plasma membranes

Membranes control the movement of substances across them.

They:

• control the passage of substances into and out of cells and organelles
• sperate the content from thier environment
• seperate different areas within the cell (organelles)
• seperate different areas with organelles
• provide a surface for attachment for enzymes and receptors

All membranes within cells have:

• phosopholipid bilayer
• protiens
• cholesterol

Phospholipid molecules have a polar head (with uneven charge distribution) that mix with water (hydrophillic) and two non-polar tails (with even charge distribution) that do not mix with water (hydrophobic).

The hydrophobic tails turn towards each other, so that they face away from the water. The polar heads turn to face the water. This form a bilayer.

the phospholipid bilayer if fluid and molecules are constantly moving around in relation to each other.

Membrane Fluidity: Unsaturated hydrocarbon tails of phospholipids have kinks that heep the molecules from packing together, enchancing membrane fluditiy

Fluid Mosaic Model

Proteins carry out a variety of roles within membranes:

• channel proteins provide a channel for passive movement of substances
• carrier protiens transport substances across membranes by changing thier shape (movement can by passive (by diffusion) or active)
• glycoproteins are involved in cell adhesion in tissues and act as receptors for chemical signals
• enzymes are the site of chemical reactions

Intrinsic: protiens run through both layers of the plama membrane

Extrinsic: proteins are present on only one side of the phospholipid bilayer

Glycoproteins have carbohydrae (sugar chains) attached to them.

They are involved in:

• cell adhesion, to hold cells together in a tissue
• cell signaling/communcation

Cell signalling is communication between cells to co-ordinate thier activty. cell signalling molecules such as hormones and neurotransmitters allow cells to work together and co-ordinate thier activities. they can bind to glycoprotein receptors on the surface of the cells and can trigger responses inside the cell..

glycoproteins can also be binding sites for transport proteins, attach to water molecules to stabilise the cell membrane and act as antigens, aiding in the recognition of cells as self or non-self (foreign)

Glycolipids are lipids with an attached carbohydrate chain.

They are often called cell markers.

Cholesterol is a lipid with a hydrophillic end and a hydrophobic end. It regulates fluidity of the membrane. They are positioned between phospholipids preventing them grouping too closely and crystallisng an so maintaining fluidity at low temperatures. At high temperatures they restrain movement.

Temperature

• increasing temperature increases kinetic energy
• phospholipids move around faster creating gaps between molecules that substances can move across
• at high temperatures, the membrane will break down completely
• carrier and channel proteins in the membrane denature affecting membrane permeability

Solvents

• water is a polar solvent essential for maintaining membrane structure
• less polar and non-polar solvents (e.g. alcohol) entre the membrane between the phospholipids and disrupt the membrane si it becomes more fluid and more permeable
• high concentrations of solvents dissolve the membrane completely

The size and charge of molecules affects thier abillity to move across the membrane. Charged molecules do not pass across the phospholipid bilayer easily becuase the fatty acid tails of phospholipids are hydrophobic.

• small molecules move across more easliy than large molecules
• hydrophobic/non-polar molecules move across more easily than hydrophillic/polar molecules
• polar molecules move across membranes more easily than ions

When molecules move randomly due to thier kinetic energy, they spread out and reach an even distribution, called equilibrium.

Random movement of the molecules continues after equilibrium has been reached.

There is a NET (overall) movement of molecules from where they are in a high concentration to where they are in a low concentration. There is a net movement of molecules DOWN this CONCNETRATION GRADIENT. This type of PASSIVE movement is called DIFFUSION.

If the molecules can pass across a cell membrane they will move passively down thier concentration gradient until equilibrium is reached. They may be able to pass through the phospholipid bilayer or through transport proteins.

Diffusion is the net movement of particles from a region where they are in high concentration to a region where they are in low concentration until they are in equilibrium.

Factors Affecting the Rate of Diffusion

• temperature - the higher the temperature, the faster the rate of diffusion as particles have more kinetic energy so they move faster
• concentration gradient - the greater the difference in concentration, the faster the rate of diffusion between the two areas
• surface area - the larger the surface area to diffuse across, the faster the rate of diffusion
• diffusion pathway - the longer the diffusion path, the more collisions occur, slowing down the rate of diffusion

Fick's Law tells us that the rate of diffusion changes in proportion to the change in surface area, concentration gradient and diffusion pathway.

Small and hydrophobic particles diffuse through the phospholipid bilayer.

Small polar molecules such as water can diffuse through the bilayer at a very slow rate. Membranes are therefore partially permeable.

Polar molecules and ions have to cross the phospholipid bilayer through membrane proteins. Membranes with protein channels are selectively permeable.

• Channel Proteins from a pore through with polar molecules and ions can travel across the membrane. The channels are specific to one type of molecule or ion. Some of there channels are gated and only open in response to a stimulus.
• Carrier Proteins bind to specific moleules and change shape, transporting the molecules across the membrane. This is a passive process, binding of the molecule brings about the change in shape without the use of engery from the cell.

The number of transport proteins in a membrane can be used to control the rate of diffusion of sustances into or out of cells.

Osmosis is a type of diffusion involving water moving across a partially permeable membrane down a water potential gradient.

It is a passive process.

Water potential is the potenital energy of water in a given volume in relation to pure water.

Pure water has the highest water potentail of 0KPa.

Active transport is the movement of molecules or ions across a cell membrane against a concentration gradient (from low concentration to high concentration ) using carrier proteins and energy from cell respiration (ATP). A carrier protein that brings about active transport is often known as a PUMP.  

1. The molecule/ion binds to the recpetors in the carrier protein

2. ATP binds to the carrier protein and is hydrolysed to ADP and Phosphate

3. The phosphate binds to the carrier portein

4. The protein changes shape and the molecule/ion is released ad the carrier protein returns to its orginal shape.

Large molecules such as hormones and whole cells are moved across the membrane in vesicles.

Bulk transport INTO cells is called endocytosis. There are two types:

• uptake of solids is called Phagocytosis
• uptake of liquids is called Pinocytosis

Process:

• The cell surface invaginates (bends inwards) around the material until it forms a vesicle
• The vesicle pinches off and moves into the cytoplasm

Bulk transport OUT OF cells is called exocytosis.

Process:

• Vesicles, usually formed by the Golgi, move towards and fuse with the cell surface membrane
• The contents of the vesicles are released outside the cell