Cell Structure and Function

The biological universe consists of two types of cells: 1. Prokaryotic cells 2. Eukaryotic cells

Prokaryotic Cells: Single celled organisms Simple internal organization Lack a defined nucleus Less than 1 micrometer in diameter

A Prokaryotic Cell

Cell Wall: Offers protection for the cell. Composed of carbohydrate- protein complex called Peptidoglycan. Plasma Membrane: Found on inside of cell wall. Responsible for the passing of various nutrients across the membrane. Capsule: Cell may have additional carbohydrate layer that disables it from interacting with various surfaces

Pili: Extensions that are longer than Cilia Mainly used for Sexual reproduction with other bacteria. DNA is exchanged via pili. Not all bacteria have pili Cilia: May contain cilia. Used to swipe molecules towards the cell for endocytosis. Flagella: May Contain Flagella. Longer than Pili Used for motility

Ribosomes: Most ribosomes (protein synthesizing particles) are found in the DNA free region of the cell. Site of Protein Synthesis Appear Granular on images

Nucleoid Region The Genetic material (DNA) is found in a single circular DNA molecule that is folded back on itself many times in the central region of the cell. It appears as a dark dense mass in the center of the cell. It may contain small circular, independent units of DNA called Plasmids. – May contribute in ability to adapt to rare environments. – Used in biotechnology (See genetics – Topic 4)

Mesosome – Infolding of the plasma membrane to assist in compartmentalization (ex. Enzymes can be found here)

Most prokaryotes studied in laboratories are bacteria. I.e. – Escherichia Coli (E. Coli) found in animal intestine

Cyanobacteria (Previously known as Blue-green Algae)

Binary Fission Bacteria go through binary fission as a method of reproduction (Asexual Reproduction) Steps involved: DNA is copied Separation of DNA molecules toward two ends of cell Elongation of cell Division of cytoplasm and plasma membrane Microtubules help separate DNA. It is a protein called FtsZ

Eukaryotic Cells: Includes all members of plant and animal kingdoms Surrounded by a plasma membrane that separates it from the external environment Range from micrometers Internal membranes enclose specific compartments, organelles and separates them from the rest of the cytoplasm (Region of cell lying outside the nucleus.)

Largest organelle in Eukaryotic cell is the nucleus. It is a membrane bound organelle and holds most of the cellular DNA.

Other organelles present in nearly all eukaryotic cells: 1. Mitochondria – responsible for cells energy metabolism 2. Rough and smooth endoplasmic reticulum – network of membranes in which certain proteins and lipids are synthesized.

3. Golgi Vesicles – Direct membrane components to appropriate places in the cell. 4. Peroxisomes – Fatty acids and amino acids are degraded. 5. Lysosomes - Animal cells, not plant cells - Degrade worn-out cell constituents and foreign materials taken in by the cell

6. Chloroplasts – Plant cells and certain single celled organisms 7. Vacuoles - plant and some single celled eukaryotes - large fluid filled organelles - store nutrients and waste compounds

8. DNA in nucleus – packaged into linear structures called chromosomes

The Cell Membrane and The Fluid Mosaic Membrane Model A phospholipid molecule contains: 1. 2 fatty acid chains 2. Glycerol backbone.

The head of the molecule is Polar and Hydrophilic The tail of the molecule is non-polar and hydrophobic. A molecule that is both hydrophilic and hydrophobic is called AMPHIPATHIC.

What happens when phospholipids are put in water… Attraction and repulsion of polar head group and non-polar tail group, respectively. Forming a spontaneous spherical bilayer – Hydrophilic heads facing the outside Hydrophobic tails facing eachother

The Cell Membrane The cell membrane is composed of a phospholipid bilayer. There are also other components within the bilayer: – Proteins – Glycoproteins – Glycolipids – Cholesterol

The Fluid-Mosaic Membrane Model Phospholipid molecules and proteins that are embedded in the membrane tend to drift sideways. This supports the idea of the cell membrane having a Fluid Consistency.

In the cell membrane we find: Proteins Carbohydrates Cholesterol Why do we need these components in the cell membrane?

Proteins There are two types of membrane proteins: 1. Peripheral or Extrinsic Proteins They do not interact with hydrophobic core of phospholipid bilayer. Bound to polar head of the phospholipid or Bound to the integral or intrinsic proteins

2. Integral or Intrinsic Proteins Embedded in the phospholipid bilayer They can be : Transmembrane (Span the entire phopholipid bilayer) Lipid-anchored (Anchored to one of the phospholipid layers)

Function: On the Extracellular surface 1. Cell-cell signaling 2. Cell surface receptors (I.e. Hormones use these receptors to transmit messages into the cell)

Within the Membrane 1. Proteins form channels and pores to move molecules across the membrane

On the Cytosolic Face of membrane 1. Proteins anchor cytoskeletal proteins that support the membrane and send signals to various parts of the cell.

Carbohydrates Are found bound to either Proteins – called glycoproteins Lipids – called glycolipids

Purpose: 1. They help increase hydrophilic character of lipids and proteins 2. Stabilize conformation (or shape) of many membrane proteins.

Cholesterol Generally, the cell membrane is – Fluid-like – And has drastic changes in physical properties over a very narrow temperature range: High temperature – Very Fluid Low temperature – Gel-like

This steroid sits between the phospholipid tails of the bilayer. Cholesterol maintains the favored fluid-like consistency.

How does cholesterol maintain the fluid-like consistency? 1. At higher temperatures, cholesterol makes the membranes less fluid by making the fatty acid chains less mobile 2. At lower temperatures, cholesterol keeps membranes fluid by preventing the fatty acid chains of the phospholipid from binding to one another.

Recap of New terms: 1. Peripheral or Extrinsic Proteins 2. Integral or Intrinsic Proteins – Transmembrane – Lipid-anchored 3. Extracellular surface 4. Cytosolic Face 5. Glycoproteins 6. Glycolipids 7. Fluid-like 8. Gel-like