Download The Structure and Function of Cells

The Structure and Function of Cells
The Cell is the basic functional unit of ALL living things. There are 2 basic types
of cells:
1. Eukaryotic Cells = contain plasma membrane (cell membrane), organelles,
nucleus, cytoplasm
 Most cells are of this type (body cells, plant cells, Protists, etc.)
 Evolutionarily this is a recent advancement
2. Prokaryotic Cells = contain plasma membrane (cell membrane) and
cytoplasm but NO TRUE ORGANELLES, NO NUCLEUS
 Bacteria cells are of this type
 Ribosomes are the only other real internal structure
 The oldest of cell types are prokaryotic
IMPORTANT PARTS OF THE CELL
1. Plasma Membrane: separates the internal metabolic events from the
external environment surrounding the cell
 Function = control the movement of materials into and out of the cell
 Structure = a double phospholipid membrane (Lipid Bilayer), Polar
hydrophilic “water loving” heads pointing toward outside and inside of the
cell; Nonpolar hydrophobic “water fearing” tails point toward the inside of
the membrane.
Fluid Mosaic Model = describes the structure of the cell
membrane. PROTEINS are scattered throughout the flexible
phospholipid bilayer. These proteins can be of several different types:
1. Peripheral Proteins: attached loosely to the inner or outer surface of
the membrane
2. Integral Proteins: extend into the membrane
3. Transmembrane Proteins: an integral protein that spans across the
membrane and appears at both surfaces
The CELL MEMBRANE is Semi-Permeable = only small, uncharged , polar,
hydrophobic molecules pass freely across the membrane
 Permeable (Pass freely) = H2O , CO2 , O2 , lipid soluble hydrocarbons
 NON-Permeable = large molecules, glucose, all ions
2. Proteins in the plasma membrane provide a wide range of functions
including:
 1. Channel Proteins: provide passageways through the membrane for
certain water soluble substances (i.e. polar & charged molecules)
 2. Transport Protein: use energy to transport materials across the
membrane against a concentration gradient
 3. Recognition Proteins (Glycoproteins): used to distinguish the
identity of neighboring cells
1
 4. Adhesion Proteins: attach cells to neighboring cells, provide stability
 5. Receptor Proteins: provide a binding site for hormones or other
chemical triggers
 6. Electron Transfer Proteins: transfer electrons from one molecule to
another during chemical reactions
3. Cholesterol = provide some rigidity to the fluid nature of the membrane in
animal cells. Act as a “Cellular Antifreeze” to keep a cell from freezing in cold
temps. or melting in warm temps.
4. Glycocalyx = carbohydrate coat covering the outer surface of the cell
membrane. Used for cell to cell recognition.
ORGANELLES OF THE CELL
Organelle = a structure in the cytoplasm of the cell that serves a specific function
related to the metabolism of the cell. They can be thought of as “tiny organs” that
carry out the life functions of a cell
1. Nucleus: contains DNA, the hereditary information of the cell.
 Surrounded by a nuclear envelope – allows for the movement of proteins
in , ribosome subunits out, mRNA out – of the nucleus
 Normal DNA = Chromatin: spread out in a threadlike matrix
 Chromosomes: visible when the cell begins to divide
2. Nucleolus: concentrations of DNA involved in the process of manufacturing
(synthesizing) and storage of the components of Ribosomes
3. Ribosomes: assemble amino acids into proteins in the cytoplasm – structure
is comprised of a small and large subunit (one on top of the other)
4. Endoplasmic Reticulum (ER): stacks of flattened sacs involved in the
production of various products, looks like a maze when viewed in crosssection, often connected to the nucleus
 Rough ER: has ribosomes on it, specializes in protein synthesis
 Smooth ER: NO ribosomes, specializes in the production of lipids,
detoxification, storage etc. (depends on the cell type)
5. Golgi Apparatus (Complex or Body): a group of flattened sacs arranged like
a stack of bowls
 Function to modify and package proteins and lipids
 Puts the products into Vesicles
 Contents of vesicles get released outside the cell
6. Lysosomes: vesicles that contain digestive enzymes. NOT FOUND IN
PLANT CELLS
2
7. Peroxisomes: breakdown various substances
 O2 + H2  H2O2
 Common in liver and kidney cells
8. Mitochondria: carry out aerobic respiration, create energy, the
“POWERHOUSE” of the cell – kidney bean shaped structure – double
membrane stack with internal foldings called Cristae
 Energy in the form of ATP is obtained from carbohydrates
9. Chloroplasts: carry out photosynthesis in plants; this is where plants
incorporate energy from the sun to make carbohydrates
STRUCTURE: a double membrane sac containing stacks of membranes
called thylakoids (grana)
10. Cytoplasm: The Semifluid medium found inside the cell
 “Suspends” all internal organelles – keeps them in place
 provides a medium through which substances can travel and move in the
cell
11. Cytoskeleton: internal structure of the cytoplasm
 Microtubules = provide support and motility, made of protein tubulin
(spindle fibers during cell division; cilia & flagella)
 Intermediate Filaments = support, maintain cell shape
 Microfilaments = made of protein actin, involved in cell motility
12. Flagella and Cilia: protrude from the cell membrane, used for movement
 Flagella = long and few
 Cilia = short and many
 Both arranged in “9+2” arrangement
13. Centrioles and Basal Bodies: Microtubule organizing centers
14. Microtubules: thin cylinders containing 13 rows of the globular protein
tubulin. Found in various organelles such as Cilia and Flagella in animal
cells,and the centriole which is used for attachment to spindle fibers during
cell division
15. Cell Wall: found in plants; outside of plasma membrane, provides support,
made of Cellulose
16. Vacuoles and Vesicles: fluid filled, membrane bound bodies
 Transport Vesicles = move materials between organelles
 Food Vacuoles = temporary food storage
 Storage Vacuoles = store starch, pigments, and toxins
3


Central Vacuole = fill up most of the interior in plant cells only, maintain
rigidity of the cell, how plant cells can grow larger in size – take in more
water to fill central vacuole
Contractile Vacuoles = collect and pump excess water out of a cell
17. Cell Junctions: serve to anchor cells to one another
 Desmosomes = act as “spot welds” to hold adjacent animal cells together
in high stress tissues (skin, heart muscle)
 Tight Junctions = “stitched seams” between animal cells, prevent
movement of material between the cells (cells lining the digestive tract).
 Gap Junctions = narrow tunnels between animal cells, allow passage of
small ions and molecules, communication through electrical impulses
 Plasmodesmata = narrow channels between plant cell
Plant vs. Animal Cells
1. Plants = cell walls, large vacuole, chloroplasts
2. Animal = lysosomes, centrioles
Prokaryotes vs. Eukaryotes
Prokaryotes
No nucleus
No Organelles
Single stranded “naked” DNA
Only Bacteria






Eukaryotes
Have a nucleus
Lots of Organelles
DNA in chromosomes
All other cells
The Movement of Substances
The cell membrane and the membrane of other organelles are Semipermeable = only certain substances can cross the membrane
Selective Permeability = allowing only specific substances to pass
Water = solvent; dissolved particles = solute
Movement of substances may be from higher to lower concentrations (down
or with the concentration gradient) or the reverse (up or against the gradient)
The movement of substances may be active or passive
Solute concentrations may be compared:
 Hypertonic = higher concentration of solutes
 Hypotonic = lower concentration of solutes
 Isotonic = equal concentration of solutes
………relative to another region.
Bulk Flow = collective movement of substances in the same direction in
response to some force or pressure (blood moving through a vessel)
4
Passive vs. Active Transport
Passive Transport = Movement of molecules down a concentration gradient
(high  low)
Does not require the cell to expend any energy!
1. Diffusion: due to the random nature of molecules in motion (dye in a beaker
of water)
Rates of diffusion are determined by:
a. Concentration gradient
b. Shape and size of the molecules
c. Temperature (higher temp. = faster diffusion time)
d. Liquid vs. gas (slower in liquids)
2. Osmosis: the diffusion of water molecules across a semi-permeable
membrane
 Osmotic Pressure = force water exerts against the inside of a cell
 Turgor Pressure = pressure water exerts against the inside of a plant cell,
used for structural support
3. Dialysis: diffusion of solutes across a semi-permeable membrane
4. Plasmolysis: movement of water out of a cell, results in the collapse of the
cell
5. Facilitated Diffusion: use channel proteins embedded in the membrane to
transport solutes
Active Transport = requires the cell to expend energy to move substances
against the concentration gradient (low  high)
Usually utilizes transport proteins to move: small ions (Na+, K+, Cl-, H+),
amino acids, simple sugars
1. Exocytosis: Large particles are released from the cell by vesicles forming
and fusing with the cell membrane
2. Endocytosis: Large particles enter the cell by vesicles forming and fusing
with the cell membrane
 Phagocytosis = cell eating (white blood cells attacking bacteria)
 Pinocytosis = cell drinking
 Receptor Mediated Endocytosis = special receptors attract certain
molecules (cholesterol transport in the blood, hormones)
5