2.3Cells

Plasma Membrane — AP BIOLOGY Revision Notes & Topical Questions

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Lesson summary

2.3 Plasma Membrane — Notes — AP BIOLOGY notes: Describe the roles of each of the components of the cell membrane in maintaining the internal environment of the cell.Membranes — they surround all living cells and form the structure of many internal organellesComponentsPhospholipi…

Thickness and bonding
Phospholipid
Cholesterol
Glycoproteins
Fluid and mosaic
Earlier model

Section 1 of 9Membrane components

Membrane components

Membranes

Membranes they surround all living cells and form the structure of many internal organelles

Components

Phospholipids.
Proteins.
Glycoproteins and glycolipids.
Steroids such as cholesterol.

Thickness and bonding

Thickness and bonding membranes are about 7–10 nm thick and held together mainly by hydrophobic interactions, not covalent bonds
This lets membranes reseal and change shape.

Section 2 of 9Phospholipid bilayer

Phospholipid

Phospholipid a polar phosphate head and two nonpolar fatty acid tails
The head is hydrophilic and interacts with water.
The tails are hydrophobic and cannot interact with water.

Bilayer

Bilayer in water the heads face the watery cytoplasm and extracellular fluid and the tails face each other in the middle,
forming the basic structure of membranes

Monolayer

Monolayer phospholipids can also form a monolayer on the surface of water

Barrier

Barrier the hydrophobic core blocks ions and polar molecules, but small nonpolar molecules such as O₂ and CO₂ and lipid-soluble substances pass freely

Fluidity

Fluidity unsaturated tails with kinks keep the membrane fluid,
whereas saturated tails pack tightly and make it more rigid, and longer tails also reduce fluidity

Cholesterol

Cholesterol a steroid lipid in animal membranes that regulates membrane fluidity
At warm temperatures it restricts movement and makes the membrane less fluid.
At low temperatures it prevents tight packing and keeps the membrane fluid.

Section 3 of 9Membrane proteins

Roles of embedded proteins

Cell recognition and cell signalling.
Membrane transport, including channels, carriers and pumps.
Membrane-bound enzymes.
Intercellular joining and attachment to the cytoskeleton or extracellular matrix.

Regions

Regions embedded proteins may be hydrophilic, hydrophobic or both
Hydrophilic regions are folded inside the protein or exposed to the aqueous cytosol.
Hydrophobic regions lie on any surface that interacts with the fatty acids in the membrane interior.

Type	Position	Examples of function
Integral (transmembrane) proteins	Span the bilayer, with hydrophobic regions in the core and hydrophilic regions at the surfaces	Channels and carriers for transport, receptors, ATP synthase
Peripheral proteins	Loosely attached to the membrane surface, often to integral proteins; hydrophilic	Enzymes, anchoring the cytoskeleton, signalling

Section 4 of 9Glycoproteins, glycolipids and asymmetry

Glycoproteins

Glycoproteins proteins with attached carbohydrate chains, and glycolipids = lipids with attached carbohydrate chains

Roles

Roles both are involved in cell recognition and cell signalling, such as ABO blood group antigens and self-recognition by the immune system

Position

Position the chains are on the outer surface only and form the glycocalyx, which also acts in cell adhesion

Asymmetry

Asymmetry the two layers differ in lipid composition and proteins are oriented in a specific way, with carbohydrate facing outward

Section 5 of 9Fluid mosaic model

Statement

Statement plasma membranes have a structural framework of phospholipids with embedded proteins, steroids, glycoproteins and glycolipids
The embedded components can move around the surface of the cell within their membrane layer.

Fluid and mosaic

Fluid and mosaic it is fluid because phospholipids and many proteins move sideways,
- and mosaic because proteins are scattered through the bilayer like tiles

Evidence

Mouse and human cells fused with differently labelled proteins showed the labels intermixing in the hybrid membrane over about 40 minutes at 37 °C,
- so proteins move laterally.
Freeze-fracture electron microscopy shows proteins embedded in the lipid layer.

Earlier model

Earlier model a lipid bilayer coated on both sides by protein layers could not explain proteins with both hydrophobic and hydrophilic regions

Section 6 of 9Membrane structure and cell function

Membrane structure and cell function

Organelle membranes

Organelle membranes they use the same bilayer but differ in protein content, for example the inner mitochondrial membrane holds electron transport proteins and ATP synthase and the thylakoid membrane holds photosystems

Receptors

Receptors they bind signalling molecules such as hormones that cannot cross the bilayer,
- and binding changes the receptor shape and starts signal transduction inside the cell

Recognition

Recognition glycoproteins are needed for tissue formation, the immune response and compatibility of blood types and transplants

Cell walls

Cell walls plants (cellulose), fungi (chitin) and prokaryotes (peptidoglycan) have a wall outside the membrane that gives structural support

Section 7 of 9More from the specification

Section 8 of 9Key terms

Term	Meaning
Plasma membrane	Selectively permeable boundary of the cell.
Fluid mosaic model	Phospholipids and embedded proteins that move laterally.
Integral protein	Protein that spans the membrane, often a channel or carrier.
Peripheral protein	Protein loosely attached to the membrane surface.
Cholesterol	Steroid that regulates membrane fluidity in animal cells.
Glycoprotein	Protein with carbohydrate chains used in cell recognition.
Amphipathic	Having both hydrophilic and hydrophobic regions.

Section 9 of 9Exam technique

Describing the membrane as a single layer — it is a bilayer.

Saying proteins form a layer on the outside, as in the old sandwich model.

Thinking the membrane is static — phospholipids and many proteins move.

Confusing glycoproteins (protein with carbohydrate) with glycolipids (lipid with carbohydrate).

Last updated: 18 October 2026