Membranes [structure + functions]

What is the structure of a phospholipid?

• Hydrophilic charged phosphate head → Oriented outwards to interact with the aqueous medium

• 2 non polar hydrophobic hydrocarbon tails → Oriented inwards away from the aqueous medium → Hydrophobic core

*Amphipahic

What are the interactions that hold phospholipids in the membrane together?

• Weak hydrophobic interactions between the non polar hydrophobic hydrocarbon chains

• Weak interactions that allow phospholipids to move laterally within the membrane plane → Fluidity

What does membrane fluidity depend on?

• Length of hydrocarbon chains

  • Longer chains → More hydrophobic interactions between phospholipids → Phospholipids move less → Fluidity decreases

• Degree of saturation of hydrocarbon chains

  • Higher degree of saturation → Phospholipids can pack closely → Fluidity decreases

What is the structure of cholesterol?

• Polar hydrophilic hydroxyl group → Interacts with phosphate heads of phospholipids

• Non-polar hydrophobic fused four-ringed structure → Interacts with hydrocarbon chains of phospholipids

What is the function of cholesterol?

Regulates membrane fluidity

• Warmer temperatures → Restricts phospholipid movement → Prevents membranes from becoming overly fluid

• Colder temperatures → Prevents the close packing of phospholipids → Prevents solidification of membrane

What kinds of membrane proteins are there?

• Integral / intrinsic proteins

  • Unilateral proteins → Reach partway across the membrane

  • Transmembrane proteins → Span across the membrane thickness

• Peripheral / Extrinsic proteins

What are the structure and properties of integral proteins?

• Hydrophilic regions → Mostly amino acids with polar or charged (acidic/basic) R groups → Exposed to the aqueous medium on both or either side of the membrane

• Hydrophobic regions → Mostly amino acids with non-polar R groups → Lie in the hydrophobic core of the bilayer

How are peripheral membranes attached to the membrane?

Loosely attached to the surface of the membrane or to integral proteins through weak ionic and hydrogen bonds

How are membrane proteins attached to the membrane?

• May be attached to the fibers of the extracellular matrix (substance where animal tissue cells are embedded) on the exterior side of the plasma membrane

• May be held by filaments of the cytoskeleton on the cytoplasmic side of the plasma membrane

• Attachment helps maintain cell shape and fix locations for some proteins

What is the cytoskeleton?

• Network of microtubules, microfilaments and intermediate filaments branching throughout the cytoplasm

• Serves a variety of mechanical and transport functions in the cell

How are glycoproteins and glycolipids formed?

Carbohydrates tend to be covalently bonded to either lipids or proteins

How are the carbohydrates of glycoproteins and glycolipids found?

• Projecting out of the cell into the extracellular matrix

• Varies from species to species, individuals of the same species and different cell types

What are the functions of glycoproteins and glycolipids?

• Cell-cell recognition

  • Diversity → Good cell markers differentiating one cell type from another

• Cell receptors

  • Receptors for hormones in cell signaling

What is cell-cell recognition?

The ability of a cell to determine if other cells it encounters are alike or different from itself

Basis for

• Sorting out an animal’s embryo cells into tissues and organs

• Rejection of foreign cells by the immune system

How do membrane proteins get embedded in the plasma membrane?

1. Glycosylation adds carbohydrates to proteins in the endoplasmic reticulum, making them glycoproteins

2. Glycoproteins undergo further carbohydrate modification inside the Golgi apparatus + Lipids acquire carbohydrates to form glycolipids

3. Glycoproteins and glycolipids are transported via vesicles to the plasma membrane

4. Vesicles fuse with the membrane and the glycoproteins and glycolipids are presented outside the membrane

*Carrier and channel proteins are embedded the same way

Why is the fluid-mosaic model used to describe membranes?

Fluid → Phospholipids and proteins that are free to move laterally within a layer

Mosaic pattern → Random arrangement of embedded proteins amongst the phospholipid molecules

What are the functions of membranes? (6)

1. Regulate the movement of substances across the membrane

2. Compartmentalisation

3. Localization of proteins of a related function across a membrane

4. Increase the surface area for chemical reactions

5. Cell-cell recognition and adhesion

6. Signal transduction

How do membranes regulate the movement of substances across themselves?

• Selectively permeable

• Non polar, uncharged molecules → Can diffuse through the hydrophobic core

• Polar, charged molecules → Repelled by the hydrophobic core → Requires transport proteins

  • Ions, amino acids, organic acids, simple sugars

How do membranes carry out compartmentalization and what are the applications?

• With the ability to regulate the movement of substances across the membrane

• Formation of unique environment with optimum conditions for highly specialized activities

• Accumulation of charged ions and formation of chemical gradients across membranes

• Storage of food source

Why do membranes localize proteins of a related function along a membrane?

Functionally-related proteins can be grouped together so that sequential biochemical processes are facilitated

How may membranes increase the surface area of chemical reactions?

Highly folded → cristae of mitochondria

How do membranes carry out cell-cell recognition and adhesion?

• With glycoproteins and glycolipids

• Glycoproteins and glycolipids are also reponsible for unique surface topography on cell surfaces

  • Membrane proteins of adjacent cells may hook together → Various junctions like tight or gap junctions

  • Drugs, chemical messengers and viruses dock onto such molecules

How do membranes carry out signal transduction?

• Some transmembrane proteins = cell surface receptors

• Receptors transfer information from the environment into the cell when specific molecules bind to them