Cells and Their Environment Chapter 4 Cells and Their Environment

Section 1: Passive Transport Cell need to maintain Homeostasis Controlling what moves across the cell membrane 1) Energy is need to move materials = ACTIVE TRANSPORT 2) Energy is NOT need to move materials = PASSIVE TRANSPORT

Passive Transport Molecules enter and exit an area because of their own motion Molecules will ALWAYS move from areas of HIGH concentration to areas of LOW concentration A difference in concentration of substances is called a CONCENTRATION GRADIENT

Passive Transport All cells strive for EQUILIBRIUM Is a condition in which the concentration of a substance is equal through a space DIFFUSION is the movement of molecules from an area of high concentration to an of low concentration Small molecules or Nonpolar molecules can diffuse across the cell membrane Diffusion is the simplest type of Passive Transport

Osmosis Is the diffusion of WATER through the cell membrane Is a type of Passive Transport Occurs as “free” water molecules move into the solution with lower concentration 3 possibilities for the direction of water movement

Osmosis – Directions of Movement #1 – Water Moves Out (Hypertonic) Cell Shrinks Outside the cell = Lower Concentration Inside the cell = Higher Concentration #2 – Water Moves In (Hypotonic) Cell Swells Outside the cell = Higher Concentration Inside the cell = Lower Concentration

Osmosis – Directions of Movement #3 –No Net Water Movement (Isotonic) Cell Stays the same Free water molecules inside & out are EQUAL Cell is in a state of Equilibrium Hypotonic solution could cause the cell to burst Cell walls in plants & fungi help prevent bursting

Crossing the Cell Membrane Most ions & polar molecules can NOT cross the cell membrane But CAN cross with the help of Channels ION CHANNELS Na, K, Ca, and Cl ions Is a transport protein with a polar pore through which ions can pass Some channels are always open Some channels have gates

Crossing the Cell Membrane Diffusion of ions through ion channels is a form of Passive Transport No energy is used because ions move down their concentration gradients Movement of ions is influenced by their charge Inside the cell  Negatively Charged “Opposites Attract”

Facilitated Diffusion (Pg. 80 - Figure 4) CARRIER PROTEINS – a kind of transport protein that can bind to a specific substance on one side and be carried to the other side of a cell Transport – Sugar & Amino Acids When carrier proteins transport Sugar & amino acid is is called FACILITATED DIFFUSION Facilitated diffusion is a type of Passive Transport

Section 1 Review 1. What is the difference between diffusion and osmosis? 2. Describe how diffusion of ions across a cell membrane differs from the diffusion of nonpolar molecules across the cell membrane. 3. How does facilitated diffusion work? 4. T/F Diffusion occurs as a result of the random motion of substances. 4. Why are green leafy vegetables sprayed with water at supermarkets?

Section 2: Active Transport Moves substances AGAINST their concentration gradients From an area of LOW concentration to an area of HIGH concentration REQUIRES ENERGY Energy comes from ATP Membrane Pumps

Sodium-Potassium Pump Transports 3 Na ion out & 2 K ions into the cell ATP supplies the energy to move the Na and K ions FIGURE 5 – page 82 – STUDY! Important for 2 Reasons #1 – Prevents Na from accumulating inside If accumulation occurs the cell could burst #2 – Helps Maintain the concentration gradient

Movement in Vesicles Large molecules are moved across the cell membrane by VESICLES Proteins & Polysaccharides Movement of substances into the cell by vesicles is called ENDOCYTOSIS Cell membrane forms a pouch around a substance Pinches off & forms a vesicle

Movement in Vesicles The movement of substances outside of a cell is called EXOCYTOSIS Vesicles fuse with the cell membrane and expel their contents outside the cell Cells use exocytosis to transport proteins that are modified by the Golgi Apparatus

Membrane Receptor Proteins Signal Molecules  Hormones RECEPTOR PROTEINS – pick up these signals and respond to the ones that are important Signal Molecules binding to a receptor protein cause a change in receiving cells

Changes in Receiving Cell 1) Change in Permeability Cause the ion channel to open Important in the nervous system 2) Second Messengers Act as a signal molecule in the cytoplasm Activating enzymes, triggering chemical reactions or change the permeability of the cell

Changes in Receiving Cells 3) Enzyme Action Receptor Protein may act as an Enzyme Speeds up chemical reactions Activate other enzymes Drug affect the binding sites of molecules to receptor proteins

Section 2 Review 1. What is the difference between passive and active transport? 2. Which ion(s) move against a concentration gradient in a sodium-potassium pump? 3. What serves as an “escort” to enable substances to move across a cell membrane by active transport and by endocytosis or exocytosis? 4. What are the 3 functions of receptor proteins?