The Cell/Plasma Membrane and Cellular Processes

Homeostasis- maintaining balance Cells must keep the proper concentration of nutrients and water and eliminate wastes. The plasma membrane is selectively permeable – it will allow some things to pass through, while blocking other things.

Embedded with proteins and strengthened with cholesterol molecules. Cell membrane Lipid bilayer – two sheets of lipids (phospholipids). Found around the cell, the nucleus, vacuoles, mitochondria, and chloroplasts. Embedded with proteins and strengthened with cholesterol molecules.

Cell membrane Phospholipid bilayer: hydrophilic heads and hydrophobic tails Hydrophilic: water loving Hydrophobic: hates water Transport proteins (passive transport channels) Ion pumps (active transport pumps) Receptor proteins (neurons, hormones, immune system) Carbohydrate chains ( identification cards)

What’s a phospholipid? It’s a pair of fatty acid chains and a phosphate group attached to a glycerol backbone. Polar (water-soluble) heads face out and the nonpolar fatty acids hang inside.

Figure 04.UN00a Title: A phospholipid Caption: A phospholipid.

Figure 04.UN00b Title: A phospholipid Caption: A phospholipid bilayer.

Membrane Proteins

Function of Membrane proteins 1. Determine what particles can pass through the membrane. 2. Serve as enzymes (may speed reactions). 3. Act as markers that are recognized by chemicals and molecules from the inside and the outside of the cell (the immune system).

Figure 04.1 Title: The plasma membrane is a fluid mosaic Caption: The plasma membrane is a bilayer of phospholipids in which various proteins are embedded. Many proteins have carbohydrates attached to them, forming glycoproteins. The wide variety of membrane proteins fall mostly into three categories: transport proteins, receptor proteins, and recognition proteins.

No energy required Passive transport

Passive Transport Molecules move down the concentration gradient Molecules move from an area of high concentration to low concentration.

Passive Transport Diffusion – movement of particles from an area of high concentration to an area of low concentration. Caused by Brownian motion (movement of particles because of the movement of their atoms). Continues until an equilibrium is reached (no gradient). Dynamic equilibrium – particles move freely and are evenly distributed.

Types of Passive Transport 1) simple diffusion -molecules are small enough and can pass directly through the lipid bilayer 2) facilitated diffusion – need transport proteins (molecules are either to large or can’t pass through the lipid bilayer themselves) 3) osmosis – movement of water

Figure 04.3a Title: Diffusion through the plasma membrane Caption: Simple diffusion through the phospholipid bilayer: Gases such as oxygen and carbon dioxide and lipid-soluble molecules can diffuse directly through the phospholipids.

Figure 04.3b Title: Diffusion through the plasma membrane Caption: Facilitated diffusion through a channel: Some molecules cannot pass through the bilayer on their own. Protein channels (pores) allow some water-soluble molecules, principally ions, to enter or exit the cell.

Figure 04.3c Title: Diffusion through the plasma membrane Caption: Facilitated diffusion through a carrier: Carrier proteins may bind a specific molecule and, as a result, change their own shape, passing the molecule through the middle of the protein to the other side of the membrane.

The diffusion of water Osmosis

Isotonic: the same amount of dissolved solute Isotonic: the same amount of dissolved solute. (cell and solution have equal solute concentration) Hypertonic: more dissolved solute.(cell placed in solution with higher concentration of solute) Hypotonic solution: less dissolved solute.(cell placed in solution with lower concentration of solute)

Osmosis

Figure 04.5 Title: The effects of osmosis Caption: Red blood cells are normally suspended in the fluid environment of the blood and cannot regulate water flow across their plasma membranes. (a) If red blood cells are immersed in an isotonic salt solution, which has the same concentration of dissolved substances as the blood cells do, there is no net movement of water across the plasma membrane. The red blood cells keep their characteristic dimpled disk shape. (b) A hypertonic solution, with too much salt, causes water to leave the cells, shriveling them up. (c) A hypotonic solution, with less salt than is in the cells, causes water to enter, and the cells swell.

Requires energy Active transport

Molecules move against the concentration gradient Requires energy Na+- K+ pump is a protein carrier that requires energy

Active Transport Endocytosis: molecules are coming into cells (examples- phagocytosis and pinocytosis) Exocytosis: release of molecules out of the cell (examples- proteins, hormones)

Figure 04.7 Title: Three types of endocytosis Caption: (a) Pinocytosis: A dimple in the plasma membrane deepens and eventually pinches off as a fluid-filled vesicle, which contains a random sampling of the extracellular fluid. (b) Receptor-mediated endocytosis: Receptor proteins selectively bind molecules (for example, nutrients) in the extracellular fluid. The receptors migrate along the plasma membrane to dimpling sites (coated pit). The membrane dimples inward, carrying the receptor-captured molecule with it. The end of the coated pit buds off a coated vesicle into the cell’s cytoplasm. The vesicle contains both extracellular fluid and a high concentration of the molecules that bind to the receptors. (c) Phagocytosis: Extensions of the plasma membrane, called pseudopodia, encircle an extracellular particle (for example, food). The ends of the pseudopodia fuse, forming a large vesicle (a food vacuole) containing the engulfed particle.

Figure 04.8 Title: Receptor-mediated endocytosis Caption: These electron micrographs illustrate the sequence of events in receptor-mediated endocytosis. (a) This type of endocytosis begins with a shallow depression in the plasma membrane, coated on the inside with a protein (dark, fuzzy substance in the micrographs) and bearing receptor proteins on the outside (not visible). (b,c) The pit deepens and (d) eventually pinches off as a coated vesicle. The protein coating is eventually recycled back to the plasma membrane.