or blood vessels
* Whiter in colour due to lack of myoglobin
* Used for short bursts of high-intensity activity

Slow twitch muscle summary

* Long contraction- relaxation cycle
* Energy released slowly
* Denser network of capillaries
* ATP from mostly aerobic respiration
* Many mitochondria and blood vessels
* Small sore of calcium ions in SR
* Small amount of glycogen
* Fatigue slower due to reduced production of lactate
* Redder in colour due to myoglobin
* Used in endurance activities

Fast Twitch fibres contraction speed

* Contract rapidly
* Myosin heads bind and unbind from the actin-binding sites five times faster than slow twitch muscle fibres
* This means large amounts of calcium ions have to be present

Fast twitch muscle fatigue

* Fatigue quickly due to the lactate produced from anaerobic respiration
* Therefore suited for short bursts of high intensity activity

Where are fast twitch muscle fibres found?

* Found in high proportions in the limbs of animals that have to run at high speeds
* Lots in human eyelids

Capillaries in fast twitch muscles

* Fewer capillaries
* Blood containing glucose and oxygen flow through capillaries
* Meaning slow supply of oxygen and glucose for aerobic respiration

Myoglobin levels in fast twitch muscle fibres

* Low amounts therefore muscles are paler
* Myoglobin stores oxygen and increases rate of oxygen absorption from the capillaries

What is myoglobin

* Red pigmented molecule that is similar to haemoglobin
* Basically haemoglobin muscle version

Slow twitch fibres contraction speed

* Contract more slowly
* Therefore suited to sustained activities

Slow twitch muscle fatigue

* Fatigue less quickly as less lactate produced
* As they rely more on aerobic respiration for ATP production

Where are slow twitch muscle fibres found?

* Found in high proportions in limbs of animals that migrate or stalk prey over long distances
* Human back muscles have a high proportion of slow twitch muscle fibres as they contract for long periods of time in order to keep the skeleton erect

Capillaries in slow twitch

* Denser network of capillaries
* Blood containing glucose and oxygen flows through the capillaries
* Meaning short diffusion distance and good supply of O2 and glucose for aerobic respiration

Myoglobin in slow twitch

* Myoglobin stores oxygen
* Therefore high amount of myoglobin and dark red pigment

What is the structure of thick filaments?

* Made up of myosin
* These are fibrous protein molecules with a globular head
* Fibrous part of the myosin molecule anchors the molecule into the thick filament
* Many myosin molecules lie next to each other with the globular heads all pointing away from the M line

What is the structure of the thin filaments?

* Made up of actin molecules
* Globular protein molecules
* Many actin molecules link together to form a chain
* Two actin chains twist together to form one thin filament
* A fibrous protein known as tropomyosin is attached to the actin chains at regular intervals

How does the sliding filament theory start?

* Impulses travel along a motor neurone causinging acetylcholine to be released into the synapse of the neuromuscular junction


* The muscle end plate depolarises (sarcolemma) causing Ca ions to be released from the sarcoplasmic reticulum

What is the neuromuscular junction?

* A specialised synapse between a motor neuron nerve terminal and its muscle fibre

THE SLIDING FILAMENT THEORY

* **Calcium** ions attach to the troponin molecules, causing them to move
* This causes the tropomyosin on the actin filaments to shift position, exposing myosin binding sites on the actin filaments
* Myosin heads bind to myosin binding sites on actin filament, forming cross bridges
* ADP and Pi on the myosin head are released
* Myosin changes shape, causing the myosin head to nod forward.
* Actin dragged toward the M-line, shortening the sarcomere
* The attached actin moves over the myosin
* An ATP molecule binds to the myosin head. This causes the myosin head to detach
* An ATPase on the myosin head hydrolyses the ATP forming ADP and Pi
* The hydrolysis causes a change in the shape of the myosin head, It returns to its upright position and enables cycle to start again at the next impulse

What happens after the muscle stimulation stops?

* Calcium ions leave their binding sites on troponin molecules
* They are actively transported back to SR
* Without calcium ions bound to them, the troponin molecules return to their original shape
* This pulls tropomyosin molecules in a position that blocks the actin-myosin binding sites
* Since no cross bridges can form between actin and myosin no muscle contraction can occur
* Sarcomere lengthens again as actin filaments slide back to their relaxed position

What is the action potential in the sliding filament theory?

* Triggers an influx of calcium ions
* This depolarises the sarcolemma
* Myosin heads can now bind to troponin

What is the role of calcium ions in the sliding filament theory?

* Released from sarcoplasmic reticulum and binds to troponin
* Troponin changes shape so tropomyosin moves
* Myosin binding site on actin is now exposed so actomyosin bridges can form
* Its presence also activates ATPase

What is a metabolic pathway?

* A series of chemical reactions

Respiration word equation

glucose + oxygen → carbon dioxide + water + energy

Respiration Balanced symbol equation

**C6H1206 + 6O2 →  6CO2 + 6H20 + 2870kJ**

What is the main respiratory substrate used by cells?

* Glucose

Is glucose the only respiratory substrate used?

* No
* Organisms can break down other molecules like fatty acids or amino acids to be respired

What is aerobic respiration?

* The process of breaking down a respiratory substrate in order to produce ATP using oxygen

What is the energy that is released during aerobic respiration used for?

* Phosphorylate ADP to form ATP
* ATP provides energy for biological processes in cells (cannot leave the cell)

What are the four stages of aerobic respiration?

* Glycolysis
* The link reaction
* The krebs cycle
* Oxidative phosphorylation

Where do the 4 main stages occur?

* Glycolysis: cytoplasm of the cell (NOT IN THE MITOCHINDRIA)
* Link reaction: matrix of the mitochondria
* Krebs cycle: matrix of the mitochondria
* Oxidative phosphorylation: inner membrane of mitochondria

Enzymes inside the cell

* Reactions in each stage controlled by enzymes inside the cell
* The enzyme that catalyses these reactions the slowest will determine the overall rate of aerobic respiration

Why are coenzymes required for respiration

* Coenzymes are required to transfer various molecules involved in the process

Which coenzymes are used in aerobic respiration

* NAD and FAD are coenzymes responsible for transferring hydrogen between molecules
* They can reduce or oxidise a molecule depending on whether they give or take a hydrogen
* Coenzyme A is responsible for the transfer of acetate from one molecule to another

Structure of mitochondria

* Two phospholipid membranes
* Outer membrane
* Inner membrane
* Intermembrane space
* Mitochondrial matrix

Features of the outer membrane

* Smooth
* Permeable to several small molecules

Features of inner membrane

* Folded (cristae)
* Less permeable
* Site of the electron transport chain (ETC)
* Location of ATP synthase enzymes

Features of the intermembrane

* Low PH because high conc of protons
* Concentration gradient across inner membrane is formed during oxidative phosphorylation and is essential for ATP synthesis

Features of the matrix

* Aqueous solution within the inner membranes of the mitochondrion
* Contains ribosomes, enzymes and circular mitochondrial DNA necessary for mitochondria to function

Why does energy in the chemical reactions have to be released gradually?

* A sudden release of a large amount of energy would result in an increase in body temperature levels that would denature enzymes

First stage of respiration

* Glycolysis
* Does not require oxygen to take place and is therefore the first step for both aerobic and anaerobic respiration

What is glycolysis (brief description)?

* Splitting of one molecule of glucose into two molecules of pyruvate

Steps of glycolysis

* Phosphorylation of glucose (hexose sugar (6C))
* Splitting of glucose phosphate
* Oxidation of triose phosphate (TP)

Phosphorylation of glucose

* 2ATP are oxidised to form 2ADP + 2Pi
* The two phosphates are added onto the glucose molecule (phosphorylated)

Splitting of glucose phosphate

* Glucose phosphate is a high energy molecule and is therefore unstable and has a lower activation energy
* This causes it to split into two molecules of triose phosphate (3C) which are at a lower energy state

Oxidation of triose phosphate (TP)

* Enzyme controlled reactions convert each TP into 3C pyruvate (TP is oxidised)
* Hydrogen removed from TP and reduces the H carrier NAD to form NADH


* In the process of oxidation of 1 TP, 2 ATP are produced
* As there are 2 TP molecules per one glucose, the products are doubled
* The 2 pyruvates go into the mitochondrial matrix for the link reaction

Products of glycolysis (per glucose molecule)

* 2 Pyruvate
* 2 NADH
* 2 ATP (net gain as 2ATP used in the phosphorylation)

Travelling to Link reaction

* Pyruvate contains a lot of chemical energy that can be further utilised in respiration to produce more ATP
* Enzymes and coenzymes required for link reaction found in mitochondrial matrix
* When oxygen is available pyruvate enters the matrix and aerobic respiration continues
* Pyruvate is actively transported across the double membrane of the mitochondria
* requires a transport protein and some ATP

Why is it called the link reaction?

* It links glycolysis to the krebs cycle

Steps of the link reaction

* Pyruvate is oxidised by enzymes to produce acetate
* The H ion lost is accepted by NAD to form NADH (reduction)
* Pyruvate is decarboxylated in the form of CO2
* Acetate then binds with coenzyme A to form acetylcoenzyme A

Products of link reaction (per glucose molecule)

* 2Acetylcoenzyme A
* 2CO2
* 2NADH
* NO ATP

Word equation for the link reaction

pyruvate + NAD + CoA → acetyl CoA + carbon dioxide + NADH

Krebs Cycle

* The Acetyl fragment (2C) from Acetyl CoA is accepted by Oxaloacetate (4C) and forms citrate (6C)
* Coenzyme A is released and reused in the next link reaction
* Citrate is then converted back into oxaloacetate through a series of redox reactions

What are the redox reactions that regenerate oxaloacetate?

* Citrate is decarboxylated
* The 2 carbons bind to oxygen to form the waste gas 2CO2
* Citrate is then oxidised
* Releases H ions that reduce 3 NAD and FAD
* Substrate linked phosphorylation
* A phosphate is transferred from one of the intermediates to ADP, forming 1 ATP

Products of krebs cycle per glucose molecule

* 6 NADH
* 2 FADH
* 2 ATP
* 4 CO2

Oxidative phosphorylation

* Last stage of aerobic respiration
* Energy carried by electrons from reduced coenzymes is used to make many molecules of ATP and water as a waste product

What are the 2 parts of oxidative phosphorylation

* ETC
* Chemiosmosis

What is the electron transport chain?

* Proteins/ electron carriers embedded in the membrane
* Proteins close together to allow electrons to pass from carrier to carrier
* The inner membrane of the mitochondria is impermeable to hydrogen ions so these electron carriers are required to **pump the protons across the membrane** to establish the concentration gradient

ETC process

* Reduced enzymes diffuse into the ETC on the inner membrane and dissociate into the coenzyme, hydrogen and electrons
* Coenzymes return to the first three stages
* Electrons are accepted by the first electron carrier- it is reduced
* Electrons passed from the first e- carrier to the second
* First carrier is oxidised, second is reduced
* As electrons pass along the ETC, some of their energy is used to pump H ions across the intermembrane space
* An electrochemical gradient (proton gradient). They cannot diffuse back across the inner membrane as it is impermeable to protons
* High concentration in intermembrane space, low conc in matrix
* Oxygen (the final electron acceptor) accepts the electrons at the end of the ETC and combines with H ions to form water

Chemiosmosis process

* Due to the electrochemical gradient of the H ions, they diffuse (facilitated diffusion) through the channel protein. This is known as chemiosmosis
* This changes the shape of the ATP synthase causing ADP and Pi to be joined to form ATP

Products of oxidative phosphorylation

* 3 ATP molecules for every NADH molecule
* 2 ATP molecules for every FAD molecule
* 38 ATP for every glucose molecule

Metabolic poisons

Some metabolic poisons target electron carriers

* Preventing the passing of electrons in oxidative phosphorylation
* Stopping e¯ from moving down ETC, stopping chemiosmosis
* NADH and FADH aren’t oxidised so none for Krebs cycle
* ATP synthesis in the cells ends up seriously reduced


* Not enough ATP to fuel ATP-regulating cellular processes

What happens when cells have little to no oxygen?

* No final e acceptor from ETC
* ETC stops functioning
* No more ATP produced via oxidative phosphorylation
* NADH and reduced FAD are not oxidised By an electron carrier
* No reduced NAD and FAD available for dehydrogenation in krebs cycle
* Krebs cycle and therefore link reaction stops
* Anaerobic respiration can still take place

What is the only source of ATP production that does not require oxygen?

The 2 net gain of ATP from glycolysis

How can glycolysis keep functioning?

* 2 NADH made during glycolysis must be reoxidised for glycolysis to repeat

What are the 2 ways to oxidise NADH

* Animals: lactate fermentation
* Fungi/yeast: Ethanol fermentation

Lactate fermentation

* NADH is oxidised into NAD so that pyruvate can be reduced into lactate by an enzyme dehydrogenase
* pyruvate is H acceptor
* NAD can now be reused in glycolysis
* Lactate can be further metabolised
* Small amount of ATP produced

Build up of lactate

* Lactate can build up in cells
* As lactate is reduced pyruvate, the build up of H ions reduces the PH, making the cytoplasm more acidic
* Only some H are accepted by NAD (in glycolysis)
* Enzymes are denatured
* Muscles may stop contracting and cause fatigue

How is lactate processed?

* It can be oxidised back to pyruvate which is then channelled into the Krebs cycle for ATP production
* It can be converted into glucose by the liver cells for use during respiration or for storage (in the form of glycogen)

What is the oxygen debt

* It is the oxygen required to oxidise all of the lactate
* The oxygen debt is why animals breathe faster and deeper after exercise

Ethanol fermentation in yeast

* Pyruvate decarboxylated to ethanal
* Catalysed by pyruvate decarboxylase
* NADH oxidised and donates H atoms to ethanal
* Ethanal reduced to ethanol

Why can yeast rely more on anaerobic respiration?

* Yeast is a facultative anaerobe- can live without oxygen
* This is because it lives in anaerobic conditions
* But high conc of ethanol can kill the yeast

What is homeostasis

* How different control systems ensure the internal conditions are kept relatively constant

What do physiological control systems do?

* Maintain the internal environment (within restricted limits) in response to changes in the external environment

What detects changes in the external environment?

* Sensory cells called receptors