biological membranes

Question 1

role of membranes

Answer 1

Membranes are vital structures found in all cells
The cell surface membrane creates an enclosed space separating the internal cell environment from the external environment

Intracellular membranes (internal membranes) form compartments within the cell, such as organelles (including the nucleus, mitochondria and RER) and vacuoles

Membranes not only separate different areas but also control the exchange of materials passing through them; they are partially permeable
Membranes form partially permeable barriers between the cell and its environment, between cytoplasm and organelles and also within organelles

Substances can cross membranes by diffusion, facilitated diffusion, osmosis and active transport

Membranes play a role in cell signalling by acting as an interface for communication between cells

Question 2

fluid mosaic model

Answer 2

The fluid mosaic model also helps to explain:
Passive and active movement between cells and their surroundings
Cell-to-cell interactions
Cell signalling

The fluid mosaic model describes cell membranes as ‘fluid’ because:
The phospholipids and proteins can move around via diffusion
The phospholipids mainly move sideways, within their own layers
The many different types of proteins interspersed throughout the bilayer move about within it (a bit like icebergs in the sea) although some may be fixed in position

The fluid mosaic model describes cell membranes as ‘mosaics’ because:
The scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above

The fluid mosaic model of membranes includes four main components:
Phospholipids
Cholesterol
Glycoproteins and glycolipids
Transport proteins

Question 3

phospholipids

Answer 3

Phospholipids form the basic structure of the membrane (the phospholipid bilayer)

The tails form a hydrophobic core comprising the innermost part of both the outer and inner layer of the membrane

Phospholipids bilayers act as a barrier to most water-soluble substances (the non-polar fatty acid tails prevent polar molecules or ions from passing across the membrane)
This ensures water-soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell and unwanted water-soluble molecules cannot get in

Phospholipids can be chemically modified to act as signalling molecules by:
Moving within the bilayer to activate other molecules (eg. enzymes)
Being hydrolysed, which releases smaller water-soluble molecules that bind to specific receptors in the cytoplasm

Question 4

cholesterol

Answer 4

Cholesterol increases the fluidity of the membrane, stopping it from becoming too rigid at low temperatures (allowing cells to survive at lower temperatures)

This occurs because cholesterol stops the phospholipid tails packing too closely together

Interaction between cholesterol and phospholipid tails also stabilises the cell membrane at higher temperatures by stopping 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

The impermeability of the membrane to ions is also affected by cholesterol
Cholesterol increases the mechanical strength and stability of membranes (without it membranes would break down and cells burst)

Question 5

glycolipids and glycoproteins

Answer 5

Glycolipids and glycoproteins contain carbohydrate chains that exist on the surface (the periphery/extrinsically), which enables them to act as receptor molecules
The glycolipids and glycoproteins bind with certain substances at the cell’s surface

There are three main receptor types:
Signalling receptors for hormones and neurotransmitters
Receptors involved in endocytosis
Receptors involved in cell adhesion and stabilisation (as the carbohydrate part can form hydrogen bonds with water molecules surrounding the cell

Some glycolipids and glycoproteins act as cell markers or antigens, for cell-to-cell recognition (eg. the ABO blood group antigens are glycolipids and glycoproteins that differ slightly in their carbohydrate chains)

Question 6

transport proteins

Answer 6

Transport proteins create hydrophilic channels to allow ions and polar molecules to travel through the membrane. There are two types:
Channel (pore) proteins
Carrier proteins
Carrier proteins change shape to transport a substance across the membrane

Each transport protein is specific to a particular ion or molecule
Transport proteins allow the cell to control which substances enter or leave

Question 7

temperature on membrane permeability

Answer 7

Proteins and lipids (the major components in cell membranes) are both affected by temperature
As temperature increases, lipids become more fluid
This increased fluidity reduces the effectiveness of the cell membrane as a barrier to polar molecules, meaning polar molecules can pass through

At higher temperatures, any diffusion taking place through the cell membrane will also occur at a higher speed (due to increased kinetic energy)
Changes in membrane fluidity are reversible
If temperatures decrease, the lipids will return to their normal levels of fluidity)

At a certain temperature (often around 40°C) many proteins (including those in cell membranes) begin to denature
This disrupts the membrane structure, meaning it no longer forms an effective barrier
As a result, substances can pass freely through the disrupted membrane
This process is irreversible

Question 8

solvent concentration on permeability

Answer 8

Organic solvents can increase cell membrane permeability as they dissolve the lipids in the membrane, causing the membrane to lose its structure

Question 9

diffusion

Answer 9

Diffusion is a type of transportation that occurs across the cell membrane

It can be defined as:
The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration.

The molecules or ions move down a concentration gradient
The random movement is caused by the natural kinetic energy of the molecules or ions

As a result of diffusion, molecules or ions tend to reach an equilibrium situation (given sufficient time), where they are evenly spread within a given volume of space
The rate at which a substance diffuses across a membrane depends on several factors

Question 10

facilitated diffusion

Answer 10

Certain substances cannot diffuse through the phospholipid bilayer of cell membranes.
These include:
Large polar molecules such as glucose and amino acids
Ions such as sodium ions (Na+) and chloride ions (Cl-)

These substances can only cross the phospholipid bilayer with the help of certain proteins
This form of diffusion is known as facilitated diffusion
There are two types of proteins that enable facilitated diffusion:
Channel proteins
Carrier proteins
They are highly specific (they only allow one type of molecule or ion to pass through)

Question 11

channel proteins

Answer 11

Channel proteins are water-filled pores
They allow charged substances (eg. ions) to diffuse through the cell membrane

The diffusion of these ions does not occur freely, most channel proteins are ‘gated’, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore
This allows the channel protein to control the exchange of ions

Question 12

carrier proteins

Answer 12

Unlike channel proteins which have a fixed shape, carrier proteins can switch between two shapes
This causes the binding site of the carrier protein to be open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape
The direction of movement of molecules diffusing across the membrane depends on their relative concentration on each side of the membrane
Net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient (from an area containing many of that specific molecule to an area containing less of that molecule)

Question 13

active transport

Answer 13

Active transport is the movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration

Active transport requires carrier proteins (each carrier protein being specific for a particular type of molecule or ion)
Although facilitated diffusion also uses carrier proteins, active transport is different as it requires energy

The energy is required to make the carrier protein change shape, allowing it to transfer the molecules or ions across the cell membrane
The energy required is provided by ATP (adenosine triphosphate) produced during respiration. The ATP is hydrolysed to release energy

Question 14

active transport is important in

Answer 14

The reabsorption of useful molecules and ions into the blood after filtration into the kidney tubules

The absorption of some products of digestion from the digestive tract

The loading of sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant

The loading of inorganic ions from the soil into root hairs

Question 15

endocytosis and exocytosis

Answer 15

The processes of diffusion, osmosis and active transport are responsible for the transport of individual molecules or ions across cell membranes

However, the bulk transport of larger quantities of materials into or out of cells is also possible

Examples of these larger quantities of materials that might need to cross the membrane include:
Large molecules such as proteins or polysaccharides

Parts of cells

Whole cells eg. bacteria

Bulk transport into cells = endocytosis
Bulk transport out of cells = exocytosis
These two processes require energy and are therefore forms of active transport

Question 16

endocytosis

Answer 16

Endocytosis is the process by which the cell surface membrane engulfs material, forming a small sac (or ‘endocytic vacuole’) around it

There are two forms of endocytosis:
Phagocytosis:
This is the bulk intake of solid material by a cell
Cells that specialise in this process are called phagocytes
The vacuoles formed are called phagocytic vacuoles
An example is the engulfing of bacteria by phagocytic white blood cells

Pinocytosis:
This is the bulk intake of liquids
If the vacuole (or vesicle) that is formed is extremely small then the process is called micropinocytosis

Question 17

exocytosis

Answer 17

Exocytosis is the process by which materials are removed from, or transported out of, cells (the reverse of endocytosis)

The substances to be released (such as enzymes, hormones or cell wall building materials) are packaged into secretory vesicles formed from the Golgi body

These vesicles then travel to the cell surface membrane
Here they fuse with the cell membrane and release their contents outside of the cell
An example is the secretion of digestive enzymes from pancreatic cells

Question 18

osmosis

Answer 18

All cells are surrounded by a cell membrane which is partially permeable
Water can move in and out of cells by osmosis

Osmosis is the diffusion of water molecules from a dilute solution to a more concentrated solution across a partially permeable membrane
In doing this, water is moving down its concentration gradient

The cell membrane is partially permeable which means it allows small molecules (like water) through but not larger molecules (like solute molecules)

Osmosis can also be described as the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane

Water potential describes the tendency of water to move out of a solution. This term is used to avoid confusion between water concentration and concentration of a solution

A dilute solution has a high water potential and a concentrated solution has a low water potential

Question 19

water potential

Answer 19

The water potential of pure water (without any solutes) at atmospheric pressure is 0kPa, therefore any solution that has solutes will have a water potential lower than 0kPa (it will be a negative value)

the more negative the water potential, the lower the water potential (the further it is away from pure water which has a water potential of 0 kPa).

Question 20

osmosis in animal cells (losing water)

Answer 20

If an animal cell is placed in a solution with a lower water potential than the cell (such as a concentrated sucrose solution)

Water will leave the cell through its partially permeable cell surface membrane by osmosis and the cell will shrink and shrivel up

This is crenation (the cell has become crenated), which is usually fatal for the cell

Crenation occurs when the cell is in a hypertonic environment (the solution outside of the cell has a higher solute concentration than the inside of the cell)

Question 21

osmosis in animals cells (gaining water)

Answer 21

If an animal cell is placed in pure water or a dilute solution, water will enter the cell through its partially permeable cell surface membrane by osmosis, as the pure water or dilute solution has a higher water potential

The cell will continue to gain water by osmosis until the cell membrane is stretched too far and the cell bursts (cytolysis), as it has no cell wall to withstand the increased pressure created

This is fatal for the cell
Lysis occurs when the cell is in a hypotonic environment (the solution outside of the cell has a lower solute concentration than the inside of the cell)
This is why a constant water potential must be maintained inside the bodies of animals

Question 22

osmosis in animal cells (isotonic environment)

Answer 22

If an animal cell is in an isotonic environment (the solution outside of the cell has the same solute concentration as the inside of the cell)

The movement of water molecules into and out of the cell occurs at the same rate (no net movement of water) and there is no change to the cells

Question 23

osmosis in plant cells (losing water)

Answer 23

If a plant cell is placed in a solution with a lower water potential than the plant cell (such as a concentrated sucrose solution), water will leave the plant cell through its partially permeable cell surface membrane by osmosis

As water leaves the vacuole of the plant cell, the volume of the plant cell decreases

The protoplast gradually shrinks and no longer exerts pressure on the cell wall

As the protoplast continues to shrink, it begins to pull away from the cell wall

This process is known as plasmolysis – the plant cell is plasmolysed

Question 24

osmosis in plant cells (gaining water)

Answer 24

If a plant cell is placed in pure water or a dilute solution, water will enter the plant cell through its partially permeable cell surface membrane by osmosis, as the pure water or dilute solution has a higher water potential than the plant cell

As water enters the vacuole of the plant cell, the volume of the plant cell increases
The expanding protoplast (living part of the cell inside the cell wall) pushes against the cell wall and pressure builds up inside the cell – the inelastic cell wall prevents the cell from bursting

The pressure created by the cell wall also stops too much water from entering and this also helps to prevent the cell from bursting

When a plant cell is fully inflated with water and has become rigid and firm, it is described as fully turgid
This turgidity is important for plants as the effect of all the cells in a plant being firm is to provide support and strength for the plant – making the plant stand upright with its leaves held out to catch sunlight

If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant wilts