UNIT 2: Transport across membranes

What are the 3 main functions of plasma membranes ?

1. To control the transport of substances into and out of the cell or organelle. Membranes allow certain molecule to pass through, but not other - partially permeable

2. To act as a recpetor site to recognise chemicals which need to enter the cell or organelle

3. To seperate off the cell from the environement and the different reactions of the celll from each other by forming the organelles

What is the Cell-Surface membrane described as and why ?

• Fluid mosaic model: This is due to the mixture and movement of the

1. Phospholipid

2. Membrane Proteins

3. Glycoprotein

4. Glycolipid

5. Cholesterol

How is the Membrane structured overall

• All of these molecules arranged within the phospholipid bilayer create the partially permeable membrane, that is the cell-surface and organelle membrane

How are the Phospholipids arranged in the bilayer ?

• The Hydrophobic tails all point towards each other

• The Hydrophillic heads point out into the water

Draw a labelled diagram of a Phospholipid

Function of Peripheral Proteins

• Peripheral proteins do not extend completely across the membrane

• They provide mechanical support, as well as carbohydrate chains that are connected to proteins or lipids to make Glycoproteins and glycolipids

• Function of these is Cell recognition, as receptors

Function of Integral Proteins

• These are protein carriers or channels proteins involved in the transport of a molecule across the membrane

• Protein channels: Form tubes that fill with water to enable water-soluble ions to Diffuse

• Carrier Protein: These bind with other larger molecules e.g glucose, amino acids and change shape to transport them to the inside of the cell or organelle

Function of Cholesterol

• This restricts the Lateral movement of other molecules making up the membrane and increases strength and stability of membranes by making them less flexible

• Helps prevent water loss and dissolved ions from the cell

State some Molecules that pass through the plasma membrane

• Lipid soluble substances - hormones

• Very small molecules e.g CO₂, O₂, H₂O

State molecules that cannot pass through the membrane

• Water-soluble (polar) substances e.g Sodium ions

• Large molecules - Glucose

What are the 4 Key types of Transport ?

1. Simple Diffusion

2. Facilitated Diffusion

3. Active Transport

4. Osmosis

Explain what Simple Diffusion is ?

• The net movement of molecules from an area of high concentration to an area of low concentration until equilibrium is reached

• This process does not require ATP and therefore it is a Passive process

• Movement of molecules by simple diffusion is due to Kinetic energy they possess to enable them to constantly move in fluids

What most molecules be for simple diffusion to take place ?

• For molecules to diffuse across the membrane they must be lipid-soluble and small

What are the main two adaptations to increase the rate of these 4 types of transport ?

• Involves increasing the surface area

• Increasing the number of protein channels and carrier molecules in the membrane

What are the factors that affect Diffusion and what is the Law called ?

• Fick’s Law

• Surface area x difference in concentration / Thickness of exchange surface

To increase Diffusion rate what must we do to each of the factors

• Maximise the surface area

• Maximise the concentration difference

• Minimise the thickness of the exchnage surface

Explain the process of Facilitated Diffusion ?

• This is a Passive process (does not require ATP)

• This differs from simple diffusion as now membrane proteins are used to transport molecules

• Ions are polar molecules, which cannot simply diffuse, can be transported across the membrane by facilitated diffusion using protein channels and carrier proteins

Explain the proccess of Osmosis

• This is the net movement of water from an area of high water potential to an area of low water potential across a partially permeable membrane

Define the term water potential and what is it measures in ?

• Water potential is the pressure created by water molecules

• Measured in kPa

What is the Water potential of Pure water ?

• Pure water has a water potential of Zero

• When solutes are dissolved in water the water potential will become negative

What does it mean if the Water potential is Negative ?

• The more negative the water potential, the more solute must be dissolved in it

Name the 3 Types of solution, explain what they are and affect on animal cells ?

1. Isotonic solution: when the water potential of the solution is the same as the water potential of the cell

• When animal cells are placed in this solution, there is no net osmotic movement of water (no water potential gradient) and so the shape and size of the cell remains the same

2. Hypotonic solution (Dilute solution/ high water potential) : A dilute solution will have a high water potential than the cell

• Water will enter the cell by osmosis. The cell swells and eventually bursts, when the pressure inside the cell becomes too great - osmotic lysis

3. Hypertonic Solution (concentrated solution/low water potential): This is when the water potential is more negative than the cell

• Water will leave the cell by osmosis, causing the cell to shrink - crenate

What happens to Plant cells when placed in a Hypertonic solution ?

• Lower water potential than the cell

• Water leaves the plants cytoplasm and vacuole by osmosis

• This cases the cytoplasm to shrink away from the cell wall leaving gaps between the cell membrane and the cell wall - filled with concentrated soltuion of salt

• Plasmolysis

What happens to Plant cells when placed in Hypotonic solution?

• A dilute solution- have a higher water potential than the cell

• Water enters the cell by osmosis

• The cyctoplasm and vacuole gains some water, but because of the high tensile strength of the cellulose cell wall the cell will not burst - it becomes turgid

Explain the process of Active Transport

• The movement of molecules and ions from an area of low concentration to an area of high concentration against the concentration gradient, using ATP and carrier proteins

• The Carrier proteins act as pumps to move substances across the membrane

• Process is very selective, as only certian molecules can bind to the carrier proteins to be pumped

What role does ATP play in changing the shape of carrier proteins during active transport?

• Certain molecules can bind to the receptor site on carrier proteins. ATP will bind to the protein on the inside of the membrane and is hydrolysed into ADP and Pi. This causes the protein to change shape and open towards the inside of the membrane.

• This causes the molecule to be released on the other side of the membrane. The Pi molecule is then released from the protein, and this results in the protein reverting to its original shape. This is how ATP and carrier proteins are used in active transport.

Why may Co-transport be used ?

• To absorb glucose from the lumen of the intestines into the

  epithelial cells, there must be a higher concentration of

  glucose in the lumen compared to the epithelial cell (for

  facilitated diffusion). However, there is usually more glucose in

  the epithelial cells and this is why active transport is needed.

Explain the process of Co-transport of Glucose and Amino acids

1. Sodium Ions (Na⁺) are actively transported out of the epithelial cell into the blood in the capillary

• Only recognises Sodium ions and glucose as there are binding sites with a specific tertiary structure - complementary shape

2. This reduces the Sodium ion concentration of the epithelial cell

3. Sodium ions can then diffuse from the lumen down their concentration gradient into the epithelial cell

4. The protein, the sodium ions diffuse through is a co-transporter protein, so either glucose or amino acids also attach and are transported into the epithelial cell against their concentration gradient

5. Glucose then moves by facilitated diffusion from the epithelial cell to the blood