Chapter 2 - Cell Membranes

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Cell Membranes

Every living cell is surrounded by a cell membrane. It is called the plasma membrane or cell surface membrane.

Majority of cells have membranes. Nucleus, Mitochondria and Chloroplasts have two membranes making up an envelope.

They are partially permeable, and can control what enters and leaves.

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Membranes Function

Membrane are partially permeable controlling what can pass through them. - Plasma membrane allows small or uncharged particles through it. Protein channels and transporters control the passage of larger or charged particles.

Membranes produce different compartments inside cells. - Mitochondria are surrounded by two membranes, which isolate the reactions taking place inside from reaction taking place in the cytoplasm

Membranes are important in cell signalling. - A substance produced by one cell docks into a receptor in the plasma membrane of another, causing something to happen.

Membranes can allow electrical signals to pass along them. - The membrane of the axon of a motor neurone transmits action potentials from the central nervous system to a muscle

Membranes provide attatchment sites for enzymes and other molecules involved in metabolism. - The inner membrane of a chloroplast contains molecules needed for the production of ATP. The inner membrane of a chloroplast contains chlorophyll needed for photosynthesis.

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Structure of Cell Membranes

between 7nm and 10nm thick

Formed by a phospholipid bilayer.
Heads (Hydrophillic) have a tiny charge attracting them to water and the tails (Hydrophobic) are repelled from water molecules.


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Phospholipid Bi-layer

Cytoplasm inside a cell contains a lot of water. So does the fluid outside the cells

The hydrophillic heads are attracted to the water inside the cytoplasm. The heads are also attracted to the fluid outside of the cell.
The tails are repelled creating a bilayer.


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Phospholipid Bi-layer


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Components and their roles

Phospholipid - forms bilayer, which is basis of membrane, all components embedded into it
provides a barrier to water-soluble (hydrophillic) substances such as charged ions 

Cholesterol - helps maintain the fluidity of membrane preventing it from becoming to stiff or fluid

Protein & Glycoprotein -

  • Form channels through which hydrohpilic substances can pass through.
  • acts as transporters that can move substances across the membrane up their concentration gradients with use of ATP
  • acts as receptor sites, allowing specific molecules from outside the cell such as hormones to bind with them and set up responses within the cell
  • act as recognition sites because their precise structure may be specific to a particular type of cell or individual
  • act as enzymes 
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Cell Signalling

3 ways that cell signalling can occur.

1. Receptor acts as a ion channel

2. Receptor activates a G-Protein

3. Receptor acts as an enzyme

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Receptors acts as an ion channel

Signal molecule is a chemical that attatches to a protein or glycoprotein acting as a ion channel.

Chemical (signal molecule) attatches to receptor and it open up the channel for proteins to pass through.


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Receptor activates a G-protein

Receptor in plasma membrane interacts with a G-protein (molecule)

When the signal molecule atttatches to the receptor, the G-protein is activated which moves acroos and attatches to the enzyme, bringing about a reaction in the cell.

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Receptors acts as an enzyme

The receptor this time is also the enzyme. Receptor is made up of 2 parts. When the signal molecule arrives, it slots into both of these parts, connecting them to one another and forming them into an active enzyme.

Then enzyme then brings about a reaction inside the cell.

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Movement across cell membranes

Many substances move into and out of the cells through their plasma membrane. Some of these substance move passively (no energy needed) and some move actively (require energy).

Passive Processes

Facilitated Diffusion

Active Processes

Active Transport 

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Diffusion is the net movement of ions down a concentration gradient. They move from an area of high concentration to low concentration.

Only small particles such as oxygen and carbon dioxide and do not have a charge can easily slip through the phospholipid bilayer.

How does oxygen enter a cell by diffusion?

1. Inside the cell, aerobic repiration for the cell uses up the oxygen. This means there is a low concentration of oxygen.

2. If there is a higher concentration of oxyegn outside the cell, then there is a concetration gradient present.

3. Oxygen particles will hte pass through the membrane both ways but as there is a higher concentration on the outside of the cell the net flow will be into the cell.

Oxygen moves from outside the cell through the plasma membrane into the cytoplasm.

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Facilitated Diffusion

Many ions are too big or have a too high charge to move through the plasma membrane.

E.g. Chloride Ions Cl- has an electrical charge.

Cells need to provide special pathways for larger molecules to travel into the cell. These special pathways are called protein channels.

Protein channels are found in the plasma membrane and they form a hydrophilic channel which ions can pass through.

Ions pass through by diffusion down a concentration gradient. Process is called facilitated diffusion.

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Water molecule even though they carry a charge can still pass through the bilayer because they are very small.

The movement of water down a concentration gradient through a partially permeable membrane is called osmosis.

In water it isnt called concentration it is called water potential. Water potential is a measure of how much water the solution contains in relation to other substances.

Pure water has a water potential of 0.

The movement of water goes from high water potential to low water potential down a water potential.

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Osmosis and Animal Cells

If the water potential is very high outside of the cell and much lower inside. This would result in a lot of water moving into the cell. If too much moves in however the cell will become lysed.

If the water potential is very low outside of the cell and much higher inside. This would result in water moving out of the cell. This leaves the cell shrivelled and it is called 'Crenated'

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Osmosis and Plant Cells


Hypotonic - water diffuses into the cell through its partially permeable membrane. The cell and its contents expand. The force of the wall pushing on the cell contents is caled the pressure potential. The cell is tugid.

Hypertonic - Water diffuse out through the partially permeable membrane. The plasma membrane is pulled away from the wall in place and the cell is plasmolysed.

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Active transport

Active transport requires ATP to go against the gradient and transport ions from low concentration to high concentration.

Usually the case with sodium and pottasium ions. Cells usually need a high concentration of potassium ions and a lower concentration of sodium ions than the concentration outside the cell.

To do this cells pump sodium ions out and potassium ions in up against their concentration gradients. This requires energy so is called active transport.

Active transport is carried out by transporter proteins in the plasma membrane. ATP supplies the energy. ATP changes the shape of the transporter proteins. 

A example of this is the sodium potasssium pump.

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Sodium Potassium Pump


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Endocytosis & Exocytosis

Cells can also move substances in bulk across the membrane.

Moving substance out of the cell is called Exocytosis.
The substance that is released is contained in tiny membranes-bound sac called a vesicle. The vesicle is moved to the plasma membrane along microtubules. The membrane around the vesicle fuses with the plasma membrane, emptying the vesicle outside the cell.

Endocytosis is the movement of substances into the cell.
1 example of this is a phagocyte (white blood cell) enguls a bacterium. The cell puts a ring around the bacterium and encloses it in a vesicle.

Endocytosis and Exocytosis are both active processes and require energy.

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Endocytosis & Exocytosis Diagram


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How temperature affects membrane permeability

Increase in Temperature

  • If you have a increase in temperatue the membranes become more permeable.
  • As phosphilipid molecules get hotter, they vibrate more and more, leaving gaps in the membrane.
  • The protein molecules lose thier shape as the vibrate more and more, leaving gaps.

Decrease in Temperature

  • Very low temperatures have the opposite effect and cold temperatures decrease membrane permeability
  • The phospholipid molecules vibrate a lot less, packing together tightly and limiting pathways through.
  • Protein channels remain normal, but transporter proteins struggle because the low temperatures make it difficult for cell to get ATP.
  • All molecules and ions move a lot slower so less frequency of bouncing into membrane and less passses through it.
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