Ch. 2: Cell Structure and Function


(text pp. 41-60)
 Human Biology (BIOL100)

The cell theory states that all living organisms are composed of cells, and new cells are formed from pre-existing cells.

In chapter 2 we will explore how the organic molecules we encountered in chapter 1 are organized into structures called cell organelles and cells:
1) atoms.
2) molecules.
3) cell organelles.
4) cells.
Cell organelles are structures contained within the cell in which specific cellular functions take place (eg. cell membrane transport and cellular respiration).
Cells are the fundamental unit of structure and function of living organisms.

In chapter 2 we will also explore two cellular functions common to all cells:
a. Cell membrane transport - moving specific substances in and out of cells.
b. Cellular respiration in mitochondria - the release of energy from organic molecules to form ATP.

In later chapters we will focus on specialized cell structures and functions. For example:
a. Muscle cells specialized for contraction.
b. Nervous cells specialized for conducting nervous impulses.


I. Cell Size
Cells are small in order to provide an optimal surface area to volume ratio, ensuring an adequate surface area for cell membrane transport in order to meet the needs of the functions occurring within the cell organelles.
II. Microscopy and Cell Structure (fig. 2.2)
 a. Compound light microscope.
b. Transmission electron microscope (TEM).
c. Scanning electron microscope (SEM).



III. Cellular Organization (fig. 2.3 and table 2.1)
 A human cell is bounded by a plasma membrane which encloses a central nucleus surrounded by cytoplasm .
The cytoplasm contains cell organelles , each bounded by plasma membrane.
The cytoplasm also contains the cytoskeleton .

A. The Plasma Membrane
a) Structure (fig. 2.4)
The plasma membrane is a bilayer of phospholipids (fig. 1.21) with embedded or attached proteins (fig. 1.25).
Glycoproteins and glycolipids serve as identification markers.
Embedded proteins function as hormone receptors, transport channels, and enzymes.
b) Function
The plasma membrane keeps the cell intact and due to its property of being selectively permeable , regulates the movement of specific substances in and out of the cell.
There are 3 mechanisms of transport across the plasma membrane:

1) Diffusion
 Diffusion is the random movement of particles from an area of higher concentration to an area of lower concentration = down the concentration gradient.

 Simple Diffusion:
-The movement of a small and/or hydrophobic molecule across the phospholipid bilayer down its concentration gradient.
-Simple diffusion does not consume cellular energy.
-Simple diffusion is not selective.
-Examples: O2, CO2, steroids, fatty acids.

Osmosis: (fig. 2.5)
 Osmosis is a specialized form of simple diffusion.
-The simple diffusion of water across the phospholipid bilayer.
Normally the intracellular solution has the same concentration of solutes (osmotic pressure) as does the extracellular fluid = isotonic .
A solution of greater solute concentration (greater osmotic pressure) = hypertonic.
 A solution of smaller solute concentration (smaller osmotic pressure) = hypotonic.








2) Transport by Protein Carriers
 There are two types of transport by protein carriers across the plasma membrane:

a. Facilitated Diffusion:
-The movement of an ion or hydrophilic molecule through a protein carrier down its concentration gradient.
-Facilitated diffusion does not consume cellular energy.
-Facilitated diffusion is selective.

b. Active Transport: (fig. 2.6)
 -The movement of an ion or hydrophilic molecule through a protein carrier against its concentration gradient.
-Active transport involves the consumption of cellular energy (ATP).
-Active transport is selective.


B. The Nucleus
 a) Structure (fig. 2.7)
Single spherical structure bounded by a double-layered plasma membrane with nuclear pores. Contains one or more nucleoli.
b) Function
The nucleus stores biological information in the form of DNA .
DNA is present in the nucleus as chromatin , but condenses to form chromosomes prior to cell division.
DNA determines the structure and function of the body's cells by directing the production of proteins at the ribosomes (protein synthesis) via mRNA and thus the structure and function of the organism as a whole:
DNA ----> mRNA -----> protein -----> cell structure and function


C. The Membranous Canals and Vesicles of the Cell Cytoplasm
1) The Endoplasmic Reticulum
 a) Structure (fig. 2.8)
 The endoplasmic reticulum (ER) is a series of plasma membranes that form tubular channels within the cytoplasm.
b) Function
The rough ER contains ribosomes where protein synthesis takes place.
The smooth ER lacks ribosomes and produces different compounds in different cells.
Molecules produced in the ER are often transported in vesicles to the Golgi apparatus.





2) The Golgi Apparatus
a) Structure (fig. 2.9)
Similar to the structure of the ER.
b) Function
The Golgi apparatus processes, packages, and distributes the proteins and other molecules produced in the ER.

3) Lysosomes
 a) Structure
Small, spherical structures bounded by plasma membrane. Multiple lysosomes are present within a cell.
b) Function
 Lysosomes contain hydrolytic enzymes that digest unwanted materials inside the cell, including worn-out cell parts.


D. Mitochondria
 a) Structure (fig. 2.10)
Bean-shaped structure bounded by a double-layered plasma membrane with inner matrix and cristae. Multiple mitochondria are present within a cell.
b) Function
Mitochondria are involved with the release and transfer of the chemical energy present in some types of organic molecules (glucose, fatty acids, amino acids) to the production of ATP molecules in the process of aerobic cellular respiration.


E. The Cytoskeleton
a) Structure
Microtubules and actin filaments form a cytoskeleton that serves as a framework for the cell's interior.
b) Function
The cytoskeleton maintains cell shape and allows for the movement of cell parts.


IV. Cellular Metabolism and Cellular Respiration
A. Metabolism:
The sum of all the chemical reactions taking place inside the cells of the body.
These chemical reactions are organized as metabolic pathways in which the product of one chemical reaction serves as the reactant for another chemical reaction:

A --1 --> B --2 --> C --3 --> D
Reactants and products (letters A, B, C, D).

B. Enzymes: (fig. 2.13)
Every chemical reaction requires a specific protein called an enzyme (numbers 1, 2, 3).
Enzymes increase the rates of chemical reactions and contain an active site for substrate.
 Some enzymes require the presence of cofactors or coenzymes.

 C. Cellular Respiration (fig. 2.14)
An example of a complex metabolic pathway which forms part of cellular metabolism and is present in every cell is cellular respiration.
 The enzymes of cellular respiration break down certain organic molecules such as glucose in order to transfer the chemical energy to ATP.
C6H12O6 + 6O2 ------> 6CO2 + 6H2O + 36 ATP
ATP is used to fuel all energy-consuming chemical reactions in the body.

Aerobic cellular respiration consists of 3 distinct metabolic pathways:
1) Glycolysis
 2) Krebs Cycle
 3) Electron Transport System (ETS)
1) Glycolysis or Anaerobic Cellular Respiration:
 -Occurs in the cell cytoplasm.
-Occurs in the absense of O2.
-Reactant glucose is broken down to product pyruvate.
-Produces 2 ATP and 2 NADH (to ETS) per glucose molecule.
-In the absense of O2 glycolysis results in the formation of product lactic acid.
-In the presense of O2 pyruvate is used as the initial reaction for Krebs cycle.

Aerobic Cellular Respiration
 2) Krebs Cycle:
 -Occurs in the mitochondrial matrix.
-Occurs in the presence of O2.
-Reactant pyruvate is broken down to product CO2.
-Produces 2 ATP and 10 NADH (to ETS) per glucose molecule.

3) Electron Transport System (ETS):
 -Occurs in the mitochondrial cristae.
-Occurs in the presence of O2.
-chemical energy of NADH used to produce 32 ATP per glucose molecule.
-H2O is formed.

At the end of aerobic cellular respiration the cell will have gleaned enough energy from the breakdown of glucose to produce 36 ATP which represents about 40% of the total energy released from glucose (the remaining 60% is lost as heat).
Now the ATP neccessary to fuel energy-consuming reactions is available.