d. Facilitated Diffusion
Since the lipid layer is amphipathic, most polar molecules cannot pass through the nonpolar region. Since most organic molecules are polar, they are unable to pass through the cell membrane by simple diffusion. For example, glucose enters cells by facilitated diffusion. This process does not require energy.
The transport of largely hydrophilic molecules across the cell membrane depends upon integral membrane proteins called transport proteins. The transport proteins are highly selective. The tertiary and even quaternary structures of the transport proteins determine which molecules are transported. These transport proteins are called permeases.
Types of transport proteins.
1)Uniport:carries a single molecule across the membrane.
2)Symport: moves two different molecules at the same time in the same direction. Both molecules must bind to the protein for transport.
3)Antiport: exchanges two molecules by moving them in opposite directions.
These proteins can be inhibited by molecules that resemble the molecule normally carried by the protein.
e. Active Transport
This type of transport requires energy and membrane proteins. Active transport occurs in situations where a substance is moved across the cell membrane and against its concentration gradient.
f. Vesicle Mediated Transport
Vesicles or vacuoles can fuse with the cell membrane. Vesicles formed inside the cell can move to the cell membrane, fuse with the outer cell membrane, and expel their contents outside the cell into the surroundings. This process is called exocytosis. In exocytosis, vesicles formed at the surface of the cell can capture substances outside the cell, and deposit their contents into the cell. There are three types of endocytosis.
1)Phagocytosis (cell eating): When the substance taken into the cell is a solid, the process is called phagocytosis. A vesicle forms around the object taken into the cell. Once the solid is in the cell, a lysosome joing with the vesicle and enzymes digest the solid.
2) Pinocytosis (cell drinking): When the substance taken into the cell is a fluid, the process is called pinocytosis.
3)Receptor mediated Endocytosis: The molecule attaches to a specific receptor on the cell surface before a vesicle forms around the molecule.
g. Cell-Celll junction
In multicellular organisms, cells are organized into tissues. Cells need to communicate with each other directly. Some communications are accomplished by chemical signals produced by the cell. They are thenexported through exocytosis, moved into the target cell or activate a second messenger within the target cell which will complete the message.
M. CYTOSKELETON
1. Background
a. All cells have a distinct shape that varies with time and function. The cytoskeleton may enable a cell to change its shape.
b. Cells have a high degree of internal organization. Organelles have patterns and form relationships that often change. Organelles and even cytoplasmic enzymes may be held in place by anchoring them to the cytoskeleton. Organelles can also be transported along the cytoskeleton which acts like a ‘railroad track’.
c. Some cells have the ability to move. Movement of organisms like ourselves depends on the same mechanisms used by the cells.
2. Three Characteristics Of the Cytoskeleton
a. Cytoskeleton elements are non-membrane bounded organelles.
b. Most cytoskeleton organelles have the ability for self-assembly
c. The cytoskeleton organelles have no specific lengths.
3. Functions and Characteristics of the Cytoskeleton
a. They are involved with the transport of organelles and cytoplasmic streaming.
b. The organelles transport soluble products.
c. They are altered when the cell comes into contact with a substrate; this may allow for cell to cell communication.
d. These organelles are not dependent on the nucleus for assembly.
e. The organelles for the cytoskeleton are inherited maternally.
4. Three Organelles that make up the Cytoskeleton
a. Microtubules
b. Actin fibrils
c. Intermediate fibrils
5. Microtubules
a. These are about 25nm in diameter.
b. Microtubules are 200nm to 25mm in length.
c. The microtubules are hollow tubes that are constructed from globular proteins called tubulins. A microtubule may elongate by the addition of tubulin proteins to one end of the tubule. Microtubules may be disassembled and their tubulin used to build microtubules elsewhere in the cell.
d. These may exist in singles, groups, or complex arrangement.
e. They are composed of alpha and beta tubulin dimers.
Microtubules extend outwards from an organizing center (centrosome) that is near the nucleus to near the cell surface. Plants do not have centrioles.
6. Functions of Microtubules
a. Microtubules play an important role in cell division. They move things within the cell; for example, chromosomes during mitosis. Mitochondria, plastids and vesicles can move along microtubule tracks.
b. They help maintain the structure of the cell. It is believed that microtubules act as a temporary scaffolding for the construction of other cell structures.
c. Microtubules probably guide secretory vesicles from the Golgi complex to the plasma membrane.
d. Cilia and flagella are formed through a specialized arrangement of microtubules. Microtubules are associated with motor proteins called dynein and kinesin.
7. Microfilaments- Characteristics and Functions
a. They are 6-7nm in diameter
b. Microfilaments are composed of the protein actin in helical chains (globular protein subunits).
c. They are involved with cytoplasmic streaming and pseudopodia movement.
d. They help arrange organelles
e. Microfilaments such as actin, and myosin are the main component of muscle cells. Microfilaments are best known for their role in muscle contraction. 10% of all the protein in a cell is actin. Actin filaments are associated with motor proteins called myosin.
8. Intermediate Fibrils- Characteristics and Function
a. They are 8-12nm in diameter.
b. Intermediate filaments comprise a diverse class of cytoskeletal elements, differing in protein composition from one type of cell to another.
c. Intermediate fibrils are permanent and very stable.
d. There are 5 classes of intermediate fibrils.
e. They radiate from the nuclear envelope and associate with microtubules- forming a cage in which the nucleus sits.
f. Experiments suggest that intermediate filaments are important in reinforcing the shape of a cell and fixing the position of certain organelles.
N. HOW CELLS MOVE
All cells show some type of movement. Cytoplasmic streaming, movement or chromosomes, and changes in shape are examples of movement that occur within the cell.
Some cells move in their environment. Two different mechanisms are responsible for assembly.
1) Assemblies of cilia and flagella (undulipodia).
2) Assemblies of micro filaments (actin proteins).
1. Undulipodi: Cilia and Flagella
Undulipodi move the eukaryotic cell by undulating. There is no structural difference between eukaryotic flagella and cilia. If a protist has a flagellum, it is classified with the flagellates. If a protist has cilia, it is a ciliate. Flagella are longer and few per cell. Cilia are shorter, beat differently and are usually abundant.
Undulipodia seen in a cross section are about 0.25 mm in diameter and how a circle of pairs of microtubules (minute cylinders). There are nine fused microtubules that form a ring surrounding two additional fused microtubules (in the center). Microtubules are composed of tubulins.
Traces of RNA have been found inside the base of the undulipodium. It is hypothesized that unduliodia were once free living spirochetes. These spirochetes formed association with heterotrophic protists. The spirochete began to propel the host through the environment. There is a weakness in this theory; no genetic material has been found in the undulipodia. The genes that determine the amino acid sequence of the tubulin proteins that make up the undulopodia are found in the nucleus of the cell.
2. Basal Bodies and Centriole
a. Basal Body
These structures have the same diameter as cilia (about 0.2mm). They are composed of microtubules arranged in nine triplets instead of pairs. Cilia and flagella are formed from basal bodies.
b. Centriole
These are small cylinders (about 0.2mm) which contain nine microtubule triplets (identical to the basal bodies). Distribution in the cell is different. Within the centrosome of an animal cell are a pair of centrioles. When a cell divides, the centrioles replicate.
In a nondividing animal cell, centrioles lie in pairs at right angles to each other near the nuclear envelope, where the microtubules radiate. During cell division, centrioles organize the spindle. The spindle appears at the time of cell division and is involved with chromosome movement during mitosis. The spindle is composed of numerous microtubules.
Centrioles are not required for spindle formation. Plant cells, which lack centrioles, form spindles.
3. Actin and other Proteins
In cells that exhibit movement, actin is associated with a motor protein called myosin. These proteins are found in organisms that exhibit cytoplasmic streaming. Actin has been found in concentration bundles near the moving edge of the cell. Actin also helps pinch a dividing cell into daughter cells during animal cell division.
Actin and myosin are the contractile proteins found in vertebrate muscle cells.
O. CELL SURFACE
1.Cell Walls
One of the features that distinguishes plant cells from animal cells is the cell wall. The cell wall protects the plant cell, maintains its shape, and prevents excessive uptake of water.
Plant cell walls are much thicker than the plasma membrane and range from 0.1 to several um. Although the exact chemical composition of the wall varies from species to species and from one cell type to another in the same plant, the basic design of the wall is consistent. Fibers made of the polysaccharide cellulose are embedded in a matrix of other polysaccharides and protein.
A young plant cell first secretes a relatively thin and flexible cell wall called the primary cell wall. Between the primary walls of adjacent cells in the middle lamella, a thin layer rich in sticky polysaccharides is called pectins. The middle lamella glues the cells together. When the cell matures and stops growing, it strengthens its wall. Some cells do this by secreting hardening substances into the primary wall. Other plants cells add a secondary wall between the cell membrane and the primary wall. The secondary wall, made of several layers, has a strong and durable matrix that affords the cell protection and support. Wood consists of mainly secondary walls.
2. Glycocalyx
Animal cells lack structured walls, but many have a fuzzy coat called the glycocalyx. The glycocalyx is made of sticky oligosaccharides. The glycocalyx strengthens the cell surface and helps glue the cells together. The olgisaccharides also contribute to cell cell recognition by serving as unique identification tags for specific types of cells.
3. Intracellular Junctions
The many cells of an animal of plant are integrated into one functional organism. Neighboring cells often adhere, interact, and communicate through special patches of direct physical contact. The walls of plants are perforated with channels called plasmodesmata through which strands of cytoplasm connect the living contents of adjacent cells. In animals there are three main types of intracellular junctions: tight junctions, desmosomes and gap junctions.