Notes for biology B2.1 - B2.3 and B3.1 IB dp program
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Module
Biology
Institution
Fifth Year / 11th Grade
These notes cover the International Baccalaureate (IB) Diploma Programme Biology topics B2.1 to B2.3 and B3.1. They are designed to provide a comprehensive overview and a detailed understanding of key biological concepts, processes, and mechanisms essential for the IB curriculum.
B2.1 - Cell Str...
B2. Form and function Cells / B2.1 Membranes and membrane transport
Lipid bilayer
Overview of membrane structure
Membranes surround cells and organelles. These membranes act as barriers controlling the exchange of
materials between the internal and external environmnet
These membranes are composed of:
- lipid (phospholipids, glycolipids and sterols)
- Proteins
- Small amounts of carbohydrates in the form of glycolipids and glycoproteins
These lipids and some proteins that compose the membrane are amphipathic molecules with both
hydrophobic and hydrpohilic regions.
Lipid bilayer
Lipids are essential components of membranes, and include three major classes of lipids:
- Phosolipids
- glycolipids
- Sterols (cholesterol)
Membrane phosoplipids are the most abundant and diverse lipids.
A phosolipids molecule comprises a backbone composed of mostly
glycerol, a three-carobn alcohol.
- A negatively charged phosphate molecule linked to molecules
like choline or serine forming a polar, hydrophilic, ‘head’ group.
The polar head easily forms hydrogen bonds with water.
- two non-polar fatty acid chains forming the hydrophobic ‘tails’. The fatty acids could be saturated
or unsaturated. Unsaturated fatty acids result in kinks in the tail.
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,Lipid bilayers are barriers
Biological membranes seperate the internal environment of the cell or organelle from the external
environmnet.
The hydrocaron tails of both layers extend inward to from a continuous hydrophobic interior; this has an
important role to determine the permeability of the membrane
- As the interior of the bilayer is hydrophobic,
non-polar, lipid-soluble molecules like steroids can
easily pass through the lipid bilayer.
- On the other hand, for the same reason, ions like Na+
cannot pass through the membrane.
- Uncharged polar molecules like glucose are
hydrophilic in nature. These molecules are ‘repelled’
by the cell membrane, i.e. the membrane is mostly
impermeable to them.
- Small, uncharged molecules can readily pass through the lipid bilayer. Thus, polar molecules like
water and ethanol or non-polar molecules like oxygen and carbon dioxide can easily enter or
leave cells.
The permeabilit of biological membranes to molecules depends on the size of th emolecules and their
hydrophilic/hydrophocic nature.
Simple diffusion of moelcuels
One of the easiest ways of molecules to mvoe across the membrane is by simple diffusion. Simple
diffusion is the movement of molecules of a substance down a concentration gradient.
From a region of where its concentration is higher to a region where its concentration is lower. Diffusion
is acspontaneous process and the movement of molecules eventually results in equilibrium.
One example of simple diffusion is the movement of non-polar
molecules like oxygen and carbon dioxide, which plays an
important role in gas exchange. The erythrocytes or red blood cells
transport oxygen from lungs to the cells of the body.
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,The oxygen diffuses from the oxygen-rich air in the alveoli down the concentration gradient to blood in
the capillaries surrounding thre alveoli.
This oxygen is carried to tissues. → in tusses the oxygen diffuses from erythrocytes where oxygen
concentration is higher to metablolically active cells where oxygen concentration is lower.
In tissues, carbon dioxide diffuses from the cells where its
concentration is higher to blood where its concentration is lower.
The carbon dioxide is then carried to the lungs. In the lungs, the
carbon dioxide fissuses from blood to the alveoli dwon the
concentration gradient.
Membrane proteins and hteir functions
Integral and peripheral proteins
Proteins are vital components of biological membrane. Membrane proteins differ in location, structure
and function.
Based on their association witht eh lipid bilayer, membrane proteins can be calssifieed as:
- Integral proteins
- Peripheral proteins
Integral protenis
Proteins that are embedded in the plasma membrane and cannot be easily extracted from the membrane,
These proteins are difficult to isolate as extraction techniques involve disrupting the bilayer.
These integral proteins are amphiphathic molecules.
The hydrophociv regions of the integral proteins interact with the hydrohobiic interior or the lipid bilayer,
causing them to be embedded in the biayer.
Most integral proteins are transmebrane proteins; they extend
across the membrane. Others are only found on one side of the
bilayer.
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, Peripheral proteins
Proteins that are attached to the plasma membrane that can be easily extracted.
They are hydrophilic in nature.
These proteins are normally found on the surface of the membrane and interact only with the hydrophilic
regions of the integral proteins, and at times, with the hydrophilic heads of the phoholipids.
Functions of membrane proteins
- Transport proteins – Membrane proteins facilitate the movement of molecules in and out of the
cell. (channel proteins and carrier proteins)
→ Channel proteins are transmembrane proteins that form channels or pores for the passage of
molecules. Carrier proteins on the other hand undergo a conformational change to transfer the
molecules from one side of the membrane to the other.
- Recognition – Membrane proteins help in cell–cell recognition acting as ‘name tags’ for the cells.
- Receptors – Membrane proteins act as receptors
for chemical signals and are binding sites for
molecules like hormones and neurotransmitters.
- Enzymes – Membrane proteins show enzymatic
activity and catalyse reactions.
Osmosis
Osmosis is the diffusion of water across a selectively
permeable membrane from lower to higher solute
concentrations.
Water diffuses from a region of lower soluite
concentration to a region with higher solute
concentration through the membrane.
Water always moves by osmosi from a region of higher
water concentration to a region of lower water
concentration.
Aquaporins
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