1. Transport across cell membranes
CELL BIOLOGY!
Transport across cell membranes

2. Cell membrane: A fluid mosaic
REMINDER: Why are phospholipids so special?
The membrane is about as fluid as salad dressing, and the components drift about
phospholipids are constantly moving and bumping into each other. It’s like a crowded party!
ACTIVITY: Membrane Structure

3. A beautiful mosaic…
The membrane has various proteins embedded in its fluid framework
These proteins have different functions, depending on the type of cell.

4. Cholesterol
This steroid helps to stabilize the membrane at warm temperatures, and also helps keep it fluid at low temperatures

5. Cell Membrane – A Mosaic

6. Functions of Membrane Proteins
(a) Transport – active vs. passive
PASSIVE  if molecules are moving from [high] to [low]
ACTIVE  if molecules are moving from [low] to [high]
Active transport requires ATP
(b) Enzymes for metabolic pathways
(c) Receptors for chemical messages.
“Signal Transduction”
ACTIVITY: Signal Transduction

7. Functions of membrane proteins
(d) Cells need to recognise other cells around them, for example white blood cells move across the lining of blood vessels
(e) Cells join to one another by protein junctions. When the space gets too crowded they know to stop dividing
(f)Help maintain cell shape by attaching to the cytoskeleton & extracellular matrix

8. How do substances cross?
When molecules diffuse down their concentration gradient, they move from an area of [high] to [low] until equilibrium is reached
At equilibrium molecules still move but at an equal rate in and out of the cell.

9. Selectively Permeable
Activity: Selective Permeability of Membranes

10. Passive Transport ACTIVITY: Diffusion
A cell does not have to do work to achieve equilibrium
Molecules will still move back and forth randomly, but will always stay in equilibrium
Two or more substances diffuse independently of each other, down its own concentration gradient.

11. Simple Diffusion
Simply movement of molecules from high to low concentration across the cell membrane.
No protein is needed
Ex. Water (slowly), I2 (a non- charged molecule), alcohol, oxygen, carbon dioxide
Macromolecules (ex. Proteins) are too large and will not diffuse. Polarity is also a factor.

12. Amazing Real Life Example…
O2 travels from a [high] in the alveoli through cell membranes into pulmonary capillaries, where O2 is in [low] in the blood.

13. What affects the rate of diffusion?
Concentration Gradient
Diffusion will be faster if the concentration gradient is large
Size of Molecules
Smaller molecules diffuse faster
Charge
Ions need proteins to diffuse across the membrane
The number of proteins in the membrane will affect diffusion
Temperature
Diffusion is faster at higher temperatures
Chemical nature of molecules

14. Facilitated Transport
Polar or charged particles can move down their concentration gradient only if there is a special pathway for them!
Does not require energy
Examples
Sugars (glucose), amino acids, Ions (Ca2+)
Water in kidney cells, red blood cells, and plant cells by the protein aquaporin.
FUN FACT! This allows the entry or exit of up to 3 billion water molecules a second!
ACTIVITY: Facilitated Diffusion

15. Osmosis Simple diffusion of water across a phospholipid bilayer
Water moves with its concentration gradient
Water molecules will move to a [lower] until equilibrium is reached
ACTIVITY: Osmosis and Water Balance in Cells

16. Hyper, hypo, and isotonic solutions
Isotonic – [equal] in & out of cell. Water moves in and out equally
Hypotonic – lower [solute] outside cell. Water enters the cell. Bursts, or turgid
Hypertonic - higher [solute] outside cell. Water exits the cell. Shriveled.

17. Active Processes Energy is required to move the molecules Three types:
Active Transport
Endocytosis
Exocytosis
GET IT?!?! He’s transporting through the cell wall!

18. Active Transport Transport proteins are needed
Solute binds to protein, but cannot pass through
ATP  ADP + P provides energy for protein to change shape
Ex. Sodium, potassium

19. Exocytosis Exo = outside Kytos = cell
Used to export bulky materials such as proteins or polysaccharides
Vesicle fuses with plasma membrane to secrete material outside of cell
This requires energy, so it is an active process
EX. When the Toronto Maple Leafs lose and you cry, cells in your tear glands use exocytosis to export a salty solution containing proteins
EX.2. steroids (estrogen, testosterone), other hormones (adrenalin, thyroxin)

20. Endocytosis
The formation of a transport vesicle by the in-folding of the cell membrane
White blood cells do this A LOT!
After the vesicle forms, it will either be released into the cytoplasm or digested by enzymes in lysozomes.
Also needs ATP, so is an active process
There are 2 types
Phagocytosis
Pinocytosis

21. Phagocytosis – “Cellular Eating”
Phagocytosis takes in large chunks, such as bacteria, viruses, or old cells
A lysosome will fuse with this vacuole and hydrolytic enzymes will digest the contents

22. Pinocytosis – “Cellular Drinking”
The cell “gulps” droplets of fluid in tiny vesicles
Any solutes dissolved in the droplets will enter the cell
ACTIVITY: Exocytosis and Endocytosis

23. Bioflix Time BIOFLIX: Membrane Transport
Fill out the flow chart in your notes!