Biochemical Processes in Cells Revision Notes for HSC SSCE Biology

Biochemical Processes in Cells

Introduction

Living cells carry out complex chemical reactions to maintain life. Eukaryotic cells contain many membrane-bound organelles, each housing specific enzymes that catalyse particular biochemical processes. These reactions within cells require energy, which is provided in the form of ATP (adenosine triphosphate).

The two major biochemical processes that occur in living cells are:

Photosynthesis - occurs in plant cells
Cellular respiration - occurs in all living cells

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For example, lysosomes use strong digestive enzymes in an acidic environment to break down compounds brought into the cell. The lysosome membrane prevents these powerful enzymes from damaging other parts of the cell, demonstrating how organelles create specialized environments for specific biochemical processes.

Energy transfer between reactions

The role of glucose and energy

Energy provision is essential for all life processes to occur. All cells use glucose as their primary energy source to drive the thousands of chemical reactions happening constantly within them. When glucose breaks down through aerobic cellular respiration, it releases a large amount of stored energy.

This released energy is captured and stored in small packets within many high-energy molecules called adenosine triphosphate (ATP). Cells access the energy in ATP to power all cellular functions. ATP is a small, mobile molecule that can deliver energy wherever it is needed in the cell.

ATP structure and the ATP-ADP cycle

ATP is composed of:

Adenosine - a complex molecule consisting of adenine attached to a ribose sugar group
Three phosphate groups - hence the name adenosine tri-phosphate

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The bond attaching the third phosphate group is a high-energy bond. When the cell requires energy, this bond breaks and releases its stored energy. This produces ADP (adenosine diphosphate) and a free phosphate group ( $P$ ).

To store energy produced by the cell, a free phosphate group attaches to ADP through a high-energy bond, reforming ATP. This creates a continuous cycle of energy storage and release between ADP, free phosphate groups, and ATP.

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The ATP-ADP Cycle:

The energy storage and release cycle can be summarised as:

Energy release: $\text{ATP} \rightarrow \text{ADP} + P + \text{energy}$
Energy storage: $\text{ADP} + P + \text{energy} \rightarrow \text{ATP}$

This cycle is fundamental to all cellular processes and is often called the energy currency of the cell.

Photosynthesis

What is photosynthesis?

Photosynthesis is the process by which plants use light energy (usually from the sun) to produce food. During this process:

Light energy is trapped by chlorophyll (contained in the chloroplast)
This energy breaks apart water ( $H_2O$ ) and carbon dioxide ( $CO_2$ ) molecules
The atoms are rebuilt into oxygen ( $O_2$ ), energy-storing glucose molecules ( $C_6H_{12}O_6$ ), and water

Photosynthesis equations

Photosynthesis can be summarised by the word equation:

$\text{carbon dioxide} + \text{water} \xrightarrow[\text{chlorophyll}]{\text{light energy}} \text{glucose} + \text{oxygen}$

The balanced chemical equation is:

$6CO_2 + 6H_2O \xrightarrow[\text{chlorophyll}]{\text{light energy}} C_6H_{12}O_6 + 6O_2$

The two stages of photosynthesis

While the general equation summarises photosynthesis, the actual process consists of two stages, each involving many chemical reactions. These reactions occur in the chloroplasts of green plant cells and in some photosynthetic bacteria.

Phase 1: the light-dependent stage

The light-dependent stage (also called photolysis, meaning "light-splitting") occurs in the grana of chloroplasts. Here's what happens:

Chlorophyll in the thylakoid membranes absorbs light energy
This energy splits water ( $H_2O$ $H_{2} O$ ) molecules into:
- Hydrogen ions ( $H^+$ )
- Oxygen ( $O_2$ )
The oxygen is released into the atmosphere
The hydrogen ions are carried to the next phase
ATP is also formed at this stage

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Key point: This stage requires light and produces oxygen as a by-product. Without light, this stage cannot proceed, which is why photosynthesis only occurs during daylight hours.

Phase 2: the light-independent stage

The light-independent stage (also called carbon fixation) occurs in the stroma of chloroplasts. Here's what happens:

Carbon dioxide ( $CO_2$ ) combines with the hydrogen ions produced in the light-dependent stage
This forms glucose ( $C_6H_{12}O_6$ )
The ATP formed in the light-dependent stage provides the energy for this reaction
No light is required in this stage (hence "light-independent")

Uses of glucose from photosynthesis

Glucose produced by photosynthesis can be converted by plants into:

Complex carbohydrates
Lipids
Proteins

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These end-products are stored by plants and serve as the source of organic nutrients for:

The plants themselves
Consumer organisms that eat the plants

This makes photosynthesis the foundation of most food chains on Earth, as it converts light energy into chemical energy that can be used by other organisms.

Cellular respiration

Overview of cellular respiration

All organisms break down glucose to release energy for cellular metabolism. Glucose can be broken down in two ways:

Aerobic cellular respiration - in the presence of oxygen
Anaerobic cellular respiration - in the absence of oxygen

These processes produce different products and release different amounts of energy.

Anaerobic cellular respiration

Many organisms (such as some bacteria and archaea) live in oxygen-poor environments. Sometimes cells (like muscle cells) cannot obtain enough oxygen but still need to release energy through anaerobic pathways in the cytosol. Two common biochemical pathways enable energy production without oxygen:

Alcoholic fermentation

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Example: Alcoholic Fermentation

In alcoholic fermentation, yeast, many bacteria, and plants (or parts of plants) carry out anaerobic respiration. This process:

Breaks down glucose in the absence of oxygen
Forms ethanol (an alcohol) and carbon dioxide
Produces two molecules of ATP

Word equation: glucose → ethanol + carbon dioxide + adenosine triphosphate

Chemical equation: $C_6H_{12}O_6 \rightarrow 2CH_3CH_2OH + 2CO_2 + 2\text{ ATP}$

Human applications: Humans use this process to produce beer, wine, and bread.

Lactic acid fermentation

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Example: Lactic Acid Fermentation

Lactic acid fermentation occurs when organisms cannot produce enough energy through aerobic respiration. This happens during strenuous exercise when our bodies cannot deliver enough oxygen to muscles to meet energy demands. In this process:

One glucose molecule breaks down in the absence of oxygen
Produces two molecules of lactic acid
Produces two molecules of ATP

Word equation: glucose → lactic acid + adenosine triphosphate

Chemical equation: $C_6H_{12}O_6 \rightarrow 2CH_3CH(OH)COOH + 2\text{ ATP}$

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Important note: Once strenuous exercise finishes and more oxygen becomes available, the lactic acid converts to a compound called pyruvate. This pyruvate is then converted by aerobic respiration into carbon dioxide, water, and energy (ATP).

Aerobic cellular respiration

Aerobic cellular respiration is a chain of many biochemical reactions that occur in cells in the presence of oxygen.

General equations for aerobic respiration

Word equation: $\text{glucose} + \text{oxygen} \xrightarrow{\text{many chemical reactions}} \text{carbon dioxide} + \text{water} + \text{energy (ATP)}$

Balanced equation: $C_6H_{12}O_6 + 6O_2 + \text{ADP} + P \xrightarrow{\text{many chemical reactions}} 6CO_2 + 6H_2O + 36\text{ ATP}$

There are at least 20 separate reactions, each catalysed by a specific enzyme, that make up this overall pathway.

Stage 1: Glycolysis

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The first step in breaking down glucose is glycolysis. This process:

Occurs in the cytosol of the cell
Breaks down glucose (a six-carbon molecule) into two pyruvate molecules (each with three carbon atoms)
Releases energy in the form of two molecules of ATP
Actually consists of 10 smaller reactions, each catalysed by a specific enzyme

Stage 2: Mitochondrial reactions

The two pyruvate molecules formed from one glucose molecule then enter the mitochondria, where the remaining reactions of aerobic cellular respiration occur. In the mitochondria:

Each reaction is catalysed by a specific enzyme
More energy is produced and released as 34 ATP molecules
Carbon dioxide and water are also produced
Total ATP production: $2 + 34 = 36$ ATP molecules per glucose molecule

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Summary of ATP production:

Glycolysis (in cytoplasm): 2 ATP
Mitochondrial reactions: 34 ATP
Total per glucose molecule: 36 ATP

This is 18 times more efficient than anaerobic respiration, which is why aerobic respiration is the preferred pathway when oxygen is available.

The relationship between photosynthesis and aerobic cellular respiration

How the processes are connected

Photosynthesis and aerobic cellular respiration are closely related and interdependent processes. When you look at their general equations, they might appear to be the reverse of each other. However, this is not the case because each process consists of very different series of chemical reactions.

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The Interdependence of Photosynthesis and Respiration:

Despite their differences, these processes are intimately connected:

The products of photosynthesis (glucose and oxygen) are used as inputs for aerobic cellular respiration
The products of aerobic cellular respiration (carbon dioxide and water) are used as inputs for photosynthesis

This creates a vital cycle that sustains life on Earth.

Where both processes occur

In plant cells that contain both chloroplasts and mitochondria, both processes occur in the same cell. This creates an efficient energy cycle within the cell.

In plant cells that do not contain chloroplasts (such as root cells), the products of photosynthesis (glucose and oxygen) must be transported from other cells that do perform photosynthesis. These products are then used for aerobic cellular respiration to occur.

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Energy distribution: Of the ATP energy produced through respiration:

40% is used for cell functions
60% is released as heat energy

This heat energy helps maintain body temperature in warm-blooded organisms.

Removal of cellular products and wastes

What are cellular wastes?

Wastes are products of cellular reactions that are not required by the organism. They need to be:

Transported to other cells, or
Removed from the cell for efficient functioning

Methods of waste removal

Diffusion

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Removal from cells usually occurs by diffusion through the cell membrane. Substances removed this way include:

Oxygen and carbon dioxide gases (easily diffuse through the membrane)
Wastes formed from protein breakdown
Water-soluble waste chemical products

Osmosis

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Excess water not required by the cell moves out through osmosis. Water-soluble wastes are removed at the same time as the water.

Special note: Oily, fat-soluble substances cannot dissolve in water. The cell attaches these substances to small electrically charged molecules, making them water-soluble so they can be removed when water diffuses out.

Lysosomal breakdown

When parts of the cell break down, wear out, or die, they need to be removed. Lysosomes (which contain digestive enzymes) break down:

Old cell parts
Other cellular waste

Any wastes that cannot be eliminated by the lysosome are packaged and removed by exocytosis.

Exocytosis

Exocytosis is used to remove:

Wastes packaged in vesicles that lysosomes cannot break down
Useful cellular products (not wastes) such as hormones, enzymes, and mucus

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Important distinction:

Excretion: when wastes leave a cell
Secretion: when useful products leave a cell to be used elsewhere

Understanding this difference is crucial for comprehending cellular processes.

Remember!

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Key Points to Remember:

ATP is the energy currency of cells. It stores energy in high-energy phosphate bonds and delivers it wherever needed through the ATP-ADP cycle.
Photosynthesis converts light energy into chemical energy. It occurs in two stages: the light-dependent stage (in grana) splits water to produce oxygen and hydrogen ions, while the light-independent stage (in stroma) uses these hydrogen ions with carbon dioxide to produce glucose.
Aerobic respiration produces 36 ATP molecules per glucose. Glycolysis in the cytoplasm produces 2 ATP, while mitochondrial reactions produce an additional 34 ATP, totalling 36 ATP per glucose molecule.
Photosynthesis and respiration are interdependent. The products of one process serve as the reactants for the other, creating a vital cycle that sustains life on Earth.
Cells have multiple waste removal systems. Wastes can be removed through diffusion, osmosis, lysosomal breakdown, or exocytosis, ensuring efficient cellular function.

Biochemical Processes in Cells (HSC SSCE Biology): Revision Notes