Biological membranes

Outline the properties and function of 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

• Net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient

Outline the properties and functions of channel proteins

• 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

How do substances that can't normally pass through the PLBL manage to pass through

• what are the proteins that help with this

This form of diffusion is known as facilitated diffusion

• Channel and carrier proteins enable FD - they're highly specific

What substances can't diffuse across the phospholipid cell membrane

• Large polar molecules such as glucose and amino acids

• Ions such as sodium ions (Na+) and chloride ions (Cl-)

How do properties of ions/molecules effect the rate of diffusion

(Large molecules, polar/non-polar)

Large molecules

• Large molecules diffuse more slowly than smaller ones as they require more energy to mov

Uncharged AND non-polar

• Uncharged and non-polar molecules diffuse directly across the phospholipid bilaye

Non-polar/polar

• Non-polar molecules diffuse more quickly than polar ones as they are soluble in the non-polar phospholipid bilaye

How does surface area effect the rate of diffusion

• what happens to a cell as it increases in size

• The greater the surface area, the greater the number of molecules or ions that can cross it at any one moment and therefore the faster diffusion occur

• As a cell increases in size, the surface area to volume ratio decreases which slows the rate substances can diffuse through a cell as the distance required becomes too great

How does temperature effect the rate of diffusion

• Molecules and ions have higher kinetic energy at higher temperatures

• They move faster, resulting in a higher rate of diffusion

How does the 'steepness' of the concentration gradient effect diffusion

A greater difference in concentration means a greater difference of the number of molecules passing in two directions, and therefore a faster rate of diffusion

Define diffusion

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 random movement is caused by the natural kinetic energy of the particles

Is the phosphate head of a fatty acid hydrophobic or hydrophilic

Hydrophilic

What are the two types of carrier protein

• Channel (pore proteins)

• Carrier proteins

  • Carrier proteins change shape to transport a substance across the membrane

Outline the role of transport proteins

• Transport proteins create hydrophilic channels to allow ions and polar molecules to travel through 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

What enables glycolipids and glycoproteins to become receptor molecules and cell markers

• Glycolipids and glycoproteins contain carbohydrate chains that exist on the surface which enables them to act as receptor molecules

  • he glycolipids and glycoproteins bind with certain substances at the cell’s surface

• Some glycolipids and glycoproteins act as cell markers or antigens, for cell-to-cell recognition

How does the interaction between cholesterol and the phospholipid tails stabilise the cell membrane at high temperatures

• 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

Outline the structure and function of cholesterol in a cell membrane

• increases the fluidity of the membrane, stopping it from becoming too rigid at lowtemperatures

• 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 increases the mechanical strength and stability of membranes (without it membranes would break down and cells burst)

How are phospholipids chemically modified to act as signalling molecules

• 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

Outline the structure and function of phospholipids in a cell membrane

• Form the basic structure of the membrane (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

What are the 4 main components of the fluid mosaic model

• Phospholipids

• Cholesterol

• Glycoproteins and glycolipids

• Transport proteins

Why does the fluid mosaic model describe cell membranes as mosaics

The scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above

Why does the fluid mosaic model describe cells as fluid

• 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 although some may be fixed in position

What does the fluid mosaic model help to explain

• this was first devised in 1972

• explains how biological molecules are arranged to form cell membranes

• Passive and active movement between cells and their surroundings

• Cell-to-cell interactions

• Cell signalling

Outline the vital roles of membranes

• Creates an enclosed space separating the internal cell environment from the external environment

• Intracellular membranes form compartments within the cell, such as organelles and vacuoles

• control the exchange of materials passing through them

• 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

Define Active Transport

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

What does active transport require

Carrier proteins and energy

Why is energy required in Active transport

The energy is required to make the carrier protein change shape, allowing it to transfer the molecules or ions across the cell membrane

How is the energy required of Active transport made

Provided by ATP during respiration

• the ATP is hydrolysed to release energy

What is active transport important in

• 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