Cell Plasma Membrane Structure and Function

• A plasma membrane surrounds the cell interior, separating it from its surroundings
• It serves as a selective barrier, regulating the movement of substances into and out of the cell

Fluid Mosaic Model

• The plasma membrane is described by this model
• It's fluid because lipids and proteins move relative to each other within the membrane
• It's a mosaic because phospholipids act like grout, and proteins are like tiles

Plasma Membrane Components

• Consists of a phospholipid bilayer with hydrophobic fatty acid tails facing inward
• Has hydrophilic phosphate heads facing outward toward water
• Includes embedded or attached proteins
• Has small carbohydrates attached to proteins (glycoproteins) or lipids (glycolipids)
• Animal cell membranes contain cholesterol, a four-ringed lipid steroid

Membrane Proteins

• Transmembrane proteins have regions inserted into the hydrophobic interior of the phospholipid bilayer
• Lipid-anchored proteins are covalently attached to a lipid
• Peripheral membrane proteins are non-covalently attached at the periphery

Factors Affecting Membrane Fluidity

• Length of phospholipid tails (shorter tails increase fluidity)
• Presence of double bonds in phospholipid tails (more double bonds increase fluidity)
• Cholesterol content
• Temperature

Membrane Permeability and Fluidity

• Lipids become more permeable and fluid with:
  • Short, unsaturated hydrocarbon tails
  • Less cholesterol
  • Higher temperatures

Glycosylation

• A process covalently attaches a carbohydrate to a lipid or protein
• Glycoproteins and glycolipids protect the cell surface
• They also aid in cell recognition
• Occurs in the endoplasmic reticulum (ER) and Golgi apparatus

Membrane Transport

• Membrane transport refers to the movement of ions and molecules across biological membranes

Passive Transport

• Does not require energy input
• Includes simple diffusion, osmosis, and facilitated diffusion

Active Transport

• Requires energy input

Diffusion

• Movement of substances from an area of high concentration to an area of low concentration
• Moves substances down their concentration gradient
• It may occur through a selectively permeable membrane

Simple Diffusion

• Movement of solutes across a selectively permeable membrane through the phospholipid bilayer from high to low concentration

Facilitated Diffusion

• Movement of solutes across a selectively permeable membrane from high to low concentration
• Requires the help of a transport protein

Solutions

• Solutes are dissolved ions or molecules
• Solvents are substances doing the dissolving
• Solutions are a combination of solutes and solvents

Permeability

• High permeability typically involves small, nonpolar molecules
• Moderate permeability may include small, uncharged polar molecules
• Low permeability is often associated with large, charged polar molecules
• Very low permeability may include large, charged ion molecules, requiring transport proteins

Transmembrane Gradients

• All living cells have these of solutes like molecules and ions
• Types include concentration and electrochemical gradients

Concentration Gradients

• Solute concentration is higher on one side of a membrane compared to the other

Electrochemical Gradients

• Include a chemical gradient (difference in ion concentration)
• Also includes an electrical gradient (difference in charge)

Osmosis

• Diffusion of water across a selectively permeable membrane

Tonicity

• Iso means isotonic (equal)
• Hypo means hypotonic (less than)
• Hyper means hypertonic (greater than)

Isotonic Solutions

• Have the same solute concentration on both sides of a membrane

Hypertonic Solutions

• Have a greater solute concentration outside the cell

Hypotonic Solutions

• Have a lesser solute concentration outside the cell

Osmosis Considerations

• Iso, hypo, and hyper refer to solutes in a solution
• Focus is on water movement
• If solute concentration outside a cell is higher, its water concentration is lower
• If solutes cannot move across a membrane, water will move to balance solute concentration

Osmosis in Animal Cells

• Isotonic: Water moves in and out at the same rate, no change
• Hypotonic: Water moves into the cell, potentially causing it to burst
• Hypertonic: Water moves out of the cell, causing it to shrink (crenation)

Osmosis in Plant Cells

• Isotonic: water moves in and out at the same rate, no change
• Hypotonic: Water moves into the cell, causing turgor pressure
• Hypertonic: Water moves out of the cell, causing the cell to shrink (plasmolysis)

Osmosis Adaptations

• Contractile vacuoles remove excess water in some freshwater organisms
• Blood cells maintain normal shape in an isotonic solution
• Cell walls prevent plant cells from bursting in hypotonic solutions

Turgor Pressure

• Water build-up creates turgor pressure in plant cells

Plasmolysis

• Occurs when a plant cell in a hypertonic solution loses water
• The plasma membrane pulls away from the cell wall

Transport Types (Carrier-Based)

• Uniport: A single solute moves in one direction
• Symport: Two solutes move in the same direction
• Antiport: Two solutes move in opposite directions

Active Transport

• Moves solutes from low to high concentration
• Requires energy input
• Substances move up their concentration gradient
• Requires a transporter or pump, and it can directly couple its shape change to an energy source, like adenosine triphosphate (ATP)

Primary Active Transport

• Directly uses energy to transport a solute
• Directly involves a functioning pump (e.g., H+ pump)

Secondary Active Transport

• Uses a pre-existing gradient to drive solute transport
• For example, an H+/sucrose symporter uses the H+ electrochemical gradient to move sucrose against its concentration gradient

Exocytosis

• The process of which material inside the cell, packaged into vesicles is sent out of the cell into the extracellular environment

Endocytosis

• The process of which the membrane folds inward to form a vesicle to bring substances into the cell

Phagocytosis

• The process of the membrane surrounding a smaller cell or food particle and engulfing it
• "Cell eating"
• A food vacuole may fuse with a lysosome for digestion
• For example, white blood cells engulf and destroy bacteria

Pinocytosis

• The process of the membrane surrounding a fluid and engulfs it to bring into the cell
• "Cell drinking"

Receptor-Mediated Endocytosis

• Selective endocytosis where specific substances bind with protein receptors in the plasma membrane and are selectively brought into the cell

Energy

• The ability to promote change or do work

Forms of Energy

• Kinetic energy is the energy of movement
• Potential energy is the energy a substance possesses due to structure or location

Kinetic Energy

• Thermal energy is the kinetic energy of molecular movement

Potential Energy

• Chemical energy is potential energy stored in atoms and bonds between atoms

Potential Energy Examples

• A rock at the top of a cliff
• Bonds in a sugar molecule
• Energy in a battery

Kinetic Energy Examples

• Wheel turning
• Water falling over a waterfall
• Heat
• Light

Thermodynamics

• Thermodynamics is the study of interconversions

First Law of Thermodynamics

• Energy is conserved and cannot be created nor destroyed
• Energy can be transferred or transformed (e.g., electrical energy to light and heat)

Entropy

• A measure of disorder or randomness in a system

Second Law of Thermodynamics

• Transfer or transformation of energy increases entropy
• The entropy of the universe is always increasing
• Energy from the sun helps decrease the entropy of living organisms

Endothermic Reactions

• Requires the taking of heat

Exothermic Reactions

• Reactions that release heat

Entro(S)py Defined

• Entropy (S) is a measure of disorder in a system
• Represents unusable energy

Heat

• Often a disordered and unusable form of energy

(G)ibbs Free Energy

• Gibbs free energy (G) is the energy available to do work
• Represents usable energy

Ent(H)alpy

• Enthalpy (H) is total energy
• Includes both usable and unusable energy

(T)emperature

• Temperature (T) is degrees in Kelvin

Delta Symbol

• The delta symbol (△) stands for "change in"

Spontaneous Reactions

• Occur without being driven by an addition of energy

Gibbs Free Energy and Spontaneity

• If △G is negative, the reaction is spontaneous
• If △G is positive, the reaction is not spontaneous unless energy is provided

Exergonic Reactions

• Release free energy
• Have a negative change in free energy (△G)
• Are spontaneous
• Reactants have more potential energy than products

Endergonic Reactions

• Require input of free energy
• Have a positive change in free energy (△G)
• Are not spontaneous
• Products have more potential energy than reactants

Activation Energy

• Both exergonic and endergonic reactions need this to get them started

Coupled Reactions

• An endergonic reaction is coupled with an exergonic reaction
• The exergonic reaction releases enough energy to power the endergonic reaction

ATP Hydrolysis

• Cells often use ATP hydrolysis to drive endergonic reactions

Chemical Equilibrium

• Chemical reactions reach a state of chemical equilibrium
• The rate of product formation equals the rate of reactant formation

ATP (Adenosine Triphosphate)

• Made of adenosine (adenine and ribose) and three phosphates
• Carries energy in its phosphate bonds