Cell Structure and FunctionCell Structure and Function

The biological universe consists of two types of cells:

1. Prokaryotic cells 2. Eukaryotic cells

Prokaryotic Cells:Prokaryotic Cells:

Single celled organismsSimple internal organizationLack a defined nucleus Less than 1 micrometer in diameter

A Prokaryotic CellA 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 piliCilia: May contain cilia. Used to

swipe molecules towards the cell for endocytosis.

Flagella: May Contain Flagella. Longer than Pili Used for motility

 

Ribosomes: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 RegionNucleoid 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 Most prokaryotes studied in laboratories are bacteria.laboratories are bacteria.

I.e.– Escherichia Coli (E.

Coli) found in animal intestine

Cyanobacteria (Previously known as Blue-green Algae)

Binary Fission 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:Eukaryotic Cells:

Includes all members of plant and animal kingdoms

Surrounded by a plasma membrane that separates it from the external environment

Range from 5-100 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 The Cell Membrane and The Fluid Mosaic Membrane Fluid Mosaic Membrane

ModelModel 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 What happens when phospholipids are put in water…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 MembraneThe 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 The Fluid-Mosaic Membrane ModelModel

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:In the cell membrane we find:

ProteinsCarbohydrates Cholesterol

 Why do we need these components in the

cell membrane?

 

ProteinsProteinsThere are two types of 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 orBound 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: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.

CarbohydratesCarbohydrates

Are found bound to either

Proteins – called glycoproteinsLipids – called glycolipids

Purpose:

1. They help increase hydrophilic character of lipids and proteins

2. Stabilize conformation (or shape) of many membrane proteins.

CholesterolCholesterol

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 How does cholesterol maintain the fluid-like maintain the fluid-like

consistency?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 termsRecap 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