2.1.5 Biological Membranes

What are the roles of membranes

• Separates the components of the cell from their environments

• Separates the different organelles within cells from each other and the cytosol

• Acts as the interface for communication

• Uses diffusion, osmosis and active transport to move substances across it

What is the structure of phospholipids

• Only 2 fatty acids (the third has been replaced by a phosphate ion)

• Contains a phosphate group, glycerol and 2 fatty acid tails

What is the structure of the plasma membrane

• Formed by a phospholipid bilayer

  • The hydrophilic phosphate heads of the phospholipids form both the inner and outer layer of the membrane

  • The hydrophobic fatty acid tails form the hydrophilic centre

• Fluid - phospholipids and proteins are free to move within the layer (mostly sideways) via diffusion

• Mosaic - The proteins embedded in the layer vary in shape, size and position (in the very same as the tiles of the mosaic

What is a micelle

• If a phospholipid is mixed/shake with water they form spheres called a micelle

• Hydrophilic head faces outwards

• Hydrophobic fatty acid tails face inwards

Name the parts of the phospholipid bi-layer protein

• A = Glycolipid

• B = Extrinsic/peripheral protein

• C = Glycoprotein

• D = Carbohydrate part of the glycoprotein

• E = Phospholipid

• F = Cholesterol

• G = Transport/Intrinsic/Integral Protein

What is cholesterols role in the phospholipid bi layer

• Regulates the fluidity of the membrane by being between the phospholipids

• They prevent phospholipids from packing to closely together and crystallising

• Also allows membrane to be impermeable to ions, to increase strength & stability

What is the structure of cholesterol

Lipid with a hydrophilic end and a hydrophobic end

What makes membranes less fluid

• More saturated fatty acid chains, pack together tightly, high number of intermolecular forces between the chains

  • This is due to the double bonds of the saturated fatty acid chains having a double bond, meaning there’s less space and therefore less movement

• Lower temperatures, molecules have less energy, not moving as freely, structure becomes more closely packed

What makes membranes more fluid

• More unsaturated fatty acid chains, pack tether less tightly, less molecular forces between the chains

• Higher temperatures, molecules have more energy, move freely, structure becomes more fluid

What are the roles of glycolipds and glycoproteins in the membrane

• Can act as receptor molecules for hormones and neurotransmitters

• Can act as receptor molecules for signalling

• Can act as antigens

• Can be used for recognition/identification of cells

• Can act as receptor/binding site on transport proteins

• Can be used for cell adhesion

• Attaches to water to stabilise the cell

What is the structure of a glycoprotein

• Intrinsic proteins embedded in the cell surface membrane

• Carbohydrate chains of varying lengths and shapes are attached

What is the structure of a glycolipid

Lipids with attached carbohydrate chains

What are the roles of intrinsic proteins

• Channel (pore) proteins: Create hydrophilic channels which allow ions and polar molecules to travel through membranes

• Carrier proteins - the protein changes shape to allow this to happen

• Each transport protein is specific to what it is transporting

• The more intrinsic proteins inside a membrane, the faster the rate of diffusion

• They are embedded through both layers of a membrane

How do membranes act as a site of chemical reactions

• The enzymes of photosynthesis are found on membrane stacks within the chloroplasts

• The proteins must be in particular positions for this to work

• For example, the electron carriers and the enzyme ATP must be in the correct positions within the cristae for the production of ATP in respiration

What is the purpose of a centriole

They help to move chromosomes during cell division

What is diffusion

The net movement of a substance from a region of its higher concentration to a region of its lower concentration

Why does diffusion happen

• Molecules move down a concentration gradient caused by natural kinetic energy

• Eventually particles become evenly spread due to random movement of particles

What are the factors affecting the rate of diffusion

• Steepness of the concentration gradient

• Temperature

• Surface area

• Properties of molecules/ions (size, charge)

• The presence of carrier/channel proteins

How do the properties of molecules or ions affect the rate of diffusion

• Large molecules diffuse slower than smaller ones as they require more energy to move

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

• Non-polar molecules diffuse quicker then polar molecules as they are soluble in the non-polar phospholipid bi-layer

What is facilitated diffusion

• Diffusion that uses Chanel and carrier proteins to assist certain substances that can’t use standard diffusion

• These include

  • large polar molecules like glucose and amino acids

  • ions like sodium and chloride

What is osmosis

The et movement of water molecules from a region of high water potential to a region of low water potential

How does osmosis of water into a plant cell work

• Plants placed in a dilute solution - water enters the plant cells cytoplasm & vacuole by osmosis - volume of cell increases

• Plant cells have protoplasts (living portion of a plant cell)

  • These push up against the cell wall Which increases pressure which makes the cell become turgid

• Turgidity provides support and strength for a plant - e.g. allows them to stand up straight

How does osmosis of water out of a plant cell work

• Plant cell placed in a concentrated solution

• Water leaves the plant cell’s cytoplasm and vacuole by osmosis - volume of cell decreases

• Protoplast shrinks and pulls away from cell wall - decreases pressure - cell eventually becomes plasmolysed

How does having no cell wall affect animals cells during osmosis

They feel the effects more severely

What is a hypertonic solution

• The outside solution is more concentrated with solutes

• The net movement of water inwards

• Cells swell - and may burst

What is a hypotonic solution

• The outside solution is more diluted with water

• Net movement of water out of cell

• The cell shrivels

What is an isotonic solution

• Outside solution and cytoplasm a similar solute concentration

• No movement

What is 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

How does active transport work

• The process required specific carrier proteins

• The energy is required to make the protein change shape

  • So that molecules/ions can transfer across the membrane

It is used in:

• Réabsorption in the kidney tubules

• Absorption in digestion

• Loading sugar and inorganic ions into phloem and root hair cells

What is co-transport

The coupled movement of substances across a cell membrane via a carrier protein (facilitated diffusion and active transport combined)

E.g.

• NA & glucose ions are transported into epithelial cells via diffusion (Facilitated)

• NA is then actively transported out of the cell, into the blood (which helps maintain a conc. gradient for sodium)

• Glucose exits the cell and enters the blood again via facilitated diffusion

What organelles are adapted for faster transport

• Neurone & muscle cells - Cell membrane read have specific channel proteins for sodium, potassium and calcium ions

• Kidney cells - Contain very high number of Aquaporins

  • Aquaporins = specific channel proteins that allow facilitated diffusion