GENERALIZED ANIMAL CELL

A cell is the basic, living, structural and functional unit of the body.

Cytology is the science concerned with the study of cells.

he principal parts of a cell are the:

Cell (plasma ) membrane - separates inside of cell from external environment.

Nucleus - contains the cell’s DNA and nucleolus.

Cytoplasm -contents of the cell between the nuclear envelope and cell membrane.

cytosol - contains proteins, enzymes, nutrients, ions, and other small molecules.

organelles - highly organized structures with characteristic shapes that are specialized for specific cellular activities.

inclusions - are temporary structures in the cytoplasm that contain secretions and storage products of the cell.

Cell Membrane Structure:

Made primarily of phospholipids and proteins - fluid mosaic model

Lipids:

Phospholipids - form the basic structure of the membrane (bilayer)

Glycolipids - the target of certain bacterial toxins

are important for adhesion among cells

may mediate cell-to-cell recognition and communication

contribute to regulation of cell growth and development.

Cholesterol - strengthen the membrane but decrease flexibility

Proteins:

Determine to a large extent what the cell can do.

Integral proteins : Extend from one side of the membrane to the other.

Channels

Transporters

Receptors

 

Peripheral proteins: loosely attached to the inside or outside of the cell.

Enzymes

Cell identity markers

Cell Membrane Function:

cellular communication

electrochemical gradient - resulting in a membrane potential

selective permeability -

lipid solubility - nonpolar, hydrophobic molecules can pass size - only a few, small uncharged polar molecules can pass electrical charges - these prevent passage

the presence of channels and transporters

 

MOVEMENT OF MATERIALS ACROSS PLASMA MEMBRANES

Passive Transport : depends on concentration and kinetic energy

Diffusion

Facilitated Diffusion

Osmosis

Osmotic Pressure

Tonicity

Isotonic

Hypertonic - crenation

Hypotonic - hemolysis

Active Transport:

Primary Active Transport

Secondary active transport

symports or cotransport

antiports or countertransport

 

Vesicular transport

Endocytosis

Pinocytosis

Phagocytosis

Receptor-Mediated Endocytosis

Exocytosis

Transcytosis

Organelles

    Endoplasmic reticulum

        Smooth E R - Site of lipid synthesis

                Detoxification of chemicals

                Sarcoplasmic reticulum holds calcium for muscle contractions

        Rough E R:

                Rough due to the presence of ribosomes

                Site of protein sythesis - primarily for export

                Proteins are then sent to the Golgi apparatus for processing

Ribosomes

free in cytoplasm or on E R

made of ribosomal RNA and proteins

"Workbenches" where proteins are put together

Golgi Apparatus

Made of flattened membranous sacs or cisterns

Processes proteins and lipids made in the E R

Forms vesicles for transport of proteins

Forms lysosomes and peroxisomes

Lysosomes

Tiny, membrane sacs containing digestive enzymes

Function in:
    intracellular digestion(phagocytosis, pinocytosis and receptor- mediated endocytosis)

    digestion of worn-out organelles (autophagy), digestion of cellular contents (autolysis) after death and in some pathological conditions

 

extracellular digestion of debris at infection site

Peroxisomes
Contain peroxidases which oxidize organic substances and produce H2O2
 

Also contain catalase :

Detoxify alcohol and other organic substances

Synthesize bile salts

Breakdown very long chain fatty acids

Mitochondria

Double membrane - inner membrane has folds called cristae

Cellular respiration - requires oxygen

Most of the cell’s ATP is produced by these "powerhouses"

Have unique DNA and RNA’s

Cytoskeleton

Microfilaments - made of actin - aid in movement

Microtubules - made of tubulin - rigid, helps maintain cell shape

Intermediate filaments - many proteins - structural reinforcement

Centrioles - sets of microtubules at right angles

Form spindle fibers during cell division

Found in cilia and flagella

 

Cilia

Short, numerous hairlike projections

Contain a modified centriole called a basal body

Flagella

Single, long projection - found only in sperm in human

Vesicles

Membrane sacs used for transport within the cell.

Inclusions

Substances produced by the cell NOT surrounded by a membrane

Usually temporary

Glycogen, lipids, melanin

Nucleus

Enclosed in a double membrane called the nuclear envelope

Pores in the nuclear envelope allow for large molecules to leave

Contains nucleoplasm.

Chromatin - DNA and histones- chromosomes during cell division

Nucleolus - site of ribosome production

 

CELL DIVISON

Division of the cytoplasm - Cytokinesis

Nuclear division

mitosis - makes identical copies of somatic cells

meiosis - makes gametes (eggs or sperm) for reproduction

Homologous chromosomes - "matching" chromosomes or members of a pair.

One homolog is received from each parent, and homologous chromosomes carry the same basic information, or genes. The genes may code for different forms of the same trait.

Somatic cells contain 23 pairs of chromosomes or 46 chromosomes

The Cell Cycle

Interphase - "resting stage" - cells are carrying on growth and all the normal functions of the cell.

G1 - Gap 1 or growth phase

S phase - DNA synthesis occurs during this phase.

DNA replication is semiconservative - each half makes a new strand.

Once DNA is duplicated, the cell is committed to division.

G2 - Gap 2

Mitosis - actually one continuous process that we describe in stages:

Prophase :

Chromosomes condense and become visible

Sister chromatids are held together by a centromere

Nucleoli disappear

Mitotic spindle forms

Prometaphase : or late prophase

Nuclear envelope disappears

Spindle fibers attach to kinetochores - protein complexes attached to centromeres.

Metaphase :

Chromosomes line up in the center of the cell at the metaphase plate.

Anaphase :

the centromere of each chromatid pair splits, and the microtubules shorten and pull one of each duplicated chromosome to opposite ends of the cell ensuring each new cell gets a copy of each chromosome.

Telophase :

the mitotic spindle disappear

new nuclear membranes form

the nucleoli can be seen again

the chromosomes unwind

Cytokinesis:

Begins in anaphase and finishing after telophase

In anaphase, cells form a contractile ring perpendicular to the axis of the mitotic spindle.

This ring pinches the cell into two, using some of the same proteins used in muscle tissue.

Cell Differentiation - the process where a cell changes from an unspecialized cell to a cell suited for a specific function.

Control of Cell Division

Cells have a "mitotic clock" - they reproduce a certain number of times, then stop

Apoptosis - programmed cell death - DNA breaks up, cytoplasm shrinks, cell is phagocytized (vs. Necrosis)

Controlled by intracellular levels of kinases and cyclins

Cell size

Hormones

Growth factors

Contact inhibition

Loss of Control of Cell Division

Tumor formation

Benign - remains localized

Malignant (cancerous) - spreads into surrounding tissue or metastasizes

Oncogenes - activate other genes which increase cell reproduction rate

Tumor suppressor genes - control the rate of cell reproduction

Genes may be damaged by:

toxic chemicals

radiation ( including sunlight)

natural mutations

viruses