Lecture 3 13/11/2018

Transcription

1 Lecture 3 13/11/2018 1

2 Plasma membrane ALL cells have a cell membrane made of proteins and lipids. protein channel Cell Membrane Layer 1 Layer 2 lipid bilayer protein pump Lipid bilayer allows water, carbon dioxide, oxygen to pass through easily. The PM constitutes a barrier against movement of ions, and large molecules between the extracellular and intracellular fluid compartments. Membrane transport protein in the PM facilitate the movement of molecules and ions across the membrane. 2

3 Membrane transport protein A membrane transport protein is a membrane protein involved in the movement of ions, small molecules, or macromolecules, such as another protein, across PM. channel protein : serves as a tunnel across the membrane into the cell. More specifically, channel proteins help molecules across the membrane via passive transport, a process called facilitated diffusion. Carrier proteins: bind with molecules or ions that are to be transported; conformational changes in the protein molecules then move the substances through the interstices of the protein to the other side of the membrane. 3

4 Protein channels Transmembrane proteins can form channels across the membrane to allow substances to pass across the membrane Ion channels: tiny openings, specific amino acid composition only allows specific ions through (ex: Na+ channels, K+ channels) - Leaky ion channels: always open, ion of choice can leak in and out based on gradients - Gated ion channels: channel changes shape in response to a specific cue, gate opens to allow ion through, only then can it move with its gradient Water channels: water is allowed in and out of the cell through specific pores called aquaporins. 4

5 Gated Channels Gated channels are divided into two categories: i. Voltage-gated channels (Voltage-gated channels are the channels which open whenever there is a change in the electrical potential across the PM). ii. Ligand-gated channels (Some protein channel gates are opened by the binding of a chemical substance (hormone) (a ligand) with the protein; this causes a conformational or chemical bonding change in the protein molecule that opens or closes the gate, e.g.: the effect of acetylcholine on the acetylcholine channel. 5

6 Passive and active transports Passive transport: is the movement molecules across a membrane from an area of high concentration to low concentration WITHOUT the need of energy. Types: Simple diffusion Osmosis Facilitated Diffusion Active transport: is the movement of molecules across a membrane from a region of their lower concentration to a region of their higher concentration. It requires energy. Types: Primary active transport Secondary active transport Vesicular transport 6

7 passive transport and gradients Molecules move passively when there is a gradient across the membrane without using energy. There are two types of gradients: - Concentration gradient It is the process of particles move through a solution or gas from an area with a higher number of particles to an area with a lower number of particles. - Electrical gradient More positively charged molecules (ions) in one region vs. another 7

8 ELECTRICAL GRADIENT 8

9 Types of passive transport 1- Diffusion: Movement of molecules from areas of high concentration to low concentration gradient without the aid of an intermediary such as a integral membrane protein. Diffusion results in equilibrium (all molecules occupy the same amount of space) For example: lipid layer of the cell membrane is permeable only to lipid-soluble substances like oxygen, carbon dioxide and alcohol 9

10 Types of passive transport 2- Facilitated diffusion of ions takes place through proteins, or assemblies of proteins, embedded in the plasma membrane. These transmembrane proteins (protein carrier and protein channels) form a waterfilled channel through which the ion can pass down its concentration gradient. examples of molecules that must use facilitated diffusion to move in and out of the cell membrane are glucose, sodium ions, and potassium ions. 10

11 Types of passive transport 3- Osmosis: The diffusion of water across the cell membrane. The direction of osmosis depends on the concentration of solutes inside and outside the cell. Solution: solute + solvent mixed together - Solute: molecules in solution - Solvent: water 11

12 Types of solutions: Hypertonic Solutions: contain a high concentration of solute relative to another solution (e.g. the cell's cytoplasm). When a cell is placed in a hypertonic solution, the water diffuses out of the cell, causing the cell to shrivel. Hypotonic Solutions: contain a low concentration of solute relative to another solution (e.g. the cell's cytoplasm). When a cell is placed in a hypotonic solution, the water diffuses into the cell, causing the cell to swell and possibly explode. Isotonic Solutions: contain the same concentration of solute as another solution (e.g. the cell's cytoplasm). When a cell is placed in an isotonic solution, the water diffuses into and out of the cell at the same rate. The fluid that surrounds the body cells is isotonic. 12

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14 Active transport Sometimes cells must move molecules against the concentration or electrical gradients. Energy is required as molecules must be pumped against the concentration gradient. Carrier proteins that work as pumps are called protein pumps. Molecule to be transported Molecule being 14 transported

15 Types of active transport 1- Primary active transport: utilizes energy in form of ATP to transport molecules across a membrane against their concentration gradient. Example: Sodium and potassium pump. Sodium-potassium pump: a protein maintains the internal concentration of potassium ions [K + ] higher than that in the surrounding medium (blood, body fluid, water) and maintains the internal concentration of sodium ions [Na + ] lower than that of the surrounding medium. The pump, which has adenosine-triphosphatase (ATPase). activity, traverses the cell membrane and is activated by external [K + ] and internal [Na + ]. 15

16 Proton pump: hydrogen ion is actively transported across the cell membrane by the carrier protein called hydrogen pump. It also obtains energy from ATP by the activity of ATPase. the gastric hydrogen potassium ATPase or H + /K + ATPase is the proton pump of the stomach. It exchanges potassium from the intestinal lumen with cytoplasmic hydronium and is the enzyme primarily responsible for the acidification of the stomach contents and the activation of the digestive enzyme pepsin. 16

17 2- Secondary active transport: is transport of molecules across the cell membrane utilizing energy in other forms than ATP. This energy comes from the electrochemical gradient created by pumping ions out of the cell. This Co-Transport can be either via antiport or symport. Example : Na+ / glucose co-transporter The formation of the electrochemical gradient, which enables the co-transport, is made by the primary active transport of Na+. Na+ is actively transported out of the cell, creating a much higher concentration extracellularly than intracellularly. This gradient becomes energy as the excess Sodium is constantly trying to diffuse to the interior. This mechanism provides the energy needed for the co-transport of other ions and substances. This is evident in co-transporters such as the Sodium-glucose cotransporter. The Na+ gradient created by the Na+/K+ ATPase is used by the Na+/Glucose co-transporter to transport glucose and Na+ back into the cell. 17

18 3- Vesicular transport: are associated with the transport of macromolecules such as big protein molecules which can neither travel through the membrane by diffusion nor by active transport mechanisms Types of vesicular transport: A-Endocytosis is the process of capturing a substance or particle from outside the cell by engulfing it with the cell membrane, and bringing it into the cell. B- Exocytosis describes the process of vesicles fusing with the plasma membrane and releasing their contents to the outside of the cell. 18

19 C- Receptor-mediated endocytosis is when the material to be transported binds to a specific molecule in the membrane. Example: transporting cholesterol into cytoplasm 19

20 D- Phagocytosis is the type of endocytosis where an entire cell is engulfed. Example: engulfing bacterial cell by white blood cell. E- Pinocytosis is when external fluid is engulfed. Example: nursing of human egg cells from surrounding cells 20

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22 Cell (Intercellular) junctions Cell junctions are specialized regions of contact between the plasma membranes of adjacent cells. Cell junctions consist of protein complexes that provide contact between neighboring cells. Types of intercellular junction: i. Gap junction ii. Tight junction iii. Desmosome iv. Hemidesmosome v. Adheren 22

23 Gap Junction Gap junctions consist of intercellular channels in the plasma membrane of adjacent cells. Consist of six connexon proteins arranged to form a flower shape structure. Play role in cardiac muscle contraction (for calcium movements) 23

24 Tight Junction The borders of two cells are fused together forming a continuous belt like junction known as a tight junction. Prevent leakage of fluid. It regulate the movement of water and solutes between epithelial layer. Example: epithelial cells in gastrointestinal tract are attached by tight junction. 24

25 Desmosomes Form cell to cell junction. Intracellular adaptor proteins connect to intermediate filament and form cytoplasmic plaque. Cadherin joins the cytoplasmic plaques of two cells. Found in epidermis of skin and muscle tissues. Cadherin 25

26 Adherens Junctions They are composed of Cadherins bind to the catenins that are connected to the actin filaments. Provide strong mechanical attachments between adjacent cells. They serve as a bridge connecting the actin cytoskeleton of neighboring cells through direct interaction. 26

27 Hemidesmosome Hemidesmosomes are very small stud-like structures found in epidermis of skin that attach to the extracellular matrix (ECM). Found in epithelial cells connecting the basal epithelial cells to the basal lamina. 27