cells - jackson county faculty sites! · 1 cells cells are the structural units of all living...

8
1 Cells Cells are the structural units of all living organisms ranging from unicellular to multicellular organisms. Biochemical activities of cells are dictated by cell shape and specific subcellular structures. A cells shape dictates its function. Human cells have 3 basic parts the plasma membrane, the cytoplasm and the nucleus. The plasma membrane is the outer boundary of the cell. The cytoplasm is the intracellular fluid that is packed with organelles, small structures that perform specific cell functions. The nucleus contains the genetic material and controls cellular activities. Plasma Membrane (Cell Membrane) separates the intracellular fluid within the cell and the extracellular fluid outside the cell. The fluid mosaic model depicts the cell membrane as a lipid bilayer with proteins embedded in it. The lipid bilayer is composed of phospholipids with a few molecules of cholesterol and glycolipids. Phospholipids have a polar, hydrophilic phosphate head and a nonpolar, hydrophobic fatty acid tail. The outer membrane surface (20%) contains lipid rafts, assemblies of saturated phospholipids associated with sphingolipids and lots of cholesterol. These patches are more stable and are assumed to be platforms for molecules needed for cell signaling. Glycocalyx - specific biological markers (carb) for cell to cell recognition; allows immune system to recognize "self" and "non self".

Upload: truongkhanh

Post on 04-Jun-2018

216 views

Category:

Documents


0 download

TRANSCRIPT

1

Cells Cells are the structural units of all living organisms – ranging from unicellular to multicellular organisms. Biochemical activities of cells are dictated by cell shape and specific subcellular structures. A cells shape dictates its function.

Human cells have 3 basic parts – the plasma membrane, the cytoplasm and the nucleus. The plasma membrane is the outer boundary of the cell. The cytoplasm is the intracellular fluid that is packed with organelles, small structures that perform specific cell functions. The nucleus contains the genetic material and controls cellular activities.

Plasma Membrane (Cell Membrane) – separates the intracellular fluid within the cell and the extracellular fluid outside the cell. The fluid mosaic model depicts the cell membrane as a lipid bilayer with proteins embedded in it. The lipid bilayer is composed of phospholipids with a few molecules of cholesterol and glycolipids.

Phospholipids have a polar, hydrophilic phosphate head and a nonpolar, hydrophobic fatty acid tail.

The outer membrane surface (20%) contains lipid rafts, assemblies of saturated phospholipids associated with sphingolipids and lots of cholesterol. These patches are more stable and are assumed to be platforms for molecules needed for cell signaling.

Glycocalyx - specific biological markers (carb) for cell to cell recognition; allows immune system to recognize "self" and "non self".

2

6 functions of membrane proteins – integral & peripheral proteins 1. Transport (channels and carriers) – Integral proteins provide a hydrophilic channel across

the membrane that is selective for a particular solute. Some transport proteins hydrolyze ATP as an energy source to actively pump substances across the membrane.

2. Enzymatic activity – A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. A team of several enzymes in a membrane may catalyze sequential steps of a metabolic pathway.

3. Receptors for signal transduction – A membrane protein exposed to the outside of the cell may have a binding site with a specific shape that fits the shape of a chemical messenger – such as a hormone. The external signal may cause a conformational change in the protein that initiates a chain of chemical reactions in the cell.

4. Intercellular joining – Membrane proteins of adjacent cells may be hooked together in various kinds of intercellular junctions. Some membrane proteins (cell adhesion molecules – CAMs) provide binding sites that guide cell migration and other cell-to-cell interactions.

5. Cell-cell recognition – Some glycoproteins are specifically recognized by other cells. 6. Attachment to the cytoskeleton and extracellular matrix (ECM) – Elements of the

cytoskeleton and the extracellular matrix may be anchored to membrane proteins, which help maintain cell shape and fix the location of certain membrane proteins. Others play a role in cell movement or bind adjacent cells together.

Membrane Junctions – 1. Tight junctions (impermeable junctions) – integral proteins fuse together in adjacent cells;

this prevents molecules from passing through the extracellular space. Example: Tight junctions between epithelial cells lining the digestive tract prevent digestive enzymes and microorganisms from entering the bloodstream.

2. Desmosomes (anchoring junctions) – adjacent cells are held together by thin protein filaments; anchor cells together at plaques (thickenings on plasma membrane). Keratin filaments extend through the cytosol to opposite plaque giving stability to cell to reduce possibility of tearing. Example: Abundant in skin and heart muscle.

3. Gap junctions – is a communicating junction between adjacent cells; cells are connected by hollow cylinders called connexons (pores). These pores allow small molecules to pass from cell to cell. Example: Spread of ions, simple sugars and other small molecules between cardiac or smooth muscle cells – to help synchronize their electrical activity and contraction.

3

Membrane Transport – The plasma membrane is selectively permeable – some molecules pass through easily, but some do not. Two ways substances cross the membrane: passive or active. 1. Passive Transport – requires no ATP; substances move down its concentration gradient.

a. Diffusion – nonpolar and lipid-soluble materials diffuse directly through the lipid bilayer. Examples of diffusible materials: oxygen, carbon dioxide, fats

b. Facilitated Diffusion – certain molecules are transported by binding to protein carriers or move through proteins channels – use leakage or gated channels. Examples of diffusible materials: glucose and some ions

c. Osmosis – movement of water; water moves until hydrostatic pressure and osmotic pressure equalize. Osmosis causes cells to shrink or swell.

Tonicity is the ability of a solution to alter cell's water volume.

Isotonic: Solution with same solute concentration as cytosol. Cells stay the same size.

Hypertonic: Solution with higher solute concentration than cytosol. Cells shrink.

Hypotonic: Solution with lower solute concentration than cytosol. Cells swell.

Filtration is the process that forces water and solutes through a membrane or capillary wall by hydrostatic pressure. Filtration is a passive process that involves a gradient. The pressure gradient pushes solute-containing fluid (filtrate) from a higher pressure to a lower pressure -- occurs in the kidneys. 2. Active Transport – requires ATP; substances move against its concentration gradient –

requires carrier proteins (pumps). Includes cotransport of two solutes across the membrane. Examples: ions, amino acids, charged solutes

4

3. Vesicular Transport – transport of large particles/fluids across membrane in vesicles (membranous sacs); requires ATP

Exocytosis – transport out of the cell; secretion or ejection of substance from a cell.

Endocytosis – transport into the cell; phagocytosis (large external particles become enclosed in vesicle), pinocytosis (membrane edge fuse, forming a fluid-filled vesicle), receptor-mediated endocytosis (external substances binds to membrane receptors)

Resting Membrane Potential – produced by separation of oppositely charged particles across membrane in all cells – resting cells are polarized.

Cell-Environment Interactions:

5

Cells interact directly or indirectly by responding to extracellular chemicals – the glycocalyx is always involved. 1. Cell adhesion molecules (CAMs) are found in every cell – play a key role in embryonic

development and wound repair. Assist in movement of cells past one another, attract WBCs to injured/infected areas and transmit intracellular signals to direct cell migration, proliferation, and specialization.

2. Plasma membrane receptors – a) Contact signaling – touching and recognition of cells; example – in normal

development and immunity b) Chemical signaling - interaction between

receptors and ligands alter activity of cell proteins. Same ligand can cause different cell responses and response is determined by what receptor linked to inside cell.

6

The Cytoplasm & Cytoplasmic Organelles Cytoplasm is the cellular material between the plasma membrane and the nucleus. The cytosol is the fluid in which cytoplasmic organelles are suspended. Each organelle carries out specific functions.

Organelles Functions Mitochondria Provide most of the cells energy (ATP) via cellular respiration

Ribosome Free ribosomes synthesize soluble proteins that function in cytosol or other organelles. Membrane-bound ribosomes (forming rough ER) synthesize proteins to be incorporated into membranes, lysosomes, or exported from cell.

Endoplasmic Reticulum – ER

Interconnected tubes and parallel membranes enclosing cisterns – smooth ER and Rough ER. Rough ER - manufactures all secreted proteins, synthesizes membrane integral proteins and phospholipids

• Assembled proteins move to ER interior, enclosed in vesicle, go to Golgi apparatus

Smooth ER – contains enzymes for lipid metabolism, detoxification of drugs, storage/release of Ca

Golgi Apparatus

Modifies, concentrates, and packages proteins and lipids from rough ER • Transport vessels from ER fuse; proteins modified, tagged for delivery,

sorted, packaged in vesicles

The sequence of events from protein synthesis on the rough ER to the final distribution of those proteins.

Peroxisomes Membranous sacs containing powerful oxidases and catalases that detoxify harmful or toxic substances.

• Neutralize dangerous free radicals (highly reactive chemicals with unpaired electrons)

7

Lysosomes Membranous bags containing digestive enzymes that digest ingested bacteria, viruses toxins and nonfunctional organelles

• Breakdown bone to release Ca2+

Cytoskeleton Elaborate series of rods throughout cytosol; proteins link rods to other cell structures Includes microtubules, microfilaments and intermediate filaments

Centrosome & Centrioles

Microtubule organizing center that contains centrioles that produce spindle fibers for separating chromosomes during nuclear division

Cilia & Flagella Cilia move substances across cell surfaces and flagella propel whole cells.

Nucleus Contains genetic material (DNA) that controls all cellular processes genetic library with blueprints for nearly all cellular proteins

• Responds to signals; dictates kinds and amounts of proteins synthesized

Nuclear Membrane

Double-membrane barrier – outer layer is continuous with rough ER and inner lining maintains shape of nucleus

• Pores allow substances to pass -- regulates transport of large molecules into and out of nucleus

Nucleolus Spherical bodies within the nucleus - Involved in rRNA synthesis and ribosome subunit assembly

Chromatin Threadlike strands of DNA (30%), histone proteins (60%), and RNA (10%) • Condense into barlike bodies called chromosomes when cell starts to

divide

The Cell Cycle – series of events a cell goes through for growth and division. 1. Interphase – cell grows & DNA replicates 2. M phase – nucleus divides; includes prophase, metaphase, anaphase, telophase

a. Meiosis – nuclear division producing gametes; produces 4 genetically different, haploid daughter cells

b. Mitosis – nuclear division producing body cells; essential for body growth and tissue repair; produces 2 genetically identical, diploid daughter cells

3. Cytokinesis – cytoplasm divides to complete divide the cell. Cell division controls - mechanisms regulate cycle.

“Go” signals - contact inhibition, critical volume of cells, chemicals (growth factors & hormones)

“Other” control signals – repressor genes inhibit cell division

8

Cell cycle stages – creating body cells:

Protein Synthesis – includes transcription and translation. 1. Transcription – mRNA copies a segment of

DNA; introns are spliced out and mRNA rejoins – exons (coding regions) leave the nucleus and go to the ribosome.

2. Translation – mRNA is read by the ribosome and the information is translated into specific amino acid sequences to form a protein.

Developmental Aspects of Cells

All cells of body contain same DNA but cells not identical. Chemical signals in embryo channel cells into specific developmental pathways by turning some genes on and others off.

Cell differentiation – the development of specific and distinctive features in cells; cell specialization leads to structural variation – different organelles come to predominate in different cells.

Apoptosis – programmed cell death. This process of controlled cellular suicide eliminates cells that are stressed, unneeded, excessive or aged.

Hyperplasia – accelerated growth

Atrophy – decrease in size of an organ or body tissue, can result from loss of normal stimulation