Cell Membranes

Cell Membranes

Cell membranes function to enclose the contents of the cell, separating

the intracellular components from the external environment

This allows for the control of internal conditions within the cell and the

maintenance of homeostasis

Cell membranes possess two key qualities that function to promote

homeostatic regulation:

Semi-permeability: Only certain materials are able to freely cross the cell

membrane

Selectivity: The cell can control the passage of any material that cannot

freely cross the membrane

Phospholipids

Phospholipids are the base unit for

cell membranes. They consist of:

A hydrophilic (‘water loving’)

phosphate head

Two hydrophobic (‘water fearing’) fatty

acid (lipid) tails

Because there is both hydrophilic and

hydrophobic regions on the

phospholipid molecule, it is referred

to as amphipathic

The Phospholipid Bilayer

 In an aqueous environment, the phospholipids spontaneously form a

bilayer, with the hydrophobic ends pointing together inside the bilayer

(away from the water)

Membrane Proteins

 The phospholipid bilayers are embedded with proteins, which may be:

 Integral Proteins

 Integral proteins penetrate the phospholipid

bilayer to remain permanently attached to

the membrane

 Peripheral Proteins

 Peripheral proteins are only temporarily

associated with one side of a membrane

and are either attached to integral proteins,

polar heads, or the cytoskeleton

Membrane Proteins

Membrane proteins can carry out a variety of functions in a cell:

Junctions – Serve to connect and join two cells together

Enzymes – Fixing to membranes localises metabolic pathways

Transport – Responsible for facilitated diffusion and active transport

Recognition – May function as markers for cellular identification

Anchorage – Attachment points for cytoskeleton and extracellular matrix

Transduction – Function as receptors for peptide hormones

Glycosylation

Phospholipids and membrane proteins can have carbohydrate chains

attached via the process of glycosylation

Glycolipid – carbohydrate attached to phospholipid

Function – recognition between cells (Example – ABO blood group antigens)

Glycoprotein – carbohydrate attached to membrane protein

Function – attachment point for other cells (Example – sperm binding to egg)

Glycosylation

Glycoproteins and glycolipids also play

an important role in maintaining the

structural integrity of the extracellular

matrix

The extracellular matrix is a network for

external molecules that provide structure

and biochemical support to surrounding

cells

The carbohydrate chains can link these

extracellular molecules together to help

make the matrix a cohesive network

Fluid Mosaic Model

The fluid mosaic model describes the structure of the plasma

membrane as a mosaic of components (including phospholipids,

cholesterol, proteins, and carbohydrates), that gives the membrane a

fluid character

Fluid – the phospholipid bilayer is viscous and individual phospholipids can

move position, meaning they are not fixed in position and can adopt

different shapes and break and reform

Mosaic – the phospholipid bilayer is embedded with proteins and

cholesterol, resulting in a mosaic of components

Membrane Fluidity 

The fluidity of a membrane is affected by the composition of fatty acid

tails within the phospholipid bilayer

Unsaturated fatty acids have kinks in their lipid tails which means they pack less

tightly together, have a lower melting point, and remain more fluid

Saturated fatty acids have straight lipid tails which means they pack more tightly

together, have a higher melting point, and are stronger and more stable at higher

temperatures

Membrane Fluidity

Cholesterol is an amphipathic molecule located within the phospholipid

bilayer that plays an important role in regulating fluidity in animal cell

membranes

At high temperatures it functions to stabilise the membrane and raises the

melting point (lowering fluidity)

At low temperatures it intercalates between the phospholipids, preventing

stiffening and crystallisation (raising fluidity)

It also makes the membrane less permeable to very small water-

soluble molecules that would otherwise freely cross, and can help secure peripheral proteins

Membrane Transport

Cells need to move certain molecules and products across the

membrane for normal functioning to continue

Cell requirements such as oxygen or nutrients need to be moved in

Cell products and waste materials need to be moved out

This can occur via a number of methods:

Passive transport methods do not require an input of energy to occur and

generally will move down their concentration gradient

Active transport methods do require an input of energy to occur and can

move against their concentration gradient

Transport Mechanisms

Uniport - A single molecule is transported across the membrane in one direction

Symport - Two molecules are transported across the membrane in the same direction

Antiport - Two molecules are transported across the membrane in opposite directions

Passive Transport

Simple diffusion

Passive movement of small and non-

polar (lipophilic) molecules across cell

membranes down their concentration

gradients

Example - O2 and CO2, glycerol

Facilitated diffusion

Passive movement of molecules (larger

or polar) that are unable to freely cross

the bilayer via the aid of a membrane

protein

Two main types:

Channel Mediated - Protein channels create

hydrophilic pores that allow some solutes,

usually ions, to pass through

Example – Ca2+, Cl-

Carrier Mediated - Protein channels change

shape to translocate the molecule across the

membrane.

Osmosis

Osmosis is the passive movement of water molecules across the membrane from a

region of low solute concentration to a region of high solute concentration (until

equilibrium is reached)

Some water can diffuse directly through the lipid bilayer, but movement is also

aided by specific protein channels called aquaporins

Osmosis

Whether there is net movement of

water into or out of the cell and which

direction it moves depends on the

solute concentration of the cell and the

solution

Isotonic solution = same concentration

inside and outside the cells

Hypertonic solution = higher solute

concentration than in cells

Hypotonic solution = lower solute

concentration than in the cells

Active Transport

Active transport requires an input of energy in

order to move the materials (often against their

concentration gradient)

Primary active transport

The energy comes from chemical energy, usually in the

form of ATP (adenosine triphosphate)

Secondary active transport (cotransport)

An electrochemical gradient is generated by active

transport which can then be used to move another

molecule against its gradient

Primary Active Transport

The sodium-potassium pump

Used by nerve cells to generate an electrochemical gradient

Example of pump mediated cotransport (antiport)

Uses ATP to exchange three sodium ions for two potassium ions

Secondary Active Transport

The sodium-glucose cotransporter

Used in the kidneys and small intestine to absorb glucose

Example of carrier mediated cotransport (symport)

Sodium gradient is used as an energy source instead of ATP (indirect active

transport)

Vesicular Transport

The association between the phospholipids

allows for the spontaneous breaking and

reforming of the bilayer, as the phospholipids

can move around and be rearranged

As a consequence, materials can enter or leave

the cell without having to cross the membrane

(membrane breaks and reforms around the

material as a vesicle)

Vesicular transport is an active process involving

the formation of vesicles or vacuoles

Endocytosis – move substances into the cell

Exocytosis – move substances out of the cell

Vesicular Transport

There are three forms of

endocytosis:

Phagocytosis: engulfment of

solid particles

Pinocytosis: engulfment of

liquid particles

Receptor mediated:

engulfment of specific

particles according to

membrane receptors

Summary of Membrane Transport

Passive transport - processes that

don’t require additional energy to

occur

Diffusion, osmosis, facilitated diffusion

Active transport – processes that do

require additional energy to occur

Primary – direct use of ATP

Secondary – use of gradient (indirect

use of ATP)

Pumps, endocytosis etc

Summary of Membrane Transport

Diffusion – passive movement through the bilayer down/along its concentration gradient

(from high to low concentration)

Facilitated diffusion – passive movement assisted by membrane proteins

Channel mediated – protein provides path for specific molecules, usually ions

Carrier mediated – protein changes shape to transport molecule

Osmosis – passive movement of water from an area of low solute concentration to high solute

concentration

Net movement depends on concentration of solution compared to cell (isotonic, hypertonic,

hypotonic)

Pumps – active movement molecules against their concentration gradients (sodium-potassium

pump, proton pumps etc)

Cytosis – active transport involving the formation of membrane-bound vesicles or vacuoles

(endocytosis into the cell, exocytosis out of the cell)