Cell Structure and Function PDF

CELL STRUCTURE AND FUNCTION Dr. P. De Gabriele MD DipWH(ICGP) MMCFD  The Cell The basic, living, structural and functional unit of the body A cell can be divided into 4 principal parts: – Plasma (cell) Membrane – Cytoplasm – Organelles – Inclusions The Plasma Membrane ...

CELL STRUCTURE AND FUNCTION Dr. P. De Gabriele MD DipWH(ICGP) MMCFD  The Cell The basic, living, structural and functional unit of the body A cell can be divided into 4 principal parts: – Plasma (cell) Membrane – Cytoplasm – Organelles – Inclusions The Plasma Membrane  The Plasma Membrane The outer limiting membrane separating the cell’s internal parts from the extracellular fluid and the external environment. It is an exceedingly thin structure. – 6.5 – 10 nm in thickness. The Plasma Membrane - Structure Plasma membranes consist of – Phospholipids – Proteins – Cholesterol – Glycolipids – Water – Carbohydrates – Ions  Phospholipids They are arranged in two parallel rows forming a lipid bilayer. A phospholipid consists of: – a hydrophilic head-group – 2 long hydrophobic hydrocarbon chains. In water, such lipids spontaneously form a two- layered film with the hydrophilic heads of each layer pointing out towards the water and the hydrophobic tails packed next to one another.  Glycolipids These are always located on the outer surface of the plasma membrane. They have an oligosaccharide in the hydrophilic head projecting like hair into the surroundings.  Proteins Proteins are the main functional elements embedded in the lipid matrix. – Some proteins extend through the whole membrane. – Others are only anchored in the outer or inner layer. Some provide channels through which water and ions can diffuse in and out of the cell. The proteins are arranged so that their hydrophobic parts are within the lipid membrane, with the polar or hydrophilic groups on their outer surfaces. Many proteins on the outside of the plasma membrane are glycoproteins. – They provide receptor sites that enable a cell to recognise other cells, hormones, nutrients and other chemicals.  Cholesterol These molecules are almost equal in number to the phospholipids. They are interspersed among the phospholipids. They stabilise the cell membrane. FUNCTIONS OF THE PLASMA MEMBRANE 1. A flexible boundary that encloses the cellular contents and separates them from the external environment. 2. It facilitates contact with other body cells or with foreign cells or substances. 3. It provides receptors for chemicals such as hormones, neurotransmitters, enzymes, nutrients and antibodies. FUNCTIONS OF THE PLASMA MEMBRANE 4. The plasma membrane mediates the entrance and exit of certain substances and restricts the passage of other substances. (Plasma membranes permit some substances to pass more readily than others.) This is selective permeability. The permeability of the plasma membrane is due to several factors: 1. Size of the molecules Small molecules such as water and amino acids can enter and exit the cell easily. Most proteins are too large to pass. Such molecules do not enter the cell because they are larger than the minute channels in the integral proteins. 2. Solubility in lipids Lipid-soluble substances such as oxygen, carbon dioxide and steroid hormones pass through the cell membrane more rapidly. 3. Charge on ions The charge on the ion trying to pass through the plasma membrane determines the ease by which the ion enters and leaves the cell. If an ion has a charge opposite that of the membrane, it is attracted to the membrane and passes through more readily. 4. Presence of carrier molecules Some integral proteins are capable of attracting and transporting substances across the membrane regardless of the factors mentioned above.  CYTOPLASM The cytoplasm is the substance inside the cell’s plasma membrane and external to the nucleus. It is the matrix or ground substance of the cell. It is a thick, semitransparent, elastic fluid containing the cytoskeleton. The cytoplasm consists of: – Water – 75-90% of cytoplasm – Proteins – Carbohydrates – Lipids – Inorganic substances  CYTOPLASM The cytoplasm is the substance in which some chemical reactions occur. – It is the site of: synthesis of new substances where chemicals are packaged for transport to other parts of the cells. It also facilitates the excretion of waste materials.  ORGANELLES Organelles are specialised portions of the cell that assume roles in – Growth – Maintenance – Repair – Control They include: – NUCLEUS – RIBOSOMES – ENDOPLASMIC RETICULUM – GOLGI COMPLEX – MITOCHONDRIA – LYSOSOMES – CENTROSOME AND CENTRIOLES Nucleus Usually round or oval The largest structure in the cell It is the “control centre” of the cell. It is separated from the cytoplasm by a nuclear membrane, which contains several small pores. The nucleus contains the genetic material DNA as well as RNA that controls protein synthesis. Ribosomes These are tiny granules consisting of a type of RNA called ribosomal RNA (rRNA). They may be found on the rough endoplasmic reticulum or scattered within the cytoplasm. They are the sites of protein synthesis. Endoplasmic Reticulum  Endoplasmic Reticulum Consists of a system of parallel membranes surrounding a network of channels. It is continuous with the nuclear membrane. There are 2 types of ER – GRANULAR (rough) ER dotted with ribosomes – AGRANULAR (smooth) ER without ribosomes The ER – Provides a surface area for chemical reactions to occur – Acts as a transport channel through which various substances may pass from one part of the cell to another – Serves as a storage site for synthesised molecules – Plays a role in the packaging of molecules together with the Golgi Complex. Golgi Complex Usually situated near the nucleus Is made up of four to eight flattened membranous sacs stacked upon one another. Its main functions are: – To process, sort and deliver proteins to various parts of the cell – Lipid packaging and secretion Mitochondria These are sausage-shaped structures scattered throughout the cytoplasm. Each mitochondrion consists of two membranes: – The outer membrane – smooth – The inner membrane – folded upon itself forming cristae that provide a very large surface area where chemical reactions can take place. Mitochondria provide energy for the cell by producing ATP. GLUCOSE ↓ GLYCOLYSIS PYRUVIC ACID + fatty acids & amino acids ↓ CITRIC ACETYL-CoA ↓  KREBS’ CYCLE ↓ ADP ATP Lysosomes These are small spherical structures dispersed throughout the cytoplasm. They contain powerful digestive enzymes. Their function is to remove the debris from within the cell. Centrosome & Centrioles The centrosome is a dense area of the cytoplasm located near the nucleus. It contains a pair of cylindrical structures called centrioles. These play a part in spindle formation during cell division.  CELL INCLUSIONS Melanin – a pigment stored in cells of the skin, hair and eyes – protects the body by screening out harmful ultraviolet rays from the sun Glycogen – polysaccharide stored in the liver, skeletal muscle cells and the vaginal mucosa – It is broken down by the liver into glucose. Lipids – Stored in adipocytes – It may be decomposed producing energy. Mucus – provides lubrication and protection  THE CYTOSKELETON This is a collection of protein filaments and cylinders. – It determines the shape of the cell. – It lends it structural support. – It organizes its contents. – It moves substances through the cell. THE CYTOSKELETON Microfilaments – 6nm thick – Made of protein actin – Keep phospholipids together Intermediate Filaments – 8-10nm thick – Stiffer – Resist stresses Microtubules – 25nm in diameter – Cylinder made up of filaments