2.5: Biological membranes

What is the fluid mosaic model?

a model proposed to describe the structure of cell membranes with proteins embedded in a sea of phospholipids

What does the fluid part of the fluid mosaic model mean?

Phospholipids form a bilayer in which the phospholipid molecules are constantly moving.

What does the mosaic part of the fluid mosaic model mean?

There are proteins of different sizes and shapes embedded in the phospholipid bilayer (so it looks a bit like a mosaic)

What are some of the key components of cell membranes?

• phospholipid bilayer

• cholesterol

• proteins

• glycoproteins

• glycolipids

What are phospholipids?

• they form the basic structure of the membrane (the phospholipid bilayer)

• hydrophilic phosphate head with a charge

• hydrophobic fatty acid tails

What two structures can phospholipids form when exposed to water?

• micelle - circular with hydrophilic tails on the inside

• bilayer - water-soluble substances can’t pass through but lipid-soluble substances can

What two categories can proteins in the cell membrane be categorised into?

• intrinsic proteins

• extrinsic proteins

What are intrinsic proteins?

(or integral) proteins are embedded through both sides of the phospholipid bilayer e.g channel and carrier proteins which transport large molecules and ions across the membrane

What are extrinsic proteins?

(or peripheral) proteins are present on only one side of the phospholipid bilayer. These proteins provide support to the membrane or may be involved in cell signalling

What are the two types of transport proteins and what do they do?

Transport proteins create hydrophilic channels to allow ions and polar molecules to travel through the membrane and control which substances enter or leave

• Carrier proteins change shape to transport a substance across the membrane in both directions(ATP usually required for active transport)

• Channel proteins allow charged particles to move down their concentration gradient through a hydrophilic pore

What does cholesterol do to the cell membrane?

it gives it mechanical strength and flexibility and controls membrane fluidity (hexagonal chain)

How does cholesterol increase the fluidity of the membrane?

cholesterol stops the phospholipid tails from packing too closely together at low temperatures

How does cholesterol stop the membrane from becoming too fluid?

Cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them and causing phospholipids to pack more closely together at higher temperatures

What are glycoproteins?

they consist of intrinsic proteins attached to a carbohydrate chain

What are glycolipids?

they consist of lipids attached to a carbohydrate chain

What are glycolipids and glycoproteins involved in?

1. Cell adhesion - This is the attachment of cells to one another. 

2. Cell recognition - This allows cells to recognise one another.

3. Cell signalling - This is communication between cells. 

What is the glycocalyx?

all the carbohydrate molecules on the exterior of a cell surface membrane, formed by carbohydrate chains attached to lipids or proteins (glyco-)

What is the role of the glycocalyx in the membrane?

regulates the movement of fluids between the endothelial cells and functions as barriers to macromolecules

What are the roles of the cell membrane?

• isolating

• controlling what enters and exits

• act as a barrier

• communication - cell signalling

• cell recognition

• allows transport of substances

• site of chemical reactions

Describe the role of the cell membrane in isolating

isolating organelles from the rest of the cytoplasm allows cellular processes to occur separately

Describe the role of the cell membrane in terms of controlling what enters and exits it

• proteins in the membrane allow the passage of large, charged water-soluble substances

• ions that are too large can be transported by carrier or channel proteins

Describe the role of the cell membrane in acting as a barrier

• hydrophilic head and hydrophobic tail forms a bilayer (micelle in water)

• small non-polar molecules e.g O2 diffuse across rapidly

• small polar molecules e.g water diffuse slower

What is a liposome?

a bilayer that has twisted on itself to make a spherical 3D structure

Describe the role of the cell membrane in cell to cell communication

• cells can signal to each other via messengers e.g hormones

• some can enter easily, others that are too large use cell membrane receptors

Describe the role of the cell membrane in cell recognition

• glycoproteins and glycolipids are restricted to the outside surface

• they help a cell recognise itself as ‘self’ cells rather than ‘foreign’ cells

Describe the role of the cell membrane in fluidity

it is fluid due to its phospholipid bilayer and it allows its components to move and flow, which is necessary for its function

Why does the cell membrane need to be fluid?

• to allow diffusion

• exocytosis and endocytosis

• phagocytosis and shape change

What happens to the phospholipids at high temperatures in the membrane?

• phospholipids get more kinetic energy, so increase in fluidity

• proteins drift around so become ineffective

• cell signalling phagocytosis affected

What happens to cholesterol at high temperatures in the membrane?

reduces the increase in fluidity so buffers the effect of the heat

What happens to phospholipids at low temperatures in the membrane?

phospholipids pack closely and tightly together

What happens to cholesterol at low temperatures in the membrane?

maintains membrane fluidity and prevents it from freezing

What is the effect of heat on the proteins (+permeability)?

• too much heat denatures proteins along with the cytoskeleton increasing the number of permeable holes in the plasma membrane

• membrane bound organelles will no longer function as they are denatured

What is happening at point 1 of this graph?

At low temperatures (below 0°C) - Phospholipids do not have much kinetic energy. They are packed closely together to form a rigid cell membrane, decreasing the permeability of the membrane. 

What is happening at point 2 of this graph?

At medium temperatures (0 - 40°C) - As temperature increases, phospholipids have more kinetic energy so they move faster and aren't packed as closely together. This increases the permeability of the membrane. 

What is happening at point 3 of this graph?

At high temperatures (above 40°C) - The phospholipid bilayer breaks down. Channel and carrier proteins denature, meaning they cannot control what enters or leaves the cell. These effects increase the permeability of the membrane.

What effect do solvents have on the permeability of membranes?

When cells are placed in a solvent such as ethanol, the phospholipids dissolve, causing the membrane to become more fluid. This disrupts the structure of the cell membrane to make it more permeable. 

Increasing the concentration of the solvent will further increase the permeability of the cell membrane. 

What is the composition of cell membranes in neurones?

• myelin sheath - formed by flattened cells wrapped around them several times giving several layers of cell membrane

• protein channels and carriers allow the entry and exit of ions to bring about the conduction of electrical impulses

What is the composition of the plasma membrane in white blood cells?

they contain special protein receptors that enable them to recognise the antigens on foreign cells

What is the composition of the membrane of root hair cells?

they have carrier proteins to actively transport nitrate ions from the soil into the cells

Define diffusion

the net movement of particles from an area of higher concentration to an area of lower concentration (maybe across a membrane), not requiring energy

What are the two types of diffusion?

simple diffusion and facilitated diffusion

What is simple diffusion?

Some molecules can diffuse directly across cell membranes in this process

Give an example in the body of simple diffusion?

oxygen and carbon dioxide diffuse into cells via simple diffusion. Carbon dioxide diffuses down the concentration gradient from the blood to the alveoli whilst oxygen diffuses from the alveoli into the capillaries

What about particles allows simple diffusion?

This is because they are: 

• Small - This means that they can pass through the spaces between phospholipids.

• Non-polar - This means that they can dissolve in the hydrophobic core of the cell membrane. 

What is facilitated diffusion?

when large or polar molecules can cross the cell membrane via carrier and channel proteins

Why can only one or two molecules pass at a time during facilitated diffusion?

because each carrier or channel protein is highly specific

What do carrier proteins usually transport across the membrane?

large molecules

How do carrier proteins carry these molecules across the membrane?

1. a large molecule attaches to a carrier protein

2. this causes the carrier protein to change shape

3. the carrier protein releases the molecule on the opposite side of the membrane

What do channel proteins mainly transport across the cell membrane?

ions

How do channel proteins carry transport these molecules across the membrane?

These proteins form pores in the cell membrane, which ions can travel through. 

What factors affect the rate of diffusion?

• surface area

• concentration gradient

• thickness of membrane

• temperature

• size of diffusing molecule

• number of carrier or channel proteins

How does the surface area affect the rate of diffusion?

Larger surface areas mean more particles can cross the membrane at once, making diffusion faster

How does the concentration gradient affect the rate of diffusion?

The steeper the concentration gradient, the faster the rate of diffusion

How does the thickness of membrane affect the rate of diffusion?

The particles travel shorter distances through thin exchange surfaces, so diffuse faster

How does the temperature affect the rate of diffusion?

At higher temperatures, particles have more kinetic energy and diffuse faster

How does the size of the diffusing molecule affect the rate of diffusion?

the smaller the molecule, the faster it diffuses across a surface

How does the number of channel or carrier proteins affect the rate of diffusion?

The more of these proteins, the faster the rate of facilitated diffusion

Define osmosis

the net movement of water molecules from an area of higher water concentration to an area of lower water concentration, through a partially-permeable membrane

Define water potential

a measure of the tendency of water molecules to diffuse from one region to another

How do water molecules move in terms of water potential?

they always move from a region of higher water potential to a region of lower (more negative) water potential

What is water potential measured in?

kilopascals (kPa),  (Ψ)

What has the highest water potential?

pure water - 0kPa

Hypotonic solutions in plant cells

• solution has a higher water potential than the cell

• Water molecules move into the cell. 

• The cell swells and becomes turgid (cell wall protects)

Isotonic solutions in plant cells

• solution has the same water potential as the cell

• There is no net movement of water into or out of the cell

• The cell stays the same size

Hypertonic solutions in plant cells

• solution has a lower water potential than the cell

• Water molecules move out of the cell

• he cell shrinks and becomes plasmolysed - flaccid

Hypotonic solutions in animal cells

• solution has a higher water potential than the cell

• Water molecules move into the cell

• The cell swells and bursts (no cell wall) - cytolysis

Isotonic solutions in animal cells

• solution has the same water potential as the cell

• There is no net movement of water into or out of the cell

• The cell stays the same size

Hypertonic solutions in animal cells

• solution has a lower water potential than the cell

• Water molecules move out of the cell

• The cell shrinks - crenation

What happens in normal CFTR channel?

1. water potential becomes lower as chloride ions move out of the cell

2. this causes water molecules to move out of the cell, causing a running liquid mucus

What happens in a mutated CFTR channel?

1. chloride ions can’t pass through

2. thus means that water molecules can’t pass through either and so mucus will sit on the top

Define active transport

the movement of particles from an area of lower concentration to an area of higher concentration. This process requires energy from respiration in the form of ATP

What proteins does active transport use?

carrier proteins

How do these proteins transport molecules across membranes?

1. The molecule or ion binds to the carrier protein

2. ATP binds to the carrier protein

3. Hydrolysis of ATP to ADP and phosphate (Pi) causes the carrier protein to change shape. This releases the molecule or ion on the opposite side of the membrane

4. The phosphate (Pi) is released from the carrier protein, causing the carrier protein to return to its original shape, ready to be used again

What additional factor affects the rate of active transport?

Rate of respiration - The more respiration, the more ATP available for active transport

How do large molecules such as enzymes that can’t be transported through channel or carrier proteins move into and out of cells?

via bulk transport

What are the two types of bulk transport?

1. Endocytosis - This transports materials into cells. 

2. Exocytosis - This transports materials out of cells. 

What happens in endocytosis?

a segment of the plasma membrane surrounds and encloses the particle and brings it into the cell, enclosed in a vesicle

What are the two forms of endocytosis?

• Phagocytosis - The uptake of solid materials.

• Pinocytosis - The uptake of liquid materials.

What happens in exocytosis?

Vesicles move towards and fuse with the cell-surface membrane where materials are released outside the cell

Why is beetroot used in permeability investigations?

Beetroot contains betalain pigment in vacuoles; damage to membranes causes pigment to leak out, so permeability can be easily measured

What variable is typically measured in the beetroot practical?

Absorbance of leaked pigment using a colorimeter

What does a high absorbance mean?

lots of pigment leaked out = high membrane permeability

What are the independent variables in membrane permeability experiments?

Temperature, solvent concentration (e.g., ethanol)

What are key controlled variables in the beetroot experiment?

Size/shape of beetroot cylinders, washing beetroot, pH, volume of solution, time left in solvent, temperature (if not being tested)

What trend is expected when temperature increases?

Absorbance increases due to more pigment leakage; sharp increase at high temperatures from protein denaturation

What trend is expected when ethanol concentration increases?

Membrane permeability increases; more pigment leaks out → higher absorbance