1. Functions and Transport
The Cell Membrane
Functions and Transport

2. Cell (Plasma) membrane- Introduction
A basic principle in biology is that all living things are made of cells. While they come in a wide variety of shapes and sizes, all cells have a fluid inside (called cytoplasm) surrounded by a protective layer.

3. Cell (Plasma) membrane- Introduction
The cell membrane, also called plasma membrane, is the boundary that separates all living cells from their nonliving surroundings. This membrane: 1. Controls what enters and leaves the cell 2. Maintains homeostasis 3. Protects the cell from the outside environment 4. Is selectively permeable, meaning it allows some substances to cross more easily than others.

4. Cell (Plasma) membrane- Components
The cell membrane is made up of three types of organic compounds
Lipids (phospholipids and steroids)
Carbohydrates
Proteins
The membrane is flexible and made of lipid molecules. It is mixed with large protein molecules that act as channels through which other molecules enter and exit the cell.

5. Components of cell Membrane- Lipids
Phospholipids- make up the majority of the cell membrane
The heads are polar (hydrophilic- “water loving”)
The tails are nonpolar (hydrophobic- “water fearing”)
By forming a bilayer, the hydrophobic tails are sheltered
from the water that surrounds the cell inside and out.

6. Components of cell Membrane- Lipids
2. Steroids (or sterols)- a second, less abundant type of lipid found in the cell membrane.
The function is to regulate the fluidity of the cell membrane.
At high temperatures, steroids help hold the cell membrane together.
At low temperatures, steroids keep the bilayer from packing too tightly together.
Cholesterol is the main membrane steroid found in animal cells.
Cholesterol is made by all animals and is found in varying quantities in the food you eat.
HDL= Good cholesterol (Healthy)
LDL= Bad Cholesterol (want Less of this)

7. Components of cell membrane- proteins
Membrane proteins- used to transport material in to the cell (things that cannot easily pass through the membrane itself)
There are 2 types of membrane proteins:
Peripheral proteins- located on the surface of the cell membrane (inside and outside)
Integral proteins- pass through the lipid bilayer

8. Components of cell membrane- carbohydrates
Carbohydrates are usually attached to the outer surface of the cell membrane.
They act as “antenna” and are used by the cells to identify each other.
There are 2 types, depending on what they are attached to:
Glycolipids- a carbohydrate attached to a lipid
Glycoprotein- a carbohydrate attached to a protein

9. Maintaining homeostasis- buffers
How do cells use buffers?
Carbonic Acid System
Maintains blood pH at 7.4
Controlled by respiration (breathing)
Carbonic acid is converted to carbon dioxide
Illnesses can result from too much carbonic acid or from the blood becoming too basic
Dihydrogen Phosphate System
Operates in the cytoplasm
Cytoplasm has a pH range of 6.9 to 7.4
The ions work to maintain this

10. Movement of material across a membrane
Passive Transport

11. Passive Transport
The movement of material across the cell membrane without using energy. There are 2 types you need to know.
Diffusion and Osmosis

12. Passive transport-diffusion
Diffusion- The process by which molecules of a substance (not water) move from an area of high concentration to an area of low concentration Equilibrium is reached when the concentration is the same throughout.

13. Passive transport-diffusion
Dynamic Equilibrium-
Movement into the cell is at the same rate as the movement out of the cell
The cell has achieved a “steady state”…
HOMEOSTASIS
Rate of water entering = Rate of water leaving Hence the level of water is constant

14. Passive transport-diffusion
What does it all mean?
HIGH Concentration: lots of solute
LOW Concentration: little solute
The goal: To reach EQUILIBRIUM
CONCENTRATION GRADIENT: difference in concentration on each side of a membrane, the larger the difference, the greater the gradient
Factors that influence rate of diffusion:
1. temperature
2. pressure
3. concentration

15. OTHER EXAMPLES OF DIFFUSION
The diffusion of solute particles through solvent particles.
The diffusion gas particles through the air.

16. Passive transport- facilitative diffusion
Facilitate= “to help”
Facilitative diffusion- molecules are moved across the membrane from an area of higher to lower concentration with the help of a carrier protein
Does not require energy
Needed when the solute particle is too large to pass through
The carrier protein opens up a larger hole in the membrane to allow the solute to pass through.

17. Passive transport- osmosis
The movement of water molecules across a selectively permeable membrane.
Movement is from high to low concentration
The goal is to create equilibrium
Since water is a small molecule, it can move quickly
Water will move to the area of higher solute concentration to dilute it

18. Passive transport- osmosis
How does osmosis affect solutions?
Hypertonic- a solution with a higher concentration than another solution
Hypotonic- a solution with a lower concentration than another solution
Isotonic- a solution with an equal concentration to another solution

19. Passive transport- osmosis
How does osmosis affect cells?
Cytolysis- the bursting of an animal cell when placed in a hypotonic solution
Plasmolysis- the shrinking of a plant or animal cell when placed in a hypertonic solution
Turgor pressure- the force that pushes out on a plant’s cell wall when placed in a hypotonic solution

20. Movement of material across a membrane
Active transport

21. Active transport
Requires energy to get material across the cell membrane against a concentration gradient (think paddling against a current)
Substances are forced to move from an area of lower concentration to an area of higher concentration
There are 3 different types you should know
Pumps
Endocytosis
Exocytosis

22. Active transport- pumps
Pump- a special type of carrier protein that is used to push molecules from lower to higher concentration
Ex. Sodium (Na+) and Potassium (K+) pumps
Energy is required for pumps
The carrier will take three Na+ ions out of the cell and pick up two K+ to pump into the cell.
This unequal exchange of ions creates a charge on the cell. The cell uses this charge to send signals to other cells.

23. Active transport- endocytosis
Endocytosis- taking material into a cell by forming a pocket around it (the cell membrane folds around something in the external environment)
Phagocytosis- intake of large particles or cells (solid particles) through the cell membrane within a vesicle
Pinocytosis- using a vesicle or pocket to take in liquid or dissolved substances (solutes)

24. Active transport- exocytosis
Exocytosis- a vesicle made by the cell fuses with the cell membrane, releasing its contents outside of the cell
Proteins made by ribosomes in the cell are packaged into transport vesicles
These transport vesicles fuse with the cell membrane and then the proteins are secreted out of the cell (e.g. insulin) to help maintain glucose levels