1. More than just boundaries….
Biological Membranes
More than just boundaries….

2. Organelles with Membranes
Eukaryotes have their cells divided into organelles with membranes, including:
The external cell membrane
The nuclear membrane
The golgi complex, vacuoles, lysosomes, the e.r., the mitochondrion, the chloroplast, and more!
Prokaryotes (bacteria) lack these internal membranes, but they still have an external cell membrane

3. The Cell Membrane
Fluid Mosaic Model (many separate pieces that can move)
Phospholipid Bilayer with proteins embedded in it

4. Phospholipid Molecules
Phosphate “Head”
(Phosphorus + Oxygen)
Hydrophilic
(Loves Water)
Lipid “Tails”
(Fatty Acid Chains)
Hydrophobic
(Fears Water)
Simplified Phospholipid
Detailed Phospholipid

5. Phospholipid Bilayer
Phosphate heads are attracted to water (hydrophilic) so they form the inner and outer layers of the membrane in contact with the intracellular and extracellular fluids
Extracellular Fluid
Lipid tails avoid water (hydrophobic) so they form the interior of the membrane where they are protected from water
Intracellular Fluid
(Cytoplasm)

6. Selective Permeability
Permeable = Molecules can cross through it (a t-shirt is permeable to water)
Impermeable = Molecules can not cross through it (plastic is impermeable to water)
Semi-permeable = Some molecules can cross through it, others can’t
Selectively Permeable = The membrane CHOOSES which molecules can cross through it – and it chooses which direction they can cross!

7. Membrane Proteins
The phospholipid bilayer has many different proteins embedded in it.
Each type of protein has its own function, such as choosing which molecules are allowed to enter or exit the cell.

8. Channel Proteins I
Low Concentration
High Concentration
Channel Proteins allow molecules to cross the membrane by making a tunnel for them.

9. Channel Proteins II
Molecules are moving from high to low concentration
Energy is NOT needed
Passive transport
Facilitated diffusion

10. Molecules inside are exchanged for other molecules outside the cell
Membrane Pump
Molecules inside are exchanged for other molecules outside the cell
Energy is required
Active transport

11. Carrier Proteins
Specific molecules are grabbed and carried across the membrane by this protein
Energy is required
Active transport

12. This changes the activities of the cell that receives the message
Receptor Proteins
Hormones and neurotransmitters send messages from one cell to another by binding with this protein
This changes the activities of the cell that receives the message

13. Marker Proteins Cells can recognize each other with these proteins
Different kinds of cells have different markers
Very important to have matching marker proteins for organ transplants and blood transfusions

14. This steroid strengthens animal cell membranes
Cholesterol
This steroid strengthens animal cell membranes
Diets high in cholesterol are associated with high blood pressure and heart disease

15. The entire membrane is involved with this process
Endocytosis
If the cell “swallows” small particles (like water) it is called “pinocytosis!”
If the cell “swallows” large particles (or other cells) it is called “phagocytosis!”
The entire membrane is involved with this process
Large molecules are surrounded and engulfed when the cell forms a vacuole around them
Amoebas get food this way, and your white blood cells attack bacteria by the same process

16. Exocytosis
The golgi complex packages hormones, enzymes, and neurotransmitters into vesicles
The vesicles merge with the cell membrane, and release the molecules outside of the cell where the molecules can go to work
This is known as secretion, since these molecules are not waste molecules (if they were wastes, it would be called excretion)

17. The Concentration Gradient
5 % Salt
The concentration gradient is the difference in concentration of a molecule between the inside and outside of a cell.
1 % Salt
Hi Concentration Gradient
1.5 % Salt
1 % Salt
Lo Concentration Gradient

18. Passive Transport vs. Active Transport
No Energy Needed
Molecules cross membrane from high to low concentration (with the concentration gradient)
Includes diffusion, osmosis, and facilitated diffusion
Energy from ATP required
Molecules cross membrane low to high concentration (against the concentration gradient)
Includes carrier proteins, endocytosis, and exocytosis

19. “The Virtual Cell Website”
References
All images courtesy of
“The Virtual Cell Website”