Unit 1.3: Cell
Membranes and
Transport
Part 1: Membrane Structure and
Passive Transport
1. The cell surface membrane
• Also called the plasma membrane. Forms the boundary between the living cell and
its surroundings. About 7 to 8 nm thick.
• Functions: controls what enters and leaves, lets nutrients in and waste out,
maintains internal conditions, allows secretion, holds receptor proteins, allows cell
recognition.
Key definition
The membrane is selectively permeable: some substances cross easily, others are blocked or need
a transport protein.
2. The fluid mosaic model
• Proposed by Singer and Nicolson in 1972. Describes how phospholipids, proteins,
cholesterol and carbohydrate chains are arranged in the membrane.
Word Why it applies
Fluid Phospholipids can move sideways within the bilayer, so the membrane is
flexible and not fixed
, Mosaic Proteins are scattered irregularly through the bilayer, creating a
patchwork pattern
3. Phospholipid bilayer
• The basic structure is a double layer of phospholipids. Each has one phosphate
group and two fatty acid chains.
Part Nature Faces
Phosphate heads Polar, hydrophilic Outwards, towards water on both sides (tissue
fluid/external environment and cytoplasm)
Fatty acid tails Non-polar, hydrophobic Inwards, towards each other, forming the
hydrophobic core
The hydrophobic core lets small non-polar molecules (e.g. O2, CO2) cross directly, but
blocks ions and large polar molecules, making the membrane a selective barrier.
4. Membrane proteins
Type Position Roles
Intrinsic Embedded in the bilayer, some Channel proteins, carrier proteins, pumps,
spanning the whole membrane receptors. Hydrophobic regions touch the
fatty acid tails; hydrophilic regions touch
water/solutes
Extrinsic On the inner or outer surface only, do Structural support, attach to the
not span the bilayer cytoskeleton, act as receptors, help cell
recognition
Channel proteins vs carrier proteins
Feature Channel proteins Carrier proteins
Structure Form hydrophilic pores across the Have specific binding sites
membrane
How they work Substances pass through the pore; the The protein changes shape to move the
protein does not usually change shape substance across, then releases it
Used in Facilitated diffusion only Facilitated diffusion, active transport and
co-transport
Typically Ions (e.g. Na+, K+, Cl-) Larger molecules or ions (e.g. glucose)
transport
Gating Some are gated: open/close in response Not gated in the same way
to a signal
Membranes and
Transport
Part 1: Membrane Structure and
Passive Transport
1. The cell surface membrane
• Also called the plasma membrane. Forms the boundary between the living cell and
its surroundings. About 7 to 8 nm thick.
• Functions: controls what enters and leaves, lets nutrients in and waste out,
maintains internal conditions, allows secretion, holds receptor proteins, allows cell
recognition.
Key definition
The membrane is selectively permeable: some substances cross easily, others are blocked or need
a transport protein.
2. The fluid mosaic model
• Proposed by Singer and Nicolson in 1972. Describes how phospholipids, proteins,
cholesterol and carbohydrate chains are arranged in the membrane.
Word Why it applies
Fluid Phospholipids can move sideways within the bilayer, so the membrane is
flexible and not fixed
, Mosaic Proteins are scattered irregularly through the bilayer, creating a
patchwork pattern
3. Phospholipid bilayer
• The basic structure is a double layer of phospholipids. Each has one phosphate
group and two fatty acid chains.
Part Nature Faces
Phosphate heads Polar, hydrophilic Outwards, towards water on both sides (tissue
fluid/external environment and cytoplasm)
Fatty acid tails Non-polar, hydrophobic Inwards, towards each other, forming the
hydrophobic core
The hydrophobic core lets small non-polar molecules (e.g. O2, CO2) cross directly, but
blocks ions and large polar molecules, making the membrane a selective barrier.
4. Membrane proteins
Type Position Roles
Intrinsic Embedded in the bilayer, some Channel proteins, carrier proteins, pumps,
spanning the whole membrane receptors. Hydrophobic regions touch the
fatty acid tails; hydrophilic regions touch
water/solutes
Extrinsic On the inner or outer surface only, do Structural support, attach to the
not span the bilayer cytoskeleton, act as receptors, help cell
recognition
Channel proteins vs carrier proteins
Feature Channel proteins Carrier proteins
Structure Form hydrophilic pores across the Have specific binding sites
membrane
How they work Substances pass through the pore; the The protein changes shape to move the
protein does not usually change shape substance across, then releases it
Used in Facilitated diffusion only Facilitated diffusion, active transport and
co-transport
Typically Ions (e.g. Na+, K+, Cl-) Larger molecules or ions (e.g. glucose)
transport
Gating Some are gated: open/close in response Not gated in the same way
to a signal
