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Cell Membranes
Created
Class

@February 12, 2025 4:49 PM
Cells to Systems

Cell Membranes
Surrounds cells
Acts as barrier between cell and environment
Partially permeable- Controls what substances exit and enter the cell

Fluid Mosaic Structure
Describes molecule arrangement in membrane
Phospholipids form bilayer which constantly moves
Bilayer contains cholesterol molecules and proteins scattered in the layer
Receptor proteins on membrane surface detects chemical release
Certain proteins have polysaccharide chain attached to it, making it a
glycoprotein

Phospholipids
Contains head and tail, head being hydrophilic and tail is hydrophobic
Molecules arrange into bilayer
Centre of layer is hydrophobic
Side of the membrane is hydrophilic

Cell Membranes

1

Cholesterol
Type of lipid
In all cell membranes
Fits between phospholipids
Restricts phospholipid movement making structure more rigid
Maintains animal cell shape

Temperature Increases Membrane Permeability
Temp below 0 Degrees
Causes channel and carrier proteins to deform
Temp Between 0 and 45 degrees
Phospholipids have slight movement
Tightly packed together
Makes membrane partially permeable
Temp increase means phospholipids are more energetic
Increases permeability of membrane
Temp Above 45 degrees
Phospholipid melts increasing membrane’s permeability
Due to water expansion inside cell, water pressure increases
Channel and carrier proteins deform

Diffusion
Net movement of particles from high concentration to lower concentration

Cell Membranes

2

Movement continues until particles are equally distributed
Particles diffuse down concentration gradient
Passive process allows particles to diffuse across cell membrane
Simple diffusion occurs when molecules diffuse directly through
membrane

Facilitated Diffusion
Large molecules take more time diffusing through bilayer
Also happens to ions and polar particles due to water soluble attributes
Using carrier and channel proteins makes process quicker
Carrier Proteins
Moves larger molecules down concentration gradient
Able to facilitate diffusion of various molecules when target molecule
attaches itself to protein and makes it change shape
Channel Proteins
Forms pores in membrane for ion diffusion
Different channel proteins facilitate different ions

Diffusion Rate Factors
Simple Diffusion
Concentration gradient
Higher the gradient, quicker the diffusion is
Concentration difference decreases until equilibrium is reached
Diffusion gradually slows down

Cell Membranes

3

Exchange Surface Thickness
Thin exchange surface= Short travel distance for particles means
faster diffusion rate
Surface Area
Larger surface means faster diffusion rate

Facilitated Diffusion
Concentration gradient
High concentration gradient means fast f

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