Cell BiologyCellular Functions

Cells Tissues Organs Body systems Body

• The cell Theory states that:1. All living things are composed of one or more

cells2. The cell is the smallest unit that retains the

properties of life3. New cells arise from existing cells

• Any cell is defined (bounded) by a cell membrane also called plasma membrane.

• All the material inside the cell is called the protoplasm, in other words the cytoplasm and its content including the nucleus.

• Inside the cell usually near the center, is the nucleus.

• The cytoplasm is the fluid that contains all the cellular organelles enclosed inside the cell membrane

Animal Cell

The cell Structure:

1] The Nucleus: is the control center of the cell, it is bound by a double membrane, called the nuclear membrane, this membrane has numerous pores, which allow the passage of certain molecules in and out of the nucleus, and prevents others. Inside the nucleus the hereditary (genetic) information is organized into threadlike structures called the chromosomes. Along the length of the chromosomes are the genes, in other words genes are certain sections of chromosomes, the genes are the characteristic-determining units, each gene contains the instructions (a code) to produce a certain protein. Each chromosome is made of nucleic acids and proteins, the nucleic acid in chromosomes is deoxyribonucleic acid (DNA), the genetic material of life!

Eukaryotic Cell Nucleus

Nucleolus

Chromatin

Pore

The Nucleus and Rough ER

• Cells could be classified according to the presence or absence of a nuclear membrane (nuclear envelope), into two types:

1. Prokaryotic cells: are those cells which have the nuclear material distributed all over the cell, with no nuclear envelope.e.g.: Bacteria and Cyanobacteria

2. Eukaryotic cells: are those cells which have the nucleus bound with a double membrane, the nuclear membrane.e.g.: Plant cells and Animal cells

Prokaryotic Bacterial Cell

Chromatin

ribosomes

flagellum

Cell wall

Plasma membrane

• Inside the nucleus there is a dark stained structure called the nucleolus. The nucleolus is involved in the synthesis of a genetic material called ribosomal RNA, which is a second type of nucleic acid (ribonucleic acid), ribosomal RNA (rRNA) synthesis is directed and controlled by DNA, in turn (rRNA) directs the synthesis of ribosomes, these are specialized structures that are important for protein synthesis.

• Pores in the nuclear envelope retain DNA inside, however they allow the passage of molecules like those of messenger RNA (mRNA) which is a copy of the genetic information contained in DNA, this (mRNA) attaches to the ribosomes, where the information is used for the synthesis of proteins.

2] The cell membrane: is the outermost boundary of the cell, it is composed of protein and a bilayer of lipid (fat) molecules, the cell membrane holds the cell contents in place and regulates the passage of materials in and out of the cell, it also contains receptor sites that allow the entry of materials that affect cell activity, which means that it is a selectively permeable membrane that allows the entry of some materials while keeps other out.

• It is important to be aware that the cell membrane is fluid in nature, in other words it is a lipoid double layer that bounds the cell.

• Some cells are equipped with flagella or cilia, both are continuous with the cell membrane and have contractile proteins that allow them to move.e.g.: sperms have flagella (singular: flagellum)

cells of the trachea have cilia (singular: cilium)

Phospholipids

• Phospholipids are composed of a glycerol molecule to which are attached two fatty acids and a highly polar phosphate group.

• The phosphate group may be thought of as a polar head and the fatty acids as long non-polar tails.

• The polar head is hydrophilic and the non-polar tails are hydrophobic.

Phospholipids and the cell membrane

The cell membranes are mostly composed of phospholipids.

3] The cytoplasm: is the fluid that contains all the parts of the cell, enclosed by the cell membrane. Most of the cell’s chemical activities take place in the cytoplasm, absorbed nutrients are transported and processed in the cytoplasm, the cytoplasm also stores the waste material until proper disposal

The Cell organelles (cytoplasmic organelles)

1] Mitochondria: are tiny oval shaped organelles. Mitochondria have a smooth outer membrane and an inner folded membrane, the finger like projections of the inner membrane are called cristae. Each mitochondrion provides the cell with the needed energy in a series of chemical process called cellular respiration, it is inside the mitochondria that this process takes place, with the help of enzymes that speed the chemical reaction, sugar molecules combine with oxygen to form carbon dioxide and water, with the release of energy from the break down of the chemical bonds of sugar, the energy is stored in a compound called adenosine triphosphate (ATP).

• It should also be noted that mitochondria have their own DNA which means that they have to replicate before cell division.

Structure of a Mitochondrion

2] The Ribozomes: Ribosomes are the sites of protein synthesis. Cell growth and reproduction requires the synthesis of different protein molecules. There are 20 different amino acids from which proteins are composed, the properties of proteins are determined by the number and the sequence of amino acids in the chain.

• Amino acids are chemically bonded together by

enzymes at the ribosomes, the specific sequence of amino acids is determined by instructions encoded in the DNA, a change in the position of a single amino acid can result in a different protein.

• Ribosomes exist floating freely in the cytoplasm, and also bound to the endoplasmic reticulum (ER). ER bound to ribosomes is called rough ER because the ribosomes appear as black dots on the ER in electron microscope photos, giving the ER a rough texture.

• Ribosomes are made of rRNA and proteins, measuring just 20 nm (nanometers) ribosomes are among the smallest organelles in the cytoplasm, yet they make up a great portion of the cytoplasm.

• Ribosomes disassemble into two subunits when not actively synthesizing protein.

3] The Endoplasmic Reticulum (ER): is a network of interconnecting channels that transport materials throughout the cytoplasm, the (ER) reaches from the cell membrane to the nuclear membrane.

• There are two types of the endoplasmic reticulum, the rough endoplasmic reticulum (RER) that has many ribosomes attached to it and the smooth endoplasmic reticulum (SER) that has no ribosomes.

• The (RER) is associated with cells involved in enzyme secretion, such as those of the liver and the pancreas.

• The (SER) is found in the cells of developing seeds and in animal cells that secret steroid hormones.

The Rough Endoplasmic Reticulum (RER)

4] The Golgi Apparatus (Golgi Complex) (Golgi Body): • The Golgi apparatus stores, modifies, and packages

proteins from the rough endoplasmic reticulum.• The Golgi apparatus is a group of membranous sacs piled

on top of each other.• These membrane-bound structures pinch off at the end to

produce smaller protein sacs called vesicles. When these vesicles reach the cell membrane, they fuse with it and release their content outside the cell, this process is called exocytosis, when other vesicles coming from outside the cell fuse with the cell membrane and release their contents into the cell, this process is called endocytosis.

Golgi Body

5] The Lysosomes: are formed by the Golgi apparatus and are bound by a single membrane.

• Lysosomes contain enzymes that break large molecules into smaller ones within the cytoplasm.

• Lysosomes play an important role in the body’s self defence mechanism, as their digestive enzymes destroy the invading bacteria after being engulfed by the white blood cells, this also results in the death of the white blood cells, the result is the formation of pus which consists of dead cells, fluid and protein fragments.

The Cytoskeleton

• Is NOT an organelle • Is transparent in standard light and electron microscope

preparations, and is therefore "invisible. It is usually left out of drawings of the cell, but it is an important, complex, and dynamic cell component. The cytoskeleton maintains the cell's shape, anchors organelles in place, and moves parts of the cell in the processes of growth and motility.

• The cytoskeleton includes 3 types of filaments, the microtubules, the microfilaments (Actin) and the intermediate filaments.

Intermediate Filaments (Model)

Cytoskeleton Functions:

• Establishing cell shape• Providing mechanical strength• Locomotion• Chromosome separation in mitosis and meiosis• Intracellular transport of organelles

Special Structures of Plant Cells

1] Plastids: are chemical factories and storehouses for food and colour pigments.

• There are many types of plastids, namely chloroplasts, chromoplasts, and amyloplasts.

a. Chloroplasts: are prevalent in all green plants, they contain the green pigment chlorophyll, chloroplasts specialize in photosynthesis, a process by which plants combine carbon dioxide from the air, with water from the roots, in the presence of light to produce sugar and oxygen.

• Like mitochondria, chloroplasts contain their own DNA and ribosomes, which means that during cell division they replicate.

b. Chromoplasts: store orange and yellow pigments, found in many fruits and flowers.

c. Amyloplasts: are colourless plastids, and they are the storehouses of starch, the are found in many seeds and roots, such as potato tubers.

Chloroplast Structure

2] Vacuoles: are fluid filled spaces (usually one big vacuole) that store sugar, minerals and proteins, they increase the surface area of the cell allowing more absorption, they also provide the cell with rigidity and support, through turgor pressure.

3] Cell Wall: most of the plant cells are surrounded by a cell wall composed of cellulose, its main function is to protect and support plant cells.

Comparison between Animal and Plant Cells

Cell

StructureAnimal Cell Plant Cell

Lysosomes Yes No

Centrosome YesNo (most plant

cells)

Vacuole Small (if present) Yes

Plastids No Yes

Cell Wall No Yes

Application: Cell Research in Medicine

• Hybridomas are cells made from white blood cells and cancer cells, they produce large amounts of antibodies for defence against disease

• The secretion of digestive enzymes by lysosomes (as the membrane vesicles burst) may explain arthritis and its pain, when cortisone (a drug) is given the swelling and pain decreases as it strengthens the lysosome membrane

• Cells grown in tissue culture, can be used for skin, liver and other cell transplants.

• Foetal brain tissue transplants into patients with Parkinson’s disease, are helped greatly through this technique, however ethical considerations remain controversial