Cell Physiology

Cell:

Basic living unit of structure & function of the body. > 100 trillion cells in body. Very small (10-5 m in diameter). Highly organized. Variety of shapes & sizes. Each type of cells has a special function.

Cell (continued)

All Cells share certain characteristics:

General cell structure & components. General mechanisms for changing nutrients to Energy. Deliver end products into their surrounding fluid. Almost all have the ability to reproduce.

General Cell structure:

The cell has two major compartments: the nucleus & the cytoplasm.

The cytoplasm contains the major cell organelles & a fluid called cytosol.

3 principal parts: Plasma (cell) membrane. Cytoplasm & organelles. Nucleus.

General Cell Structure & FunctionComponentStructureFunction

Plasma (cell) membrane

Membrane composed of double layer of phospholipids in which proteins are embedded

Surrounds, holds cell together & gives its form; controls passage of materials into & out of cell

CytoplasmFluid, jellylike substance b/w cell membrane & nucleus in which organelles are suspended

Serves as matrix substance in which chemical reactions occur.

Nucleus:- Nuclear

envelope

- Nucleolus

- Chromatin

Double-layered membrane that surrounds nucleus, composed of protein & lipid molecules

Dense nonmembranous mass composed of protein & RNA molecules

Fibrous strands composed of protein & DNA

Supports nucleus & controls passage of materials b/w nucleus & cytoplasm

Produces ribosomal RNA for ribosomes

Contains genetic code that determines which proteins (including enzymes) will be manufactured by the cell

Plasma (cell) membrane

Plasma membrane:

Surrounds, holds cell together & gives its form. 75 to 111 Å thickness. Not solid. Separates cell’s internal structures from extracellular

environment. Is selectively permeable, & controls passage of

materials into & out of cell. Participates in intracellular communication.

Plasma (Cell) Membrane

Composed of:

Double layer of phospholipids (hydrophobic/ hydrophilic parts).

Proteins span, or partially span the membrane.

Negatively charged carbohydrates attach to the outer surface.

Plasma Membrane (continued)

General composition of cell membrane

Proteins ……………………. 55% Lipids ……………………….. 41%

- Phospholipids … 25%

- Cholesterol ……. 12% Lipids

- Glycolipids …….. 4%

Carbohydrates …………… 3%

Cell membrane phospholipids

Consists of:

a. Glycerol head that contains phosphate group

(polar & hydrophilic).

b. 2 fatty acid ‘tails’ (nonpolar & hydrophobic).

The hydrophobic parts restricts the passage of H20 & H20- soluble ions.

1. Integral proteins: / Internal or intrinsic proteins

- span the membrane. - transport proteins. - provide structural channels or pores.

2. Peripheral proteins: / external or extrinsic proteins

- embedded in one side (face) of the membrane. - carrier proteins. - bind with substances to be transported. - include hormone receptors & cell surface antigens.

Cell membrane proteins

General functions of cell membrane proteins

1. Provide structural support.2. Transport molecules across the membrane.3. Enzymatic control of chemical reactions at cellular surface.4. Some function as receptors for hormones.5. Some function as regulatory molecules, that arrive

at outer surface of the membrane.6. Some act as antigens and induce the process of

antibody formation.

Cell membrane carbohydrates

Primarily attached to the outer surface of the membrane as:

- Glycoproteins … (most of it).

- Glycolipids …… (1/10).

1. Attach cells to each other.

2. Act as receptor substances.

3. Some enter in immune reactions.

4. Give most of cells overall –ve surface charge, which affects the interaction of regulatory molecules with the membrane.

General functions of cell membrane carbohydrates

Cytoplasm & Organelles

Cytoplasm, Organelles, Nucleoli (continued)

Cytoplasm

The aqueous content of a cell (fluid, jellylike

substance), that lies b/w cell membrane & nucleus

in which organelles are suspended.

Serves as matrix substance in which chemical

reactions occur.

‘cytosol’ is the term used to describe fluid portion of

the cytoplasm.

Organelles (excluding nucleus)

Subcellular structures within the cytoplasm that perform specific functions.

Generalized view of a mammalian cell showing organelles common to all cells (such as the Golgi

complex) as well as specialized structures (e.g., cilia) found only in some cells.

Cytoplasmic Organelles: Structure & Function

ComponentStructureFunction

Endoplasmic reticulum

System of interconnected membrane-forming canals & tubules

Agranular (smooth) ER metabolizes nonpolar compounds & stores Ca2+ in striated muscle cells; granular (rough) ER assists in protein sysnthesis

RibosomesGranular particles composed of protein & RNA

Synthesize proteins

Golgi complexCluster of flattened membranous sacs

Synthesizes carbohydrates & packages molecules for secretion. Secretes lipids & glycoproteins

MitochondriaMembranous sacs w folded inner partitions

Release energy from food molecules & transform energy into usable ATP

LysosomesMembranous sacsDigest foreign molecules & damaged organelles

An illustration of the processing of proteins by the granular endoplasmic reticulum & Golgi complex. Notice the formation of vesicles at the ends of some of the flattened sacs of the Golgi complex.

The endoplasmic reticulum. Agranular ER has ribosomes attached to its surface, whereas granular ER lacks ribosomes.

A model structure of a ribosome. It is composed of two subunits: smaller (lighter) & larger (darker) subunits. The space between the two subunits accommodates a molecule of transfer RNA, needed to bring amino acids to the growing polypeptide chain.

The structure of a mitochondria. The outer mitochondrial membrane & the infoldings of the inner membrane. The fluid in the center is the matrix.

ComponentStructureFunction

PeroxisomesSpherical membranous vesicles

Contain enzymes that detoxify harmful molecules & break down hydrogen peroxide

CentrosomeNonmembranous mass of 2 rodlike centrioles

Helps to organize spindle fibers & distribute chromosomes during mitosis

VacuolesMembranous sacsStore & release various substances within the cytoplasm

Microfilaments & microtubules

Thin, hollow tubesSupport cytoplasm & function as cytoskeleton, transport materials within the cytoplasm

Cilia & flagellaMinute cytoplasmic projections that extend from the cell surface

Move particles along cell surface, or move the cell

Cytoplasmic Organelles: Structure & Function (continued)

The formation of the cytoskeleton by microtubules. Microtubules are also important in the motility (movement) of the cell, & movement of materials within the cell.

Nucleus

Cell Nucleus Is a large spheroid body. Largest of organelles. Contains the genetic material (DNA). Most cells have a single nucleus. Enclosed by inner & outer membrane (nuclear

envelope). Outer membrane is continuous with ER.

Nuclear pore complexes fuse inner & outer membranes together. Selective active transport of proteins & RNA.

Cell Nucleus

(a) The cell nucleus is enclosed in a double membrane called the nuclear envelope. Pores in the envelope permit the passage of molecules in & out of the nucleus. The outer layer of the nuclear envelope is continuous with the endoplasmic reticulum, so that the lumen of the ER is continuous with the perinuclear space. In the nondividing nucleus, DNA is visible as chromatin. The nucleolus plays a role in the synthesis of ribosomes from RNA. (b) A nuclear pore is formed from the fusion of the two layers of the nuclear envelope. Proteins are thought to be located in the pores.

Cell Nucleus (continued)

Nucleoli: Dark areas within the nucleus, not surrounded by

membrane. Centers for production of ribosomes.

Chromatin: Threadlike material that makes up chromosomes.

Intercellular Junctions

Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact

Intercellular junctions facilitate this contact

There are several types of intercellular junctions1.Tight junctions

2.Desmosomes

3 .Gap junctions

Tight Junctions, Desmosomes, and Gap Junctions

-At tight junctions, membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid

-Desmosomes (anchoring junctions) fasten cells together into strong sheets

-Gap junctions (communicating junctions) provide cytoplasmic channels between adjacent cells

Tight junction

0.5 µm

1 µm

Desmosome

Gap junction

Extracellularmatrix

0.1 µm

Plasma membranesof adjacent cells

Spacebetweencells

Gapjunctions

Desmosome

Intermediatefilaments

Tight junction

Tight junctions preventfluid from movingacross a layer of cells