cell membranes

fluid mosaic model

fluid - can move

mosaic - made of lots of different parts

phospholipid bilayer structure

• Polar, charged heads are hydrophilic (attracts/interacts with water) 

• Non-polar, uncharged fatty acid tails are hydrophobic (repel water) 

bilayer function

PARTIALLY PERMEABLE AND BARRIER 

• Hydrophobic/ non-polar molecules can pass through as they do not repel the fatty acid tails

• Small molecules can fit between the phospholipids e.g. O₂ CO₂ H₂O 

• Polar/ charged molecules/ ions are water soluble so cannot pass through on their own as they repel the fatty acid tails 

• Electrical insulator as charged molecules/ ions cannot pass through

cholesterol

function

• Maintains fluidity, stability and strength of the membranes 

• Restricts movement of the phospholipids

• Reduce lateral movement of molecules and phospholipids 

• Makes membrane less fluid at high temperatures to prevent damage 

• Prevents loss of water and dissolved ions as tails are hydrophobic and closer together 

structure

type of lipid, fits in between the phospholipids

glycolipid

FUNCTIONS

• Helps cells attach together to form tissues 

• Recognition sites 

• Maintains stability of membranes 

• Cell-surface receptor sites 

glycoprotien

FUNCTIONS

• Allows cells to recognise each other e.g lymphocytes as own cells 

• Helps cells attach together to form tissues 

• Recognition sites 

protein channels

• Form pores in the membrane which charged/polar/hydrophilic particles diffuse through. 

carrier protein

• Move large molecules across the membrane. 

• Specific large molecule attaches to a specific binding site on  carrier protein in the membrane.

• Then the protein changes shape due to its tertiary structure being altered  to release the molecule on the opposite side of the membrane.

simple diffusion

The net movement of particles from a region of high concentration to a region of low concentration, down the concentration gradient through a partially permeable membrane. Without the use of metabolic energy (ATP from respiration) as it is a passive process. 

WHAT CAN GO THROUGH MEMBRANE BY SIMPLE DIFFUSION AND WHY? 

• Gases - Oxygen and carbon dioxide diffuse into and out of the cell for respiration. 

• Hydrophobic molecules - don’t repel from the hydrophobic fatty acid tails 

• Non-polar/uncharged - don’t repel from the hydrophobic fatty acid tails 

• Water - small molecule 

• Lipid-soluble (capable of dissolving in lipids) - dissolve in phospholipid bilayer to move through it 

• Small molecules - fit between phospholipids 

factors affecting simple diffusion

Concentration gradient 

Greater = faster 

As diffusion takes place, conc difference reduces until equilibrium so diffusion slows down until it is reached

Surface area 

Greater = faster

More area/space is exposed for particles to use and diffuse through

Distance/thickness of surface

Shorter/thinner = faster

Particles have less distance to cover  

Temperature 

Warmer = faster 

More kinetic energy, move faster 

Particle size/mass 

Smaller/lighter = faster 

At any given temperature, the diffusion of a smaller particle is faster than bigger as smaller particles can move faster.

facilitated diffusion

Facilitated diffusion is the passive net movement of particles across a cell membrane from an area of high concentration to an area of low concentration, down a concentration gradient by means of a transport protein located in the cell membrane. 

WHAT CAN PASS THROUGH BY FACILITATED DIFFUSION?

• Larger molecules like glucose or amino acids.

• Charged/polar/hydrophilic molecules 

• Water soluble molecules 

• Molecules that can’t readily pass through the cell membrane via simple diffusion.

factors affecting facilitated diffusion

Concentration gradient 

Greater = faster 

Up to a point if all proteins are in use. As equilibrium reaches, rate plateaus. 

Number of protein channels/carriers

More = faster 

Once all in use, the rate of diffusion cannot increase even if other factors increase.

osmosis

The movement of water from an area of higher water potential (less negative) to an area of lower water potential (more negative) over a partially permeable membrane. This is a passive process and doesn’t require metabolic energy (ATP from respiration). 

WATER POTENTIAL

Water potential (Ψ) is the likelihood (potential) of water molecules to diffuse out of or into a solution. 

Adding solute lowers water potential, making it more negative. More concentrated solution is more negative with more solute. 

Pure water is 0

Higher water potential in hypotonic.

Lower water potential in hypertonic.

Water moves from:

Hypotonic → Hypertonic 

High Ψ → Low Ψ

types of solutions

Hypotonic - Ψ of solution is higher (closer to 0) than Ψ of cell. 

Isotonic - Ψ same inside and outside cell. 

Hypertonic - Ψ of solution is lower (more -ve) than Ψ of cell. 

factors affecting osmosis

Water potential gradient 

Higher gradient = faster

As osmosis takes place, the difference in water potential on either side of the membrane decreases, so the rate of osmosis levels off over time.

Thickness of exchange surface 

Thinner = faster 

Shorter distance for the water molecules to travel 

Surface area of exchange surface 

Larger = faster 

More area for the water molecules to travel through in a set amount of time.

Active transport

Movement of substances from an area of low conc to an area of high conc, against the concentration gradient, requiring energy from ATP and carrier proteins.

process of active transport

1. A molecule binds to a receptor complementary to its shape on the carrier protein. Each carrier protein is specific and will only transport one type of molecule/ ion. 

2. ATP binds to carrier protein from inside of the cell. 

3. ATP hydrolysed into ADP and Pi 

4. Pi attached to protein causing phosphorylation.

5. This produces a conformational change of the protein/change in tertiary structure so the molecule is released on the other side of the membrane. 

6. Carrier protein returns to its original shape when inorganic phosphate ions are released after the molecule/ ion is transported. 

ENERGY FOR ACTIVE TRANSPORT

A lot of ATP (metabolic energy) is required. This is released in mitochondria during respiration so lots of mitochondria will be present in cells undergoing lots of active transport. 

ATP → ADP + Pi 

CO-TRANSPORT 

Transport of two molecules, one going down concentration gradient and one against concentration gradient. Energy for active transport indirectly from the conc gradient of the other molecule. 

co transport process

(Glucose absorption in ileum) Ileum→ epithelial cells → capillary 

1. High concentration of glucose in ileum following digestion. Low concentration of glucose in epithelial cells lining the ileum. 

2. Meaning the glucose can enter by facilitated diffusion by carrier proteins. 

3. Rate of facilitated diffusion slows as the concentration gradient falls. So can’t absorb all glucose by facilitated diffusion, so glucose is absorbed by active transport as well.) 

Sodium potassium pump  (carrier protein between bloodstream and epithelial cell) 

4. Na+ from epithelial cell→ bloodstream by active transport in the sodium potassium pump. 

   1. K+ from blood → epithelial cells  

5. As ileum has a high concentration of Na+ ions following digestion, concentration of Na+ ions now lower in epithelial cells than ileum, creating a concentration gradient between lumen of ileum and epithelial cells. 

Sodium glucose co-transporter (protein in membrane of epithelial cell)

6. Na+ ions into the epithelial cells from ileum via facilitated diffusion down concentration gradient. Attaches to complementary shape receptor.

7. In the same co-transporter (symport), glucose then attaches to it and is absorbed into epithelial cells from the ileum, against its concentration gradient. 

8. The Na+ is then released on the other side which then enables glucose to also be released. Transported together. 

9. Glucose is absorbed by active transport which ensures glucose is absorbed at a fast rate.  

10. The energy for active transport of glucose comes indirectly from the concentration gradient of the Na+ ion.

11. Glucose then enters the bloodstream by facilitated diffusion. Glucose conc. is high in the epithelial cells and is carried straight away in blood as it always flows, so there is always a low concentration in the bloodstream.