Biological Science 101 – General Biology
Lecture Two: Cell Membrane
Chapter 7, pg. 125-139
Composition of cell membranes
Plasma membrane: The membrane that surrounds the cell.
- Other membranes around: nucleus, mitochondria, Golgi apparatus.
- Membranes are selectively permeable – “some things can go in and some things
can go out”.
Lipids and proteins are in the membrane (they also contain carbohydrates).
- Lipids and proteins are bond together in a way that makes up a “fluid mosaic”
model; because it is a fluid model it is flexible.
- It is very thin 8 x 10 (8,000 membranes = thickness of one paper page).
Lipids = Phospholipids that have a hydrophilic end and a hydrophobic end.
Proteins Present in Membrane
Amphipathic = Hydrophilic + Hydrophobic
Embedded Proteins (integral) and External
(peripheral). Carbohydrates in membrane -> short-chain (oligosaccharides).
Functions of Carbohydrates:
1) Cell-to-cell recognition.
E.g. Foreign cells.
2) Determine cell features.
E.g. Blood types (A, B, O, AB)
Functions of Proteins:
1) Cell-to-cell recognition:
- Some glycoproteins serve as identification
tags that are specifically recognized by
membrane proteins of other cells. This type
of cell-to-cell binding is usually short-lived
compared to that shown in intercellular
2) Intercellular joining:
- Membrane proteins of adjacent cells may
hook together in various kinds of junctions
(see textbook figure 6.32).
- This type of binding is more long-lasting
that that shown in cell-to-cell recognition.
3) Attachment to the cytoskeleton and
extracellular matrix (ECM):
- Microfilaments of other elements of the
cytoskeleton may be noncovalently bound to
membrane proteins, a function that helps
maintain cell shape and stabilizes the
location of certain membrane proteins.
Proteins that can bind to ECM molecules can
coordinate extracellular and intracellular
changes (see textbook figure 6.30).
Movement of Molecules:
Hydrophilic End --- Hydrophobic End
- Sugars and amino acids are difficult to transport.
- Water is very easy. Passive Transport = No energy required.
1) Diffusion: Molecules spread into a space, from high concentration to low
2) Osmosis: The movement of water, from a
low solute (hypotonic) to a high solute
- It equalizes the distribution of solute, which
is important for cell viability.
3) Facilitated Diffusion: Diffusion with the
help of transport proteins.
- Specific for molecules to transport.
- Water uses “aquaporins” (water channels). Active Transport = Moving molecules against a concentration gradient.
- Gradient from low concentration to high concentration.
- Against concentration gradient.
- Cells uses energy in the form of ATP:
1) Sodium – Potassium Pump: Sodium out, Potassium in.
- Cells need high K inside.
- Potassium ions have to enter and Sodium ions have to leave.
2) Proton Pump (Electrogenic Pump):
- Hydrogen ions go out of the cell.
- Negative charge inside.
- Electrical potential created (membrane potential)
- 50 mV to -200 mV
- Creates a pathway for ions to enter cell.
- Solute transport is accompanied by the proton pump.
E.g. Plants – transport sucrose into the cell. Bulk Transport: Moving large molecules
E.g. Carbohydrates and Proteins.