ANATOMY CH 3

What are cells?

Cells are the basic units of structure and function in living things

How many cells are in the human body?

50-100 trillion

Cell Theory

1. A cell is the basic structural and functional unit of living organisms

2. The activity of an organism depends on the collective activities of its cells

3. According to the principle of complementarity, the biochemical activities of cells are dictated by their structure (anatomy) which determines their function (physiology)

4. Continuity of life has a cellular basis

Most cells are composed of 4 elements:

1. Carbon

2. Hydrogen

3. Oxygen

4. Nitrogen

What percentage of cells is water?

Cells are 60% water

A cell has three main regions or parts:

1. nucleus

2. cytoplasm

3. plasma membrane

Describe the nucleus

• control center of the cell

• contains genetic material known as deoxyribonucleic acid (DNA)

• DNA is needed for building proteins

• DNA is necessary for cell reproduction

• Three regions:

1. Nuclear envelope (membrane)

2. Nucleolus

3. Chromatin

nuclear envelope

• Nuclear envelope (membrane)

• Consists of a double membrane that bounds the nucleus

• Contains nuclear pores that allow for exchange of material with the rest of the cell

-Encloses the jellylike fluid called the nucleoplasm

Nucleolus

• Nucleus contains one or more dark-staining nucleoli

• sites of ribosome assembly

• ribosomes migrate into the cytoplasm through nuclear pores to serve as the site of protein synthesis

Chromatin

• Composed of DNA wound around histones (proteins)

• Scattered throughout the nucleus and present when the cell is not dividing

• Condenses to form dense, rodlike bodies called chromosomes when the cell divides

The Plasma Membrane

• transparent barrier for cell contents

• contains cell contents

• separates cell contents from surrounding environment

fluid mosaic model

fluid mosaic model is constructed of:

-Two layers of phospholipids arranged "tail to tail"

-Cholesterol and proteins scattered among the phospholipids

-Sugar groups may be attached to the phospholipids, forming glycolipids

Phospholipid arrangement in the plasma membrane

• Hydrophilic ("water loving") polar "heads" are oriented on the inner and outer surfaces of the membrane

• Hydrophobic ("water fearing") nonpolar "tails" form the center (interior) of the membrane

• This interior makes the plasma membrane relatively impermeable to most water-soluble molecules

Role of proteins in plasma membrane

Responsible for specialized membrane functions:

• Enzymes

• Receptors for hormones or other chemical messengers

• Transport as channels or carriers

Role of Sugars in the Plasma Membrane

• Glycoproteins are branched sugars attached to proteins that abut the extracellular space

• Glycocalyx is the fuzzy, sticky, sugar-rich area on the cell's surface

cell membrane junctions

Cells are bound together in three ways:

1. Glycoproteins in the glycocalyx act as an adhesive or cellular glue

2. Wavy contours of the membranes of adjacent cells fit together in a tongue-and-groove fashion

3. Special cell membrane junctions are formed, which vary structurally depending on their roles

Main types of cell junctions

1. tight junctions

2. desmosomes

3. gap junctions

Tight junctions

• Impermeable, bind cells together into leak-proof sheets

• Plasma membranes fuse like a zipper to prevent substances from passing through extra cellular space between cells

Desmsomes

• anchoring junctions, like rivets, that prevent cells from being pulled apart as a result of mechanical stress

• Created by buttonlike thickenings of adjacent plasma membranes

Gap junctions (communicating junctions)

• allow communication between cell

• Hollow cylinders of proteins (connexons) span the width of the abutting membranes

• molecules can travel directly from one cell to the next through these channels

Cytoplasm

• Cellular material outside the nucleus and inside the plasma membrane

• site of most cellular activities

• Includes cytosol, inclusions, and organelles

The three major components of the cytoplasm:

1. cytosol

2. inclusions

3. organelles

Cytosol

Fluid that suspends all elements and contains nutrients and electrolytes

Inclusions

Chemical substances, such as stored nutrients or cell products, that float in the cytosol

Organelles

Metabolic machinery of the cell that perform functions for the cell

• many are membrane-bound, allowing for compartmentalization of their functions

Mitcohondria

• Powerhouse of the cell

• mitochondrial wall consists of a double membrane with cristae on the inner membrane

• Carry out reactions in which oxygen is used to break down food ATP molecules

Ribosomes

• made of protein and ribosomal RNA

• Sites of protein synthesis in the cell

• found at two locations:

-Free in the cytoplasm

-As part of the rough endoplasmic reticulum

Endoplasmic Reticulum (ER)

• fluid failed tunnels (or canals) that carry substances within the cell

• continuous with the nuclear membrane

• 2 types:

• Rough ER

• Smooth ER

Rough Endoplasmic Reticulum

• studded with ribosomes

• synthesizes proteins

• transport, vesicles move proteins within cell

• Abundant and cells that make an export proteins

Order of synthesized protein in the ER

1. As the protein is synthesized on the ribosome, it migrates into the rough ER tunnel system.

2. in the tunnel, the protein folds into its functional shape. Short chains may be attached to the protein for a glycol protein.

3. The protein is packaged in a tiny membranous sac called a transport vesicle

4. the transport vesicle buds from the rough ER and travels to the Golgi apparatus for further processing

Smooth Endoplasmic Reticulum

• lacks ribosomes

• functions and lipid metabolism

• Detoxification of drugs and pesticides

Golgi apparatus

• appears as a stack of flattened membranes associated with tiny vesicles

• Modifies and packages proteins, arriving from the rough via transport vesicles

• produces different types of packages:

• secretory vesicles (pathway 1)