2.5. Biological Membranes

Question 1

What are the roles of the cell surface membrane?

Answer 1

• It forms a partially permeable barrier between the cell and the environment, controlling the entry and exit of substances
• It is the site of some chemical reactions (like the hydrolysis of ATP)
• It is the site of cell-cell communication and the reception of signalling molecules

Question 2

What are the roles of intracellular plasma membranes?

Answer 2

• Compartmentalise organelles, allowing for conditions specific to the reactions taking place in the organelles and the formation of concentration gradients
• Further compartmentalise areas within organelles
• Control the entry and exit of substances between the cytoplasm and the organelles
• Facilitate some chemical reactions and provide an attachment site for certain enzymes

Question 3

What is the general structure of cell membranes?

Answer 3

• They consist of two layers of phospholipids
• As they are usually in an aqueous environment, the polar phosphate heads will point outwards and the non-polar fatty acid tails will point inwards, attracted to one another due to their mutual hydrophobicity
• The phospholipid bilayer extends around whatever it is containing, such as the contents of a cell
• Phospholipids are free to move within their layer, so cell membranes are referred to as fluid

This is the case for the overwhelming majority of membranes in all organisms

Question 4

What are the different types of proteins that form part of the cell surface membrane?

Excluding ones with carbohydrate attachments

Answer 4

• Intrinsic, or integral proteins, which are embedded through both layers of the bilayer
• Intrinsic proteins have hydrophobic R groups on their surface to keep them in place
• They can either be channel proteins, which provide hydrophilic routes down concentration gradients through the membrane, or carrier proteins, which actively change shape to bring certain substances across the membrane
• Extrinsic, or peripheral proteins, which are present on one side of the membrane and have hydrophilic R groups on their external surface
• Extrinsic proteins can be found on either layer and sometimes move between layers

As a result of these different components within the membrane, this understanding of cell membranes is termed the ‘fluid-mosaic model’

Question 5

What components of the cell membrane contain carbohydrate groups and what are their functions?

Answer 5

• Glycoproteins, which are usually intrinsic proteins and have a carbohydrate chain attached to their surface
• Glycoproteins play a role in cell-cell adhesion and function as receptors, eliciting a response from the cell after the binding of drugs, hormones or neurotransmitters
• Glycolipids, which are carbohydrate chains attached to the phosphate head of a phospholipid in the bilayer
• They are used as cell markers or antigens by the immune system, helping it ascertain whether cells are self or non-self

Question 6

How are cholesterol molecules positioned in cell membranes and what is their purpose?

Answer 6

• Cholesterol molecules are amphipathic
• They are positioned within membranes with the hydrophobic part interacting with the fatty acid tails in the phospholipid bilayer and the hydrophilic part interacting with the phosphate heads
• They regulate the fluidity of the membrane by pulling fatty acids tails together while preventing them from becoming too close and solidifying
• In this way, they prevent both excess fluidity in high temperatures and rigidity in low temperatures

Question 7

Diagram of the fluid-mosaic model to visualise how each component is positioned

Turn over

Answer 7

Question 8

How does temerature affect membrane permeability?

Answer 8

• As temperature increases, the kinetic energy of phospholipids increases, meaning they move more
• This results in membranes being more fluid and thus more permeable as the loss in structural rigidity makes it easier for substances to move across through gaps
• However, at very high temperatures, the permeability of cell membranes to certain substances can decrease as carrier and channel proteins denature

Question 9

How does the presence of a solvent affect membrane permeability?

Answer 9

• Polar solvents like water are essential for maintaining the structure of the phospholipid bilayer
• However, non-polar solvents like benzene or less polar solvents like alcohol are able to dissolve the cell membrane as they can dissolve the hydrophobic centre of the bilayer
• In low concentrations, non-polar will disrupt the rigidity of the bilayer, increasing the concentration of gaps and thus increasing permeability

• When a cell membrane becomes too permeable, the cell can die from the entry of toxins and the function of the cell may no longer be able to be carried out
• This means the presence of non-polar solvents can kill cells

Question 10

What are the two types of diffusion that can take place across cell membranes?

Answer 10

• Simple diffusion
• Facilitated diffusion

• Both require no metabolic energy; they rely on the kinetic energy of the particles themselves
• Both processes simply involve the net, or overall, movement of particles

Question 11

Which molecules can move across cell membranes via simple diffusion?

Answer 11

• Non-polar molecules move across the phospholipid bilaye efficiently while charged or polar molecules are repelled by the central fatty acid tails
• Although it is possible for polar molecules like water to move across the membrane via simple diffusion, they do so very slowly; water instead relies on carrier proteins for rapid movement
• Small molecules move across more easily than large molecules

Question 12

What is the process of facilitated diffusion?

Answer 12

• The passive net movement of substances across a membrane through proteins
• This can be through either channel proteins or carrier proteins
• Channel proteins provide routes for ions or polar substances to quickly diffuse down
• It must be down a concentration gradient and occur without the input of metabolic energy to be considered diffusion
• This process is selective; each type of membrane protein usually only facilitates the movement of one substance
• The normal factors, along with density of membrane proteins, influence the rate of facilitated diffusion

Question 13

How is active transport carried out across cell membranes?

Answer 13

• Using carrier proteins
• A substance binds to the inside of the channel of a carrier protein
• ATP binds to the carrier protein inside the cell and is hydrolysed into ADP and a phosphate molecule
• This phosphate molecule binds to the carrier protein, causing it to change shape and forcing the substances bound to the channel through
• The phosphate molecule is released and recombines with ADP to form ATP
• The carrier protein returns to its original shape

• This process only requires metabolic energy if the substance is moving against a concentration gradient
• If the substance is moving down a concentration gradient, it is facilitated diffusion, not active transport

Question 14

What is the process of endocytosis?

Answer 14

• The bulk transport of substances too large to travel across the membrane into cells
• Phagocytosis is the movement of solids into cells and pinocytosis is the movement of liquids into cells
• The cell surface membrane invaginates, surrounding the material to the transported
• Eventually, the membrane will completely surround the material and fuse to form a vesicle, which is pinched into the cytoplasm

Changing the shape of the membrane and forming vesicles requires metabolic energy

Question 15

What is the process of exocytosis?

Answer 15

• The active, bulk transport of substances that are too large to move across the membrane from inside the cell to the extracellular environment
• Vesicles inside the cell fuse with the cell surface membrane
• Their contents are released outside the cell

Changing the shape of the membrane and moving the vesicle inside the cell with the cytoskeleton requires metabolic energy

Question 16

What is water potential (Ψ) and how is it measured?

Answer 16

• The potential energy of water molecules in an area, measured in Pa as it incorporates pressure
• Water potential indicates the tendency for water to move from one place to another via osmosis
• Pure water is taken as a reference point, so has a water potential of 0
• Adding any solute causes the water potential to become negative, indicating that pure water will diffuse to these regions
• Water potential gradients are ultimately what drive osmosis

Question 17

What is the difference between hydrostatic pressure and water potential (Ψ)?

Answer 17

• Hydrostatic pressure is the force exerted per unit area on the walls of a container with water inside it
• Water potential incorporates hydrostatic pressure but is a thermodynamic and more comprehensive understanding of what drives the movement of water (osmosis) beyond pressure gradients that acknowledges the influence of solute concentration

Question 18

What are the definitions of the terms hypertonic solution, hypotonic solution and isotonic solution?

Answer 18

• Hypertonic refers to having a higher solute concentration and therefore a lower water potential than another solution
• Hypotonic refers to having a lower solute concentration and therefore a higher water potential than another solution
• Isotonic means having the same solute concentration and water potential

Question 19

What is osmotic pressure?

Answer 19

• The minimum pressure required to oppose osmosis
• It will increase with increasing water potential gradient

Question 20

What happens to cells placed in hypotonic solutions?

Answer 20

• Animal cells, with no cell walls, burst in a process called cytolysis
• Plant cells, with cell walls, simply experience an increase in hydrostatic pressure called turgidity; this turgor pressure acts as osmotic pressure, preventing further osmosis

• Therefore, animal cells must only be surrounded by isotonic solutions like the blood plasma
• This is something plant cells are unable to do, which is why they can cope with hypotonic solutions

Question 21

What happens to cells placed in hypertonic solutions?

Answer 21

• Animal cells lose water through osmosis, causing parts of the cell membrane to fold inwards in a process called crenation
• Plant cells similarly lose water and have the protoplast fold inwards away from the cell wall
• This process is called plasmolysis and is reversed by the entry of water in a process called deplasmolysis