CELL STRUCTURE

Animal cell

Cell surface membrane

• Structure:
• A partially permeable barrier.

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• Function:
• controls the movement of substances in and out of the cell.

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Nucleus

• Structure
• Largest cell organelle, enclosed by an envelope of two membranes that is perforated by nuclear pores.
• It contains chromatin which is the extended form of chromosomes.
• It also contains a nucleolus.

Nucleus

• Functions
• Chromosomes contain DNA, the molecule of inheritence.
• DNA is organised into genes which control all the activities of the cell.
• Responsible for nuclear division.
• The nucleolus manufactures ribosomes.

Cytoplasm

• Structure
• The material between the nucleus and the cell surface membrane is known as cytoplasm.
• Cytoplasm is an aqueous (watery) material, varying from a fluid to a jelly-like consistency.
• Many small structures can be seen within it. These have been likened to small organs and hence are known as organelles.
• Functions:
• Site of many chemical reactions.

Organelle

• An organelle can be defined as a functionally and structurally distinct part of a cell.
• Organelles themselves are often surrounded by membranes so that their activities can be separated from the surrounding cytoplasm.
• This is described as compartmentalisation.
• Having separate compartments is essential for a structure as complex as an animal or plant cell to work efficiently.
• Since each type of organelle has its own function, the cell is said to show division of labour, a sharing of the work between different specialised organelles.

Mitochondrion

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• Structure:
• Surrounded by an envelope of two membranes, the inner being folded to form cristae.
• Contains a matrix with a few ribosomes, a circular DNA molecule and phosphate granules.
• Functions:
• Specialised for aerobic respiration

Golgi apparatus

• Structure:
• A stack of flattened membrane-bound sacs.
• Function:
• Internal processing and transport system.
• Golgi vesicles are formed and transport the materials to other parts of the cell or to the cell surface membrane for secretion.

Endoplastic reticulum

• Structure:
• A system of flattened, membrane-bounded sacs called cisternae, forming tubes and sheets.
• It is continuous with the outer membrane of the nuclear envelope.
• Function:
• If ribosomes are formed on its surface it is called rough ER, and transports proteins made by the ribosomes through the cisternae.
• Smooth ER is a site of lipid and steroid synthesis.

Ribosomes

• Structure:
• Very small organelles consisting of a large and a small subunit.
• They are made of roughly equal parts of protein and RNA.
• Slightly smaller ribosomes are found in mitochondria and chloroplasts in plants.
• Function:
• Incolved in protein synthesis

Lysosome

• Structure:
• A simple spherical sac bounded by a single membrane and containing digestive (hydrolytic) enzymes.
• Function:
• Mainly concerned with breakdown of structures or molecules.
• For example,
• Get rid of old organelles, digest bacteria taken in by phagocytosis.

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Cell wall

• Structure:
• Consists of cellulose
• Function:
• Provides mechanical support and protection.
• It allows a pressure potential to be developed which helps in support.
• It prevents osmotic bursting of the cell.

Plasmodemata

• Plant cells are linked to neighbouring cells by means of fine strands of cytoplasm called plasmodesmata (singular: plasmodesma), which pass through pore-like structures in their walls.
• Structure:
• A fine cytoplasmic thread linking the cytoplasm of two neighbouring cells through a fine pore in the cell walls.
• Function:
• Enables a continuous system of cytoplasm, the synpolast, to be formed between neighbouring cells for transport of substances between cells.

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Vacuole

• Although animal cells may possess small vacuoles such as phagocytic vacuoles, which are temporary structures, mature plant cells often possess a large, permanent, central vacuole.
• Structure:
• A sac bounded by a single membrane called the tonoplast.
• It contains cell sap, a concentrated solution of various subtances, such as mineral salts, sugars, pigments, organic acids and enzymes.

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Vacuole

• Function:
• Storage of various subtances including waste substances including waste products.
• It makes an important contribution to the osmotic properties of the cell.

Chloroplasts

• Structure:
• Contains chlorophyll for photosynthesis.
• It is surrounded by an envelope of two membranes and contains a gel-like stroma through which runs a system of membranes that are stacked in places to form grana. It may store starch.
• The stroma also contains ribosomes, a circular DNA molecule and lipid droplets.

Chloroplasts

• Function:
• It is the organelle in which photosynthesis takes place, producing sugars from carbon dioxide and water using light energy trapped by chlorophyll.
• Light energy is converted to chemical energy.