1. Transport across cell membranes

2. The Cell Wall These are found in plant cells.
They are made of cellulose.
The cell wall is slightly elastic, so it can stretch a certain amount if the cell is full.
It is completely permeable.
i.e. it has no control over what enters and exits the cell.

3. The Cell Membrane
All cells are surrounded by a partially (selectively) permeable cell membrane.
the membrane is partially permeable because it allows only small molecules through but not larger molecules.
Its function is to control what goes in and out of the cell.
The membrane is made of phospholipids and proteins.

4. The cell membrane cont
phospholipids have hydrophilic heads and hydrophobic tail.
Embedded in this layer are different proteins. Some have channels running through them that allow molecules through.
Other proteins sit on the surface, acting as receptors for hormones or antibodies.
Some are structural proteins, others help to actively pump certain molecules against a diffusion gradient.

5. Cell membrane

6. Passive transport
This is passive because it does not need energy.

7. diffusion
Remember! The particles of matter are constantly moving, those in a gas move faster and bump into each other more often than those in a liquid.
The movement of each particle is random, but the net movement can be in one direction.
The particles tend to spread evenly until they fill the space evenly.

8. diffusion

9. diffusion
This is the net movement of molecules from an area of high concentration to an area of low concentration, down a concentration gradient.
If there is a large difference in the concentration then the gradient is steep and diffusion is faster.

10. diffusion
Oxygen and carbon dioxide move into and out of cells by diffusion.
Many minerals are absorbed by diffusion.

11. Factors affecting Rate of diffusion
The size of the molecules, small molecules move faster than larger ones.
The temperature of the liquid or gas, higher temperature means faster diffusion.
The state of the matter, gases are faster than liquids.
The concentration of the chemicals. (diffusion gradient.)

12. Concentration gradients

13. Fascilitated diffusion
Some proteins called transport proteins, found within the membrane help or facilitate diffusion.
This allows certain chemicals across the membrane faster than others.
E.g. in the liver, certain transport proteins will carry glucose across.

14. Fascilitated diffusion
The transport proteins are thought to act specifically, rather like enzymes.
They are specialised for the chemical they carry across the membrane.
diffusion

15. osmosis
This is the net movement of water from an area of high water concentration to an area of low water concentration through a partially permeable membrane.
Does not require energy so is passive.

16. terminology
Hypotonic solution – this is a weak solution, i.e. it has a high water concentration.
Isotonic solutions – these have the same water concentration.
Hypertonic solution – this is a strong solution i.e. it has a low water concentration.

17. osmosis
osmosis and diffusion

18. osmoregulation
This is the control of water concentration.

19. Osmoregulation in plants
If the vacuole inside plant cells loses water, the cell becomes floppy and the cytoplasm pulls away from the cell wall.
The cells are said to be plasmolysed.
If there is a slight loss of water pressure the cell is said to be flaccid, and the plant will wilt.
When more water passes into the vacuole, it swells until the cell is tight or turgid.

20. Osmoregulation in plants
The cell is prevented from bursting by the presence of the cell wall.
in plants

21. Osmoregulation in animals
Since animal cells have no cell wall, they will burst (lyse) if water pours into them.
If water moves out they shrivel.
This is why it is so important for our blood system, liver and kidneys to keep our cells in the correct osmotic balance.

22. Single-celled animals
Small single-celled animals living in water have the constant problem of water flooding into their cells.
They overcome this problem by having a small pump called a contractile vacuole, which gathers up excess water and pumps it out of the animal at regular intervals.

23. Freshwater fish
Freshwater fish are often in danger of drowning as water pours into their bodies by osmosis.
They overcome this problem by producing copious amounts of urine.

25. Marine animals
Marine invertebrates e.g. starfish keep their bodies at the same level of salinity as the sea.
Bony fish tend to lose water because their blood is less salty than the sea. These fish drink a lot of sea water, and they excrete salt from special glands in the gills.
Sea birds and turtles also drink a lot of sea water, and weep salty tears from special glands near the eye.

26. Marine animals
Cartilaginous fish manage to keep the osmotic pressure of their blood slightly higher than the sea water. They do this by retaining some urea in their blood.

27. humans
We have a waterproof skin, so we are able to bath or swim with no problems.

28. Active transport
This is the movement of substances from an area of low concentration to an area of high concentration, against the concentration gradient.
This requires energy.
E.g. marine fish secrete salt from the blood vessels in their gills. They do this even though their sodium level is far lower than sea water.

29. Active transport

30. Active transport
Recent studies suggest that certain proteins in the cell membrane pick up specific ions and transport them across the cell membrane.
How do we know that energy has been used?
Heat is given off
Oxygen is used up.
Carbon dioxide is produced.
Glucose is used up.

31. Active transport
If the cell has a large number of mitochondria, we can assume that it needs a lot of energy.
of active and passive transport

32. endocytosis Some cells have the ability to engulf large particles.
This process is called endocytosis.
If the engulfed particle is solid, the process is called phagocytosis.
If the engulfed material is liquid, the process is called pinocytosis.

34. examples
Amoeba and our white blood cells surround a solid piece of food or a foreign particle with protrusions called pseudopodia.
They then engulf the food into a food vacuole, lysosomes pour digestive enzymes into this vacuole and the food is broken down.
With pinocytosis the cell membrane buckles in to form a depression which eventually becomes pinched in from the surface, thus forming a vacuole.

35. exocytosis This occurs when the reverse process takes place.
The vacuoles fuse with the membrane and expel their contents outside the cell.
This is how cells in the lining of the digestive canal release digestive enzymes.
and exocytosis