# The Need for Cellular Respiration ## Why Cells Need Respiration - Living organisms are made up of cells that constantly carry out **activities and processes** to maintain life - All work in a living organism requires **energy** and **usable carbon compounds**

Cellular respiration is the process that releases energy stored in organic molecules and transfers it to ATP

## Essential Work Within Organisms - Cells perform many types of work that all demand energy | **Type of work** | **Examples** | |------------------|--------------| | Transporting substances across membranes | • Active transport using the sodium-potassium pump in cell membranes
• Exocytosis of digested bacteria from white blood cells | | Anabolic reactions | • Synthesis of DNA from nucleotides
• Synthesis of protein from amino acids | | Movement | • Cellular movement of chromosomes via the spindle
• Mechanical contraction of muscles | | Maintaining body temperature | • Only occurs in mammals and birds | ### Active Transport Example - Think of the **sodium-potassium pump** as a biological revolving door that uses energy tickets (ATP) - Three sodium ions get pushed out of the cell while two potassium ions are pulled in - Both ion movements go **against their concentration gradients** – like pushing objects uphill - This requires a carrier protein **and ATP** to power the transport

Energy is never "created" – it is transformed from one form to another according to the laws of thermodynamics

## The Source of Energy & Materials ### The Sun as Primary Energy Source - For nearly all organisms, the **sun** is the ultimate source of energy - Light energy from the sun is **transformed** (not created) into chemical potential energy through photosynthesis ### Photosynthesis Stores Energy - Light energy → chemical potential energy stored in carbohydrates - Carbohydrates can then be: - **Broken down** in respiration to synthesize ATP - **Combined and modified** to form all usable organic molecules essential for metabolic processes within the plant ### Two Types of Organisms

**Autotrophs** are organisms that synthesise their own usable carbon compounds from carbon dioxide via photosynthesis

**Heterotrophs** cannot make their own usable carbon compounds and must obtain pre-made organic molecules from their food

## Respiration Releases Energy - Respiration in all living cells releases energy from the **breakdown of organic molecules** - Chemical potential energy from nutrient molecules (carbohydrates, fats, proteins) is transferred into a **usable energy form** through ATP synthesis - ATP can then be used for work within the organism ### Glucose Respiration Summary $$\text{glucose + oxygen} \rightarrow \text{carbon dioxide + water + energy}$$ $$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + 2870\text{kJ}$$

Approximately 2870 kJ of energy is released per mole of glucose respired aerobically

## Energy Transfer Between Autotrophs & Heterotrophs - **Photosynthesis** (in autotrophs like plants): - Light energy + carbon dioxide + water → organic compounds + oxygen - Energy is **stored** in chemical bonds - **Cellular respiration** (in all living cells): - Organic compounds + oxygen → carbon dioxide + water + ATP - Energy is **released** from chemical bonds ### The Cycle in Action - Imagine a forest ecosystem: - Trees (autotrophs) capture sunlight and produce glucose through photosynthesis, releasing oxygen as a by-product - A deer (heterotroph) eats leaves from the tree, obtaining pre-made organic compounds - Both the tree and deer use cellular respiration to break down glucose and release energy as ATP - Carbon dioxide from respiration returns to the atmosphere, where plants can use it again for photosynthesis - This creates a continuous cycle of energy transformation and material recycling

Photosynthesis is carried out by the first organism in a food chain (producers) while respiration occurs in all living organisms

## Key Examples of Energy-Requiring Processes ### The Sodium-Potassium Pump - Found on many cell membranes - A prime example of **active transport** requiring ATP - For every cycle: - **3 sodium ions** are transported out of the cell - **2 potassium ions** are transported into the cell - Both movements occur **against their concentration gradients** - Critical for: - Maintaining cell volume - Generating electrical signals in nerve and muscle cells - Creating concentration gradients for secondary transport ### Muscle Movement and Contraction - Muscles require **substantial amounts of energy** to function - Think of a sprinter exploding off the starting blocks: - Muscle fibres rapidly contract and relax - Thousands of protein filaments slide past each other - Each sliding movement requires ATP molecules to be broken down - During intense exercise, muscles can consume ATP at an enormous rate - This is why respiration rate increases dramatically during physical activity

More active cell types (like muscle cells) have more mitochondria per cell than less active cells (like fat cells)

## The Relationship Between Photosynthesis and Respiration - These two processes are **complementary** – the products of one become the reactants of the other - **Photosynthesis:** - Takes in: carbon dioxide + water + light energy - Produces: glucose + oxygen - **Respiration:** - Takes in: glucose + oxygen - Produces: carbon dioxide + water + ATP ### Material Cycling - Carbon dioxide released during respiration can be used by autotrophs in photosynthesis - Oxygen produced during photosynthesis can be used by all organisms in aerobic respiration - This interconnection links autotrophs and heterotrophs in ecosystems - Energy flows **one way** (from sun → chemical energy → heat) - Materials **cycle** between organisms and the environment

Energy cannot be created or destroyed, only transformed – be careful not to say energy is "created" in photosynthesis or respiration

## Why ATP Matters - ATP (adenosine triphosphate) is the **universal energy currency** of cells - All the energy-requiring processes mentioned need ATP: - Active transport mechanisms - Synthesis reactions (making proteins, DNA, etc.) - Movement and contraction - Maintaining body temperature in endotherms - Without cellular respiration to generate ATP, none of these essential processes could occur - Life depends on the continuous production and use of ATP