Cell Membrane Fluidity and Composition

Cell Structure & Membranes

• Lipids and proteins can move laterally within the membrane.
• The hydrophobic interior of the membrane prevents polar molecules from diffusing through it.
• Membranes differ in chain length, saturation, and polar groups.

Composition Affects Fluidity

Chain Length

• Longer chains result in less fluidity and slower movement.

Degree of Saturation

• More double bonds lead to more fluidity because they take longer to freeze and increase the rate of diffusion, resulting in less friction

Temperature

• Colder temperatures lead to less fluidity.
• Some organisms alter their composition, specifically the double bonds, in cold environments.

Membranes Contain Proteins

• The number of proteins varies.
• Chloroplasts and mitochondria contain a lot of proteins.

Integral Membrane Proteins

• Integral membrane proteins are at least partly embedded in the membrane.

Transmembrane Proteins

• Transmembrane proteins extend through the bilayer.
• They can have different functions on the inside and outside of the membrane.

Peripheral Membrane Proteins

• Peripheral membrane proteins only touch the hydrophilic surface of the membrane.

Anchored Membrane Proteins

• Anchored membrane proteins are covalently attached to fatty acids or lipid groups to anchor the protein to the membrane, and are permanently attached.

Glycolipid

• A glycolipid is a carbohydrate attached to a lipid.
• The polar part of glycolipids faces out.

Glycoprotein

• A glycoprotein is a protein with attached oligosaccharides.
• Glycoproteins typically have 8-12 sugars normally.

Proteoglycan

• A proteoglycan is a protein with covalently attached polysaccharide chains.

Selective Permeability

• Selective permeability does not allow all substances to pass through.

Transport

• Passive transport does not require energy.
• Active transport requires energy.

Diffusion

• Diffusion is the movement of particles toward a state of equilibrium from high to low concentrations.

• t = L²/4D, where t = time, L = distance, and D = diffusion constant

• Diffusion is fast for short distances but not as effective for long distances.

Factors Affecting Diffusion

• Size: Smaller molecules diffuse faster.
• Temperature: Higher temperature increases diffusion.
• Concentration Gradient: A greater concentration gradient results in faster diffusion.
• Area/Distance: A larger area and shorter distance increase diffusion.

Diffusion in Complex Solutions

• In a complex solution (multiple solutes), the diffusion of each solute depends only on its own concentration.
• Higher concentration inside the cell causes the solute to diffuse out and vice versa

Simple Diffusion

• Oxygen, carbon dioxide, and small, nonpolar, lipid-soluble molecules can cross the membrane unaided.

Tonicity

• Isotonic: Equal solute concentrations.
• Hypotonic: Solution has a lower solute concentration.
• Hypertonic: Solution has a higher solute concentration.
• Water follows solute.

Facilitated Diffusion

• Facilitated diffusion is passive transport of solutes down a gradient with the help of integral transmembrane proteins.
• Aquaporin and ion channels are examples of proteins that aid in facilitated diffusion

Ion Channels

• Most ion channels are gated and open up when a stimulus causes the protein to change shape.
• Examples include ligand-gated and voltage-gated channels.

Active Transport

• Active transport requires ATP to move substances against the concentration gradient.

Types of Active Transport

• Primary Active Transport: Involves direct hydrolysis, such as the sodium-potassium pump, where one ATP moves two K+ ions and three Na+ ions.
• Secondary Active Transport: Uses energy stored in concentration or electrical gradients established by primary transport.
• The sodium-potassium pump creates a concentration gradient of Na+, then passive diffusion of Na+ back into the cell provides energy for glucose transport.

Exocytosis

• Exocytosis moves materials out of the cell via a vesicle.

Endocytosis

• Endocytosis brings macromolecules into eukaryotic cells.

Phagocytosis

• Phagocytosis engulfs large particles or a cell, forming a phagosome.

Pinocytosis

• Pinocytosis brings in fluids and dissolved substances via smaller vesicles.

Receptor-Mediated Endocytosis

• Receptor-mediated endocytosis brings specific large molecules into the cell.

Intracellular Components

• Diffusion moves molecules around the cell.
• Some cells have foldings to increase surface area.

Cytoskeleton

• The cytoskeleton is the meshwork of protein filaments.
• It supports and maintains cell shape.
• It maintains the position of organelles or particles in the cell.
• It can move organelles or particles in the cell.
• It interacts with structures outside the cell to keep it in place.

Types of Cytoskeletal Protein Filaments

Microfilaments

• Microfilaments are made of actin polymers from actin monomers.
• They attach at the "plus end."
• They shorten or lengthen quickly to assemble and disassemble the cytoskeleton.
• They help maintain cell shape.
• Cytoplasmic streaming involves movement to the side walls of the cytoplasm.
• Actin filaments and the motor protein myosin are responsible for cell contractions.

Intermediate Filaments (IF)

• Intermediate filaments are made of fibrous proteins organized into tough, ropelike assemblages.
• They also help maintain shape.
• They are more permanent than other filaments.
• They are not found in plant cells but are found in animal cells.
• Keratin is an example.
• They are the most stable and least soluble components of the cytoskeleton.

Microtubules

• Microtubules are polymers made up of tubulin (protein) of alpha and beta.
• Microtubules have a hollow core.
• They have positive and negative ends and dynamic instability.
• Kinesin walks along microtubules, releasing ADP and accepting and hydrolyzing ATP during each step.

Extracellular Matrix

• The extracellular matrix absorbs water to form mats of collagen and gel-like proteoglycans.
• It holds cells together in tissues.
• It is a part of cartilage, skin, and bone.
• It helps filter material passing between different tissues, in kidney cells for example.
• It helps orient cell movements during tissue repair.

Integrins

• Integrins connect the cell membrane to the extracellular matrix.
• Reversible binding sites connect to microfilaments or IF inside the cell and collagen in the extracellular matrix.

Cell Junctions

• Tight Junctions:Prevent substances from moving between cells, as seen in the digestive tract.
• Desmosomes: Hold cells with stable protein connections, permit materials to move around them (very small space), as seen in skin.
• Gap Junctions: Channels that run between membrane pores, allowmaterials and electrical signals to pass.

Cell Types

• Prokaryotes have no nucleus or membrane-enclosed internal compartments
• Do use protein-based capsules to separate substances.
• Some prokaryotes (cyanobacteria) have an internal membrane for photosynthesis.
• Prokaryotic DNA is a singular, circular molecule in the nucleoid.
• Ribosomes contain proteins and RNAs, also the site of protein synthesis into peptide sequences.

Eukaryotes

• Eukaryotes have a nucleus
• Contain cell DNA
• Each organelle has a specific role
• Compartmentalization allows for regulation and efficiency

Nucleus

• Site of DNA replication and RNA transcription

Endoplasmic Reticulum

• Rough ER contains ribosomes attached for protein synthesis
• Smooth ER contains interconnected sheets and tubules for lipid synthesis helps detox small molecules and stores Ca2+

Golgi Complex

• Functionally inked with endoplasmic reticulum
• Vesicles move protiens to gogli to be modified
• Vesicles fuse with cis-gogli and proteins exit through vesicles that bud off

Lysomes

• Formed by the fusion of vesicles post-gogli that contain enzymes used to hydrolyze macromolecules
• Two Types: Primary and Secondary

Types Post-Gogli Vesicles

• Primary vesicles originate from gogli
• Secondary vesicles formed by phagocytois of a primary lisosame

Non-Endomembrane System

Mitochondria

• Cristae for oriented pH gradient
• Energy is harvested and stored as ATP
• Has some DNA, Ribosomes and RNA
• Contains an inner membrane folded into Cristae used for large surface area to ↑ ATP production

Plastids (Chloroplasts)

• Contains 2 membranes + Thylakoid membranes to store light energy
• Has circular DNA that divides autonomously
• Carbohydrates synthesized in Stroma
• Stores Sugars

Vacuoles

• Primarily in plants and fungi
• Store waste and toxic compounds

Peroxisomes

• Peroxisomes break down toxic peroxides
• All Around cell

Glyoxysomes

• Glyoxysomes (Plants) convert stored lipids to carbohydrate
• Reporduction takes places in flowers