Biology 120 Notes (Part 9) Phospholipids, Plasma Membrane, Diffusion, and Osmosis

Phospholipids: Membrane Components

• ==Phospholipids== consist of one glycerol molecule linked to two fatty acids and a modified phosphate group.
• Phospholipids are amphipathic and their function is to form plasma membranes.
• Phospholipids also tend to form a phospholipids bilayer.

Plasma Membrane Structure and Function

• The ==plasma membrane== is common to all cells, separates cell contents from outside of the cell, allows for communication between cells, allows for cells to attach to each other, and is selectively permeable.
• The plasma membrane’s main components are ==the phospholipid bilayer protein molecules, carbohydrates, and cholesterol.==
• Membrane fluidity decreases as temperature drops.
• The plasma membrane defines the intro-cellular environment.
• There are three mechanisms of membrane transport: ==diffusion, facilitated diffusion through transports of carriers, and active transport== (this mechanism requires energy for transport.)
• Permeability of the plasma membrane is affected by water moving across it, via aquaporins (a type of channel protein.)
• Double bonds can cause a “kink” in the hydrocarbon chain.
• Saturated hydrocarbon chains have fewer spaces and stronger hydrophobic interactions.
• Longer tails make the membrane less permeable (increased hydrophobic reactions.)

Membrane Proteins

• ==Membrane proteins== function as transporters, receptors, and enzymes or in binding and adhesion.
• There are two types of membrane proteins: ==integral and peripheral==.

Carbohydrate Chains

• ==Carbohydrate chains== are attached to proteins or phospholipids, and they function as cellular “fingerprints.”

Membrane Permeability

• Many factors influence the behavior of the membrane such as ==the number of double bonds in phospholipid tails, length of the tails, the number of cholesterol molecules in the membranes, and the temperatures.==
• ==Cholesterol== reduces membrane permeability.
• Adding cholesterol to membranes increases the density of the hydrophobic section.
• Membrane fluidity decreases as the temperature drops.

How Substances Move Across Lipid Bilayer: Diffusion and Osmosis

• ==Diffusion== is the net movement of molecules down a concentration gradient. The movement stops when it hits equilibrium.
• A ==solution== consists of a solvent (liquid) and a solute (dissolved liquid.)
• Diffusion is a type of passive transport.
• Membrane proteins facilitate diffusion.
• ==Facilitated diffusion== is a passive transport involving transmembrane proteins.
• Channel proteins facilitate diffusion. Electrical gradients are a combo of two gradients.
• ==Osmosis== is the diffusion of water across a selectively permeable membrane.
• The movement of water is toward a low water (high solute) concentration.
• ==Isotonic solutions== describe how the amount of solute and water are equal on both sides of the membrane. There is not net gain or loss concentrations of water in the cell.
• ==Hypotonic solutions== describe the concentration of solute in the solution is lower inside the cell.
• ==Hypertonic solutions== describe the concentration of solute being higher in the solution than inside the cell.

More on Osmosis

• Cells placed in a hypertonic solution will shrink, this is called ==crenation== and occurs in animal cells.
• ==Plasmolysis== occurs in plant cells.
• Cells placed in a hypotonic solution will swell which causes turgor pressure in plants.
• This may cause animal cells to ==lyse== (rupture.)

More on Active Transport

• ==Active transport== moves substances against their gradient and requires an input of energy (ATP.)
• ==Bulk transport== is the movement of large particles across the plasma membrane using vesicles.
• There are two varieties: ==exocytosis and endocytosis.==
• ==Pinocytosis== describes how vesicles form around a liquid or very small particles
• ==Receptor-Medicated Endocytosis== describes a specific form of pinocytosis using receptor proteins and a coated pit.
• ==Phagocytosis== describes how the plasma membrane engulfs a smaller cell or food particle. Large, solid material is taken in and digested and is common in unicellular organisms.

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