Diffusion/ osmosis etc
Membrane Transport
▪ Solution—homogeneous mixture of two or more components
o Solvent—dissolving medium; typically water in the body
o Solutes—components in smaller quantities within a solution
Membrane Transport
▪ Intracellular fluid
o Nucleoplasm and cytosol
o Solution containing gases, nutrients, and salts dissolved in water
▪
Interstitial fluid
o Fluid on the exterior of the cell
o Contains thousands of ingredients, such as nutrients, hormones,
neurotransmitters, salts, waste products
Membrane Transport
▪ The plasma membrane is a selectively permeable barrier
o Some materials can pass through while others are excluded
o For example:
▪ Nutrients can enter the cell
▪ Undesirable substances are kept out
Membrane Transport
▪ Two basic methods of transport
o Passive processes
▪ No energy (ATP) is required
o Active processes
▪ Cell must provide metabolic energy (ATP)
Passive Processes
▪ Diffusion
o Particles tend to distribute themselves evenly within a solution
o Driving force is the kinetic energy (energy of motion) that causes the
molecules to move about randomly
Passive Processes
▪ Diffusion
o Molecule movement is from high concentration to low concentration, or
down a concentration gradient
o Size of molecule and temperature affects the speed of diffusion
▪ The smaller the molecule, the faster the rate of diffusion
▪ The warmer the molecule, the faster the rate of diffusion
Passive Processes
▪ Example of diffusion:
o Pour a cup of coffee and drop in a cube of sugar
o Do not stir the sugar into the coffee; leave the cup of coffee sitting all day,
and it will taste sweet at the end of the day.
o Molecules move by diffusion and sweeten the entire cup
Passive Processes
▪ Molecules will move by diffusion if any of the following applies:
o The molecules are small enough to pass through the membrane’s pores
(channels formed by membrane proteins)
o The molecules are lipid-soluble
o The molecules are assisted by a membrane carrier
Passive Processes
▪ Types of diffusion
o Simple diffusion
▪ An unassisted process
▪ Solutes are lipid-soluble or small enough to pass through
membrane pores
Passive Processes
▪ Types of diffusion (continued)
o Osmosis—simple diffusion of water
▪ ighly polar water molecules easily cross the plasma membrane
through aquaporins
▪ Water moves down its concentration gradient
Passive Processes
▪ Osmosis—A Closer Look
o Isotonic solutions have the same solute and water concentrations as cells
and cause no visible changes in the cell
o Hypertonic solutions contain more solutes than the cells do; the cells will
begin to shrink
o Hypotonic solutions contain fewer solutes (more water) than the cells do;
cells will plump
Passive Processes
▪ Types of diffusion (continued)
o Facilitated diffusion
▪ Transports lipid-insoluble and large substances
▪ Glucose is transported via facilitated diffusion
▪ Protein membrane channels or protein molecules that act as
carriers are used
Passive Processes
▪ Filtration
o Water and solutes are forced through a membrane by fluid, or hydrostatic
pressure
o A pressure gradient must exist
▪ Solute-containing fluid (filtrate) is pushed from a high-pressure area
to a lower-pressure area
▪ Filtration is critical for the kidneys to work properly
Active Processes
▪ Sometimes called solute pumping
▪ Requires protein carriers to transport substances that:
o May be too large to travel through membrane channels
o May not be lipid-soluble
o May have to move against a concentration gradient
▪ ATP is used for transport
Active Processes
▪ Active transport
o Amino acids, some sugars, and ions are transported by protein carriers
known as solute pumps
o ATP energizes solute pumps
o In most cases, substances are moved against concentration (or electrical)
gradients
Active Processes
▪ Example of active transport is the sodium-potassium pump
o Sodium is transported out of the cell
o Potassium is transported into the cell
Active Processes
▪ Vesicular transport: substances are moved without actually crossing the plasma
membrane
o Exocytosis
o Endocytosis
▪ Phagocytosis
▪ Pinocytosis
Active Processes
▪ Vesicular transport (continued)
o Exocytosis
▪ Moves materials out of the cell
o Material is carried in a membranous sac called a vesicle
o Vesicle migrates to plasma membrane
o Vesicle combines with plasma membrane
o Material is emptied to the outside
o Refer to Pathway 1 in Figure 3.6
Active Processes
▪ Vesicular transport (continued)
o Exocytosis docking process
▪ Transmembrane proteins on the vesicles are called
v-SNAREs (v for vesicle)
▪ Plasma membrane proteins are called t-SNAREs
(t for target)
▪ v-SNAREs recognize and bind t-SNAREs
▪ Membranes corkscrew and fuse together
Active Processes
▪ Vesicular transport (continued)
o Endocytosis
▪ Extracellular substances are engulfed by being enclosed in a
membranous vescicle
▪ Vesicle typically fuses with a lysosome
▪ Contents are digested by lysosomal enzymes
▪ In some cases, the vesicle is released by exocytosis on the
opposite side of the cell
Active Processes
▪ Vesicular transport (continued)
o Types of endocytosis
▪ Phagocytosis—“cell eating”
• Cell engulfs large particles such as bacteria or dead body
cells
• Pseudopods are cytoplasmic extensions that separate
substances (such as bacteria or dead body cells) from
external environment
• Phagocytosis is a protective mechanism, not a means of
getting nutrients
Active Processes
▪ Vesicular transport (continued)
o Types of endocytosis
2. Pinocytosis—“cell drinking”
• Cell “gulps” droplets of extracellular fluid containing
dissolved proteins or fats
• Plasma membrane forms a pit, and edges fuse around
droplet of fluid
• Routine activity for most cells, such as those involved in
absorption (small intestine)