3.1 Cellular Energetics + Photosynthesis & Cellular Respiration

Enzymes: Structure and Function

Activation Energy

  • Chemical reactions require an initial energy input called activation energy.

  • This energy barrier prevents cells from releasing energy uncontrollably.

  • Activation energy can be overcome by:

-Adding energy (e.g., heat)

-Using a catalyst

Catalysts

  • Catalysts:

-Lower activation energy

-Increase the rate of a reaction

-Are not changed or consumed in the reaction

Enzymes as Biological Catalysts

  • Enzymes are biological catalysts.

  • They work by:

-Changing the shape (conformation) of molecules

-Making molecules more favorable for reactions

Key Enzyme Terms

  • Active Site:

-The specific region of the enzyme where the reaction occurs

  • Substrate:

-The molecule that binds to the enzyme

Environmental Effects on Enzyme Activity

Temperature:

  • Increased temperature:

-Speeds up reactions

-Increases molecular movement and collisions

  • Excessive heat:

-Causes denaturation (loss of structure)

-Prevents enzyme function

pH:

  • Changes in pH:

-Affect hydrogen bonding

-Alter enzyme shape and efficiency

  • Effects may be reversible if conditions return to normal

Concentration Effects

  • Higher substrate concentration:

-Increases reaction rate (more collisions)

  • Higher enzyme concentration:

-Also increases reaction rate

Enzyme Inhibitors

Competitive Inhibitors:

  • Compete with substrate for the active site

  • Block substrate binding

  • Slow or stop the reaction

Noncompetitive Inhibitors:

  • Bind to a different site (not the active site)

  • Change enzyme shape (conformation)

  • Reduce enzyme efficiency

The Role of Energy in Living Systems

Importance of Energy

  • All living organisms require energy to survive.

  • Energy is needed to:

-Maintain organization

-Carry out cellular processes

-Support growth and movement

Bioenergetics and Metabolism

  • Bioenergetics:

-Study of energy transformations in living organisms

  • Metabolism:

  • All chemical reactions involving energy in organisms

  • Includes processes such as:

-Photosynthesis

-Cellular respiration

-Movement

Energy in Living Systems

  • Energy input must exceed energy loss to sustain life.

  • Energy is stored in molecules with chemical bonds, such as:

-Sugars

-Fats

Anabolism vs. Catabolism

Anabolism:

  • Builds molecules

  • Stores energy in chemical bonds

Catabolism:

  • Breaks down molecules

  • Releases energy from chemical bonds

  • These processes are often linked, so released energy is immediately used by cells.

Energy Storage and Efficiency

  • Large molecules (fats and sugars) store large amounts of energy.

  • Breaking them down directly can release more energy than needed.

  • To prevent energy waste:

-Cells convert large energy molecules into smaller, usable forms.

ATP

  • Adenosine triphosphate (ATP) is the main energy molecule used by cells.

  • ATP provides a controlled and efficient way to supply energy.

ATP Breakdown

  • ATP can be broken down into:

-ADP (adenosine diphosphate)

-AMP (adenosine monophosphate)

  • These breakdown processes occur during:

-Respiration

-Fermentation

  • Each step releases usable energy.

ATP and Cellular Processes

  • ATP breakdown is linked to most cellular activities.

  • This allows cells to:

-Efficiently capture and use energy

-Directly power biological processes

The Processes of Photosynthesis

Overview of Photosynthesis

  • Photosynthesis converts light energy into chemical energy (glucose).

  • Overall equation:

6CO₂ + 6H₂O + Light → C₆H₁₂O₆ + 6O₂

  • Inputs:

-Carbon dioxide (CO₂)

-Water (H₂O)

-Light energy

  • Outputs:

-Glucose (C₆H₁₂O₆)

-Oxygen (O₂)

Evolution of Photosynthesis

  • First evolved in cyanobacteria.

  • Led to the oxygenation of Earth’s atmosphere.

  • Forms the basis for photosynthesis in modern plants.

Two Main Phases of Photosynthesis

A. Light-Dependent Reactions (Photolysis)

  • Location: Thylakoid membranes of chloroplasts

  • Key processes:

-Light absorbed by chlorophyll

-Production of:

  • ATP

  • NADPH (electron carrier)

  • Water is split → releases oxygen

B. Light-Independent Reactions (Calvin Cycle)

  • Location: Stroma of chloroplasts

  • Key processes:

Uses:

  • CO₂

  • ATP

  • NADPH

Produces:

3-carbon sugars → combine to form glucose

Importance of Glucose and Carbon Fixation

  • Glucose:

-Used to produce ATP (energy)

-Stores energy for later use

  • Fixed carbon:

-Organic carbon molecules (like sugars)

-Used to build cellular components

  • Calvin cycle:

-Removes CO₂ from atmosphere

-Stores energy in chemical form

Photosystems and Light Harvesting

  • Light-dependent reactions rely on photosystems:

-Photosystem II (PSII)

-Photosystem I (PSI)

  • Located in thylakoid membranes

  • Structure:

-Light-harvesting complex (pigments + proteins)

Non-Cyclic Photophosphorylation

  • Main process of light reactions

  • Produces:

-ATP

-NADPH

  • Involves electron movement from water → NADP⁺

Step-by-Step: Electron Flow

Step 1: Photosystem II (PSII)

  • Light excites electrons

  • Electrons passed to an acceptor

  • Water split to replace electrons:

-Releases O₂

  • Electrons enter electron transport chain (ETC)

Step 2: Electron Transport Chain (ETC)

  • Electrons lose energy as they move

  • Energy used to pump H⁺ ions into thylakoid space

  • Creates a proton gradient

Step 3: ATP Formation (Chemiosmosis)

  • H⁺ ions flow through ATP synthase

  • Drives conversion of ADP → ATP

  • This process is called photophosphorylation

Step 4: Photosystem I (PSI)

  • Electrons arrive at PSI

  • Light re-excites electrons

  • Electrons passed to another ETC

Step 5: NADPH Formation

  • High-energy electrons used to convert:

NADP⁺ → NADPH

Connection to Calvin Cycle

  • ATP and NADPH from light reactions:

-Used in the Calvin cycle

-Help convert CO₂ into glucose

Photosynthesis

Cellular Respiration & Fermentation

  • All forms of life break down biological macromolecules (like sugars and fats) to produce ATP.

  • Two main processes:

-Cellular respiration (uses oxygen)

-Fermentation (does not require oxygen)

  • Both involve transferring energy through electron carriers and metabolic pathways.


Cellular Respiration in Eukaryotes

  • Breaks down glucose into carbon dioxide (CO₂) and water (H₂O).

  • Occurs in multiple stages with energy passed along an electron transport chain (ETC).

Step 1: Glycolysis

  • Location:

Cytosol

  • Process:

1 glucose (6-carbon) → 2 pyruvate (3-carbon each)

  • Products:

2 ATP

2 NADH

  • Key events:

ADP → ATP

NAD⁺ → NADH

Step 2: Pyruvate Oxidation

  • Location:

Mitochondria

  • Process:

Pyruvate → acetyl group → binds to coenzyme A → acetyl CoA

  • Products:

NADH

  • Notes:

No ATP produced

Step 3: Krebs Cycle (Citric Acid Cycle)

  • Location:

Mitochondrial matrix

  • Type:

Aerobic respiration

  • Process:

Acetyl CoA is broken down

  • Products (per cycle):

1 NADH

1 FADH₂

2 CO₂

1 ATP (or GTP)

Step 4: Oxidative Phosphorylation

  • Location:

Inner mitochondrial membrane

  • Includes:

Electron transport chain (ETC)

Chemiosmosis

  • Process:

NADH and FADH₂ donate electrons to the ETC

Energy released pumps H⁺ ions into intermembrane space

Creates a proton gradient

H⁺ flows back through ATP synthase, producing ATP

  • Final electron acceptor:

Oxygen (O₂) → forms water (H₂O)

Cellular Respiration in Prokaryotes

  • Glycolysis, pyruvate oxidation, and citric acid cycle:

Occur in the cytosol

  • Oxidative phosphorylation:

Occurs in the cell membrane

  • Some anaerobic prokaryotes:

Use alternative final electron acceptors (not oxygen)

Special Case: Heat Production

  • In some cells:

Electron transport chain is decoupled from ATP production

Energy is released as heat instead of ATP

Fermentation (Anaerobic Conditions)

  • Occurs when oxygen is not available

  • Only glycolysis continues

  • Pyruvate is converted into:

Lactic acid OR Alcohol (ethanol)

  • Purpose:

Regenerates NAD⁺ from NADH

Allows glycolysis to keep producing ATP

Cellular Respiration

Molecular Diversity & Cellular Response to Environmental Changes

  • Living organisms have multiple, overlapping mechanisms to produce ATP for immediate energy needs.

Adaptations in Energy Production

  • Cells can adjust how they produce ATP based on environmental conditions:

With oxygen → use cellular respiration

Without oxygen → switch to fermentation

Molecular Diversity in Photosynthesis (Plants)

  • Plants contain different types of chlorophyll molecules.

  • These variations allow plants to:

Absorb different wavelengths of light

Maximize energy capture under varying light conditions

Importance of Molecular Diversity

  • The presence of diverse molecules allows organisms to:

-Adapt to changing environments

-Maintain efficient energy production

-Increase chances of survival and reproduction

Connection to Evolution

  • These adaptive mechanisms contribute to:

Natural selection

Long-term evolutionary success of species