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6: Cell Membranes - Biology

6: Cell Membranes - Biology



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6: Cell Membranes

6: Cell Membranes - Biology

Investigate cell parts and organelles within organisms especially membranes.
Unit looks at biochemistry of cellular membrane structures. Discusses the structures and functions of the molecules within the membrane and how these membranes lead to specialized functioning of organelles and organs within organisms. Also looks at cell communication and how chemical structures affect function.
Unit looks at these concepts in action using the osmoregulation function of the excretory system. The unit discusses the importance of membrane structure and function to the structure and function of animals by describing the structure and function of various organs in the excretory, endocrine, and nervous systems and how they are structurally, chemically, and functionally related. It explains how thermoregulation involves physiological and behavioral adjustments. It explains how an animal’s nitrogenous wastes are correlated with its phylogeny and habitat and how nervous and hormonal feedback circuits regulate kidney functions. The unit explains how membrane potentials arise from differences in ion concentrations between cells’ content and the extracellular fluid. It explains how the interactions between myosin and actin are essential to movement In looking at plant transport, the unit describes how the differences in water potential drives water transport in plants and how the phloem translocates sap sugar from source to sink

Notes & Lectures
Mrs Fitzgerald's Powerpoint on Cells: find at the bottom of the page
Mrs Fitzgerald's Powerpoint on Membranes & Water Balance: find at the bottom of the page

Mr. Knuffke's Prezi on Cells: Endomembrane Systems
Mr. Knuffke's Prezi on Cells: Part 2: Matter & Energy Processing
Mr. Knuffke's Prezi on Cells: Part 3: Structure & Support
Mr. Knuffke's Prezi on Cells: Part 4: Transport

Bozeman Vodcast: A Tour of the Cell
Bozeman Vodcast: Biology Essentials #43: Cellular Organelles
Bozeman Vodcast: Biology Essentials #17: Compartmentalization
Bozeman Vodcast: Cell Membrane
Bozeman Vodcast: Biology Essentials #15: Cell Membranes
Bozeman Vodcast: Biology Essentials #7: Transport Across Cell Membranes
**Bozeman Vodcast: Thermoregulation, Osmoregulation, and Excretion - Watch this one!

Textbooks & Readings
University of Utah: Learn Genetics Cell Information
Activities & Labs
Bozeman Biology walks you through a Diffusion Demo
Study Guides & Reviews

Other Links
BBC's The Cell movie on Youtube- split into 6 parts


Cell Membrane - Cell Wall

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

Cell walls are only found in plant cells and are used primarily to create structure, but cell membranes are found in all cells. The bulk of the membrane is composed of a double layer of lipids called the lipid bilayer. Inside the bilayer there is a layer of cholesterol to keep the membrane fluid. Imbedded in the membrane are various proteins which provide channels for specific molecules to pass through, making the membrane semi-permeable.

The outside of a cell in many kinds of cells is covered with a rigid structure called the cell wall whether you're a plant, a fungi, many of the bacteria as well as the protest group called algae. Now the cell wall is made out of a material called cellulose or chitin or peptidioglycan. There's others out there. Some of the algae even use glass, silicon dioxide as their cell walls but in general they all provide structure and support for the cell that's inside of that cell wall.

Let's take a quick look at this plant cell over here and you can see the thick outer cell wall that helps keep the cell inside protect it plus, since they don't have skeletons like you and I do, if they didn't have this rigid structure outside, they would just collapse and instead of having tall trees, you'd just have little blobs of green goo. Now if we go back here, the cell membrane often referred to as the plasma membrane, is what you find immediately underneath that cell wall, and it's made up of something called a phospholipid bilayer which I'll discuss later. Now phospholipid bilayer is one of the key things that allows the cell membrane or plasma membrane to control what can come in or out of the cell. In fact because of some of other things that are embedded within the phospholipid bilayer, it's also able to change what it can allow in from moment to moment which gives it not just the semi-permeable nature of the phospholipid bilayer but it's actually a subset of that called a selectively permeable barrier.

Now, another thing that is caused by not the phospholipids but some of the other molecules that are part of the membrane is that it's involved in cell to cell communication. So again if we take a look at our plant cell here, here's the cell wall. Immediately underneath that is the thin little phospholipid bilayer. Now it's called a phospholipid bilayer because this structure here and this structure here, each of them is a phospholipid and you see one, two layers, duh.

Now, a phospholipid is a special kind of lipid or a fat molecule and it has one end that has a phosphate head. Phosphate is an ion which means has a strong negative charge. These little wiggly bits down at the end are the two fatty acids tails, they're called sometimes. I've often thought they look like little legs of some guy wearing a swim vest and just kind of floating around in the water. These legs are the fat part and fats as you may have seen if you've ever had to do the dishes, fats greases don't mix with water. So these here are hiding from the water that's on the inside and outside of the cell. I've often thought of a phospholipid, the phosphate here which can interact with the water and the fatty acid tails that can't, it's kind of like you and your two younger siblings. You're awesome, you're cool. Everybody wants to interact with you, everybody being the water. You can hydrogen bond with them as you interact with them at a party.

Your younger siblings, nobody wants to interact with them and they don't really care about interacting with anybody else. They just want to sit there and look at their Pokemon cards or whatever it is that they're into. So this phosphate head interacts with the water, the Pokemony down here doesn't and what you wind up doing is if you wind up going to a party and your parents shackle your younger siblings to you, you'll hide them behind you. And you may form a wall of older siblings with the younger siblings hiding behind them and in fact you could form this double layer here. Anything that tries to get through, has to be able to interact with the fatty acid tails and not too many chemicals can. You'd have to be uncharged and generally you'd have to be small. And most molecules can't do it. Which is why this is impermeable to most molecules, that means that they can't penetrate through. Some molecules can, like oxygen gas and carbon dioxide gas. They can permeate straight through it because sometimes some things can other things cannot, that's called semi-permeable.

Now, I mentioned previously that the cell membrane is actually selectively permeable because it can change what is allowed in and what can't come in. This is what your cell membrane would actually look like. Here these purple globs represent protein that are embedded within the membrane. Here we see our phospholipid bilayer. These little orange things are steroid or cholesterol molecules that help, give some rigidity to the membrane. And they're flowing around. This is sometimes called the fluid mosaic model because it's fluid and it's made up of small pieces much like mosaic.

Now, these proteins, some of them are pumps or channels. Pumps like the name implies can pump things in or out of the cell if it spends some energy. Channels have little doors that they can open or close, open when they want something to come in or out close when they don't.

Now some of these other things that are attached are sugarcanes, polysaccharides that, especially when they attached to proteins, they're called glycoproteins, when they're attached to some of the phosphate heads here, they're called glycolipids because a phospholipid is a kind of lipid. These are what I was talking about earlier when we talked about how cell membranes are involved in cellular communication and recognition.

Your white blood cells for example are constantly investigating all the cells of your body, using these other molecules, these proteins and glycoproteins and glycolipids to recognise what cells belong in your body, which ones don't. And for example if a cell is trying to communicate with another cell let's say a neuron in your brain is trying to communicate with another neuron to say hey, I thought of something. It does it by releasing chemicals through the membrane which go to the next cell and interact with proteins on its membrane.

So there you go. The cell wall is on the outside of the cell and it's made of rigid structures like cellulose or chitin and it gives protection and structure to the cell. Inside the cell wall, you'll have the phospholipid bilayer of the plasma membrane. It's made out of these phospholipids plus a bunch of other chemicals such as proteins.


Function and Structure of Cell membrane

A Cell membrane is a thin semi-permeable membrane that surrounds the cytoplasm of a cell, enclosing its contents. The membrane also gives a cell its shape and enables the cell to attach to other cells, forming tissues. The cell membrane also serves as a base of attachment for the cytoskeleton in some organisms and the cell wall in others. Thus, it helps to support the cell and maintain its shape. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. The basic function of the cell membrane is to protect the cell from its surroundings.

The Cell membrane is primarily composed of a mix of proteins and lipids. While lipids help to give membranes their flexibility, proteins monitor and maintain the cell’s chemical climate and assist in the transfer of molecules across the membrane.

The cell membrane is a selectively permeable membrane that is composed of hydrophilic heads and hydrophobic tails. This allows the membrane to form a lipid bilayer. The cell membrane is also embedded with cell membrane proteins. These proteins float in the bilayer and allow other solutes to enter and exit the cell. These proteins also act as messengers to the cell by binding to objects outside of the cell and transferring a message to the cell’s nucleus.

The Cell membranes are involved in a variety of cellular processes such as cell adhesion, ion conductivity and cell signalling and serve as the attachment surface for several extracellular structures, including the cell wall, glycocalyx, and intracellular cytoskeleton. Cell membranes can be artificially reassembled.

Functions of Cell membranes

A cell membrane protects the structures within the cell. Cell membranes are semipermeable, meaning that only certain objects are able to pass through them. Cell membranes also give shape to the cell and support its structure.

Some functions of the cell membrane are:

  • To maintain the physical integrity of the cell – that is to mechanically enclose the contents of the cell, and also
  • To control the movement of particles e.g. ions or molecules, into and out of the cell.
  • The cell membrane maintains the physical integrity of the cell. It’s most obvious in the cases of animal cells (because they don’t have cell walls) that the cell membrane holds the cell together by enclosing the cytoplasm and organelles within it.
  • The cell membrane forms a barrier between the inside of the cell and the environment outside the cell – enclosing cytoplasm and any organelles within the cell, and enabling different chemical environments to exist on each side of the cell membrane.
  • The cell membrane physically separates the intracellular components (e.g. organelles in eukaryotic cells) from the extracellular environment. The cell membrane protects the cell from some harmful chemicals in its external environment.
  • It also protects the cell from loss of useful biological macromolecules held within the cell by its plasma membrane.

  • The cell membranes that enclose cells (inside the cell wall in the cases of plant cells and prokaryotic cells) are selectively permeable. That is, the structure of these membranes is such that they allow certain particles, incl. e.g. molecules, – but not others – to pass through the membrane, hence into or out of the cell.
  • Endocytosis is the process in which cells absorb molecules by engulfing them. The plasma membrane creates a small deformation inward, called an invagination, in which the substance to be transported is captured. Endocytosis is a pathway for internalizing solid particles (“cell eating” or phagocytosis), small molecules and ions (“cell drinking” or pinocytosis), and macromolecules. Endocytosis requires energy and is thus a form of active transport.
  • Proteins and lipids make up the composition of a cell membrane. There are three different types of proteins found within a cell membrane: structural protein, transport protein and glycoprotein. These support cell structure and shape, move molecules through the membrane and transmit signals between cells.
  • Cell membranes often include receptor sites for interaction with specific biochemicals such as certain hormones, neurotransmitters and immune proteins. In this way the cell can recognize and process some signals received from the extracellular environment.

Structures of Cell membranes

The cell membrane is primarily composed of a mix of proteins and lipids. Depending on the membrane’s location and role in the body, lipids can make up anywhere from 20 to 80 percent of the membrane, with the remainder being proteins. While lipids help to give membranes their flexibility, proteins monitor and maintain the cell`s chemical climate and assist in the transfer of molecules across the membrane.

The Cell Membrane is a Fluid Mosaic: A cell is the basic unit of life, and all organisms are made up of one or many cells. One of the things that all cells have in common is a cell membrane. It is a barrier that separates a cell from its surrounding environment. It is composed of four different types of molecules:

The fluid mosaic model describes the structure of a cell membrane. It indicates that the cell membrane is not solid. It is flexible and has a similar consistency to vegetable oil, so all the individual molecules are just floating in a fluid medium, and they are all capable of moving sideways within the cell membrane.

Phospholipids: Phospholipids are a major component of cell membranes. Phospholipids form a lipid bilayer in which their hydrophillic (attracted to water) head areas spontaneously arrange to face the aqueous cytosol and the extracellular fluid, while their hydrophobic (repelled by water) tail areas face away from the cytosol and extracellular fluid. The lipid bilayer is semi-permeable, allowing only certain molecules to diffuse across the membrane.

Cholesterol: Cholesterol is another lipid component of animal cell membranes. Cholesterol molecules are selectively dispersed between membrane phospholipids. This helps to keep cell membranes from becoming stiff by preventing phospholipids from being too closely packed together. Cholesterol is not found in the membranes of plant cells.

Glycolipids: Glycolipids are located on cell membrane surfaces and have a carbohydrate sugar chain attached to them. They help the cell to recognize other cells of the body.

Carbohydrates: Carbohydrates, or sugars, are sometimes found attached to proteins or lipids on the outside of a cell membrane. That is, they are only found on the extracellular side of a cell membrane. Together, these carbohydrates form the glycocalyx.

Cell Membrane Proteins: The cell membrane contains two types of associated proteins. Peripheral membrane proteins are exterior to and connected to the membrane by interactions with other proteins. Integral membrane proteins are inserted into the membrane and most pass through the membrane. Portions of these transmembrane proteins are exposed on both sides of the membrane. Cell membrane proteins have a number of different functions. Structural proteins help to give the cell support and shape. Cell membrane receptor proteins help cells communicate with their external environment through the use of hormones, neurotransmitters, and other signaling molecules. Transport proteins, such as globular proteins, transport molecules across cell membranes through facilitated diffusion. Glycoproteins have a carbohydrate chain attached to them. They are embedded in the cell membrane and help in cell to cell communications and molecule transport across the membrane.


Why are cells so small?

Cells are so small that you need a microscope to examine them. Why? To answer this question we have to understand that, in order to survive, cells must constantly interact with their surrounding environment. Gases and food molecules dissolved in water must be absorbed and waste products must be eliminated. For most cells, this passage of all materials in and out of the cell must occur through the plasma membrane.Each internal region of the cell has to be served by part of the cell surface. As a cell grows bigger, its internal volume enlarges and the cell membrane expands. Unfortunately, the volume increases more rapidly than does the surface area, and so the relative amount of surface area available to pass materials to a unit volume of the cell steadily decreases.Finally, at some point, there is just enough surface available to service all the interior if it is to survive, the cell must stop growing. The important point is that the surface area to the volume ratio gets smaller as the cell gets larger. Thus, if the cell grows beyond a certain limit, not enough material will be able to cross the membrane fast enough to accommodate the increased cellular volume. When this happens, the cell must divide into smaller cells with favorable surface area/volume ratios, or cease to function. That is why cells are so small.


Questions in Short Answer Questions (SA)

Q1) Name the scientist who invented the first microscope.

Q2) Who coined the term "cell" ?

Q3) Briefly describe the three essential basic parts of a cell.

Q4) The cell membrane is called selectively permeable. Why ?

Q5) State the difference between

(ii) Cytoplasm and protoplasm

(iii) Cell wall and cell membrane

Q6) List the major differences between a plant cell and an animal cell.

Q7) Briefly discuss the importance of chromosomes to an organism.

Q8) Fill in the blanks with the terms given below :

Pigments, wall, pre-existing , cell, vacoules

The _______ is the structural unit of all living things.

All cells arise from __________ cells.

Animal cells have no cell ______.

Plastids contain __________.

_________ are filled with water and dissolved substances.

Q9) Try to find the names of four cell organelles hidden in this maze.(hint : The hidden words can appear horizontally or vertically forwards or backward or even mixed up). Write them in the lines provided. For example : "NUCLEUS" in the last row, seven backward letters .

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Q1) Name the scientist who invented the first microscope.

Q2) Who coined the term "cell" ?

Q3) Briefly describe the three essential basic parts of a cell.

Q4) The cell membrane is called selectively permeable. Why ?

Q5) State the difference between

(ii) Cytoplasm and protoplasm

(iii) Cell wall and cell membrane

Q6) List the major differences between a plant cell and an animal cell.

Q7) Briefly discuss the importance of chromosomes to an organism.

Q8) Fill in the blanks with the terms given below :

Pigments, wall, pre-existing , cell, vacoules

The _______ is the structural unit of all living things.

All cells arise from __________ cells.

Animal cells have no cell ______.

Plastids contain __________.

_________ are filled with water and dissolved substances.

Q9) Try to find the names of four cell organelles hidden in this maze.(hint : The hidden words can appear horizontally or vertically forwards or backward or even mixed up). Write them in the lines provided. For example : "NUCLEUS" in the last row, seven backward letters .


Membranes

The cell membrane is the barrier that separates the cytoplasm from the external world. The cell membrane consists primarily of phospholipids in a bilayer. Phospholipids are amphipathic with a polar head (phosphate group) and a hydrophobic tail (2 hydrocarbon chains). Due to the chemical properties of the heads being attracted to water and the tails having a desire to avoid water, phospholipids self assemble into micelles. Cell membranes form from a phospholipid bilayer where the lipid tails interact with each other and the phosphate heads face the external water environment or the internal cytoplasm of the cell.
The cell membrane does not solely consist of phospholipids but also have proteins and cholesterol inserted into the bilayer. As the image of the bilayer above indicates, the molecules are constantly moving and flow in a lateral motion. Cholesterol modulates the fluidity of this motion. Proteins associated with the membrane may sit on either side ( peripheral proteins ) of the membrane or pass through both layers of the membrane ( transmembrane proteins ). The model that describes the components of the cellular membrane is referred to as the Fluid Mosaic Model . This model states that the cell membrane is a mosaic of 1)Phospholipids 2)Proteins 3) cholesterol that move about in a side to side motion.

The fluid mosaic of phospholipids, proteins and cholesterol that create the selective barrier between the interior and the exterior of the cell. Evidence for the fluid mosaic model comes from a process called freeze-fracture electron microscopy. A cell is essentially frozen and etched in a way that the leaves of the lipid bilayer are exposed. Using electron Microscopy, the features of the leaves of the bilayer can be examined using a scanning electron microscope.


Small uncharged molecules pass through the double layer of phospholipids. Polar, charged or large molecules have great difficulty passing through the membrane and require the aid of transmembrane proteins. An example of a transmembrane protein that facilitates movement of a polar substance is aquaporin, which permits the free movement of water.

1) a single transmembrane α-helix (bitopic membrane protein). 2) a polytopic transmembrane α-helical protein. 3) a polytopic transmembrane β-sheet protein. The membrane is represented in light green.

This site is a reading guide for the Molecular and Cell Biology BIO3620 Course.


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