Cellular Energy, Enzymes, and Membrane Function

Harnessing Cellular Structures

Cells: Monocentric Units of Life

Cells are the fundamental units of life, responsible for energy processing and the production of various products. They control their chemical environment using specialized structures and molecules.

• Energy: Required for all cellular activities.

• Enzymes: Biological catalysts that speed up chemical reactions.

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

Cell-based nanotechnology may be used to power microscopic robots. (Additional info: This is an emerging field in biotechnology.)

Conservation of Energy

Definition and Forms of Energy

Energy is defined as the capacity to cause change. It exists in various forms and can be transformed from one form to another.

• Kinetic energy: The energy of motion.

• Potential energy: Stored energy, such as that possessed by an object because of its location or structure.

Life depends on countless conversions 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 contained in 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 the potential energy stored in the arrangement of atoms and can be released by a chemical reaction.

• Examples: Food, gasoline, and other fuels.

• Living cells and automobile engines use the same basic process to release energy.

Cellular Energy

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

• Cells store energy in a form that can be used to perform work.

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

Food Calories

• A calorie (cal) is the amount of energy that can raise the temperature of 1 gram of water by 1°C.

• Food calories are kilocalories (kcal), equal to 1,000 calories.

• The energy in food is used to fuel the activities of life.

ATP and Cellular Respiration

ATP: The Energy Currency of the Cell

Chemical energy released by the breakdown of organic molecules during cellular respiration is used to generate molecules of ATP (adenosine triphosphate).

• ATP acts like an energy shuttle.

• Stores energy obtained from food.

• Releases it later as needed.

Equation:

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

Phosphate Transfer

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

• This energy transfer helps cells change shape, enables the transport of ions and other dissolved substances across membranes, and 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 are consumed and recycled each second in a working muscle cell.

Enzymes

Role and Function

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

Activation Energy

• Activation energy is the energy that must be invested to start a reaction (e.g., activating the reactants, triggering a chemical reaction).

• Enzymes enable metabolism to occur by reducing the amount of activation energy required to break the bonds of reactant molecules.

Structure and Function: Enzyme Activity

• An enzyme is very selective in the reaction it catalyzes.

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

• The active site has a shape and chemistry that fits the substrate molecule.

• This interaction is called induced fit because the entry of the substrate induces the enzyme to change shape slightly, making the fit between the substrate and active site stronger.

• After the product is released, the enzyme can accept another substrate molecule.

• The ability to function repeatedly is a key characteristic of enzymes.

• Many enzymes are named for their substrates but with an -ase ending (e.g., lactase).

How an Enzyme Works

• Enzyme binds to substrate at the active site.

• Enzyme-substrate complex forms, catalysis occurs, and product is released.

• Enzyme is ready to catalyze another reaction.

Enzyme Inhibitors

• Certain molecules inhibit a metabolic reaction by binding to an enzyme or disrupting its function.

• Some inhibitors are substrate imposters that plug up the active site.

• Other inhibitors disrupt the function of an enzyme by altering its shape.

Membrane Function

Plasma Membrane Structure

• The plasma membrane consists of a double layer of fat (phospholipid bilayer) with embedded proteins.

• It regulates the flow of materials to and from the environment.

Movement Across Membranes

• Diffusion: The movement of molecules to spread out evenly into the available space. (Diffusion causes a net movement of molecules down their concentration gradient.)

• Passive transport: The diffusion of a substance across a membrane without the input of energy.

• In passive transport, substances move from areas of higher concentration to lower concentration.

• Substances that do not cross membranes spontaneously or very slowly can be transported via proteins (facilitated diffusion).

Osmosis and Water Balance

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

• Solute: A substance that is dissolved in a liquid solvent, such as salt in water. The resulting mixture is called a solution.

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

Water Balance in Plant Cells

• Plant cells have rigid cell walls.

• Plant cells in a hypotonic environment will not burst due to the cell wall.

• Animal cells can burst in a hypotonic environment.

Active Transport: The Pumping of Molecules Across Membranes

• Active transport requires that a cell expend energy to move molecules across a membrane against their concentration gradient.

• Cellular energy (usually provided by ATP) is used to drive a transport protein that pumps a solute across the membrane.

Bulk Transport: Exocytosis and Endocytosis

• Exocytosis: The movement of material out of the cytoplasm of a cell via membranous vesicles or vacuoles that fuse with the plasma membrane.

• Endocytosis: The process by which a cell takes in material via vesicles that bud inward. Phagocytosis (cellular eating) is a type of endocytosis where a cell engulfs a particle and packages it within a food vacuole.

Osmoregulation

• Osmoregulation is the control of water balance within a cell or organism.

Evolution Connection: The Origin of Membranes

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

• A plasma membrane that allows cells to regulate their chemical exchanges with the environment is a basic requirement for life.