Chapter 7

A View of the Cell

Cell Theory

•      Before microscopes, people thought diseases were caused by curses and supernatural spirits

–  They had no idea microorganisms, such as bacteria and viruses existed

•      When the microscope was invented, scientists discovered cells exist

•      Cells are the basic units of living organisms

Cell Theory

•      Anton van Leeuwenhoek (Dutch) used the first simple light microscope in the 1600s

–  It had one lens (page 1064)

•      Gradually, lenses got better

•      Compound light microscopes use a series of lenses to magnify objects in steps

–  Can magnify up to 1500 times

–  The microscopes we use in the lab

 

Cell Theory

•      Robert Hooke (1600s, English) used a compound light microscope to study cork, the dead cells of oak bark

•      He saw small, geometric shapes that reminded him of the small rooms monks lived in called cells

•      Cells are the basic units of all living things

Cell Theory

•      Matthias Schleiden (1830s, German) discovered that all plants are made of cells

•      Theodor Schwann (German) discovered that all animals are made of cells

•      Rudolf Virchow discovered that all cells come from existing cells

•      All these scientists (plus others) contributed ideas that are now summed up in what we call the cell theory

Cell Theory

•             All organisms are composed of one or more cells.

•             The cell is the basic unit of structure and organization of organisms.

•             All cells come from preexisting cells.

Electron Microscopes

•      Electron Microscopes let us see a much greater magnification than light microscopes

–  They use beams of electrons to produce images

•      Scanning Electron Microscope

–  Used for scanning 3-D surfaces

Electron Microscopes

 

•      Transmission Electron Microscope

–  Used for scanning interior structures of cell

Types of Cells

•      Organelles are small, specialized structures within cells

–   Many, but not all are surrounded by membranes

•      Prokaryotes are organisms that do not have membrane-bound organelles  (biology.arizona.edu)

–   Kingdom Monera ( now split into Eubacteria & Archaebacteria)

•      Eukaryotes are organisms that do have membrane-bound organelles

–   Kingdoms Animalae, Plantae, Fungi, Protista

Types of Cells

•      Robert Brown (Scottish) observed that eukaryotic cells have a prominent structure, the nucleus

–  the nucleus manages cellular functions

 

•      Rudolf Virchow concluded that this prominent structure was responsible for cell division

The Plasma Membrane

•      Cells must maintain proper conditions within itself to function

•      The plasma membrane is a flexible boundary between the cell and its environment

–   It allows a steady supply of nutrients into and out of the cell at the appropriate levels

•      The plasma membrane is selectively permeable

–   This means that it allows some molecules to pass through while keeping others out.

–   Like a screen in a window

The Plasma Membrane

•      Phospholipids

•      The “head” is polar, or slightly charged

–   It is hydrophilic (likes water)

•      The “tail” is nonpolar, or not charged

–   It is hydrophobic (repels water)

•      The phospholipids arrange themselves form a bi-layer with the fatty acid  tails forming the interior  and the heads facing the  watery environments  outside and inside the cell

 

Plasma Membrane

•      The plasma membrane is called a  phospholipid bilayer

•      Arranged in this manner, a barrier is created that is water-soluble at its outer surfaces and water-insoluble in the middle

•      Keeps out both   water soluble  and water  insoluble  molecules

Plasma Membrane

•      The current model of the plasma membrane is called the fluid mosaic model.

•      The phospholipids actually move around like a fluid.

•      The other components of the membrane (proteins, carbohydrates, cholesterol, etc.) move around as well

 

Plasma Membrane

•      Cholesterol helps to stabilize the phospholipids by preventing their fatty acid tails from sticking together

•      Transport proteins move span the membrane and move needed substances or waste materials through the plasma membrane

•      Other proteins and carbohydrates that stick out help cells to identify surface signals and other cells

–   These proteins play an important part in protecting cells from infection

Cell Wall

•      Fairly rigid structure located outside the plasma membrane of some cells that provides additional support and protection

–  Plants, fungus, most bacteria

•      The cell wall is very porous, made of a tough mesh of fibers

–  Like framing of a house

Nucleus

•      Membrane-bound (called nuclear envelope) organelle that contains the directions to make proteins

•      Nuclear pores allow passage through the nuclear envelope

•      Chromatin - uncondensed strands of DNA

–   When cell is dividing, DNA condenses into chromosomes

•      Nucleolus - organelle within the nucleus that make ribosomes

 

Organelles

•      Ribosomes are the sites where the cell produces proteins according to the directions of DNA

–  One organelle without a membrane

•      Cytoplasm is the clear, gelatinous fluid inside the cell

–  Acts as a medium for things to move around in the cell

–  “Organelles can’t fly”

Organelles

•      The endoplasmic reticulum (ER) is arranged in a series of highly folded membranes suspended in the cytoplasm

•      In general, ER is involved in the movement of materials throughout the cell

•      Rough ER has ribosomes, which is where protein synthesis takes place

•      Smooth ER has no ribosomes and is involved in numberous biochemical activities, including the production and storage of lipids

Organelles

•      The Golgi apparatus is a flattened stack of tubular membranes that modifies the proteins

•      The Golgi sorts proteins into packages and packs them into membrane-bound structures called vescicles for later transport (like UPS)

Organelles

•      Vacuoles are membrane-bound compartments for temporary storage of materials

–   Store food, enzymes, water, waste

–   Plant cells have one very large vacuole

 

Organelles

•      Lysosomes are organelles that contain digestive enzymes

•      Digest old organelles, food, viruses, bacteria, etc.

•      Can fuse with a vacuole to dump waste or give enzymes

•      Lysosomes digest a tadpole’s tail

 

Organelles

•      Plastids are a group of organelles used for storage

–   Named for the color or pigment they contain

•      Chloroplasts are organelles that capture light energy and convert it to chemical energy (photosynthesis occurs here)

–   Plastid containing chlorophyll, which actually traps the light and gives plants green color

Organelles

•      Mitochondria are membrane-bound organelles in plant and animal cells that transform stored chemical energy into a usable form for the cell (ATP)

•      Has an outer and inner highly folded membrane

–   ATP produced on inner folds

Organelles

•      The cytoskeleton forms a framework for the cell

–  A network of tiny rods and filaments

•      Cytoskeleton is constantly changing its shape

•      Microtubules are thin, hollow cylinders made of protein

•      Microfilaments are smaller, solid protein fibers 

•      Give support for cell

•      Provide “highway system” for organelles to move throughout cell

 

Organelles

•      Centrioles are organelles made up of microtubules that play a part in cell division

–   In animals and most protists

•      Cilia are short, numberous projections that look like hairs

–   Made of microtubules and help organelles move and feed

•      Flagella are longer projections that move in a whip-like motion

–   Made of microtublules - help with movement

–   Cells usually only have 1 or 2