cells

The Cell Theory• A cell is the basic structural and functional unit of

living organisms.• The activity of an organism depends on both the

individual and collective activities of its cells• According to the principle of complementarity,

the biochemical activities of cells are dictated by the specific subcellular structures of cells.

• Continuity of life has a cellular basis.

Parts of a cell• Plasma membrane – boundary, maintains cell integrity• Nucleus – contains chromosomes, has a double

membrane• Nucleolus – organelle inside the nucleus, where

ribosomal subunits are manufactured• Rough endoplasmic reticulum – extensive system of

membranes, contiguous with nucleus, studded with ribosomes

• Ribosomes – small dark-staining granules composed of protein and rRNA, some are located on the outer surface of the RER and some are free in the cytoplasm, sites of protein synthesis, each is composed of 2 subunits, each subunit is made of rRNA (ribosomal RNA) + protein

Cell Anatomy• After synthesis, proteins transported into lumen of RER

where they fold into their final 3 dim. shape, some are modified by attachment of carb., packaged into transport vesicles which carry them to Golgi complex (apparatus)

• Golgi apparatus – site of synthesis of most carbohydrates, also where proteins are further processed by addition of other CHO chains + packaged into secretory granules (vesicles) for exocytosis or into lysosomes in which intracellular digestion is performed.

• Smooth endoplasmic reticulum – where phospholipids, steroids and some carbohydrates are synthesized; processes exogenous chemicals (drugs)

Cell Anatomy cont.• The Golgi complex is like the cell's packaging and

shipping department. It is made up of a stack of flattened membrane sacs. Some of the protein being transported through the canals of the endoplasmic reticulum ends up in the Golgi complex. Here it may be joined with other molecules before being "packaged". The packages are little pieces of the Golgi complex which break off and form "vesicles". The vesicles move to the cell membrane and fuse with it. It may then squirt its contents outside of the cell as a secretion, or the product assembled in the Golgi complex may become a new piece of the cell membrane itself. In that case the vesicle fuses with the membrane and becomes a part of it

• Lysosomes and peroxisomes – types of vesicles, they contain enzymes

Plasma membrane

Electron photomicrograph of two adjacent plasma membranes

Fluid Mosaic Model• The plasma membrane is composed of

– A lipid bilayer of phospholipids:• The polar phosphate-containing heads are hydrophilic and

face the internal and external aqueous environments• The non-polar fatty acid chains are hydrophobic and are

directed inward to the center of the membrane

– Cholesterol is embedded in the lipid portion of the membrane

– Two populations of proteins are present:• Integral proteins span the entire membrane• Peripheral proteins are associated with the inner and outer

surfaces of the membrane

Other molecules associated with cell membranes

• Glycolipids—externally facing phospholipids with attached sugar groups

• Glycoproteins—branching sugar groups associated with proteins on the external surface of the cell. The glycoproteins function as biological markers and are important in cell recognition.

How do peripheral proteins differ from integral proteins?

Functions of membrane proteins

• Most integral proteins are transmembrane proteins that span the entire membrane and function in transport either as carriers or by forming channels for the transport of water-soluble molecules

• Peripheral proteins are not embedded in the lipid bilayer—some support the membrane, others function as enzymes

Phospholipid molecule

Cholesterol makes the lipid bilayer less deformable and decreases its permeability to small water-soluble molecules.

Cytoplasm • The cellular material between the plasma

membrane and the nucleus• Three major components

– Cytosol—the viscous fluid in which the other cytoplasmic elements are suspended; mostly water with proteins, salts, sugars and other solutes

– Cytoplasmic organelles—tiny machines that perform specific functions for the cell

– Inclusions—stored nutrients and pigments found in some cells

Cell Organelles• Mitochondria (sing. – mitochondrion) -

sites of ATP synthesis, the power plants of the cell, they provide most of the cell’s energy in the form of ATP. The more metabolically active the cell, the greater the # of mitochondria. They have a double membrane. Inner membrane is folded. Folds are called cristae. Interior of mito called the matrix.

Cell Organelles• Golgi apparatus: consists of stacked,

flattened membranous sacs and vesicles; it modifies and packages the proteins synthesized on the RER and lipids manufactured in the smooth ER.

• Lysosomes: membrane-bound organelles containing digestive enzymes, abundant in phagocytes and necessary for intracellular digestion.

Cellular Extensions• Microvilli, cilia and flagella – 3 types of cell

membrane modifications• Microvilli increase surface area of cell, function

to increase absorption, also called brush border • Cilia: whip like, motile cellular extensions found

on some cells, cilia function in movement of substances directionally along cell surfaces

• Flagella: much longer than cilia, only one per cell—in the human body found only on sperm cells; function in propelling the cells themselves

cilia

Name the cellular extensions visible in the photo.

Sperm cells with flagella

Nucleus• Control center of the cell• All body cells are nucleated except mature

red blood cells• Bound by a nuclear envelope with nuclear

pores• Contains one or more nucleoli, sites where

ribosome subunits are made• Contains DNA (the genetic material)

arranged in chromosomes

Name the RNA molecules that travel through the nuclear pores and carry the code for protein synthesis.

Protein Synthesis, 1• A gene is a segment of a chromosome

(DNA molecule) that carries instructions for creating a protein

• The code for building a protein is copied by a messenger RNA molecule in a process called transcription

• mRNA leaves the nucleus and attaches to a ribosome where the code is “read” by transfer RNA molecules

Protein Synthesis, 2

• tRNA molecules match amino acids to the triplet codons of mRNA in a process known as translation

• The newly synthesized protein may remain in the cell or be packaged and secreted for use in another location

Some Important Proteins

• Keratin remains within epidermal cells as a structural protein

• Digestive enzymes are secreted by exocytosis for extracellular functions

• Hemoglobin in produced by red blood cells when they are forming in bone marrow; it remains in the cells and functions in transport of blood gases