Table of Contents

BIOLOGY 1110: BIOLOGY - Notes

Professor: NANCY KEENE

Chapter 4-5 Unit III

Membranes and Cells: Lecture Notes

Membrane Function Summary

Membranes define or separate the cell from its internal environment; they compartmentalize the cell ( in eukaryotic cells); they serve as a location or surface for specific cellular functions; they control and regulate the movement of substances into and out of the cell and its compartments; and they serve as a means of sampling the external environment and detecting the environment's conditions.

Membrane Structure

In this discussion we'll focus on the plasma membrane, that is to say, the membrane which surrounds the cell, separating the internal contents from the external environment. The internal membranes of the cell have basically the same structure but vary in the types of proteins one may find associated with them.

As we discuss the structure of the membrane, you should think of the membrane as a three-dimensional structure rather than a flat surface. As an analogy, the membrane is like a sandwich with several layers of toppings rather than a uniform layer of plastic wrap. The currently accepted model of the structure of cell membranes is referred to as the fluid mosaic model. In this model, the membrane is composed of a phospholipid bilayer, cholesterol, proteins and other molecules which give the membrane its structure and functionality. All of this structure is sandwiched into a structure approximately 8 nm thick (one nanometer is 1x10-9meters).

The Phospholipid Bilayer - The primary structure of membrane is composed of two layers of phospholipid molecules. Recall that phospholipids are molecules with a phosphate group at one end and two long fatty acids at the other. Often these are referred to as the fatty acid tails of the phospholipid. The end of these molecules with the phosphate group is very polar and thus water soluble. It is said to be hydrophilic or 'water loving'. The fatty acid tails are very non-polar or hydrophobic, 'water hating' portions of the molecule.

The phospholipid molecules orient themselves so that they form two layers. The phosphate group end of the molecules are oriented toward the outside and inside surfaces of the membrane (it's these surfaces which are in contact with the watery solutions found in the outside environment and the cell's cytoplasm). The fatty acid tails of the phospholipids are oriented toward the middle of the membrane. These non-polar portions form a hydrophobic zone within the membrane. This hydrophobic zone accounts for much of the membrane's ability to allow some compounds to cross the membrane while preventing others from entering or leaving the cell.

Cholesterol - Within the inner portion of the membrane, molecules of cholesterol are found among the fatty acid tails. Like the fatty acid tails, cholesterol is a hydrophobic molecule. The cholesterol molecules appear to maintain the membrane's fluidity, its ability to change its shape and to have the phospholipid molecules move past one another. Experiments have found that if the cholesterol is removed from the membrane, fluidity is lost and the membrane is no longer able to function properly.

Associated Proteins - Essential to the overall function of the membrane are a large variety of proteins associated with it. These proteins are embedded within the phospholipids. Some of the proteins pass through the entire membrane so that it's exposed on either side of the membrane. Other proteins only extend part way into the phospholipids so that they are only exposed on the outside or the inside surface of the membrane.

An analogy you might use to visualize this model of membrane structure is that the phospholipids are like the water molecules in the ocean. Floating on this ocean are protein 'boats' which extend to various depths in the ocean. You are of course looking at only one surface of the ocean. This membrane ocean has two surfaces to it.

Glycoproteins - Glycoproteins are found on the outside of the plasma membrane only. These molecules, a complex of both protein and carbohydrate, are used as a means of communicating between the cell and the environment.

Membrane Transport

The cell's membranes are said to be selectively permeable. This means that some substances are able to cross the membrane while others are not. Whether a substance can cross the membrane depends on factors such as the size of the substance, the substance's solubility in the plasma membrane (basically how lipid soluble it is), and whether or not there are transport proteins available to allow the substance to move across the membrane.

If the substance is large, it will be unable to squeeze between the phospholipid molecules of the membrane. Small molecules like water or carbon dioxide can easily pass between the phospholipids. Larger, polar molecules or molecules which have a strong charge such as ions have a very difficult time passing through the non-polar, hydrophobic layer of the cell's membrane. For some of these large or charged molecules, specific transport proteins may either provide tunnels or passageways through the membrane or the proteins may physically bond to the molecules and carry them to the other side of the membrane.

Thus there are three mechanisms for a substance to move through the cell's membrane. If it is able to pass through on its own, the process is called simple diffusion. If it passes through a protein channel it is called facilitated transport or facilitated diffusion. If it uses a protein carrier it is called active transport. A fourth mechanism to move material into or out of the cell that does not require the material to move across a membrane is also discussed below.

Simple diffusion - Simple diffusion is a spontaneous event which requires no energy expenditure by the cell. The rate of movement of the substance and its direction of movement are controlled by physical laws. The result is that simple diffusion across a membrane depends on a concentration gradient existing across the membrane.

If able to move freely, molecules will spontaneously move from a region of high concentration to a region of lower concentration, that is, molecules will move down a concentration gradient. This is why you are able to smell perfume from an opened bottle across the room. The perfume molecules have diffused from a high concentration inside the bottle to the lower concentrations found in the air of the room.

When water diffuses across a selectively permeable membrane like those found in the cell, the process is given a special name, osmosis. The force or pressure needed to counteract this natural tendency of diffusion is called osmotic pressure. When we talk about osmotic pressure here and later you can think of it as the amount of force water is applying to cross a cell membrane.

Facilitated transport - Facilitated transport utilizes proteins in the cell's membrane which form tunnels or channels that allow certain molecules or ions to pass. These molecules or ions move in accordance with concentration gradients, moving through the channel from an area of higher concentration to an area of lower concentration. In most instances the cell can control whether or not the channel is open, thus regulating when these substances move. When the channels are open the process is spontaneous and does not require the cell to expend energy.

Active transport - Both simple diffusion and facilitated transport can only move material down a concentration gradient. For many substances, it is necessary for the cell to maintain either a higher concentration of a substance inside the cell or a lower concentration than what is found outside. That is the cell must work against the diffusion gradient. This is accomplished by a process call active transport.

In active transport, the molecule or ion to be moved is bound to a protein carrier which physically moves the molecule or ion to the other side of the membrane. This movement requires the cell to expend energy. The cell's energy which is used is packaged in a molecule called adenosine triphosphate or ATP (this molecule will be discussed in greater detail in future topics).

Endocytosis & Exocytosis - Large molecules (or even whole cells) which can not be transported by the above processes may be moved from inside to out or vise-versa by the processes of endocytosis and exocytosis. If something is to be moved into the cell, the cell's plasma membrane can wrap around the object and then pinch off to form a membrane 'bubble', called a vesicle, inside the cell. Then the cell is free to act on that membrane bubble as necessary (secrete digestive enzymes into it for example). Just the opposite may also occur. A vesicle inside the cell may fuse with the plasma membrane, releasing its contents to the outside environment. When material is brought into the cell by this process it is called endocytosis. When material is removed from the cell by this process it is called exocytosis.

Tonicity

Tonicity is a term and concept that relates to the osmotic pressures existing across membranes. Specifically, tonicity is referring to the concentration of solutes, the molecules and ions, dissolved in water (water is the solvent). We will always be using this term when comparing the osmotic pressure conditions on either side of the cell's membrane.

Isotonic - Isotonic refers to the situation in which the water solution outside the cell has the same concentration of solutes found in the water solution inside the cell. Therefore the osmotic pressures inside and outside are the same and there is no net movement of water molecules into or out of the cell. Note that we said net movement. Water molecules will still be moving in and out of the cell, but the number moving in equals the number moving out.

Hypertonic & Hypotonic - These terms are used relative to each other. If one solution has a higher concentration of dissolved solutes than another, the one with the higher concentration is referred to as a hypertonic solution and the one with the lower concentration is referred to as a hypotonic solution.

If the molecules dissolved in the water solutions are able to move freely, these solute molecules would move by diffusion from their higher concentration to their lower concentration. However, a cell membrane generally prevents the solutes from moving freely, often only the water molecules are able to freely move about. This can get a little confusing but if you think about it, if there is a high concentration of solute molecules there will be room for fewer water molecules in a given space. Thus, a hypertonic solution with a high solute concentration will have a lower water molecule concentration. A hypotonic solution, which has a lower solute concentration, will have a higher water molecule concentration. Since water can freely move across the cell's membrane, water will move, by simple diffusion, from the hypotonic solution to the hypertonic solution.

In the illustration below, the bag containing the 2M sucrose solution is modeling a cell. The bag material is selectively permeable, allowing water to pass through be not sucrose molecules. When the bag is placed in the distilled water (left beaker), there is a net movement of water into the bag causing the bag to swell. The distilled water is hypotonic relative to the solution inside the bag. When the bag is placed in the beaker with the 10M sucorse solution (middle beaker), there will be a net movment of water out of the bag because the outer solution is hypertonic relative to the inner solution. Finally, when the bag is placed in the beaker with the 2M sucrose solution (the right beaker), the two solutions have the same concentrations so there is no net movement of water. These solutions are isotonic.

If a cell's contents are hypotonic to the environment (which would mean the environment solution is hypertonic), the water in the cell would move out. This causes the cell to lose volume or shrink. In animal cells this shrinking is called crenation, in plant cells this shrinking is called plasmolysis. Many times this is lethal to the cell.

On the other hand, if a cell's contents are hypertonic to the environment, the water outside the cell will move into the cell causing the cell's volume to increase or swell. In animals this swelling will eventually cause the cell to burst like a balloon, this is called cell lysis. In plant cells, the cell wall surrounding the cell prevents to much swelling. Basically the cell wall pushes back, counteracting the osmotic pressure of the water trying to move in. Plant cells which have their membranes pressed tight against their cell walls are called turgid.

 

Eukaryotic vs. Prokaryotic Cells

Living organisms may be categorized into two broad categories, the eukaryotes and the prokaryotes. This division is based on differences in the structure of their cells as well as other metabolic differences. Eukaryotic organisms are those which we are most familiar and aware of in our daily lives. These include all the animals, the plants, and fungi. In contrast, the only prokaryotes we generally hear about and occasionally see are the bacteria. Prokaryotic cells will be examined in more detail later in the course.

Here are some of the characteristics of eukaryotic and prokaryotic cells:

Summary of the structure of eukaryotic cells.The virtual cell web site.ProkaryotesEukaryotes

lack membrane bound organelles

DNA, is not stored within a membrane bound organelle

single, circular chromosome

generally much smaller

with membrane bound structures called organelles

genetic material (DNA) is stored within the nucleus

linear chromosomes

larger than prokaryotic cells

Illustration of a prokaryotic cellIllustration of an animal cell (eukaryotic)Illustration of a plant cell (eukaryotic)

Models of plant and animal cells.

 

A plant cell

 

An animal cell.

The following discussion focuses on the structure and function of eukaryotic cells.

Organelles

Nucleus - The nucleus is the organelle in which chromosomes are stored and protected from the activities occurring in the cell's cytoplasm. The synthesis of RNA and ribosomes also occurs here. The area in the nucleus where ribosomes are synthesized is the nucleolus. One or more of these are present in non-dividing nuclei, usually appearing as a dark patch.

 

The cell's nucleus.

The nucleus is surrounded by two membranes, making a double membrane. This is often referred to as the nuclear envelope. The membranes are perforated by holes called nuclear pores. Around the margin of each pore are proteins which appear to regulate what passes through. These pores are essential for RNA and ribosomes to exit the nucleus.

Ribosomes - Ribosomes are not bound by membranes and so not cellular organelles but we will introduce them here. Ribosomes are structures composed of proteins and RNA. As we will see in future topics, they are essential for protein synthesis in cells. Both prokaryotic and eukaryotic cells have ribosomes however the ribosomes differ in their composition. Some antibiotics take advantage of this difference to kill bacteria (prokaryotic) while not harming ourselves (eukaryotes).

Endoplasmic Reticulum - The endoplasmic reticulum is a network of channels in the cytoplasm formed by highly folded membranes. Ribosomes may or may not be attached to these membranes. If ribosomes are attached, the endoplasmic reticulum appears bumpy and is called rough endoplasmic reticulum. If ribosomes are absent, the endoplasmic reticulum is referred to as smooth. These two forms of endoplasmic reticulum are continuous with each other.

The rough endoplasmic reticulum is the site of protein synthesis for those proteins that are destined to be transported from the cell. The newly synthesized proteins are transported to the inside of the ER, called the lumen, where they can begin to take on their complex shape. The proteins are transported through the lumen of the ER to the smooth ER where further processing of the protein may occur. Eventually a vesicle pinches off and transports these proteins to other cell organelles or to be secreted from the cell.

Information about the structure and function of the endoplasmic reticulum.

 

Rough ER

 

Smooth ER

Golgi Apparatus - The Golgi apparatus is also sometime called the dictyosome in plant cells. This organelle receives many of the vesicles produced by the smooth ER. Once inside the Golgi apparatus, further processing of the proteins occurs resulting in a functional protein product. The Golgi apparatus also 'sorts' the proteins. Some it will package in vesicles for secretion from the cell. Other proteins will be packaged to produce other organelles such as lysosomes that are used for cellular digestion.

 

The Golgi apparatus.

Vesicles - Vesicles are small organelles formed by a pocket of membrane pinching off from the endoplasmic reticulum or from the Golgi apparatus. These are used to transport material from one site to another in the cell or to transport material to or from the plasma membrane.

Vacuoles - These are larger organelles used largely for storage of material. They may be used to store poisonous metabolic waste products, store food molecules such as starch, and store pigment molecules as seen in the petals of flowers. They may also be used to help regulate water balance in the cell. This is seen in the contractile vacuoles of Paramecium. Plant cells frequently have a very large central vacuole filled with water.

Mitochondria- Mitochondria are sometimes referred to as the power houses of the cell. They function in the metabolic process of aerobic respiration which results in the production of the molecule adenosine triphosphate (ATP). ATP is a high energy molecule used to power the majority of the cell's activities.

Mitochondria have several features which distinguish them from most other organelles. Two membranes surround the mitochondria. The inner membrane is highly folded creating structures called cristae. This folding of the inner membrane creates a large surface area on which many of the organelle's reactions occur. The mitochondria also contain their own chromosome and are able to self replicate.

 

Mitochondria

 

Chloroplasts - Chloroplasts are organelles found only in plants and some algae. It is responsible for the metabolic process of photosynthesis in which light energy is transformed into chemical energy. The chloroplast is in many respects similar to the mitochondria. Like the mitochondria, the chloroplast is surrounded by two membranes. The inner membrane forms folds called thylakoids. Many of these form stacks called grana. Also like the mitochondria, the chloroplasts contain their own circular chromosome and are able to reproduce themselves.

 

Chloroplast

The cell's nucleus is unable to produce new chloroplasts or mitochondria, they must be derived from preexisting chloroplasts or michondria. This, as well as the other features of these organelles, suggests that they were derived in eukaryotic cells differently than the other organelles. The current theory regarding the origin of chloroplast and mitochondria is called the endosymbiotic theory. This theory suggests that both chloroplasts and mitochondria are derived from bacteria-like organisms that were engulfed but not digested by ancestral cells.

Cytoskeleton

The cytoskeleton consists of a number of protein filaments that create a framework around which the cell's structures are organized. It provides mechanical support to the cell and helps maintain cell shape. It also enables the cell to change shape and is associated with the cell's ability to move. The cytoskeleton is composed of three types of filaments: microfilaments, intermediate filaments, and microtubules.

 

 

Micrograph showing the cell's nucleus (purple) and cytoskeleton (yellow)

Flagella

Flagella and cilia consist of hair-like extensions of the plasma membrane around a core of microtubules. The microtubules are arranged in a pattern in which two run down the center of the flagella or cilia with nine pairs of microtubules found in a ring around the center. This is sometimes referred to as the 9+2 arrangement of microtubules. Flagella differ from cilia in that flagella are usually quite long and only a small number are found on the cell while cilia are relatively short and quite numerous. Both function in allowing the cell to move or move substances near the cell.

 

Structure of Flagella and Cilia


 


Practice Tests Unit III

Chapter 4:

The smallest living unit is a _____.

                A)           proton

                B)            molecule

                C)            cell

                D)           tissue

Feedback: Yes; the cell is the basic unit of life and contains the necessary components to present life.

 

2 Protein synthesis in a bacterial cell is done by _____.

                A)           ribosomes

                B)            centrioles

                C)            flagella

                D)           thylakoids

Feedback: Yes; ribosomes are structurally different in bacteria but still are responsible for protein synthesis.

 

3 The outermost boundary of prokaryotic cells is their _____.

                A)           plasma membrane

                B)            nuclear envelope

                C)            cytoplasm

                D)           cell wall

Feedback: Yes; the cell wall defines the osmotic boundary of the cell and helps to maintain osmotic regulation.

 

4 Sperm cells move by means of _______.

                A)           flagella

                B)            cilia

                C)            pseudopods

                D)           actin filaments

Feedback: Yes; the long whip-like action of flagella allows sperm cells to move.

 

5 Skeletal muscle cells will have a large number of _____ due to their high demand for energy.

                A)           chloroplasts

                B)            lysosomes

                C)            flagella

                D)           mitochondria

Feedback: Yes; mitochondria is the source of ATP production and is found in high concentrations in energy requiring muscle cells.

 

6 All cells contain the following items except ______.

                A)           ribosomes

                B)            cytoplasm

                C)            plasma membrane

                D)           nucleus

Feedback: Yes; prokaryotes have regions of DNA/RNA molecules but no defined nucleus.

 

7 The stacks of disks containing chlorophyll in a chloroplast are the ______.

                A)           stroma

                B)            plastids

                C)            microfilaments

                D)           grana

Feedback: Yes; this structure contains pigments and enzymes needed for photosynthesis.

 

8 Large storage structures typically found in the center of plant cells are ______.

                A)           plastids

                B)            mitochondria

                C)            Golgi bodies

                D)           central vacuoles

Feedback: Yes; central vacuoles is a common storage site for substances in plants such as water, etc.

 

9 Which of the following statements is false?

                A)           Some cells are large enough to be seen by the naked eye.

                B)            Cells with infoldings and outfoldings of their plasma membranes are able to move.

                C)            All cells are shaped like cubes or are round.

                D)           In Eukaryotic cells, the central nucleus controls activities in the cell.

Feedback: Yes; this is a false statement because there are some muscle and nerve cells which are flat, or long and fiber-like.

 

10 Cells of more complex organisms are different from bacterial cells in which of the following ways?

                A)           The presence of cytoplasm

                B)            The presence of a plasma membrane

                C)            The variety of organelles and their complexity.

                D)           The presence of DNA.

Feedback: Yes; eukaryotic cells have much more complexity and size than bacterial components.

 

11 Which of these organelles contain digestive enzymes?

                A)           Mitochondria

                B)            Ribosomes

                C)            Lysosomes

                D)           Golgi bodies

Feedback: Yes; this organelle breaks down cell debris and large molecules.

 

12 Synthesis of lipids occurs in which major organelle?

                A)           Chloroplasts

                B)            Mitochondrian

                C)            Smooth endoplasmic reticulum

                D)           Rough endoplasmic reticulum

Feedback: Yes; smooth endoplasmic reticulum is the major site of lipid synthesis.

 

13 The description "9+2 array" in cells refers to _______.

                A)           microfilaments

                B)            spindles

                C)            vacuoles

                D)           microtubules

Feedback: Yes; microtubules are found in cilia, flagella, and centrioles as important structures in movement.

 

14 When a cell grows, which of the following occurs?

                A)           They increase in size only.

                B)            They will develop multiple nuclei.

                C)            They will increase the volume faster than the surface area of the cell.

                D)           They will lose their nuclear membranes.

Feedback: Yes; volume does increase faster than the surface area in cells.

 

15 The large surface area needed for the participants in cellular respiration is provided by the _____ of the mitochondria.

                A)           stroma

                B)            thylakoids

                C)            cristae

                D)           matrix

Feedback: Yes; cristae are the membrane structures found in the interior of the chloroplast where diffusion of hydrogen ions powers ATP production.

 

16 Which of the following will NOT be found in abundance in an insulin-producing pancreas cell?

                A)           Rough endoplasmic reticulum

                B)            Golgi apparatus

                C)            Chloroplasts

                D)           Ribosomes

Feedback: Yes; chloroplasts are functional organelles in plants, not animals.

 

17 Debris that is trapped in mucus is moved away from the lungs toward the throat by _____.

                A)           flagella

                B)            cilia

                C)            pseudopods

                D)           actin filaments

Feedback: Yes; large beds of cilia use wave-like motion to move mucus away from the lungs.

 

18 The _____ participates in ribosome synthesis and is found in the nucleus.

                A)           plasma membrane

                B)            smooth endoplasmic reticulum

                C)            Golgi apparatus

                D)           nucleolus

Feedback: Yes; the nucleolus is found in the interior of the cell and is the site of the synthesis of ribosome components.

 

19 The endosymbiotic hypothesis attempts to explain _____.

                A)           the origin of chloroplasts and mitochondria

                B)            how cells reproduce

                C)            how photosynthesis occurs

                D)           the differences between plant and animal cells

 

Chapter 5:

The plasma membrane is composed primarily of _____.

                A)           phospholipids

                B)            carbohydrates

                C)            waxes

                D)           nucleic acids

 

2 The tails of the phospholipids are _____.

                A)           nonpolar

                B)            hydrophobic

                C)            directed inwards in the bilayer

                D)           All of these

 

3 The permeability of most membranes is reduced by the presence of _____ in the membrane.

                A)           nucleotides

                B)            proteins

                C)            cholesterol

                D)           glycolipids

 

4 Particular molecules or ions freely cross the plasma membrane with assistance from _____.

                A)           receptor proteins

                B)            cell recognition proteins

                C)            channel proteins

                D)           enzymatic proteins

 

5 A person's blood type is determined by the presence of particular _____ in the red blood cells' membranes.

                A)           phospholipids

                B)            glycoproteins

                C)            steroids

                D)           nucleic acids

 

6 Which of the following does NOT move freely (without energy or a carrier protein) across the plasma membrane?

                A)           H2O

                B)            Steroids

                C)            Na+

                D)           O2

 

7 Osmosis is specifically about the movement of _____ in and out of cells.

                A)           sugars

                B)            proteins

                C)            water

                D)           oxygen

 

8 If the inside of a cell is 1% NaCl, which solution is isotonic to the cell?

                A)           0.01% NaCl

                B)            0.1% NaCl

                C)            1% NaCl

                D)           10% NaCl

 

9 When a plant cell is placed in a hypotonic solution which of the following will occur?

                A)           The cell will swell and burst.

                B)            Nothing will occur.

                C)            The central vacuole gains water.

                D)           The cell will shrink or shrivel up.

 

10 The term hemolysis refers to ______.

                A)           normal red blood cells

                B)            red blood cells that burst after being placed in a hypotonic solution

                C)            shrinking of the cytoplasm after a cell is placed in a hypertonic solution

                D)           the loss of turgor pressure in a plant cell

 

11 Which of the following correctly describes facilitated transport?

                A)           It requires the expenditure of energy.

                B)            Molecules move from high concentration to low concentration.

                C)            Vacuoles form to move materials.

                D)           All of the above.

 

12 Insulin leaves insulin-secreting cells by _____.

                A)           phagocytosis

                B)            receptor-mediated endocytosis

                C)            osmosis

                D)           exocytosis

 

13 The peptide hormone, follicle stimulating hormone, would activate its specific target cells by _____.

                A)           diffusing into the cells

                B)            binding a receptor protein in the plasma membranes

                C)            phagocytosis

 

14 Heart cells are able to contract in unison because of the flow of ions through _____.

                A)           gap junctions

                B)            tight junctions

                C)            adhesion junctions

 

15 People may get injections of _____, a component of the extracellular matrix of animal cells, to get rid of their wrinkles.

                A)           collagen

                B)            cholesterol

                C)            cellulose

                D)           chromatin

 

16 Which of the following correctly describes active transport?

                A)           Carrier proteins are needed.

                B)            It requires the expenditure of energy.

                C)            Molecules move from areas of low concentration to areas of high concentration.

                D)           All of these.

Feedback: Yes; all of these are characteristics or factors involved in active transport.

 

17 Macrophages, immune system cells, engulf bacteria by _____.

                A)           exocytosis

                B)            facilitated transport

                C)            phagocytosis

                D)           osmosis

 

18 The amount of phospholipid extracted from red blood cells led researchers to propose that ____.

                A)           the phospholipids form a single layer around the cell

                B)            the phospholipids form a bilayer around the cell

                C)            there are not enough phospholipids to create a membrane around the cell

 

19 According to the fluid-mosaic model, _____ are partially or wholly embedded in a fluid phospholipid bilayer.

                A)           nucleic acids

                B)            monosaccharides

                C)            proteins

                D)           triglycerides

 

20 Transplant rejections are the result of the immune system's response to foreign ______ and __________.

                A)           DNA, RNA

                B)            glycoproteins, glycolipids

                C)            plasma membranes; glycolipids