2021 Ch 6. La membrana plasmática (The plasma membrane).

 Período 1. Capítulo 3. Semana 7.

Ø    Preliminares

Tiempos

Desde (03-05-2021) hasta (07-05-2021). Nota de puntualidad 50.

Desde (08-05-2021) hasta (14-05-2021). Nota de puntualidad 40.

Desde (15-05-2021) hasta (16-05-2021). Nota de puntualidad 30.

Después del (16-05-2021) Nota de puntualidad 20, más penalización de 5 puntos en cada actividad.

Eje temático

- La célula.

Objetivos

-Describir la membrana plasmática y sus propiedades.

Ø    Contents and activities

           Engagement

(6.1) Draw the following pictures in your notebook.

(6.2) Look at the following pictures and write three ideas for each picture in English and Spanish that relate them to the ideas of (theory) and (cell).

        Exploration

(6.3) What do you think the edges of a cell are made of?.

(6.4) How would you describe the shape of a cell? (a) circular and flat like an arepa, or (b) spherical like a balloon. Discuss it with your parents and explain your answer.

Fig 1. A model of the plasma membrane. The plasma membrane is composed of a phospholipid bilayer. The polar heads of the phospholipids are at the surfaces of the membrane; the nonpolar tails make up the interior of the membrane. Proteins embedded in the membrane have various functions.                

 Explanation

          (6.5) Read the following text

All cells have an outer membrane called the plasma membrane, which acts as the boundary between the outside and inside of a cell. The integrity and function of the plasma membrane are vital to a cell because this membrane acts much like a gatekeeper, regulating the passage of molecules and ions into and out of the cell.

The structure of the plasma membrane plays an important role in its function. In all cells, the plasma membrane consists of a phospholipid bilayer with numerous proteins embedded in it (Fig. 1). In the bilayer, the polar (hydrophilic) heads of the phospholipids are oriented in two directions. In the outer layer of the membrane, the phospholipid heads face toward the external environment. On the interior layer of the membrane, the heads of phospholipid molecules are directed toward the interior cytoplasm of the cell. The nonpolar (hydrophobic) tails of the phospholipids point toward each other, in the space between the layers, where there is no water. Cholesterol molecules, present in the plasma membrane of some cells, lend support to the membrane, giving it the general consistency of olive oil.

The structure of the plasma membrane (Fig. 1) is often referred to as the fluid-mosaic model, since the protein molecules embedded in the membrane have a pattern (a mosaic) within the fluid phospholipid bilayer. The actual pattern of proteins varies according to the type of cell, but it may also vary within the membrane of an individual cell over time. For example, the plasma membrane of a red blood cell contains over 50 different types of proteins, and they can vary in their location on the surface, forming a mosaic pattern

Short chains of sugars are attached to the outer surface of some of these proteins, forming glycoproteins. The sugar chain helps a protein perform its particular function. For example, some glycoproteins are involved in establishing the identity of the cell. They often play an important role in the immune response against disease-causing agents entering the body.

Functions of Membrane Proteins

The proteins embedded in the plasma membrane have a variety of functions. Channel Proteins Channel proteins form a tunnel across the entire membrane, allowing only one or a few types of specific molecules to move readily through the membrane (Fig. 2a). For example, aquaporins are channel proteins that allow water to enter or exit a cell. Without aquaporins in the kidneys, your body would soon dehydrate.

Transport Proteins

Transport proteins are also involved in the passage of molecules and ions through the membrane. They often combine with a substance and help it move across the membrane, with an input of energy (Fig. 2b).

Fig 2. Membrane protein diversity. Each of these types of proteins provides a function for the cell.

For example, a transport protein conveys sodium and potassium ions across a nerve cell membrane. Without this transport protein, nerve conduction would be impossible.

Cell Recognition Proteins

Cell recognition proteins are glycoproteins (Fig. 2c). Among other functions, these proteins enable our bodies to distinguish between our own cells and the cells of other organisms. Without this distinction, pathogens would be able to freely invade the body.

Receptor Proteins

A receptor protein has a shape that allows a specific molecule, called a signal molecule, to bind to it (Fig. 2d). The binding of a signal molecule causes the receptor protein to change its shape and thereby bring about a cellular response. For example, the hormone insulin binds to a receptor protein in liver cells, and thereafter these cells store glucose.

Enzymatic Proteins

Some plasma membrane proteins are enzymatic proteins that directly participate in metabolic reactions (Fig. 2e). Without enzymes, some of which are attached to the various membranes of a cell, the cell would never be able to perform the degradative and synthetic reactions that are important to its function.

Junction Proteins

Proteins are also involved in forming various types of junctions (Fig. 4.5f) between cells. The junctions assist cell-to-cell adhesion and communication. The adhesion junctions in your bladder keep the cells bound together as the bladder swells with urine.

All cells have a plasma membrane that consists of phospholipids and embedded proteins.

Phospholipids, also known as phosphatides, are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group, and two hydrophobic "tails" derived from fatty acids, joined by a glycerol molecule. The phosphate group can be modified with simple organic molecules such as choline, ethanolamine or serine.

Fig 3. Simple models for one phospholipid and a phospholipid bilayer devoid of proteins and other elements of a true plasma membrane.

Phospholipids are a key component of all cell membranes. They can form lipid bilayers because of their amphiphilic characteristic. In eukaryotes, cell membranes also contain another class of lipid, sterol, interspersed among the phospholipids. The combination provides fluidity in two dimensions combined with mechanical strength against rupture. Purified phospholipids are produced commercially and have found applications in nanotechnology and materials science. The first phospholipid identified in 1847 as such in biological tissues was lecithin, or phosphatidylcholine, in the egg yolk of chickens by the French chemist and pharmacist Theodore Nicolas Gobley.

Elaboration

(6.5) Draw the current model of the cell membrane with as much detail as possible, each name must be given in Spanish and English. Use a full legal and letter size page.

(6.6) Complete a two-page summary in Spanish of the explanation section.

(6.7) Identify the basic components that make up the structure of the plasma membrane. (Give your answer in English)

(6.8) Explain why the plasma membrane is described as a fluid-mosaic model. (Give your answer in Spanish)

(6.9) Distinguish among the types of membrane proteins by function. (Give your answer in Spanish)

(6.10) Explain what is the difference between a true plasma membrane and a simple phospholipid bilayer. (Give your answer in Spanish)

(6.11) Explain what is the difference between a true plasma membrane and a simple phospholipid bilayer. (Give your answer in Spanish)

(6.12) What is the relationship between the initial images and the topic explained?

Evaluation

Enzymes are powerful components of the plasma membrane. In Fig. 4., a comparison of two chemical processes is observed, one occurs without an enzyme and the other with an enzyme. The energy required on the (y) axis and the amount of reaction on the (x) axis are compared.

Fig 4. Energy comparison of a process with and without enzymes.

(6.13) Which process uses the most energy?

(a) with enzymes

(b) without enzymes

(6.14) What is the benefit to the cell of evolving powerful enzymes in its metabolic reactions?

Impacto vital

(6.16) Como parte de las actividades del PROYECTO AMBIENTAL ESCOLAR PRAE, resolver la actividad de Impacto Vital. Traduzca el texto y transcríbalo con las imágenes (u otras que sirvan para el mismo propósito).

1. People have always had trash. Only cave men didn’t have a big problem with it. In those days, there was plenty of room for trash_.

2. Today trash is a big problem. More people mean more trash and more different kinds of trash: cans, papers, bottles, clothes, cars, etc. In time all becomes T-R-A-S-H

3. Every year each one of us throws away almost one ton of trash. If you piled this trash in your living room, it would come up to your shoulders.

4. In one year all our trash amounts to 360,000,000 tons. We have a problem with trash that cave men never had. To get rid of it we have tried:

5 … burn it. But burning trash can cause air pollution.

6 … dump it in the ocean. But dumping can pollute water too.

₇ … bury it. But we are running out of empty land near cities.

8. Some scientists have even suggested shooting it off into empty space. But who wants old trainers or can orbiting the Earth?

9. In the past few years however we have found a new way to get rid of some of our trash. It’s called RECYCLING.

Recycling means reusing our trash instead of getting rid of it. This solves the problem of what to do with our trash and it also helps us with another problem. By using the same materials over and over again, we save our natural resources.

Recycling means shredding old cans and cars

and melting the pieces to make new metal

for new cars and cans …

… chopping up grass and garbage to burn for

energy or to make fertilizer to help new

plants grow.

... crushing bottle into tiny glass bits and

melting these bits to make new glass.