Cellular Energy, Enzymes, and Membrane Function: Study Notes for General Biology

Harnessing Cellular Structures

Cells: Monocentric Functions

Cells are the fundamental units of life, responsible for a variety of essential processes that sustain organisms.

• Energy processing: Cells convert energy from one form to another to power biological activities.

• Production of various products: Cells synthesize proteins, lipids, and other molecules necessary for life.

Cells Control Their Chemical Environment Using:

• Energy: Required for all cellular processes.

• Enzymes: Biological catalysts that speed up chemical reactions.

• The plasma membrane: Regulates the movement of substances into and out of the cell.

Cell-based nanotechnology may be used to power microscopic robots. Additional info: This is an application of cellular machinery in biotechnology.

Conservation of Energy

Energy: Definition and Forms

Energy is defined as the capacity to cause change. It exists in various forms and is essential for all biological processes.

• Kinetic energy: The energy of motion. Example: a moving object.

• Potential energy: Stored energy due to an object's position or structure. Example: energy stored in chemical bonds.

Life depends on the conversion of energy from one form to another. The conservation of energy principle states that energy cannot be created or destroyed, only transformed.

Heat and Entropy

• Heat: A type of kinetic energy associated with the random motion of atoms and molecules.

• All energy conversions generate some heat.

• Entropy (S): A measure of disorder or randomness in a system. Every time energy is converted from one form to another, entropy increases.

Chemical Energy

Chemical energy is stored in the bonds of molecules such as food, gasoline, and other fuels. This energy can be released by chemical reactions.

• Chemical energy: The potential energy available for release in a chemical reaction.

• Living cells and automobile engines: Both use the same basic process to release energy from fuel molecules.

Cellular Energy

• Cellular respiration: The process by which cells obtain energy from food molecules.

• Only about 34% of the energy in food is converted to useful work; the rest is lost as heat.

Food Calories

• Calorie (cal): The amount of energy needed to raise the temperature of 1 gram of water by 1°C.

• Food calories are actually kilocalories (kcal), where 1 kcal = 1,000 cal.

• The energy in food is used to fuel cellular activities.

ATP and Cellular Respiration

ATP: The Energy Currency of the Cell

Cells use chemical energy released by the breakdown of organic molecules to generate ATP (adenosine triphosphate), which powers cellular work.

• ATP: Stores energy obtained from food and releases it as needed.

• Acts like an energy shuttle within the cell.

Additional info: The ATP cycle involves the conversion of ATP to ADP (adenosine diphosphate) and inorganic phosphate (Pi), releasing energy for cellular processes.

Phosphate Transfer

• ATP energizes other molecules in cells by transferring phosphate groups to them.

• This transfer enables molecules to change shape and facilitates the transport of ions and other substances across membranes.

• Drives the production of large molecules.

The ATP Cycle

• Cells spend ATP continuously.

• ATP is recycled when ADP and phosphate are combined using energy released by cellular respiration.

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

Enzymes

Metabolism and Enzyme Function

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.

Activation Energy

• Activation energy: The energy required to start a reaction by activating the reactants and triggering a chemical reaction.

• Enzymes lower the activation energy needed, enabling metabolism to occur efficiently.

Structure and Function: Enzyme Activity

• Enzymes are highly selective for the reactions they catalyze.

• Each enzyme recognizes a substrate (the reactant molecule).

• The active site of the enzyme binds the substrate, often changing shape to fit (induced fit model).

• After the reaction, the product is released, and the enzyme can catalyze another reaction.

• Enzyme names often end with -ase.

How an Enzyme Works

• Substrate binds to the enzyme's active site.

• Enzyme catalyzes the conversion of substrate to product.

• Product is released, and the enzyme is free to catalyze another reaction.

Enzyme Inhibitors

• Certain molecules can inhibit enzyme activity by binding to the enzyme or altering its shape.

• Some inhibitors resemble the substrate and block the active site (competitive inhibition).

• Others bind elsewhere, changing the enzyme's shape (noncompetitive inhibition).

Membrane Function

Plasma Membrane Structure and Function

The plasma membrane is a selectively permeable barrier that regulates the flow of materials into and out of the cell. It consists of a phospholipid bilayer with embedded proteins.

• Maintains the internal environment of the cell.

• Allows for communication and transport between the cell and its surroundings.

Transport Across Membranes

• Diffusion: The movement of molecules from an area of higher concentration to lower concentration.

• Passive transport: Diffusion across a membrane without energy input.

• Facilitated diffusion: Passive transport aided by transport proteins.

• Osmosis: The diffusion of water across a selectively permeable membrane.

Osmosis and Water Balance

• Solute: A substance dissolved in a solution.

• Hypertonic solution: Higher concentration of solute.

• Hypotonic solution: Lower concentration of solute.

• Isotonic solution: Equal concentration of solute.

• Osmoregulation: Control of water balance.

Solution Type	Solute Concentration	Effect on Cell
Hypertonic	Higher outside cell	Cell loses water, shrinks
Hypotonic	Lower outside cell	Cell gains water, swells
Isotonic	Equal inside and outside	No net water movement

Water Balance in Plant Cells

• Plant cells have rigid cell walls.

• In a hypotonic environment, plant cells become turgid (firm) due to water uptake.

• In a hypertonic environment, plant cells lose water and may wilt.

Active Transport

• Requires energy (usually ATP) to move substances against their concentration gradient.

• Involves transport proteins that pump molecules across the membrane.

Bulk Transport: Exocytosis and Endocytosis

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

• Endocytosis: Movement of materials into the cell via vesicles that bud inward from the plasma membrane.

• Phagocytosis: A type of endocytosis where the cell engulfs large particles or cells.

Evolution Connection: The Origin of Membranes

• A membrane can enclose a solution that is different in composition from its surroundings.

• The plasma membrane allows cells to regulate chemical exchanges with the environment, a basic requirement for life.

Key Equations

• ATP Hydrolysis:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• Calorie Definition:

$1\ \text{calorie} = \text{energy required to raise 1 g of water by 1}^\circ\text{C}$

• Entropy (S):

$\Delta S = S_{\text{final}} - S_{\text{initial}}$