Biology | Cellular energetics

Chapter: Cellular Energetics - The Energy of Life!

Hi everyone! Welcome to one of the most exciting topics in Biology: Cellular Energetics. Ever wonder where you get the energy to think, run, or even just breathe? Or how plants seem to make their own food out of thin air and sunlight? This chapter holds the answers!

We're going to explore the amazing chemical reactions happening inside every living cell that capture, store, and release energy. Think of it as learning the secrets of the cell's power plants and factories. Don't worry if it sounds complicated; we'll break it down step-by-step with simple examples. Let's get started!

1. Metabolism: The Cell's Chemical Factory

Every living organism is like a busy city, and the cells are the factories. All the chemical reactions happening inside these factories are collectively called metabolism. Metabolism has two main "departments": one for building things up, and one for breaking things down.

Anabolism: Building Up

Anabolism is all about building larger, complex molecules from smaller, simpler ones. This process requires energy.

Analogy: Think of building a LEGO castle (a complex molecule) from individual LEGO bricks (simple molecules). You need to put in energy to connect the bricks!

• Example in cells: Photosynthesis, where plants use carbon dioxide and water (simple molecules) to build glucose (a complex molecule).

Catabolism: Breaking Down

Catabolism is the opposite. It's about breaking down large, complex molecules into smaller, simpler ones. This process releases energy.

Analogy: Imagine smashing your LEGO castle back into individual bricks. Energy is released (and you can hear it!).

• Example in cells: Cellular respiration, where glucose (a complex molecule) is broken down to release energy for the cell to use.

Key Takeaway: Metabolism

Think Anabolism = Adding together (building up, requires energy) and Catabolism = Cutting apart (breaking down, releases energy).

2. Enzymes: The Super-Fast Workers

Metabolic reactions need to happen incredibly fast to sustain life. That's where enzymes come in. They are biological catalysts – special proteins that speed up chemical reactions without being used up themselves.

Properties and Roles of Enzymes

• They are catalysts, meaning they speed up reactions.
• They are highly specific. Each enzyme is like a key that fits only one specific lock (substrate).
• They are affected by temperature and pH.
• They are essential for almost every process in a cell, from digestion to DNA replication.

The Active Site and Specificity

The magic of an enzyme's specificity comes from its unique 3D shape, which includes a special region called the active site. The shape of the active site is complementary to the shape of its specific substrate molecule(s).

Analogy: The "Lock and Key" model. The enzyme is the lock, and the substrate is the key. Only the correctly shaped key (substrate) can fit into the keyhole (active site).

When the substrate binds to the active site, an enzyme-substrate complex is formed, and the reaction happens much faster.

Factors Affecting Enzyme Activity

Enzymes are picky about their working conditions! If the conditions aren't right, they can't work properly.

1. Temperature:

• As temperature increases, enzyme activity increases because molecules move faster and collide more often.
• However, every enzyme has an optimum temperature where it works best.
• If the temperature gets too high, the enzyme's shape changes permanently. This is called denaturation. A denatured enzyme's active site no longer fits its substrate, so it stops working.

2. pH:

• Similar to temperature, each enzyme has an optimum pH.
• Extreme pH values (either too acidic or too alkaline) can change the shape of the enzyme and cause it to denature.
• Example: Pepsin, an enzyme in your stomach, works best at a very acidic pH of around 2, while trypsin in the small intestine prefers a slightly alkaline pH of around 8.

3. Inhibitors:

• Inhibitors are substances that slow down or stop enzymatic reactions. They can interfere with the enzyme's active site.

Enzymes in Everyday Life

We use enzymes all the time!

• Biological washing powders: Contain enzymes like proteases and lipases to break down protein and fat stains on clothes.
• Food industry: Pectinase is used to clarify fruit juices, and rennet (containing enzymes) is used to make cheese.

Did you know?

Without enzymes, the chemical reactions in your body would be so slow that you wouldn't be able to live! A single reaction could take millions of years to happen on its own.

Key Takeaway: Enzymes

Enzymes are specific protein catalysts with an active site. Their activity is maximised at optimum temperature and pH. Extreme conditions cause them to denature.

3. Photosynthesis: Making Food from Sunlight

Photosynthesis is the amazing anabolic process that plants, algae, and some bacteria use to convert light energy into chemical energy in the form of glucose (food). It's the foundation of almost all life on Earth!

The overall word equation is:

Carbon dioxide + Water --(in the presence of light & chlorophyll)--> Glucose + Oxygen

Where Does Photosynthesis Happen?

The main site is the leaf, which is perfectly adapted for the job. Inside the leaf cells, the process happens in tiny organelles called chloroplasts.

• Chloroplasts contain chlorophyll, the green pigment that absorbs light energy.
• They have two key areas:
  - The grana (stacks of discs) where the first stage happens.
  - The stroma (a fluid-filled space) where the second stage happens.

The Two Stages of Photosynthesis

Think of photosynthesis as a two-part factory assembly line. Don't worry, you only need to know the outline of the major steps!

Stage 1: Photochemical Reactions (Light-Dependent Reactions)

This stage happens in the grana and requires light.

1. Light Absorption: Chlorophyll absorbs light energy.
2. Photolysis of Water: The absorbed light energy is used to split water molecules (H₂O) into oxygen, protons (H⁺), and electrons. The oxygen is released as a waste product (lucky for us!).
3. Generation of ATP and NADPH: The energy from the light and the products from splitting water are used to generate two crucial energy-carrying molecules: ATP (the main energy currency) and NADPH (an electron carrier).

Think of ATP and NADPH as charged-up batteries and delivery trucks, ready to power the next stage.

Stage 2: Carbon Fixation (The Calvin Cycle / Light-Independent Reactions)

This stage happens in the stroma and does not directly require light, but it depends on the products from the light-dependent stage (ATP and NADPH).

1. Carbon Dioxide Fixation: Carbon dioxide (CO₂) from the air enters the cycle and combines with a 5-carbon acceptor molecule to form an unstable 6-carbon compound, which immediately splits into two 3-carbon (3-C) compounds.
2. Reduction: Using the energy from ATP and the reducing power of NADPH (from Stage 1), the 3-C compounds are converted into another 3-C compound (triose phosphate).
3. Glucose Formation: Some of these triose phosphates leave the cycle and are used to make glucose and other organic molecules.
4. Regeneration: Most of the triose phosphates are used to regenerate the original 5-carbon acceptor molecule, using more ATP. This allows the cycle to continue.

What happens to the glucose?

Plants are smart! They don't just use glucose for energy. They can convert it into:

• Starch for storage (like in potatoes).
• Cellulose to build cell walls.
• Lipids (fats and oils) for storage in seeds.
• Proteins for growth (by adding nitrogen and other elements from the soil).

Factors Affecting the Rate of Photosynthesis

The rate can be limited by certain factors. The one in shortest supply is called the limiting factor.

• Light Intensity: As light intensity increases, the rate increases, up to a certain point. After that, another factor (like CO₂ concentration) becomes limiting.
• Carbon Dioxide Concentration: As CO₂ concentration increases, the rate increases, until another factor becomes limiting.

Key Takeaway: Photosynthesis

Photosynthesis uses light energy, water and CO₂ to make glucose. It has two stages: 1. Light-dependent reactions (making ATP & NADPH) and 2. The Calvin Cycle (using ATP & NADPH to fix CO₂ into sugar). The rate is affected by limiting factors like light and CO₂ levels.

4. Cellular Respiration: Releasing Energy for Life

Cellular respiration is the main catabolic process that breaks down glucose and other food molecules to release chemical energy. This energy is then stored in the form of ATP.

ATP: The Cell's Energy Currency

ATP (Adenosine Triphosphate) is the universal energy molecule for all living things. It's like a tiny, rechargeable battery. When a cell needs energy to do work (like muscle contraction or active transport), it 'spends' ATP, which breaks down to release energy. Respiration is the process that 'recharges' these batteries.

There are two main types of respiration: aerobic (with oxygen) and anaerobic (without oxygen).

Aerobic Respiration: The Main Pathway

This is the most efficient way to get energy from glucose, and it requires oxygen. It happens in three main stages.

The overall equation is:

$$ \text{Glucose} + \text{Oxygen} \rightarrow \text{Carbon Dioxide} + \text{Water} + \text{Large amount of ATP} $$

Stage 1: Glycolysis

• Location: Cytoplasm.
• Requires Oxygen? No.
• What happens? A 6-carbon glucose molecule is broken down into two 3-carbon molecules called pyruvate.
• Energy produced: A small net gain of ATP and some NADH (another electron carrier, like NADPH's cousin).

Stage 2: The Krebs Cycle

• Location: Mitochondrion.
• Requires Oxygen? Yes (indirectly).
• What happens? Before the cycle, pyruvate is converted into a 2-carbon molecule called acetyl-CoA.
  - Acetyl-CoA (2-C) combines with a 4-carbon (4-C) molecule to form a 6-carbon (6-C) molecule.
  - Through a series of reactions, this 6-C molecule is broken down, releasing carbon dioxide and regenerating the original 4-C molecule.
• Energy produced: A small amount of ATP, and a large amount of energy carriers (NADH and FADH₂).

Stage 3: Oxidative Phosphorylation

• Location: Inner membrane of the mitochondrion.
• Requires Oxygen? Yes, directly.
• What happens? This is the big energy payoff stage!
  - The energy carriers (NADH and FADH₂) from the first two stages drop off high-energy electrons.
  - As these electrons are passed down a chain of proteins, their energy is used to produce a large amount of ATP.
  - At the very end of the chain, oxygen acts as the final electron acceptor, combining with protons to form water.

Quick Review: Aerobic Respiration

Glycolysis (cytoplasm) splits glucose. Krebs Cycle (mitochondrion) finishes breaking it down. Oxidative Phosphorylation (mitochondrion) uses oxygen to make LOTS of ATP.

Anaerobic Respiration: The Backup Plan

What happens when there's no oxygen? The cell can't do the Krebs cycle or oxidative phosphorylation. Instead, it uses anaerobic respiration to produce a small amount of ATP from glycolysis alone. The main purpose is to recycle NADH back to NAD⁺ so that glycolysis can continue.

In Muscle Cells (during strenuous exercise)

When you exercise hard, your muscles can't get oxygen fast enough. They switch to anaerobic respiration.

Pyruvate is converted into lactic acid. The build-up of lactic acid contributes to muscle fatigue and soreness.

In Yeast

Yeast is a single-celled fungus that performs anaerobic respiration in the absence of oxygen.

Pyruvate is converted into ethanol and carbon dioxide. This process, called fermentation, is very useful!

• Industrial applications: The CO₂ produced makes bread rise, and the ethanol produced is used in brewing beer and wine.

Key Takeaway: Respiration

Respiration breaks down glucose to produce ATP. Aerobic respiration uses oxygen, occurs in the cytoplasm and mitochondria, and produces a lot of ATP. Anaerobic respiration does not use oxygen, occurs only in the cytoplasm, and produces much less ATP, along with lactic acid (in muscles) or ethanol and CO₂ (in yeast).

5. Comparing Photosynthesis and Respiration

These two processes are like two sides of the same coin. They are closely linked and complementary.

Feature	Photosynthesis	Aerobic Respiration
Metabolic Process	Anabolic (builds up glucose)	Catabolic (breaks down glucose)
Energy Conversion	Converts light energy to chemical energy	Releases chemical energy from glucose to make ATP
Reactants	Carbon dioxide and water	Glucose and oxygen
Products	Glucose and oxygen	Carbon dioxide, water, and ATP
Location in Eukaryotes	Chloroplasts	Cytoplasm and mitochondria
When it occurs	Only in the presence of light	All the time (day and night)

A final thought: Notice how the products of photosynthesis are the reactants for respiration, and vice versa! This beautiful cycle sustains life on our planet. You've now learned the fundamental principles of how life is powered at the cellular level. Great job!