Cell Structure and Function
The cell is the smallest unit of life that displays all of the
characteristics of life. Its purpose has always been to maintain the DNA which it houses.
Cells may specialize to extremes in larger organisms, but do not deviate from their
. Components of the Cytoskeleton
- Early observations revealed an unseen world:
- Galileo saw the facets of an insects eyes.
- Robert Hooke saw small compartments in cork, which he named cells.
- Van Leeuwenhoek observed several types of living cells, including sperm.
- Schleiden and Schwann proposed the idea that all living things were composed of cells.
- Virchow concluded that all cells come from cells.
- Basic Aspects of Cell Structure and Function
- Structural Organization of Cells
- A plasma membrane separates each cell from the environment, permits the flow of
molecules across the membrane, and contains receptors that can affect the cells
- A DNA-containing region localizes the hereditary material, which can be copied and read.
- The cytoplasm contains membrane systems, particles (including ribosomes), filaments (the
cytoskeleton), and a semifluid substance (cytosol).
- There are basically two kinds of cells in nature:
- Eukaryotic cells contain distinctive arrays of organelles, including a
- Prokaryotic cells (bacteria) have no nucleus.
- The Lipid Bilayer of Cell Membranes
- The lipid bilayer of plasma membranes forms a boundary between inside and outside of the
cell, subdivides the cytoplasm into compartments, and regulates the entry/exit of
- Proteins positioned in the plasma membrane serve as channels, pumps, or receptors.
- Cell Size and Cell Shape
- As the surface area of a cell increases to the square of the diameter, the volume
increases to the cube of the diameter.
- The Defining Features of Eukaryotic Cells
Eukaryotic cells exhibit great complexity individually, but
demonstrate little variability as a group.
- Major Cellular Components
- The nucleus controls access to DNA and permits
easier packing of DNA during cell division.
- Ribosomes, either "free" or attached to membranes (RER) are the
assembly sites of polypeptide chains.
- The endoplasmic reticulum (ER) modifies proteins and
is also involved with lipid synthesis.
- Golgi bodies also modify proteins, sort and ship proteins, and play a role in
the biology of lipids for secretion or internal use.
- Various vesicles transport, store, and digest various
materials within the cell.
- Mitochondria have enzymes responsible for ATP formation.
- The cytoskeleton determines cell shape and internal
organization; it also provides for motility.
- Organelles form compartmentalized portions within the cytoplasm allowing reactions to be
separated with respect to time (allowing proper sequencing) and space (allowing
incompatible reactions to occur in close proximity).
- Typical Organelles in Plant Cells
- Typical Organelles in Animal Cells
- The Nucleus
- The nucleus isolates DNA, which contains the code for
protein assembly, from the sites, ribosomes in cytoplasm, where proteins will be
- Nuclear Envelope
- The nuclear envelope consists of two lipid bilayers with pores.
- The inner surface has attachment sites for protein filaments, which anchor the DNA
molecules and keep them organized.
- The outer surface is studded with ribosomes.
- It is a region where RNA subunits of ribosomes are
prefabricated before shipment out of the nucleus.
- Chromatin refers to the total collection of DNA and proteins.
- Each chromosome is a single molecule of DNA and its
associated proteins; it may take on different appearances depending on the events
currently happening within the cell.
- What Happens to the Proteins Specified by DNA?
- Within the cytoplasm, newly formed polypeptide chains may be stockpiled in solution or
may enter the cytomembrane system (ER, Golgi bodies, and vesicles).
- Some of the proteins will be used within the cell in which they were made, other will be
exported for use elsewhere.
- The Cytomembrane System
- The cytomembrane system is a series of organelles in which lipids are assembled and new
polypeptide chains are modified into final proteins.
- Endoplasmic Reticulum
- The endoplasmic reticulum is a collection of interconnected tubes and flattened sacs
that begins at the nucleus and winds its way through the cytoplasm.
- Two kinds of ER may be found in a cell:
- Rough ER consists of stacked, flattened sacs with many ribosomes attached;
oligosaccharide groups are attached to polypeptides as they pass through on their way to
other organelles or to secretory vesicles.
- Smooth ER has no ribosomes; it is the area from which vesicles carrying
proteins and lipids are budded; it also inactivates harmful chemicals.
- Golgi Bodies
- A Golgi body consists of flattened sacsresembling a stack of pancakeswhose
edges break away as secretory vesicles.
- Here proteins and lipids undergo final processing, sorting,
- A Variety of Vesicles
- Lysosomes are vesicles that bud from Golgi bodies; they carry powerful
enzymes that can digest the contents of other vesicles, worn-out cell parts, or bacteria
and foreign particles.
- Peroxisomes are small vesicles that contain enzymes using oxygen to degrade
fatty acids and amino acids, forming a harmful byproduct, hydrogen peroxide, which is then
converted to water.
- Mitochondria are the primary organelles for transferring the energy in carbohydrates to
ATP under oxygen-plentiful conditions.
- Each mitochondrion has an outer membrane and an inner folded membrane (cristae).
- The membranes form two compartments.
- Hydrogen ions and electrons move between the compartments during ATP formation
- Mitochondria have their own DNA and ribosomes, a fact which points to their origination
from ancient bacteria engulfed by predatory cells.
- Specialized Plant Organelles
- Chloroplasts and Other Plastids
- Chloroplasts are oval or disk shaped, bounded by a double membrane, and are critical to
the process of photosynthesis.
- In the stacked disks (grana), pigments and enzymes trap
sunlight energy to form ATP.
- Sugars are formed in the fluid substance (stroma)
surrounding the stacks.
- Pigments such as chlorophyll (green) confer distinctive
colors to the chloroplasts.
- Chromoplasts store red and brown pigments that give color to petals, fruits, and roots.
- Colorless amyloplasts store starch granules.
- Central Vacuole
- In a mature plant, the central vacuole may occupy 50 to 90 percent
of the cell interior.
- Central vacuoles store amino acids, sugars, ions, and wastes.
- The vacuole enlarges during growth and greatly increases the cells outer surface
- The cytoskeleton gives cells their internal organization, shape, and capacity to move.
- It forms an interconnected system of bundled fibers, slender threads, and lattices that
extends from the nucleus to the plasma membrane.
- The main components are microtubules, microfilaments,
and intermediate filamentsall assembled from protein subunits.
- Some portions are transient, such as the "spindle" microtubules used in
chromosome movement during cell division; others are permanent, such as filaments
operational in muscle contraction.
- Microtubules, the largest structural elements in the cytoskeleton, are composed of tubulin subunits that compose a cylinder.
- Microtubules govern the division of cells and some aspects of their shape as well as
many cell movements.
- Microfilaments, the thinnest elements, consist of two helically twisted polypeptide
chains assembled from actin monomers.
- Microfilaments are particularly important in movements that take place at the cell
surface; they also contribute to the shapes of animal cells.
The Structural Basis of Cell Motility
Cell Surface Specializations
- Mechanisms of Cell Movements
- Through controlled assembly and disassembly of their subunits, microtubules, and
microfilaments grow or diminish in length, thereby the structures attached to them are
thereby pushed or dragged through the cytoplasm (example: pseudopod movement in Amoeba.
- Flagella and Cilia
- Microtubular extensions of the plasma membrane have a 9 + 2
cross-sectional array that arises from a centriole (a type of MTOC) and are useful in
- Flagella are quite long, not usually numerous, and found on one-celled protistans
and animal sperm cells.
- Cilia are shorter and more numerous and can provide locomotion for free-living
cells or may move surrounding water and particles if the ciliated cell is anchored.
- Eukaryotic Cell Walls
- Cell walls are carbohydrate frameworks for mechanical support in bacteria, protistans,
fungi, and plants; cell walls are not found in animals.
- In growing plant parts, bundles of cellulose strands form a
primary cell wall that is pliable enough to allow enlargement under pressure.
- Later, more layers are deposited on the inside of the primary wall to form the secondary
- Lignin composes up to 25 percent of the secondary wall in woody plants; it makes
plant parts stronger, more waterproof, and less inviting to insects.
- Cell-to-Cell Junctions
- In plants tiny channels called plasmodesmata cross
the adjacent primary walls and connect the cytoplasm
- Animal cells display three types of junctions:
- Tight junctions occur between cells of epithelial tissues in which
cytoskeletal strands of one cell fuse with strands of neighboring cells causing an
- Adhering junctions are like spot welds at the plasma membranes of two
adjacent cells that need to be held together during stretching as in the skin and heart.
- Gap junctions are small, open channels that directly link the cytoplasm of
Prokaryotic Cells The Bacteria and Cyanobacteria
Prokaryotic cells exhibit great variability as a group, but
demonstrate little complexity as individuals.
- The term prokaryotic ("before the nucleus") indicates existence of
bacteria before evolution of cells with a nucleus; bacterial DNA is clustered in a
distinct region of the cytoplasm (nucleoid).
- Bacteria are some of the smallest and simplest cells.
- Bacterial flagella project from the membrane and permit rapid movement.
- A somewhat rigid cell wall supports the cell and surrounds the plasma membrane, which
regulates transport into and out of the cell.
- Ribosomes, protein assembly sites, are dispersed throughout the cytoplasm.