1. Cells and Their Environment
Chapter 4
Cells and Their Environment

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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”

10. 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

11. 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?

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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?