Download Plasma Membrane

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Anatomy & PhysiologyI
Chapter 3 (Cellular Form & Function)
Cells are the living structural & functional units of the body.
Humans start out as a single cell (fertilized ovum).
Adult humans have 75 trillion cells.
Cytology is the study of cells
Cell physiology is the study of cell function.
Structure of a Generalized Cell:
Cells develop into different shapes, sizes & types to carry out varied functions
Plasma MembraneIt is the selectively permeable covering of cells that separates a cell from outside.
Cell dies when the cell membrane is broken.
Cell membrane is composed of phospholipids, proteins, carbohydrates & cholesterol.
Phospholipid molecules are two-layered:
1. Outer polar phosphate layers are hydrophilic (in contact with extra- & intra-cellular fluid) &
2. Middle non-polar lipid layer is hydrophobic (water does not mix with it).
Protein molecules are scattered throughout phospholipids molecules. Their function is to:
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Support the membrane
Form channels that allow water & water-soluble substances to pass into the cell
Act as carrier molecules to transport ions & molecules across the membrane
Act as receptor sites for hormones & chemicals to attach to (insulin)
Act as markers for cell identification (to identify cell as self -- not foreign).
Act as enzymes to catalyze chemical reactions.
A) Transport of substances across plasma membrane
To maintain homeostasis, cell has to get essential nutrients in and waste products out.
Concentration is the amount of solute in a solution.
There is a concentration gradient (difference in the amounts of a solute) between the intracellular
and interstitial fluids (eg. more K+ inside cells, more Na+ outside cells).
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-This maintains osmotic balance between cytosol and interstitial fluid & generates electrical signals.
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Types of cellular transport:
1. Passive transport (diffusion & osmosis) moves solutes down a concentration gradient.
i) Simple Diffusion – movement of atoms, molecule & ions down concentration gradient
-Lipid soluble substances (O2, Co2, N2, steroids, fat soluble vitamins) diffuse through lipid bi-layer.
-Ions (K+, Ca++, Na++, Cl-) pass via gated channels in the integral proteins
ii) Facilitated Diffusion – movement of a molecule (glucose) down concentration gradient with the
help of a protein carrier molecule (insulin). Energy (ATP) is not used.
iii) Osmosis – movement of water down concentration gradient, across a membrane
Energy (ATP) is not used.
Osmotic pressure is exerted by a solute on a membrane that it cannot pass through.
The higher the solute concentration, the higher the osmotic pressure exerted on the membrane.
The osmotic pressure of cytosol and interstitial fluid is the same.
-This prevents a cell from shrinking or bloating due to osmosis.
Intravenous isotonic solution (normal saline - 0.9% NaCl) is used so that RBCs maintain their shape
In a hypotonic solution, water enters to first bloat and then burst the RBCs (hemolysis)
In a hypertonic solution, RBCs lose water and shrink (crenation)
2. Active transport uses cell energy (ATP) to move solutes up (low to high) concentration gradient.
There are two types of active transport:
a) Primary active transport – Uses energy from ATP to transport Na+, K+, H+, I-, Cl-, Ca++.
-It expels Na+ and imports K+ inside the cell against a concentration gradient.
-It is, therefore, called Na+ / K+ pump (or sodium pump) –
b) Secondary active transport – uses energy stored in Xs interstitial Na+ to transports substances
Symporters are integral proteins that simultaneously move substances in the same direction
(Na+ as well as glucose or amino acids enter bowel cells)
Antiporters simultaneously move substances in opposite direction (Na+ in & Ca++ or H+ out)
3. Vesicular transport – a tiny membrane sac is detached from cell membrane to move substances.
-It uses energy from ATP
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Two types of vesicular transport are:
i) Endocytosis – Ingestion of solids (phagocytosis) & liquids (pinocytosis) in vesicles. (phagocytes)
a) Phagocytosis - Phagocytes ingest virus, bacteria or dead cell & takes it to lysosome
b) Pinocytosis – Most body cells take in solutes dissolved in tiny drops of ECF
ii) Exocytosis – Excretion of cell products in vesicles.
-Waste products, digestive enzymes, milk, insulin and neurotransmitters are excreted this way.
CYTOPLASM –
It is gel-like intracellular fluid (cytosol) in which organelles are suspended.
Inclusion bodies may also be suspended temporarily in cytoplasm (vacuoles, glycogen, pigment)
1. Cytosol – Surrounds organelles, forms 55% of cell volume and is 75-90% water.
-It contains ions, glucose, lipids, proteins, ATP and wastes.
2. Organelles – ‘little organs’ in the cytoplasm having a specific shape & function.
a) Cytoskeleton – “maintains shape of the cell & anchors and moves some organelles”
It is made of protein microtubules & microfilaments.
b) Centrosome – “Aids distribution of chromosomes during cell division”
Dense centrosome near nucleus contains a pair of centrioles.
The 2 centrioles are rod-shaped microtubules placed perpendicular to each other.
c) Cilia – Short hair-like projections on cell membrane that move substances along cell surface.
d) Flagella – Long thread-like projections that whip the cell into motion (tail of sperm, T. Vaginalis)
e) Ribosomes -
“Protein synthesizer”
They are small RNA granules in the cytoplasm that make proteins.
Free ribosomes make protein for use in the cells.
Ribosomes attached to endoplasmic reticulum make proteins for export outside the cell.
Proteins synthesized in the ribosomes provide structure and become enzymes & hormones.
f) Endoplasmic Reticulum –
“transport path” “protein, lipid & detox- factory”
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These are membranous channels extending from the nuclear membrane to the cell membrane.
Rough endoplasmic reticulum has ribosomes attached to it that make proteins.
Smooth endoplasmic reticulum has no ribosomes and makes lipids, steroids & detoxifies drugs.
g) Golgi complex - “packaging & shipping”
Composed of 3-20 cisterns (flattened membranous sacs) near nucleus & connected to the ER.
-They modifiy & package proteins in secretory vesicle for export & to make plasma membrane.
-They also package enzymes in the lysosomes (a type of storage vesicle).
h) Lysosomes –
“enzyme store”
They contain enzymes that destroy cellular debris and foreign matter (bacteria, virus, dead cells).
Normally lysosomal membrane is impermeable to enzymes, preventing self-digestion (autolysis).
-In inflammation & anoxia, enzymes escape causing autolysis (eg. muscle & organ atrophy)
-After death, autolysis of cells occurs because enzymes leak from lysosomes.
i) Peroxisomes oxidize (remove H) from amino acids, fatty acids & toxic chemicals (alcohol).
-Peroxisomes in the liver & kidneys contain peroxidase & catalase that destroys toxic H2O2
j) Mitochondria –
“power house of the cell”
They are oval fluid-filled sacs in the cytoplasm.
Their membrane is 2 layered - outer smooth layer and inner convoluted layer called Crista.
Matrix is the fluid inside crista. It contains enzymes that converts glucose to ATP.
Enzymes that convert the nutrients we eat to ATP (energy) are located along the cristae.
They have some DNA & ribosomes that make proteins
k) Nucleus-
“control center”
It is the control center of the cell directing it’s metabolic activities.
Cells have one or more nuclei. Some cells like RBCs lose the nucleus as they mature.
A double-layered nuclear membrane separates nucleus from cytoplasm.
Nuclear membrane has pores that allow large RNA molecules to pass.
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Nucleus contains the genetic material of the cell (DNA) and one or more nucleoli.
-DNA is long thread-like in a non-dividing cell called Chromatin.
-Chromatin coils tightly to become rod-like Chromosomes in a dividing cell.
Chromosomes contain thousands of genes in a specific order for a species (human, sheep etc)
Humans have 23 pairs of chromosomes that direct synthesis of > 100, 000 proteins.
l) Nucleolus –
“house of RNA that makes ribosomes”
There are 1-4 dense nucleoli at the center of the nucleus that stain dark.
It has no covering membrane.
Nucleolus has high concentration of RNA and makes ribosomes.
-It is very large & distinct in growing cells & cells that make a lot of proteins.
J) Inclusions –
Inclusions freely float around in the cytoplasm.
They include fat, pigments, glycogen, viruses, bacteria etc.
They are not essential for cell survival.
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