Cellular Functioning

Chapter 5

CELLULAR MEMBRANES

Plasma Membrane’s Role • Physical isolation

– Ions & nutrient in, wastes & secretions out– Allows a concentration gradient to develop

• Regulates exchange with environment– Maintains homeostasis– Selective permeability

• Polarity (hydrophobic vs. hydrophilic)• Charge (charged vs. uncharged)• Size (large vs. small)

• Genes not necessary to arrange– Structure and function similar in all life

The Fluid Mosaic Model• Integral proteins– Channels – Carriers

• Peripheral proteins• Cell – cell recognition

• Phospholipid bilayer– Hydrophilic heads– Hydrophobic tails

• Fluidity– Cholesterol– Temperature

Types of Transport

Passive Active• Energy required• Movement against a

concentration gradient• Maintains disequilibrium

• Energy not required• Movement ‘down’ a

concentration gradient• Maintains dynamic

equilibrium• Specific types

– Diffusion– Osmosis

Reviewing Terms

• Solute

• Solvent

• Solution

• Concentration

Simple Diffusion• Movement of MOLECULES ‘down’ their concentration

gradient– Small, nonpolar molecules

• E.g. O2 in and CO2 out of red blood cells

– Each substance is independent

• Continues until equilibrium = no NET movement

Facilitated Diffusion• Integral proteins move MOLECULES ‘down’ their

concentration gradient– Large, polar molecules

• E.g sugars, AA’s, ions, and water– Are specific to substances

• Channels can open or close; carriers change shape• Rate increases with an increase in protein number

Osmosis• Movement of WATER ‘down’ its concentration

gradient– Water binds to solute in solution– More solute = less free water = less water available to move

• Depends on TOTAL solute concentration and permeability

watermolecules

glucosemolecules

Tonicity• Ability of a solution to cause a cell to gain or lose water

– Depends on [solutes] that can’t cross PM relative to that in the cell• Hypotonic solutions have a ___?__ [solute] than the cell

– Water moves ____?______• Animal = lyse• Plant = turgor pressure (central vacuole)

• Hypertonic solutions have a ___?__ [solute] than the cell– Water moves ____?______

• Animal = crenation• Plant = plasmolysis

• Isotonic solutions have ___?__ [solute] as the cell– Water shows no NET movement

• Plant = flaccid

Active Transport• Movement of molecules against their concentration

gradient• ATP is energy source• Maintains disequilibrium

Applying These Concepts

• Diffusion overview• Practice problem– “Cell” is impermeable to sucrose

• Movement of solutes?• Movement of water?• Solution type?• Resulting ‘cell’ shape?

Bulk Transport

• Exocytosis removes ‘stuff’ from inside the cell– Golgi apparatus to PM

• Endocytosis brings ‘stuff’ into the cell– PM pinches in to form vesicles

• Phagocytosis• Pinocytosis• Receptor-mediated

ENERGY REACTIONS

Energy• Capacity to cause change or rearrange matter– Kinetic energy: energy of movement or objects in motion

• Heat: random movement of particles associated with KE– Potential energy: stored energy as a result of structure or

location• Chemical energy: PE available for release to do work

• Cells transform chemical energy into usable energy

Chemical Reactions

• Exergonic releases energy– Reactants have more PE than products– Cellular respiration converts stored energy to usable energy

• Endergonic needs a net input of energy– Products have greater PE than reactants– Photosynthesis converts energy-poor reactants to energy rich sugars

• Degree of energy change is equal to the differences in PE

The Importance of ATP• Powers all cellular work• ATP + H2O ADP + phosphate + E release– Reversible because phosphate can rejoin ADP– Process of phosphorylation, phosphate binds to a

molecule to energize it

The Role of Enzymes• Proteins that increase the rate of reaction w/o being

consumed– Generally end in ‘-ase’ and are named for substrates– Lower the EA barrier

• Energy of activation (EA) is the energy needed to be overcome to start a reaction

• Net change of energy is the same with or without

Enzymatic Reactions

• 3D shape determines reactivity

• Synthetic or degradative• Enzyme activity factors– Temperature and pH denature– Buffers help regulate– Concentrations

Enzyme Inhibitors• Competitive– Resembles substrate and competes for binding– Increasing [substrate] can compensate

• Noncompetitive– Binds elsewhere than at active site– Causes conformational change so substrate can’t bind

• Facilitates feedback inhibition which prevents overproduction of a substance by the cell