Revision:Edexcel Biology Unit 1 Molecules and Cells - Cells

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Cellular organization

Aggregations of cells

An overview of a multicellular organism. Where cells make up the entire body. This is as follows

Cells-->Tissues->Organs-->individuals

There are distinct differences between an animal cell and a plant cell. Among the many, they are:

1. Vacuoles which are found to be much larger in plant cells
2. Chloroplasts which are found more abundantly in plant cells
3. Cell walls are usually present in plant cells

Cell membrane

• The membrane functions as a passage of control for the substances into and out of the cells. The membrane is not only present on the surface of the membrane but also at the membrane of organelles (mitochondrion, golgi apparatus, chloroplsts etc.)
• It follows the fluid mosaic model at where fluid represents the phospholipid molecules having a fluid arrangement and mosaic represents the random arrangement of proteins, which moves above the phospholipid bilayer.
• The model shows phospholipids pointing head up. This is because the head section of the phospholipds is hydrophilic (soluble in water) where else the tail is hydrophobic (insoluble in water)
• Thus, having to know that, the bilayer only allows non-polar molecules to diffuse through a concentration gradient. Molecules which are sufficiently small (such as water, oxygen and carbon dioxide diffuses through the membrane freely regardless of being polar or not. Polar molecules are only allowed to enter if they are incorporated into a carrier protein at where it is actively transported into the cell or undergo facilitated diffusion (diffusion using a protein molecule).
• Glycolipids are also present on the cell surface membrane since it allows cell recognition and also to promote an immune response
• Carbohydrates and cholesterol functions to reduce the fluidity of the membrane and maintain its stability to ensure that no leaking occurs.
• Proteins which are present can ether be intrinsic or extrinsic and is important in maintaining the stability of the membrane.

Endoplasmic Reticulum (ER)

Smooth ER

• Important with the synthesis of lipids and the production of steroids.

Rough ER

• Important at protein synthesis since it has ribosomes at the top of its surface.

Golgi apparatus

• It is a stack of flattened cistern and associated vesicles.
• It functions to modify proteins which are produced from the process of protein synthesis
• After the synthesis of the proteins, the product is brought out of the cells with the cell through a secretary vesicle. As the secretory vesicles fuses with the membrane, it releases its contents into the surroundings through the process of exocytosis.
• The Golgi apparatus is also important in the process producing lysosomes, which contains hydrolytic enzymes. These hydrolytic enzymes cause the breakdown of unwanted organelles. It is also important to digest phagocytic vesicles.

Nucleus

• Contains a double layers membrane which separates the genetic material from the cytoplasm.The nuclear membrane sometimes contains ribosomes for protein synthesis.
• Among the contents of the Nucleus are chromosomes, chromatin, histones, rRNA.

Mitochondrion

• It is rod shaped with a double layered membrane consisting of cristae for increasing the surface area.The membrane of the mitochondria is called the matrix.
• It is involved in the production of ATP from cellular respiration.

Chloroplasts

• Contains a double layered membrane with molecules of chlorophyll anchored on it. It also has stacks of granum (formed by stacks of thylakoids) and its cytoplasm Is called the stroma.
• It also contains starch, lipid droplets and circular DNA.
• Involved in the process of photosynthesis.

Microtubules

• Fine tubular organelles which contribute to the network of fibrous protein.
• It is necessary for microtubules to be present since it provides the maintenance and shape of certain organelles such as centrioles and is also present in the flagellum of some bacterias.

Cell walls

• The composition of cell wall varies from one organism to another
• In fungi, its cell wall composition is made out of chitin.
• In bacteria, its cell wall composition is made out of murein
• In plant cell walls, its composition is made out of cellulose.
• The main function of plant cell walls are to provide mechanical strength and the resist expansion of plant cells when subjected to high hydrostatic pressure.

The difference between prokaryotic cells and eukaryotic cells

• Prokaryotes are basically cells with a smaller size wherelse eukaryotic are larger
• Prokaryotes have smaller (70s) ribosomes whereas eukaryotes have larger (80s) ribosomes
• Prokaryotes lack a true nucleus whereelse a eukaryotes has a real nucleus present
• Prokaryotes have no membrane bounded organelles whereas eukaryotes do.
• Flagellum present in prokaryotes lacks a 9+2 arrangement of microtubules whereelse eukaryotes does not.

Transfer across cell membranes

Diffusion

• There are 2 types of diffusion which are facilitated diffusion and simple diffusion.
• Simple diffusion involves the transfer of molecules across a membrane or barrier following a concentration gradient.
• Facilitated diffusion involves the transfer of molecules across a barrier or membrane following a concentration gradient with the help of a protein carrier. This is because as the molecules bind on the the protein, it changes its shape in a way that it allows the molecule to be detached form the protein and get released into the cytoplasm of the cell or vice versa.

Osmosis

• It is the process whereby water is moved according a concentration gradient across a semi-permeable membrane.

Active transport

• It is a process whereby molecules are moved across a membrane or barrier against the concentration gradient with the help of a protein carrier and a molecule of ATP.
• The molecules bind on to the protein and ATP releases a phosphate group to the protein which alters the protein shape for it to release its contents against the concentration gradient.

The Cell cycle (mitosis)

Interphase

• Separated into 3 phases which are the G1,S and G2 phase.
• In the G1 stage, the cell is undergoing a period of rapid growth.
• In the S stage, DNA molecules are replicating and therefore the number of chromosomes are now doubled here. Histones are built up and the chromosomes are divided into two chromatids.
• In the G2 stage, more cell growth is taking place at where some organelles divide and there is an accumulation of energy stores

Prophase

• This phase can be seen clearly through a light microscope since it is positively charged and it takes up the orcein staind much easier.
• Chromosomes become shorter and thicker on this stage, nucleoli starts to disappear.

Metaphase

• Chromosomes get attached to the spindle fibres and the chromosomes align at the equator of the cell

Anaphase

• Centromers divide and the spindle fibres shorten and pulls the centromers to opposite poles. (attracted by centrioles)

Telophase

• Chromosomes begin to uncoil from histones and the nuclear envelopes begins to form.

Cytokinesis

• A process whereby furrows are formed after the process of anaphase .It only occurs in animal cells .A plant cell forms cell walls between the two nucleus formed to separate the cells.

Also See

• Edexcel Biology Unit 1 Molecules and Cells - Molecules
• Edexcel Biology Unit 1 Molecules and Cells - Nucleic Acids and Enzymes
• Edexcel Biology Unit 1 Molecules and Cells - Cells
• All A Level Biology Revision Notes

Comments

These notes are aimed at people studying Edexcel Biology A Level, but will be suitable for other exam boards too.

Originally written by MalaysianDude on TSR Forums.