Cellular Transport and Bulk Flow

• Cellular membranes control the passage of substances into and out of cells.
• The plasma membrane controls the movement of substances in a cell.
• The nucleus, chloroplasts, and mitochondria, along with peroxisomes and vacuoles, have membranes that control the flow of substances.

Common Substances

• Most molecules and ions essential for life are dissolved in water; water is the solvent.
• Solutes are substances dissolved in a solution.

Bulk Flow

• Bulk flow (mass flow) involves the movement of substances from one location to another due to differences in potential energy.
• Potential energy is the stored energy of an object based on its position.
• Water's potential energy is referred to as water potential.
• Water flows from areas of higher water potential to those with lower water potential.

Diffusion

• Diffusion results in the uniform distribution of a substance down a concentration gradient until equilibrium is reached.
• Water, carbon dioxide, oxygen, and other simple uncharged molecules diffuse freely across the plasma membrane.
• A selectively permeable membrane allows some substances to pass through while limiting others.
• Osmosis is the movement of water from an area of higher water potential to lower water potential.

Measuring Osmotic Potential

• Distilled water is separated from a solution by a selectively permeable membrane.
• Water diffuses into the tube, increasing the volume.
• Gravity exerts pressure proportional to the column height.
• Equilibrium is reached when water potential is equal on both sides.
• The pressure needed to return the solution to its initial level is the osmotic potential.

Imbibition

• Imbibition is the movement of water molecules into substances like wood or gelatin, causing swelling.
• Occurs in seeds and is crucial for germination.

Turgor Pressure

• Plants concentrate strong salt and molecule solutions within vacuoles.
• Osmosis draws water in, building internal hydrostatic pressure.
• Turgor pressure develops in a plant cell due to osmosis, keeping the cell stiff.
• Wall pressure is the equal and opposite mechanical pressure exerted by the cell wall.
• Plasmolysis is the result of water loss.

Membrane Structure

• All cells have the same basic structure, including a lipid bilayer embedded with globular, transmembrane proteins.
• Two major types of lipids in the plasma membrane are phospholipids and sterols.
• Peripheral proteins are associated with membranes, while integral proteins are bound to the membrane.
• The membrane structure is fluid, with proteins floating in a lipid sea.

Newer Model

• The newer model suggests a less fluid structure with a higher protein ratio.
• Proteins are organized into large, patchy complexes.
• Oligosaccharides attach to protruding proteins (glycoproteins) on eukaryotic cells.
• These sugars are recognition plays a role.
• Glycolipids are another type of recognition molecule.
• Single and multi-pass transmembrane proteins exist.

Membrane Transport

• Carrier proteins bind specific solutes, causing a conformational change.
• Channel proteins permit selected solutes, such as ions, to pass through water-filled pores:
• Channels can be gated.
• Active transport (pumps) moves solutes against the gradient using energy.
• The plasma membrane and tonoplast contain aquaporins.

Solute Passage

• Uniport transports only one solute.
• Symport depends on the transfer of another solute in the same direction.
• Antiport depends on the transfer of another solute in the opposite direction.

Sucrose Transport

• With primary active transport, a proton pump moves protons against their gradient.
• With secondary active transport, as protons flow back down the gradient, sucrose molecules are co-transported against the gradient.
• This allows accumulation of concentrations at much higher levels inside the cell.

Vesicle-Mediated Transport

• Vesicle-mediated transport is used for large particles like proteins, polysaccharides, microorganisms, and cellular debris via vesicles that bud off from or fuse with the plasma membrane.
• Exocytosis releases contents after fusing with the membrane, which includes the secretion of substances from the Golgi body.
• Endocytosis takes up material from outside the cell.

Endocytosis Forms

• Phagocytosis involves contact with particulate matter.
• Pinocytosis involves plasma membrane pouches inward to bring in extracellular liquid.
• Receptor-mediated endocytosis involves molecules binding to specific proteins in pits.

Cell Communication

• Communication occurs through chemical signals to the target cell.
• This involves reception, transduction, and induction.

Plasmodesmata

• Plasmodesmata are narrow strands of cytoplasm interconnecting the protoplasts of neighboring plants.
• Symplast is plasmodesmata + protoplasts.
• Symplastic transport involves movement from cell to cell via plasmodesmata.
• Apoplast is the movement of substances in the cell wall continuum surrounding the symplast;
• Primary plasmodesmata form during cytokinesis.
• Secondary plasmodesmata form afterwards and are essential for cellular communication between cells of different lineage, and are typically branched.