Unit 5: Cell Transport

Plasma membrane

• controls the environment of materials in/out of the cell

• key regulation of homeostasis of cell

• maintenance of life functions

Homeostasis

≠ equilibrium

Phospholipids

• specialized triglycerides that make up most of the membrane

• consist of phosphate “heads” and fatty acid “tails”

Phospholipid structure

leads to “phospholipid bilayer”

Phosphate “heads”

• attract to water

• polar/hydrophilic

Phosphate ”tails”

• avoid water

• non-polar/hydrophobic

Bilipid layer

• remains fluid while preventing polar molecules (like water) from crossing through

• molecules within, e.g. phospholipids, can move as freely as they want; membrane can bend

Plasma membrane structure/proteins

• multiple other structures reside within the plasma membrane to its function

• can use a fluid mosaic model

Cholesterol

• provides fluidity to membrane

• lubricates phospholipids (prevents coagulation)

Integral (membrane) proteins

embedded (within the bilayer)

Peripheral (membrane) proteins

along the side/edge of the membrane

Glycocalyx

• surface “fuzz” involved in cell to cell recognition

• made of oligosaccharides, attached to glycoproteins and glycolipids

Channel proteins

• “aquaporins” (mainly facilitate transport of water between cells) - small, polar molecules by simple diffusion

• substances pass through the channel unimpeded by non-polar region

Membrane proteins

• integral proteins and peripheral proteins

• perform various functions

Carrier proteins

• like a revolving door, e.g. glucose could go in - facilitate passage of large and/or polar molecules

• may function via simple diffusion (passive transport) or by ATP (active transport)

Marker protein

• specialized protein involved in cell to cell recognition

• help form the glycocalyx

Receptor protein

• specialized protein involved in “receptor mediated endocytosis”

• binds to other substances and allows them to change the activity of a cell

Oligosaccharides

• short carbohydrate chains

• 5-10 monosaccharides stuck together

Enzymes

• promote chemical reactions

• found as integral or peripheral proteins

Passive transport

movement of materials across a membrane which requires no energy other than kinetic energy of molecules

Simple diffusion

• driven completely by kinetic energy of particles

• may or may not include a membrane

• movement of materials from area of high concentration to area of low concentration

Concentration gradient

exists when there is a difference in concentrations over a distance or across a membrane

Equilibrium

no concentration gradient; equal concentrations

Rate of diffusion

due to diffuson pressure, mass of molecule, and temperature

Greater concentration gradient =

greater diffuson pressure = greater rate of diffusion

Semi-permeable membrane

• lets only selected substances through

• based on size and polarity of molecules

Factors affecting rate of diffusion

• mass

• concentration gradient

• temperature

Factors affecting ability to diffuse

• molecular size

• polarity

Facilitated diffusion

• form of passive transport in which substances move from an area of high conc. to an area of low conc. with the assistance of proteins within the membrane

Osmosis

passive transport of water across a membrane from high conc. to low conc.

Isotonic environment

• state of equilibrium

• equal concentrations throughout, both inside and outside cell

• no concentration gradient exists, zero diffusion pressure

Hypotonic environment

concentration of solutes outside is lower than concentration of solutes inside the cell (therefore waer is greater outside)

Hypertonic environment

concentration of solutes outside the cell is greater than the concentration inside the cell (therefore water outside is less)

Turgor pressure

• pressure that builds up in a cell due to the inward flow of water

• some organisms use a contractile vacuole to expel excess water to lessen turgor pressure

Plasmolysis

shrinking of a cell from loss of water

Cytolysis

swelling and bursting of a cell caused by internal pressure

Active transport

• passage of a substance across a semipermeable membrane that requires the use of ATP energy

• may also be the movement of materials against the conc. gradient

• or movement of molecules too large for the membrane pores

Ion pumps

• carrier proteins used to move ions across the membrane against the conc. gradient

• conformational change of carrier proteins require ATP

• used to maintain steep conc. gradient of ions across a cell membrane

• ex: sodium-potassium pump

Endocytosis

• taking in large molecules by a cell w/out moving through pores of the cell membrane

• formation of vesicles as the cell membrane pinches around the substance

Receptor-mediated endocytosis

acquiring specific materials from the environment through use of receptor proteins found at specific sites (coated pits) on the outer surface of the cell membrane

• when receptor proteins bind to the appropriate macromolecules outside the cell, the coated pit invaginates (folds inward) to form a vacuole

Phagocytosis

• engulfing whole cells or one-celled organisms

• formation of vacuoles

• broken down by digestive enzymes of lysosome

Pinocytosis

• taking in liquids or dissolved solutes by a cell

• formation of vacuoles

• broken down by digestive enzymes of lysosome

Exocytosis

• passage of large molecules to the outside of the cell w/out going through pores of the cell membrane

• vesicles fuse with the cell membrane as the molecule is released outside the cell

Endomembrane system

the fluid plasma membrane works in conjunction with all other membrane-bound organelles to promote both exocytosis and endocytosis of materials

Substances that can move via passive transport between phospholipids

• small, non-polar (simple diffusion)

• oxygen and carbon dioxide

Substances that require a channel protein to move across membrane

• small, polar

• ions like sodium, potassium, calcium, and chloride; water

Substances that require a carrier protein to move across membrane

• large and/or polar

• sugars and amino acids

Passive transport

no input of energy is required

Diffusion

• movement of molecules across the membrane down their concentration gradient

• process continues until there are equal concentrations on both sides of the membrane

Equilibrium

concentration gradient of molecules and diffusion pressure are zero

Passive transport of molecules across a membrane continues

molecules are in constant, random motion