The Cell Membrane AKA Plasma Membrane

3.4 Plasma Membrane Structure and Function The plasma membrane regulates entrance and exit of substances from the cell to help maintain homeostasis. The fluid-mosaic model of the plasma membrane structure: Phospholipids form a bilayer, with the hydrophilic polar heads facing inside and outside of the cell and hydrophobic tails facing each other consistency of light machine oil (~fluid) Peripheral proteins are partially embedded on one side of the membrane Integral proteins span the entire membrane and may protrude into one or both sides; they move laterally Steroids, glycolipids, and glycoproteins are also present UNIT A Chapter 3: Cell Structure and Function Section 3.4

carbohydrates strung together in chains are attached to proteins ("glycoproteins") or lipids ("glycolipids") of membrane. They function as identification markers for cell recognition (helps immune system identify which cells belong to body and which are invaders). Cholesterol helps to both maintain the integrity of the cell membrane by making sure it isn’t too fluid by blocking the movement of phospholipids and to maintain the fluidity of the cell membrane by making sure phospholipids cannot clump together and crystalize.

UNIT A Chapter 3: Cell Structure and Function Section 3.4 TO PREVIOUS SLIDE Figure 3.15 Fluid-mosaic model of plasma membrane structure.

Functions of the Membrane Proteins Plasma membranes of different cell types have unique combinations of proteins. Generally, Peripheral proteins play a structural role by helping to stabilize and shape the plasma membrane Integral proteins determine a membrane’s specific functions. The following are types of integral proteins: channel proteins carrier proteins cell recognition proteins receptor proteins enzymatic proteins UNIT A Chapter 3: Cell Structure and Function Section 3.4 TO PREVIOUS SLIDE

Functions of the Membrane Proteins Channel proteins are involved in the passage of molecules through the cell membrane by forming a channel. UNIT A Chapter 3: Cell Structure and Function Section 3.4 TO PREVIOUS SLIDE Carrier proteins are involved in the passage of molecules through the cell membrane by combining with the substance. From Figure 3.16 Examples of membrane protein diversity.

Functions of the Membrane Proteins Cell recognition proteins are glycoproteins involved in cell recognition of pathogens. UNIT A Chapter 3: Cell Structure and Function Section 3.4 TO PREVIOUS SLIDE Receptor proteins bind specific molecules, which causes a cell response. From Figure 3.16 Examples of membrane protein diversity.

Functions of the Membrane Proteins UNIT A Chapter 3: Cell Structure and Function Section 3.4 TO PREVIOUS SLIDE Enzymatic proteins catalyze cell reactions. From Figure 3.16 Examples of membrane protein diversity.

3.5 The Permeability of the Plasma Membrane The plasma membrane is selectively permeable, allowing passage of only certain molecules. UNIT A Chapter 3: Cell Structure and Function Section 3.5 TO PREVIOUS SLIDE Figure 3.17 How molecules cross the plasma membrane. Molecules that can diffuse across the plasma membrane are shown with long back-and-forth arrows. Substances that cannot diffuse across the membrane are indicated by the curved arrows.

Passage of Molecules Across the Membrane UNIT A Chapter 3: Cell Structure and Function Section 3.5 TO PREVIOUS SLIDE Some substances freely cross the membrane. They move “down” their concentration gradient (from high concentration to low concentration). Some substances are unable to freely cross and are transported by proteins or vesicles. They may go “up,” or against, their concentration gradient.

Diffusion Diffusion is the movement of molecules down their concentration gradient. It does not require energy. The rate of diffusion is affected by factors such as temperature, pressure, and molecule size. A solution contains a solute in a solvent. Diffusion occurs until there is an equal distribution of solute and solvent. Figure 3.18 Process of Diffusion. UNIT A Chapter 3: Cell Structure and Function Section 3.5 TO PREVIOUS SLIDE

Three Ways of increasing the rate of diffusion: 1. increase the temperature 2. increase the concentration gradient 3. decrease the size of the diffusing molecules

Diffusion of Oxygen Only a few types of molecules can diffuse across the plasma membrane. Gases can diffuse across the bilayer Oxygen enters cells and carbon dioxide leaves In lungs, oxygen moves from the alveoli to blood in the capillaries Figure 3.19 Gas exchange in lungs. Oxygen (O 2 ) diffuses into the capillaries of the lungs because there is a higher concentration of oxygen in the alveoli (air sacs) than in the capillaries. UNIT A Chapter 3: Cell Structure and Function Section 3.5 TO PREVIOUS SLIDE

Osmosis Osmosis is the diffusion of water molecules across a selectively permeable membrane due to a difference in concentration. There is a net movement of water and changes in solute concentration on both sides of the membrane Figure 3.20 Osmosis demonstration. UNIT A Chapter 3: Cell Structure and Function Section 3.5 TO PREVIOUS SLIDE

Definitions: Osmosis: the net movement of water molecules from the area of greater concentration to the area of lesser concentration across a selectively-permeable membrane. Solute: particles which are dissolved in water Solvent: liquid which dissolves the solute. This is water when we are talking about osmosis. Solution: combination of solute and solvent. Osmotic Pressure: the pressure due to the flow of water from the area of greater concentration to the area of lesser concentration. The greater the concentration difference across the membrane, the greater the osmotic pressure.

UNIT A Chapter 3: Cell Structure and Function Section 3.5 TO PREVIOUS SLIDE Isotonic solutions have the same concentration of solute and solvent as the solution inside the cell, and water will not enter or leave the cell. Hypotonic solutions have a lower concentration of solute than solution inside the cell, and water will enter the cell. Hypertonic solutions have a higher concentration of solute than solution inside the cell, and water will leave the cell. Isotonic, Hypotonic, and Hypertonic Solutions Prefixes: iso: the same as hypo: less than hyper: more than _____________ tonicity: refers to osmotic pressure

UNIT A Chapter 3: Cell Structure and Function Section 3.5 TO PREVIOUS SLIDE Figure 3.21 Osmosis in animal and plant cells.

Transport by Carrier Proteins The plasma membrane stops the passage of most molecules into and out of the cell. However, biologically important molecules do pass. They do so because of carrier proteins that exist in the plasma membrane. Carrier proteins are specific and each binds to specific molecules Carrier proteins are required for both facilitated transport and active transport of substances across the plasma membrane UNIT A Chapter 3: Cell Structure and Function Section 3.5 TO PREVIOUS SLIDE

Figure 3.22 Facilitated transport. UNIT A Chapter 3: Cell Structure and Function Section 3.5 TO PREVIOUS SLIDE Assists in transport of molecules across the membrane by binding to those molecules Occurs down a concentration gradient and does not require ATP Facilitated Transport

Active Transport Assists transport of substances across the membrane by binding to them Occurs against a concentration gradient and requires energy, usually in the form of ATP Proteins involved in active transport are often called pumps because they use energy to pump substances against their concentration gradient. One important carrier protein pump is the sodium- potassium pump. It moves sodium ions to the outside of the cell and potassium ions to the inside of the cell. UNIT A Chapter 3: Cell Structure and Function Section 3.5 TO PREVIOUS SLIDE

Figure 3.23 The sodium- potassium pump. The same carrier protein transports sodium ions (Na + ) to the outside of the cell and potassium ions (K + ) to the inside of the cell because it undergoes an ATP-dependent change in shape. Three sodium ions are carried outward for every two potassium ions carried inward. Therefore, the inside of the cell is negatively charged compared to the outside. UNIT A Chapter 3: Cell Structure and Function Section 3.5 TO PREVIOUS SLIDE

Bulk Transport Macromolecules are transported into and out of the cell by vesicle formation, called membrane-assisted transport in energy-dependent processes. Exocytosis is a way substances can exit a cell Endocytosis is way substances can enter a cell UNIT A Chapter 3: Cell Structure and Function Section 3.5 TO PREVIOUS SLIDE

Exocytosis During exocytosis, a vesicle fuses with the membrane and the substance it is carrying is secreted outside of the cell. Neurotransmitters, hormones, and digestive enzymes are examples of substances secreted in this way UNIT A Chapter 3: Cell Structure and Function Section 3.5 TO PREVIOUS SLIDE Figure 3.24 Exocytosis. Exocytosis deposits substances on the outside of the cell and allows secretion to occur.

Endocytosis During endocytosis, cells take in substances by vesicle formation. The plasma membrane folds in on itself and then pinches off to form an intracellular vesicle Endocytosis occurs in one of three ways. Phagocytosis Pinocytosis Receptor-mediated endocytosis UNIT A Chapter 3: Cell Structure and Function Section 3.5 TO PREVIOUS SLIDE

UNIT A Chapter 3: Cell Structure and Function Section 3.5 TO PREVIOUS SLIDE From Figure 3.25 Three methods of endocytosis. a. Phagocytosis occurs when the substance to be transported into the cell is large. Amoebas ingest by phagocytosis. Digestion occurs when the resulting vacuole fuses with a lysosome. During phagocytosis, the material being taken into the cell is large, such as a food particle or another cell. Common in unicellular organisms and occurs in certain types of human white blood cells Phagocytosis

UNIT A Chapter 3: Cell Structure and Function Section 3.5 TO PREVIOUS SLIDE From Figure 3.25 Three methods of endocytosis. b. Pinocytosis occurs when a macromolecule such as a polypeptide is transported into the cell. The result is a vesicle (small vacuole). During pinocytosis, vesicles form around liquid or very small particles. Common in blood cells, intestinal cells, and plant root cells Pinocytosis

UNIT A Chapter 3: Cell Structure and Function Section 3.5 TO PREVIOUS SLIDE From Figure 3.25 Three methods of endocytosis. c. Receptor-mediated endocytosis is a form of pinocytosis. Receptor-mediated endocytosis is a type of pinocytosis. It involves receptor proteins that only bind to certain molecules. The receptors are in coated pits. Once vesicles form, they become uncoated and fuse with lysosomes. Empty vesicles fuse with the plasma membrane and receptors return to their previous locations. Receptor-Mediated Endocytosis