Energy Systems
🔋 Energy Metabolism – Introduction
🍞🥑🥩 Dietary macronutrients (carbohydrate, fat, protein) determine energy availability
🗄 Storage differences
🟡 Fat → easily stored (adipose tissue), metabolized more slowly
🔵 Protein → no storage capacity, metabolized first after meals
🔄 Order of metabolism after a mixed meal
🥩 Protein → 🍞 Carbohydrate → 🥑 Fat
🚫 Not all food energy is usable
🌾 Fibre cannot be digested → energy lost in faeces
🧪 Some metabolic by-products (e.g. urea, ammonia) still contain energy → lost in urine
⚡ Metabolizable energy
✔ Only the energy that can be digested, absorbed, and used is relevant for human energy metabolism
⚙ Metabolism
🔄 Metabolism → all chemical processes required to maintain life
🔬 Two phases of metabolism:
🧱 Anabolism (constructive phase)
➕ Builds larger molecules from smaller ones
🍬 Example: glucose → glycogen
🧨 Catabolism (destructive phase)
➖ Breaks larger molecules into smaller ones
🥑 Example: triglycerides → glycerol + fatty acids
🔋 The Role of Mitochondria
⚡ Energy requirement → all cells need energy for growth, maintenance, and function
💪 Muscle cells → require energy to produce force during contraction
🧬 Mitochondria
🏭 Cell organelles responsible for energy provision
🫁 Only site where oxygen is used in cells
❌ Absent only in red blood cells
🔄 Aerobic metabolism occurs in mitochondria, including:
🔁 Krebs cycle
⚙ Electron transport chain
🔋 ATP production
Krebs cycle and related processes produce large amounts of ATP
🍞🥑 Carbohydrates and fats are main energy substrates
🥩 Proteins also contribute (~15% of resting energy metabolism)
🧪 All macronutrients can be used to produce ATP in mitochondria
🔋 The Energy Currency of the Cell — ATP
🔄 Cell respiration → catabolic reactions convert biochemical energy into ATP
⚛ ATP structure
🧱 Composed of adenosine + three phosphate groups
🔗 Bonds between phosphate groups are energy-rich
💥 ATP breakdown (hydrolysis)
💧 ATP + water → ADP + phosphate + energy
⚡ Energy is released when the final phosphate is removed
🔁 ATP synthesis (phosphorylation)
➕ ADP + phosphate → ATP
🔌 Central role of ATP
🔗 Links catabolic (energy-releasing) and anabolic (energy-requiring) reactions
🍞🥑 Chemical energy from food must be transferred to ATP before use
💡 Why ATP matters
🌍 Universal energy carrier in cells
💱 Often called the “energy currency of the cell”
💪 Energy for Muscle Contraction
🧬 Muscle fibres
Each muscle fibre is a single elongated cell
All muscle fibres can produce ATP from carbohydrates and fats
⚡ ATP and contraction
Muscle contraction is driven by ATP
Contractile proteins (actin and myosin) use ATP to power contraction
All movement in sport and exercise depends on ATP in muscle fibres
⏱ Limited ATP stores
Stored ATP in muscle supports only ~2 seconds of activity
🔁 Energy systems
Exercise lasting longer than ~2 seconds requires ATP resynthesis
ATP is regenerated via energy systems (biochemical pathways) within muscle cells
These catabolic reactions generate ATP to sustain muscle contraction
🏃 Performance link
The effectiveness of energy systems influences muscle contraction
This affects the ability to perform different types of exercise
🍞 Carbohydrate Metabolism
🍽 Digestion
Carbohydrates are broken down into monosaccharides
Includes glucose, fructose, and galactose
🩸 Absorption & transport
Monosaccharides are absorbed into the bloodstream
Transported to the liver
🏭 Liver processing
Fructose and galactose are converted into glucose
🚚 Distribution
Glucose is released from the liver
Transported to other organs and tissues
Breakdown of Glucose to Pyruvate for Energy
🧫 Location
Occurs in the cytosol of all cells
🔄 Process: Glycolysis
Series of reactions that break down glucose → pyruvate
Produces ATP (energy)
🫁 With oxygen (aerobic)
Pyruvate enters the mitochondria
Oxidized to carbon dioxide and water
🚫🫁 Without oxygen (anaerobic)
Pyruvate converted to lactate
🔁 Fate of lactate
Transported to the liver for gluconeogenesis (glucose re-formed)
Or oxidized back to pyruvate in muscles
🧬 Conversion of Glucose to Glycogen for Storage
🍬 Excess glucose
In liver and muscle cells, glucose is stored when intake exceeds demand
🧱 Glycogen
Storage form of glucose
🔗 Glycogenesis
Many glucose molecules are linked together to form glycogen
🔓 Glycogenolysis
Breakdown of glycogen when more glucose is needed
🏋 Muscle glycogen
Broken down to glucose-6-phosphate
Used only by the muscle for immediate energy
🫀 Liver glycogen
Broken down to glucose
Supplies glucose to all other organs
Reaction Summaries:
Glycolysis – The breakdown of glucose into pyruvate
Glycogenesis – The use of glucose to make glycogen (when more glucose is eaten/present than is required)
Gluconeogenesis - The production of glucose from lactate
Glycogenolysis – The breakdown of glycogen into glucose (when there isn’t enough glucose in blood/muscle)
Lipolysis – the breakdown of triglycerides into glycerol and three fatty acids
Beta oxidation – the breakdown of fatty acids from the methyl end into acetyl-CoA
⚡ Glycolysis (Glycolytic Energy System)
📍 Where it happens
Occurs in the cytoplasm of the cell
Does not require mitochondria
Can occur with or without oxygen
🚦 First step in energy production
Glycolysis is the first stage of breaking down glucose for energy
All other energy pathways depend on glycolysis happening first
🍬 What happens to glucose
One glucose molecule (C₆H₁₂O₆) is split into two pyruvate molecules (C₃H₄O₃)
This splitting allows energy to be released and captured
🔋 ATP production
Produces 2 ATP molecules
This is a small but fast supply of energy
Useful for short, high-intensity activity
🔁 NAD⁺ → NADH
Converts NAD⁺ into NADH
NADH acts as an energy carrier
It is used later in aerobic processes to help make more ATP
⏱ Why glycolysis is important
Provides quick energy
Starts energy production when exercise begins
Supports both anaerobic and aerobic pathways depending on oxygen availability
Aerobic (oxidative) - glucose
🧪 Requires oxygen to produce energy
🍞 Fuel source: glucose (from carbohydrates)
🔄 Glycolysis produces pyruvate in the cytosol
❌➡🧬 With oxygen present, pyruvate does NOT become lactate
🔁 Pyruvate is converted into acetyl-CoA
🔢 Each pyruvate → acetyl-CoA (2-carbon compound)
🌫 1 carbon dioxide (CO₂) released during this conversion
➕ 1 glucose molecule → 2 acetyl-CoA molecules
🔄 Acetyl-CoA enters the Krebs cycle (in the mitochondria)
⚙ Krebs cycle reactions release energy used later to make ATP
🔋 Large ATP yield, but slower energy production
🏃♂ Best suited to long-duration, lower-intensity exercise (e.g. distance running, cycling)
🔋 Aerobic (Oxidative) Energy System – Fatty Acids (Beta oxidation)
🫁 Oxygen must be present
🧘 Used mainly at low to moderate exercise intensities
🥑 Uses fats (fatty acids) as the fuel source
✂ Fatty acids are broken down by removing 2 carbon atoms at a time
🔄 Each removal produces 1 Acetyl-CoA
🔢 Number of Acetyl-CoA molecules depends on fat length
➡ 1 Acetyl-CoA per 2 carbon atoms
📊 Example: a fatty acid with 18 carbons → 9 Acetyl-CoA
🔁 Acetyl-CoA enters the Krebs Cycle
🔋 Very large ATP yield
⏱ Slow energy release
🧈 Saturated fats are harder to metabolize
Require more steps and more oxygen
🏃 Best for:
Long-duration, low-intensity exercise
e.g. walking, long-distance cycling
Aerobic (oxidative) (krebs cycle)
Krebs Cycle (Citric Acid Cycle)
🔁 Acetyl-CoA enters the Krebs Cycle
Comes from glucose (via glycolysis) or fatty acids
🫁 Only occurs when oxygen is available
Without oxygen, this pathway cannot continue
⚠ Oxygen is not directly used in the Krebs Cycle
Oxygen is needed later in the electron transport chain
🏭 Takes place in the mitochondrial matrix
Inside the mitochondria = aerobic energy production
⚙ What Happens in the Krebs Cycle?
🔥 Acetyl-CoA is broken down
Its carbon atoms are released as carbon dioxide (CO₂)
🌫 Major source of CO₂ production
🟢 2 CO₂ molecules per Acetyl-CoA
🔋 Small amount of ATP produced
🟢 1 ATP per Acetyl-CoA
📦 High-energy carriers are produced
NADH (very important)
🔌 Link to the Electron Transport Chain (ETC)
🔄 NADH carries energy forward
Transfers energy to the electron transport chain
⚡ ETC uses oxygen to make large amounts of ATP
This is where most ATP is produced aerobically
🧠 Key idea:
Krebs Cycle = energy preparation stage, not the main ATP maker
⚡ Electron Transport Chain (ETC)
🏭 Occurs on the inner mitochondrial membrane (cristae)
🫁 Requires oxygen as the final electron acceptor
🔄 Uses NADH and FADH₂ from glycolysis & Krebs
🔋 Produces the largest amount of ATP
🔢 ~34 ATP per glucose
💧 Forms water as a by-product
⚡ Glycolysis → Lactate (Anaerobic Pathway)
🚫🫁 Low oxygen available
When oxygen supply cannot meet energy demand (e.g. sprinting), cells switch to an anaerobic pathway.
🔄 Pyruvate → Lactate
Pyruvate is converted into lactate (lactic acid) instead of entering the mitochondria.
♻ NADH → NAD⁺
This conversion regenerates NAD⁺, which is essential for glycolysis to keep running.
⏩ Allows fast ATP production
By restoring NAD⁺, the cell can continue producing ATP quickly through glycolysis.
⚠ No extra ATP produced
The pyruvate → lactate step does not make any additional energy.
🏃♂ Used in high-intensity exercise
Important during short, intense efforts where energy is needed faster than oxygen can be delivered.
⚡ Anaerobic Energy System: Phosphagen (ATP-PCr) (creatine phosphate)
🧪 What is Creatine Phosphate (PCr)?
🧱 Creatine phosphate (PCr) is a high-energy storage molecule found in muscle cells
🔋 It stores energy in a phosphate bond, similar to ATP
🔄 How PCr Regenerates ATP
🛑 At rest, ATP gives a phosphate to creatine → forms PCr + ADP
🚀 During maximal or explosive exercise, PCr donates its phosphate back to ADP
⚡ This rapidly reforms ATP, allowing muscles to keep contracting
⏱ When the Phosphagen System Is Used
🏃 Used during very high-intensity, short-duration activities
⏲ Provides energy for up to ~20 seconds of all-out effort
🏋 Examples: sprint starts, jumps, throws, heavy lifts
🔒 Why PCr Is Important
🧊 PCr is more stable than ATP, so it can be stored for longer
⚡ Allows instant energy release without oxygen
🚫 Works anaerobically (no oxygen required)
⚠ Limitations of the Phosphagen System
🔻 PCr stores are very limited
🔄 Once PCr is depleted, another energy system must take over
🧠 Usually followed by the glycolytic (lactate) system to continue exercise
Hormonal regulation - insulin
Hormonal Regulation of Energy Metabolism 🧪⚡
🧠 Energy metabolism is regulated by hormones:
Insulin
Glucagon
Epinephrine
Cortisol
Growth hormone
After a meal (high blood glucose) 🍽⬆
🩸 Rising blood glucose → pancreas releases insulin
🔑 Insulin enables glucose entry into cells (especially skeletal muscle & liver)
🚪 GLUT4 transporters act as “doors” allowing glucose into muscle cells
⚡ Glucose used for energy production inside cells
🧊 Promotes glycogenesis (glucose → glycogen)
🔥 Stimulates glycolysis
⛔ Inhibits:
Gluconeogenesis
Lipolysis (fat breakdown)
Protein breakdown
📉 Overall effect: lowers blood glucose
Hormonal regulation - glucagon
During fasting or exercise (low blood glucose) 🏃♂⬇
🩸 Falling blood glucose → pancreas releases glucagon
🔄 Glucagon acts opposite to insulin
🧊 Stimulates glycogenolysis (glycogen → glucose)
🔁 Stimulates gluconeogenesis
🛢 Activates lipolysis (fat → energy)
📈 Overall effect: raises blood glucose
Hormonal regulation - exercise
Epinephrine (stress/exercise hormone) 🚨
📉 Low blood glucose also stimulates epinephrine
⚡ Increases:
Glycogen breakdown
Lipolysis
🧠 Supports rapid energy availability during exercise
Characteristics of energy systems and their contributions during exercise
⚙ All energy systems work together to meet ATP demand during exercise
⏱ Rate of ATP production varies:
🧈 Fat oxidation = slowest
🧪 Glycolytic system = fast
⚡ Phosphagen (PCr) = fastest
Immediate & High-Intensity Exercise 🚀
⚡ PCr system supplies energy at the start of muscle contraction
⏳ Dominates during brief, maximal efforts
⛔ Contribution stops after ~20 seconds of all-out exercise
Light to Moderate Intensity Exercise 🚶♀🏃
🍞 Energy mainly from glucose
🧪 Initially from anaerobic glycolysis
🌬 As exercise continues, aerobic metabolism increases
🚪 Muscle contraction increases glucose uptake via non-insulin pathways
📉 Insulin levels fall during exercise → reduced insulin-driven glucose transport
Across Exercise Intensities 🔄
🍞 Glucose used at all intensities (anaerobic + aerobic)
🧈 Fatty acids:
Used only at lower intensities
⛔ Not used above ~90% maximal intensity
Variable Exercise Patterns 🔁
🏀 Team sports:
Repeated high-intensity bursts → ⚡ PCr & 🧪 glycolytic systems
Lower-intensity periods → 🌬 aerobic recovery
🏃 Endurance events:
Pace changes (start, surges, sprint finish) alter system contribution