B2.1 Membranes and membrane transport

Structure of phospholipids

• Hydrophobic tails made of a glycerol and a phosphate molecule

• Hydrophilic tails made of fatty acid chains

Molecular size affects

Membrane permeability

Diffusion

Net movement of particles from a higher to a lower concentration

When can simple diffusion only happen

If phospholipid bilayer is permeable to particles

State and define the two membrane proteins

• Integral proteins: Penetrate the phospholipid bilayer to remain permanently attached to the membrane

• Peripheral proteins: Only appear on one side of protein

Examples of integral proteins

• Glycoproteins

• Ion channels

• Carrier proteins

• Protein pumps

Examples of peripheral proteins

• G proteins

• Any other protein that are receptor complexes involved in cell signalling

Membrane protein functions (JETRAT)

• Junctions – Connect + join two cells together 

• Enzymes – Fixing to membranes localises metabolic pathways

• Transport – Responsible for facilitated diffusion + active transport 

• Recognition – Markers for cellular identification

• Anchorage – Attachment points for cytoskeleton and extracellular matrix

• Transduction – Function as receptors for peptide hormones 

Aquaporins

Water channels in some cells

Channel proteins

Transmembrane proteins that connects the cytoplasm to the aqueous solution inside the cell

3 different ways of pump protein in membranes

• Using energy (active transport)

• Only move particles across membrane (one direction)

  • Usually moves particles against concentration gradient

Semi-permeable membrane

Allows passage of certain small solutes and is freely permeable to the solvent

Glycoproteins

Conjugated proteins with carbohydrates as the non-polypeptide component

Formation of carbohydrate

Single monosaccharide between 2 and 4 sugar units

Functions of carbohydrate chains

• Cell recognition

• Attachement points for other cells

Glycocalyx

Carbohydrate-rich layer on the outer face of the plasma membrane of an animal cells

Extracellular matrix

Network for external molecules that provide structure and biochemical support to surrounding cells

Draw the fluid mosaic model and include:

peripheral and integral proteins, glycoproteins, phospholipids and cholesterol

(draw and label)

Outline the factor of what the fluidity of a membrane is affected by

The composition of fatty acids within the phospholipid bilayer

Distinguish between unsaturated and saturated fats

Unsaturated	Saturated
high viscosity	low viscosity
kinky hydrocarbon tail	straight hydrocarbon tail
low melting point	high melting point
has double bonds	has no double bonds
packed together	loosely packed together
high fluidity	low fluidity

Example(s) of adaptations in membrane composition in relation to habitat

• Increasing USFAs at lower temperatures

• Increasing SFAs at higher temperatures

Cholesterol

An amphipathic sterol in charge of membrane fluidity

Explain the role of cholesterol in the plasma membrane

• At high temps, it stabilises membrane, increases MPs, decreases fluidity

• At low temps, it inserts itself between P.L.B.L and prevents stiffness and crystallisation, increases fluidity

Vesicle

Small sac of membrane with a droplet of fluid inside

Facilitated diffusion

Specific ions passing across a membrane in either direction

Endocytosis

Large substances enter a cell without passing through the membrane

• Invagination of membrane envelopes the molecule

• Invagination is sealed off to form an intracellular vesicle

Phagocytosis

Solid substances are ingested

Pinocytosis

Liquids/dissolved substances are ingested

Exocytosis

Large substances leave the cell without crossing the membrane

• Vesicles fuse with the plasma membrane

• This process adds vesicular phospholipids to cell membrane

Ion channels

Integral membrane protein that has a hydrophilic inner pore which ions pass

Acetylcholine

Neurotransmitter released from the nerve cells to stimulate adjacent cells

Binding causes a:

Conformational change

Binding of acetylcholine is

Reversible

Voltage-gated ion channels + example

Cycle between an open and closed conformation according to the transmembrane voltage

• E.g., Sodium-potassium channels

• Sodium channels: IN to the neuron (depolarisation)

• Potassium channels: OUT of the neuron (repolarisation)

Ligand-gated channels

Change their conformation in response to the binding of a specific chemical (ligand)

• Muscles contain nicotinic acetylcholine receptors that will trigger the opening of an ion channel when activated

Draw and distiguish voltage-gated and ligand-gated ion channels

(draw and label)

VOLTAGE

• One (+ -) side, one (+ +) side

LIGAND

• No + or - sides

Antitransporters

Move two molecules in opposite directions across the membrane 

Glucose molecule

(draw and label)

Cotransporters

Link the movement of an ion along its concentration gradient to the movement of a solute against its concentration gradient

Example of cotransport

The absorption of glucose in the kidneys and small intestine (contransported with sodium ions)

CAMs

Cell-adhesion molecules

Metastasis

Movement of spread of cancer cells

Outlien 4 processes with examples that allow substances to pass through the plasma membrane

1. Osmosis Diffusion of water from an area of high water potential

2. Simple diffusion Diffusion from a high concentration to low concentration (e.g. steroid hormones)

3. Exocytosis Diffusion of vesicles with membranes to release a large molecule (e.g. nuerotransmitters)

4. Endocyosis The infolding of membranes to form a vesicle and take in a large molecule; (e.g. macrophages engulfing pathogens)

What is happening in I and II?

I: Simple diffusion

II: Facilitated diffusion

Locations of lipid bilayers in cells.

Nucleus, mitochondria and chloroplasts

Function of cell membrane

Acts as a barrier and has ability to control what substances are able to move in and out of it

Anchoring junctions

Holds cells together to strengthen contact within tissues

Tight (occluding) junctions

Create tight seals that result in an impermeable barrier to diffusion

Gap junctions

Links cells together to allow the movement of material between them 

CAMs are typically..

proteins with phospholipid bilayer domains