Cell Membranes, Transport, and Enzyme Function: Chapter 5 Study Notes

Cell Membrane Structure and Function

Fluid Mosaic Model

The fluid mosaic model describes the structure of biological membranes, emphasizing their dynamic and diverse nature. This model is foundational for understanding how membranes function in cells.

• Fluid: Refers to the movement and flexibility of the membrane, primarily due to unsaturated fatty acid tails in phospholipids and the presence of cholesterol, which prevents tight packing of lipids.

• Mosaic: Indicates the presence of many different types of proteins embedded within the membrane, each with specific functions such as transport, signaling, and enzymatic activity.

Selective permeability is a key property of membranes, allowing certain substances to cross more easily than others. This is due to the specific arrangement and types of membrane proteins.

• Membrane proteins perform various functions, including transport, attachment, cell recognition, and enzymatic activity.

Transport Across Membranes

Passive Transport

Passive transport is the movement of substances across a membrane without the expenditure of cellular energy (ATP). It relies on the natural tendency of particles to move from areas of higher concentration to areas of lower concentration.

• Diffusion: The movement of small, nonpolar molecules (e.g., oxygen, carbon dioxide) down their concentration gradient.

• Osmosis: The diffusion of water across a selectively permeable membrane. Water moves from areas of lower solute concentration to areas of higher solute concentration.

Tonicity and Water Movement

Tonicity describes the ability of a surrounding solution to cause a cell to gain or lose water. It is determined by the relative concentrations of solutes inside and outside the cell.

• Hypotonic solution: Lower solute concentration outside the cell; water rushes into the cell, which may cause it to swell or lyse.

• Isotonic solution: Equal solute concentration inside and outside; water moves in and out at equal rates, and the cell remains normal.

• Hypertonic solution: Higher solute concentration outside the cell; water rushes out, causing the cell to shrink or become flaccid.

Solution Type	Solute Level	Effect on Animal Cell	Effect on Plant Cell
Hypotonic	Lower outside	Lysed (swells)	Turgid (normal)
Isotonic	Equal	Normal	Flaccid
Hypertonic	Higher outside	Shriveled	Plasmolyzed

Facilitated Diffusion

Facilitated diffusion is a type of passive transport that allows polar or charged molecules to cross the membrane with the help of transport proteins. It does not require energy and moves substances down their concentration gradient.

• Transport proteins are specific for the solute they move and increase the selective permeability of the membrane.

Active Transport

Active transport requires energy (usually from ATP) to move substances against their concentration gradient. This process is essential for maintaining internal concentrations of ions and other molecules.

• Transport proteins change shape to move solutes across the membrane.

• Cotransport: A type of active transport where two substances are moved simultaneously.

Bulk Transport: Exocytosis and Endocytosis

Large molecules such as proteins and polysaccharides are transported across membranes via vesicles in processes called exocytosis and endocytosis.

• Exocytosis: The process by which large molecules are moved out of the cell. Example: Insulin secretion by pancreatic cells.

• Endocytosis: The process by which large molecules are taken into the cell. Includes phagocytosis (engulfment of particles) and receptor-mediated endocytosis (specific uptake via membrane receptors).

Energy and the Cell

Forms and Transformations of Energy

Cells require energy to perform work. Energy exists in various forms and can be transformed from one type to another.

• Kinetic energy: Energy of motion, including thermal energy (heat).

• Potential energy: Stored energy, such as chemical energy in bonds.

Thermodynamics is the study of energy transformations in matter.

• First Law: Energy cannot be created or destroyed, only transferred or transformed.

• Second Law: Every energy conversion increases the entropy (disorder) of the universe.

Exergonic and Endergonic Reactions

Chemical reactions in cells can either release energy (exergonic) or require an input of energy (endergonic).

• Exergonic reaction: Reactants have more energy than products; energy is released.

• Endergonic reaction: Products have more energy than reactants; energy is absorbed.

ATP: The Energy Currency of the Cell

Adenosine triphosphate (ATP) is the primary energy carrier in cells. It consists of adenosine and three phosphate groups. The bonds between phosphate groups are unstable and release energy when broken by hydrolysis.

• ATP hydrolysis: $\text{ATP} \rightarrow \text{ADP} + \text{Phosphate} + \text{Energy}$

• Energy released is used to drive endergonic reactions via phosphorylation (transfer of a phosphate group to another molecule).

Enzymes and Their Function

Enzyme Structure and Catalysis

Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy required. They are not consumed in the reaction and are highly specific for their substrates.

• Active site: The region of the enzyme where the substrate binds.

• Catalytic cycle: The process by which an enzyme binds to its substrate, converts it to products, and releases the products.

Activation energy is the energy barrier that must be overcome for a reaction to proceed. Enzymes lower this barrier.

Enzyme Specificity and Environmental Effects

Enzyme activity is affected by environmental factors such as temperature and pH. Extreme conditions can denature enzymes, reducing their effectiveness.

• Optimal temperature and pH: Each enzyme has specific conditions under which it functions best.

Cofactors and Coenzymes

Cofactors are nonprotein helpers required for enzyme activity. They may be inorganic ions (e.g., zinc, iron, copper) or organic molecules called coenzymes (e.g., vitamins).

• Coenzyme Q10 (CoQ10): An important coenzyme involved in electron transfer during cellular respiration.

Enzyme Inhibition

Enzyme activity can be regulated by inhibitors.

• Competitive inhibitors: Bind to the active site, blocking substrate binding.

• Noncompetitive inhibitors: Bind elsewhere on the enzyme, causing a change in shape that reduces activity.

• Feedback inhibition: A product of a metabolic pathway inhibits an enzyme involved earlier in the pathway, regulating the pathway's activity.

Type of Inhibitor	Binding Site	Effect
Competitive	Active site	Blocks substrate binding
Noncompetitive	Allosteric site (elsewhere)	Changes enzyme shape, reduces activity

Examples of Enzyme Inhibition

• Ibuprofen/Motrin/Advil: Inhibits enzymes involved in prostaglandin production, reducing inflammation and pain.

• Antibiotics: Many inhibit enzymes in disease-causing bacteria. Example: Penicillin blocks the active site of an enzyme bacteria use to make cell walls.

Summary Table: Key Concepts

Concept	Definition	Example/Application
Fluid Mosaic Model	Describes membrane structure as flexible and diverse	Cell membrane
Passive Transport	Movement without energy input	Osmosis, diffusion
Active Transport	Movement against gradient, requires energy	Na+/K+ pump
Enzyme	Protein catalyst	Sucrase, DNA polymerase
ATP	Energy currency of the cell	Drives cellular work