BMSC11002 Lecture Notes - Lecture 3: Hexose, Polar Regions Of Earth, Amphiphile
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WEEK 3: CELLULAR LEVEL OF ORGANISATION
3.1 THE PRINCIPAL PARTS OF A CELL
• Plasma membrane – forms the cells flexible outer surface, separating the cell’s internal
environment from its external environment
o This selective barrier regulates the flow of materials into and out of a cell to help establish
and maintain the appropriate environment for normal cellular activities
o Plays a key role in communication among cells and between cells and their external
environment
• The cytoplasm consists of all the cellular contents between the plasma membrane and the nucleus
o The cytoplasm consists of cytosol and organelles
▪ Cytosol contains water, dissolved solutes and suspended particles
• Also called intracellular fluid
▪ There are several types of organelles
• Each organelle has a characteristic shape and specific functions
• E.g. endoplasmic reticulum, Golgi complex, lysosomes, peroxisomes, and
mitochondria
• The nucleus is a large organelle that houses most of a cell’s DNA
o Each chromosome, a single molecule of DNA associated with several proteins, contains
thousands of hereditary units called genes that control most aspects of cellular structure
and function
3.2 THE PLASMA MEMBRANE
• The plasma membrane, is a flexible yet sturdy barrier that surrounds and contains the cytoplasm
of a cell, described using the fluid mosaic model
o The molecular arrangement of the plasma membrane resembles a continually moving sea
of lipids that contains a mosaic of many different proteins
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THE LIPID BILAYER
• The head of a phospholipid is polar and hydrophilic and the two fatty acid tails are hydrophobic,
which orientates them in a certain position
• Cholesterol molecules are found in both layers of the plasma membrane
o They are steroids with an attached OH group, which is the only polar region of cholesterol,
and forms the hydrogen bonds with the polar heads of the phospholipids and glycolipids
o Most of the cholesterol molecule is non-polar and fits in with the rest of the phospholipid
• Glycolipids are lipids with an attached carbohydrate group that form a polar head
o They only appear in the layer of the plasma membrane that faces the extracellular fluid
▪ This accounts for the asymmetric appearance of the bilayer
ARRANGEMENT OF MEMBRANE PROTEINS
• Integral proteins extend into or through the lipid bilayer among the fatty acid tails and are firmly
embedded in it
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o Most integral proteins are transmembrane proteins, which means they span the entire
length of the bilayer and extend into the cytosol and extracellular fluid
o They have hydrophilic regions and hydrophobic regions
• Peripheral membranes are not as firmly embedded in the membrane
o They are attached to the polar heads of membrane lipids or to integral proteins at the
inner or outer surface of the plasma membrane
• Glycoproteins, proteins with carbohydrate groups attached to the ends that protrude into the
extracellular fluid
o They have a sugary coating called glycocalyx (which is the signature of a glycoprotein)
MEMBRANE PERMEABILITY
Plasma membranes are asymmetric: the interior of the membrane is not identical to the exterior of the
membrane. In fact, there is a considerable difference between the array of phospholipids and proteins
between the two leaflets that form a membrane . On the interior of the membrane, some proteins serve
to anchor the membrane to fibers of the cytoskeleton. There are peripheral proteins on the exterior of
the membrane that bind elements of the extracellular matrix. Carbohydrates, attached to lipids or
proteins, are also found on the exterior surface of the plasma membrane. These
carbohydrate complexes help the cell bind substances that the cell needs in the extracellular fluid. This
adds considerably to the selective nature of plasma membranes.
Asymmetry in Plasma Membranes
The exterior surface of the plasma membrane is not identical to the interior surface of the same
membrane.
Recall that plasma membranes are amphiphilic; that is, they have hydrophilic and hydrophobic regions.
This characteristic helps the movement of some materials through the membrane and hinders the
movement of others. Lipid-soluble material with a low molecular weight can easily slip through the
hydrophobic lipid core of the membrane. Substances such as the fat-soluble vitamins A, D, E, and K
readily pass through the plasma membranes in the digestive tract and other tissues. Fat-soluble drugs
and hormonesalso gain easy entry into cells and are readily transported into the body's tissues and
organs. Molecules of oxygen and carbon dioxide have no charge and so pass through membranes by
simple diffusion.
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Document Summary
Integral proteins extend into or through the lipid bilayer among the fatty acid tails and are firmly embedded in it. Plasma membranes are asymmetric: the interior of the membrane is not identical to the exterior of the membrane. In fact, there is a considerable difference between the array of phospholipids and proteins between the two leaflets that form a membrane . On the interior of the membrane, some proteins serve to anchor the membrane to fibers of the cytoskeleton. There are peripheral proteins on the exterior of the membrane that bind elements of the extracellular matrix. Carbohydrates, attached to lipids or proteins, are also the plasma membrane. These carbohydrate complexes help the cell bind substances that the cell needs in the extracellular fluid. This adds considerably to the selective nature of plasma membranes. the exterior surface of found on. The exterior surface of the plasma membrane is not identical to the interior surface of the same membrane.
