2.2 Cell Structure and Function

What I will be learning....

I will be learning the structure and function of cells.  

Why I will be learning this......

I will be learning this because cells are the basic unit of life.

How I will know I learned it....

I will be able to discuss the basic types of cells and the  organelles inside of them.  I will know cell theory.

Cell Structure and Function 

Explore this Phenomenon

The pictures above show different types of cells. The cell types look different, even though they all have to perform the same functions to stay alive.

1. What do you think causes the cells to look different?
2. What are three questions you could investigate to figure out why cells look different

THE HISTORY OF OUR KNOWLEDGE OF CELLS:

Our knowledge of cells and their function has increased over time.  In  1609, Galileo Galilei, arranged two glass lenses in a cylinder- placed an insect under it and described the geometric patterns of its eye.  The same man who turned magnification to his use to discover the Galilean moons of Jupiter, then experimented with using magnification to see very small things.  In the 1600's Robert Hooke looked at cork, the bark of a special tree that grows in southwest Europe and North Africa, and is used as a stopper in wine bottles. When he looked he saw tiny compartments which he named "Cellulae".  A "cell" was a tiny room.

 

In 1676, Antony Van Leeuwenhoek who improved and crafted excellent lenses, was the first to observe bacteria.  In 1839 Theodor Schwan and Matthias Schlieden concluded that all animal and plant tissues were made of cells.   In 1855, Rudolph Virchow created "CELL THEORY".

 

CELL THEORY:

1. All organisms are composed of one or more cells

2. The cell is the basic unit of organization for all organisms

3.  All cells arise from pre-existing cells

THE SHAPE OF CELLS

Not all cells are shaped the same.  Their shape serves their function.  These are nerve cells, meant to send signals from one place to the next in the body.  Red Blood cells are the next picture down. Then a drawing of an intestinal cell. Why do you think they are shaped like this?

 

A nerve cell has to send a signal along a long path, sometimes from the end of your toe to your brain.  Its long branches, called axons and dendrites, allow the signal to travel quickly, chemically transmitted from nerve cell to nerve cell.  A blood cells smooth edges help it travel without getting stuck up and down blood vessels.  An intestinal cell has many folds in its membrane called microvilli that increase its surface area so it can absorb the nutrients you are digesting.

CHARACTERISTICS COMMON TO ALL CELLS:

basic components of the cell, which include the:

1. Cell Membrane- All cells have a plasma membrane, a phospholipid bilayer, that separates the inside of the cell from the outside environment.

2. Cytoplasm- the jelly like fluid and internal structures of a cell

3. Genetic Material- the DNA 

4. Ribosomes- small particles that synthesize proteins

TWO BASIC TYPES OF CELLS

There are two basic kinds of cells, Prokaryotic and Eukaryotic.  A Prokaryotic Cell is any cell that lacks a distinct nucleus and other organelles due to the absence of internal membranes. The area where inside a prokaryotic cell where the DNA is is called the "nucleoid". A Eukaryotic Cell is any cell that contains a clearly defined and contained nucleus. 

Bacteria and Archaea are Prokaryotic, single cell organisms.  Bacteria are single cell organisms that can be beneficial or detrimental to human beings. Archaea are similar to bacteria, but they evolved separately.  They are a group of micro-organisms that are similar to, but evolutionarily distinct from bacteria. Many archaea have been found living in extreme environments, for example at high pressures, salt concentrations or temperatures. These types of organisms are called extremophiles.

Plant, animal and fungus cells are eukaryotic.  They can be single celled or multi-celled organisms.  

Cell walls and cell mobility can be either prokaryotic or eukaryotic characteristics.  Plants and bacteria have cells walls.  Some animal and bacteria cells have mobility. 

Prokaryotes reproduce by binary fission, where two identical cells are cloned from a parent cell. Eukaryotic cells reproduce by Mitosis,  which is asexual reproduction, and Miosis, which is sexual reproduction with special cells called gametes. 

THE STUCTURE OF EUKARYOTIC, ANIMAL, CELLS

We will focus first on Eukaryotic cells, and specifically animal cells, since these are our cells.

Cells are the basic unit of life.  Cells are very, very small.  The width of a human hair is about 20 to 180 micrometers.  A single cell is about 1 micrometer in width.  It would take about 2000 blood cells to make a line across your thumbnail.  There are exceptions. The yolk of a birds egg are a single cell.  The ostrich egg is the largest cell in the world. Fish eggs can be seen with the eye, as well as the cells in the red part of a watermelon.  

We live together with lots of other single celled organisms.  Scientists estimate their are more bacterial cells in the human body, than human cells, concentrated in the intestines and on the skin.  Different kinds of cells have different life spans.  The cells of your intestines are replaced every 3 days, while the cells in the lens of your eye have to last your entire life.  

You would think, that because they are so small, they would be very simple, but they are not.  Cells  and cellular processes are very complex.  Eukaryotic cells have many "Organelles", specialized structures within a cell made by more phospholipid membrane.  

STRUCTURES:

The Plasma Membrane and Cytoplasm

All cells have a cell membrane. The cell membrane is a double layer of specialized lipids, known as phospholipids, along with many special proteins. The function of the cell membrane is to control what moves in and out of the cell. The cell membrane is semipermeable, which means that some molecules can move through the cell membrane while others can't. Without a cell membrane, a cell would be unable to maintain a stable internal environment separate from the external environment. Cytoplasm is everything inside the cell membrane, fluid and organelles  Cytosol is the jelly like fluid inside a cell.  The cytosol is composed of water and other molecules, including enzymes that speed up the cell’s chemical reactions.  

The plasma membrane is made of a double layer of phospholipids, with proteins dotted throughout the membrane. a phospholipid has a "hydrophilic" phosphate head and two hydrophobic fatty acid tails. The "head face the outside of the the membrane, both external and internal of the cell, and the tails are on the inside of the membrane. The little kink in the fatty acid tail keeps the phospholipids from making a very hard dense surface.  Instead it is a flexible, permeable surface. Look at these diagrams to see the structure of a phospholipid and the structure of the phospholipids all together making up the cell membrane, dotted with various types of proteins that regulate substances going in and out of the cell.

 

 

The Nucleus and Chromosomes

The nucleus is a membrane-enclosed structure that contains the genetic material, or DNA, of the cell. The nucleus is surrounded by a double membrane, two layers of phospholipid bilayer, which controls which molecules go in and out of the nucleus.  There is an inner and an outer layer and this protects the DNA.  There are pores through the which proteins called RNA and Ribosomes can move in and out of the nucleus.  Under a microscope it will appear that inside the nucleus is a very dark spot.  This is called the nucleolus, where ribosomes are made.  DNA never leaves the nucleus.  Instead a special protein called messenger RNA, or mRNA, takes copies of instructions from the DNA out of the nucleus to give instructions to the rest of the cell. 

Inside the nucleus are the chromosomes, which consist of DNA that is wrapped in special proteins.  In eukaryotic cells the DNA is made in long strings.  The genetic information on the chromosomes is stored, made it available to the cell when necessary, and also duplicated when it is time to pass the genetic information on when a cell divides. All the cells of a species carry the same number of chromosomes. For example, human cells each have 23 pairs of chromosomes, 46 total. A fruit Fly has 8 chromosomes. Each chromosome in turn carries hundreds or thousands of genes that encode proteins that help determine traits as varied as tooth shape, hair color, or kidney function.

 

Other Organelles

ORGANELLE	LOCATION	DESCRIPTION	FUNCTION
Cell Wall	Outside the cell- plants, bacteria, fungus	Rigid, strong covering, stiff, nonliving	protects and supports cell
Cell Membrane	all cells- the outer layer of a cell	Flexible, semi-permeable, phospholipid	Controls what goes in and out of cell
Nucleus	Inside cell, toward the middle	Contains DNA and Ribosomes	directs cells activities
Nuclear Envelope	surrounds the nucleus	double layer with pores	Protects the DNA
Nucleolus	Middle of nucleus	dark spot in middle of nucleus	where ribosomes are made
Endoplasmic Reticulum (rough and smooth)	surrounds the nucleus	flat disks and tubes, hollow, dotted with ribosomes	processes proteins and lipids
Ribosomes	throughout the cytoplasm and in the ER	Particles that help make proteins	Where amino acid chains are made for proteins
Vacuoles	though out the cytoplasm	Sacs filled with fluid and nutrient- large in plants	Storage
Vesicles	though out the cytoplasm	form from phospholipid membranes and fuse with phospholipid membranes	moving substances around the cell
Golgi Bodies	cytoplasm-near the edge of the cell	like a stack of flattened discs- hollow	Package and ship proteins and lipids
Lysosome	though out the cytoplasm	spheres	digest wastes
Mitochondria	cytoplasm	shaped like a kidney bean	power house of the cell
Chloroplasts	in cytoplasm near the edge of the cell of plants/ photosynthesizes	oval shape	Converts sunlight to sugar

THE ENDOMEMBRANE SYSTEM

The ENDOMEMBRANE SYSTEM is a group of organelles in that work together to modify, package and transport protiens, lipids and some carbohydrates for the cell or outside of the cell. 

Each of the following organelles is part of the endomembrane system:

Ribosomes

Rough and Smooth endoplasmic reticulum (ER)

Vesicles

Golgi body ( also called golgi apparatus) 

Lysosomes

Cell Membrane

Messenger RNA "transcribes", copies a section of DNA, leaves the nucleus through a pore in the nuclear membrane, and ends up on a ribosome attached to the endoplasmic reticulum (ER).  On the Ribosome, a long chain of amino acids ( the monomer of a protein) is made and then dropped inside the ER.  There the amino acid chain is twisted up into new shapes, and carbohydrate branches are added.   The Smooth ER helps to make various lipids (fats).  Vesicles enclose the proteins or fat as it comes out of the ER, travel along the cytoskeleton and bring the cargo to the Golgi Body.  The Golgi body packages the  protein or lipid, puts a sugar shipping tag on it, and then the cargo leaves by vesicle to the cell membrane.  The vesicle fuses with the cell membrane, releasing its contents outside of the cell.  We will learn a lot more about making proteins in a future lesson. Lysosomes digest any waste left from the process.

 

Plant Cells

Plant cells differ in some ways from animal cells. Use the table above to identify organelles that are found in plant cells but not in animal cells. Consider the functions of these organelles. How does having a different structure affect the function of plant cells?

First, plant cells are unique in having a large central vacuole that holds a mixture of water, nutrients, and wastes. A plant cell's vacuole can make up 90% of the cell’s volume. In animal cells, vacuoles are much smaller.

Second, plant cells have a cell wall, animal cells do not. A cell wall gives the plant cell strength, rigidity, and protection. Although bacteria and fungi also have cell walls, a plant cell wall is made of a different material.

Third, plant cells have chloroplasts. These organelles convert light energy from the sun into chemical energy that can be stored and used by cells.