CELLS

Cells are small compartments that hold all of the biological equipment necessary to keep an organism alive and successful on Earth. The main purpose of a cell is to organize. Cells hold a variety of pieces and each cell has a different set of functions. It is easier for an organism to grow and survive when cells are present. If you were only made of one cell, you would only be able to grow to a certain size. You don't find single cells that are as large as a cow. Also, if you were only one cell you couldn't have a nervous system, no muscles for movement, and using the internet would be out of the question. The trillions of cells in your body make your life possible.

One Name, Many Types

There are many types of cells. In biology class, you will usually work with plant-like cells and animal-like cells. We say animal-like because an animal type of cell could be anything from a tiny microorganism to a nerve cell in your brain. Plant cells are easier to identify because they have a protective structure called a cell wall made of cellulose. Plants have the wall; animals do not. Plants also have organelles like the chloroplast (the things that make them green) or large water-filled vacuoles.

We said that there are many types of cells. Cells are unique to each type of organism. Humans may have hundreds of types of cells. Some cells are used to carry oxygen (O2) through the blood (red blood cells) and others might be specific to the heart. If you look at very simple organisms, you will discover cells that have no defined nucleus (prokaryotes) and other cells that have hundreds of nuclei (multinucleated). The thing they all have in common is that they are compartments surrounded by some type of membrane.

CELLS

Plasma Membrane
• Partially permeable: O2 and CO2 pass through by simple diffusion (also ethanol; drugs)
o Glucose, amino-acids, etc. enter by facilitated diffusion
o Water enters by osmosis
o Na+, K+, Cl – glucose etc, enter by active transport (sodium pump) – needs ATP
• Selectively permeable: controls entry and exit of specific materials.
• Keeps cell contents together allowing efficient coordination of its activity.
• Homeostasis - interior of cell maintained at optimum pH etc for efficient metabolism

Cytoplasm
• The living contents of a cell excluding the nucleus and large vacuoles.
• A complex solution in which the cell’s organelles are suspended.
• Many biochemical processes take place here, e.g., glycolysis, protein synthesis.

Nucleus

• Present in eukaryotes only – NOT prokaryotes (i.e. bacteria).
• Contains DNA (on chromosomes), the hereditary material.
• The genes are present in the DNA.
Passes on genes to next generation of cell (through mitosis)
• DNA (deoxyribonucleic acid) is visible as chromatin in active cells
• In chromosomes during mitosis and meiosis.
• Not present in a red blood cells (or bacteria!).

Nuclear Pores
• Large molecules can pass between the cytoplasm and the nucleus through these pores.
• E.g. mRNA from nucleus to cytoplasm. – and nucleotides the other way.

Chromatin
Chromatin is the very fine thread-like combinations of DNA and protein in non-dividing nuclei. The protein assists in the efficient packaging and regulation of DNA activity. A human nucleus contains 46 such fine threads of chromatin.

Chromosomes
A chromosome is a ‘condensed chromatin’ thread only visible during mitosis and meiosis.
Haploid nuclei (n) have one set of chromosomes i.e. one of each kind of chromosome. Diploid nuclei (2n) have two sets of chromosomes i.e. two of each chromosome.
The nuclei of human somatic (= body) cells are diploid (2n), with 46 chromosomes i.e. 23 x two sets.
One set from the mother in the haploid egg cell, the other from the father’s haploid sperm cell.
Sex Chromosomes: the 23rd pair. Female: XX Male XY
Each chromosome has a unique set of genes. Each gene has a specific locus – it is on a particular chromosome at a specific site.
Different (mutant) forms of a gene (A, B, O for the blood-group gene) are known as alleles.

Centrioles
Have a ‘9 + 2’ arrangement of protein tubes (tubulin).
Two sets of tubes, each at right-angles
Divides to from spindle at cell division.

Lysosomes
Contain digestive enzymes for recycling macromolecules within the cell
Responsible for ‘cell death’ – burst to carry out ‘autolysis’


Mitochondria:
• The aerobic steps of respiration occur here – Krebs Cycle and Electron Transport System.
• 36 of the 38 ATPs from one molecule of glucose are produced in the mitochondrion.
• Liver, muscle and nerve cells are rich in mitochondria.
• Sperm cells have a ‘mid-piece’ containing many mitochondria
• Bone and fat cells have low numbers of mitochondria.
• Found in low numbers in plant cells



Flagellae / cilia
Composed of ‘9 + 2’ protein structure; have basal body at base, which provides power
Falgellae, few, long; cilia many, short, synchronised beating (metachronal rhythm)
Responsible for cell movement (sperm) or fluid movement (trachea and Fallopian tubes)

Ribosomes:
• Composed of RNA and protein.
• Function in protein synthesis – translation of mRNA into a specific sequence of amino acids.
• 70s – prokaryotes & mitochondria and chloroplasts – 80s all eukaryotes

Chloroplast
• Photosynthesis is the major function of the chloroplast.
• The light stage takes place in the green internal membranes.
• The dark stage occurs in liquid part of the chloroplast.
• Starch may be stored in the chloroplast



Large Plant Cell Vacuole
• Storage of water, food (sugar, amino acids), ions, wastes.
• Role in cell elongation during plant growth.