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الكلية كلية العلوم للبنات
القسم قسم علوم الحياة
المرحلة 3
أستاذ المادة قاسم محمد علي العامري
4/24/2011 6:04:34 PM
Cell Physiology
Cell Structure and Function
What is a Cell?
Cells are the microscopic fundamental unit that makes up all living things. Every living
thing has cells: bacteria, protozoans, fungi, plants, and animals are the main groups
(Kingdoms) of living things. Some organisms are made up of just one cell (e.g. bacteria
and protozoans), but animals, including human beings, are multicellular. An adult human
body is composed of about 100 trillion cells! Each cell has basic requirements to sustain
it, and the body s organ systems are largely built around providing the many trillions of
cells with those basic needs (such as oxygen, food, and waste removal).
There are about 200 different kinds of specialized cells in the human body. When many
identical cells are organized together it is called a tissue (such as muscle tissue, nervous
tissue, etc). Various tissues organized together for a common purpose are called organs
(e.g. the stomach is an organ, and so is the skin, the brain, and the uterus).
Ideas about cell structure have changed considerably over the years. Early biologists saw
cells as simple membranous sacs containing fluid and a few floating particles. Today s
biologists know that cells are infinitely more complex than this. Therefore, a strong
knowledge of the various cellular organelles and their functions is important to any
physiologist. If a person s cells are healthy, then that person is healthy. All physiological
processes, growth and development, and disease can be described at the cellular level.
Specialized Cells of the Human Body
Although there are specialized cells - both in structure and function - within the body, all
cells have similarities in their structural organization and metabolic needs (such as
maintaining energy levels via conversion of carbohydrate to ATP and using genes to
create and maintain proteins).
Here are some of the different types of specialized cells within the human body.
• Epithelial cells: Functions of epithelial cells include secretion, absorption,
protection, transcellular transport, sensation detection, and selective permeability.
Epithelium lines both the outside (skin) and the inside cavities and lumen of
bodies.
• Exocrine cells: These cells secrete products through ducts, such as mucus, sweat,
or digestive enzymes.
• Endocrine cells: These cells are similar to exocrine cells, but secrete their
products directly into the bloodstream instead of through a duct. Endocrine cells
are found throughout the body but are concentrated in hormone-secreting glands
such as the pituitary.
• Blood Cells: The most common types of blood cells are:
o red blood cells. The main function of red blood cells is, to collect oxygen
in the lungs and deliver it through the blood to the body tissues. Gas
exchange is carried out by simple diffusion (To see this in action please
click here).
•
o various types of white blood cells. They are produced in the bone marrow
and help the body to fight infectious disease and foreign objects in the
immune system. White cells are found in the circulatory system, lymphatic
system, spleen, and other body tissues.
Cell Size
Cells are the smallest living units within our body, but play a big role in making our body
function properly. Many cells never have a large increase in size after they are first
formed from a parental cell. Typical stem cells reproduce, double in size, then reproduce
again. Most Cytosolic contents such as the endomembrane system and the cytoplasm
easily scale to larger sizes in larger cells. If a cell becomes too large, the normal cellular
amount of DNA may not be adequate to keep the cell supplied with RNA. Large cells
often replicate their chromosomes to an abnormally high amount or become
multinucleated. Large cells that are primarily for nutrient storage can have a smooth
surface membrane, but metabolically active large cells often have some sort of folding of
the cell surface membrane in order to increase the surface area available for transport
functions.
Cellular Organization
Several different molecules interact to form organelles with our body. Each type of
organelle has a specific function. Organelles perform the vital functions that keep our
cells alive.
Cell Membranes
The boundary of the cell, sometimes called the plasma membrane, separates internal
metabolic events from the external environment and controls the movement of materials
into and out of the cell. This membrane is very selective about what it allows to pass
through; this characteristic is referred to as "selective permeability." For example, it
allows oxygen and nutrients to enter the cell while keeping toxins and waste products out.
The plasma membrane is a double phospholipid membrane, or a lipid bilayer, with the
nonpolar hydrophobic tails pointing toward the inside of the membrane and the polar
hydrophilic heads forming the inner and outer surfaces of the membrane.
The molecular structure of the cell membrane.
Protein and Cholesterol
Proteins and cholesterol molecules are scattered throughout the flexible phospholipid
membrane. Peripheral proteins attach loosely to the inner or outer surface of the plasma
membrane. Integral proteins lie across the membrane, extending from inside to outside. A
variety of proteins are scattered throughout the flexible matrix of phospholipid molecules,
somewhat like icebergs floating in the ocean, and this is termed the fluid mosaic model of
the cell membrane.
The phospholipid bilayer is selectively permeable. Only small, uncharged polar
molecules such as H2O and CO2, and hydrophobic (nonpolar) molecules like O2 and lipid
soluble molecules such as hydrocarbons can pass freely across the membrane. Other
molecules need the help of a membrane protein to get across. There are a variety of
membrane proteins that serve various functions:
• Channel proteins: Proteins that provide passageways through the membranes for
certain hydrophilic or water-soluble substances such as polar and charged
molecules. No energy is used during transport, hence this type of movement is
called facilitated diffusion.
• Transport proteins: Proteins that spend energy (ATP) to transfer materials
across the membrane. When energy is used to provide passageway for materials,
the process is called active transport.
• Recognition proteins: Proteins that distinguish the identity of neighboring cells.
These proteins have oligosaccharide or short polysaccharide chains extending out
from their cell surface.
• Adhesion proteins: Proteins that attach cells to neighboring cells or provide
anchors for the internal filaments and tubules that give stability to the cell.
• Receptor proteins: Proteins that initiate specific cell responses once hormones or
other trigger molecules bind to them.
• Electron transfer proteins: Proteins that are involved in moving electrons from
one molecule to another during chemical reactions.
Passive Transport Across the Cell Membrane
Passive transport describes the movement of substances down a concentration gradient
and does not require energy use.
• Bulk flow is the collective movement of substances in the same direction in
response to a force, such as pressure. Blood moving through a vessel is an
example of bulk flow.
• Simple diffusion, or diffusion, is the net movement of substances from an area of
higher concentration to an area of lower concentration. This movement occurs as
a result of the random and constant motion characteristic of all molecules, (atoms
or ions) and is independant from the motion of other molecules. Since, at any one
time, some molecules may be moving against the gradient and some molecules
may be moving down the gradient, althought the motion is random, the word
"net" is used to indicate the overall, eventual end result of the movement.
• Facilitated diffusion is the diffusion of solutes through channel proteins in the
plasma membrane. Water can pass freely through the plasma membrane without
the aid of specialized proteins.
• Osmosis is the diffusion of water molecules across a selectively permeable
membrane. When water moves into a body by osmosis, hydrostatic pressure or
osmotic pressure may build up inside the body.
• Dialysis is the diffusion of solutes across a selectively permeable membrane.
Active Transport Across the Cell Membrane
Active transport is the movement of solutes against a gradient and requires the
expenditure of energy, usually in the form of ATP. Active transport is achieved through
one of the following two mechanisms:
1. Transport proteins in the plasma membrane transfer solutes such as small ions
(Na+, K+, Cl-, H+), amino acids, and monosaccharides.
2. Vesicles or other bodies in the cytoplasm move macromolecules or large particles
across the plasma membrane. Types of vesicular transport include:
• Exocytosis, which describes the process of vesicles fusing with the plasma
membrane and releasing their contents to the outside of the cell. This process is
common when a cell produces substances for export.
• Endocytosis, which describes the capture of a substance outside the cell when the
plasma membrane merges to engulf it. The substance subsequently enters the
cytoplasm enclosed in a vesicle.
There are three kinds of endocytosis:
• Phagocytosis or cellular eating, occurs when the dissolved materials enter
the cell. The plasma membrane engulfs the solid material, forming a
phagocytic vesicle.
• Pinocytosis or cellular drinking occurs when the plasma membrane folds
inward to form a channel allowing dissolved substances to enter the cell.
When the channel is closed, the liquid is encircled within a pinocytic
vesicle.
• Receptor-mediated endocytosis occurs when specific molecules in the
fluid surrounding the cell bind to specialized receptors in the plasma
membrane. As in pinocytosis, the plasma membrane folds inward and the
formation of a vesicle follows.
Note: Certain hormones are able to target specific cells by receptor-mediated
endocytosis.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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