The Cell and Cell Processes

Beth

CytologyThe study of the structure and function of cells.

Structures Functions

• Cell Membrane• Cytosol• Cytoskeleton• Microvilli• Cilia• Centrioles• Ribosomes• Mitochondria• Nucleus• Nucleolus• Rough Endoplasmic Reticulum• Smooth Endoplasmic Reticulum• Golgi Apparatus• Lysosomes• Peroxisomes• Nuclear Pores

• Isolation and protection• Distributes materials by diffusion• Internal Support• Absorption of extracellular material• Movement of materials over surfaces• Movement of chromosomes during cell division• Protein Synthesis• Produce 95% of the ATP required by the cell• Control of metabolism• Site of RNA synthesis• Secretory protein synthesis• Lipid and carbohydrate synthesis• Storage, alteration, and packaging of secretory products

and lysosomes• Intracellular removal of damaged organelles or pathogens• Neutralization of toxic compounds• Allow the movement of ions and small molecules

Cell Theory1. Cells are the building blocks of all plants and

animals.2. Cells are produced by the division of

preexisting cells.3. Cells are the smallest units that perform all

vital physiological functions.4. Each cell maintains homeostasis at the

cellular level.5. Homeostasis at the tissue, organ, system, and

individual levels reflects the combined and coordinated actions of many cells.

Two most common methods used to study cell and tissue structure: •Light Electron Microscopy

•Scanning Electron Microscopy

Light Microscopy

Can magnify cellular structures about 1,000 times and show details as fine as 0.25 µm (micrometer or micron). With a light microscope one can identify cell types and see large intracellular structures.

Electron Microscopy

Electrons pass through an ultrathin section to strike a photographic plate. Transmission Electron Microscopy shows the fine structure of cell membranes and intracellular structures.

Electrons bouncing off exposed surfaces create a scanning electron micrograph. Although Scanning Electron Microscopy provides less magnification, it provides a three-dimensional perspective on cell structure.

Transmission Electron Microscopy Scanning Electron Microscopy

The Study of Cell Structure

THE CELL

Cytoplasm

Organelles

Membranous Organelles

Non-Membranous Organelles

Cytosol

Cell Membrane

Non-membranous Organelles

• Cytoskeleton• Microvilli• Centrioles• Cilia• Flagella• Ribosomes

Membranous Organelles

• Mitochondria• Nucleus• Endoplasmic

Reticulum• Golgi

Apparatus• Lysosomes• Peroxisomes

The permeability of a membrane is a property that

determines its effectiveness as a barrier.

If nothing can cross a membrane, it is described as

impermeable.

If any substance can cross without difficulty, the

membrane is freely permeable.

Cell membranes fall somewhere in between and are

thus said to be selectively permeable.

The net movement of material from an area where its concentration is relatively high to an area where its concentration is relatively low.

The difference between the high and low concentrations represents a concentration gradient and diffusion takes place until that gradient has been eliminated.

Diffusion

Important factors that influence Diffusion rates:Distance- The

greater the distance, the longer the time required.

Size of Gradient- The larger the concentration gradient, the faster diffusion proceeds.

Molecular Size- Ions and small organic molecules such as glucose diffuse faster than large proteins.

Temperature- The higher the temperature, the faster the diffusion rate.

Diffusion (continued)Three major factors determine whether or

not a substance can diffuse across a cell membrane.

These major factors are lipid solubility, channel size, and electrical interactions.

Osmosis

Three characteristics of osmosis should be remembered:

1. Osmosis is the diffusion of water molecules across a membrane.

2. Osmosis occurs across a selectively permeable membrane that is freely permeable to water but not freely permeable to solutes.

3. In osmosis water will flow across a membrane toward the solution that has the highest concentration of solutes, because that is where the concentration of water is lowest.

The movement of water across a semipermeable membrane toward a solution containing a relatively high solute concentration.

Osmosis (continued)Isotonic is one that has the same solute

concentration as does the cytoplasm.

Hypotonic solution has a solute concentration lower than that of the cytoplasm.

Hypertonic solution has a solute concentration higher than that of the cytoplasm.

Facilitated Diffusion

In this process an extracellular molecule, such as

glucose, binds to a receptor site on a carrier protein.

The binding alters

the shape of the

protein, which then

releases the

molecule to diffuse

into the cytoplasm.

Receptor-Mediated Endocytosis

• Resembles pinocytosis, but is far more selective.

• Produces vesicles that contain a specific target molecule in high concentrations.

• Begins when materials in the extracellular fluid bind to receptors on the membrane surface.

Filtration• Movement of water, usually with solute, by

hydrostatic pressure; requires membrane filter.

Phagocytosis

• Produces vesicles containing solid objects that may be as large as the cell itself.

Active Active Transport Endocytosis

• Carrier molecules work regardless of any concentration gradients.

• Creation of vesicles containing fluid or solid material.

Cell Attachments

• Gap Junctions- Two cells are held together by an interlocking of membrane proteins.

• Tight Junctions- Partial fusion of the lipid portions of the two cell membranes.

CELL ATTACHMENTS (CONTINUED)

Intermediate Junctions- The opposing cell membranes, while remaining distinct, are held together by a thick layer of proteoglycans.

Desmosomes- There is a very thin proteoglycan layer between the opposing cell membranes.

Junctional Complexes Hold together the cells

lining passageways such as the digestive tract and respiratory tract.

Interphase

G1 Phase- the cell manufactures enough mitochondria, centrioles, cytoskeleton elements, endoplasmic reticulum, ribosomes, Golgi bodies, and cytosol to make 2 functional cells.

G0 Phase- the cell is not preparing for mitosis.

• S Phase- over the next 6-8 hours the cell duplicates itself.

Interphase(continued)

Gm Phase- begins mitosis.

G2 Phase- the cell does last minute protein synthesis.

MITOSIS4 stages

Begins when the chromosomes coil so tightly that they become visible as individual structures. As a result of DNA replication during the S phase, there are 2 copies of each chromosome, called chromatids, connected at a single point, the centromere.

As the chromosomes appear, the 2 pairs of centrioles move apart. Spindle fibers extend between the centriole pairs; smaller microtubules radiate into the disappearance of the nuclear envelope.

Stage 1:Prophase

The chromatids now move to a narrow central zone called the metaphase plate. A microtubule of the spindle apparatus attaches to each centromere.

Stage 2: Metaphase

As if responding to a single command, the chromatid pairs separate and the

daughter chromosomes move toward opposite ends of the cell.

Stage 3: Anaphase

This stage is in many ways the reverse of prophase, the nuclear membranes form, the nuclei enlarge, and the chromosomes gradually uncoil. Once the chromosomes disappear, nucleoli reappear and the nuclei resemble those of interphase cells.

Stage 4:Telophase

Is the separation of the two daughter cells. It usually begins in late Anaphase and continues through Telophase. The completion of cytokinesis marks the end

of cell division.

Cytokinesis