Shannon Rufus

1. Phospholipids#

The main structural component, arranged in a bilayer:

• Hydrophilic (water-loving) head: Phosphate group + glycerol
• Hydrophobic (water-fearing) tails: Two fatty acid chains

1
      Hydrophilic head (attracted to water)
2
              ┌───┐
3
              │ P │
4
              └─┬─┘
5
                │
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      ┌─────────┴─────────┐
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      │    Hydrophilic    │
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      │      region       │
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      ├─────────┬─────────┤
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      │         │         │
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  Fatty      Fatty    Fatty      Fatty
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  acid       acid     acid       acid
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  tail       tail     tail       tail
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   (hydrophobic)

This arrangement means:

• Hydrophilic heads face outward (towards aqueous environment)
• Hydrophobic tails face inward (away from water)
• Creates a stable barrier in aqueous environments

2. Proteins#

Integral proteins (embedded):

• Channel proteins - Form pores for specific molecules
• Carrier proteins - Change shape to transport substances
• Glycoproteins - Protein + carbohydrate (cell recognition)

Peripheral proteins (surface):

• Attached to membrane surface (often to integral proteins)
• Involved in signalling and structural support

3. Cholesterol#

• Fits between phospholipid tails
• Regulates membrane fluidity:
  • At low temperatures: Prevents packing, maintains fluidity
  • At high temperatures: Restricts movement, provides stability

4. Glycolipids#

• Lipid + carbohydrate chain
• Found on outer surface only
• Important for cell recognition and immunity

Simple Diffusion#

Movement of molecules down a concentration gradient (high → low concentration).

Characteristics:

• No energy required (ATP)
• Small, non-polar molecules only
• Examples: Oxygen, carbon dioxide, glycerol

1
High concentration ───→ Low concentration
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    [●●●●]           [●]
3
      ║                ╲
4
      ╲                 Diffusion
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       ╲                  ╲
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        ▼                  ▼
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    [●●●]              [●●●]

Fick’s Law of Diffusion:

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Rate ∝ (Surface area × Concentration gradient) ÷ Thickness of membrane

Factors affecting diffusion rate:

• Surface area - Larger area = faster diffusion
• Concentration gradient - Steeper gradient = faster diffusion
• Thickness - Thinner membrane = faster diffusion
• Temperature - Higher temp = faster particle movement

Facilitated Diffusion#

For molecules that cannot pass through the phospholipid bilayer:

Channel proteins:

• Form water-filled pores
• Specific to certain ions or molecules
• Gated (open/close in response to signals)
• Examples: Ion channels in neurons

Carrier proteins:

• Bind to specific molecule
• Change shape to transport across
• Example: Glucose transport in red blood cells

1
Channel protein:          Carrier protein:
2
┌────────┐               ┌────────┐
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│  ││││  │               │   ⊕    │  →  ┌────────┐
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└────────┘               └────────┘           │   ⊕    │
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   Pore                                     └────────┘

NOTE

Facilitated diffusion is still passive - no energy required, moves down concentration gradient!

Osmosis#

Special case of diffusion - water moves from high water potential to low water potential across a partially permeable membrane.

Water potential (Ψ) = Pure water at standard conditions = 0

Adding solutes decreases water potential (makes it more negative):

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Solution type        Ψ value    Water movement
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Pure water           0          -
3
Dilute solution      -500       → More concentrated
4
Concentrated         -2000      ← From less concentrated

Key terms:

Effects on cells:

• Animal cells in hypotonic: Burst (lysis)
• Animal cells in hypertonic: Shrink (crenation)
• Plant cells in hypotonic: Turgid (rigid, supports plant)
• Plant cells in hypertonic: Plasmolysed (membrane pulls away from wall)

IMPORTANT

Water potential = Pressure potential (Ψp) + Solute potential (Ψs). Pressure is positive in turgid cells, solute potential is always negative!

Primary Active Transport#

Direct use of ATP to pump substances across membrane.

Sodium-Potassium Pump (crucial for nerve impulses):

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1. 3 Na⁺ bind inside           2. K⁺ bind outside
2
   ╱                          ╱
3
  │   ┌─────┐                 │   ┌─────┐
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  │   │ Na⁺ │                 │   │ K⁺  │
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  └───┤  +  │  ATP → ADP + Pi └───┤  +  │
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      └─────┘  ────────→         └─────┘
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        ╲  ╲                      ╲  ╲
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         ╲  ╲ 2. Pump changes shape,  ╲  ╲ 4. Pump changes
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          ╲  ╲ releases Na⁺ outside    ╲  ╲ shape, releases K⁺
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           ╲  ╲                        ╲  ╲inside
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            ╲  ▼                        ╲  ▼

• Pumps 3 Na⁺ out for every 2 K⁺ in
• Maintains electrochemical gradient
• Uses ~1/3 of body’s ATP at rest!

Secondary Active Transport#

Uses energy from electrochemical gradient created by primary active transport.

Cotransport:

• Symport: Both substances move same direction
• Antiport: Substances move opposite directions

Example: Glucose absorption in small intestine:

• Na⁺ gradient (created by Na⁺/K⁺ pump) drives glucose uptake
• Na⁺ moves down gradient, glucose moves against gradient

Exocytosis#

• Secretion of substances from cell
• Vesicle fuses with plasma membrane
• Contents released outside
• Examples: Neurotransmitters, hormones, enzymes

1
Inside cell                   Outside
2
    ┌─────┐   Fusion         ●●●●
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    │ ●●● │  ─────────────→  ●●●●
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    │ ●●● │                  ●●●●
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    └─────┘
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   Vesicle

Endocytosis#

• Uptake of substances into cell
• Membrane engulfs material, forming vesicle

Types: