Unit 1 notes for IB Biology HL students. I took my exams in July 2022 and got a 7 in Biology HL. The notes are a combination of information from different sources with a main focus on the IB Biology Textbook.
1.4 Membrane Transport
Objectives:
❏ Understanding:
❏ Particles move across membrane by simple diffusion, facilitated diffusion, osmosis and active
transport
❏ The fluidity of membranes allows materials to be taken into cells by endocytosis or released by
exocytosis
❏ Vesicles move materials within cells
❏ Applications:
❏ Structure and function of sodium - potassium pumps for active transport and potassium
channels for facilitated diffusion in axons
❏ Tissues or organs to be used in medical procedures must be bathed in a solution with the same
osmolarity as the cytoplasm to prevent osmosis
❏ Skills
❏ Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions
(Practical 2)
❏ Nature of Science
❏ Experimental design: accurate quantitative measurements in osmosis experiments are essential
Endocytosis:
- Endocytosis = the taking in of materials via vesicles and using ATP
- Vesicle = small sac of membrane with a droplet of fluid inside; small vacuoles
used to transport materials in the cell
- Normally present in eukaryotic cells
- They are a very dynamic feature of cells
- They are constructed, moved around and then deconstructed
- Can happen because of the fluidity of membranes, which allows
structures surrounded by a membrane to change shape and move
- Formation of a vesicle:
- A small region of a plasma membrane is pulled from the rest of the
membrane and is pinched off
a. Proteins in the membrane carry out this process, using energy
from ATP
- Vesicle is formed on the inside of the plasma membrane
- Vesicle contains materials that was outside the cell (this is a method of
taking materials into a cell)
- Vesicles often contain larger molecules needed by the cell that cannot pass across
the plasma membrane
- E.g. in the placenta, proteins from the mother’s blood (including
antibodies) are absorbed into the fetus by endocytosis
- Some cells take in large undigested food particles by endocytosis
- This also happens in unicellular organisms including Amoeba and
Paramecium
- Phagocytosis = some types of white blood cells take in pathogens
including bacteria and viruses by endocytosis and then kill them as part of the body’s response to
infection
, Vesicle movement in cells:
- Vesicles move materials in their contents within cells
- Sometimes it is proteins in the membrane of the vesicle that are the reason for vesicle movement
- An example of moving the vesicle contents occurs in secretory cells
1. Protein is synthesised by ribosomes on rER and accumulates inside rER
2. Vesicles containing the proteins bud off rER and carry them to the Golgi apparatus
3. The vesicles fuse with the Golgi apparatus, which processes the protein into its final form
4. When this has been done, vesicles bud off the Golgi apparatus and move to the plasma membrane,
where the protein is secreted
- In a growing cell, the area of the plasma membrane needs to increase
- Phospholipids are synthesised next to the rER and become inserted into the rER membrane
- Ribosomes on the rER synthesizes membrane proteins which also become inserted into the
membrane
- Vesicles bud off the rER and move to the plasma membrane
- They fuse with it, increasing the area of the plasma membrane by a very small amount
- Can also be used to increase the size of organelles in the cytoplasm such as lysosomes
and mitochondria
Exocytosis:
- Vesicles can be used to release materials from cells
- If a vesicle fuses with the plasma membrane, the contents are then outside the
membrane and therefore outside the cell
- Digestive enzymes released from gland cells by exocytosis
1. The polypeptides in the enzymes are synthesised by the rER, processed in
the Golgi apparatus and then carried to the membrane in vesicles for
exocytosis
2. Release is referred to as secretion because a useful substance is being
released, not a waste product
- Exocytosis can also be used to expel waste products or unwanted materials
- E.g. the removal of excess water from the cells of unicellular organisms
1. The water is loaded into a vesicle (sometimes called a contractile
vacuole)
2. The moved to the plasma membrane for expulsion by exocytosis
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