Membrane Structure and Function

Membrane Structure & Function

Fluid Mosaic Model

  • Glycoprotein: A protein with a carbohydrate attached.

  • Glycolipid: A lipid with a carbohydrate attached.

  • Phospholipid Bilayer: The fundamental structure of the cell membrane, formed by phospholipids.

  • Peripheral Membrane Protein: Proteins loosely associated with the internal or external surfaces of the membrane, possible functions include enzymes, signaling, structural attachments.

  • Integral Membrane Protein: Proteins that penetrate the hydrophobic core of the membrane, can span across completely or partially.

  • Cholesterol: A lipid that stabilizes membrane fluidity.

  • Protein Channel: A specialized integral protein that facilitates molecule transport across the membrane.

  • Cytoskeletal Filaments: Filaments in the cytoplasm that provide structural support and shape to the cell.

Structure

  1. Lipids

  2. Proteins

  3. Carbohydrates

A. Lipids

  1. Phospholipids

    • Phosphate Head: Hydrophilic (water-attracting) component.

    • Lipid Tails: Hydrophobic (water-repelling) fatty acid tails.

    • Most Abundant Lipid: Phospholipids are the most common type of lipid in the membrane.

    • Structure: Typically arranged in a bilayer, with hydrophobic tails facing inward.

    • Functionality: Creates a semi-permeable barrier allowing selective passage of substances.

    • Variability: Four major types of phospholipids contribute to the 'sidedness' of the membrane.

B. Proteins

  1. Peripheral Proteins

    • Loosely associated with the membrane's surface, can be dissociated easily.

    • Functions include: enzymes, signaling, and structural attachments.

  2. Integral Proteins

    • N-terminus and C-terminus: Ends of the protein with specific sequence orientations.

    • Integral proteins penetrate the hydrophobic core of the membrane, some extend completely across while others are monotopic or bitopic.

    • Function examples: receptors, enzymes, attachment proteins, transport proteins, identity proteins.

  3. Transport Proteins

    • Facilitate the movement of substances across the membrane.

C. Carbohydrates

  1. Glycolipids & Glycoproteins

    • Structure: Carbohydrate chains (2-60 monosaccharides) attached to integral proteins or lipids, found only on the exterior surface of the membrane, forming glycocalyx (sugar coating).

    • Functions: Add complexity to protein structures, involved in cell identity.

Membrane Function

  • Transport Regulation: Membranes are selectively permeable, regulating what substances can cross.

A. Solutes/Molecules Movement

  1. Easily Cross: Small, non-polar/hydrophobic substances (e.g., O2, CO2, small hydrocarbons).

  2. Difficult to Cross: Large or polar/hydrophilic substances (e.g., water, ions, sugars).

B. Transport Proteins

  • Channel Proteins: Facilitate transport of specific solutes (e.g., aquaporins for water transport).

  • Carrier Proteins: Change shape to carry solute across, do not interact chemically with solute.

C. Passive Movement

  1. Simple Diffusion: Movement of substances down their concentration gradient without assistance (e.g., gases).

  2. Facilitated Diffusion: Movement through transport proteins; passive transport of larger molecules.

  3. Osmosis: Diffusion of water across a semi-permeable membrane.

D. Tonicity

  • Hypertonic: Solution with a higher total non-permeable solute concentration that leads to water moving out of cells.

  • Hypotonic: Solution with a lower total non-permeable solute concentration that leads to water moving into cells.

  • Isotonic: Solutions on either side of the membrane have equal concentrations, resulting in no net movement of water.

E. Active Transport

  1. Active Transport: Requires energy input (ATP) to transport substances against their concentration gradient using carrier proteins.

  2. Cotransport: Movement of one solute to power the active transport of a second solute against its gradient.

    • Transporters: Include uniporters, symporters, and antiporters.

F. Transport of Large Molecules

  1. Exocytosis: Process by which a cell expels materials via vesicles.

  2. Endocytosis: Process by which cells ingest external fluids, macromolecules, and large particles.

    • Phagocytosis: Cell eating via engulfing solid particles.

    • Pinocytosis: Cell drinking via engulfing liquids.

    • Receptor-Mediated Endocytosis: Selective uptake of molecules via receptors.

Conclusion:

  • Students should be able to discuss membrane composition, explain how fluidity is influenced by temperature, differentiate between types of transport proteins, and understand how large molecules are transported across membranes.

Cells

Concept 1: Cell Theory

  • Explanation of the fundamental framework for understanding all cellular life on Earth.

Concept 2: Common Features of All Cells

  • Regardless of type, all cells share basic structures and functions crucial for life.

A. Prokaryotic Cells

  • Structures: fimbriae, bacterial chromosome, nucleoid, ribosomes, plasma membrane, cell wall, glycocalyx, flagella.

B. Eukaryotic Cells

  1. Plant Cells

    • Structures: nuclear envelope, nucleus, nucleolus, chromatin, rough and smooth ER, Golgi apparatus, mitochondria, peroxisome, central vacuole, cytoskeleton, chloroplasts, cell wall.

  2. Animal Cells

    • Structures: nucleus, nucleolus, mitochondria, lysosomes, Golgi apparatus, and more.

Relevant Cell Structures

  1. Plasma Membrane: Hydrophilic regions, hydrophobic regions, phospholipids, and proteins.

  2. Cell Wall: Provides structural support for plant cells.

  3. Nucleus: Contains genetic material and plays a key role in cell function.

Organelles and Their Functions

  1. Endomembrane System: This includes lysosomes, Golgi apparatus, vesicles, endoplasmic reticulum, and nuclear envelope.

  2. Mitochondria: The site of aerobic respiration, converting energy in food into ATP.

  3. Chloroplasts: The site of photosynthesis, converting light energy and CO2 into sugar.

Endosymbiotic Theory

  • Mitochondria and chloroplasts are believed to have originated from ancestral prokaryotes that were engulfed by ancestral eukaryotes.

Other Important Structures

  • Peroxisomes: Play a role in lipid metabolism and detoxification.

  • Cytoskeleton: Composed of microtubules, microfilaments, and intermediate filaments that maintain cell shape and facilitate movement.

  • Cell Junctions: Include tight junctions, desmosomes, and gap junctions that link cells together and facilitate communication.

Biological Molecules

Concept 1: Organic Molecules

  • Generally contain carbon skeletons along with various chemical groups that confer different properties.

Important Chemical Groups in Biomolecules

  1. Hydroxyl:

    • Structure: -OH

    • Property: Polar, can hydrogen bond; found in alcohols and sugars.

  2. Carbonyl:

    • Structure: -C=O

    • Property: Polar; found in sugars (e.g., ketones, aldehydes).

  3. Carboxyl:

    • Structure: -COOH

    • Property: Acidic; found in carboxylic acids.

  4. Amino:

    • Structure: -NH₂

    • Property: Acts as a base, found in amino acids.

  5. Phosphate:

    • Structure: -PO₄

    • Property: Contributes a negative charge; found in organic phosphates.

  6. Methyl:

    • Structure: -CH₃

    • Property: Found in methylated compounds.

Macromolecules

Types of Biological Macromolecules

  1. Carbohydrates: Consist of monosaccharides (simple sugars), oligosaccharides, and polysaccharides.

    • Functions: Energy storage, structural roles, cell identity.

  2. Proteins: Made from amino acids building blocks.

    • Functions: Enzymatic, structural, storage, signaling.

  3. Nucleic Acids: Composed of nucleotides; includes DNA and RNA.

    • Functions: Information storage, transfer, structure, and enzymatic activity.

  4. Lipids: Not true polymers, consist of fatty acids and include fats, oils, steroids, and phospholipids.

    • Functions: Long-term energy storage, insulation, cellular structure, and signaling.

Polymerization Reactions

  • Dehydration Reactions: Mechanism to build polymers by removing water.

  • Hydrolysis Reactions: Mechanism to break down polymers by adding water.

Overview of Carbohydrates

Monomers and Polymers

  • Monomer: Monosaccharides (e.g., glucose, fructose).

  • Polymer: Oligosaccharides (3-9 units) and polysaccharides (long chains).

Key Polysaccharides

  1. Starch: Energy storage in plants.

  2. Glycogen: Energy storage in animals.

  3. Cellulose: Structural component in plant cell walls.

Overview of Proteins

Amino Acids Structure

  • Central carbon, amino group, carboxyl group, and variable side chain (R group).

Protein Structure Levels

  1. Primary Structure: Sequence of amino acids.

  2. Secondary Structure: α-helix and β-pleated sheet formation through hydrogen bonding.

  3. Tertiary Structure: Three-dimensional folding due to side chain interactions.

  4. Quaternary Structure: Complex of multiple polypeptides.

Lipids Overview

  1. Saturated Fats: Contain no double bonds; typically solid at room temperature (e.g., butter).

  2. Unsaturated Fats: Contain one or more double bonds; usually liquid at room temperature (e.g., olive oil).

  3. Trans Fats: Processed fats that have been hydrogenated to improve shelf life.

Nucleic Acids Overview

Structure

  • Comprised of nucleotides (sugar, phosphate, base).

DNA vs RNA Bases

  • DNA: Adenine, Guanine, Cytosine, Thymine.

  • RNA: Adenine, Guanine, Cytosine, Uracil.

Function

  • Storage and transfer of genetic information.

Final Review

Understandings for Students

  1. Recognition and description of key chemical groups.

  2. Distinction among major classes of organic molecules.

  3. Differences in carbohydrate types.

  4. Differences between types of fats.

  5. Understanding protein structures and functions.

  6. Key structural elements of nucleic acids.

  7. The importance of molecular shape in biological function.