2.5 Biological Membranes

OCR A level biology

What is a plasma membrane?

a cell surface membrane

What are some of the functions of membranes?

• partially permeable

• involved in cell signalling

• provide attachment sites for enzymes and other molecules involved in metabolism

• allow electrical signals to be passed

• allow compartmentalisation

Why is it important that membranes allow compartmentalisation?

It allows cellular compartments to have different conditions required for optimum reaction rates, as the membrane acts as a barrier

Explain why phospholipids form a bilayer in plasma membranes

• phospholipids have a polar phosphate group head, which is hydrophilic and will face the aqueous solution

• the fatty acid tails are non-polar and hydrophobic so will move away from the aqueous solution

• as both the tissue fluid and the cytoplasm are aqueous, phospholipids form 2 layers with the hydrophilic tails facing inwards and the hydrophobic phosphate groups facing outwards

Which part of a phospholipid is polar and hydrophilic?

phosphate heads

Which part of a phospholipid is non-polar and hydrophilic?

fatty acid tails

True or false? Phospholipids act as a barrier to most water-soluble substances, such as dissolved glucose and amino acids

True

Why is the structure of the plasma membrane often called the “fluid mosaic model”?

• ‘fluid’ as the phospholipids are free to move laterally within the layer

• ‘mosaic’ due to the scattered pattern of intrinsic and extrinsic proteins in the membrane

• ‘model’ as the agreed structure is based on experimental and chemical evidence

True or false? Proteins (both intrinsic and extrinsic) cannot move through the phospholipid bilayer

false - they can move depending on the number of phospholipids with unsaturated fatty acids in their tails

What is an intrinsic or integral protein?

a protein that is embedded in both layers of the phospholipid bilayer

What is an extrinsic or peripheral protein?

a protein that is present in only one layer of the phospholipid bilayer

How are intrinsic proteins fixed in position in the phospholipid bilayer?

The hydrophobic R-groups in their amino acids interact with the hydrophobic core of the membrane

How are extrinsic proteins held in place?

The hydrophilic R-groups interact with the polar heads of the phospholipids or with intrinsic proteins?

True or false? All extrinsic proteins can move between the layers of the phospholipid bilayer

false - only some can

What type of molecules can diffuse directly through a membrane?

small, non-polar, fat soluble molecules

Give 2 examples of transport proteins?

channel and carrier proteins

What characteristic of molecules means they need a transport protein to transport them through a membrane?

polar

Fill in the blanks and choose the correct words: channel proteins are (p)____ in the membrane that provide a hydrophilic/hydrophobic channel to allow the passive/active of small/large or polar/non-polar molecules through membranes down a ___ ___. Some may be (g)____ to control movement by opening and closing.

pores, hydrophilic, passive, large, polar, gated

What are carrier proteins?

proteins that change shape to carry molecules across a membrane

True or false? Carrier proteins always require energy

false - some are passive

Are glycoproteins intrinsic or extrinsic proteins?

intrinsic

What are the carbohydrates chains (of varying lengths) that protrude from glycoproteins and glycolipids?

glycolax

On a diagram, how can you tell the difference between a glycolipid and a glycoprotein?

the glycolax (carbohydrate chain) of a glycoproteins is usually more highly branched

How do glycoproteins help to stabilise the cell membrane?

they form hydrogen bonds with the water molecules surrounding the cell

Give 3 functions of glycoproteins

• help stabilise the membrane 

• involved in cell adhesion (for building cell tissues)

• act as cell receptors for cell signalling, neurone transmission, hormones, and drugs

What is the function of glycolipids?

• immune recognition - act as cell identity markers or antigens allowing the immune system to recognise its own cells and distinguish them from pathogens, preventing self destruction

• receipts for drugs and hormones 

What does amphiphilic mean?

a molecule is both hydrophobic and hydrophilic

Is cholesterol an amphiphilic molecule?

yes

How is cholesterol positioned within a membrane?

it is found in small amounts positioned in between the phospholipids with the hydrophilic end interacting with the phospholipid heads and hydrophobic end with tails, pulling them together

Choose the correct word: cholesterol - at a high temperature membranes becomes more/less fluid, so more/less stable, as cholesterol releases/binds/modifies the phospholipids together, so they are packed more loosely/tightly.

less, more, binds, tightly

How does cholesterol affect membrane fluidity at low temperatures?

cholesterol maintains the membranes fluidity by preventing the phospholipid molecules from grouping too closely together.

What are the 2 functions of cholesterol in cell membranes?

• regulate fluidity

• increase mechanical strength (less likely to burst)

A - phospholipid head, fatty acid tail, G - intrinsic channel protein, F - extrinsic protein

B - glycolipid, C - protein component of glycoprotein, D - carbohydrate chain of glycoprotein

cholesterol

What 2 factors can affect membrane permeability?

temperature and solvents

How does a temperature below 0ºC affect membrane permeability?

• the phospholipids don’t move as they have little energy, so they are packed closely together causing the membrane to become rigid increasing permeability

• proteins denature increasing permeability

• ice crystals may pierce the membrane, making it very permeable when it thaws

How does a temperature between 0-45ºC affect membrane permeability?

phospholipids have increasing kinetic energy so move around more, meaning they are less tightly packed together so permeability increases

How does a temperature above 45ºC affect membrane permeability?

Fill in the blanks: the bilayer starts to ___, ____ permeability. Water inside the cell ___ putting more ___ on the cell surface membrane. Proteins begin to ___, disrupting the membrane (s)___ so it is no longer an effective (b)___ and allows substances pass through freely.  This process is (i)___

break down, increasing, expands, pressure, denature, structure, barrier, irreversible

How do organic solvents affect the permeability of the phospholipid bilayer?

they increase the permeability by dissolving the phospholipids in the membrane

Give an example of an organic solvent

alcohols

Even though water is a solvent, why does it benefit the phospholipid bilayer?

it has hydrophilic interactions with the phospholipid heads of the phospholipids which help to keep the membrane intact