Chapter 7: A View of the Cell

7.1. The Discovery of Cells

The Cell Theory (History)• The microscope was

invented by Anton van Leeuwenhoek.

• The first person to see a cell (in cork) was Robert Hooke.

• Matthias Schleiden concluded that all plants have cells

• Theodore Schwann observed that animals were also composed of cells

The Cell Theory• 3 main ideas:

– All living things are composed of one or more cells

– The Cell is the basic unit of organization of organisms

– All cells come from cells

The Light Microscope• Uses light and lenses• The Simple light

Microscope used one lens and natural light (Leeuwenhoek)

• The Compound light microscope: Uses multiple lenses– Magnifies up to 1500

times

The Electron Microscope• Invented in the 1940s• Uses a beam of electrons• Magnifies up to 500,000

times• Two Kinds:

– Scanning electron microscope (SEM): Scans the surface of cells.

– Transmission electron microscope (TEM): Allows for study of structures inside cells.

The Electron Microscope

SEM TEM

Two Basic Cell Types• Prokaryotes: Cells lacking internal membrane-bound

structures• Eukaryotes: Cells containing internal membrane-

bound structures– The membrane-bound structures are called organelles– Contains a nucleus: organelle that manages cellular function.

First observed by Robert Brown. Rudolf Virchow concluded that it was responsible for cell division.

Chapter 7: A View of the Cell

7.2 The Plasma Membrane

Maintaining a Balance• The Plasma membrane is the boundary

between the cell and it’s environment• Needs to let the good stuff (e.g. nutrients) in

and the bad stuff (waste) out• The plasma membrane maintains

homeostasis.

Plasma Membrane

The Plasma Membrane• Maintains Homeostasis: regulates

internal environment [Good in (but not too much), Bad Out]

• Selective permeability: Allows some molecules into the cell and keeps some out.

• Some molecules can cross the plasma membrane (i.e. water). Others must go through channels (i.e. Na, Ca, etc)

Structure of the Plasma Membrane• Composed of a

phospholipid bilayer.– A Lipid with a phosphate

group attached– Has only 2 fatty acid

tails– Forms a sandwich

• The phosphate group forms the polar head

• The fatty acid tails form the nonpolar tail

Fluid Mosaic Model• The membrane is fluid: It is flexible and

phospholipids can move in the membrane like water in a lake.

• The membrane is mosaic: There are proteins embedded in the membrane that also move (like boats in the lake)

Other components• Cholesterol: Helps stabilize the plasma membrane,

and prevents the phospholipids from sticking together.

• Transport Proteins: Proteins that span the entire membrane and form channels for specific molecules to enter and leave (like a door).

• Other Proteins and carbohydrates on the external surface: Helps with identification.

• Proteins on internal surface: Provides flexibility by attaching the plasma membrane to the cell’s internal structure.

Chapter 7: A View of The Cell

7.3 Eukaryotic Cell Structure

Cellular Boundaries• Plasma membrane

surrounds the cell• In plants, fungi, most

bacteria and some protists, the cell wall surrounds the plasma membrane– Fairly rigid– Provides support and

protection– Made up of the

carbohydrate cellulose– Has pores to allow

molecules through

Nucleus and cell control• The Nucleus is the

leader of the cells– Gives directions for

the making of proteins

• The master set of directions is in chromatin

• During cell division, chromatin condenses to form chromosomes.

Nucleus and Cell Control• Inside the nucleus there is also the

nucleolus– Makes ribosomes

• Ribosomes are sites where proteins and other enzymes are made, according to instructions from DNA

• Ribosomes leave the nucleus, into the cytoplasm in order to make proteins

• Cytoplasm: The fluid inside the cell

Nucleus and cell control• The Nuclear

envelope is a double membrane that surrounds the nucleus.– Made up of 2

phospholipid bilayers– Contains small

nuclear pores

Assembly and Transport• The endoplasmic reticulum: A

series of highly folded membranes– Where cellular chemical

reactions take place– Like a large workspace

• Some parts have ribosomes attached (rough endoplasmic reticulum - RER)

• Others don’t (smooth endoplasmic reticulum – SER)

Assembly and Transport• RER: Proteins made

in the RER may:– form part of the

plasma membrane– be released from the

cell– transported to other

organelles

• SER: involved in production and storage of lipids.

Assembly and Transport• The Golgi apparatus:

flattened system of tubular membranes and vessicles– Modifies proteins– Sorts and packages

proteins

• It’s kind of like the post office: Sorts the mail and sends it to the right place

Assembly and Transport

RER Golgi apparartus vessicles

Vacuoles• A vacuole is a sac surrounded by

membrane• Used for temporary storage of

– Food– Enzymes– Waste

• Plant cells usually have one large vacuole, animal cells have many smaller ones

Lysosomes and recycling• Lysosomes are organelles

that contain digestive enzymes– They digest food particles,

organelles and engulfed viruses or bacteria

• Can fuse with vacuoles and digest the contents.

• Can also digest cells that contain them.– i.e. tadpole’s tail

Energy Transformers• For all the cellular processes to happen,

energy is needed• Two organelles provide that energy:

– Choloroplasts (in plants)– Mitochondria (in animals and plants)

Chloroplasts• Chloroplasts are

organelles that captures light energy and produces food to store for later

• Has a double membrane (like the nucleus)

• The inner membrane folds in to form stacks of membranous sacs called grana/thylakoids.

Chloroplasts• In the thylakoid

membrane there is the green pigment called Chlorophyll– Traps light energy– Gives leaves and

stems their green color

Mitochondria• Mitochondria

produces energy in a form that can be used by the cell when necessary.

• Has an outer membrane and a highly folded inner membrane.– Provides large surface

area.

Structures for Support and Locomotion• Cytoskeleton: forms the

framework of the cell– Maintains shape– Composed of:

• Microtubules: thin hollow cylinders made of protein

• Microfilaments: thin, solid protein fibers

Structures for Support and Locomotion• Cilia and flagella : Structures that aid

in locomotion and feeding.– Composed of pairs of microtubules,

with a central pair surrounded by 9 additional pairs.

– Cilia are short, numerous, hair-like projections that move in a wavelike motion

– Flagella are longer projections, move in a whip-like motion.