1. Lipids and Cell Membranes
Cell (plasma) membranes are composed of mostly lipids (phospholipids)
Proteins are also an important part of the structure of a membrane. They have several roles: transport, cell signalling and communication, receptors, positioning etc.
Other structures such as cholesterol and modified lipids and proteins (eg: glyco-proteins / glycolipids) also exist in the plasma membrane.

2. Phospho-lipid Bilayer

4. The Fluid Mosaic Model
The fluid mosaic model of the cell membrane describes how a membrane behaves.
Proteins are the mosaic and Phospholipids are the fluid matrix.
Phospholipids have a hydrophobic tail x 2 and a hydrophilic head (a bit like soap).
They form two layers (a bilayer) but the tails face each other and the heads face out.
This forms a selectively permeable barrier in which some substances can pass through but others can not.

5. The Fluid Mosaic

6. Membranes have many functions
As well as the plasma (cell) border, other
membranes have several functions within cells.
Create the intracellular / extracellular border (plasma membrane)
Isolates enzymes (lysosomes)
Cell communication (plasma membrane)
Transport within cells (E’R’)
Packaging (golgi)
Energy transfer (mito / chloro)
Containment of DNA (nuclear envelope)
Role in protein / lipid synthesis

7. Moving through membranes
Substances pass in and out of cells by four main processes. 1. Diffusion (passive) 2. Osmosis (passive) 3. Active Transport (requires energy) 4. Bulk Transport or Endo/Exocytosis (requires energy)

8. 1. Passive Transport : Diffusion
The movement of materials across membranes without the energy is called passive transport. Diffusion: is the (net) movement of particles from an area where they are in high concentration to a region where they are at a lower concentration. The difference in concentration of the particles (solute) is called the concentration gradient.

10. Diffusion
Diffusion will always take place where a concentration gradient exists across a membrane.
It continues until all the particles are distributed evenly.
This results in a state described as Equilibrium.
Substances such as water, carbon dioxide and other small uncharged particles can pass through a membrane by simple diffusion

12. Factors influencing the rate of simple diffusion
The size of the concentration gradient
The temperature
Molecule size

13. 2. Passive Transport: Osmosis
Osmosis is a special type of diffusion
requiring no energy input.
Osmosis: is the (net) movement of a solvent (water) across a selectively permeable membrane.
The movement occurs from areas of high water concentration (low solute) to areas of low water concentration (high solute).

15. Hypotonic / Isotonic / Hypertonic
Solutions can be described as Hypotonic: if they have a low solute concentration compared to the cell contents. Isotonic: if they have the same solute concentration as the cell contents Hypertonic: if they have a high solute concentration compared to the cell contents

16. Hypotonic / Isotonic / Hypertonic
Plant and animal cells (& others) will respond to being in hypotonic, isotonic or hypertonic environments.
Some small organisms (unicellular protazoa) have specialised organelles to help balance their water content if they live in hypo/hyper tonic environments.

19. 3. Passive Transport: Facilitated Diffusion
Facilitated Diffusion occurs when carrier / channel proteins open and close along the plasma (cell) membrane. Larger molecules (glucose / amino acids) and charged Ions (Ca2+, K+ ) use this mode of transport across a bilayer. These particles are still moving down a concentration gradient: no energy required.

21. 1. Energy required: Active Transport
Use when cells need to transport particles (substances) against a concentration gradient. Moving from an are of low solute concentration to an area of high concentration. Carrier proteins are also involved but an input of energy is required to push substances across. Eg: glucose pushed into lining cells of gut during digestion / sodium in nerve cells.

22. Sodium / Potassium Pump

23. Transport across membranes

24. 2. Energy Required: Bulk Transport
Endocytosis: When cells engulf (eat) larger particles and bring them into the cell. Exocytosis: Secretory cells use this method to release materials out of the cell (hormones) Some cells are highly specialised in this type of transport. (Eg: Phagocytic white blood cells, Ameoba)