BY1 - Cell membranes and transport

They form the basis of the membrane structure and can form bilayers, with one sheet of phospholipid molecules opposite another.

The hydrophillic heads either point inwards, or outwards, interacting with the water in the cytoplasm and the water surrounding the cell. 

The phospholipids allow lipid-soluble molecules across the membrane, but not water-soluble molecules.

Extrinsic proteins: on either surface of the bilayer. They provide structural support and form recognition sites, by identifying cells and receptor sites for hormone attachment.

Intrinsic proteins: span both layers of the bilayer. Some are carriers, transporting water-soluble substances across, others allow active transport of ions across, by forming channels.

This arrangement was proposed by Singer and Nicolson in 1972.

It's called fluid-mosaic as the indiviudal phospholipid molecules can move within the layer (fluid) and the proteins embedded in the bilayer vary in shape, size and pattern (mosaic).

The membrane also contains cholesterol which makes the membrane more rigid and stable, as well as glycoproteins and glycolipids which have roles as hormone receptors or in cell-to-cell recognition.

Lipid-soluble substances (vitamin A) and small molecules (oxygen and carbon dioixide) dissolve in the phospholipid and diffuse across the membrane. The phospholipid layer is hydrophobic so lipid-soluble substances move through the cell membrane easier than water-soluble.

Water-soluble substances (glucose), polar molecules and ions can't diffuse through the phospholipids and must pass through the intrinsic protein molecules which form water-filled channels across the membrane.

The cell-surface membrane is selectively permeabe to water and some solutes.

The concentration gradient - greater difference in concentration, the more molecules diffuse.

Thickness of exchange surface/distance of travel - thinner memrbane/shorter distance, more molecules diffuse.

Surface area - larger surface area, more molecules diffuse.

Size of diffusing molecules - smaller molecules diffuse faster than larger ones.

Nature of the diffusing molecules - fat-soluble molecules diffuse faster then water-soluble molecules, non-polar diffuse faster than polar.

Temperature - increased temperature increases rate, molecules have more kinetic energy.

Rate of diffusion = (surface area x difference in concentration) / length of the diffusion path

Channel proteins are molecules with pores lined with polar groups. Channels are hydrophilic, and water-soluble ions can pass through. Channels open and close according to the needs of the cell.

Carrier proteins allow diffusion of larger, polar molecules such as sugars and amino acids. A molecule attaches to its binding site, on the carrier protein and the carrier protein changes shape, releasing the molecule to the other side.

Ions and molecules move from an area of lower to higher concentration, against the concentration gradient.

It requires energy from ATP therefore whatever affects respiration will affect active transport.

The process occurs through instric carrier proteins.

The rate is limited by the number/availability of carrier proteins.

Muscle contraction, nerve impulse transmission, reabsorption of glucose in the kidney and mineral uptake into plant root hours.

1) Molecule or ion combines with a specific carrier protein on the outside of the membrane.

2) ATP transfers a phosphate group to the carrier protein on the inside of the membrane.

3) Carrier protein changes shape, carries the molecule/ion across the membrane, to the inside of the cell.

4) Molecule/ion is released into the cytoplasm.

5) Phosphate ion is released from the carrier molecule, back to the cytoplasm and recombines with ADP to form ATP.

6) Carrier protein reverts to original shape.

A type of facilitated diffusion that brings molecules and ions into cells, together on the same transport protein molecule.

A measure of the free energy of water molecules, the tendency for water to move. It's measured in kilopascals. 

Pure water has a water potential of 0.

The higher the concentration, the more strongly water molecules are pulled in, the lower/more negative the water potential.

It measures how easily water molecules move out of a solution.

water potential of cell = pressure potential + solute potential

A force which increases the tendency of water to move out.

If water potential of external solution is less negative/higher than the internal solution, it's hypotonic to the cell, water flows into the cell.

If water potential of external solution is more negative/lower than the internal solution, its hypertonic to the cell and water flows out of the cell.

If the cell has the same water potential as the surrounding solution, external solution and cell are isotonic, no net movement of water.

They lose water by osmosis. 

The vacuole shrinks and the cytoplasm draws away from the cell, plasmolysis, when it is complete the cell is flaccid/floppy. 

Turgid - cytoplasm pushed against cell wall

Incipient plasmolysis - cytoplasm beginning to pull away from the cell wall

Plasmolysed - cytoplasm compeltely pulled away from the cell wall

Endocytosis - occurs when material is engulfed. There are two types of endocytosis.

Phagocytosis - uptake of solid materials that are too large to be taken in by diffusion or active transport. Products are absorbed into the cytoplasm.

Pinocytosis - uptake of liquid by the same mechanism. 

The process by which substances leave the cell, having been transported through the cytoplasm in a vesicle, which fuses with the cell membrane.