Week 2

Cellular Respiration  and Movement of Molecules 

Topic 1: Cellular energy and respiration

  • Describe how energy is stored and transported within cells.

  • Summarise the important events and products of glycolysis, the citric acid cycle and electron transport.

  • Describe the difference between aerobic vs anaerobic respiration.


Topic 2: Movement of molecules

  • Describe the major fluid compartments in the body

  • Describe the different methods of transport across the cell plasma membrane

  • Define and describe diffusion and the difference between ligand-gated ion channels, mechanically gated ion channels or voltage-gated ion channels

  • Define and describe osmosis and what would happen to a cell in a hypertonic, isotonic and hypot5onic solution


Cellular Respiration

Cellular respiration takes place inside our cells, specifically in the mitochondria (except for glycolysis, which occurs in the cytoplasm).

It is the process of breaking down food (glucose) in the presence of oxygen to produce ATP, the cell’s usable energy.

Overall Equation

Three Main Stages of Cellular Respiration

  1. Glycolysis (in the cytoplasm)

  2. Krebs Cycle (in the mitochondrial matrix)

  3. Electron Transport Chain (ETC) (inner mitochondrial membrane)

1⃣ Glycolysis

Location: Cytoplasm
What happens:

  • One glucose molecule (6 carbons) is split into two pyruvate molecules (each with 3 carbons).

  • A small amount of energy is produced.

Energy produced per glucose:

  • 2 ATP

  • NADH (electron carrier molecule)

NADH is formed when high-energy electrons are transferred to NAD⁺ along with a hydrogen ion. These high-energy electrons will be used later in the Electron Transport Chain.

After glycolysis:

  • Pyruvate moves into the mitochondria.

  Pyruvate Oxidation (Link Reaction)

Inside the mitochondria:

  • Each 3-carbon pyruvate is converted into a 2-carbon molecule called Acetyl-CoA.

  • One carbon is removed and released as carbon dioxide (CO₂).

  • More NADH is produced.

Now Acetyl-CoA enters the Krebs Cycle.

2⃣ Krebs Cycle (Citric Acid Cycle)

Location: Mitochondrial matrix

What happens:

  • Acetyl-CoA is broken down further.

  • More carbon dioxide is released.

  • A small amount of ATP is produced.

  • High-energy electrons are added to:

    • NAD⁺ → NADH

    • FAD → FADH₂

These electron carriers (NADH and FADH₂) carry high-energy electrons to the final stage.

3⃣ Electron Transport Chain (ETC)

Location: Inner mitochondrial membrane

This is where most ATP is made.

Step 1: Electron Movement

  • NADH and FADH₂ drop off high-energy electrons.

  • Electrons move through a series of proteins embedded in the membrane.

Step 2: Proton Pumping

  • The energy from electrons is used to pump hydrogen ions (H⁺) across the membrane.

  • This creates a concentration gradient (more H⁺ on one side).

Step 3: ATP Production

Hydrogen ions flow back across the membrane through a protein called:

ATP synthase

ATP synthase works like a turbine:

  • As hydrogen ions flow through it,

  • It attaches a phosphate to ADP,

  • Forming ATP.

This process generates large amounts of ATP.

Step 4: Formation of Water

  • After passing through the chain, hydrogen ions combine with oxygen.

  • This forms water (H₂O).

  • Oxygen is the final electron acceptor.

Big Picture Summary

  • Glycolysis: small ATP production (2 ATP)

  • Krebs Cycle: small ATP production

  • Electron Transport Chain: produces most ATP

  • Oxygen is essential because it accepts electrons at the end

  • Carbon dioxide is released as waste

  • Water is formed at the end


Cellular Energy and Aerobic Respiration

All cells need energy to stay alive.

Most cells get their energy through aerobic respiration (also called cellular respiration).

  • Aerobic means “with oxygen.”

  • Oxygen reacts with glucose.

  • Glucose is broken down into:

    • Carbon dioxide (CO₂)

    • Water (H₂O)

  • Energy is released and stored in the chemical bonds of ATP (adenosine triphosphate).

Stages of Aerobic Respiration

There are three main stages:

  1. Glycolysis

  2. Krebs Cycle

  3. Electron Transport Chain (ETC)

1⃣ Glycolysis

Location: Cytoplasm
Oxygen required? ❌ No

  • Glucose is broken down into pyruvic acid (pyruvate).

  • This produces:

    • 2 ATP

    • Hydrogen ions (H⁺)

  • Hydrogen ions are picked up by NAD, forming NADH.

Think of NADH as a taxi that carries hydrogen ions and high-energy electrons to the mitochondria for further processing.

Even if oxygen is not present, glycolysis can still occur because it does not require oxygen.

2⃣ Krebs Cycle

Location: Mitochondria
Oxygen required? ✅ Yes (indirectly)

  • Occurs inside the mitochondria.

  • Produces:

    • 2 more ATP

    • Carbon dioxide

    • More NADH (and other electron carriers)

3⃣ Electron Transport Chain (ETC)

Location: Mitochondria
Oxygen required? ✅ Yes

This stage produces most of the ATP.

  • About 32 ATP molecules are produced.

  • Oxygen is essential because it acts as the final electron acceptor.

  • Without oxygen, this stage stops.

What Happens During Heavy Exercise?

During intense exercise, oxygen levels in cells can become low.

What still happens?

  • Glycolysis continues, because it does not require oxygen.

  • It still produces 2 ATP.

  • NADH is formed.

The problem:

The Krebs cycle and ETC require oxygen. Without oxygen:

  • NADH cannot unload its electrons.

  • NAD is not regenerated.

Lactic Acid Formation (Anaerobic Respiration)

To solve this problem, the cell:

  • Converts pyruvic acid into lactic acid, especially in muscles.

  • Lactic acid accepts electrons from NADH.

  • This converts NADH back into NAD.

  • Now NAD can allow glycolysis to continue.

Important:

  • Lactic acid does not produce additional ATP.

  • It simply allows glycolysis to keep going.

Why Do Muscles Hurt?

During heavy exercise:

  • Your body first uses aerobic respiration.

  • If oxygen becomes limited, anaerobic respiration occurs.

  • Lactic acid builds up in muscles.

  • This build-up contributes to muscle pain and fatigue.

After exercise:

  • You breathe heavily to take in more oxygen.

  • Oxygen is used to break down lactic acid.

  • This is called repaying the oxygen debt.



Movement of molecules

The plasma membrane separates the body’s major fluid compartments, the intracellular fluid within cells and the extracellular fluid outside the cells. The transport of molecules (e.g. ions, nutrients, waste) across the plasma membrane into and out of the cell

Why Fluid Is Essential

Maintaining fluid, electrolyte, and acid–base balance is one of the body’s most important functions.

Fluid:

  • Is a major component of the body

  • Serves as a medium for metabolic reactions

  • Transports nutrients and waste

  • Acts as a lubricant

  • Provides insulation

  • Functions as a shock absorber

  • Regulates body temperature

How Fluid Enters & Leaves

Enters through:

  • Food

  • Drinks

Leaves through:

  • Urine

  • Faeces

  • Skin (sweat)

Total Body Water (TBW)

TBW depends on:

  • Age

  • Body mass

  • Sex

  • Body fat percentage

Key values:

  • Infants: ~73% water or more

  • Healthy young male: ~60% water

  • Healthy young female: ~50% water

  • Old age: ~45% water

Why the difference?

  • Adipose tissue (fat) = ~20% water (least hydrated tissue)

  • Skeletal muscle = ~75% water

  • Even bone contains more water than fat

  • More muscle → higher total body water

Example:
70 kg healthy male × 60% = 40 litres total body water

🌊 2. FLUID COMPARTMENTS & ELECTROLYTES

Two Main Fluid Compartments

1⃣ Intracellular Fluid (ICF)

  • Inside cells

  • ~2/3 of total body water

  • ~25 L in average adult male

  • ~40% of body weight

2⃣ Extracellular Fluid (ECF)

  • Outside cells

  • ~1/3 of total body water

  • ~15 L

  • ~20% of body weight

  • Forms the body’s internal environment

ECF is divided into:

  • Plasma (fluid portion of blood) → 3 L (20% of ECF)

  • Interstitial fluid (between tissue cells) → 12 L (80% of ECF)

  • Other fluids (~1%): lymph, cerebrospinal fluid, joint fluid, pleural, peritoneal, and pericardial fluids

Electrolytes & Solutes

Both ICF and ECF contain solutes, including:

  • Nutrients

  • Waste products

  • Ions (electrolytes)

There are important compositional differences:

Major Cations (Positive Ions)

  • Sodium (Na⁺) → higher in ECF

  • Potassium (K⁺) → higher in ICF

Their concentrations are nearly opposite in ICF vs ECF.

Major Anions (Negative Ions)

  • Chloride (Cl⁻) → higher in ECF

  • Phosphate → higher in ICF (major intracellular anion)

Plasma and interstitial fluid have similar electrolyte levels, but:

  • Plasma contains higher protein levels than interstitial fluid.

⚖ 3. HOMEOSTASIS & FLUID MOVEMENT

Homeostasis = the body’s ability to maintain stable internal conditions despite external changes.

Fluid and electrolytes are essential for proper cellular function.

To maintain balance:

  • Fluids continuously move between compartments.

  • Electrolytes move in and out of cells.

  • Oxygen moves:

    • Plasma → Interstitial fluid → Cells

  • Cellular waste moves:

    • Cells → Interstitial fluid → Plasma → Out of body

Movement occurs across selectively permeable membranes by:

  • Diffusion

  • Osmosis

  • Active transport

  • Filtration

These mechanisms maintain the optimal internal environment necessary for survival.


Methods of transport across plasma membrane

We will now look at different methods of transport across the plasma membrane — methods like bulk transport of large molecules in and out of the cell respectively, and methods like diffusion, which is a passive process, where particles are transported across their concentration gradient.

Bulk transport is a type of active transport in which large molecules or particles are moved into or out of a cell via vesicles. This process requires energy (ATP) and is used for substances that are too large to pass through membrane proteins. There are two main types of bulk transport:

  • Endocytosis — transport of substances into the cell, where a portion of the cell membrane folds in on itself and surrounds the particles outside the cell. A vesicle then forms. The vesicle can take the particles inside the cell and release them into the cytoplasm.  

  • Exocytosis — transport out of the cell, a vesicle containing waste or cell particles moves towards the cell membrane. The membrane surrounding the vesicle fuses with the cell membrane. The contents of the vesicle are released or secreted from the cell.


1⃣ Plasma Membrane & Interstitial Fluid

Cells are surrounded by interstitial fluid, which is derived from blood. This fluid contains amino acids, sugars, fatty acids, vitamins, hormones, neurotransmitters, salts, and waste products — essentially a nutrient-rich environment around every cell.

To remain healthy, each cell must take in exactly what it needs and remove what it does not. This is controlled by the plasma membrane, which is a selectively (differentially) permeable barrier. It allows nutrients and oxygen to enter, allows waste products to leave, and blocks undesirable substances.

There is constant movement across the membrane, either directly through the lipid bilayer or through protein channels or carrier proteins embedded in the membrane.

2⃣ Passive Transport (No Energy Required)

Passive transport moves substances without energy (ATP) and always down their concentration gradient (high → low concentration).

Diffusion is the movement of solutes from high to low concentration until evenly distributed. For example, oxygen diffuses into cells.

Osmosis is a special type of diffusion involving water. If a solute cannot cross the membrane, water moves across instead. Water moves from an area of low solute concentration (more water) to an area of high solute concentration (less water) until concentrations balance.

Key idea: Passive transport requires no ATP and moves substances along their gradient.

3⃣ Active Transport (Requires Energy)

Active transport requires metabolic energy (ATP) and moves substances against the concentration gradient (low → high concentration).

This process requires a carrier protein (pump) that binds specifically and reversibly to the substance being transported. ATP is hydrolyzed into ADP, which causes phosphorylation of the transport protein. This phosphorylation changes the protein’s shape, allowing it to move the solute across the membrane and release it on the other side.

Important ions that rely on active transport include sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), and hydrogen (H⁺). These pumps create unequal distributions of ions across the membrane, which are essential for nerve impulses, muscle contraction, and overall cellular function.



1⃣ Diffusion — Basic Principle

Diffusion is the movement of substances from an area of high concentration to low concentration until evenly distributed. It requires no energy and is driven by the kinetic energy of the molecules themselves.

The speed of diffusion depends on:

  • Molecule size (smaller = faster; oxygen moves faster than glucose)

  • Temperature (higher temperature = faster movement)

  • Concentration gradient (greater difference = faster diffusion)

2⃣ Simple Diffusion vs Facilitated Diffusion

Simple diffusion occurs when non-polar or lipid-soluble substances pass directly through the lipid bilayer. Examples include:

  • Oxygen (O₂)

  • Carbon dioxide (CO₂)

  • Fat-soluble vitamins

Some substances cannot pass through the lipid bilayer because they are polar or too large. These use facilitated diffusion, which is still passive (no ATP required) but requires membrane proteins.

There are two types:

Channel-mediated facilitated diffusion

  • Uses transmembrane channel proteins

  • Forms water-filled pores

  • Usually transports ions or water

  • Selective based on size and charge of amino acids lining the channel

Carrier-mediated facilitated diffusion

  • Uses carrier proteins

  • Specific for certain molecules (e.g., glucose, amino acids)

  • Protein changes shape to bind and release the substance

3⃣ Channels & Regulation

Some channels are leak channels, which are always open and allow ions or water to move according to their concentration gradient.

Other channels are gated channels, which open and close in response to signals:

  • Chemically (ligand) gated → open when a chemical (e.g., neurotransmitter) binds

  • Voltage gated → open in response to changes in membrane potential

  • Mechanically gated → open due to physical deformation (e.g., touch or pressure receptors)

When these channels open, ions diffuse rapidly from high to low concentration. These channels are especially important in nerve signalling.


1⃣Osmosis

Osmosis is a special type of diffusion that occurs when the solute cannot cross the membrane. Instead, water moves across a selectively permeable membrane to balance solute concentrations. Water moves from low solute concentration (more water) to high solute concentration (less water). It is passive and requires no ATP.

2⃣Osmolarity (Driving Force)

The driving force of osmosis is osmolarity, which refers to the total number of solute particles per litre of solution (osmol/L), not their size. Water movement depends on the number of dissolved particles because each particle displaces water.

  • NaCl dissociates into Na⁺ and Cl⁻.
    1 mole NaCl → 2 particles →     2 osmol/L

  • Glucose does not dissociate.
    1 mole glucose → 1 particle →     1 osmol/L

  • 1 mole NaCl + 1 mole glucose → 3 particles → 3 osmol/L

Inside cells, osmolarity is about 300 mOsm/L. Water moves depending on the difference between intracellular and extracellular osmolarity.

3⃣Tonicity & Effects on Cells

Water movement changes cell volume. This effect is called tonicity, which is a solution’s ability to change a cell’s shape by altering its internal water volume.

  • Isotonic (≈300 mOsm/L): Same as inside the cell. No net water movement. Cell keeps normal shape.
    Examples:     0.9% saline, 5% glucose solution.

  • Hypertonic (higher than 300 mOsm/L): More solute outside. Water moves out. Cell shrinks (crenates). Used clinically to reduce swelling.

  • Hypotonic (lower than 300 mOsm/L): Less solute outside. Water moves in. Cell swells and may burst (lysis). Used carefully to rehydrate tissues.