Lecture 2- Chapter 3: pages 66-72 and 89-100
The Cellular level of Organization
An introduction to Cells
- Cell theory
1. Cells are the building blocks of all plants and animals
2. Cells are produced by the division of pre-existing cells
3. Cells are the smallest units that perform all vital physiological functions
4. Each cell maintains homeostasis at the cellular level.
- Homeostasis at the level of the tissue, organ, organ system, and organism reflects the
combined and coordinated actions of many cells.
- 2 types of cells : SEX CELLS ( sperm for males, and oocytes for females) and SOMATIC
CELLS (all other cells in the human body)
- The extracellular fluid in most tissues is called INTERSTITIAL FLUID.
- A plasma membrane separates the cell content or cytoplasm from the extracellular fluid.
- The cytoplasm can itself be subdivided into
1. The cytosol, a liquid.
2. Intracellular structures collectively known as organelles.
3.1 The plasma membrane separates the cell from its surrounding environment and
performs various functions.
- the outer boundary of the cell is the plasma membrane, also called the cell membrane.
1. Physical isolation: separates the cell from the surrounding extracellular fluid. The inside
and outside the cell are very different, and those differences must be maintained to
2. Regulation of Exchange with the Environment: The plasma membrane controls the entry
of ions and nutrients, such as glucose; the elimination of wastes and the release of
3. Sensitivity to the environment: affected first if there is changes in the composition,
concentration or pH of the extracellular fluid. It also contains a variety of receptors that
allow the cell to recognize and respond to specific molecules in its environment. For
instance, the plasma membrane may receive chemical signals from other cells. The binding of just one molecule may trigger the activation or deactivation of enzymes that
affect many cellular activities.
4. Structural support: gives tissues stability!
- The plasma membrane is extremely thin. This membrane contains lipids, proteins, and
- a.k.a. phospholipid bilayer, because the phospholipid molecules in it form 2 layers---
hydrophilic end (the phosphate portion) and the hydrophobic end (the lipid portion).
- Phospholipids lie with their hydrophilic heads at the membrane surface and the
hydrophobic tails on the inside.
- Ions and water-soluble compounds cannot enter the interior of a micelle, because the
lipid tails of the phospholipid molecules are hydrophobic and will not associate with
water molecules. For the same reason, water and solutes cannot cross the lipid portion
of the plasma membrane. Thus, the hydrophobic compounds in the center of the
membrane isolate the cytoplasm from the surrounding fluid environment. Such isolation
is important because the composition of cytoplasm is very different from that of
extracellular fluid, and the cell cannot survive if the differences are eliminated.
- there are 2 general structural classes of membrane protein : INTEGRAL PROTEIN AND
- Integral proteins are part of the membrane structure and cannot be removed without
damaging or destroying the membrane. Also known as trans-membrane proteins as
they span the width of the membrane one or more times.
- Peripheral proteins are bound to the inner or outer surface of the membrane and are
easily separated from it.
- Have a variety of specialized functions. i.e.
1. Anchoring proteins: attach the plasma membrane too other structures and stabilize
its position 2. Recognition proteins: the cells of the immune system recognize other cells as
normal or abnormal based on the presence or absence of characteristic recognition
proteins. Many important proteins are glycol-proteins.
3. Enzymes: may be integral or peripheral protein. They catalyze reactions in the
extracellular fluid or in the cytosol, depending on the location of the protein and its
4. Receptor proteins : are sensitive to the presence of specific extracellular molecules
5. Carrier: bind solutes and transport them across the membrane. May require ATP as
an energy source.
6. Channels: form passageway across plasma membrane. The channel permits the
movement of water and small solutes across the membrane. Ions do not dissolve in
lipids, so they cannot cross the phospholipid bilayer. Thus, ions and other small
water-soluble materials can cross the membrane only by passing through channels.
- the carbohydrate portions of these large molecules extend beyond the outer surface of
the membrane, forming a layer known as the GLYCOCALYX. The glycocalyx has a
variety of important functions, including the following,
1. Lubrication and protection: the glycoproteins and glycolipids form a
viscous later that lubricates and protects the plasma membrane.
2. Anchoring and Locomotion: because the components are sticky, the
glycocalyx can help anchor the cell in place. It also participates in the
locomotion of specialized tem.cells.
3. Specifying and binding: glycoproteins and glycolipids can function as
receptors, binding specific extracellular compounds. Such binding can alter
the properties of the cell surface and indirectly affect the cell’s behaviour
4. Recognition: glycoprotein and glycolipids are recognized as normal or
abnormal by cells involved with the immune response. The characteristics of
the glycocalyx are genetically determines. The body’s immune system
recognized its own members of glycoproteins and glycolipids as self rather
than as foreign. PART 2: PG 89-100
3.5 diffusion is a passive transport mechanism facilitating membrane passage
- Permeability is the property of the plasma membrane that determines precisely which
substance can enter or leave the cytoplasm
- A membrane though which nothing can pass is impermeable
- Any substance can pass- free permeable
- Selective stuff can pass- selectively permeable
- The distinction may be based on size, electrical charge, molecular shape, lipid solubility,
- Passage across the membrane is either passive or action
- Passive- move ions or molecules across the plasma membrane with no expenditure of
energy by the cell. Active – requires that the cell expend energy, generally in the form of
- Results from the random motion and collusion of ions and molecules
- Passive process
- The difference between high and low concentration is a concentration gradient
- Diffusion tends to eliminate that gradient
- Because diffusion tends to spread materials from a region of high concentration to one
of lower concentration, it is often described as proceeding “down a concentration
- To be effective, the diffusion of nutrients, waste products, and dissolved gases must