4.2.44 · Cell Membranes and Transport
Endocytosis and Exocytosis — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
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Lesson summary
4.2.4 Endocytosis and Exocytosis — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe bulk transport by endocytosis and exocytosis and explain why these processes need energy.
- Why it is needed
- Phagocytosis
- Pinocytosis
- Receptor-mediated endocytosis
- Vesicle movement
- Membrane shape changes
Section 1 of 10Bulk transport
Why it is needed
- Why it is needed proteins, polysaccharides, parts of cells, whole bacteria and large volumes of fluid are too large to cross through channels or carriers.
Vesicles
- Vesicles the material is moved in membrane-bound vesicles, so it does not pass through the membrane itself.
Direction
- Direction bulk transport into cells is endocytosis and bulk transport out of cells is exocytosis.
Energy
- Energy both need ATP, so they are forms of active transport, and both rely on the fluidity of the bilayer and on the cytoskeleton.
Section 2 of 10Endocytosis
1Endocytosis
Endocytosis bulk movement of material into a cell,
- when the cell surface membrane folds inwards, surrounds the material and pinches off as a vesicle.
2Phagocytosis
("cell eating") — solid particles such as bacteria are engulfed.
- The membrane extends around the particle with pseudopodia to form a phagocytic vesicle.
- In neutrophils and macrophages, lysosomes fuse with the vesicle and release hydrolytic enzymes that digest the contents.
- Amoeba feed in the same way.
3Pinocytosis
("cell drinking") — small droplets of liquid with dissolved substances are taken in by small vesicles.
4Receptor-mediated endocytosis
- Receptor-mediated endocytosis specific molecules bind to receptors in a pit on the membrane and are taken in efficiently.
- Low-density lipoprotein, which carries cholesterol, enters cells this way.
Section 3 of 10Exocytosis
1Exocytosis
Exocytosis bulk movement of material out of a cell, the reverse of endocytosis.
2Process
- Process vesicles carrying the material, often budded from the Golgi apparatus, move along cytoskeleton tracks to the cell surface membrane and fuse with it.
- The contents are released outside and the vesicle membrane becomes part of the surface membrane.
3Examples
- Examples digestive enzymes from pancreatic cells, neurotransmitter at a synapse after calcium ions enter the presynaptic knob, hormones such as insulin, collagen,
- and mucus from goblet cells.
4Plants
- Plants exocytosis delivers cell wall materials to the outside of the cell surface membrane.
Section 4 of 10Pathway of a secreted protein
1
The protein is made on ribosomes bound to the rough endoplasmic reticulum and enters its cisternae.
2
Transport vesicles bud off and carry the protein to the Golgi apparatus.
3
The Golgi modifies the protein, for example by adding carbohydrate, and packages it in secretory vesicles.
4
Secretory vesicles move to the cell surface membrane and fuse with it, releasing the protein by exocytosis.
Section 5 of 10Why energy is needed
Vesicle movement
- ATP powers motor proteins that move vesicles along the cytoskeleton.
Membrane shape changes
- ATP is used to form vesicles.
Membrane fusion
- ATP is used for fusion, which is possible because the fluid mosaic membrane lets phospholipids rearrange.
Evidence
- Evidence both processes slow or stop when respiration is inhibited.
Membrane balance
- Membrane balance membrane added to the surface by exocytosis is balanced by membrane removed in endocytosis, so surface area stays roughly constant.
Section 6 of 10Comparing the transport processes
| Process | Substances | Energy | Protein or vesicle |
|---|---|---|---|
| Simple diffusion | Small, non-polar | No ATP | None |
| Facilitated diffusion | Polar molecules, ions | No ATP | Channel or carrier |
| Osmosis | Water | No ATP | Bilayer or aquaporin |
| Active transport | Ions, small molecules | ATP | Carrier |
| Endocytosis and exocytosis | Large particles, bulk fluid | ATP | Vesicle |
1Cells specialised for secretion
- Cells specialised for secretion they have extensive rough endoplasmic reticulum, a large Golgi apparatus, many mitochondria and numerous vesicles near the surface.
2Phagocytes
- Phagocytes they have numerous lysosomes and an irregular, extended surface.
Section 7 of 10More from the specification
1Endocytosis mechanisms
- Cells ingest extracellular macromolecules through invagination of the plasma membrane,
- forming endocytic vesicles via phagocytosis or pinocytosis.
2Exocytosis and vesicular secretion
- Intracellular transport vesicles from the Golgi body fuse with the plasma membrane, releasing secretory proteins or neurotransmitters.
3Investigating plant tissue water potential
- Immersing potato cylinders in graduated sucrose solutions identifies the external solute concentration where no net water movement occurs.
4Osmosis experiments with potato cylinders
- Osmosis experiments with potato cylinders investigating water potential in potato tissue involves measuring mass changes when cylinders are placed in graded sucrose solutions.
5Selectively permeable membrane transport
- Selectively permeable membrane transport selectively permeable plasma membranes regulate solute exchange, permitting lipid-soluble molecules to cross while restricting large polar substances.
6Bulk vesicle transport mechanisms
- Bulk vesicle transport mechanisms endocytosis and exocytosis transport large macromolecules across membranes using energy supplied by cellular respiration without concentration limitations.
7Permeable
- Allowing substances to pass through.
8Sucrose
- The sugar mainly transported in phloem.
9Indicator
- A chemical that changes colour depending on pH.
10Water potential
- A measure of the tendency of water to move; pure water has the highest.
11Selectively permeable
- Allowing only some substances through.
12Dilute solution
- Solution with a high water concentration and low solute concentration.
13Animal cell
- A cell with a nucleus, cytoplasm, cell membrane and mitochondria but no cell wall, chloroplasts or large vacuole.
14Concentration gradient
- The difference in concentration of a substance between two regions.
Section 8 of 10More on this topic
- Very large particles or whole cells move by endocytosis, and secreted proteins leave by exocytosis.
| Feature | Process |
|---|---|
| ATP used | Active transport or bulk transport |
| Movement against the concentration gradient | Active transport |
| Protein used but no ATP | Facilitated diffusion |
| Water crossing a selectively permeable membrane | Osmosis |
| Very large particles or whole cells such as bacteria | Endocytosis |
Section 9 of 10Key terms
| Term | Meaning |
|---|---|
| Endocytosis | Bulk uptake of material into a cell by infolding of the membrane to form a vesicle. |
| Exocytosis | Bulk release of material from a cell when a vesicle fuses with the surface membrane. |
| Phagocytosis | Endocytosis of solid particles. |
| Pinocytosis | Endocytosis of liquid droplets. |
| Vesicle | Small membrane-bound sac used for transport inside cells. |
| Lysosome | Organelle containing hydrolytic enzymes. |
| Golgi apparatus | Organelle that modifies and packages proteins into vesicles. |
Section 10 of 10Exam technique
1
Saying exocytosis needs no energy because it releases material — ATP is needed for vesicle movement and fusion.
2
Confusing phagocytosis (solids) with pinocytosis (liquids).
3
Forgetting that a vesicle membrane becomes part of the surface membrane after exocytosis.
4
Describing endocytosis as diffusion through the membrane — the material is enclosed in a vesicle and the membrane is not crossed.
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