Cell Structure and Function Ms. Taylor

5-2 The Plasma Membrane Has Many and Various Functions

The plasma membrane is a phospholipid bilayer with embedded proteins Plasma membrane is a fluid phospholipid bilayer – Polar head of a phospholipid is hydrophilic – Nonpolar tails of a phospholipid are hydrophobic Fluid-mosaic model – Mosaic - proteins embedded in the membrane have a pattern – Fluid - pattern varies according to the particular membrane and also within the same membrane at different times 5-3

The plasma membrane is a phospholipid bilayer with embedded proteins. The proteins have various functions. plasma membrane glycoprotein glycolipid hydrophobic tails Outside of cell hydrophilic heads phospholipid bilayer cholesterol filaments of cytoskeleton peripheralprotein Inside of cell carbohydrate chain 5-4

Proteins in the plasma membrane have numerous functions Plasma membrane of different cells and membranes of various organelles have specific proteins – Channel proteins – Carrier proteins – Cell recognition proteins – Receptor proteins – Enzymatic proteins – Junction proteins 5-5

Channel proteins In some cases the channnel proteins simply act as a passive pore. Molecules will randomly move through the opening in a process called diffusion. This requires no energy, molecules move from an area of high concentration to an area of low concentration.

Carrier proteins Carrier proteins assist molecules across the cell membrane. This process is called carrier mediated transport. The makeup of the amino acid chains in the protein determines the size and shape of the carrier protein. This, in turn, determines what molecule can be received by the carrier protein to cross the cell membrane. Remember that each molecule has a specific carrier protein that assists the molecule cross the cell membrane. That molecule can only cross the cell membrane if the particular carrier protein is available. There are two types of carrier proteins. The first type does not require energy to move the substance across the cell membrane - this is called facilitated diffusion and is described above. The second type uses energy and the process is called active transport.

Cell recognition proteins Recognition proteins are a protein in the plasma membrane that provides a fingerprint on the outside-facing surface of the cell, making it recognizable to other cells.

Receptor proteins Usually found embedded within the plasma membrane surface of a cell that receives chemical signals from outside the cell.

Enzymatic proteins Catalyze (speed up) the biochemical reactions that occur in cells.

Junction proteins

Homework Write a two page essay on a disease that can be caused by a malfunctioning membrane protein. Must be two FULL pages, typed (1.5 spaced, 12 pt Times New Roman font). Include: What is the problem protein, what is the disease, how is it diagnosed (at what age usually?), what are the long term effects? What has been done to treat this disease? MUST have proper grammar, proper capitalization and MUST have sources cited. No citations=no credit. Due in class: Monday, November 2, No late papers will be accepted for any reason.

Suggested Diseases Cystic Fibrosis PKU QT Syndrome

5-14 The Plasma Membrane Regulates the Passage of Molecules Into and Out of Cells

Diffusion across a membrane requires no energy Diffusion – Molecule moves from a high concentration to a low concentration until it is distributed equally – Concentration gradient Passive form of transport – Requires no energy to be added Very few molecules can simply diffuse through the hydrophobic portion of the plasma membrane 5-15

5-16 Some molecules can move freely by simply diffusing across a membrane. H2OH2O membrane Time red dye

Diffusion across a membrane requires no energy Facilitated diffusion – Requires a transporter but no energy – Carrier protein or channel protein – Transporter is specific 5-17 During this type of facilitated diffusion, a carrier protein combines with and assists solute movement across the membrane. carrier protein OutsideInside

Diffusion across a membrane requires no energy Osmosis – Diffusion of water across the plasma membrane due to concentration differences – Which way the water moves is dependent on the solute versus water concentration on both sides of the membrane Isotonic solution – cell neither gains or loses water Hypotonic solution – cell gains water – Hemolysis – Turgor pressure Hypertonic solution – cell loses water – Plasmolysis 5-18

5-19 When cells are in an isotonic solution, they remain normal. When cells are in a hypotonic solution, they gain water, and when they are in a hypertonic solution, they lose water to the environment. SolutionIsotonicHypotonicHypertonic Animal cells normal cellcell shrivelscell swells, bursts Plant cells normal cell Normal turgid cell cytoplasm shrinks from cell wall

Active transport across a membrane requires a transporter and energy Active transport – Molecules or ions move across the plasma membrane, accumulating on one side of the cell – Movement of molecules against their concentration gradients requires both a carrier protein and ATP Sodium-potassium pump undergoes a change in shape when it combines with ATP allowing it to combine alternately with sodium ions and potassium ions 5-20

5-21 During active transport, a protein carrier assists solute movement toward a higher concentration, and energy is required. P P carrier protein OutsideInside ATP

Bulk transport involves vesicle formation Bulk transport – Occurs when fluid or particles are brought Into a cell by vacuole formation - endocytosis Out of a cell by evagination – exocytosis – Phagocytosis occurs when the material taken in is large – Pinocytosis occurs when vesicles form around a liquid or around very small particles – Receptor-mediated endocytosis - receptors for particular substances are found in the plasma membrane 5-22

Bulk transport into the cell is by endocytosis Endocytosis 5-23