Chapter 5: The Working Cell – Energy, Enzymes, and Membrane Function

Chapter 5: The Working Cell

Introduction to Cellular Function

Cells are dynamic units that perform a variety of essential functions necessary for life. These include movement, energy processing, and the production of various products. Understanding how cells harness and regulate energy, utilize enzymes, and control their internal environment is fundamental to biology.

• Movement: Cells can move themselves or their internal components to perform tasks.

• Energy Processing: Cells convert energy from one form to another to sustain life.

• Production of Products: Cells synthesize molecules required for growth, repair, and communication.

Additional info: Cell-based nanotechnology is an emerging field that may use cellular structures to power microscopic robots.

Basic Energy Concepts

Forms and Properties of Energy

Energy is the capacity to cause change and is essential for performing work in biological systems. There are different forms of energy, each with unique properties and roles in cellular processes.

• Kinetic Energy: The energy of motion. Example: Muscle contraction.

• Potential Energy: Stored energy due to an object's location or structure. Example: Chemical bonds in glucose.

• Chemical Energy: A type of potential energy stored in the arrangement of atoms within molecules. Released during chemical reactions.

Conservation of Energy: Energy cannot be created or destroyed, only converted from one form to another.

Heat: A form of kinetic energy resulting from the random motion of atoms and molecules. Energy conversions often generate heat.

Entropy: A measure of disorder or randomness in a system. Every energy conversion increases entropy.

Energy Transformations in Cells

Cellular Respiration and Energy Use

Cells obtain energy by breaking down fuel molecules in a process called cellular respiration. This process releases energy that is stored in a form the cell can use to perform work.

• Cellular Respiration: The energy-releasing chemical breakdown of fuel molecules and the storage of that energy in ATP.

• Humans convert about 34% of food energy to useful work; the rest is released as heat.

Food Calories: A calorie (cal) is the amount of energy needed to raise the temperature of 1 gram of water by 1°C. Food calories are actually kilocalories (1,000 calories).

ATP and Cellular Work

Role and Structure of ATP

ATP (Adenosine Triphosphate) is the primary energy carrier in cells. It acts as an energy shuttle, storing energy from food and releasing it as needed for cellular work.

• ATP consists of adenosine attached to three phosphate groups.

• Energy is released when ATP loses a phosphate group, forming ADP (adenosine diphosphate).

Phosphate Transfer: ATP energizes other molecules by transferring phosphate groups, enabling cellular processes such as shape change, transport, and synthesis of large molecules.

ATP Cycle: Cells continuously recycle ATP. Cellular work spends ATP, which is regenerated from ADP and phosphate using energy from cellular respiration.

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i$

$\text{ADP} + \text{P}_i \rightarrow \text{ATP}$

Additional info: Up to 10 million ATP molecules are consumed and recycled each second in a working muscle cell.

Enzymes and Metabolism

Enzyme Function and Activity

Metabolism is the sum of all chemical reactions in an organism. Most metabolic reactions require enzymes, which are proteins that speed up chemical reactions without being consumed.

• Each enzyme is specific to a particular reaction and substrate.

• Enzymes lower the activation energy required to start a reaction.

• The active site of an enzyme binds the substrate, often changing shape for a snug fit (induced fit).

• Enzymes can be reused for multiple reactions.

Enzyme Inhibitors: Certain molecules can inhibit enzyme activity by binding to the enzyme and disrupting its function, often by blocking the active site or altering its shape.

Membrane Function and Transport

Structure and Role of the Plasma Membrane

The plasma membrane regulates the flow of materials into and out of the cell. It consists of a double layer of phospholipids (phospholipid bilayer) with embedded proteins.

• Phospholipids: Form the basic structure of the membrane.

• Membrane Proteins: Serve functions such as transport, attachment, recognition, and enzymatic activity.

Transport Across Membranes

Cells use various mechanisms to move substances across membranes, maintaining homeostasis and responding to environmental changes.

• Passive Transport: Movement of molecules without energy input. Includes diffusion and facilitated diffusion.

• Diffusion: Movement of molecules from high to low concentration.

• Facilitated Diffusion: Passive transport assisted by proteins for molecules that cannot cross the membrane easily.

• Osmosis: Diffusion of water across a selectively permeable membrane.

Osmotic Environments

Solution Type	Solute Concentration	Effect on Cell
Hypertonic	Higher outside cell	Cell loses water, shrivels
Hypotonic	Lower outside cell	Cell gains water, may burst (animal) or become turgid (plant)
Isotonic	Equal inside and outside	No net water movement

Osmoregulation: The control of water balance is vital for cell survival.

Active Transport

Active transport requires energy (usually ATP) to move molecules against their concentration gradient via transport proteins. This allows cells to maintain internal concentrations different from their environment.

Bulk Transport: Exocytosis and Endocytosis

Large molecules are transported across membranes by vesicle-mediated processes.

• Exocytosis: Movement of materials out of the cell via vesicles that fuse with the plasma membrane.

• Endocytosis: Intake of materials by forming vesicles from the plasma membrane. Includes phagocytosis (“cellular eating”).

Evolutionary Connection: The Origin of Membranes

Membranes and the Origin of Life

Phospholipids are key components of membranes and likely among the first organic compounds formed on early Earth. The spontaneous formation of membrane-enclosed collections of molecules was a critical step in the evolution of the first cells.

• Membranes allow cells to regulate chemical exchanges with the environment.

• All cells are enclosed by a plasma membrane, illustrating the evolutionary unity of life.