Biology 101 Chapter 4

Cells as the Basic Unit of Life

The Cell Theory

Major Contributors:

Galileo = first observations made with a microscope

Robert Hooke = first to observe small compartments in dead plant tissue, coined the term "cell"

Antony van Leeuwenhoek = first to observe living, mobile cells and bacteria

Robert Brown = first to observe the nucleus

Rudolf Virchow = every new cell comes from a pre-existing cell

Schleiden and Schwann = plants and animals are composed of cells and cell products

Tenements of the Cell Theory:

1) All organisms are composed of one or more cells.

2) The cell is the smallest unit having the properties of life.

3) The continuity of life arises directly from the growth and division of single cells.

Cell size and cell function: Surface area to volume ratio!

- The larger the ratio, the better off the cell!

- Impact of surface area to metabolism

What is the largest cell in the human body? The smallest?

Cell Structures and Their Functions

All cells are placed in one of 2 classes:

Prokaryotic = lack a nucleus (bacteria)

Eukaryotic = have a nucleus (protists, fungi, plants,

and animals)

PROKARYOTIC CELLS

· very, very small

· very simple structure

Parts:

A) Plasma (cell) membrane = encloses cytoplasm of cell

B) Nucleoid Region = where DNA is at (not a nucleus)

C) Ribosomes = assembles proteins with info from DNA

D) Bacterial Cell Wall = a rigid outer layer that surrounds

the cell membrane, protects the cell, maintains shape

E) Capsule = a sticky outer layer over cell wall

F) Pili and Fimbriae = numerous short projections that

help with adherence

G) Prokaryotic Flagella = longer projections that help with

motility

H) Plasmids = extra-chromosomal pieces of DNA

EUKARYOTIC CELLS

· have a nucleus

· very, very large

· complex internal organization

· compartmentalized

· membrane bound organelles

Organelle = "small organ", membrane enclosed structures found inside the cell, each for a specialized function. All chemical activities of the cell occur within organelles.

Benefits of Organelles:

1) Separate environments for chemical reactions

2) Increased membrane surface area

Eukaryotic Cells Broken Up into 3 Regions:

1. Cell Membrane

2. Cytoplasm (cytosol and organelles)

3. Nucleus

Organelles:

1. Nucleus *

2. Endoplasmic Reticulum (ER, smooth and rough)

3. Golgi Apparatus (or Body)

4. Vesicles (lysosomes and peroxisomes)

5. Mitochondria

6. Chloroplasts (only in plants)

7. Storage Vacuole (mainly in plants)

8. Centriolus (only in animal cells)

Other Structures:

1. Ribosomes

2. Cell Wall (in plants)*

3. Cell Membrane

4. Cytoskeleton

a. Microtubules

b. Microfilaments

c. Intermediate Filaments

5. Flagella and Cilia (mainly in animals)

6. Nucleolus

The Nucleus

· Cell's genetic control center

· Double membrane

· Nucleoplasm

· Nuclear envelope

· Nuclear pores

· Chromatin = DNA + associated proteins

· Nucleolus = internal structure of nucleus, site of ribosome assembly

· Chromatin vs. Chromosome

The Cytomembrane System

· Function: internal transport, importing and exporting of cell

· 3 parts:

1. ER

2. Golgi Apparatus

3. Vesicles

Endoplasmic Reticulum

· Single, continuous membrane

· Pipes, tubes and tunnels in cell

· Continuous with nuclear envelope

· Superhighway of the cell

· 2 kinds: Rough ER + Smooth ER

Rough ER

- Flattened connected sacs

- Studded, or covered, with ribosomes

- Major site of protein synthesis

- Synthesis of new membrane

Smooth ER

- Lacks ribosomes

- Continuous with rough ER

- Functions:

1. Transport

2. Synthesis of lipids

3. detoxification

4. Storage of calcium ions

Golgi Apparatus

· Stack of flattened, pancake looking sacs located near cell membrane

· Handles export and import of material for cell

· Not continuous with ER, NOT physically connected

· Functions:

1. Storage, packaging, sorting and final touches and modification of proteins before exportation

2. The UPS of the cell

Vesicles

· General, short term transport, some storage, single membrane

· 3 special types:

Transport Vesicles

1) Used to transport material from ER to Golgi Apparatus

2) Transport of finished product from Golgi to Cell Membrane for export (process reversed for import)

Lysosomes

1) Contain digestive (hydrolytic) enzymes

2) Breakdown cell's food and wastes

Peroxisomes

1) Breakdown lipids

2) Detox alcohols and hydrogen peroxide

Vacuoles

· Very large, single membrane sacs

· Functions:

1. Work with lysosomes for digestion

2. Storage of food and water

3. Stores wastes, excess water

4. Turgor pressure in plants

Ex. Large Central Vacuole of plants

Mitochondria

· Found in all eukaryotic cells

· Carry out cellular respiration to produce energy for the cell

· Cell's "power house"

· Composed of 2 membranes

· Cristae = folds of inner membrane, site of energy production

· Matrix = fluid inside mitochondria

Chloroplasts

· Found only in green plant cells and algae

· Site of photosynthesis

· Contain the pigment chlorophyll

· Composed of 3 membranes

· Grana = stacks of discs of inner membrane, actual site of photosynthesis

· Stroma = fluid inside chloroplast

Centriolus

· Also referred to as basal bodies and MTOCs

· Composed of two centrioles in a membrane

· Used for anchoring, microtubule growth

· Centrioles also used in cell reproduction

Structures Based on Microtubules

The Cytoskeleton

· Framework of protein fibers inside cell

· Support and movement (dynamic)

· Composed of:

1. Microfilaments = thinnest (actin)

2. Intermediate filaments = (composition varies)

3. Microtubules = thickest (tubulin)

Cilia & Flagella

· Used in locomotion

1. Cilia = numerous, very short

2. Flagella = few, very long

· "9 + 2" arrangement of microtubules

§ 9 outer pairs

§ 2 single central

Plant Cell Wall

· Surrounds the cell external to cell membrane

· Very stiff, rigid structure

· Supports cell, gives it shape, protects it

· Composed of a complex sugar called cellulose

· Note: some protistans and all fungi also have a cell wall

COMPOSITE ANIMAL CELL

COMPOSITE PLANT CELL

Some notes on microscopes:

Stereomicroscopes (dissecting)

Compound light microscopes

Scanning Electron Microscope (SEM)

Tunneling Electron Microscope (TEM)

Scanning-Tunneling Electron Microscope (STM)

Drawbacks on electron microscopes