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Cell structure and function

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Cells are the smallest building unit of living organisms that can carry out all processes required for life. Almost all cells are too small to see without the aid of a Microscope. Although glass lenses used to magnify images for hundreds of years, they were not enough to reveal individual cells. The invention of Compound microscope was in the late 1500s by the Dutch eyeglass maker Zacharias Janssen. In 1665, the English scientist Robert Hooke was the first to identify cells, and named them.

After long studies, the accumulated research can be summarized in the cell theory, which is considered as the first unifying concepts in biology. The Major principles of the Cell Theory:

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1. All organisms are made of cells.
2. All existing cells are produced by other living cells.
3. The cell is the most basic unit of life.

Cell Types:
The variety of cell types found in living things is staggering; the human body alone is made of trillions of cells of many different shapes, sizes and functions.

Despite this variety, the cells in the body share many similar characters. All cells are Microscopic in size, Composed of similar building blocks, and are enclosed by a membrane that controls the movement of materials into and out of the cell. Within this membrane the cell is filled with Cytoplasm, and Organelles. Organelles are structures specialized to perform distinct processes; mostly they are surrounded by a membrane, which take us to the first classification of Cells.

Prokaryotic Cells:
Is type of cells that Do not have a nucleus or other membrane-bound Organelles, the Cell’s DNA is suspended in the Cytoplasm. Most prokaryotes are microscopic single-celled organisms. E.g.: Bacteria. Eukaryotic Cells: Is type of cells that Have a nucleus and other membrane bound Organelles, The nucleus is the largest organelle where it encloses the genetic information (DNA), Eukaryotes may be Multi-cellular or Single-celled organisms.

Prokaryotic cell

The origin of the term “Prokaryotes” and “Eukaryotes” is the Greek root “Karuon” which means “ nuts” or “kernel” that refers to the nucleus, where “Eu= true” and “pro= before” , the cell with true nucleus is Eukaryote.

General Questions on the Section:
1. What are the major principles of the cell theory?
2. What characteristics are shared by most cells?
3. How do prokaryotes and eukaryotes differ?
4. In what way are cells similar to atoms?

Section 3.2:
Cell Organelles:
We mentioned that Eukaryotic cells share many similar structures, these structures include similar organelles involved in processing the cell functions. In this chapter we will discuss each Organelle structure and function.

Cell internal structure:
There are some similar characters in all cells that shapes its body and protects its identity, these characters are: 1. Cell Membrane: Is a double layered membrane that forms a boundary between a cell and the outside environment and controls the passage of materials into and out of the cell.

2. Cytoskeleton: Is a network of proteins that is constantly changing to meet the needs of a cell. It is made of small protein subunits that form long threads, or fibers that crisscross the entire cell. Cytoskeleton prevents the cell from being a random jumble of suspended organelles.
Cytoskeleton is divided into three types:
1) Microtubules: Are long hollow tubes, that gives the cell its shape. 2) Itermediate filaments: Smaller than microtubules, and gives the cell its strength. 3) Microfilaments: The smallest of three, are tiny threads that enable cells to move and divide.

3. Cytoplasm: It is the jelly like substance that contains dissolved molecular building blocks, such as proteins, nucleic acids, minerals and ions. Cytoplasm is the cell matrix that fills the space between the nucleus and the cell membrane containing the organelles. The fluid portion excluding the organelles is called Cytosol, and it consists mostly of water, which shows the importance of water as an essential component for life.
This is the structure of plant and animal cell, you will find that they have so many similar characters and organelles, with little different organelles in each one that are found to serve a certain function in the cell. We will discuss this difference in details later on, in the unit, now we will discuss the organelles according to their roles in the cell.

Organelles involved in making and processing Proteins:

1. The Nucleus:
Is the storehouse for genetic information, or DNA (deoxyribonucleic acid) in the cells.The nucleus has two roles: a) DNA must be carefully protected.
b) DNA must be available for use at proper times.

The molecules that would damage DNA need to be kept out of the nucleus, while the proteins involved in turning the genes on and off have to access the DNA at certain times.

The Nucleus Rough & SmoothEndoplasmic Reticulum

2. Endoplasmic Reticulum and Ribosomes (ER):
Is an interconnected network of thin folded membranes.It forms a maze of enclosed spaces, the inner side of the maze called Lumen. Numerous processes
including production of proteins and lipids occur both on the surface of the ER and inside the Lumen. There are two types of ER: Smooth ER and Rough ER.

Rough ER: Has Ribosomes on its surface which give it the bumpy or gritty look. Ribosomes: Are tiny organelles that link amino acids together to form proteins, they may be bonded on the ER surface or suspended in the Cytoplasm. Smooth ER: They don’t have Ribosomes on its surface; they make lipids and other specialized functions such as breaking down drugs and alcohol.

3. Golgi Apparatus:It consists of closely layered stacks of membrane-enclosed spaces, that package and deliver proteins. 4. Vesicles:
Small membrane-bound sacs that enclose some materials from the rest of the cytoplasm and transport them from place to place.

Organelles having various functions:
1. Mitochondria:
Is the main energy supplier to the cell.
It is a bean shaped with two membranes,The inner membrane has many folds that form a compartment for many series of chemical reactions that convert food into usable energy. It has its own DNA.

2. Vacuole:
It is a fluid-filled sac used for storage of materials needed by the cell. In plant cell, Vacuole takes most of the space inside, where it filled with watery fluid that strengths the cell and helps to support the entire plant.

3. Lysosomes:
Are membrane-bound organelles that contain enzymes.
They defend the cell from invading bacteria and viruses, They also break down damaged or worn out cell parts.

4. Centrosome and Centriols:
Centrosome: Is a small region of cytoplasm that produce microtubules. In animal cells, it contains two small structures called Centrioles.
Centerioles: Are cylinder-shaped organelles made of short microtubules arranged in a circle, where it doubles and form spindle fibers that attach to DNA, at the time of cell division. It plays role in animal cell division.

Lysosome Centerioles

Spindle fibers of centrosome Plant Cell Special structures:
Plant cell has two feattures not shared by animal cells.
Cell walls that provide rigid support and Chloroplasts that carry on photosynthesis.
1. Cell Walls: It is a rigid layer that surrounds the cell membrane of Plants, algea, fungi, and most of bacteria, to give protection, support and shape to the cell. Cell walls of Plants and algea is made of Cellulose.

2. Chloroplasts: Are organelles that carry out photosynthesis, which a series of complex chemical reactions that convert solar energy into usable energy(chemical energy). Chloroplasts are highly compartmentalized organelles having both inner and outer membranes. Chlorophyll is the main component of chloroplasts which is alight-absorbing molecules that gives the plant its green colour,it’s found in sacs called Thylakoids. Chloroplasts and Mitochondria works together in plant cells to capture and convert energy,having their own DNA.

Animal Cell

Plant Cell

Chapter 4
Cell Membrane and Types of Transport
Section 4.1:
Cell Membrane:
1. Structure:
Cell membrane or Plasma membrane, forms a boundary between the cell and the outside enviroment and controls the passage of materials into and outside the cell. It consists of a double layer of phospholipids interperesed with a variety of other molecules, like proteins and carbohydrates. As we mentioned before that Phospholipid molecule consists of three basic parts: Charged phosphate group (PO₄⁻) – Glycerol – Two fatty acid chains. Glycerol and the phosphate group form the “head” and the fatty acids forms the “tail” The “head” has a charge , so it is Polar and forms Hydrogen bonds with water molecules. i.e.: The polar head is Soluble in water as it is Polar solvent. The fatty acid “tail” is Non-polar and Cannot form hydrogen bonds with water, so Non-polar tails are attracted to each other and repelled by water.

The Cell membrane touches the Cytoplasm inside the cell and the watery fluid outside the cell (both are polar solvents) which makes the Phospholipids to arrange themselves in layers like a Sandwich, where the “heads” are like bread forming the outer layer,and the “tails” are like the filling hiding from watery enviroment. The “Head” is called Hydrophilic which means “water-loving”, while the “tail” is called Hydrophobic which means “water-fearing”.

2. Properties:
Selective Permiability:
The cell membrane has the property of selective permiability, which means it allows some,but not all materials to cross. Therefore, the cell membrane is called Semipermiable.
Selective permiability enables the cell to maintain homeostasis(balance), inspite of unexpected changes outside the cell. The cell only permit molecules and ions it needs to get in, whether they are at high concentration or not outside the cell. Molecules cross the membrane in several ways, Some of these methods require energy and others not, it depends on the molecule size, polarity and concentration inside versus outside.

A receptor is a protein that detects a signal molecule and performs an action
in response. It recognize and binds to only certain molecules, ensuring that the right cell gets the right molecule. The molecules that binds to the receptor is called Ligand.

There are Two types of receptors:
a. Intracellular Receptors:
Is a receptor within or inside the cell, it binds to the molecules that can cross the cell membrane which are generally small and non-polar. b. Membrane Receptors:
Is a receptor that is located between the spaces of the phospholipid bilayer, i.e.:on the cell surface, It binds to the molecules that cannot cross the cell.
Types of transport across the cell:
There are many types of transport cross the cell membrane and as we said before it almost depends on the molecule size, polarity, and concentration inside versus outside the cell. Cells continually import and export substances, but if they have to expend energy to move every molecule, cells would require enormous amount of energy to stay alive. There are two types of transport:

I. Passive Transport: It is type of transport where the molecules move across the cell membrane without energy input from the cell, where the molecules move from higher to lower concentration. It is also described as the diffusion of molecules a cross the membrane.

II. Active Transpot: It is a type of transport where the cell uses energy to move a substance against the concentration gradient. i.e.: Move the substance from lower to higher concentration.

We will discuss every type in details, to know its subtypes and conditions. I. Passive Transport:
It can be expressed or divided to two terms, depending on the concentration of molecules, or concentration of water, These terms are: 1. Diffusion:
Is the movement of molecules in a fluid or gas from a region of higher conc. To a region of lower conc.It results from the natural motion of particles, which causes molecules to collide and scatter Concentration: is the number of molecules of a substance in a given volume and it can vary from one region to another.E.g.: 20mg/100ml. Molecules diffuse down their concentration gradient, from a region of higher conc. To a region of lower conc. Concentration Gradient: Is the difference in the concentration of a substance from one location to another. Difffusion plays important role in moving substances across the cell membrane. Small lipids and other non-polar molecules such as Carbon dioxide and oxygen, easily diffuse across the membrane. Most of cells continually consume oxygen, which means that the oxygen conc. Is always higher outside the cell than inside, as a result oxygen diffuses into the cell without cell’s expending energy.

2. Osmosis:
It is the movement of Water molecules across a semipermiable membrane from area of higher conc. Of water to area of lower conc. The higher the conc. Of dissolved particles in a solution, the lower the conc. Of water molecules, in the same solution. A solution may be Isotonic, Hypertonic, or Hypotonic, relative to another solution. i.e.: a solution may be described as isotonic only in comparison with another solution. Hypo = Less or low, Iso = Equal , Hyper = More or high.

Isotonic Solution: A solution is Isotonic to a cell if it has the same concentration of dissolved particles as the cell, where water molecules moves into and out of the cell at equal rate, so the cell size remains constant.

Hypertonic Solution: It is a solution with higher conc. Of dissolved particles than a cell, this means that water conc. Is higher inside the cell than outside, so water flows out of the cell, causing it to shrink or even die.

Hypotonic Solution: It is a solution with lower conc. Of dissolved particles than a cell, this means that water molecules are more concentrated outside the cell than inside, so water diffuses into the cell leading it to expand until it bursts.

3. Facilitated Diffusion:
Is the diffusion of molecules across the cell membrane through Transport Proteins. Transport Proteins are openings formed by proteins that pierce the cell membrane, helping some molecules to cross the membrane where they cannot easily pass. The cell doesn’t expend energy in this process, where the molecules move down the concentration gradient. Sugar molecules are large to cross the cell membrane, so they have to enter by the aid of transport proteins; thus Sugar molecules cross the cell membrane by facilitated diffusion.

II. Active Transport:
It is the movement of substances across the cell membrane against the concentration gradient, where the cell use energy to take its needed molecules from region of low conc. To region of high conc. To maintain homeostasis. Molecules pass across the membrane through Transport Proteins like facilitated diffusion but the difference here is in the energy used incase of active transport. Some of Active transport proteins bind to only one type of molecules ( like enzymes).

The Cell uses a high-energy chemical compound called ATP as source of energy to allow the passage of molecules against the concentration gradient. There are another two types of active transport but it doesn’t depend on Transport proteins where materials are transported across the membrane in vesicles, this type of transport is mainly for large substances or large amount of a substance.

1. Endocytosis:
Is the process of taking liquids or large molecules into a cell by engulfing them in a membrane, where the cell membrane makes a pocket around the substance. The pocket breaks off inside the cell and forms a vesicle, which is broken down by lysosomal enzymes to get its content. Phagocytosis: Is a word literally means “cell eating” which is widely used to express cells in Immune system called Macrophages, where these cells are responsible for finding, engulf, and destroying foreign materials in blood like bacteria.

2. Exocytosis:
Is the opposite of endocytosis, it is the release of substances out of the cell by the fusion of vesicles with the membrane. The vesicle forms around the material to be expelled out of the cell, then moves towards the cell’s surface, where it fuses with the membrane. Exocytosis is widely happen in the Nerve cells (neurons) where the chemical transmitters that carry the electrical signals from brain are stored in vesicles that fuse with the cell membrane and release these chemical transmitters outside the cell to act as stimulus to the next cell.

General Questions on the chapter3 & 4:

1) What are the types of Cytoskeleton? And what are their functions? 2) Explain the structure and function of: Mitochondrion – Nucleus- ER- Golgi apparatus- lysosomes. 3) What are the role of: Cell wall and Chloroplasts?

4) Desceibe the structure of cell membrane and how it affects the on the passage of materials in and out the cell? 5) State and compare types of transport.
6) What is the difference between Diffusion and Osmosis?
7) Explain what is the concentration gradient and what it means for a molecule to diffuse down its conc. Gradient? 8) Why facilitated diffusion doesnot require energy from the cell? 9) How are receptors , transport proteins and enzymes are similar? 10) Defineand give one example on: Endocytosis – Exocytosis.

Unit3: Cells and Energy
Chapter 1
Chemical Energy and ATP
Section 1.1:
Chemical energy used by most of cells is carried by ATP:
Sometimes you may feel that you need energy, so you eat food containing sugar. Carbohydrates and lipids are the most Important energy sources in food. How,ever this energy is only usable after these molecules are broken down by a series of chemical reactions. All cells use chemical energy carried by ATP [ adenosine triphosphate] ATP : Is a molecule that transferes energy from the breakdown of food molecules to cell processes. ATP has three phosphate groups, but the bond holding the third phosphate group is unstable and easily broken. The energy carried by ATP is released when a phosphate group is removed from the molecules.

The removal of the third phosphate group usually involves a reaction that release energy. You can think of ATP as a wallet filled with money, as long as it carries money you can spend, where ATP carries chemical energy that the cell can use. Cells use ATP for functions as building molecules and moving materials by active transport.

When the phosphate is removed, energy is released and ATP becomes ADP [adenosine Diphosphate ], which is a lower energy-molecule that can be converted into ATP by the addition of phosphate group.

ADP is nearly empty wallet, where the cell cannot spend from it.

Food and Energy:
Food that we eat do not contain ATP that the cells can use.
Food must be digested to be broken into into smaller molecules, that can be used to make ATP. Different types of food have different amounts of Calories, which are measures of energy. Organisms break down carbon-based molecules to produce ATP

The number of ATP molecules that are made from the breakdown of food is related to number of Calories. Calorie: Is the amount of Energy required to raise the temperature of 1 kilogram of water by 1 C. {i.e: Energy unit} It also depends on the type of the molecules that are broken down whether it is Carbohydrate, Lipid, or protein. Lipids stores 80% of the energy in the body.

Plants also need ATP, but plants do not eat food as animals, Plants make their own food through Photosynthesis. In Photosynthesis plants absorb sunlight and make sugars that are broken down to produce ATP.

Cellular respiration:
It is the process by which the cell releases chemical energy from sugars and other carbon-based molecules to make ATP when Oxygen is present. Cellular respiration is an aerobic Process ( aerobic respiration ), where it needs oxygen to take place. Cellular respiration takes place in Mitochondria.

The aeorbic part of cellular respiration is called Krebs cycle and makes 34 ATP molecules. Mitochondria cannot directly make ATP from food.First, food are broken into smaller molecules such as Glucose. Glycolysis: it is the process of spliting glucose into two 3-carbon molecules (pyruvate) and makes two ATP molecules.It is an anaerobic Process that doesn’t need Oxygen and takes place in Cytoplasm.

When oxygen is abscent, there is another anaerobic process that takes place rather than Krebs cycle, called Fermentation. Lactic acid Fermentation: it produces 2 ATP and allows glycolysis to continue, leading to the formation of Lactic acid at the end of the process.

Alcoholic Fermentation:
It is the same as lactic acid fermentation but it leads to the formation of alcohol at the end of process, it takes place in many yeasts and some plants.
General Questions on the Chapter:
1) How are cellular respiration and glycolysis related?
2) What are the steps taken by the cell to get ATP?
3) Illustrate the components of ATP molecule and how it could be converted to ADP? 4) What do you know about: aerobic and anerobic respiration

Cite this Cell structure and function

Cell structure and function. (2016, Jun 10). Retrieved from https://graduateway.com/cell-structure-and-function/

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