BIO210 - Cell Membranes

Membrane key concepts 

1. Fluid mosaic model

2. Membrane fluidity and composition 

3. Membrane functions 

Fluid mosaic model

Fluid comes from phospholipids and the mosaic comes from membrane proteins

What are amphipathic molecules

They form a bilayer in the water

What alters membrane fluidity 

Saturated fatty acids —> make the membrane less fluid

Unsaturated fatty acids —> makes the membrane more fluid 

What to steroids help in the cell

They make the cell more stable

When heated, steroids decrease fluidity. When cooled steroids increase fluidity(prevent solidification)

Why is membrane fluidity important

It helps with cell movement, Movement of organelles within the cell, and allows for an efficient transport process

What are the membrane functions 

1. Permeability barrier 

2. Location for anchoring proteins

3. Location for energy generation

Permeability barrier

Movement of molecules across the membrane

1. Simple diffusion

2. Osmosis

3. Facilitated movement (passive vs active transport)

Osmosis

The diffusion of water. Moving from an area of high water concentration to low concentration

Or, moving from an area of low concentration of solutes, to high concentrations of solutes

Hypotonic solution 

Makes cells lysed (oversaturated) and causes them to burst. When the concentration inside of the cell is greater than the solution surrounding it 

Isotonic solutions

The concentration inside the cell and the solution are relatively the same

Hypertonic solution

When the concentration of solute is greater outside of the cell than the inside of the cell. Causes cells to look shriveled and spiky.

Transport proteins

Integral membrane proteins that span the length of the membrane and form channels through the cellular membranes

Facilitated diffusion (passive transport)

Does not require energy expenditure, goes against the concentration gradient

Channel proteins

Less active (aquaporin)

Carrier proteins 

Somewhat selective (sodium gated channels in nerve cells)

Aquaporin

Allows water to cross the cellular membrane much faster. Channel typed facilitated diffusion

Types of co-transport

1. Symport

2. Antiport

Symport 

Two molecules move in the same direction (into the cell)

Antiport

One molecule goes out, and the other molecule goes in

Bulk transport

Only in eukaryotes, transported in vesicles

1. Exocytosis —> out of the cell

2. Endocytosis —> Into the cell

Exocytosis 

Secretion of proteins from the cell - the vesicle becomes full of protein buds from the Golgi and fuses with the cellular membrane 

Endocytosis (types)

1. Pinocytosis

2. Phagocytosis

3. Receptor-mediated endocytosis

Pinocytosis

The cell “gulps” droplets of extracellular fluid

Phagocytosis

The cell engulfs a particle by wrapping pseudopodia around it, forming a vesicle

Receptor-meditaed endocytosis 

Cells acquire bulk quantities of specific substances that bind to receptors

Functions of membrane proteins

1. Transport

2. Enzymatic activity

3. Signal transduction

4. Cell-cell junction

5. Intercellular joining

6. Attachment and adhesion

Enzymes

Selectively accelerate specific chemical reactions (catalysts)

Signal Transduction

Sense the external environment, transmit “signal” to modify cellular activity (gene expression)

Cell-cell recognition 

Important in growing embryo and tissue development 

Intercellular joining

Gap junctions and tight junctions

Attachment and adhesion

Attachment to the cytoskeleton and extracellular matrix (ECM)