1. cell
physiology

2. is the net movement of a substance
diffusion
is the net movement of a substance
from where it is in a higher concentration
to where it is in a lower concentration
this is a passive process,
using the kinetic energy of the particles
to pass across the membrane

3. factors affecting rate of diffusion
concentration
gradient
The greater the concentration gradient across the membrane, the faster the diffusion
particle
size
Small molecules diffuse faster than larger ones
Diffusion normally takes place quicker at high temperatures, as the higher temperatures give the diffusing molecules more kinetic energy
temperature

4. membrane thickness surface area
Biological membranes are generally very thin, giving a short diffusion distance, ideal for rapid diffusion
membrane
thickness
The greater the surface area, the faster the diffusion. In many cells where diffusion is important, the cell surface membrane is extended to increase the area across which diffusion can take place, e.g. by the presence of microvilli or root hairs
surface
area

5. This is the relationship between
Fick's law
This is the relationship between
the rate of diffusion, concentration gradient, surface area, and membrane thickness
at a given temperature
conc
gradient SA X
rate of
diffusion =
membrane
thickness

6. surfaces which require fast diffusion rates
will have high values on the top line and low on the bottom
e.g. alveoli, villi
surfaces which require slow diffusion rates
will have low values on the top line and high on the bottom
e.g. xerophyte leaf surface to reduce transpiration

7. there are 2 types of diffusion
simple diffusion
Substances cross the phospholipid bilayer easily if they are fat soluble (glycerol, steroid hormones, vitamins A + E) or if they are small uncharged/non-polar (O2, CO2) or
Very small charged molecules (H2O, urea)
The substances move between the fatty acid tails of the phospholipids.
Large water soluble molecules (glucose) and ions can’t pass through the membrane this way, due to the hydrophobic nature of the phospholipid bilayer

8. facilitated diffusion
Protein carriers in the cell membrane facilitate the movement of water soluble molecules and ions through the membrane.
These proteins have binding sites that match specific molecules and assist the movement of these molecules across the membrane.

9. channel proteins / ion channels
These proteins have a central pore lined with charged/polar groups making it hydrophillic.
Ions such as Ca2+, Na+, K+, Cl- and HCO32-
can move across the membrane through these pores.
Each channel protein is usually specific for one type of ion.
Some channels are permanently open, whilst others can be opened and closed; they are gated. This allows the flow of ions to be controlled.

10. ions Gated channel intrinsic protein phospholipid bilayer hydrophilic

11. carrier proteins
These proteins allow large polar molecules such as sugars and amino acids to cross the membrane.
A specific molecule attaches to the carrier protein at a particular binding site. The protein changes shape allowing the molecule to pass across the membrane.

12. draw diagram page 52

13. the rate of facilitated diffusion is dependent upon the number of
channel and carrier proteins
in the membrane

14. the use of energy from respiration
active transport
the use of energy from respiration
to move substances across the membrane,
through carrier proteins,
against the concentration gradient

15. The molecule or ion to be transported combines with a specific carrier protein.
ATP causes the carrier protein to change shape, and releases the transported substance on the other side of the membrane. Once released the carrier protein returns to its original shape.
Cells which undergo AT usually have many mitochondria and a high rate of respiration to provide the ATP energy needed for active transport.

16. The carrier proteins are specific to a particular molecule or ion.
The presence or absence of particular carrier proteins allows the membrane to be selective in the molecules or ions that they take up.

17. AT is affected by temperature, oxygen concentration and the presence of respiratory poisons such as cyanide.
Processes involving AT include:
Nerve impulse transmission
Muscle contraction
Protein synthesis
Absorption of mineral salts by root hair cells

18. ATP ADP + Pi Solute binds with carrier protein
2. Energy from cell respiration
changes shape of protein

19. 3. Carrier protein changes shape,
releasing solute into the cell

20. The carrier protein systems are often referred to as protein pumps
The carrier protein systems are often referred to as protein pumps. Some protein pumps are able to move different molecules into and out of the cells, e.g. the sodium-potassium pump

21. The sodium-potassium pump moves sodium into and potassium out of cells

22. cytosis
This method transports substances in (endocytosis) and out (exocytosis) of the cell without actually passing across the cell membrane.
It is involved in
transporting large molecules that are too big for the carriers
and bulk transport of smaller molecules such as water.
The process is active i.e. requires energy.

23. There are 2 forms of endocytosis:
The cell membrane invaginates (folds in) around the substance entering the cell to form a membrane bound sac or vesicle. This pinches off the cell membrane and the vesicle enters the cytoplasm. The membrane reforms to close the gap.
There are 2 forms of endocytosis:

24. phagocytosis pinocytosis
This involves the transport of solid material into the cell.
White blood cells take in bacteria and amoeba take in euglena by phagocytosis.
pinocytosis
This involves the transport of fluid into the cell.
Human egg cells take up nutrients from surrounding cells this way.

25. exocytosis
In this process substances pass out of the cell in a reverse of endocytosis.
This process is important in secretion e.g. digestive enzymes, hormones, mucus.
Secretory vesicles, possibly from the golgi apparatus, move to and fuse with the cell surface membrane. The contents are then released outside the cell and the membrane re-sealed.

26. endocytosis exocytosis cell surface membrane cell surface membrane
secretory
vesicle
exocytosis