BIO OCR 2.1.5

What model describes the structure of biological membranes

The fluid mosaic model

Why is the membrane described as “fluid” in the fluid mosaic model

Phospholipids and some proteins can move laterally within the membrane

Why is the membrane described as a “mosaic”

It contains a mixture of different components, including phospholipids, proteins. Cholesterol, glycoproteins and glycolipids

What are the main components of a biological membrane

• phospholipids

• Proteins

• Cholesterol

• Glycoproteins

• Glycolipids

Describe the structure of a phospholipid

• polar, hydrophilic phosphate head

• Non-polar, hydrophobic fatty acid tail

Why do phospholipids form a bilayer in an aqueous environment

The hydrophilic phosphate heads interact with water, while the hydrophobic fatty acid tails orientate away from water as they repel it, causing the PPL to form a bilayer.

Why does the PPL bilayer act as a barrier to ions and many polar substances

Ions and polar substances cannot readily pass through the hydrophobic region formed by the non-polar fatty acid tails

Which substances can pass directly through the PPL bilayer relatively easily

Small, non-polar or lipid-soluble molecule

• O2 and CO2

Why is the plasma membrane described as partially permeable

It allows some substances to cross more readily than others, depending on properties such as size, polarity and charge.

What is an intrinsic protein?

A protein that is embedded within the PPL bilayer

• channel proteins

What is the structure and function of a channel protein

A channel protein forms a hydrophilic channel through the membrane, allowing specific ions or polar substances to cross

What is the structure and function of a carrier protein

A carrier protein binds to a specific molecule or ion and changes shape transporting the membrane across the membrane

What is the key difference between channel proteins and carrier proteins

Channel proteins : forms a hydrophilic channel/pore

Carrier proteins : binds to a specific substance and changes shape

What is simple diffusion across a cell-surface membrane

The net movement of molecules from a region of higher concentration to a region of lower concentration, directly through the PPL bilayer

Give two molecules that can cross the PPL bilayer by simple diffusion

• Oxygen

• Carbon dioxide

Because they are small and non-polar

What is facilitated diffusion

The net movement of substances from a region of higher concentration to a region of lower concentration through channel or carrier proteins

Why type of process in facilitated diffusion

A passive process and does no require energy from ATP

What types of substances require facilitated diffusion rather than passing directly through the PPL bilayer

Substances that cannot readily cross the hydrophobic bilayer, such as ions, polar molecules and some larger molecules

What is active transport

The movement of substances against their concentration gradient, using carrier proteins and ATP as an immediate source of energy

Which membrane protein should you associate with active transport in OCR questions

Carrier Proteins

(Do not state that active transport occurs through channel proteins)

What is a glycoprotein

A protein with a carbohydrate chain attached

What is a glycolipid

A lipid with a carbohydrate chain attached

What are important roles of glycoproteins and glycolipids in cell membranes

They are involved in:

• cell recognition

• Cell signalling/communication

What is the glycocalyx

The carbohydrate-containing layer on the outside of the cell surface membrane, formed from carbohydrate chains associated with glycoproteins and glycolipids

• it is involved in cell recognition

What is the function of membrane-bound receptor proteins

They provide specific binding sites for signalling molecules, such as hormones, allowing information to be transmitted into the cell and a response to be produced.

• hormones and drugs can bind to membrane-bound receptors

Where is the cholesterol found in the cell-surface membrane

Cholesterol is located between phospholipids, where its hydrophobic region interact with the hydrophobic fatty acid tails.

What are the functions of cholesterol in biological membranes

• regulates membrane fluidity/flexibility

• Increases membrane stability

• Reduces permeability to some polar/water-soluble substances

How does cholesterol regulate membrane fluidity at high temperatures

Cholesterol restrict the movement of phospholipids, preventing the membrane from becoming too fluid

How does cholesterol regulate membrane fluidity at low temperatures

Cholesterol prevents phospholipids from packing too closely together, helping to maintain membrane fluidity

State three roles of membranes within cells at the cell surface

• partially permeable barriers

• Sites of chemical reactions

• Sites of cell communication/cell signalling

Why are membrane-bound enzymes useful

Membranes provide sites for enzymes, allowing chemical reactions and metabolic pathways to be organised

Explain why a charged or polar substance cannot simply diffuse through the phospholipid bilayer

• The substance is charged/polar/hydrophilic

• Therefore it cannot readily pass through the hydrophobic region of the PPL bilayer and requires the appropriate transport protein

Explain how the properties of phospholipids cause a bilayer to form in an aqueous environment

• phospholipids contain polar, hydrophilic phosphate heads

• they also contain non-polar, hydrophobic fatty acid tails

• The heads interact with the aqueous environments while the tails orientate away from water, causing a phospholipid bilayer to form.

Explain how the structure of a plasma membranes allows both lipid-soluble and water-soluble substances to cross it

• Lipid-soluble/non-polar substances can dissolve in and diffuse through the phospholipid bilayer

• The centre of the bilayer is hydrophobic

• Ions/polar or water-soluble substances cannot readily pass through this hydrophobic region

• They cross using specific channel or carrier proteins

Explain the relationship between cholesterol and membrane fluidity

• cholesterol is positioned between phospholipids

• At high temps, it restricts phospholipid movement, reducing excessive fluidity

• At low temps, it prevents phospholipids packing too closely together

• Therefore cholesterol regulates membrane fluidity

Describe the fluid mosaic model of membrane structure

• membrane consists of a phospholipid bilayer

• Contains proteins embedded within/associated with the bilayer

• Contains other components including cholesterol, glycoproteins and glycolipids

• Component can move laterally, making the membrane fluid, while the different components produce a mosaic arrangement

A molecule is too large or too polar to pass directly through the phospholipid bilayer. Explain how membrane structure enables it to cross

• The molecule can cross using a specific transport protein.

• A channel protein provides a hydrophilic pathways for suitable ions/polar substances, which a carrier protein binds to a specific substance and changes shape to transport it across the membrane.

What are the main roles of biological membranes?

• act as partially permeable barriers

• Control movement of substances between different regions

• Compartmentalise cells and organelles

• Provide sites for chemical reactions

• Provide sites for cell communication / cell signalling

• Allow cell recognition

• Form and fuse to produce vesicles for intracellular transport

What is a membrane?

Membranes act as partially permeable barriers, sites of chemical reactions and sites of cell communication

What are the roles of the cell-surface membrane?

• separates the cell contents from the external environment

• Acts as a partially permeable barrier

• Controls which substances enter and leave the cell

• Contains receptors involved in cell signalling

• Contains sell-surface molecules involved in cell recognition

Explain how the cell-surface membrane acts as a partially permeable barriers Control movement (3)

• The PPL bilayer prevents many substances from passing freely through the membrane

• Specific membrane proteins allow particular substances to cross.

• Therefore, the membrane regulates the movement of substances into and out of the cell

Explain how the cell-surface membrane is involved in cell signalling (3)

• a signalling molecule binds to a specific receptor on the cell-surface membrane of its target cell

• The receptor has a specific/ complementary shape to the signalling molecule

• Binding causes a response/change in behaviour in the target cell.

Explain how the cell-surface membranes are involved in cell recognition

• cells have cell-surface identity markers, including antigens

• These allow cells of the immune system to distinguish between cells/material recognised as belonging to the body and foreign cells/material

What is compartmentalisation

The division of a cell into separate compartments by membranes, allowing different cellular processes to occur in different regions.

Why is compartmentalisation important

• different cellular processes require different conditions

• Membranes separate these processes into different compartments

• Each compartment can maintain the conditions needed for particular reactions/processes

How does lysosome show the importance of compartmentalisation

• lysosome contains hydrolytic enzymes

• The lysosome maintains an acidic environment of about pH 5

• The cytoplasm is about pH 7.2

• The lysosome membrane keeps these environments separate, providing suitable conditions for the lysosome enzymes

How do organelle membranes control the movement of substances

• Organelle membranes are partially permeable

• They control which substances enter and leave the organelle

• This helps maintain the organelle’s internal conditions

Why are some membranes within cells folded

• folding increases surfaces area

• This provides more sites for membrane - associated enzymes/proteins

• Therefore, more membrane - associated chemical reactions can occur

How are membranes involved in transporting substances within cells

• membranes can form vesicles around substnaces

• Vesicles transport the substances within the cell

• Vesicle membranes can fuse with other membranes and reveal without losing their contents

What is diffusion

The net movement of a substance down a concentration gradient, form an area of high concentration to an area of lower concentration, until equilibrium is reached

What does ‘net movement’ mean in diffusion

• particles more in both directions

• Overall, more particles move from high concentration to low concentration

• The produces a net movement down the concentration gradient

What happens when equilibrium is reached during diffusion

• particles continue moving randomly

• They move in both directions at equal rates

• Therefore, there is no net movement

Why is diffusion described as a passive process

It does not require energy transferred by respiration. The particles move due to their own kinetic energy.random motion

How does temp affect the rate of diffusion

Higher temp:

• particles have more kinetic energy than

• Particles move faster

• Rate of diffusion increases

How does conc gradient affect the rate of diffusion

Steeper conc gradient:

• greater different in concentration

• Greater net movement per unit time

• Rate of diffusion increases

How does diffusion distance affect the rate of diffusion

Shorter diffusion distance/ thinner membrane:

• particles travel a shorter distance

• Rate of diffusion increases

How does SA:V ratio affect diffusion

Larger SA:V:

• more surface available for exchange real active to volume

• Greater rate of exchange by diffusion

How does the number of channel/carrier proteins affect facilitated diffusion

More appropriate channel/carrier proteins:

• more routes through the membrane

• Greater rate of facilitated diffusion

Which substances can cross the PPL bilayer by simple diffusion

Small nonpolar/lipid soluble molecules inc.

• O2

• CO2

What is facilitated diffusion

The passive movement of substances down then concentration gradient through specific channel or carrier proteins without ATP

What is active transport

The movement of substances against their concentration gradient using specific carrier/transport proteins using ATP as an immediate source of energy

What are the key differences between simple diffusion, facilitated diffusion ans active transport

Simple diffusion: PPL bilayer → down gradient → no ATP

Facilitated diffusion: channel/carrier protein → down gradient → no ATP

Active transport: carrier protein → against gradient → ATP required

A substance enters a cell by simple diffusion. Why does increasing its conc outside the cell increased its rate of uptake?

• increasing its external concentration creates a steeper conc gradient across the membrane

• This causes greater net movement down the conc gradient per unit time

• Therefor, the rate of simple diffusion increases

Why does a steeper conc gradient increase the rate of diffusion

• there is a greater difference in concentration between the two regions

• Therefore, there is greater newt movement down the concentration gradient per unit time

Why can facilitated diffusion reach a maximum rate even if concentration continues to increase

• there is a limited number of channel/carrier proteins

• At high concentrations, the transport proteins become saturated

• They are working at maximum capacity, so the rate cannot increase further

Why can active transport reach a maximum rate even rate even if concentration continues to increase

• there are a limited number of carrier proteins

• At high concentrations, all carriers may be occupied

• The carriers become saturated, so transport reaches its maximum rate

A substance still enters a cell when ATP production is inhibited. What does this suggest

It suggests the substance may enter by a passive process such as simple diffusion or facilitated diffusion.

Why is compartmentalisation important-