Lecture Notes - Chapter 1 - Concordia College



Lecture Notes - Chapter 5

Homework - Review questions 1, 4, 8, 10

I. An Overview of Cell Structure

All cells share a similar overall organization.

A. The Plasma Membrane Surrounds the Cell

- cell is enclosed by a phospholipid bilayer.

- proteins embedded in the layer are responsible for cell’s ability to interact with the environment.

B. The Central Portion of the Cell Contains the Genetic Material

- Nucleoid –

- Nucleus –

C. The Cytoplasm Comprises the Rest of the Cell’s Interior -

- semifluid matrix which completely fills the space between the nucleus and the plasma membrane

- contains the components, molecules, proteins needed to sustain life processes.

II. Most Cells Are Very Small

A. The Cell Theory

- describes the importance of cells to life, and includes 3 principles:

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

2) Cells are the smallest living things, the basic units of organization of all organisms.

3) Cells arise only by division of a previously existing cell.

B. Why Aren’t Cells Larger? –

- Cell size influences the speed at which materials are able to diffuse between the plasma membrane and the center of the cell.

- Smaller cells have a greater surface-to-volume ratio, allowing for better control of cell function relative to the interaction with the environment.

III. The Structure of Simple Cells: Bacteria

Membrane-bound cells which are surrounded by a rigid cell wall and contain no distinct interior compartments.

A. Strong Cell Walls

- carbohydrate matrix cross-linked by short polypeptides

- Gram-positive vs. gram-negative

B. Simple Interior Organization

- few or no membrane-bound internal compartments or organelles

- whole bacterium operates as a single unit.

C. Rotating Flagella

- locomotion and feeding

IV. The Structure of Eukaryotic Cells: An Overview

All have membrane-bound organelles to compartmentalize processes occurring in the cell. Most organelles are found in both plant and animal eukaryotic cells.

V. The Nucleus: Information Center for the Cell

Largest “organelle”, usually located near the center.

Contains all the genetic information responsible for the activities carried out by the cell.

A. Getting In and Out: The Nuclear Envelope

- composed of two phospholipid bilayer membranes.

- Outer membrane is continuous with the e.r.

- membranes are fused together in several areas to form nuclear pores.

B. The Chromosomes of Eukaryotes are Complex

- chromosomes – provide compact packaging of DNA for cell division

- chromatin – loosely packed DNA

VI. Proteins Are Synthesized on Ribosomes -

Ribosomes read/translate mRNA (copy of a gene from the DNA) to synthesize proteins.

Made of rRNA and a complex of many proteins.

Ribosomal assembly area is easily visible within the nucleus, in dark-staining regions called nucleoli.

The nucleolus is not membrane-bound.

VII. The Endoplasmic Reticulum: Compartmentalization of the Cell

Composed of very thin, interconnected membranes (lipid bilayers)

Functions to divide the cell into compartments, assist the transport of molecules through the interior of the cell, and provide surfaces on which the enzymes may act.

A. Rough E.R.: Manufacturer of Proteins for Export

- Studded with ribosomes

- Most of the proteins synthesized by the rough e.r. are eventually destined for export from the cell. Pass through the lumen of the e.r. for processing.

- Proteins synthesized by free ribosomes may also need to be processed and packaged.

- The signal hypothesis - as a new protein is being made by a free ribosome, a short amino acid sequence (signal sequence) at the tip of the protein recognizes a “docking site” on the surface of the e.r. As the protein continues to be synthesized, it passes into the interior of the e.r. When synthesis is complete, the protein can pass through the e.r. to the Golgi complex, where it can be processed and packaged for use or export from the cell.

B. Smooth E.R. - Organizer of Internal Activities

- contains few to no bound ribosomes.

- contains many enzymes which are involved in the synthesis of carbohydrates and lipids, and in certain organs (liver in particular) are capable of drug detoxification.

- Acts as a passageway for some processed proteins originating in the rough e.r.

VIII. The Golgi Complex: The Delivery System of the Cell

Golgi bodies - flattened stacks of membranes. Collectively - Golgi complex.

Functions to package and distribute the proteins and lipids synthesized on the e.r. Modifications which may occur include the addition of short sugar chains to form glycoproteins and glycolipids.

The molecules collect in areas of the Golgi body know as the cisternae. The membranes pinch off, surrounding the glycoproteins or glycolipids and forming transport vesicles known as liposomes which deliver the molecules to their appropriate destinations, such as the plasma membrane.

IX. Lysosomes: Producers of Digestive Enzymes For the Cell

Small membrane-bound sacs that contain the digestive enzymes of the cell, and assist in the breakdown of proteins, nucleic acids, lipids, and carbohydrates.

Also break down particles and foreign materials brought into the cell by phagocytosis, as well as old organelles, and recycling the components to create new organelles for the cell.

Lysosomes that are not functioning actively are called primary lysosomes. When the enzymes of a primary lysosome are activated (by fusion with a vesicle, etc.), it becomes a secondary lysosome.

X. Peroxisomes: Detoxifiers of Hydrogen Peroxide

Small membrane-bound sacs that contain the enzymes responsible for many of the oxidative reactions within the cell.

Hydrogen peroxide is prevalent in eukaryotic cells as a by-product of the oxidative activities of the peroxisomes; although it is dangerous to cells, the peroxisomes are able to enzymatically break it down to water and oxygen.

XI. Mitochondria: The Cell’s Chemical Furnaces

Bounded by two membranes (each consisting of a phospholipid bilayer).

The outer mitochondrial membrane is smooth, the inner mitochondrial membrane is organized into numerous folds called cristae.

Space between the two membranes is called the intermembranous space.

Area inside the inner membrane is known as the matrix.

Proteins embedded on and within the inner membrane are responsible for oxidative metabolism, the process by which the cell produces energy in the form of ATP.

Mitochondria contain their own DNA and are therefore capable of self-replication, though this process is tied to the presence of proteins encoded for by the DNA of the nucleus and translated by cytoplasmic ribosomes.

XII. Centrioles: Microtubule Assembly Plants

Occur in pairs located at right angles to each other near the nuclear membranes, and together form a centrosome.

Involved in the assembly of microtubules, as well as moving the chromosomes during cell division, influencing cell shape, and moving flagella and cilia (via actions of microtubules).

XII. The Cytoskeleton: Interior Framework of the Cell

Network of protein fibers within cells.

The fibers are constantly formed and disassembled by polymerization, where identical protein subunits spontaneously assemble into long chains, or are removed subunit by subunit. There are 3 types of cytoskeletal fibers present in plants and animals:

A. Actin filaments –

- long fibers about 7 nm in diameter composed of 2 protein strands loosely twisted around one another.

- strands are made of many globular proteins called actin.

- function in cellular movements such as contraction, crawling, “pinching” during division, and formation of cellular extensions.

B. Microtubules –

- hollow tubes about 25 nm in diameter, composed of a ring of 13 protofilaments, which are each composed of heterodimers of alpha and beta tubulin subunits.

- form from the centrosome near the center of the cell and radiate out toward the periphery.

- almost constantly polymerized and depolymerized.

- allow for cell movement (cilia, flagella), and are responsible for movements of materials within the cell itself (chromosomes, organelles).

- Two specific proteins which assist in this movement are kinesin (moves organelles toward cell periphery) and dynein (moves organelles toward center of cell).

C. Intermediate filaments –

- tough, fibrous protein fibers, 8-10 nm in diameter

- generally stable and are not broken down frequently.

- several types of intermediate filaments, including vimentin (important in structural support), keratin (present in epithelial cells, as well as hair and nails), and neurofilaments (intermediate filaments of the nervous system).

- Along with actin filaments, they provide mechanical support to the membrane and prevent stretching of the cell.

XIII. Flagella and Cilia: Motility for the Cell

A. Flagella

- consist of a circle of nine microtubule pairs surrounding two central microtubules - referred to as the 9 + 2 structure.

- pairs of microtubules move past one another which causes the entire flagellum to undulate rather than rotate.

- flagella are outward projections of the cell - they contain cytoplasm and are enclosed by the plasma membrane.

- the basal body is located just below the point at which the flagellum protrudes from the surface of the cell, and it is from this structure that the flagellum is derived.

B. Cilia

- also composed of the 9 + 2 arrangements of microtubules

- often arranged in rows along the cell surface, and are much shorter and more numerous than flagella.

- responsible for the movement of water or materials along the cell surface. They also function in sensory perception (hearing).

XIII. Plant Cell Specializations

A. Chloroplasts: Where Photosynthesis Takes Place

These are found primarily in the photosynthetic cells of plants and algae, and give these organisms the unique ability to manufacture their own food. Similar to the mitochondria, chloroplasts have 2 membranes, as well as a closed compartment of stacked membranes called grana that lie inside the internal membrane. The grana contain thylakoids, which contain the photosynthetic pigments capable of capturing light. Chloroplasts contain DNA, but like mitochondria, are unable to replicate without several components located in the nucleus. Chloroplasts that are deprived of light for long periods of time are called leucoplasts, and many function primarily as sites of starch storage (amyloplasts).

B. Central Vacuole

A central, membrane-bound sac that is the water-storage area for the plant cell. Also contains sugars, ions, pigments.

C. Cell Wall

A protective structure comprised of cellulose (a fibrous, rigid polysaccharide).

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