C1.2 Cell Respiration

What is ATP, & what is it composed of?

ATP (adenosine triphosphate) is a nucleotide composed of:

• Nitrogenous base - Adenine 

• Pentose sugar - Ribose 

• 3 phosphate groups

Where is ATP stored?

• ATP is stored in the bonds between phosphate groups

  • Especially high-energy bond between 2nd & 3rd phosphate (terminal bond)

What properties make ATP ideal?

Property	Why It’s Useful
Small & soluble	Easily moves around inside cells
Releases energy in small amounts	Prevents waste of energy as heat
Rapid breakdown & reformation	Can be recycled quickly & reused
Universal molecule	Used in all types of cells, across all life forms
Couples w/ many reactions	Drives both anabolic & catabolic processes

State the processes requiring ATP

1) Active transport across membranes

2) Anabolic reactions (building macromolecules)

3) Movement within or by cells

ATP-powered process: Describe ATP’s role in active transport, & give an example (3)

• ATP provides energy for carrier proteins (pumps) that move substances against their concentration gradient

• Without ATP, substances would only move passively (with the gradient)

Example: Sodium-potassium pump in nerve cells 

ATP-powered process: Describe ATP’s role in anabolic reactions, & give examples (4)

• ATP powers condensation reactions that build complex molecules from smaller ones

Examples: 

• Protein synthesis (from amino acids) 

• DNA/RNA synthesis (from nucleotides) 

• Glycogen synthesis (from glucose)

ATP-powered process: Give examples of ATP’s role in movement within/by cells (4)

Type of Movement	ATP Role
Chromosome movement	Powers spindle fibers during mitosis and meiosis
Cytoplasmic streaming	Helps move organelles through the cytoplasm
Muscle contraction	ATP is needed to detach myosin heads from actin
Flagella/cilia movement	Example: Sperm movement  ATP powers flagellum’s motor proteins like dynein

What does ADP stand for?

Adenosine diphosphate

What does Pi stand for?

Inorganic phosphate

Describe how ATP releases & stores energy (8)

1) Hydrolysis reaction: 

• ATP → ADP (adenosine diphosphate) + Pi (inorganic phosphate) + Energy 

  • A water molecule is used to break the bond between last two phosphate groups

  • Energy is released & used immediately for cellular processes 

2) Synthesis (Phosphorylation) reaction: 

• ADP + Pi + Energy → ATP

  • Energy is used to rejoin the phosphate group to ADP

  • This energy is stored in the new ATP molecule 

Define cell respiration

A controlled release of energy from organic compounds in cells, used to produce ATP

Describe the process of cell respiration (3)

1. Carbon compounds act as substrates that are broken down in a series of enzyme-catalysed steps:

• Principal substrates: Glucose & Fatty Acids

• Others (e.g. proteins)
  

2. Energy released from these reactions is used to convert ADP + Pi into ATP

3. In aerobic cell respiration, ATP is produced w/ CO₂ & H₂O as waste products

Describe the differences between cell respiration & gas exchange (4×2)

Feature	Cell Respiration	Gas Exchange
What it is	Chemical reactions that release energy	Physical process of moving gases in/out of cells
Purpose	To make ATP	To supply O₂ & remove CO₂
Location	Cytoplasm & mitochondria	Across cell membranes (e.g., lungs, leaves)
Gases used/produced	Uses O₂ (aerobic) & produces CO₂	Brings in O₂ & expels CO₂

What is the word equation for aerobic respiration?

Glucose + oxygen → carbon dioxide + water + energy (ATP)

What is the word equation for anaerobic respiration?

Glucose → lactic acid + energy (ATP)

Describe the differences between aerobic & anaerobic cell respiration in humans (5×2)

Feature	Aerobic Respiration	Anaerobic Respiration
Oxygen required?	✅	❌
Main substrate	Glucose (can also use fatty acids, amino acids)	Glucose only
ATP yield	High (~36–38 ATP per glucose)	Low (only 2 ATP per glucose)
Waste products	Carbon dioxide (CO₂) + water (H₂O)	Lactic acid (lactate)
Where in the cell?	Starts in cytoplasm (glycolysis) Continues & finishes in mitochondria (Krebs cycle + electron transport chain)	Entirely in cytoplasm (no mitochondria needed)

What does the rate of cell respiration refer to, & how is it measured? (6)

• How quickly a cell produces ATP by breaking down organic compounds (glucose or others)

• It is often measured by:

  • Oxygen consumption 

  • Carbon dioxide production 

  • Change in pH 

  • Heat released

Describe the methods of measuring rate of cell respiration

Method	Factor measured
Respirometer (with seeds or insects)	Volume of oxygen consumed
CO₂ probe	Rate of carbon dioxide production
pH meter in yeast/glucose solution	Drop in pH as CO₂ forms carbonic acid
Calorimeter	Heat produced by respiring organisms

Describe how rate of cell respiration varies with different variables

Variable	Effect
Temperature	Higher temps increase enzyme activity → faster rate until optimum Too hot = denaturation
pH	Each enzyme has an optimal pH. Deviations slow down or stop respiration.
Glucose concentration	More glucose = more fuel for higher rate (up to a saturation point)
Oxygen availability	Needed for aerobic respiration  Limited oxygen shifts cells to slower anaerobic respiration
Enzyme concentration	More enzymes = faster reaction, as long as substrate is available
Cell type or tissue type	Some cells (like muscles) have more mitochondria = higher respiration rates