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a) The internal membranes of eukaryotic cells and their importance. The structure of the following organelles: mitochondria; endoplasmic reticulum (rough and smooth); ribosomes; Golgi body; lysosomes; centrioles; chloroplasts; vacuoles; nucleus; chromatin; nuclear envelope; nucleolus; plasmodesmata. The function of these organelles. Structure of prokaryotic cells and viruses. Comparison of cell structure of eukaryote, animal and plant, prokaryote and virus.

The cytoplasm in eukaryotic cells is organised by membranous structures, such as the ER, lysosomes, mitochondria etc. The membrane of these structures is called internal membrane. It is important in providing a transport system separating areas from the rest of the cytoplasm, providing a large surface area for the attachment of enzymes and other reactants and for ATP synthesis (respiration).

There are many organelles within the cytoplasm of a eukaryotic cell:

• Mitochondria - The mitochondria are the site of aerobic respiration, where ATP is produced. It has a double membrane separated by a narrow fluid filled space, with the inner membrane folded to form cristae. The inside of a mitochondrion contains a matrix which contains mitochondrial DNA and ribosomes as the mitochondria are self-replicating. Some respiration takes place in the matrix and some takes place on the inner membrane, with the cristae giving a larger surface area for reaction. Muscles and other active cells have lots of mitochondria.

• Endoplasmic Reticulum - The ER is an extensive membrane system of flattened sacs called cisternae, and may be continuous with the external nuclear membrane and Golgi body. The **smooth ER **does not have associated ribosomes and is used in lipid and steroid synthesis. The **rough ER **has associated ribosomes and is involved in protein synthesis and transport.

• Ribosomes - Ribosomes consist of two subunits, large and small, made of rRNA and protein. Free ribsosomes exist in the cytoplasm, and fixed ribosomes exist on the RER. Ribosomes synthesise proteins using mRNA that has been transcribed from DNA in the nucleus.

• Golgi Apparatus (Golgi Body) – The golgi apparatus is a series of dynamic sacs made up of RER pinched off at the ends to form small vesicles, which then fuse together to form the golgi apparatus. The main function of the golgi apparatus is to modify/package newly synthesised proteins so they can be used in other parts of the cell, or a different part of the organism. Other functions include producing secretory enzymes, secreting carbohydrates, transporting and storing lipids and forming lysosomes.

• **Lysosomes – **Lysosomes are small vacuoles formed when part of the Golgi apparatus is pinched off. They contain and isolate digestive enzymes from the rest of the cell – they can release enzymes and destroy worn organelles. They can also digest material taken into the cell – they contain lytic enzymes, so when phagocytosis occurs, the lysosome can discharge its contents into the vesicle formed around the pathogen.

• **Centrioles – **Centrioles are located in the cytoplasm just outside the nucelus in a distinct region called the centrosome. A centriole is two hollow cylinders that are at right angles to each other. They are used in spindle formation and attachment during cell division.

• **Chloroplasts – **The chloroplasts are bound by a double membrane and contain a fluid called stroma. This contains ribosomes, lipid droplets and possibly starch grains as well as circular DNA as chloroplasts are self replicating. Inside the stroma are flattened stacks called thylakoids which are stacked to from grana, which is where photosynthetic pigments are found. The grana are connected by intergranal membranes. The main function of the chloroplasts is photosynthesis and the grana are arranged in such a way that there is a large surface area for trapping light energy.

• Cell wall – consists of cellulose fibrils embedded in a polysaccharide matrix. IT provides strength and support and permits the movement of water from cell to cell.

• **Vacuoles – **Small temporary vesicles are formed in animal cells during phagocytosis. Plant cells have a large permanent vacuole, filled with cell sap and bound by a single membrane called the tonoplast. Cell sap is a storage site for chemicals such as glucose and provides an osmotic system which supports young tissues.

• **Nucleus – **The nucleus contains the genetic material of the cell and is the site that controls cell activities. It is made up of four different parts:

• **Nuclear envelope – **double membrane which surrounds the nucleus, it has gaps in it, which are the nuclear pores.

• **Nucleolus – **Darkly staining area of the nucleus, it is the site of ribosomal RNA (rRNA) synthesis.

• **Chromatin – **Chromatin is made up of loosely coiled DNA and histone proteins.

• Nuclear Pores – Allow messenger RNA (mRNA) and nucleotides to travel between the cytoplasm and the nucleus.

• Plasmodesmata – The plasmodesmata are strands of cytoplasm that link cells together.

Prokaryotic cells have no true nucleus – the DNA lies in the cytoplasm. Prokaryotes have a cell wall containing murein and mesosomes, invaginations in the cell membrane that are the site of respiration. They have small circular chromosomes called plasmids which often carry genes for antibiotic resistance. They have small 70S ribosomes as opposed to the larger 80S found in eukaryotes.

Viruses are obligate, intracellular parasites that do not respire (they have no mitochondria) and can only reproduce inside living cells. They are only classified as living when they infect host cells, which they destroy after reproducing. Each virus is made up of a core of nucleic acid surrounded by a protein coat.

Plant Animal Prokaryote Virus
Cell Wall Cellulose - Murein -
Chloroplasts Yes - - -
Vacuole Large, permanent Small, temporary
Centrioles Yes Some - -
DNA True nucleus True nucleus In cytoplasm DNA or RNA
Organelles Membrane bound Membrane bound
Respiration Occurs in mitochondria Aerobic in mitochondria, anaerobic in cytoplasm Occurs on the mesosomes -
Ribosomes 80S 80S 70S -

b) **Levels of organisation: aggregation of cells into tissues. Brief histology of: epithelium, cuboidal and ciliated; muscle, smooth and striated; connective tissue, collagen. Aggregation of tissues into organs. **Cells are made up of organelles, which work together to perform the function of the cell. Similar cells carrying out the same function form a tissue, with several tissues carrying out the same function forming an organ.

**Epithelium – **There are 3 main types of epithelial cell: squamous, cuboidal and ciliated.

**Squamous epithelium – **composed of flattened cells, which is thin and delicate. It is ideal if substances need to diffuse across from one side to the other efficiently. It is the lining of capillaries and around the walls of the alveoli in the lungs.

**Cuboidal Epithelium – **Consists of roughly cube shaped cells that tessellate. The tissue is found lining many glands and ducts. The kidney tubules are lined with cuboidal epithelium where the surface of the cells have microvilli present to increase the surface area for reabsorption of substances.

**Ciliated Epithelium – **Have cilia on the free surface of the cells. This tissue is found lining walls of tubes and cavities where substances are ‘wafted’ along. In the respiratory tract, cilia waft mucus and dust along until it reaches the top of the trachea, where it can be sneezed or coughed out.

**Muscle – **Muscle tissue is able to contact and change length, developing tension which enables us to move, when the tissue is attached to part of the skeleton. Muscle can be classified as skeletal muscle, cardiac muscle or smooth muscle.

Skeletal Muscle – This is also known as striated muscle, and its contraction is controlled by the nervous system. It is under voluntary control, and appears stripy due to where the muscle fibres overlap.

Smooth Muscle – This is found in the wall of the gut, blood vessels and other tubes throughout the body. Normally under involuntary control, i.e. its contraction happens automatically. Each smooth muscle fibre is a single elongated, spindles shaped cell that contains thread-like micro fibrils responsible for contraction. They may be oriented circularly and control width, or longitudinally and control length.

**Connective tissue – **connective tissue binds organs and tissues together as well as filling the spaces between them. It is strong and its basic structure is a jelly like matrix. The main types are areolar, collagen, elastic and adipose.

Areolar Tissue - the most basic kind of connective tissue found under the skin and between organs. It contains two types of protein, unbranched collagen fibres and branched elastic fibres.

**Collagen tissue – **contains mostly collagen fibres and has a high tensile strength. It is quite flexible but not very stretchy, it stores enough potential energy to be used in movement. Tendons are made of collagen.

b) **Levels of organisation: aggregation of cells into tissues. Brief histology of: epithelium, cuboidal and ciliated; muscle, smooth and striated; connective tissue, collagen. Aggregation of tissues into organs. **Cells are made up of organelles, which work together to perform the function of the cell. Similar cells carrying out the same function form a tissue, with several tissues carrying out the same function forming an organ.

**Epithelium – **There are 3 main types of epithelial cell: squamous, cuboidal and ciliated.

**Squamous epithelium – **composed of flattened cells, which is thin and delicate. It is ideal if substances need to diffuse across from one side to the other efficiently. It is the lining of capillaries and around the walls of the alveoli in the lungs.

**Cuboidal Epithelium – **Consists of roughly cube shaped cells that tessellate. The tissue is found lining many glands and ducts. The kidney tubules are lined with cuboidal epithelium where the surface of the cells have microvilli present to increase the surface area for reabsorption of substances.

**Ciliated Epithelium – **Have cilia on the free surface of the cells. This tissue is found lining walls of tubes and cavities where substances are ‘wafted’ along. In the respiratory tract, cilia waft mucus and dust along until it reaches the top of the trachea, where it can be sneezed or coughed out.

**Muscle – **Muscle tissue is able to contact and change length, developing tension which enables us to move, when the tissue is attached to part of the skeleton. Muscle can be classified as skeletal muscle, cardiac muscle or smooth muscle.

Skeletal Muscle – This is also known as striated muscle, and its contraction is controlled by the nervous system. It is under voluntary control, and appears stripy due to where the muscle fibres overlap.

Smooth Muscle – This is found in the wall of the gut, blood vessels and other tubes throughout the body. Normally under involuntary control, i.e. its contraction happens automatically. Each smooth muscle fibre is a single elongated, spindles shaped cell that contains thread-like micro fibrils responsible for contraction. They may be oriented circularly and control width, or longitudinally and control length.

**Connective tissue – **connective tissue binds organs and tissues together as well as filling the spaces between them. It is strong and its basic structure is a jelly like matrix. The main types are areolar, collagen, elastic and adipose.

Areolar Tissue - the most basic kind of connective tissue found under the skin and between organs. It contains two types of protein, unbranched collagen fibres and branched elastic fibres.

**Collagen tissue – **contains mostly collagen fibres and has a high tensile strength. It is quite flexible but not very stretchy, it stores enough potential energy to be used in movement. Tendons are made of collagen.