Cell Membranes

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Roles of membrane

Major roles of membranes include:

On cell surface:

  • Seperating cell contents from outside environment
  • Cell recognition and signalling
  • Regulating transport of materials into or out of cells

In cell:

  • Seperating cell components from cytoplasm
  • Holding components of some metabolic pathways in place
  • Seperating different pH levels of different organelles
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Phospholipid bilayer

The phospholipid bilayer is the basic structural component of plasma membranes. It consists of two layers of phospholipid molecules.

  • Phosphate 'head' is hydrophillic while the two fatty acid 'tails' are hydrophobic. These properties come from the way charges are distributed across the molecule.
  • Molecules with evenly distributed charges do not dissolve or mix with water, they repel water molecules. Whereas molecules with more unevenly distributed charges can interact with water molecules quite easily. 
  • If phospholipid molecules are mixed with water, they form a layer at the water surface. Phosphate heads stick into water, while the fatty acid tails stick up out of the water. If phosphate molecules are completely surrounded by water, a bilayer can form. Phosphate heads on each side of bilayer stick into water, whilst hydrophobic fatty acid tails point towards each other - means they are held away from water molecules. In this state, phospholipid bilayer can move freely. Stability comes from the fact that phospholipid heads can't easily pass through hydrophobic region of bilayer.
  • Most membranes are about 7-10nm thick.
  • Simple phospholipid bilayer incapable of performing all functions needed, other components needed. The number and type of theses other components varies according to function of membrane. Specialisation of cell membranes is part of differentiation. 
  • Cell membranes that are permeable to water and some solutes = partially permeable.
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Fluid mosaic model

Fluid mosaic refers to the model of cell membrane structure. The lipid molecules give fluidity and proteins in the membrane give it a mosaic appearance. The molecules can move about.

The main features are:

  • a phospholipid bilayer - basic structure
  • various protein molecules floating in bilayer, some freely and some bound
  • extrinsic proteins - embedded in bilayer on the inside or outside face
  • intrinsic proteins - completely span bilayer
  • glycolipids - phospholipid molecules with a carbohydrate part attached
  • glycoproteins - protein molecules with a carbohydrate part attached

Roles of components:

  • Cholesterol - stability; fits between fatty acid tails & helps make barrier more complete.
  • Channel proteins - movement of substances that are too large/hyrophilic across membrane
  • Carreir proteins - actively move substances across membrane
  • Receptor sites - allow hormones to bind with cell so specific response can take place
  • Glycolipids and glycoproteins - signals that the cell is 'self', recognition by immune system
  • Enzymes and coenzymes - speed up metabolic processes in cell

Increased temperature = increased kinetic energy of phospholipids = more leaky

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Cell signalling

Cell signalling is when cells communicate with one another using signals. Many molecules act as signals - some signal during processes taking place inside cells; others signal from one cell to others.

In order to detect signals, cells must have receptors that are capable of receiving signals. They are often protein molecules or modified protein molecules.

In multicellular organisms, communication between cells often done through hormones. Hormones are chemical messengers, produced in specific tissues and then released into the organism. Any cell with a receptor for the hormone molecules is called a target cell.

Hormone molecule binds to receptor on target cell surface membrane because they have complementary shapes. Binding of the hormone and the receptor causes the target cell to respond in a certain way.

A number of medicinal drugs have been developed that are complementary to the shape of a type of receptor molecule - these drugs are intended to block receptors so response does not occur.

Viruses enter cells by binding with receptors on the cell's plasma membrane that normally bind the the host's signalling molecules.

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Crossing membranes - passive processes

Diffusion = movement of molecules from a region of high concentration to a region of low concentration down a concentration gradient.
Mainly lipid-based molecules, gases like oxygen and carbon dioxide 
Rate of diffusion increased by:
-Increase in temperature
-Steeper concentration gradient
-Increase in movement of molecules (stirring)
-Increase in surface area
-Decrease in distance/thickness
-Small molecules rather than larger ones

Facilitated diffusion = the passive movement of molecules across membranes down their concentration gradient, which is aided by transport protein molecules.
Carrier proteins - shaped so specific molecule can fit into them at membrane surface; when specific molecule fits, the protein changes shape to allow molecule through to other side. Mainly larger molecules, e.g. glucose and amino acids
Channel proteins - pores in membrane, often shaped so only one type of ion fits through, many are also 'gated' so they can be open and closed. Mainly charged ions, e.g. sodium and calcium ions

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Crossing membranes - active processes

Active transport = the movement of molecules/ions across membrances, which requires ATP to drive protein pumps within the membrane. The molecules go against their concentration gradient.
Uses carrier proteins that differ to the ones used in facilitated diffusion in the following ways:
- carry specific molecules one way across the membrane
- use metabolic energy in form of ATP
- carry molecules against the concentration gradient
- carry molecules at a much faster rate
Active transport used for larger or charged molecules/ions.
To ensure one-way flow, energy used in active transport changes shape of the carrier protein whilst transporting the specific molecule which means that the molecule fits into the carrier protein on one side of the membrane only as the protein is now a different shape. 

Endocytosis - process of moving large quantities of material into the cell
Exocytosis - process of moving large quantities of material out of the cell
Possible as the membrane can easily fuse, seperate and 'pinch off'. 
Requires ATP to move vesicles of material around cell.

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Osmosis = the movement of water molecules from a region of higher water potential to a region of lower water potential, down a water potential gradient, across a partially permeable membrane. 
Water potential = a measure of the tendancy of water molecules to diffuse from one place to another, measured in kPa.

Pure water has highest water potential of 0kPa. 

Cells in solution of high water potential:
1. Animal cells - water moves into cell, cell will swell up and eventually burst open.
2. Plant cells - water moves into cell, swelling cytoplasm and vacuole push cell membrane against cell wall. Cell will not burst, because the wall will eventually stop getting any larger. Osmosis will stop at this point, even though there may still be a water potential gradient. Cell becomes turgid.

Cells in solution of low water potential:
1. Animal cells - water moves out of cell, cell contents shrink and membrane will wrinkle up; it is crenated. 
2. Plant cells - water moves out of cell, cytoplasm and vacuole shrink as they lose water; making cell membrane pull away from the cell wall - it is plasmolysed.

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