Cell Membranes

Carbohydrates

• Monosaccharides

• Disaccharides (oligosaccharides)

• Glycogen polysaccharides)

Monosaccharides

• Monosaccharides are hydroxylated aldehydes and ketones

• Building blocks of other sugars

• Often exist in ring closed form

Glucose and galactose are what?

epimers

Disaccharides (oligosaccharides)

Formed by linking monosaccharides together with a glycosidic bond

Glycogen (polysaccharides)

• Glycogen can contain up to 100,000 glucose molecules

• It is highly branched = α-1,4 and α-1,6 glycosidic bonds

How does the body use sugars?

• Glycogen exists in the body as a reservoir of available energy that is stored in the chemical bonds within individual glucose monomers

• The hydrolysis of glycogen (which occurs during times like fasting) leads to the releasing of glucose monomers into the blood

How do cells use sugars?

• Extracellular matrix: Sugars help form a scaffold for cell attachment and help transmit signals that control cell growth, differentiation, and migration.

• Glycosylation: Sugars are added to proteins and lipids to form glycoproteins and glycolipids, which are important for cell recognition and signalling.

What are lipids?

They are molecules composed mainly of H and C atoms linked by non polar covalent bonds.

• Structures are very diverse

What are the roles of lipids in the body?

• Lipids are a major component of cell membranes

• They provide a valuable source of energy

• They are important signalling molecule

Name the 4 types of lipids:

• Steroid

• Fatty acid

• Triglyceride

• Phospholipid

Steroids

• What’s their structure?

• Why do different steroids arise?

• What are some examples of steroids?

• Steroids have a basic 4 ring structure with side chains

• Different steroids arise due to the functions of the rings and changing side chains

• Examples: cholesterol, oestrogen, testosterone

Fatty acids

• What’s their structure?

Fatty acids are carboxylic acids with long alkyl side chains (lipophilic). Chain lengths can vary, but 14, 16, and 18 carbons are common

Triglycerides

• What’s the importance of triglycerides?

• Where are triglycerides held?

• How are triglycerides formed?

• What mechanisms interconverts triglycerides?

• Triglycerides (fats) are important energy stores

• Held in adipose tissues

• Glycerol (alcohol) is esterified with three carboxylic (fatty) acids, which can be the same or different

• Interconverted by hydration/dehydration

Phospholipids

• Are phospholipids amphipathic?

• What does ‘n’ mean?

• Phospholipids are amphipathic (has both hydrophilic and hydrophobic parts)

• n = number of repeating CH₂ groups

What’s the structure of a cell membrane?

• Cell membrane is a lipid bilayer

• Polar head groups face water

• Lipophilic side chains face each other

• Cell membranes contains cholesterol and proteins (important for function)

• Fluid structure

What’s the point of membranes?

They create boundaries between cells and the environment. They are a selective barrier to the passage of molecules.

Name the roles of the cell membranes?

• Detects chemical signals from other cells

• Anchors cells to adjacent cells and to the extracellular matrix

What is the selective permeability of membranes?

Well, diffusion rates depend on ionic charge and size.

Membranes favour small, uncharged molecules, making diffusion selective.

Why does diffusion depend on ionic charge?

Charged particles (ions) cannot easily pass through the hydrophobic lipid bilayer because they are attracted to water and repelled by the non-polar membrane core. They usually require channels or transporters.

Why does diffusion depend on size?

Smaller molecules diffuse more easily than larger ones because they can pass between lipid molecules more readily, while large molecules are physically restricted.

Definition of Diffusion

Movement influenced by electrochemical gradient

Definition of Osmosis

Water molecules diffuse down concentration gradient until equilibrium is reached

What are protein transporters / carriers?

Membrane proteins can perform both passive and active transport of selected molecules

What is endocytosis / exocytosis?

Vesicular bulk transport into/out of the cell, often receptor mediated

Passive diffusion

Passive diffusion occurs down a concentration gradient, and net flux takes into account diffusion in both directions

Name the protein transport/carriers mechanisms:

1. Simple diffusion

2. Ion channels & porins, transport carriers (passive transport/facilitated diffusion)

3. Pumps (active transport)

Protein transporters often undergo a conformational change.

Sodium/Potassium pump

• is this, simple diffusion, facilitated diffusion, or active transport?

• How does it work?

• Active transport

• Powered by ATP, the pump moves sodium and potassium ions in opposite directions, each against its concentration gradient