Biology 2

Chapter 4: A Tour of the Cell

Light Microscope – works by passing light through a specimen.

Magnification – the increase in the apparent size of an object

Resolution – is a measure of the clarity of an object.

With a L.M., as magnification goes up, resolution goes down.

Electron Microscope – uses a beam of electrons instead of light. Much greater resolution, it can distinguish between structures as small as 2 nm

1. Scanning Electron Microscope(SEM) – uses an electron beam to scan the surface of a cell.

2. Transmission Electron Microscope (TEM) – is used to study the details of internal cellular structures.

However, electron microscopes cannot be used to study living specimens because the methods used to prepare the specimen kills the cells.

Cell Size

The maximum size of a cell is influenced by:

  • Requirement for enough surface area to obtain adequate nutrients and oxygen from the environment
  • The distance these materials must diffuse into the cell and waste products out.

Prokaryotic and Eukaryotic Cells

Things all cells have in common:

  • Bound by a plasma membrane
  • Have chromosomes made of DNA
  • Contain ribosomes
  • Cytoplasm

Prokaryotic Cells -- Bacteria and ArchaeaKingdoms

  • small, relatively simple cells – typically about one tenth the size of eukaryotic cells
  • lacks a nucleus -- the DNA is coiled in a region called the nucleoid but isn’t surrounded by a membrane.
  • Robosomes of prokaryotes are smaller and differ slightly in structure from eukaryotes. These molecular differences are the basis for the action of some antibiotics, which specifically target prokaryotic ribosomes and interrupt protein synthesis but not in eukaryotic cells of the user.
  • Capsule – a sticky, jellylike outer coating that helps “glue” them to rocks in fast moving streams or tissues within the body.
  • Pilli – also help attach prokaryotes to surfaces.
  • Flagella – propels the cell through liquid environments

Eukaryotic Cells – Animal, Plant, Protist and FungiKingdoms

  • The nucleus, with its nuclear membrane, multiple chromosomes, and nucleolus, is the most obvious difference between pro- and eu- karyotic cells
  • various membrane-bound organelles (“little organs”) that perform specific functions in the cell.

Structures and Organelles can be organized into four basic groups:

a. Manufacturing – nucleus, ribosomes, endoplasmic reticulum, and Golgi Apparatus

b. Breakdown or Hydrolysis – lysosomes, vacuoles, and peroxisomes

c. Energy Processing – mitochondira and chloroplasts

d. Structural Support, Movement, and Communication – cytoskeleton

Differences between Plant and Animal Cells:

  • Only animal cells have lysosomes, centrioles, cilia, and flagella (except for the sperm cells of some plants)
  • Only plant cells have a thick cell wall containing the polysaccharide cellulose, plasmodesmata (channels through the cell wall that connect adjacent cells), chloroplasts and a large central vacuole.

4.5 Structure of Membranes:

  • In all cells the plasma membrane forms a boundary between the living cell and its surroundings and controls what goes into and out of the cell.
  • Phospholipids are the main component of biological membranes. It has two distinct regions: a negatively charged (hydrophilic) phosphate head and two nonpolar (hydrophobic) fatty acid tails
  • Most membranes are composed of a two-layer sheet called a phospholipid bi-layer with the hydrophilic heads facing outward and their hydrophilic tails pointing inward
  • Embedded in this lipid bi-layer are proteins which selectively allow materials into and out of the cell.

4.6 The nucleus

  • Contains most of the cell’s DNA and controls the cell’s activities by directing protein synthesis.
  • The chromosomes are made up of thin threads called chromatin which is fine strands of DNA wrapped in protein.
  • The nuclear envelop encloses the nucleus. It is a double membrane with numerous protein-lined pores that control the flow of materials into and out of the nucleus.
  • The nucleolus is where ribosomal RNA is synthesized (made). Along with proteins brought in from the cytoplasm these subunits combine to make functioning ribosomes.

4.7 Ribosomes

  • Ribosomes carry out protein synthesis. Not all cells are created equal: cells found in organs that are active in making hormones, digestive enzymes, etc. have far more ribosomes than others.
  • Two types: Free ribosomes can be found suspended in the fluid of the cytoplasm while bound ribosomes are attached to the endoplasmic reticulum. Both are structurally identical and can alternate between the two locations.
  • However, free ribosomes tend to make proteins/enzymes that are used within the cell while bound proteins specialize in proteins that are exported out.

4.8 Endomembrane System

  • Within the cell there is a series of membranes that are physically connected to each other and directly transfer cellular products or transfer “works in progress” by enveloping it in a large section of the membrane and transporting it to another part of the system via small packages (called vesicles)
  • The Endomembrane System includes the nuclear envelop, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and the plasma membrane.

4.9 Endoplasmic Reticulum

  • The tubes and sacs of the ER enclose an area that is separate from the cytoplasmic fluid.
  • Two Types: Smooth ER – lacks ribosomes and Rough ER – has ribosomes attached to its outer surface.

Smooth ER – lack attached ribosomes

  • Important in the synthesis of lipids (fats) such as oils, phospholipids, and steroids.
  • The cells of the ovaries and testes have an over abundance of smooth ER due to the synthesis of steroid sex hormones.
  • Liver cells have a lot of smooth ER that help to break down drugs and harmful substances.

Rough E.R – has ribosomes attached to the outer surface of the membrane.

  1. Manufactures membranes. As the rough ER increases in size, so of the membrane is transferred to other members of the endomembrane system by vesicles.
  2. Bound ribosomes attached to the rough ER produce proteins that are usually secreted by the cell. Example: the protein insulin

Synthesis, Modification and Packaging of Proteins

  • As a long polypeptide chain is synthesized by a ribosome, it is threaded into the interior of the ER through a small pore.
  • As it enters, the new polypeptide chain folds three-dimensional into the protein structure it is destine to become.
  • Sugars attach to parts of the folding protein making the molecule a glycoprotein.
  • Now the protein is ready for export out of the ER. To do this it is packaged in a transport vesicle which buds off from the ER membrane and travels to the Golgi apparatus for further processing.
  • The Golgi apparatus consists of stacks of sacs. One side serves as a receiving dock for transport vesicles from the rough and smooth ER. Once inside, the products are further modified by enzymes as they move through.
  • The end products end up on the opposite side of the Golgi -- the shipping side – which gives rise to vesicles that bud off and
  1. Move to the plasma membrane and release the contents outside the cell.
  2. Become part of the plasma membrane (IE: plasma membrane proteins) along with the lipid bi-layer of the vesicle.
  3. Become part of another organelle such as enzymes (proteins) being added to a lysosome.

4.11 Lysosomes – are membrane sacs that contain enzymes that function in digestion and recycling within the cell.

  • FYI: both the membrane that forms the lysosome and the enzymes it contains were both produced by the rough ER

Lysosomes Have Several Types of Digestive Functions:

a)Food Vacuoles: They fuse with food vacuoles that enter the cell. Once attached the enzymes break down the food, releasing nutrients to the cell.

b)Destruction of Harmful Microbes: White blood cells ingest bacteria by encapsulating them into a vacuole as they enter the cell. Lysosomes attach to the vacuole and enzymes digest the bacteria.

c)Breakdown of dysfunctional organelles. First, the organelle is wrapped in a membrane vesicle. Next a lysosome fuses with it and enzymes dismantle it for pieces parts.

Peroxisome – an organelle containing enzymes that transfers hydrogen in harmful waste products to oxygen producing a toxic intermediate hydrogen peroxide and then through enzyme action further degrades it into water.

4.12 Vacuoles – cell organelle that stores materials such as water, salts, proteins, carbohydrates, and waste products.

4.14 Mitochondria – converts the chemical energy in food to make ATP that will supply the energy required to do work within the cell.

  • Enclosed by two phospholipid bilayers that create two internal compartments. The inner membrane is highly folded to greatly increase surface area.

Key Terms:

  1. Matrix – contents found within the inner membrane. Contains the mitochondrial DNA, ribosomes and numerous self made enzymes.
  2. Cristae – name given to the folds in the inner membrane.

4.15 Chloroplasts – organelle that performs photosynthesis in plants and some protists.

  • Absorbs sunlight and converts the solar energy into chemical energy by building glucose molecules from H2O and CO2
  • 6H20 + 6CO2 C6H12O6 + 02

Key Terms:

  1. Stroma – thick fluid inside the inner membrane that contains the chloroplast DNA, ribosomes and self produced enzymes.

Both Mitochondria and Chloroplasts

  • Have circular DNA strands similar to prokaryotes
  • Ribosomes that are closer in structure to prokaryotes than eukaryotes
  • Both reproduce by a splitting process similar to prokaryotes
  • Both are surrounded by a double membrane with the inner one very similar to the plasma membrane found in prokaryotes

This led to the Endosymbiosis Hypothesis that proposes both organelles were once small solitary prokaryotes that gained entry to the larger as undigested prey or possibly as a parasite.

4.17 Cytoskeleton – network of protein fibers in the cytoplasm of eukaryotic cells.

Types:

  1. Microfilaments – solid rods composed mainly of actin proteins that are arranged in a twisted double strand.
  • Forms a network of fibers just inside the plasma membrane that helps maintain the cell’s shape
  • The smallest of the three types
  1. Intermediate Filaments – made of several fibrous proteins and have a rope-like appearance.
  • Used to reinforce cell shape and anchor some organelles. Ex: nucleus
  1. Microtubules – straight, hollow tubes composed of tubulin proteins.
  • Shapes and supports the cell also acts as “tracks” which organelles (such as vacuoles and lysosomes) can move along.
  • Also, guides the movement of chromosomes during cellular division.
  • The main component of cilia and flagella to move cells through aqueous environments.

Skip 4.18 and 4.19

4.20 Extracellular Matrix

  • Helps hold cells together in tissues
  • Long strands of glycoproteins that protect and supports the plasma membrane
  • The most abundant glycoprotein found between the cells is collagen which forms into long fibers that give tissues strength and support

4.21 Neighboring cells in animal tissues often adhere, interact and communicate through specialized junctions between them.

Three Types of Cell Junctions:

a. Tight Junction – cells are tightly held together by a mesh or weave of proteins. This forms a continuous seal around the cells. Example: cells lining the digestive track

b. Anchoring Junctions – intermediate filaments fasten cells together in strong flexible sheets. Examples: major component of skin and heart tissue

c. Gap Junctions – channels that allow small molecules to flow through protein lined pores between neighboring cells. Example: flow of ions through heart tissue coordinates their contraction.

4.22 Cell Wall – protective layer outside the plasma membrane in plant cells, bacteria, fungi, and some protists.

  • Protects the cell and helps maintain its shape
  • In plants, it consists of cellulose fibers embedded in a matrix of sugars and proteins.
  • Plasmodesmata – channels through the cell walls that allow for the exchange of materials and communication between adjacent cells.
  • Note: the plasma membrane and the cytoplasm of adjacent cells extends through the plasmodesmata so water and other small molecules can readily pass from cell to cell.